Inorganica Chimica Acta 378 (2011) 49–55

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Inorganica Chimica Acta

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Luminescence properties of cyclopalladated complexes with Schiff base ligands

M. Dolores Santana a, Rocío García-Bueno a, Gabriel García a, Gregorio Sánchez a, Joaquín García a,José Pérez b, Luis García b, J. Luis Serrano b,⇑a Departamento de Química Inorgánica, Universidad de Murcia, 30071 Murcia, Spainb Departamento de Ingeniería Minera, Geológica y Cartográfica, Universidad Politécnica de Cartagena, Área de Química Inorgánica, 30203 Cartagena, Spain

a r t i c l e i n f o

Article history:Received 25 March 2011Received in revised form 5 August 2011Accepted 8 August 2011Available online 17 August 2011

Keywords:Schiff basesCyclometallated ligandsLuminiscenceHydroxo-complexesPalladium

0020-1693/$ - see front matter � 2011 Elsevier B.V. Adoi:10.1016/j.ica.2011.08.012

⇑ Corresponding author.E-mail address: jose.serrano@upct.es (J.L. Serrano)

a b s t r a c t

Direct reaction between the hydroxo-complexes [{Pd(l-OH)(C^N)}2] (C^N = 2-(2-pyridyl)phenyl (Phpy)I; C^N = 7,8-benzoquinolyl (Bzq) II) and N-naphtylsalycilaldimine (N-naphsal) 1 yields new mononuclearcyclometallated palladium(II) complexes [Pd(N-naphsal)(C^N)] (I1, II1). Photophysical properties wereinvestigated together with those of complexes with related ligands N-phenylsalycilaldiminate (N-Phsal)2, N-p-chlorophenylsalycilaldiminate (N-pClPhsal) 3. All the compounds absorb intensely below 300 nmvia 1LC transitions located in Bzq or Phpy ligands, and display additional low energy absorptions of mixed1MLCT–1LC character. The complexes under study are quite unusual in terms of luminescence behavior,since some of them are emissive in solution at room temperature and all display intense emissions in fro-zen CHCl3 solution, but also in solid state at 298 and 77 K. Structural characterization by X-ray diffractionof complexes I2, I3 and II2 confirmed the proposed formulae.

� 2011 Elsevier B.V. All rights reserved.

1. Introduction

In addition to their interest related to several applied fields, thesynthetic value of palladium(II) and platinum(II) di-l-hydroxo-complexes is a subject of continuous study [1]. We have contrib-uted to this area exploiting the usefulness of binuclear hydroxocomplexes of palladium, some of them with a cyclometalated back-bone [2], in the preparation of a wide variation of new compoundsby means of a simple acid–base reaction [3]. In a recent report [4]we described the preparation of several mono- and dinuclear com-plexes containing orthometallated 2-(2-pyridyl)phenyl (Phpy) or7,8-benzoquinolyl (Bzq) backbones combined with several O^O,O^N or N^S ligands. Preliminary results showed that most of thecomplexes were good emitters in solution at room temperature[4], and following studies focused on the N^S series revealed themas quite unusual in terms of luminescence behavior, since most areemissive in solution at room temperature and all display intenseemissions in frozen CHCl3 solution, but also in solid state at 298and 77 K [5]. In this sense, luminescent cyclopalladated complexesare scarce in the literature if compared with analogous platinumderivatives [6], and usually show low efficient emissions detectedonly at low temperatures [7] due to the presence of thermallyaccessible low-lying metal centered states (MC) that allow excitedstates to experience nonradiative decay [8]. There have been someexceptions, as the cyclopalladated Nile Red based complexes

ll rights reserved.

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described by Ghedini and co-workers [9]. These authors havepointed out the relationship between molecular geometries, typesof ligands and the arising of important physical properties of dinu-clear and mononuclear cyclometallated derivatives containing O^Ncomplementary ligands among others [10]. It would be well exem-plified by complexes [Pd(oxin-R)(Phpy)], that also emit in solutionat room temperature [11]. It is worth it to note that whether as acomparison probe with platinum analogues or by its own potential,the luminescent properties of cyclopalladated complexes are a sub-ject of continuous study [10,12].

On the other hand, Schiff bases have played an important role inthe development of coordination chemistry, as they readily formstable complexes with most of the transition metals exhibiting dif-ferent coordination modes and functionalities [13]. The potentialapplication of such complexes in different fields as biological mod-eling, design of molecular ferromagnets, encapsulation, liquid crys-tals, catalysis, medical imaging or crystal engineering has beenextensively explored [14]. Among Schiff base complexes, thosecontaining salicylaldimines derived from condensation of salicylal-dehyde and various mono-or diamines have attracted considerableattention in the fields above mentioned, [14a] and a great numberof them have shown luminescent properties [14f,15].

In this paper we report the synthesis of mononuclear cyclopal-ladated complexes that contain O^N Schiff base complementaryligands. The clean one-step reaction with the correspondinghydroxo-complexes has been the synthetic route of choice.Extensive spectroscopic characterization and study of photophysi-cal properties is also presented.

50 M.D. Santana et al. / Inorganica Chimica Acta 378 (2011) 49–55

2. Experimental

2.1. Materials and methods

C, H, and N analyses were carried out with a Carlo Erba instru-ment. IR spectra were recorded on a Perkin–Elmer spectrophotom-eter 16F PC FT-IR, using Nujol mulls between polyethylene sheets.NMR data (1H) were recorded on Bruker 300 spectrometer. ESI-MSanalyses were performed on an Agilent VL mass spectrometer. Theionization mechanism used was electrospray in positive and nega-tive ion full scan mode using acetonitrile as solvent and nitrogengas for desolvation.

The UV–Vis spectra were recorded on a UNICAM UV 500 spec-trophotometer for 230–800 nm range. DRUV spectra were re-corded with a Unicam UV-4 spectrophotometer equipped with aSpectralon RSA-UC-40 Labspere integrating spere. The solid sam-ples were mixed with dried silica gel to form an homogeneouspowder. The mixtures were placed in a home-made cell equippedwith a quartz window. The intensities were recorded in Kubelka–Munk units: log[R/(1 � R)2], were R = reflectance. Excitation andemission spectra were recorded on a Jobin Yvon Fluorolog 3-22spectrofluorometer with a 450 W xenon lamp double-gratingmonochromators and a TBX-04 photomultiplier. The solutionmeasurements were carried out in a right angle configurationusing degassed acetonitrile solutions of the samples in 10 mmquartz fluorescence. Quantum yields were determined relative to[Ru(bpy)3](PF6)2 in H2O, according to reported methods [16].

The cyclometallated precursors [{Pd(l-OH)(C^N)}2] (C^N = 2-(2-pyridyl)phenyl (Phpy) I; C^N = 7,8-benzoquinolyl (Bzq) II) wereprepared as described previously [4]. For comparative purposesregarding the luminescent properties, we have used complexescontaining ligands N-phenylsalycilaldiminate (N-Phsal) 2 and N-p-chlorophenylsalycilaldiminate (N-pClPhsal) 3, whose synthesiswas reported in a previous paper [4].The commercially availablechemicals were purchased from Aldrich Chemical Co. and wereused without further purification, and all the solvents were driedby standard methods before use.

2.2. Synthesis

2.2.1. Preparation of complexes [Pd(O^N)(C^N)] O^N = N-naphtylsalycilaldiminate (N-naphsal) (1)

The new complexes were obtained by treating a CH2Cl2

suspension (20 mL) of the different precursors (I, II) (0.120 g) withN-naphtylsalycilaldimine (molar ratio 1:2). The suspension wasstirred at room temperature for 1 h until a clear solution wasobtained and then it was concentrated under reduced pressure un-til ca. one fifth of the initial volume. Slow addition of diethyl ethercaused the formation of the complexes, which were filtered off,washed with diethyl ether and air-dried. The compounds wererecrystallised from dichloromethane–hexane.

2.2.2. [Pd(N-naphsal)(Phpy)] I1(0.159 g, 73%). Anal. Calc. for C28H20N2OPd: C, 66.3; H, 4.0; N,

5.5. Found: C, 66.3; H, 4.1; N, 5.2%. FT-IR (Nujol mull, cm�1):1606s, 1581s, 1557s. HPLC–MS (positive mode) m/z: 508 [Pd(N-naphsal)(Phpy)]+. 1H NMR (300 MHz, CDCl3): d (ppm) 9.31 (d, 1H,Phpy, J = 5.6 Hz), 8.59 (s, 1H, HC@N, N-naphsal), 8.01 (m, 3H, N-naphsal), 7.82 (m, 1H Phpy), 7.69 (m, 1H, N-naphsal), 7.60–6.80(m, 12H, 6H Phpy + 6H N-naphsal), 6.04 (m, 1H, N-naphsal).

2.2.3. [Pd(N-naphsal)(Bzq)] II1(0.129 g, 73%). Anal. Calc. for C30H18N2OPd: C, 67.9; H, 3.8; N,

5.3. Found: C, 68.0; H, 3.9; N, 5.3%. FT-IR (Nujol mull, cm�1):1606s, 1587s, 1571s. HPLC–MS (positive mode) m/z: 531 [Pd(N-

naphsal)(Bzq)]+. 1H NMR (300 MHz, CDCl3): d (ppm) 9.60 (d, 1H,Bzq, J = 5.4 Hz), 8.67 (s, 1H, HC@N, N-naphsal), 8.25 (d, 2H, N-naph-sal, J = 8.0 Hz), 7.92 (m, 3H Bzq), 7.56 (m, 5H, N-naphsal), 7.44 (m,4H, 1H Bzq + 3H N-naphsal), 7.25 (m, 1H, N-naphsal), 7.15 (d, 1H,Bzq, J = 8.4 Hz), 6.63 (m, 2H, Bzq).

2.3. Crystal structure determination of I2, I3 and II2

Data collection for, I2, I3 and II2 were obtained at 100(2) K on aBruker Smart CCD diffractometer with a nominal crystal to detec-tor distance of 4.5 cm. Diffraction data were collected based on a xscan run. A total of 2524 frames were collected at 0.3� intervals and10 s per frame. The diffraction frames were integrated using theSAINT package [17] and corrected for absorption with SADABS [18].The structures were solved by direct methods [19] and refinedby full-matrix least-squares techniques using anisotropic thermalparameters for non-H atoms (Table 1).

3. Results and discussion

3.1. Characterization of complexes

The cyclometallated precursors [{Pd(l-OH)(C^N)}2] (C^N =Phpy I or Bzq II) were conveniently prepared by reaction of the cor-responding acetate-bridged complexes with NBu4OH in acetone[4]. The hydroxo-complexes react with weak protic acids like sal-yciladimine ligands 1–3 to give mononuclear species of formula[Pd(N^O)(C^N)] (I, II/1, 2 or 3) following the mild conditions thatare collected in Section 2. The drawings of the acidic ligands la-beled with their abbreviations and the general reaction are shownin Scheme 1.

The new palladium complexes are air-stable and their IR spec-tra show bands around 1600 cm�1 characteristic of the corre-sponding cyclometallated backbone, partly overlapped by thoseabsorptions attributed to the incoming ligands (see Section 2).The absence of relevant bands in the region 3000–3500 cm�1 indi-cates both deprotonation of acidic ligands and complete reaction ofthe hydroxo-palladium precursors. This fact was also supported by1H NMR spectra, where no typical high-field resonances of –Pd-OHgroups are found. The 1H NMR spectra also show the correspond-ing signals of the incoming ligands, frequently overlapped in thearomatic region with those of the cyclometallated backbone. Note-worthy, the presence of only one isomer was observed by 1H NMR,although at least two could exist depending on the relative posi-tions of the C^N/O^N systems.

Additional support for the proposed formulae of the new com-plexes arises from mass spectrometry. Data of the complexes withthe m/z values for the observed fragments, that includes M+ and M+

– L^L as a common pattern together with some recombinationfragments, are collected in Section 2. The abundance of the signalsaround the parent ion is consistent with the natural isotopicabundances.

3.2. Molecular structures

As it was also possible to grow single crystals of complexes I2, I3and II2 suitable for X-ray diffraction, their molecular structures wereunambiguously determined and are shown in Figs. 1–3, respectively.Selected bond distances and angles are collected in Table 2. Thearrangement around the Pd atoms in can be described as planar.Their deviation from the planar coordination geometry has beenquantified by measures of improper torsion angles [20]. Thus, incomplex I2 w1 = 4.39� and w2 = 3.69� for Pd(1); in complex I3w1 = 2.94� and w2 = 1.81�, and in complex II2 w1 = �2.47� andw2 = �2.12� for Pd(1). These values correspond to a moderate

Table 1Crystal data and structure refinement for complexes I2, I3 and II2.

Compound I2 I3 II2

Empirical formula C24H18N2OPd C24H17ClN2OPd C26H18N2OPdM 456.80 491.25 480.82T (K) 100(2) 100(2) 100(2)k (Å) 0.71073 0.71073 0.71073Crystal system monoclinic monoclinic monoclinicSpace group P21/a P21/c CcUnit cell dimensionsa (Å) 15.0001(12) 12.2047(5) 19.6285(13)b (Å) 5.9263(5) 11.7402(5) 6.2156(4)c (Å) 20.6274(17) 14.5635(6) 15.9307(10)a (�) 90 90 90b (�) 99.7690(10) 114.4780(10) 97.1380(10)c (�) 90 90 90V (Å3) 1807.1(3) 1899.18(14) 1928.5(2)Z 4 4 4Dcalc (Mg m�3) 1.679 1.718 1.656l (mm�1) 1.045 1.137 0.984F(0 0 0) 920 984 968h range for data collection (�) 1.00–28.70 1.83–28.18 2.09–28.67Reflections collected 19 822 21 500 11 195Independent reflections 4406 4421 4533Data/parameters/restrains 4406/253/0 4421/262/0 4533/271/2Goodness-of-fit (GOF) on F2 1.230 1.043 1.061Final R indices [I > 2r(I)] R1 = 0.0410, xR2 = 0.0882 R1 = 0.0227, xR2 = 0.0543 R1 = 0.0275, xR2 = 0.0694R indices (all data) R1 = 0.0471, xR2 = 0.0969 R1 = 0.0242, xR2 = 0.0553 R1 = 0.0276, xR2 = 0.0696Maximum/minimum Dq (e Å�3) 0.953 and �0.564 0.419 and �0.335 1.634 and �0.374

Pd Pd

OH

HO

C

N

N

C

Pd

N

O

C

N

= Phpy I, Bzq IIC N

(N-naphsal-H) I1 or II1(N-Phsal-H) I2 or II2(N-pClPhsal-H) I3 or II3

+2 N OH =CH2Cl2

OH

NCH

Cl

OH

NCH

(2) N-phenylsalycilaldimine (N-Phsal-H)

(3) N-p-chlorophenylsalycilaldimine (N-pClPhsal-H)

(1) N-naphtylsalycilaldimine (N-naphsal-H)

OH

NCH

N^O =

Scheme 1.

Fig. 1. X-ray crystal structure of I2. Thermal ellipsoids are drawn at 50% probability.Hydrogens omitted for clarity.

M.D. Santana et al. / Inorganica Chimica Acta 378 (2011) 49–55 51

tetrahedral distortion from the ideal square-plane. The N1–Pd1–C7angle, 81.03(12)�, 80.84(7)� and 82.22(15)� in complexes I2, I3 andII2, respectively, is similar to that found in related complexes [4,5].Methods for the conformational classification of medium size ringshave been described [21]. Thus, the Pd1–O1–C14–C19–C20–N2 ringin the three complexes displays a screw-boat conformation,SB = 0.9920; HC = 0.0080 deformed 21� for I2, SB = 0.9932;HC = 0.0055; E = 0.0013 deformed 26� for I3 and SB = 0.9908;HC = 0.0060; E = 0.0032 deformed 24� for II2, for r = 10�. Using thesame classification method, the 5-membered palladacycles Pd–C–C–C–N exhibit conformations, for r = 10�, HC = 0.5146; E = 0.4854for I2; 0.5058 E; 0.4942 HC for I3 and HC = 0.5141; E = 0.4859 for II2.

3.3. Supramolecular structures

Packing in complexes I2 and II2 that contain the N-phenylsaly-cilaldimine ligand are similar. Thus, as seen in Fig. 4 molecules of I2are stacked along b direction with a distance between mean planedefined by coordination atoms of 3.751 Å (3.653 Å for II2). Thereare two main interactions between iminic and phenylpiridine li-

gands from consecutive molecules: the 6-membered chelating ringover pyridine ring and a significant C–H� � �p interaction betweenthe C6H5 group and the phenyl ring in the phenylpyridine. Analo-gous pattern is found in II2 with the 6-membered chelating ringover N-ring of Bzq and a significant C–H� � �p interaction betweenthe C6H5 group and Bzq. Interatomic distances shorter than thesum of the van der Waals radii were considered as relevant forthe presence of C–H� � �p interaction [22].

On the other hand, I3 molecules are stacked forming centro-symmetric ‘‘dimmers’’ with a distance between mean planes de-fined by coordination atoms of 3.687 Å and C–H� � �p interactions(Fig. 5).

Fig. 2. X-ray crystal structure of I3. Thermal ellipsoids are drawn at 50% probability.Hydrogens omitted for clarity.

Fig. 3. X-ray crystal structure of II2. Thermal ellipsoids are drawn at 50%probability. Hydrogens omitted for clarity.

Table 2Selected bond lenghts (Å) and angles (�) of complexes I2, I3 and II2.

I2 I3 II2

Pd1–C7a 2.001(3) 2.0035(17) 2.027(6)Pd1–N1 2.026(3) 2.0258(15) 2.039(3)Pd1–N2 2.043(3) 2.0511(15) 2.031(3)Pd1–O1 2.065(2) 2.0587(13) 2.055(4)C7a–Pd1–N1 81.03(12) 80.84(7) 82.22(15)C7a–Pd1–N2 100.41(12) 101.69(7) 99.61(15)C7a–Pd1–O1 168.24(11) 175.29(6) 169.13(17)N1–Pd1–N2 173.71(10) 168.18(6) 176.21(11)N1–Pd1–O1 89.42(10) 87.92(6) 87.61(11)N2–Pd1–O1 89.83(9) 89.78(5) 90.77(11)

a C7 ? C11 in II2.

52 M.D. Santana et al. / Inorganica Chimica Acta 378 (2011) 49–55

3.4. Absorption spectra and luminescence properties

Absorption data of the complexes under study are listed inTable 3. As example, absorption spectra of I2 and II2 are shownin Fig. 6. All complexes display very intense absorption bands be-low 300 nm, because of 1p–p⁄ transitions located in Bzq or Phpy li-gands (e > 20 000 L mol�1 cm�1). Moreover, a set of less intensebands in the region 300–470 nm (e 2000–20 000 L mol�1 cm�1) isobserved. The less intense low-energy absorptions (k > 300 nm)are tentatively ascribed to a mixture of spin-allowed metal-to-li-gand charge transfer (1MLCT) and ligand centered 1LC transitions,in agreement with recent studies based on related compounds[12d,23].

Fig. 4. Molecular packing diagram of complex I2.

Fig. 5. Molecular packing diagram of complex I3.

Table 3Absorption data of complexes I1–I3, II1–II3 at room temperature in chloroformsolutions and solid-state diffuse reflectance-DRUV.

Compound kmax/nm (e/dm3 mol�1 cm�1) DRUV

I1 242 (47 299), 270 (6914), 468 (3503) 317, 4302 246 (30 516), 305 (10 453), 367 (4780), 426

(3799)320, 376,430

3 253 (66 246), 305 (26 275), 365 (10 883), 427(9357)

320, 380,430

II1 248 (29 945), 304 (13 612), 383 (5431), 430

(2604)318, 398

2 247 (37 376), 305 (16 220), 387 (4645), 433(2306)

318, 394

3 305 (14 088), 390 (4999), 432 (2871) 319, 391,449

Fig. 6. UV–Vis absorption spectra of I2 and II2 in CHCl3 (10�4 M) at 298 K.

λ / nm

0

0.5

1

1.5

2

2.5

220 270 320 370 420 470 520 570

Abs

(a.

u.)

Toluene

Acetonitrile

Chloroform

Fig. 7. UV–Vis absorption spectra of I3 in different solvents (10�4 M) at 298 K.

Table 4Luminescence spectral data at 298 and 77 K, in chloroform solutions (10�6 M).

Compound 298 K U/%a 77 K

kmax

(exc)/nmkmax(em)/nm kmax

(exc)/nmkmax

(em)/nm

I1 260 430, 455, 486 (sh) 0.81 280 3332 – – 240 3913 280 434, 459, 483 (sh) 0.59 240 389

II1 270 348, 367, 385, 432, 455 0.66 240 3942 – – 240 3943 – – 260 389

sh = Shoulder.a Quantum yields were determined relative to [Ru(bpy)3](PF6)2 in H2O, U = 2.8%.

Table 5Luminescence spectral data for compounds I1–I3, II1–II3 in solid state at 298 and77 K.

Compound 298 K 77 K

kmax(exc)/nm kmax(em)/nm kmax(exc)/nm kmax(em)/nm

I1 375 537 370 560, 660 (sh)2 370 523 370 5603 370 540 370 560

II1 320 535 370 587, 6662 360 554 370 5583 380 570 370 570

sh = Shoulder.

M.D. Santana et al. / Inorganica Chimica Acta 378 (2011) 49–55 53

As shown in Fig. 6, low-energy bands in benzoquinolinate com-plexes are bathochromically shifted in relation to that of phenyl-pyridine derivatives, due to the presence of a more extendedconjugation in Bzq ligand relative to Phpy ligand [24,25]. Thesesalycilaldiminate complexes show also absorption bands in therange 430–460 nm that could be attributed to a 1MLCT assignment[9]. In general, the 1MLCT/1LLCT bands are usually sensitive to sol-vent polarity, so lower-polarity solvents such as toluene cause abathocromic shift of these bands if compared with the bands ob-served in the CHCl3 spectra. This red-shift in low-polarity solventsindicates that ground states of these complexes are more polarthan excited states, which is consistent with the charge transfernature of the excited state [26], as can be observed for I3 in Fig. 7.

The high energy (k < 300 nm) more intense absorption bands donot show any appreciable solvatochromism, suggesting theassignment to p–p⁄ mainly ligand-centered (1LC) transition [27].Solid-state diffuse reflectance UV–Vis spectra (DRUV) display sim-ilar features with those obtained in CHCl3 solution (see Table 3).

3.5. Emission spectra

All palladium complexes under study show photoluminescencein the solid state and/or in solution (Tables 4 and 5). Room-temper-ature luminescence of mononuclear palladium complexes is notcommon [7a], because of the presence of low-lying metal-centered(MC) excited states which deactivate the potentially luminescentmetal-to-ligand charge transfer (MLCT) and ligand-centered (LC)

levels through thermally activated processes. It is not surprisingthen that in our case room temperature luminescence was onlyexhibited by I1, I3 and II1. Emission quantum yields are in a rangesimilar (0.6–0.8%) to that of related complexes [5].

The emission spectra of phenylpyridine complexes I1 and I3 insolution at room temperature are shown at in Fig. 8. These com-plexes display structured emissions at kmax = 430 and 434 nm, inCHCl3 solutions, with vibronic progressions of 1278 and 1255cm�1, respectively, and lifetimes (s) in the order of 10�6 s. Thesebands can be assigned to intraligand (3IL) emission by the cyclo-metallated ligand Phpy [28].

λ / nm

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

400 420 440 460 480 500

I1I3

Em

issi

on I

nten

sity

/ a. u

.

Fig. 8. Emission spectra at r.t. of I1 and I3 in CHCl3 solutions (10�6 M), kexc = 280(I1), 240 (I3) nm.

λ / nm

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

450 500 550 600 650 700 750

Em

issi

on I

nten

sity

/ a.

u.

II1

II2

II3

Fig. 10. Solid state emission spectra of II1, II2 and II3 (kexc = 320–380 nm) at 298 K.

0.5

0.6

0.7

0.8

0.9

1I1

I2

I3te

nsity

/ a.

u.

54 M.D. Santana et al. / Inorganica Chimica Acta 378 (2011) 49–55

Complexes I1–I3 and II1–II3 are luminescent in chloroformsolutions at 77 K (Fig. 9, Table 4). These emission spectra are verysimilar and show a broad band. The emission maxima span the333–394 nm range. These structureless bands suggest that the ob-served emission includes a certain character 3CT.

0

0.1

0.2

0.3

0.4

480 530 580 630 680 730

λ / nm

Em

issi

on I

n

Fig. 11. Solid state emission spectra of I1, I2 and I3 (kexc = 370 nm) at 77 K.

3.6. Emission in solid state

Microcrystalline samples of the compounds under study exhibita broad room-temperature emission in the range 523–570 nm, sig-nificantly red-shifted if compared to the emission in rigid-matrixat 77 K. It is a general trend that the structureless solid state emis-sions are red-shifted to those of CHCl3 solutions at 77 K, and can beattributed to excimeric 3pp⁄ excited states [5]. Luminescence dataof our solid samples (see Table 5) suggest that their emission couldbe originated by such excimeric species [29]. Upon cooling to 77 K,the solid state emissions of complexes increase in intensity, butwith no notable shift in energy. Figs. 10 and 11 show a selectionof emission spectra.

Excimer formation (particularly in the solid state) for square-planar metal complexes has been reported to be quite effective

λ / nm

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

300 350 400 450 500 550

Em

issi

on I

nten

sity

/a. u

.

I1

I2

I3

Fig. 9. Emission spectra at 77 K of I1, I2 and I3 in CHCl3 solutions (10�6 M),kexc = 240–280 nm.

in generating room-temperature luminescence [7a]. Excimeric ex-cited states are indeed substantially red-shifted from MLCT or LCstates relate to isolated molecules, so that the energy gap betweenthe excimeric state and the deactivating MC states increases. Thereported excimeric emission from square-planar complexes comesfrom two different processes: (i) charge transfer emission arisingfrom excited states exhibiting substantial metal–metal interaction(metal–metal to ligand charge transfer, MMLCT, sometimes de-signed as dr⁄?p⁄ (ligand) charge transfer excited states, wherethe donor orbital is derived from direct metal–metal interaction)and (ii) pp⁄ emission from excited states derived from stackinginteractions between the aromatic moieties of coordinated ligands[30].

In the case of our mononuclear complexes that show an intensestructureless broad emission in the range 523–570 nm at roomtemperature, it can be suggested that an excimeric contributionto solid state emissions arising from the second process is possible,according to X-ray single-crystal data which indicate extended p-stacking.

4. Conclusions

The unique characteristics of di-l-hydroxo-complexes contain-ing a cyclometallated backbone as precursors in the preparation ofnew compounds when ligand deprotonation is required have beenexploited. By means of simple acid–base reaction with Schiff-base

M.D. Santana et al. / Inorganica Chimica Acta 378 (2011) 49–55 55

type ligands new complexes with luminescent properties havebeen prepared. Extensive characterization has been carried out,including X-ray single-crystal analyses, which indicate the pres-ence of relevant p-stacking interactions. These structural resultshave helped to confer luminescence in the solid state to excimericstates that could include pp⁄emission. As a series, all palladiumcomplexes under study exhibit an exceptional behavior since in-tense photoluminescence was found in both solid state and in solu-tion, at room temperature or 77 K, supporting the crucial roleplayed for auxiliary ligands in the luminescent properties of palla-dacycles suggested by other authors.

Acknowledgments

We thank Fundación Séneca (Project 08670/PI/08) for financialsupport. R.G.B. thanks Universidad de Murcia for a Grant.

Appendix A. Supplementary material

CCDC 795484, 817874, and 817875 contain the supplementarycrystallographic data for (I3), (I2), and (II2). These data can be ob-tained free of charge from The Cambridge Crystallographic DataCentre via www.ccdc.cam.ac.uk/data_request/cif. Supplementarydata associated with this article can be found, in the online version,at doi:10.1016/j.ica.2011.08.012.

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