Tautomeric forms of PPI dendrimers functionalizedwith 4-(40-ethoxybenzoyloxy)salicylaldehyde chromophores

M. Franckevicius a,⇑, R. Vaišnoras b, M. Marcos c, J.L. Serrano c, A. Gruodis a,d, N. Galikova a,d, V. Gulbinas a,d

a Institute of Physics, Center for Physical Sciences and Technology, Savanoriu Ave. 231, LT-02300 Vilnius, Lithuaniab Liquid Crystals Laboratory, Lithuanian Educological University, Studentu 39, LT-08106 Vilnius, Lithuaniac Facultad de Ciencias de Materiales de Aragon, Universidad de Zaragoza, Pedro Cerbuna 12, E-50009 Zaragoza, Spaind Department of General Physics and Spectroscopy, Faculty of Physics, Vilnius University, Sauletekio 9, LT-10222 Vilnius, Lithuania

a r t i c l e i n f o

Article history:Available online 9 January 2012

Keywords:PPI dendrimersKeto-enol tautomerismtrans–cis isomerismChromophore groupsSchiff base

a b s t r a c t

Bonding of the promesogenic unit derived from 4-(40-ethoxybenzoyloxy)salicylaldehyde to the aminoterminated PPI dendrimer chains results in formation of the salicylidenimine chromophore groups.Absorption and fluorescence investigations of the dendrimer solutions supported by the quantum chem-istry calculations revealed that the chromophore groups may exist in enol and keto tautomeric formswith relative concentrations depending on the dendrimer generation and solvent. The dendrimer fluores-cence is attributed to np⁄ states of keto tautomers which may also be formed from excited enoltautomers.

� 2012 Elsevier B.V. All rights reserved.

1. Introduction

During recent decades, light sensitive organic composites havereceived significant attention due to their importance in basic re-search and plentiful technological application perspectives [1–3].In order to improve photochromic behavior and expand applica-tion areas, many photochromic materials have been chemicallyincorporated in different matrices. Polymers, liquid crystals or den-drimers have been widely used for these purposes [4,5]. As a result,recent studies have shown that dendrimers due to their attractivestructural features are perfect candidates for implementation ofthese achievements [6].

Dendrimers are uniform, three-dimensional spherical macro-molecules composed of the central core, internal branching unitsand terminal groups on periphery [7]. Well-defined moleculararchitecture of dendrimers is responsible for versatile structuralmodifications within the entire volume [8–11]. Numerous chromo-phore groups have been used for dendrimer functionalization in or-der to create materials with controlled physico-chemical properties[12,13]. Consequently dendrimers functionalized with the lightsensitive molecules are potential candidates for the creation of arti-ficial light harvesting systems, nonlinear optical, optoelectronic orphotovoltaic devices [14–16]. Light-induced orientation of photo-chromic molecules is an extremely important feature in the devel-opment of photochromic materials [17–19]. Various light-sensitive

materials are also attractive for medical applications due to theirlight controllable host–guest properties [12,20,21].

Aromatic molecules containing Schiff base groups are especiallyimportant light sensitive materials in photochemistry. Their photo-chromic properties [22,23] are directly related to the presence ofintramolecular hydrogen bonds and proton transfer reactions.These features suggest application of Schiff base compounds in fab-rication of molecular memories, optical switches, etc. From an aca-demic point of view they are also seen as perfect materials for thestudy of the proton transfer and keto-enol tautomerism.

Photochromic behavior of Schiff base compounds is usuallyaccompanied by the photoinduced structural changes of moleculessuch as keto-enol tautomerism and trans–cis isomerization. In aro-matic Schiff bases, a hydroxyl group is responsible for the H-donating properties, while a nitrogen atom participates as anacceptor. Schiff base compounds have a dominant enol tautomericform in the ground state, while a hydrogen atom rapidly transfersand binds to the nitrogen atom upon its photoexcitation. This pro-cess is considered as the excited state intramolecular proton trans-fer (ESIPT). The created excited cis-keto tautomers undergorelaxation to the ground state, and finally back proton transfertakes place reproducing the original enol tautomeric form. TheESIPT process was found to take place on a femtosecond timescale, while the back proton transfer occurs during severalpicoseconds [22,24]. There are many different types of Schiff basecontaining compounds revealing light induced reversible tauto-merization. Salicylideneanilines are among the most widelyinvestigated [25].

0301-0104/$ - see front matter � 2012 Elsevier B.V. All rights reserved.doi:10.1016/j.chemphys.2011.12.017

⇑ Corresponding author.E-mail address: marius.franckevicius@ftmc.lt (M. Franckevicius).

Chemical Physics 404 (2012) 2–8

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Chemical Physics

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Here we present investigations of the poly(propylene-imine)(PPI) dendrimers functionalized with 4-(40-ethoxybenzoyloxy)sali-cylaldehyde moieties (PPI-SA). Functional moieties attached tocore chains form salicylidenimine chromophore groups, whichdetermine the near UV absorption and fluorescence properties ofthe dendrimers. We present absorption and fluorescence investiga-tions of the PPI-SA dendrimers in solutions combined with thequantum chemistry calculations of their electronic and conforma-tional structure. Our investigations show that the terminatinggroups may exist in several tautomeric forms with relative concen-trations depending on the dendrimer generation. Keto-enol tauto-meric changes take place in the excited state, determiningfluorescence properties of the investigated dendrimers.

2. Experimental section

The functionalized PPI-SA dendrimers consist of amino termi-nated PPI dendrimers to which 4-(40-ethoxybenzoyloxy)salicylal-dehyde chromophore moieties are attached. The structures of thefirst (G1) and fifth (G5) generation dendrimers are presented inFig. 1. This picture also shows the Schiff base formation scheme.PPI dendrimers carrying primary amino groups at their peripheryin reaction with the aldehydes tend to form well known carbonnitrogen double conjugates – commonly known as Schiff bases.Additional information about the dendrimer synthesis and theirpurity are described in detail in Ref. [26].

All spectroscopic investigations were performed with dendri-mer solvents using 10 mm path length quartz cuvettes. Three dif-ferent solvents: chloroform, toluene and methanol were used toprepare initial dendrimer solutions of concentrations of about0.02–0.1 mM. Optical densities of the samples were adjusted tobe approximately of 0.3–0.5 within the 350–400 nm range forthe absorption investigations and below 0.1 for the fluorescencemeasurements.

UV–visible absorption spectra of the PPI dendrimers were re-corded with a Jasco V-670 spectrophotometer. Steady state fluores-cence and fluorescence excitation spectra were measured with thePerkin-Elmer LS 50B fluorimeter.

Quantum chemistry calculations have been performed bymeans of Gaussian 03 using the ab initio Hartree–Fock (HF) methodand 6-311G basis set. Additional calculations using the densityfunctional (DFT) B3LYP method revealed very similar geometricalstructures. Polarization function and diffusion function were notused because of negligible influence on the final result. Electronicabsorption spectra were simulated using the ZINDO method.

3. Results and discussion

3.1. Steady state absorption

Fig. 2a shows steady state absorption spectra of PPI-SA dendri-mers of all generations in a chloroform (CHF) solution. The spectraof different generation dendrimers are significantly different. Themost significant differences could be observed between the firstgeneration and the third or higher generations dendrimers.Absorption spectrum of the second generation dendrimers is in be-tween, however closer to the high generations dendrimers. Due tothe fact that the dendrimers of the first (G1) and fifth (G5) gener-ations reveal most pronounced structural and optical differenceswe have selected them for a more detailed investigation. All den-drimers have several distinct absorption bands. The near UV andvisible absorption spectra shall be attributed to the terminatingchromophore groups because dendrimer cores have absorptionspectra below 300 nm [27]. The high-intensity absorption bandpeaking at about 310 nm is similar to dendrimers of all genera-tions. The G1 dendrimers have also a wide weak absorption bandat about 400 nm. The long wavelength band of higher generationdendrimers is shifted to the short wavelength side and has muchhigher intensity. Similar absorption features revealing severalabsorption bands were also observed for other Schiff base com-pounds, for example: salicylideneaniline, salicylaldehyde azine[28], salicylidene-aminopyridene [29] or N-triphenylmethylsalicy-lideneimine [30], and attributed to different tautomeric forms ofthe molecules. It should be noted, that no significant changes ofthe absorption spectra have been observed by changing the dendri-mer concentration in solutions.

Fig. 2b shows absorption spectra of the G1 and G5 dendrimers indifferent solvents. In addition to polar and aprotic solvent CHF, non-polar aprotic solvent toluene and polar protic solvent methanolwere used. Absorption spectra of the G5 dendrimers only weaklydepend on solvent, while spectra of G1 dendrimers are significantlydifferent in different solvents. The absorption spectrum of G1 in tol-uene has only a very weak absorption band at about 400 nm, muchweaker than in other solutions. The spectrum of the first generationdendrimer in methanol closely resembles spectra of G5 dendrimerswith the strong long wavelength absorption band shifted to theshort wavelength side. It should be noted that the solubility ofthe dendrimers in ethanol is very low, therefore their spectral fea-tures in this solvent are less reliable; they may be affected by thepresence of small unsolved dendrimer powder particles or dendri-mer clusters. However, the G1 spectrum in methanol, being similarto the spectra of higher generation dendrimers, allows us to rule out

Fig. 1. Structures of the first and fifth generation PPI-SA dendrimers and the mechanism of formation of the salicylidenimine chromophore group.

M. Franckevicius et al. / Chemical Physics 404 (2012) 2–8 3

the possibility that chromophores of the first generation dendri-mers have some chemical or structural differences. It allows us toconclude that only the dendrimer generation is responsible for dif-ferent spectral features of chromophore groups in some particularsolvent. The inset in Fig. 2b reveals one additional important differ-ence between dendrimers of different generations; G5 dendrimershave a weak absorption band at about 480 nm, which is absent inG1 solutions.

In order to characterize the absorption spectra of both genera-tions dendrimers, we have approximated them by Gaussian bandswhich are presented in Fig. 3. Good agreement between experi-mental and modeled spectra, has been obtained by using threeGaussian bands, with identical central wavelength and bandwidthvalues for the first and fifth generation dendrimers in the sameparticular solvent, by changing only the relative band intensities.The central wavelength positions were slightly different in differ-ent solvents. The possibility to approximate spectra of the two dif-ferent generations by the same set of Gaussian componentsindicates that different absorption bands correspond to differentforms of dendrimer chromophore groups and only relative concen-trations of different forms depend on the dendrimer generationand solvent.

We have also investigated spectral properties of dendrimers atdifferent temperatures ranging from 280 to 315 K. Only small spec-tral changes were observed for G5 dendrimer in chloroform (spec-tra is not presented); intensity of the absorption band at about380 nm slightly decreased at higher temperatures. However, theobserved changes were not very reliable because absorption spec-tra (relative intensities of different absorption bands) of the samedendrimer solution slightly varied depending on some uncontrol-lable conditions of the sample preparation and storage.

3.2. Quantum chemistry calculations

Quantum chemistry calculations have been performed in orderto interpret the absorption spectra. Calculation results for isolatedmolecules in vacuum indicate that chromophore groups of dendri-mers can exist in four most stabile tautomeric forms, which areshown in Fig. 4. The calculated electron density distributions inthe ground and excited electronic states show that electronic orbi-tals responsible for the optical transitions are delocalized over theentire chromophore groups. As an example, Fig. 5 shows calculateddistributions of electron densities in HOMO and LUMO+3 orbitalsof the enol closed tautomer responsible for the strong absorptionband at about 310 nm. It indicates that structures of the entirechromophore groups play an important role in formation of theiroptical properties. For this reason a direct comparison of theabsorption properties of PPI-SA dendrimers with other moleculescontaining Schiff bases and revealing keto and enol tautomericforms [22,30–32] is not straightforward.

Fig. 2. (a) Absorption spectra of all generations PPI-SA dendrimers in a chloroformsolution. Bars show calculated positions and oscillator strengths of opticaltransitions of different tautomeric forms of terminating groups. (b) Absorptionspectra of G1 and G5 dendrimers in different solvents. Inset presenting the samespectra in log scale demonstrates the weak absorption band at about 480 nm in G5solutions.

Fig. 3. Absorption, fluorescence and fluorescence excitation spectra of G1 and G5dendrimers in CHF solutions. The picture also shows deconvolution of theabsorption spectra to three Gauss components as well as fluorescence quantumyield dependences on the excitation wavelength.

4 M. Franckevicius et al. / Chemical Physics 404 (2012) 2–8

Table 1 presents total energies of different tautomers in theelectronic ground state and in three lowest energy excited statescalculated for the ground state geometries optimized by usingHF/6-311G(d,p) and HF/6-311G(2df,2pd) basis sets. The energies

are presented relatively to the ground state energy of the most sta-bile enol closed tautomeric form. Table 1 also presents wave-lengths of the electronic transitions from the ground state todifferent excited states as well as corresponding oscillatorstrengths. Electronic transitions with the largest oscillatorstrengths are shown in bold. For more demonstrative representa-tion and easier comparison with the experimental absorption spec-tra these data are also presented in Figs. 2a and 6 in a graphicalform.

Calculation results show that terminating groups have minimalenergies in the enol closed tautomeric form, i.e. possessing hydro-xyl group and a Schiff base in a closed cycle completed by a hydro-gen bond. The cis-keto tautomer with the protonated Schiff basehas energy higher by about 300 meV. Open tautomers, both in enoland keto forms, have energies higher by about 300 meV than cor-responding closed tautomers. This difference is evidently causedby the formation of the intramolecular hydrogen bonds (H-bonds)in closed forms of molecules. The H-bonds, which typically havehundreds of meV energies, shall break up upon formation of opentautomeric forms.

According to the calculations, the enol closed tautomers havestrong electronic transition to S3 state at 297 nm. There are alsotwo lower energy transitions to S2 and S1 states at 302 nm and328 nm, but their intensities are about 10 and 100 times lower,respectively. Enol open tautomers have similar electronic transi-tions, but they are shifted to the short wavelength side by about10 nm. Cis-keto and trans-keto tautomers have strong opticaltransitions to S2 states at 384 and 366 nm, respectively, and weakoptical transitions of np⁄ nature at about 425 nm. Thus, optical

Fig. 4. Calculated structures of the four tautomeric forms of terminating groups.

Fig. 5. Electron density distributions in HOMO and LUMO+3 orbitals of the enolclosed form terminal groups.

Table 1Total energies of different tautomers in the electronic ground state and in threelowest energy excited states and wavelengths of electronic transitions from theground state to the excited states as well as corresponding oscillator strengths.

Enol closed Enol open cis-keto trans-keto

E, eV k, nm E, eV k, nm E, eV k, nm E, eV k, nmf f f f

S0 0 0289 0299 0601S1 3776 328 4.06 329 3197 427 3537 422

0.0015 0.0010 0.0001 0.0002S2 4103 302 4.53 292 3526 384 4014 366

0.0099 0.0113 0.3441 0.4433S3 4161 297 4.59 288 4072 328 4374 328

0.2233 0.1350 0.0004 0.0003

Fig. 6. Energy values of the dendrimer chromophore in different tautomeric formscalculated in vacuum (symbols) and oscillator strengths of the optical transitionsfrom the ground to excited states (bars).

M. Franckevicius et al. / Chemical Physics 404 (2012) 2–8 5

transitions to the lowest energy electronic states were obtained tohave very weak oscillator strengths for all tautomeric forms. Calcu-lation results suggest that the absorption band at about 310 nmshall be attributed to the enol tautomeric form, most probably tothe enol closed tautomers. This tautomeric form has the lowest en-ergy in the ground electronic state, thus it shall dominate overother tautomeric forms and its calculated optical transition energyis close to the absorption band position.

The enol tautomers do not have optically allowed transitionsabove 328 nm. The G1 spectrum in toluene supports this state-ment; the long wavelength absorption is very weak when one tau-tomeric form, evidently enol closed, strongly dominates. Therefore,in agreement with the calculation results the absorption bands atabout 375 and 400 nm shall be attributed to the trans-keto andcis-keto tautomers, respectively. However, such attribution meetssome problems. We can calculate relative concentrations of differ-ent tautomers in solutions from intensities of the absorptionbands. Taking into account larger oscillator strength of the ketotautomers, we obtain that about 2% and 5% of terminating groupsare in cis-keto form in G1 solutions in toluene and in CHF, respec-tively. Relative concentrations of trans-keto tautomers in solutionsof G5 dendrimers reach about 20%, and about 7–12% of terminatinggroups are in cis-keto form. So high relative concentrations of ketotautomers are in apparent disagreement with the large energy dif-ference of about 10 kT, between enol closed and cis-keto tautomersand with the difference of about 20 kT between enol closed andtrans-keto tautomers. However, it shall be noted that the calcula-tions were performed for isolated molecules, therefore notaccounting for the solvent influence and interactions between ter-minating groups of the same dendrimer. It is well known that sol-vents strongly influence the keto-enol tautomerism [33,34]. Theinfluence is particularly strong in case of protic solvents, whichform hydrogen bonds with N and O atoms. As was demonstratedin Ref. [29] protic molecules, forming H-bonds with N-triphenylm-ethylsalicylideneimine, significantly reduce the energy of the ketotautomeric form. Very weak intensity of the keto form absorptionof G1 dendrimers in neutral solvent toluene strongly supports thisconclusion.

The difference between the absorption spectra of G1 and largergenerations dendrimers is another intriguing question. All dendri-mers have identical terminating groups, which are bound to identi-cal dendrimer chains. Therefore, their absorption spectra andrelative concentrations of tautomeric forms are also expected tobe identical. On the other hand, the difference may arise from inter-action between terminating groups, or between terminating groupsand dendrimer core chains. Due to a larger number of terminatinggroups such interactions and their consequences are expected tobe more expressed in dendrimers of higher generations. Three typesof interactions may be anticipated: Van der Waals interactions,excitonic interactions and formation of hydrogen bonds (H-bonds).Van der Waals and excitonic interactions may lead to shifts or split-ting of absorption bands, but hardly can change their intensities,therefore hardly can cause the observed spectral variations. TheH-bond formation may significantly influence tautomeric forms ofterminating groups in a similar way as H-bonded solvent moleculesinfluence tautomerism of salicylideneanilines [35,36]. Enol openand trans-keto tautomers have OAH and NAH groups, which mayform H-bonds with solvent molecules or with neighboring termi-nating groups, therefore they shall be very sensitive to the H-bondformation. Our calculations show that enol open and trans-keto tau-tomers may form stable H-bonded aggregate structures. Moreover,the H-bond formation stabilizes enol open and trans-keto forms,because interchromophore H-bonds partly compensate the energyincrease caused by breaking of the intrachromophore H-bonds. Thecalculated binding energy of dimers formed by two trans-ketotautomers in head-to-tail arrangement was obtained to be equal

to 0.68 eV. The binding energies of enol closed dimer and enolclosed – trans-keto heterodimer were obtained to be almost twotimes lower. Thus, dimerization shall facilitate formation of trans-keto tautomers by reducing energies of every chromophoric groupby about 0.34 eV. Such aggregated structures may be formed inhigh generation dendrimers, however, hardly can be formed in G1dendrimers because of short dendrimer chains not allowing opti-mal mutual arrangement of chromophores. Sequential arrange-ment of the interchromophore H-bonds may lead to formation ofchromophore bunches as was suggested in Ref. [37] in relation withthe liquid crystalline properties of the PPI-SA dendrimers. Accord-ing to our calculations the np⁄ transition has a very low oscillatorstrength for perfectly plain molecule conformation, but it gainsintensity upon distortion of the chromophore groups, namely upontwisting of the CAC bond connecting phenyl ring and Schiff base(see Fig. 7). Such distortion probably may take place in bunchesof the terminating groups. Therefore formation of the dendrimerbunches may also be responsible for the absorption band at480 nm attributed to np⁄ transitions.

Back folding of the terminating groups is another possiblemechanism responsible for the differences between G1 and highergeneration dendrimers. Back folding and interaction of the termi-nating groups with the dendrimer core are hardly possible in theG1 dendrimers because of the small dendrimer core and short den-drimer chains. Higher generation dendrimers may easier back-foldand more efficiently interact with the dendrimer core [38]. How-ever, we do not see what mechanism of the chromophore–coreinteraction may be responsible for so strong spectral changes,therefore we prefer the explanation based on the interchromo-phore interactions.

3.3. Fluorescence properties

Fluorescence spectra of all dendrimers are identical peaking atabout 500 nm and do not depend on the excitation wavelength.The fluorescence shall be attributed to the np⁄ excited states ofthe keto form tautomers because only keto tautomers have elec-tronic transitions in a visible spectral range. The fluorescence bandis red-shifted relatively to the lowest energy absorption bandattributed to np⁄ states by about 100 nm, which is quite typicalof large complex molecules. Fluorescence spectra of PPI-SA dendri-mers are very similar to those of different Schiff base compounds

Fig. 7. Calculated total energies of the keto form chromophore groups in the groundand first excited states as well as oscillator strengths of the two lowest energyoptical transitions as functions of the dihedral angle formed by the phenyl ring andSchiff base.

6 M. Franckevicius et al. / Chemical Physics 404 (2012) 2–8

where they have been generally attributed to the cis-keto tauto-meric form [39].

Fig. 3 shows fluorescence excitation spectra of G1 and G5 den-drimers in CHF solutions measured by detecting fluorescence at530 nm. The sample concentrations were adjusted to have identi-cal optical density of 0.07 at 375 nm, thus the fluorescence inten-sity under 375 nm excitation showed relative fluorescence yield.The excitation spectra revealed the same absorption bands as theabsorption spectra. They may be quite satisfactorily approximatedby the same three bands obtained from the approximation ofabsorption spectra, but their relative intensities are significantlydifferent. In order to estimate absolute fluorescence yield valueswe used solution of Coumarin 307 dye in methanol having thefluorescence yield close to 100% as a reference. This dye solutionhas a quite similar fluorescence spectrum to that of PPI-SA dendri-mers. We obtained the fluorescence yields of the G1 and G5 den-drimers of about 0.2% and 0.5%, respectively. Higher fluorescenceyield of G5 dendrimers correlates with the presence of the low en-ergy absorption band: oscillator strength of the np⁄ transition ishigher in G5 dendrimers, thus G5 dendrimers have a faster radia-tive relaxation rate and consequently a larger fluorescence yield.We obtained dependence of the fluorescence yield on the excita-tion wavelength by dividing the fluorescence excitation spectrumby the absorption spectrum and normalizing the obtained curveto the measured yield value at 375 nm (see Fig. 3). Fluorescenceyields of both dendrimers are very low under their excitation tothe enol-form absorption band. The yield slightly increases underthe sample excitation to the keto tautomer absorption band, andgrows up significantly under excitation to the weak lowest energyabsorption band attributed to np⁄ states. This observation is in linewith the attribution of the low energy absorption band to the geo-metrically distorted chromophore groups; upon excitation directlyto np⁄ states, we selectively excited more distorted chromophoregroups possessing larger oscillator strengths both for absorptionand fluorescence transitions.

Fluorescence excitation spectra revealing all absorption compo-nents indicate that fluorescence of keto tautomeric form appearsindependently of the tautomeric form which has been excited.Only the fluorescence yield is lower under excitation of the enoltautomeric form. Keto-enol tautomeric changes evidently takeplace in the excited state, as has been demonstrated for other mol-ecules with salicylidenimine chromophore group [2,25,28,40].Chromophore groups probably experience nonradiative relaxationduring the keto-enol tautomeric transitions as well as during ex-cited state relaxation to the low energy np⁄ states causing strongfluorescence yield dependence on the excitation wavelength. Fem-tosecond transient absorption investigations are in progress toclarify the excited state relaxation processes.

4. Summary

Bonding of the 4-(40-ethoxybenzoyloxy)salicylaldehyde moietyto the amino terminated PPI dendrimers results in formation of sal-icylidenimine chromophore groups containing a Schiff base. Steadystate absorption and fluorescence excitation spectra reveal severaltautomeric forms of terminating groups. Relative concentrations ofdifferent tautomeric forms in dendrimer solutions depend on thedendrimer generation and solvent. Based on the analysis of theabsorption spectra and quantum chemistry calculation results, weattribute the high intensity absorption band at about 300 nm tothe enol closed tautomers and lower intensity absorption bands atabout 375 and 400 nm to trans-keto and cis-keto tautomers respec-tively. A very weak absorption band at about 480 nm we attribute tonp⁄ transitions of keto tautomers, which gain intensity under thegeometrical distortion of terminal groups. Relative concentration

of keto tautomers in solutions increases with the dendrimer gener-ation. We attribute this dependence to the interaction between ter-minal groups of dendrimer, in particular to formation ofinterchromophore hydrogen bonds stabilizing the trans-keto form.

All tautomeric forms show identical fluorescence attributed tonp⁄ states of keto tautomers indicating that keto-enol transforma-tions take place in the excited states. The fluorescence yield is verylow, of the order of 0.1%, under excitation of enol tautomers, butincreases more than by order of magnitude under excitation ofketo tautomers to the np⁄ absorption band.

Acknowledgements

This research was funded by the European Social Fund underthe Global Grant measure and by the MICINN, Spain, under ProjectCTQ2009-09030, and FEDER funding.

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