Tuning the color and excited state properties of the azulenic chromophore: NIR absorbing pigments...

Journal of Photochemistry and Photobiology C: Photochemistry Reviews4 (2003) 179–194

Review

Tuning the color and excited state properties of the azulenicchromophore: NIR absorbing pigments and materials

Robert S.H. Liu∗, Alfred E. AsatoDepartment of Chemistry, University of Hawaii, 2545 McCarthy Mall, Honolulu, HI 96822, USA

Received 18 July 2003; received in revised form 1 September 2003; accepted 3 September 2003

Abstract

In this review, we summarize the work carried out at the University of Hawaii on the highly colored, non-alternant hydrocarbonazulene. Through systematic perturbation of the HOMO and LUMO molecular orbitals, we showed that the characteristic azure colorcan be changed in a manner that covers the entire visible spectrum based on this single chromophore. Some of the modified azulenesexhibit unusual excited state properties: reversal of energy gap of low lying excited states, unprecedented high fluorescence yield, andlong fluorescence lifetime for an upper state in condensed media. Highly polarized polyene aldehydes attached to azulene at C-1 as a�-electron donating end group have been successfully incorporated into bacterioopsin to generate NIR absorbing bacteriorhodopsin pig-ment analogs—the longest wavelength absorbing analog withλmax = 830 nm. Azulene-containing donor–acceptor and symmetrical dyechromophores were synthesized and their excited state absorption spectra, and other properties determined. Non-linear optical proper-ties were also examined. The azulenic dye chromophores are reverse saturable absorbers and are potentially useful for optical limitingapplications.© 2003 Japanese Photochemistry Association. Published by Elsevier B.V. All rights reserved.

Keywords: Azulene; S2-emission; Bacteriorhodopsin; Near infrared pigments; Color-tuning; Non-linear optical properties

Contents

1. Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1801.1. Alternant and non-alternant hydrocarbons. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1801.2. Polarization and charge distribution. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 181

2. New areas of research. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1812.1. New substituted azulenes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 182

2.1.1. Color tuning via selective perturbation of molecular orbitals of azulene. . . . . . . 1822.1.2. Changes of excited state properties in substituted azulenes. . . . . . . . . . . . . . . . . . . 184

2.2. Near infrared absorbing retinoid binding proteins. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1842.2.1. Azulenic bacteriorhodopsin analogs. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1842.2.2. The delocalized PSB cation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1862.2.3. Sequential formation of pigments. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1872.2.4. NIR absorbing rhodopsin analogs. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 188

2.3. Possible photochemistry from S2 of azulene. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1882.4. Azulenic non-linear optical compounds. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 189

2.4.1. Azulene-containing donor–acceptor chromophores. . . . . . . . . . . . . . . . . . . . . . . . . . . 1892.4.2. Azulene-containing symmetrical dye molecules. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 190

3. Future directions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 192Acknowledgemtns. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 192References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 192

∗ Corresponding author. Tel.:+1-808-956-5723; fax:+1-808-956-5908.E-mail address: [email protected] (R.S.H. Liu).

1389-5567/$ 20.00 © 2003 Japanese Photochemistry Association. Published by Elsevier B.V. All rights reserved.doi:10.1016/j.jphotochemrev.2003.09.001

180 R.S.H. Liu, A.E. Asato / Journal of Photochemistry and Photobiology C: Photochemistry Reviews 4 (2003) 179–194

1. Introduction

Ever since its isolation from the essential oils of medicinalherbs, azulene1 has piqued the imagination of chemists dueto its singularly most striking property—its brilliantly bluecolor (hence, the name derived from “azure”)[1]. In con-trast, even though both azulene and naphthalene have thesame chemical composition (C8H10) and degree of unsatu-ration (five double bonds and two fused rings), naphthalene2 is completely colorless. The color of azulene is readilyexplained upon inspection of its absorption spectrum whichfeatures an unusually low-lying, first excited state S1 [2],an unusually large S1–S2 energy gap[3], and a rather broadregion of transparency between∼380 and 480 nm (the blue-window of azulene). It is this gap that dictates the anomalousfluorescence of azulene from S2 [3], the first proven case inviolation of the Kasha’s rule[4]. Thereafter, we witnessed aplethora of papers on the preparation of new azulene deriva-tives with much of the early synthetic contributions madeby Nozoe[5], Heilbronner[2] and Hafner[7] that paved theway for subsequent fluorescence and other studies[6,8,9].

Our involvement with azulene and its derivatives stemsfrom our interest in preparing red-shifted visual pigmentanalogs. Our basic premise was quite simple: if we replacedthe trimethylcyclohexenyl end group of retinal (see struc-ture 4 below) with a delocalized�-electron system in con-jugation with the side chain, the new chromophore couldabsorb deep in the red region. Subsequent incorporation ofthis new red-shifted retinal analog into rhodopsin, the vi-sual protein, or similar retinal-proteins, ought to afford anartificial pigment analog absorbing deep in the near infrared(NIR) region [10]. Indeed, the effort proved fruitful as wewere able to prepare a number of NIR-absorbing azulenicbacteriorhodopsin (bR) analogs[10].

Extending the scope of our research led to a more gen-eralized program of preparing azulene derivatives for colortuning, material science studies, and for generating novelred-shifted retinal-protein analogs. In this review, we sum-marize our effort in these directions.

1.1. Alternant and non-alternant hydrocarbons

It is well known that the color of an unsaturated chromo-phore depends on its length of conjugation[11]. For exam-ple, consider the series of unsaturated compounds3–6.

1,3-Butadiene (3, λmax = 217 nm) is colorless, retinal (4,368 nm)[12] is yellow, �-carotene (5, 448 nm) is orange,and lycopene (6, 472 nm) is red[13]. The reason for thebathochromic shift of absorption maxima is the closing ofthe HOMO and LUMO gap with an increase of length ofconjugation[11].

This explanation clearly does not apply to the cyclic con-jugated hydrocarbons azulene1 and naphthalene2 both withthe same number (5) of conjugated double bonds. To ex-plain the blue color of azulene and the absence of color innaphthalene, we need to examine their respective groundand excited state�-electron distributions. The major differ-ence between the compounds is that naphthalene is an alter-nant hydrocarbon (as are the linear polyenes3–6) whereasazulene is a non-alternant hydrocarbon. It has been pointedout by many researchers that these two sets of conjugatedsystems (alternant and non-alternant) possess molecular or-bitals very different in properties which contribute to theirdrastically different transition energies of the low-lying elec-tronic states[14]. Naphthalene and azulene are no more thanrepresentative members of the two classes of conjugatedsystems.

The probabilities of locating electrons in the atomic or-bitals of the HOMO, LUMO and LUMO+ 1 orbitals ofazulene and the HOMO and LUMO orbitals of naphthaleneare depicted graphically inFig. 1 [15,16].

For the lowest excited state, the electronic configurationis that shown in the middle ofFig. 1, the singly occupiedorbitals HOMO, LUMO. For azulene, the electron distribu-tion is clearly different between HOMO and LUMO, butsimilar between HOMO and LUMO+ 1. In HOMO, muchof the electron density is localized in C1 and C3 of thesmaller ring with negligible electron density on the ring car-bon atoms on the long axis of the molecule. In contrast toHOMO, a substantial shift of electron density toward the lar-ger ring (at C4 and C8) and the carbon atoms on the longaxis (C2 and C6) is clearly evident in its LUMO. TheMO descriptions are consistent with the large ground statedipole moment for azulene (ca. 1 debye) and the reversal ofthe dipole moment for the excited state[17]. As a result,the HOMO and LUMO electron pair occupy different re-gions of the molecule with very little overlap or exchange;that is, electron–electron repulsive interactions substantiallyreduced.

R.S.H. Liu, A.E. Asato / Journal of Photochemistry and Photobiology C: Photochemistry Reviews 4 (2003) 179–194 181

Fig. 1. Probability of locating the electron (square of coefficients of eachwave-function) in the HOMO, LUMO and LUMO+ 1 of azulene (left)and naphthalene (right).

A different situation exists in naphthalene wherein orbitalpairing results in the electrons occupying the same regions inthe molecule with substantial overlap and electron–electronrepulsion[15]. Thus, in this situation the unpaired electronsin HOMO and LUMO are perforce localized on the samecarbons, i.e., they are in the “same room.” While staring ateach other, what do you think they tell each other? “YOUARE REPULSIVE!” [17].

Hence, the difference in excitation energies of these twoisomeric compounds is not due to the expected small differ-ences in HOMO and LUMO energies as a result of the dif-ferent connectivities in the two ring systems[17]. But rather,it is due to the large difference in repulsive energies betweenthe HOMO and LUMO electrons in the two cases. This dif-ference is, in fact, general for all alternant and non-alternanthydrocarbons; but clearly, it is not an effect understandablebased on one-electron HMO approach (see parallel argu-ments for singlet–triplet splitting)[18]. We shall elaboratethis point further when we discuss color tuning of the azu-lenic chromophore.

1.2. Polarization and charge distribution

Another fundamental difference between alternant andnon-alternant hydrocarbons is that the former HC’s are rela-

tively poor electron donors or acceptors. For alternant HC’s,polarization requires the presence of a donor and an ac-ceptor substituents. Charge transfer is also dependent onthe position of attachment of electron donor or acceptorgroups. For donor–acceptor alternant HC’s the functionalgroups must be in a “starred–unstarred” arrangement asillustrated for 1-amino-4-cyanonaphthalene,7. For exam-ple, in contrast, the latter HC’s are capable of substantialcharge separation when substituted with a single donor oracceptor group. The non-alternant hydrocarbon azulene canserve as either an electron donor or an acceptor depend-ing on the nature and location of the substituent. This isdue to the well known electron-rich characteristic of thefive-ring and the electron-deficient nature of the seven-ring,giving a large dipole moment for a ground state hydrocar-bon. For a substituted azulene with an electron-withdrawinggroup on C-1 (8) or an electron-donating group on C-4 (9),or C-6, is stabilized by charge delocalization and forma-tion of Hückel 6e aromatic seven- or five-member rings,respectively.

In contrast to the 1-CHO isomer8 above, azulene-2-car-boxaldehyde (10) is not resonance stabilized since the CHOgroup at an even-numbered carbon is not conjugated to theseven-member ring.

2. New areas of research

Through the early work of Heilbronner on a series ofmethylated azulenes[2], it was established that methylsubstitution at the odd carbon atoms of azulene caused ared-shift of the S1-band while substitution at the even num-bered carbon atoms caused a blue shift[9] (Table 1). Theshifts were attributed to perturbation of the weakly donating(inductive) methyl substituent on HOMO and LUMO ofazulene. When located at C-1 (or C-5), where the HOMOhas a high electron density, the substituent exerts a desta-bilizing influence and raises its energy. At the same time,the LUMO has a nodal point at C-1; hence, the methylsubstituent has no effect on the level of this MO. The netresult is closing of the HOMO–LUMO gap and, hence a redshift in the absorption spectrum. When located at C-2, themethyl substituent produced the opposite effect, destabiliz-ing the LUMO while having no effect on HOMO, hence ablue-shift.


Table 1Spectral shifts (λmax) in azulenes with inductively donating (methyl),resonance donating (fluoro), and withdrawing (CHO, CN) substituentsa

Compound S1–S0 transition

νmax (cm−1) �νmax

Azuleneb 17241 −7911-Methylb 26450 +4292-Methylb 17670 +3694-Methylb 17610 −3515-Methylb 16890 +4596-Methylb 17700 −12411-Fluoroc 16000 −23211,3-Difluoroc 14920 +12091-CHO 18450 +12091,3-Di-CHO 19724 +24832-Cyanod 15750 −14912,4-Dicyanoe 15720 −15212,6-Dicyanoe 15480 −17612,4,6-Tricyanoe 14619 −26222,4,8-Tricyanoe 14619 −26222,4,6,8-Tetracyanoe 13790 −3451

aIn hexane.b[2].c[31].d[25].e[26].

2.1. New substituted azulenes

2.1.1. Color tuning via selective perturbation of molecularorbitals of azulene

The spectral shifts induced by the weakly electron do-nating methyl group do not visibly change the characteris-tic azure color of the unsaturated chromophore. For visiblecolor changes, a more strongly electron donating substituentis clearly a better choice. The resonantly electron-donatingand inductive electron-withdrawing properties of fluorine arewell known[19]. We carried out the fluorination of azuleneand its derivatives using a twofold excess of commerciallyavailable Selectfluor® [20] and obtained both 1-fluoro- and1,3-difluoroazulene (20% each) as a blue and a green solid,respectively[17,21] (preparation of the same compound bya different procedure is also known[22]). The method wasextended to preparation of other fluorinated azulenes listedin Table 1.

The strongly resonance (�) donating[19] characteristicof fluorine, exerts a large perturbation on molecular or-bital energies and the HOMO–LUMO gap. For comparisonof substituent effects, selected data are listed inTable 1.The S2-fluorescence of the fluorinated azulenes was subse-quently examined. InTable 2, are listed in detail the absorp-

tion and emission characteristics of all fluorinated azulenes[23]. In the case of 1,3-difluoroazulene,11, the perturbedtransition energy significantly red-shifted the S1-absorptionband (Fig. 2). The result of opening of the “blue-window”of azulene allows transmission of the yellow light. Hence,11 exists as dark green crystals.

By the same token, one would expect an electron with-drawing substituent to have an opposite effect[24]. How-ever, most neutral electron-withdrawing substituents areunsaturated which complicates matters by imposing an ad-ditional conjugative effect. For example, for an aldehydesubstituent at C-1, the electron-withdrawing effect willcause a blue shift of S1 but the conjugative effect shouldcause a red-shift. And, it will also exert different effects onthe S2-level. Which of the two effects are likely to domi-nate? The answer is found in the absorption spectra of someof the compounds prepared (Fig. 2).

The S1-band of azulene-1-carboxaldehyde is clearlyblue-shifted from that of azulene. (The O–O band for eachcompound is marked with an asterisk (Fig. 2).) Hence, theelectron-withdrawing effect dominates for this non-alternanthydrocarbon. The S2-band is red-shifted (385 nm) and moreintense than the S2-band for azulene (341 nm). Interestingly,the shift of the S1-band might have created the impressionthat extended conjugation led to a blue-shift!

Thus, by manipulating the location of a resonantlyelectron-donating fluorine and an electron-withdrawingaldehyde CHO group on azulene, we were able to pre-pare azulene derivatives that exhibit colors covering theentire visible spectrum[23,24]. For example, in the caseof azulene-1-carboxaldehyde8, the electron withdrawingsubstituent caused a blue-shift of the S1-band and a si-multaneous smaller red-shift of the S2-band (Fig. 2). Thenet result is closing of the azulene “blue-window”, al-lowing a simultaneous transmission of small amounts ofthe blue and the red light; hence the compound is ma-roon in color. The presence of another CHO substituent inazulene-1,3-dicarboxaldehyde,12, resulted in the furthernarrowing of the S1–S2 energy gap such that the blue lightwas practically no longer transmitted and the simultaneoustransmission of more red-light (Fig. 2). Hence, the com-pound now appeared red. When dissolved in trifluoroacetic

acid, one of the two aldehyde groups is protonated (see struc-ture 13), (facilitated by formation of the tropylium cation)creating an even stronger electron-withdrawing substituent.Hence, the further red-shift of the S1-band permits trans-mission of yellow-orange light and led to a merged S1 and


Table 2Absorption maxima and extinction coefficients of substituted azulenesa

Compound Solvent Color λmax (S2)(nm)

ε (S2)(×103 M−1 cm−1)

λmax (S1)(nm)

ε (S1)(×102 M−1 cm−1)

S1 (O–O)(×103 cm−1)

ΦF τ (S2)(ns)

Azulene1a Hexanes Blue 341 4.01 580 3.48 14.33 0.046 1.3EtOH 340 4.20 576 3.62 14.47 0.035 1.3

Azulene-d8a Cyclohexane 354d 695d 14.05 0.072 1.7EtOH 349d 687d 14.02 0.060 1.7

1-CHO 8b Hexanes Maroon 385 1.00 542 4.67 15.43

1-Fa Hexanesa Blue 343 3.26 625 3.23 13.02 0.057 2.4EtOH 341 3.54 618 3.52 13.16 0.058 2.5

1-CHO, 3-Fb Hexanes Blue 398 8.91 590 5.37 14.02CH2Cl2 386 10.81 568 6.25 14.28

2-CHO 10b Hexanes Green 360 6.31 664 1.554-CHOb Hexanes Green 354 1.95 642 3.555-CHOb Hexanes Violet 383 9.01 571 4.50 14.516-CHOb Hexanes Blue–greenc 351 5.75 652 3.63 12.341-F, 6-CHOb EtOH Greenc 354 3.71 706 2.29 11.23 0.01 1.9

1,3-F2 11a Hexanes Greenc 342 3.29 670 1.44 11.88 0.15 6.9EtOH 341 3.39 665 2.51 12.72 0.20 9.5

1,3-F2,6-CHO 14b Hexanes Pale greenc 750 10.13EtOH 336 4.67 738 2.57 10.39 0.01 7.1

1,3-(CHO)2 12b Hexanes Red 507 16.64CH2Cl2 384 10.55 486 8.20 17.54CF3COOH Yellow 374 12.05

4-Isopropyla Hexanes 343d 682d 14.411-F-4-Isopropyla Hexanes 347d 745d 15.39 0.019 0.51,3-F2-4-Isopropyla Hexanes 28.82 12.26 16.56 0.051 2.7

1,3-Cl2a Cyclohexane 27.14 12.79 14.35 0.073 0.99EtOH 27.40 13.10 14.30 0.062 0.88

1,3-Br2a Cyclohexane 27.03 13.16 13.87 0.16 0.20

4,6,8-Trimethyla Cyclohexane 27.82 15.53 12.29 0.001 0.015EtOH 27.78 15.78 12.00 0.001 0.04

aFrom [31] unless otherwise specified.b[24].cBlue in solution.dO–O band.

Fig. 2. UV-Vis absorption spectra of azulene,1 ( ) 1,3-difluoroazulene, 11 (– · –), 1,3-difluoroazulene-6-carboxaldehyde,14, (–· · –)azulene-1-carboxaldehyde,8 (- - -), azulene-1,3-dicarboxaldehyde,12 (· · ·) in hexane and12 in trifluoroacetic acid (with asterik (∗).


S2 bands (Fig. 2). The color of the solution turned yellow.The O–O bands of these compounds along with other fluori-nated azulenecarboxaldehyde derivatives are listed inTable 2[24].

Absorption data on reported cyano-substituted azulenes[25,26] also demonstrate dramatic shift of the S1 and S2bands.

2.1.2. Changes of excited state properties in substitutedazulenes

Perhaps more surprising is that the substantial loweringof the HOMO has resulted in a reversal of energy gapsbetween the three low lying states: in 1,3-difluoroazulene,11, the S2–S1 gap is∼4,000 cm−1 wider than the S1–S0gap! Not too surprisingly, this compound exhibits unusualexcited state properties.

The unusual anti-Kasha behavior of azulene (strong emis-sion from S2 and little from S1) was attributed to the largeseparation of its S2 and S1 bands (∼14,000 cm−1, approx-imately the same as that between S1 and S0 states). TheS2-fluorescence yield is 3–5% and its lifetime being∼1 ns[27]. (It was shown recently that the S2-state of azulene hasanother sub-picosecond deactivation channel that proceedsdirectly from the initially excited species[28].) Interestingly,the S1-state with a similar 14,000 cm−1 separation from theground state, deactivates extremely rapidly (<1 ps) with afluorescence yield of∼10−6 [29]. Crossing of the potentialcurves was suggested in the early fluorescence study[3].Recent conical intersection calculations confirmed this sus-picion [30].

In the case of11, with an even larger separation ofthe S2–S1 states, its S2-fluorescence yield was found toapproach 20% (in ethanol) and with fluorescence life-time close to 10 ns[31]. Both numbers are clearly un-usually high for an upper excited state in a condensedmedium. Fluorescence yields and lifetimes for other flu-orinated azulenes are listed inTable 2 [24,31]. Clearly,there is a general trend of an increase in such val-ues upon F-substitution at 1 or 3 position. However, inthe case of 1,3-difluoro-6-azulenecarboxaldehyde (14),while the separation of the “S1 and S2” bands is fur-ther widened, there is in fact a decrease in the excitedstate lifetime and in its fluorescence yield[24]. We sus-pect possible presence of an intervening n,�-excitedstate, too low in intensity to be detected in the absorp-tion spectrum, that provides another channel of deactiva-tion.

2.2. Near infrared absorbing retinoid binding proteins

Rhodopsin and bR are heptahelical membrane boundretinal proteins. For visual pigments, rhodopsin is theequivalent of the more sensitive black and white film, andiodopsins in three different forms (blue, green and red) arepigments in the color films. All four pigments have a com-mon chromophore, 11-cis-retinal,15, attached to the proteinvia a protonated Schiff base (PSB) linkage[32]. However,their absorption maxima are different with rhodopsin muchred-shifted to 498 nm and the three iodopsins at 460 (blue),530 (green) and 560 nm (red)[32]. Spectral tuning is accom-plished in Nature by minor variation of protein structures.

Many rhodopsin analogs were known in which the11-cis-retinal chromophore was replaced by a syntheticretinal analog. Those with absorption maxima red-shiftedrelative to native rhodopsin[33–35]are listed inTable 3a.

Bacteriorhodopsin, the retinal-binding protein respon-sible for proton-pumping activity (as part of a photo-cycle) as a means for solar energy storage, contains alltrans-retinylidene PSB chromophore[36]. The all-transstructure (including the 6-s-trans conformation) contributesto the longer absorption maximum at 568 nm[35]. Thechromophore of its photoproduct is not detached from theprotein and it contains the blue-shifted 13-cis geometry(548 nm). Several bR analogs with modified all-trans reti-nal chromophores were known to absorb to the red of thenative pigment (Table 3b) [35,37,38].

2.2.1. Azulenic bacteriorhodopsin analogsIn our effort to prepare red-shifted pigment analogs, we

set our sight for analogs absorbing beyond the normal visibleranges (i.e., >700 nm). To achieve such a large red-shift,a more drastic structural modification for the new retinalanalogs was needed with some other form of perturbationthan those listed in retinal analogs inTable 3. A simpleand expedient approach was to start with a small, coloredchromophore and append it to a short dienal or trienal sidechain. Azulene was an obvious choice, the first absorptionband of which already appears partially beyond 600 nm.

At the time we started the project, the Columbia grouphad already reported on the preparation of several azulenicretinal analogs and their incorporation into bR. Surprisingly,their bR pigments were not red-shifted as expected, but,instead, were more blue shifted than the native bR[39]. Sincewe were unable to offer explanations for such “unexpectedresults”, we decided initially to examine systematically thefate of the∼600 nm S1-absorption band of azulene uponincreasing the length of the appended conjugated side chain.


Table 3Red-shifted pigments from retinal analogs

(a) Rhodopsin and analogs (nm) (b) Bacteriorhodopsin and analogs (nm)

Bovine rhodopsina (11-cis-retinal) 498 trans-bRe (all-trans-retinal) 56814-Methylretinalb 508 6-s-trans ring fusedd 5963-Dehydroretinalb 515 13-CF3-retinalb 60212-Fluororetinalc 507 4-Dimethylaminophenylf 61414-Fluororetinalc 527 Tetraenecyanineg 6626-s-cis ring fusedd 539 14-Fluororetinalh 587 and 68014-Chlororetinalc 544 14-Chlororetinalh 69013-CF3-retinalc 545 Indole-cyanine-retinali 610–755Chicken cone-reda (11-cis-retinal) 571

aSee, e.g., Y. Shichida, T. Yoshizawa, Methods Enzymol. 81 (1982) 333–354.b[33].c[34].d[35].e[36].f [37].g[47].h[49].i [38].

Much to our surprise, in our hands preparation of azu-lenic bR pigments absorbing beyond 568 nm and even be-yond 700 nm was not a problem[10]. In Table 4, are listedthe absorption maxima of all the azulenic retinal analogscontaining a dienal (16), or a trienal (17) side chain and gua-

Table 4Properties of bacteriorhodopsin analogs derived from azulene dienals, dienals and tetraenals

Retinal analog Pigmentλmax (nm) Timec

(50% pigment)Stability withNH2OHd

OS (cm−1)e

RCHOa PSB Initialb Finalb

All- trans-retinal, 15 381 440 568 640 12 min Stable (4100) 6100

Az-dienal,16 435 575 620 – 17 h 40 min (3300)4-Me, 16a 438 522 620 – 20 h 4 min (3100)6-Me, 16b 440 540 629 674 50 m 100 min (2600) 37006-iso-Pr, 16c 438 535 630 674 50 min 9 h (2800) 39006-t-Bu, 16d 436 532 628 673 53 min 10 h (2900) 39003-F-6-iso-Pr, 16e 443 526 630 672 48 min 3 h (3200) 44006-t-Bu-2′F, 16f 450 558 632 686 47 min 12 min (2900) 30004,6,8-Tri-Me,16g 444 533 629 – 96 h (2900)4-Me-7-iso-Pr, 16h 450 538 595 – 36 h 6 min (1800)

Guaiaz-1-dienal,18 465 560 642 312 h <1 and 15 minf (2300)Guaiaz-2-dienal,l9 403 443 514 – 36 h (3100)

Az-trienal, 17 455 542 645 – 4 h <1 and 40 minf (3000)2′-F-, 17a 463 572 694 748g,h 40 min (2400) 41005-t-Bu-, 17b 653 562 664 – 72 h (2700)

Guaiaz-trienal,20 482 590 660 764g,h 20 min <1 and 40 minf (1700) 38002′-F, 20a 493 640 684 799 16 min <1 and 60 minf (1000) 31003-CF3-, 2b 506 556 640 830 10 min <1 and 5 minf (2400) 6000

Guaiaz-tetraenal,21 477 614 624 774 10 min – (260) 3000

aIn ethanol.bEstimate values obtained by inspection of binding curves.cTime required for formation of 50% of final pigment, or initial pigment if the later not available.dFinal concentration of 2 M. Time for 50% depletion of final pigment.eParenthesis for initial pigment.f Instantaneous disappearance of initial pigment, then disappearance of PSB.gLess than 10% yield.hAfter curve resolution.

iazulene retinal analogs containing a dienal (18), a trienal(19) or a tetraenal (20) side chain[40–42]. Many of the pig-ments absorbing beyond 700 nm, several beyond 750 nm,and a trifluoromethylated analog (19b, 3′-CF3) was foundto absorb maximally at 830 nm.


That pigment formation was possible for azulenic alde-hydes with varying chain lengths and alkyl substituents andalkyl substituents reflects flexibility of the binding site ofthe opsin[43]. However, there is a limit in this flexibility—too long or too short chromophores do not yield the mostred-shifted, narrow band pigments (see more detailed discus-sion below). For the trienals, the length of the chromophoreis closest to that of all-trans retinal (15), thus yielding mostred-shifted pigments. There was also an unexpected prefer-ence for size (t-butyl over methyl) and location (C-6 overC-4) alkyl substitution in a study with substituted azulene-dienaldehydes[44] (see data in the next section).

Our results were in apparent contradiction to the Columbiaefforts. The only structural difference between the reportedazulenic retinal analogs (e.g.,21) [39] and ours (16–20) wasthe position of attachment of the unsaturated side chain tothe azulene ring: at the C-2 position for, e.g.,21 and a C-1for 16–20. In this structural variation must lie the key tothe unique red-shift mechanism for such polarized polyenechromophores.

2.2.2. The delocalized PSB cationAs mentioned before, in forming the pigment, the azulenic

retinals, e.g., the dienal16, are expected to bind with bac-terioopsin (bO) with a protonated Schiff base linkage, e.g.,22. The PSB side chain is an electron-withdrawing group.When attached at the 1-position of the five-membered ring,the azulene bicyclic system provides an internal mechanismfor delocalization of the iminium positive charge in a mannersimilar to that discussed above for increased charge delocal-ization for any electron-withdrawing substituent at C-1 (8a).The tropylium 6e Hückel system contributes to the stabilityof the resonance structure22′. The net result is delocalizationof the positive charge toward the 7-ring, i.e., away from thecounter anion. Such a separation of charges in an ion-pair isa known cause for red shift[45]. On the other hand, the PSBof 2-azulene dienal (23) is incapable of existing in the formof a tropylium ion stabilized resonance structure, giving in-

stead a pentadienylcation with exocyclic double bonds,23′(in a manner similar to 2-azulenecarboxaldehyde,10, dis-cussed above).

We have since demonstrated by calculations that the po-sition of the symmetry-forbidden S1-state of azulene is in-sensitive to increase of the length of the PSB side chain[23]. On the other hand, the symmetry-allowed S2-bandred-shifts rapidly upon increasing the length of the conju-gated PSB side chain when it is attached at the C-1 posi-tion. When the change of the S1-levels is minimal, for the1-substituted systems the S1, S2 states cross between analogswith two and three double bonds (Fig. 3), i.e., a reversal ofthe two low-lying state upon increase of chain length. Forthe 2-substituted analogs, crossing of S2 and S1 is avoideddue to a combination of a high energy for S2, a low energyfor S1 and the levels being relatively less sensitive to changeof length of conjugation[24]. Under these circumstances,the low intensity S1-band remains as the lowest lying state.But, they are difficult to detect because of the tail of themuch more intense S2-band. The net result is that the visi-ble absorption spectrum is dominated by the S2-band, whichappears in a region more blue shifted than that of bR.

Fig. 3. Calculated (ZINDO) levels of the forbidden S1 and allowed (bold)S2 states of protonated methyl Schiff bases of 1-substituted polenylsand 2-substituted polyenyl with oscillator strengths listed above all lines.Clearly, the two states cross for the 1-substituted compounds and do notfor the 2-substitued (data based on those reported in[23]).


Fig. 4. Binding curves obtained during incubation of bacterioopsin with azulenic dienals[46]: (a) the parent compound,16, (b) 4-methyl, 16a, (c)6-methyl,16b, (d) 6-t-butyl, 16d, (e) 3-fluoro-6-iso-propyl, 16e, and (f) 6-t-butyl-2′-fluoro, 16f, showing progressive formation of the red-shifted pigments.

2.2.3. Sequential formation of pigmentsThere are more than one pigments formed during binding

interaction of the azulenic retinal analogs with bO with thefinal pigment formation requiring as long as 2–3 weeks.Fig. 4a–f show the binding curves of six related azulenedienals. The binding curves reveal different trends rangingfrom conversion to a single product, A to B or A to C forcases a and f (presence of isosbestic points) to consecutiveconversions of A to B to C for cases b and e (absence of

isosbestic points)[45,46]. Clearly, the first pigment formedpossesses a broad absorption band which gradually givesway to a sharp, red-shifted absorption band in cases b–f.The structure of the latter band (case f, sharp, narrow bandwith a blue shoulder) is reminiscent of those of cyanine dyebR pigments[47] or azulenylazulenium salts (seeFig. 5below). Therefore, we suspect that the positive chargeis extensively delocalized in the second pigment assistedby the involvement of the stabilized tropylium-containing


ion (structure22′) as an important resonance contributingstructure. An analysis of components contributing to theopsin shift of these red-shifted band lends support to theidea of maximum delocalization of the positive charge[40,46].

In bR, there is still the interesting question of the na-ture of perturbation that led to the unusually red-shiftedfinal O intermediate prior to regeneration[48] of bR andthe previously unexplained appearance of stable red-shiftedminor pigments in cases of 14-fluoro and 14-chloro-bR[49]. The presence of a halogen atom in close proximity tothe PSB assists charge delocalization in these bR analogs.These results are in accord with results from earlier stud-ies of fluorinated retinoids, in which we demonstrated thatelectron-withdrawing atoms close to the electron withdraw-ing end group enhance delocalization of the positive charge[34,50]. (Note that the fluorine atom is not located at theproper position to contribute resonance stabilization of thecation.)

Formation of the red-shifted pigments, favoring bulkiersubstituent at C-6, likely involves extensive rearrangementof the surrounding protein residues. This is suggested by thelow stability of such pigments toward external reagents. Forexample, the final, most red-shifted pigments are much lessstable toward hydroxylamine compared to native bR[41].Their half lives, ranging from<1 min to 10 h, are listed inTable 4. The low stability of azulene-containing bR analogsas well as other artificial bR pigments derived from othersynthetic chromophores is probably due to an increased like-lihood of exposure of the PSB linkage to external reagentsas a result of a less than perfect fit of the non-native chro-mophore. (In contrast,trans-bR is stable in the presence ofhydroxylamine[36].)

It is noteworthy that proton pumping activity is likely tobe absent in the red-shifted azulenic bR analogs. In thesesystems, the characteristic photocycle of bR[33] was nolonger detectable[51]. We suspect that a highly favorabledeactivation pathway involving the low-lying S1-state ofazulene supercedes deactivation through torsional relax-ation (photoisomerization) of the side chain. Therefore,these pigments are effectively long wavelength light ab-sorbers that undergo physical relaxation processes with-out competitive chemical deactivation pathways, i.e., nophotoisomerization.

2.2.4. NIR absorbing rhodopsin analogsWe also attempted to prepare azulene-containing retinal

analogs for formation of red-shifted rhodopsin analogs[52].In this case the more labile 11-cis isomer is needed. Weobserved earlier that azulenic chromophores containing acis-double bond underwent ready thermal isomerization tothe more stabletrans form. To circumvent this dilemma,we incorporated acis-directing trifluoromethyl group intothe trienal side-chain to produce a 9-cis equivalent struc-ture 24 [34]. Unfortunately, the compound failed to bindwith opsin even though 9-cis retinal (15) readily forms a

rhodopsin analog[53]. This result, however, was not sur-prising in retrospect because earlier we found that isomersof the corresponding naphthalene retinal analog also failedto give a rhodopsin analog[54]. These results are consis-tent with a somewhat more restrictive hydrophobic bindingsite for the visual protein compared to bR[55]. Efficientbinding in rhodopsin requires a substantial ring-chain twistat the 6,7 bond. For example, for phenyl retinal analogsonly those with substituents at bothortho-positions give pig-ments in high yields[56,57]. The problem of red-shiftedrhodopsin analogs was eventually solved with the prepara-tion of pigment analogs derived from retinal analogs,25,26 [52]. The merocyanine dye like PSB (27) provides a dif-ferent internal mechanism for delocalization of the positivecharge,27′. These pigments (λmax at 653 and 710 nm, re-spectively) are potentially the beginning of a set of chem-ical sensors for detecting the threshold energy for visualtransduction[52]. (It should be noted that the 710 pigmentis only 10% as active as the 653 in visual transduction[52].)

2.3. Possible photochemistry from S2 of azulene

With the known long lifetime of the second excited singletstate of azulene (ns), one would logically anticipate possiblechemical reaction from this upper state. Indeed, recently T.-I.Ho was able to demonstrate that the photoarylation reactionof azulene originates from the second excited singlet state[58].

We have also observed that di-1,1′-azulenylmethane (28)is unstable upon prolonged irradiation with short wave-length light (exciting the S2-band). The product retainedpart of the azulene absorption[59]. Whether this is a case ofdi-�-methane rearrangement[60] from the S2-state requiresfurther structural proof of the photoproduct.


Table 5Absorption maxima,βµ-values, and melting points of30–33

Compounds λmax (nm)a βµ (10−48 esu)b Td (◦C)c

30 513 911 29031 582 1323 24732 477 906 28033 555 1400 314

aIn dioxane.bAt 1.907�m.cDSC.

2.4. Azulenic non-linear optical compounds

The encouraging results from our studies of azulene-co-ntaining retinal derivatives and their corresponding bRanalogs prompted us to undertake a systematic investiga-tion into the non-linear optical (NLO) properties of azu-lenic donor–acceptor chromophores and azulenic cyaninedye-like molecules.

2.4.1. Azulene-containing donor–acceptor chromophoresAt the onset, we sought to compare quantitatively the

donor strength of azulene and its derivatives to knownamino-substituted aromatic donors. Position of an electron-withdrawing substituent should be located at C-1, -3, -5 or-7 for reasons discussed inSection 2.1.1. Therefore, com-pounds containing electron-withdrawing substituents at C-1of azulene were prepared.

Second-order NLO coefficients (βµ-values) and thermalstabilities of representative azulenic donor–acceptor chro-mophores (29, 30, 31) were measured by the EFISH (electricfield induced second harmonic) technique and compared tothe NLO and physical properties ofpara-N,N-diethylamino-phenyl substituted donor–acceptors (32, 33) (Table 5) [61].

Theβµ-values of30 and31 were essentially identical tothose of the conventional donor–acceptor chromophore (32and 33) of equal length. However, the absorption maximafor the azulenic DA chromophores were more red-shiftedthan the corresponding nitrogen-containing derivatives. Thethermal stability of the azulene derivatives was also betterthan their nitrogen-containing counterparts.

Compounds29–31 exhibit strong solvatochromic behav-ior. For example, for compound31, the absorption maximumin cyclohexane was at 567 nm and in hexafluoro-iso-pro-panol at 632 nm. A recent paper reporting results on asimilar set of donor–acceptor molecules of the general

structure34 also described strong solvatochromic behavior[62].

A theoretical study of the NLO properties of azulenicDA molecules revealed several structural features for theenhancement of first molecular hyperpolarizabilities andβµ-values [63]. Using AM1/FF and ZINDO/S-CI ap-proaches, the calculatedβµ-values were in close agreementwith experimental results. Moreover, when substituted withboth donor and acceptor groups on either ring, the azulenemolecule was calculated to be a more efficient conjugationbridge than either benzene or thiophene. Azulene derivativeswith 1-donor and 5-acceptor groups (35) were also predictedto have exceptionally large second-order coefficients and,more importantly, relatively small dipole moments (a con-clusion not entirely evident from a single consideration ofresonance contributing structures,35a), a useful property forprocessing and fabrication of NLO electrooptical materials.

First, molecular hyperpolarizabilities for several represen-tative azulenic donor–acceptor chromophores (29–31) weredetermined by the hyper-Raleigh scattering (HRS) methodat both 1064 and 1907 nm excitation wavelengths[63]. Mea-surements at the longer wavelength presented some technicaldifficulties, but gave more accurateβ-values. Results fromthis HRS study suggested the involvement ofmore than oneexcited state, since the data could not be correlated using thewidely accepted two-level model. Basically, this same con-clusion was subsequently arrived at in a resonance Ramanstudy of DA chromophore30 in which strong dispersion inthe relative intensities of Raman lines for the charge-transfer


Table 6Absorption maxima, dipole moments and linear polarizabilities for chro-mophores36–41

Compound λmax (nm) (ε)a �µ (D) α (×10−23 esu)

36 458 (28100) 3.6 (3) 2.4037 458 (30400) 3.6 (3) 2.5838 449 (27800) 3.5 (3) 2.2439 464 (29700) 4.0 (1) 2.5340 518 (38600) 2.79 (6) 3.3441 480 (30700) 4.5 (3) 2.83

aIn PMMA film.

band was attributable to multiple state involvement[64]. Anintriguing explanation for these spectroscopic results simi-lar to the one alluded to earlier might conceivably involvemixing of the normally very low intensity azulene S1 bandwith the charge-transfer transition resulting in a concomitantdramatic increase in intensity[65]. The involvement of addi-tional excited states including a putative twisted intramolec-ular charge-transfer species has also been postulated in a fastkinetics study of the transient absorption properties of azu-lenic molecules[66]. In all cases, azulenic chromophoresexhibited negligible fluorescence.

Linear polarizabilitiesα and changes in dipole moment�µ were determined by Stark spectroscopy for a seriesof guaiazulenic fluorinated azobenzene donor–acceptor(36–41) in a PMMA matrix (Table 6) [67].

The polarization effects were modest—both dipole mo-ment and polarizability changes were quite small and dif-ficult to measure. For the monofluorinated chromophores36–38, the position of attachment of the fluorine substituentappeared to have no effect on either�µ or α. Compound40 was the most red-shifted and had the largestα of thisset of molecules, but, unexpectedly, it had the smallest�µ.The bis-trifluoromethylated derivative41 had the largest�µ

consistent with stronger electron-withdrawal for this groupcompared to fluorines directly attached to the benzene ring.

2.4.2. Azulene-containing symmetrical dye moleculesThird-order susceptibilitiesχ(3) were measured for a

series of azulenic compounds using modified degeneratefour-wave mixing (DFWM) techniques[68–71]. From thisdiverse set of compounds emerged several excellent can-didates for optical limiting—the symmetrical guaiazulenic

Fig. 5. Absorption spectra of azulenylazulenium cyanine dyes42–45 inchloroform (arbitrary absorbance).

trimethine dyes (42–45). These dye molecules, electroni-cally similar to conventional cyanine or polymethine dyes,are reverse saturable absorbers (RSAs)—compounds thatbecome more strongly absorbing as the incident light in-tensity or energy is increased. Representative absorptionspectra for dye molecules with one to four double bonds(42–45) are shown below (Fig. 5).

In general, the dye molecules are stable to light andair in the solid state. The long-wavelength absorbing hep-tamethine dye partially degrades to a new blue-shiftedspecies upon dissolution in dichloromethane, but in thepresence of lithium perchlorate, the solution is indefinitelystable. We also measured the electrochemical redox po-tentials for a number of azulenic chromophores includingthe series of dye molecules (data not shown). Excellentcorrelation of�(3) values, first and second redox potentials,conjugation length, and absorption energies was obtained[72].

Aggregation phenomenon—dimerization and H- orJ-aggregate formation—is frequently encountered with cya-nine dyes and can lead to large optical non-linearities[73].We examined the temperature and concentration dependentNMR behavior of an intrinsically achiral, ring-locked gua-iazulenic pentamethine dye and observed unexpected split-ting for the isopropyl methyls at low temperature[74]. Nosplitting of these signals was observed for dye moleculeslacking a central ring substituent (R= CH3 or C(CH3)3)creating a prochiral carbon that is a prerequisite for chiraldimeric aggregation. These observations were consistentwith a non-symmetrical dimeric “staggered brickwork”assembly (seeFig. 6) for compound46, but not with sym-metrical “face-to-face” aggregate (Fig. 6).

Dimer formation was not observed at low concentrations(10−4 to 10−6 M) or higher temperatures (RT) by UV-Visor NMR spectroscopy.


Fig. 6. Simulated staggered (brickwork) dimeric arrangement of an azu-lenylazulenium cyanine dye,46.

Excited state properties for a series of azulenic re-verse saturable absorber dyes were determined by thetwo-dimensionalZ-scan method[70,71]. In general, thesechromophores exhibited large excited state absorptioncross-sections, broad band spectral response over the visi-ble region, and extremely rapid sub-psec response times—highly desirable properties for optical limiting and otherNLO applications. In addition, excited state absorptionspectra and lifetimes for guaiazulenic RSA dyes were de-termined by picosecond transient absorption spectroscopy[66]. In keeping with HRS and Resonance Raman studiesdescribed earlier, the sub-picosecond excited state kineticmeasurements revealed the involvement of at least threedistinct excited state transient species.

Upon excitation a very short-lived Franck-CondonSn species in an “un-relaxed” solvent cage is initiallyformed. Rapid solvent reorganization then gives rise tosolvent-equilibrated S1. Subsequently, the longer lived “sol-vent relaxed” S1 species undergoes an intramolecular con-

Fig. 7. Normalized excited singlet state absorption spectra of a series of azulenic dyes in dichloromethane:29 (- - -), 42 (· · ·), 43 (—), 44 (- · -) and45 (- · · -).

formation adjustment to an even longer-lived, significantlyred-shifted twisted-excited state charge-transfer speciesthat ultimately undergoes radiationless decay to the groundstate. The normalized excited state absorption spectra of aseries of azulenic dyes (29, 42–45) effectively blankets theentire visible region (Fig. 7).

The ultrafast kinetic response times for the deactivationof the azulenic molecules are probably associated with thenovel “breathing” vibration modes localized on the bondsconnected to the central ring-fusion bond. These in-planevibrations in azulene have been calculated to provide a con-venient and rapid channel to the ground state via a so-calledconical intersection[30].

In general, the various excited state transformations ofazulenic chromophores are confined to the singlet manifoldwith little involvement of competing deactivation pathways;i.e., the quantum yields for photoisomerization, intersystemcrossing, and fluorescence are essentially negligible.

For optical limiting applications, reverse saturable ab-sorber dyes must be processible; i.e., the dyes must bethermally stable, resistant to oxidation, photostable, andsoluble in organic solvents[75]. NLO polymer films wereprepared from guaiazulenic polymethine dyes and polymerssuch as polystyrenesulfonic acid, PSSA, an acidic polyelec-trolyte, and polycarbonate copolymers. Both the absorptionspectra and color (gray–green) of a guaiazulene trimethineand PSSA dye–polymer solution in alcohol and a thin filmcast on glass from the solution were essentially identical.However, the dye–polymer film exhibited unexpected ther-mochromic behavior. Briefly warming the film resulted inits bleaching to a tan color (Fig. 8).

Upon cooling, the film to room temperature, the greencolor rapidly returned. The intensity of the main band after


Fig. 8. (a) Absorption spectra of (1) guaiazulenic trimethine dye43 inmethanol (—) and (2) a thin film of dye43 and polystyrenesulfonicacid (PSSA) on glass (- - -). (b) Absorption spectra of (1) the thermally“bleached” dye43–PSSA polymer film (- - -) and (2) the same polymerfilm sample after cooling to room temperature.

one heating–cooling cycle was somewhat lower (ca. 5%)than in the original spectrum. Mechanisms associated withthe thermochromic processes are still unclear.

3. Future directions

Along with a discussion on perchloroazulene, Lemalsuggested that perfluoroazulene would be an interest-ing target molecule[76]. Our studies suggest instead1,3,5,7-tetrafluoroazulene will likely be the polyfluorinatedderivative that will exhibit a maximum effect from theelectron-donating characteristics of the fluorosubstituents.At the even numbered carbons, fluoro-substituents willhave an opposite effect. To further enhance red-shift of theS1-band and increase of the S1–S2 gap, one will have tointroduce electron-withdrawing substituents at C-2, -4, -6and -8 positions. In fact, the synthesis of 2,4,6,8-tetracy-anoazulene has been reported[26]. The UV-Vis absorptiondata also showed an inverted state alignment: a S2–S1 gapof 13,790 cm−1 and S1–S2 gap of 10,970 cm−1. Hence2,4,6,8-tetracyano-1,3,5,7-tetrafluoroazulene (47) would bean ultimate target which might have a thermally accessibleS1-state and perhaps a very large window that will transmit

nearly all visible light because of the expected much en-larged S1–S2 gap. We might point out that the cyano groupis likely to be a

better electron-withdrawing substituent than an aldehydegroup because of its compact size, thus more likely to re-tain molecular planarity and maximum effect of conjugativeinteraction.

It should be clear that the interesting behavior describedfor the azulene chromophore is only a beginning fornon-alternant hydrocarbons in general. For example, ace-heptalene (48) is also known to have a low lying S1-state(lower than that of azulene) and a sizable S1–S2 gap[77].In fact, its S1-state is now well into the NIR region. Theincreased red-shift of the S1-band is likely due to a furtherdecrease of the electron–electron repulsive interaction withthe increase of molecular size. It will be of interest to exam-ine substituent effects and properties of materials derivedfrom 48 and other non-alternant hydrocarbons.

Acknowledgements

This project was supported by grants from the Army Re-search Office (DAAH19-99-1-0205), the US Public HealthServices (DK-17806) and the National Science Foundation(CHE01-32250). X.-Y. Li and R. Muthyala contributed muchof the experimental work described in this paper. Othercontributors to our azulene program were: I. Koukhareva,H.-R. Li, S. Shevyakov, X.-S. Wang, D. Watanabe and W.Schroeder. We also wish to acknowledge our collaboratorswho made many of the measurements of the azulene deriva-tives and contributed to understanding of the excited stateproperties of the molecules reported herein: Drs. Y. Chang,T.-I. Ho, D. Jameson, R. Larsen, V. Rao and J. Wang.

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[49] M.E. Tierno, D. Mead, A.E. Asato, R.S.H. Liu, N. Sekiya, K.Yoshihara, C-W. Chang, K. Nakanishi, R. Govindjee, T.G. Ebrey,Biochemistry 29 (1990) 5948–5953.

[50] L.U. Colmenares, A.E. Asato, R.S.H. Liu, Spectrum 14 (2001) 8–12.[51] J.R. Bell, R.S. Muthyala, R.W. Larsen, M. Alam, R.S.H. Liu, J.

Phys. Chem. A 102 (1998) 5481–5483.[52] H. Imai, T. Hirano, A. Terakita, Y. Shichida, R.S. Muthyala, R.-

L. Chen, L.U. Colmenares, R.S.H. Liu, Photochem. Photobiol. 70(1999) 111–115.

[53] R.S.H. Liu, Y. Shichida, in: V. Ramamurthy (Ed.), Phochemistryin Organized and Constrained Media, VCH, New York, 1991,pp. 817–840.

[54] M. Denny, A.E. Asato, unpublished results.[55] R.S.H. Liu, T. Mirzadegan, J. Am. Chem. Soc. 110 (1988) 8617–

8623.[56] A. Maeda, A.E. Asato, R.S.H. Liu, T. Yoshizawa, Biochemistry 23

(1984) 2507–2513.[57] L.U. Colmenares, R.S.H. Liu, Tetrahedron 52 (1996) 109–118.[58] T.-I. Ho, C.-K. Ku, R.S.H. Liu, Tetrahedron Lett. 42 (2001) 715–717.[59] H.-R. Li, A.E. Asato, unpublished results.[60] H.E. Bhameram, in: W.M. Horspool, P.S. Song (Eds.), CRC

Handbook of Organic Photochemistry and Photobiology, CRC Press,New York, 1994, pp. 184–193.

[61] A.E. Asato, R.S.H. Liu, V.P. Rao, Y.M. Cai, Tetrahedron Lett. 37(1996) 419–422.

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[64] J.N. Woodford, C.H. Wang, A.E. Asato, R.S.H. Liu, J. Chem. Phys.111 (1999) 4621–4628.

[65] W. Leng, C.H. Wang, A.E. Asato, A.M. Kelley, J. Phys. Chem. A106 (2002) 9479–9484.

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Robert S. H. Liu was born in the year of the Tiger, received his earlyeducation in Shanghai, Hong Kong, college education at Howard PayneCollege, Texas and his PhD degree from California Institute of Technology(with G.S. Hammond). He worked 4 years as a research chemist at duPont(with H.E. Simmons), then joined the chemistry department at Universityof Hawaii in 1968, where he remains as a professor. He had the pleasureto work in the laboratories of George Porter (The Royal Institution)and George Wald (Harvard Biological Laboratories) during one of his

early sabbatical leaves. In his photochemical studies (on polyenes, secondtriplet states, vitamin A isomers, visual pigments, azulenes) he attemptsto integrate results from kilo-second time-resolved photochemical studieswith ideas put forward by ultrafast spectroscopists. He was a SloanFellow (1970–72) and a Guggenheim Fellow (1974–75). In 1998, hewas one of six 60-year-old photochemists honored at the Boston ACSNational Meeting (for part of the festschrift, see: J. Phys. Chem. A(1998) 5316–5317).

Alfred E. Asato was born in Chicago, Illinois to expatriated citizens ofthe then territory of Hawaii. He attended the University of Illinois inChampaign (BS, 1964), the University of Washington, and, thereafter, theUniversity of Hawaii where he received his PhD in 1972. Thereafter, hejoined the research group of Professor Robert Liu with whom he hasenjoyed a long and fruitful association to this day. An organic synthetikerat heart, his initial research efforts focused on the synthesis of novel 7-cisanalogs of retinal as well as aromatic and fluorinated analogs for studiespertaining to the chemistry of vision. Later work included the preparationof retinoic acid derivatives for anticancer studies, azulenic retinals forbacteriorhodopsin studies, and azulene-containing donor–acceptor and dyechromophores for non-linear optical studies. His current research interestsinclude the synthesis of model compounds for study of the Hula-Twistphotoisomerization process and azulene-containing chromoproteins.

Tuning the color and excited state properties of the azulenic chromophore: NIR absorbing pigments...

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