Modulation of the luminescence quantum efficiency ...

Download PDF
2 downloads 0 Views 389KB Size Report
Comment
cIUT Paul Sabatier, Av. G. Pompidou, BP 20258, F-81104, Castres, France. E-mail: ...... 16 F. Launay, V. Alain, E. Destandau, N. Ramos, E. Bardez, P. Baret and.
PAPER

www.rsc.org/dalton | Dalton Transactions

Modulation of the luminescence quantum efficiency for blue luminophor {Al(salophen)}+ by ester-substituents† Virginie B´ereau,*a,b,c V´eronique Jub´era,d Philippe Arnaud,a,b Abdellah Kaiba,d Philippe Guionneaud and Jean-Pascal Sutter*a,b Received 3rd September 2009, Accepted 30th November 2009 First published as an Advance Article on the web 20th January 2010 DOI: 10.1039/b918235g A series of AlIII complexes of a salophen (N,N¢-bis(salicylidene)-o-phenylenediamine) derivative with an ester substituent at the C5 position of the salicylidene rings have been prepared and their luminescence properties investigated. All exhibit a bright blue emission (l em = 478 nm) that is observed neither for other metal ions nor with the non-functionalized ligand. Moreover, the ester-R group (Me, t Bu, Ph) was found to significantly affect the quantum yields (f = 12 to 20%) of the luminophor. DFT calculations have been undertaken to reveal the influence of the ester and its R-group on the frontier molecular orbitals. Preparation and photoluminescence properties in solution and solid state are reported. The crystal structure of a pyridine oxide adduct [LtBu Al(PyO)2 ]·NO3 , where LtBu stands for the tButyl-ester functionalized Schiff base, has been solved.

Introduction N,N¢-Bis(salicylidene)ethylenediamine (salen) molecules and their derivatives are some of the most widely used ligands as they are usually excellent chelating agents for a large variety of metal salts and form stable complexes.1 The principal application of these Schiff base complexes is in catalysis for various organic transformations including the epoxidation of olefins, lactide polymerization, asymmetric ring opening of epoxides, and Michael reactions.2 Among these salen metal complexes are those obtained with the Al3+ cation, exhibiting catalytic behaviour in asymmetric synthesis and polymerization reactions.3,4 In spite of the great attention devoted to this class of compounds for catalytic applications, the photophysical properties of the aluminium Schiff base derivatives have remained hardly explored until recently. In the past five years, however, derivatives of salen ligands have been shown to be interesting sensors for the spectrofluorimetric determination of trace Al3+ and an aluminium complex of a functionalised Schiff base has been used as luminescent labeling agent.5-8 Studies on aluminium salen derivatives as versatile materials for OLEDs have emerged only very recently.9-11 This is in contrast with the extensive work devoted to the aluminium quinoline derivatives showing potential in OLED and sensor applications.12-16 For both quinoline and Schiff base complexes of Al3+ , it has been shown that the emission wavelength can be modulated by ligand substituents,

a CNRS; LCC (Laboratoire de Chimie de Coordination), 205 route de Narbonne, F-31077, Toulouse, France. E-mail: [email protected] b Universit´e de Toulouse; UPS, INPT, LCC, F-31077, Toulouse, France c IUT Paul Sabatier, Av. G. Pompidou, BP 20258, F-81104, Castres, France. E-mail: [email protected] d CNRS, Universit´e de Bordeaux, ICMCB, 87 avenue du Dr A. Schweitzer, Pessac, F-33608, France † Electronic supplementary information (ESI) available: text version of cif and crystal data for PyO-4b; synthesis for 5b and 5c; photo-physical data for 3a-c, 5b, 6b and non-substituted salophen complexes; TD-DFT results. CCDC reference number 740475. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/b918235g

2070 | Dalton Trans., 2010, 39, 2070–2077

which permits the fine adjustment of the colour of the emitted light for a given luminophor.9,17-19 A further parameter of importance in today’s light emitter devices is the emission efficiency and its long term stability under ambient conditions. In the present report we show that quantum yield can also be enhanced by subtle modification of the ligand substituent without affecting the emission wavelength. A series of AlIII complexes of a salophen (N,N¢-bis(salicylidene)o-phenylenediamine) derivative with an ester substituent at the C5 position of the salicylidene rings (Chart 1) have been prepared and their luminescence properties investigated. All exhibit a bright blue emission that is observed neither for other metal ions nor with the non-functionalized ligand. Moreover, the ester-R group was found to significantly affect the quantum yields. DFT calculations have been undertaken to reveal the incidence of the ester and its R-group on the frontier molecular orbitals.

Chart 1

Ester functionalized salophen ligands.

Results and discussion Synthesis and crystal structure The functionalised Schiff bases 3a, 3b, and 3c have been prepared in three steps (Scheme 1). After the controlled formylation of 4-hydroxybenzoic acid with hexamethylenetetramine in trifluoroacetic acid through the Duff reaction,20 the resulting This journal is © The Royal Society of Chemistry 2010

Scheme 1 Synthetic pathways for ester functionalised Schiff bases 3a-c and their metal complexes: (i) 1 equiv. HMTA, CF3 COOH, inert atmosphere, 3 h reflux; (ii) 2a: H2 SO4 (1 equiv.), methanol, 3 h reflux, 2b: 1 equiv. DCC, tert-butanol, 1 h 30 min reflux; 2c: 1 equiv. phenol, 1 equiv. POCl3 , toluene, 2 h reflux, (iii) 0.5 equiv. o-phenylenediamine, methanol, 2 h RT; (iv) 4a-c: 1 equiv. Al(NO3 )3 ·9H2 O in ethanol, RT; 5b-6b: 1 equiv. M(CH3 COO)2 ·H2 O, methanol, RT.

salicylaldehyde derivative was esterified. For 2a (R = CH3 ), the esterification was classically run in methanol in the presence of sulfuric acid. For 2b (R = C(CH3 )3 ), a DCC-mediated coupling was necessary. The phenol ester salicylaldehyde 2c was obtained by generating in situ the acyl halide derivative with POCl3 . The third step was the formation of 3a-c by condensation of the ester-functionalized aldehydes with o-phenylenediamine, classically operated in alcohol. Addition of aluminium nitrate to these Schiff bases in ethanol gave the three complexes in good yields: [LMe Al(NO3 )(H2 O)2.5 ],21 4a, [LtBu Al(NO3 )(H2 O)], 4b, and [LPh Al(NO3 )(H2 O)2 ], 4c (LMe stands for N,N¢-bis-(3-methoxycarbonylsalicylidene)-1,2-phenylenediamine; LtBu for N,N¢-bis-(3tert-butoxycarbonylsalicylidene)-1,2-phenylenediamine, and LPh for N,N¢-bis-(3-phenoxycarbonylsalicylidene)-1,2-phenylenediamine). The complexes [LtBu Zn(H2 O)], 5b, and [LtBu Cu(MeOH)], 6b, were prepared from the acetate salts of the metal in methanol. The non-functionalized Schiff base salophen and its Al, Zn and Cu complexes were obtained according to literature procedures.22-24 All attempts at growing crystals of 4a, 4b and 4c suitable for X-Ray diffraction studies failed. However, when pyridine oxide (PyO) was added, pale yellow crystals of the pyridine oxide adduct [LtBu Al(PyO)2 ]·NO3 ·3H2 O, PyO-4b, were obtained. The ORTEP representation of the metal complex is shown in Fig. 1 with a selection of bond distances and bond angles. The crystallographic data are given in the ESI.† PyO-4b crystallizes in the triclinic space group and comprises one cationic metal complex, one nitrate anion and three water molecules. The complex consists of a central six-coordinate aluminium atom in a distorted octahedral geometry. The Schiff base ligand occupies the four equatorial positions, and the pyridine oxide molecules occupy the two axial ones. The most significant deviation from the Oh geometry is found for the two equatorial bond angles O13–Al1–O14 (93.79(19)◦ ) and N1–Al1–N2 (81.0(2)◦ ), reflecting the constraints imposed by the This journal is © The Royal Society of Chemistry 2010

Fig. 1 Molecular structure with ellipsoids cut at the 50% probability level of the adduct PyO-4b. The hydrogen atoms, the nitrate anion and the three water molecules have been omitted for clarity. Selected bond distances ˚ ): Al1–O13: 1.816(4), Al1—O14: 1.824(4), Al–N1: 2.007(5), Al1–N2: (A 2.000(5), Al1–O1: 1.945(4), Al1–O2: 1.953(4). Selected bond angles (◦ ): O13–Al1–O14: 93.79(19), N1–Al1–N2: 81.0(2), O1–Al1–O2: 174.2(2).

o-phenylenediimino backbone of the ligand and creating a more “open” front for the compound. The axial O1–Al1–O2 bond angle slightly deviates from linearity with 174.2(2)◦ . The axial Al–O ˚ are longer than the bond distances of 1.945(4) and 1.953(4) A ˚ ), probably equatorial Al–O bond lengths (1.816(4) and 1.824(4) A due to the steric requirement of the two axial pyridine oxide. Photophysical investigations in solution In order to perform a rigorous comparison the same solvent was used for all absorption and emission experiments reported here. DMSO was chosen to circumvent the low solubility of the nonfunctionalized salophen complexes in common organic solvents. The absorption and emission data of the Al complexes 4a-c are gathered in Table 1, those for the other compounds are given as ESI.† The UV-visible spectra are characterized by a high energy Dalton Trans., 2010, 39, 2070–2077 | 2071

Table 1 Experimental photophysical data of Al complexes 4a-c (l exc = 385 nm, 5.0 ¥ 10-6 M in DMSO) and selected calculated absorption maximum wavelengths and oscillator strengths for 4b and 4c Absorption Al complexes (R = CH3 ) 4a

(R = (CH3 )3 C) 4b

(R = C6 H5 ) 4c

a

Emission -1

l max /nm

e/M cm

280 332 378 407

-1

TD-DFT calculations

f (%)

l em /nm

58000 15800 17200 sh

14

478

278 334 382 415

50600 20400 20400 sh

12

20 294 387 421

74800 21000 sh

a

l calcd /nm

f calcd

—

—

478

266 336 368 413

0.447 0.329 0.397 0.442

478

278 312 369 415

0.794 0.445 0.417 0.494

Quinine sulfate (f = 0.55) used as standard.

band, at 280, 278 and 294 nm for 4a, 4b, and 4c respectively, with an additional band at 332 for 4a and 334 for 4b. A very large and unstructured band with a long tail is also present for the three complexes. This absorption, found between 378 and 387 nm, is gradually red-shifted as the R group of the ester varies from CH3 to C6 H5 (from 4a to 4c). All the transitions observed are assigned to p–p* intra-ligand transitions. Excitation at this lowest energy band (l exc = 385 nm) leads to a broad bright blue emission centered at 478 nm for Al-complexes 4a-c. Comparison of the emission spectra of the free ligands 3a-c and of the Al derivatives (Fig. 2) shows the significantly enlarged emission intensity exhibited by the complexes. Quantum yield measurement for 4a-c gave values between 12 and 20% (in DMSO solutions),25 values that compare favorably with those obtained for successful Al-salen luminophors.6,9 Comparison with the values obtained for the free ligands 3a (3%), 3b (1%) and 3c (7%) (see ESI†) highlights the influence of the metal-coordination on the emission efficiency.

To evaluate the actual importance of the nature of the metal ion, the luminescence properties of the Zn and Cu complexes, 5b and 6b, have been investigated. Results can be visually evaluated in Fig. 3. Both 5b and 6b, are poorly emissive with quantum yield below 1% (f = 0.002 for 5b and f = 0.0004 for 6b, see ESI†), confirming the role of Al in luminophors 4a-c. Likewise, the importance of the ester function was confirmed by the poor luminescence (f = 0.01, see ESI†) of the [Al salophen]+ complex. It can be noticed that the ester group of the salicylidene rings has very little effect on the emission maximum of the complexes, the emission wavelength of [Al salophen]+ being noted at 490 nm versus 478 nm for the ester functionalized complexes 4a-c.

Fig. 3 Pictures showing the luminescence of Schiff base 3b and its complexes 4b (Al), 5b (Zn), and 6b (Cu) under UV-irradiation (365 nm, in DMSO solutions).

Fig. 2 Emission spectra of Schiff bases 3a-c and their Al complexes 4a-c (l exc = 385 nm, 5.0 ¥ 10-6 M in DMSO).

2072 | Dalton Trans., 2010, 39, 2070–2077

Unambiguously, the remarkable properties found for compounds 4a-c rely on a conjugate contribution of the Al metal center and the ester-functions. To account for the performance of the Al-derivatives 4a-c as compared to the Zn or Cu homologues, the much stronger Lewis-acid character of AlIII might be invoked. This leads to a more ionic Al–O(salicylidene) bond and thereby This journal is © The Royal Society of Chemistry 2010

reinforces the propensity for a quinoid-type resonant structure that enhances the light absorption ability for the Al complex.19 Considering the favorable positioning of the ester functions and their electronic characteristics (i.e. sp2 bonds and lone pairs) it is likely that the ester strongly contributes to such a resonant structure. The involvement of the ester is supported by the variation of the quantum yield with the ester R-group (Me, t Bu, Ph). Replacing the alkyl R-group with Ph results in a significant enhancement of the emission efficiency with f = 20% for R = Ph versus f = 14 and 12% for R = Me and t Bu, respectively. To gain insight into the electronic contribution of these chemical groups a series of DFT calculations have been undertaken. DFT calculations The electronic structures of the two compounds 4b (R = (CH3 )3 C) and 4c (R = C6 H5 ) were investigated by quantum chemical calculations at the DFT level of theory. The geometry 4b was optimized from the X-Ray structure of PyO-4b, with replacement of the two axial pyridine oxide ligands by two water molecules; whereas geometry optimization 4c was performed after the tertbutyl groups in the geometry 4b have been substituted by two phenyl rings. For both geometries 4b and 4c, the HOMO and LUMO orbitals are ligand centred, no contribution of the Al centre is found in the frontier MO (Fig. 4 and 5) in accordance with earlier observations.9,26 For geometry 4b, the HOMO is spread over the

Fig. 5 Calculated frontier orbitals for geometry 4c.

Fig. 4 Calculated frontier orbitals for geometry 4b.

whole salen core, including a small contribution from one of the ester O atoms. The same trend applies down to HOMO-4 with however, a stronger involvement of the ester moieties (HOMO-2 and -3). The same applies for the LUMOs. Strikingly, for geometry 4c the HOMO and closest MO (HOMO-1 to -3) are constructed solely from the contributions of the ester groups, and mainly from the phenoxy units. A MO involving the salen core is found for HOMO-4. Such a difference between 4b and 4c is not seen for the LUMOs, which are very similar for both compounds. It can also be noticed that the HOMO for 4b is found at -8.725 eV whereas it is at -8.204 eV for 4c; their LUMOs being at -5.258 eV and -5.370 eV respectively. As a result the HOMO–LUMO gap is 3.467 eV for 4b and 2.834 eV for 4c. Interestingly, even though the HOMO–LUMO gap for the compounds is different, the MO schemes allow understanding of the very similar optical properties for the two compounds (Table 1). It can be recalled that for such salen type compounds the absorptions at low energies result from p–p* transitions involving the salycilidene and imino moieties of the ligand. Orbitals centred on the salycilidene are mainly found in the HOMO and HOMO1 for compound 4b but only in the HOMO-4 for 4c whereas for both complexes, imino-centred orbitals are involved in the LUMO and LUMO+1. The corresponding energy gaps are close with 3.467 eV (HOMO–LUMO) and 3.855 eV (HOMO-1–LUMO) for 4b, and 3.471 eV (HOMO-4–LUMO) for 4c, i.e. 358 and 322 nm, and 357 nm, respectively. These values deduced from calculations compared well with those observed experimentally: 382 and 334 nm for 4b and 387 nm for 4c. Such good agreement between the calculated and experimental absorption wavelengths is found over the whole of the spectra (See Table 1 and ESI†).

This journal is © The Royal Society of Chemistry 2010

Dalton Trans., 2010, 39, 2070–2077 | 2073

Table 2 Luminescence properties of the solid state materials 3b and 4-6b Data

l exc /nm

3b

4b

5b

Trichromatic coordinates (x;y)

solar lighting 350 280 350 450 280 350

(0.465;0.452) (0.412;0.572) 9 11 12 10 12

(0.427;0.462) (0.267;0.526) 3 4 5 3.3 4.4

(0.402 0.452) (0.193;0.425) 3.8 5 5 3.9 5

External quantum efficiencya Internal quantum efficiencya a

in%; Zn2 SiO4 : Mn2+ (N.B.S. 1028) used as standard (rext NBS1028 at 280 nm = 45%).

Solid state luminescence The luminescence properties of the solid samples of 3b, 4b, and 5b were also investigated. Excitation and emission spectra were recorded at room temperature and are depicted in Fig. 6. When excited at 350 nm, an emission with a peak centered at 553 nm for 3b, 498 nm for 4b, 490 nm for 5b is observed. The luminescence spectra in solid state are red shifted compared with those obtained from solutions. This is likely caused by molecular aggregation between adjacent luminophors (i.e. exciplex formation), such interaction being very much reduced in dilute solutions. The x and y trichromatic coordinates (CIE) of the powders are reported in Table 2 under solar lighting to indicate the natural coloration of the powders (yellow) and under excitation at 350 nm. In the latter case, the color is shifted from the yellow-green part of the trichromatic diagram for 3b to the bluish-green part for 5b through the yellowish green part for 4b (ESI†). The quantum efficiencies are also listed in Table 2. The strong enhancement of the Al complex 4b observed in solution is no longer seen in solid state. Both 4b and 5b exhibit very similar fluorescence efficiency. It is worthy of note that for 5b the quantum efficiency in the solid state is one order of magnitude larger than that obtained in solution.

Fig. 6 Normalized excitation (dashed lines) and emission spectra (solid lines) of 3b (red), 4b (blue) and 5b (black) in solid state at 298 K (l exc = 350 nm).

Conclusions The present study unambiguously shows that the bright blue luminescence observed for the Al complexes of the novel esterfunctionalized salophen ligands is a consequence of a conjugated 2074 | Dalton Trans., 2010, 39, 2070–2077

effect from both the metal center and the ester-group. Moreover, these compounds represent a rare example of substantial modulation of the luminescence intensity by subtle chemical alteration at the periphery of a given luminophor without affecting the emission wavelength. Hence, they open interesting perspectives in further increasing emission efficiency by judicious design of the ester-functionalized ligand. Furthermore the ester-R group was found to substantially affect the HOMO–LUMO gap, another parameter of importance in photo-voltaic devices for instance. Concomitantly, the high versatility and quite easy preparation of Schiff bases permits us to envision chemical alterations of the salophen core to tune the emission maximum. Work along these lines is in progress.

Experimental section Synthesis and characterization Reagents and solvents were obtained commercially and used without further purification. Reactions that required anhydrous conditions were carried out under an inert atmosphere of nitrogen. ˚ When specified, reactions were monitored using Silica Gel 60 A ˚, analytical TLC plates by UV detection (254 nm). Silica gel (60 A 70–200 mm) was used for column chromatography. 1 ¨ H NMR spectra were recorded using a Bruker spectrometer with working frequency 250.0 MHz for 1 H. Chemical shifts were referenced to the residual proton resonance of the deuterated solvent. Infrared spectra were recorded as KBr pressed pellets on a Genesis Series FTIR Ati Mattson. (3-Formyl-4-hydroxy)benzoic acid (1). The synthesis of 1 has been adapted from the published procedure for the ortho formylation of phenol.27 4-Hydroxybenzoic acid (15 g; 108 mmol) was suspended in 40 mL of trifluoroacetic acid under nitrogen. A solution of hexamethylenetetramine (15.3 g; 109 mmol) in 45 mL of trifluoroacetic acid was added dropwise. The resulting mixture was refluxed under nitrogen and monitored by TLC (10CHCl3 + 1.5CH3 OH). The reflux was maintained until the disappearance of the 4-hydroxybenzoic acid (ca. 2 h). After cooling to room temperature, the mixture was added to 300 mL of 4 M HCl and stirred for 3 h. The yellow precipitate was then isolated by filtration and abundantly washed with water. The yellow solid was dried under vacuum yielding 1 (7.2 g, 40%). Anal. Calcd for C8 H6 O4 : C, 57.84; H, 3.64. Found: C, 57.56; H, 3.34. 1 H NMR (CD3 OD) d 10.15 (1H, s, HC=O), 8.44 (1H, d, Ar-H), 8.20 (1H, dd, Ar-H), 7.06 (1H, d, Ar-H). n max /cm-1 1666 (CHO). This journal is © The Royal Society of Chemistry 2010

(3-Formyl-4-hydroxy)benzyl methanoate (2a). 3-Formyl-4hydroxybenzoic acid 1 (1.00 g; 6.00 mmol) was suspended in 10 mL of methanol. H2 SO4 (0.50 mL; 8.26 mmol) was added and the suspension was refluxed until completion of the reaction (around 3 h). The clear pale pink solution obtained was evaporated to dryness. The pink residue was triturated with water and extracted with ethyl acetate (3 ¥ 50 mL). The organic phase was dried over Na2 SO4 and evaporated. An orange-yellow solid was obtained. Purification was made by extraction with diethyl ether, addition of charcoal, filtration and evaporation to dryness, leading to a white solid (0.833 g, 75%). Anal. Calcd for C9 H8 O4 : C, 60.00; H, 4.48. Found: C, 60.20; H, 4.16. 1 H NMR ((CD3 )2 CO) d 11.40 (1H, s, Ar–OH), 10.17 (1H, s, HC=O), 8.47 (1H, d, Ar-H), 8.21 (1H, dd, Ar-H), 7.12 (1H, d, Ar-H), 3.90 (3H, s, CH 3 ). n max /cm-1 1712 (COO), 1666 (CHO). (3-Formyl-4-hydroxy)benzyl tert-butanoate (2b). A solution of dicyclohexylcarbodiimine (DCC) (1.24 g; 6.00 mmol) in 35 mL of tert-butanol was added dropwise to a solution of 1 (1 g; 6.02 mmol) in 40 mL of tert-butanol. The resulting solution was reflux for 1 h 30 min. The solution was then cooled down to room temperature and added to 150 mL of brine. The aqueous phase was twice extracted with 100 mL of CHCl3 . The organic phases were combined, dried with anhydrous Na2 SO4 and evaporated. The yellow oil obtained was solubilized in the minimum of acetone and then filtered to eliminate some white insoluble dicyclohexylurea (DCU). This step was repeated until no further DCU precipitated. The orange oil was purified by column chromatography (eluant: CHCl3 ) to obtain a white solid (1.34 g, 79%). Anal. Calcd for C12 H14 O4 : C, 64.85; H, 6.35. Found: C, 64.21; H, 5.92. 1 H NMR ((CD3 )2 CO) d 11.42 (1H, s, Ar–OH), 10.13 (1H, s, HC=O), 8.49 (1H, d, Ar-H), 8.28 (1H, dd, Ar-H), 7.17 (1H, d, Ar-H), 1.73 (9H, s, CH 3 ). n max /cm-1 1712 (COO), 1660 (CHO). (3-Formyl-4-hydroxy)benzyl phenolate (2c). To a mixture of phenol (425.3 mg; 4.52 mmol) and 1 (750 mg; 4.52 mmol) in 75 mL of dry toluene was added POCl3 (200 mL; 2.14 mmol). The suspension was refluxed for 4 h (the condenser was equipped with a drying tube) and then stirred at room temperature for 12 h. Toluene was then removed. The pink oil obtained was purified by column chromatography (eluant: CHCl3 ) to obtain a white oily solid (415 mg, 38%). 1 H NMR ((CD3 )2 CO) d 11.44 (1H, s, Ar– OH), 10.23 (1H, s, HC=O), 8.66 (1H, d, Ar-H), 8.37 (1H, dd, Ar-H), 7.53–7.18 (5H, m, Ar-H), 6.84 (1H, d, Ar-H). n max /cm-1 1712 (COO), 1666 (CHO). N,N¢-Bis-(3-methoxyoxycarbonylsalicylidene)-1,2-phenylenediamine (3a). A solution of o-phenylenediamine (119 mg; 1.10 mmol) in 30 mL of methanol was added dropwise to a solution of 2a (397 mg; 2.20 mmol) in 20 mL of methanol. The resulting solution was stirred at room temperature for 2 h. The volume was then reduced to 2/3 of its initial value to allow the beginning of the precipitation. It was completed by stirring the suspension for an additional 1 h. The yellow precipitate was filtered and dried under vacuum (0.45 g, 30%). Anal. Calcd for C24 H20 N2 O6 : C, 66.66; H, 4.68; N, 6.48. Found: C, 66.07; H, 4.60; N, 6.78. 1 H NMR ((CD3 )2 CO) d 13.60 (2H, s, Ar–OH), 9.08 (2H, s, HC=N), 8.32 (2H, d, Ar-H), 8.05 (2H, dd, Ar-H), 7.53 (4H, m, Ar-H), 7.08 (2H, d, Ar-H), 3.89 (6H, s, CH 3 ). n max /cm-1 1707 (COO), 1616 (C=N). This journal is © The Royal Society of Chemistry 2010

N,N¢-Bis-(3-tert-butoxycarbonylsalicylidene)-1,2-phenylenediamine (3b). A solution of o-phenylenediamine (182.7 mg; 1.69 mmol) in 45 mL of methanol was added dropwise to a solution of 2b (751.4 mg; 3.38 mmol) in 35 mL of methanol. The resulting solution was stirred at room temperature for 2 h. The yellow precipitate obtained under stirring was filtered, washed with water and dried under vacuum (0.66 g, 76%). Anal. Calcd for C30 H32 N2 O6 : C, 69.75; H, 6.24; N, 5.42. Found: C, 69.31; H, 5.86; N, 5.85. 1 H NMR ((CD3 )2 CO) d 13.64 (2H, s, Ar–OH), 9.10 (2H, s, HC=N), 8.33 (2H, d, Ar-H), 7.95 (2H, dd, Ar-H), 7.52 (4H, m, Ar-H), 7.05 (2H, d, Ar-H), 1.57 (18H, s, CH 3 ). n max /cm-1 1708 (COO), 1619 (C=N). N, N¢ - Bis - (3 - phenoxycarbonylsalicylidene) - 1, 2 - phenylenedi amine (3c). A solution of o-phenylenediamine (47 mg; 0.436 mmol) in 1 mL of methanol was added dropwise to a solution of 2c (211 mg; 0.872 mmol) in 7 mL of methanol. The resulting solution was stirred at room temperature for 2 h and evaporated to dryness. The orange residue was extracted with CHCl3 . After filtration, the CHCl3 solution was evaporated and the yellow solid obtained dried under vacuum (0.16 g, 66%). Anal. Calcd for C34 H24 N2 O6 : C, 73.37; H, 4.35; N, 5.03. Found: C, 72.85; H, 4.25; N, 5.13. 1 H NMR ((CD3 )2 CO) d 13.68 (2H, s, Ar–OH), 9.16 (2H, s, HC=N), 8.53 (2H, s, Ar-H), 8.25 (2H, dd, Ar-H), 7.63 (2H, m, Ar-H), 7.50 (6H, m, Ar-H), 7.33 (6H, m, Ar-H), 7.18 (2H, d, Ar-H). n max /cm-1 1723 (COO), 1613 (C=N). N, N¢ - Bis - (3 - methoxycarbonylsalicylidene) - 1, 2 - phenylenedi amine aluminium(III) nitrate·2.5H2 O (4a). 3a (75 mg; 0.173 mmol) was suspended in 4 mL of ethanol. Al(NO3 )3 ·9H2 O (64.8 mg; 0.173 mmol) was solubilised in 2 mL of ethanol and slowly added to the solution of 3a. The resulting yellow suspension was stirred at room temperature for 2 h. Slowly, the suspension turned clear to obtain, after 2 h of stirring at room temperature, a bright yellow solution. After evaporation to dryness, the orange oil obtained was triturated with diethyl ether leading a yellow solid, isolated by filtration and dried under vacuum (80 mg; 86%). Anal. Calcd for C24 H18 N3 O9 Al·2.5H2 O: C, 51.07; H, 4.11; N, 7.44. Found: C, 51.22; H, 3.92; N, 7.76. 1 H NMR ((CD3 )2 CO) d 9.51 (2H, s, HC=N), 8.49 (2H, s, Ar-H), 8.20 (2H, m, Ar-H), 8.06 (2H, dd, Ar-H), 7.61 (2H, m, Ar-H), 7.06 (2H, d, Ar-H), 3.87 (6H, s, CH 3 ). n max /cm-1 1698 (COO), 1615 (C=N). N,N¢-Bis-(3-tert-butoxycarbonylsalicylidene)-1,2-phenylenediamine aluminium(III) nitrate·H2 O (4b). 3b (150 mg; 0.291 mmol) was added to 8 mL of ethanol. Al(NO3 )3 ·9H2 O (109 mg; 0.291 mmol) was solubilised in 3 mL of ethanol and slowly added to the solution of 3b. The resulting yellow suspension was stirred at room temperature for 2 h. The bright yellow solution was evaporated to dryness. The orange oil was dried under vacuum and triturated with 10 mL of diethyl ether to obtained a yellow solid isolated by filtration (162 mg; 90%). Anal. Calcd for C30 H30 N3 O9 Al·H2 O: C, 57.97; H, 5.19; N, 6.76. Found: C, 57.75; H, 5.45; N, 6.50. 1 H NMR ((CD3 )2 CO) d 9.40 (2H, s, HC=N), 8.33 (2H, s, Ar-H), 8.20 (2H, m, Ar-H), 8.02 (2H, d, Ar-H), 7.58 (2H, m, Ar-H), 6.90 (2H, d, Ar-H), 1.62 (18H, s, CH 3 ). n max /cm-1 1713 (COO), 1616 (C=N). N,N¢-Bis-(3-tert-butoxycarbonylsalicylidene)-1,2-phenylenediamine aluminium(III)-bis-pyridine oxide nitrate·3H2 O (PyrO-4b). 4b (25.1 mg; 0.0407 mmol) and pyridine oxide (11.7 mg; Dalton Trans., 2010, 39, 2070–2077 | 2075

0.123 mmol) were mixed together in 5 mL of methanol leading to a clear bright yellow solution. This solution was placed in a glass hemolysis tube and allowed to slowly evaporate. Yellow plate-shaped crystals suitable for X-Ray diffraction, were collected (26.8 mg, 78%). 1 H NMR (CD3 OD) d 9.37 (2H, s, HC=N), 8.37 (6H, m, Ar-H), 8.15 (2H, m, Ar-H), 8.08 (2H, d, Ar-H), 7.67–7.56 (8H, m, Ar-H), 7.05 (2H, d, Ar-H), 1.63 (18H, s, CH 3 ). n max /cm-1 1684 (COO), 1611 (C=N). N, N¢ - Bis - (3 - phenoxycarbonylsalicylidene) - 1, 2 - phenylenedi amine aluminium(III) nitrate·2H2 O (4c). 3c (150 mg; 0.291 mmol) was added to 8 mL of ethanol. Al(NO3 )3 ·9H2 O (109 mg; 0.291 mmol) was solubilised in 3 mL of ethanol and slowly added to the solution of 3c. The resulting yellow suspension was stirred at room temperature for 2 h. The bright yellow solution was evaporated to dryness. The orange oil was dried under vacuum and triturated with 10 mL of diethyl ether to obtain a yellow solid isolated by filtration (162 mg; 90%). Anal. Calcd for C34 H22 N3 O9 Al·2H2 O: C, 60.09; H, 3.66; N, 6.35. Found: C, 60.19; H, 3.74; N, 6.18. 1 H NMR ((CD3 )2 CO) d 9.59 (2H, s, HC=N), 8.68 (2H, d, Ar-H), 8.22 (4H, m, Ar-H), 7.62 (2H, m, Ar-H), 7.51 (4H, m, Ar-H), 7.33 (6H, m, Ar-H), 7.14 (2H, d, Ar-H). IR n C=N :1718 (COO), 1614 (C=N). X-Ray diffraction The yellow single crystals of PyO-4b show a poor diffraction pattern with no Bragg peaks at high q angle ( 2s(I))= 0.073 and R(all data) = 0.153, wR2 (all data) = 0.2186. Full details are given in Table S1-ESI.† The CIF file is given as ESI. CCDC deposit number 740475. Computational method All the calculations described here were carried out with the Gaussian program.30 The 6-31G(d) basis set was used for all calculations. The ground state geometries of 4b and 4c were optimized with a C 2v and C 2 point group respectively, using DFT method with the B3LYP hybrid exchange correlation functional. For all schemes the ground state minima have been confirmed by determination of the vibrational frequencies. The electronic transition energies were computed by the time-dependent density functional theory (TD-DFT)31 method using the B3LYP functional32,33 (TD-B3LYP). Solvent effects on transition energies were evaluated by means of the polarizable continuum model (PCM) in its integral equation formalism34 and the default 2076 | Dalton Trans., 2010, 39, 2070–2077

parameters were taken from the literature.35 In continuum models, one divides the model into a solute part lying inside a cavity surrounded by the solvent part (here DMSO). Absorption and emission spectroscopy Setup for the characterization of solutions. UV-visible spectra were obtained from a Hewlett-Packard 8453 diode array spectrophotometer. Measurements were made in 1 cm path length cells at 293 K. Uncorrected emission and corrected excitation spectra were collected with a Perkin-Elmer LS50B spectrofluorimeter, using 1 cm ¥ 1 cm quartz cells at 293 K. A 1500 nm min-1 speed scan was used. Setup used for the characterization of powders. UV excitation and emission spectra were recorded with a SPEX Fluorolog FL 212 spectrofluorimeter with the possibility to add a low temperature setup (liquid nitrogen or helium cryostat). Excitation spectra were corrected for the variation of the incident flux as well as emission spectra for the transmission of the monochromator and the response of the photomultiplier. Reflectance spectra were obtained with the same equipment by simultaneous rotation of the monochromators placed before and after the sample. Quantum yields. The quantum yields for fluorescence in solution were determined using an aqueous solution of quinine sulfate in 1 N H2 SO4 . Concentrations of the solutions, including solutions of standards, were adjusted so that the absorbance at l exc was between 0.04 and 0.05. In this case, intensity of the measured emission can be considered to be proportional to the concentration of the species in solution. The emission quantum yields were then calculated using eqn (1), where fs is the emission quantum yield of the sample, fstd is the emission quantum yield of the standard, Astd and As represent the absorbance of and the sample at   the standard the excitation wavelength, while I std and I s are the integrals of the emission envelopes of the standard and the sample respectively, and h is the refractive index of the solvents used for the samples and standard solutions.

⎛ ⎜ ⎛ A ⎞⎟⎜⎜ f s = f std ⎜⎜⎜ std ⎟⎟⎜⎜ ⎜⎝ As ⎟⎠⎜⎜ ⎜⎜ ⎝

⎞⎟ ⎟⎟⎛ h ⎞2 ⎟⎟⎜⎜ s ⎟⎟ ⎟ ⎟⎟⎜⎜ ⎟⎝ hstd ⎟⎠ std ⎟ ⎟⎠

∫I ∫I

s

(1)

For the determination of the quantum efficiencies on solid samples, Zn2 SiO4 :Mn2+ (N.B.S. 1028) has been used as the reference for the measurements. All the acquisitions were performed under the same conditions. All the emission spectra were recorded at 280 nm in order to optimize the emission of the standard (rext NBS1028 at 280 nm = 45%). The internal quantum efficiency rint corresponds to the external quantum efficiency, rext , divided by the absorption spectrum obtained from the reflectance spectra. The external and internal quantum efficiencies are given in the ESI† as a function of the excitation wavelengths.

Acknowledgements This work was supported by the University of Toulouse (Universit´e Paul Sabatier) and by the CNRS. This journal is © The Royal Society of Chemistry 2010

References 1 P. A. Vigato and S. Temburini, Coord. Chem. Rev., 2004, 248, 1717– 2128. 2 P. G. Cozzi, Chem. Soc. Rev., 2004, 33, 410–421. 3 M.-A. Munoz-Hernandez, M. L. McKee, T. S. Keizer, B. C. Yearwood and D. A. Atwood, J. Chem. Soc., Dalton Trans., 2002, 410–414. 4 T. M. Ovitt and G. W. Coates, J. Am. Chem. Soc., 1999, 121, 4072– 4073. ¨ uz, ¨ S. Kuc ¨ ¸ ukkolbasy, ¨ 5 S. B. Gund O. Atakol and E. Kylyc¸, Spectrochim. Acta, Part A, 2005, 61, 913–921. 6 P. G. Cozzi, L. S. Dolci, A. Garelli, M. Montalti, L. Prodi and N. Zaccheroni, New J. Chem., 2003, 27, 692–697. 7 M. S. J. Briggs, J. S. Fossey, C. J. Richards, B. Scott and J. Whateley, Tetrahedron Lett., 2002, 43, 5169–5171. 8 M. Arduini, F. Felluga, F. Mancin, P. Rossi, P. Tecilla, U. Tonellato and N. Valentinuzzi, Chem. Commun., 2003, 1606–1607. 9 K. Y. Hwang, H. Kim, Y. S. Lee, M. H. Lee and Y. Do, Chem.–Eur. J., 2009, 15, 6478–6487. 10 J. O. Huh, M. H. Lee, H. Jang, K. Y. Hwang, J. S. Lee, S. H. Kim and Y. Do, Inorg. Chem., 2008, 47, 6566–6568. 11 K. Y. Hwang, M. H. Lee, H. Jang, Y. Sung, J. S. Lee, S. H. Kim and Y. Do, Dalton Trans., 2008, 1818–1820. 12 S.-H. Liao, J.-R. Shiu, S.-W. Liu, S.-J. Yeh, Y.-H. Chen, C.-T. Chen, T. J. Chow and C.-I. Wu, J. Am. Chem. Soc., 2009, 131, 763–777. 13 M. A. Palacios, Z. Wang, V. A. Montes, G. V. Zyryanov, B. J. Hausch, K. Jursikova and P. Anzenbacher Jr, Chem. Commun., 2007, 3708– 3710. 14 C. Perez-Bolivar, V. A. Montes and P. Anzenbacher Jr, Inorg. Chem., 2006, 45, 9610–9612. 15 J.-B. Mulon, E. Destandau, V. Alain and E. Bardez, J. Inorg. Biochem., 2005, 99, 1749–1755. 16 F. Launay, V. Alain, E. Destandau, N. Ramos, E. Bardez, P. Baret and J.-L. Pierre, New J. Chem., 2001, 25, 1269–1280. 17 A. Mishra, N. Periasamy, M. P. Patankar and K. L. Narasimhan, Dyes Pigm., 2005, 66, 89–97. 18 I. Aoki, T. Matsunaga and K. Wanatabe, Bull. Chem. Soc. Jpn., 1989, 62, 2369–2374. 19 K. Morisige, J. Inorg. Nucl. Chem., 1978, 40, 843–851. 20 W. E. Smith, J. Org. Chem., 1972, 37, 3972–3973. 21 Note: Solvent contents were determined by elemental analysis (see Experimental section and ESI). 22 S. Kashanian, M. B. Gholivand, F. Ahmadia, A. Taravati and A. Hosseinzadeh Colagar, Spectrochim. Acta, Part A, 2007, 67, 472–478. 23 D. N. Kumar and B. S. Garg, Spectrochim. Acta, Part A, 2006, 64, 141–147. 24 D. J. Darensbourg and D. R. Billodeaux, Inorg. Chem., 2005, 44, 1433– 1442. 25 Note: the same values have been obtained from MeOH solutions.

This journal is © The Royal Society of Chemistry 2010

26 M. La Deda, I. Aiello, A. Grisolia, M. Ghedini, M. Amati and F. Lelj, Dalton Trans., 2006, 330–339. 27 L. F. Lindoy, G. V. Meehan and N. Svenstrup, Synthesis, 1998, 1029– 1032. 28 G. M. Sheldrick, Programs for Crystal Structure Analysis (Release ¨ fur ¨ Anorganische Chemie der Universit¨at, Tam97-2), 1998, Institut ¨ manstrasse 4, D-3400 Gottingen, Germany. 29 L. J. Farrugia, J. Appl. Crystallogr., 1999, 32, 837–838. 30 M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, J. A. Montgomery Jr., T. Vreven, K. N. Kudin, J. C. Burant, J. M. Millam, S. S. Iyengar, J. Tomasi, V. Barone, B. Mennucci, M. Cossi, G. Scalmani, N. Rega, G. A. Petersson, H. Nakatsuji, M. Hada, M. Ehara, K. F. Toyota, R., J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, M. Klene, X. Li, J. E. H. Knox, H. P. Hratchian, J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperst, R. E. Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. W. Otchterski, P. Y. Ayala, K. Morokuma, G. A. Voth, P. Salvador, J. J. Dannenberg, V. G. Zakrzewski, S. Dapprich, A. D. Daniels, M. C. Strain, O. Farkas, D. K. Malick, A. D. Rabuck, K. Raghavachari, J. B. Foresman, J. V. Ortiz, Q. Cui, A. G. Baboul, S. Clifford, J. Cioslowski, B. B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaroni, R. L. Martin, D. J. Fox, T. Keith, M. A. Al-Laham, C. Y. Peng, A. Nanayakkara, M. Challacombe, P. M. W. Gill, B. Johnson, W. Chen, M. W. Wong, C. Gonzalez and J. A. Pople, GAUSSIAN 03, (2004) Gaussian, Inc., Wallingford, CT. 31 E. Runge and E. K. U. Gross, Phys. Rev. Lett., 1984, 52, 997–1000. 32 C. Lee, W. Yang and R. G. Parr, Phys. Rev. B: Condens. Matter, 1988, 37, 785–789. 33 P. J. Stephens, F. J. Devlin, C. F. Chabalowski and M. J. Frisch, J. Phys. Chem. A, 1994, 98, 11623–11627. 34 J. Tomasi, B. Mennucci and R. Cammi, Chem. Rev., 2005, 105, 2999– 3094. 35 M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, J. A. Montgomery Jr., T. Vreven, K. N. Kudin, J. C. Burant, J. M. Millam, S. S. Iyengar, J. Tomasi, V. Barone, B. Mennucci , M. Cossi, G. Scalmani, N. Rega, G. A. Petersson, H. Nakatsuji, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, M. Klene, X. Li, J. E. H. Knox, H. P. Hratchian, J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperst, R. E. Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. W. Otchterski, P. Y. Ayala, K. Morokuma, G. A. Voth, P. Salvador, J. J. Dannenberg, V. G. Zakrzewski, S. Dapprich, A. D. Daniels, M. C. Strain, O. Farkas, D. K. Malick, A. D. Rabuck, K. Raghavachari, J. B. Foresman, J. V. Ortiz, Q. Cui, A. G. Baboul, S. Clifford, J. Cioslowski, B. B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaroni, R. L. Martin, D. J. Fox, T. Keith, M. A. Al-Laham, C. Y. Peng, A. Nanayakkara, M. Challacombe, P. M. W. Gill, B. Johnson, W. Chen, M. W. Wong, C. Gonzalez and J. A. Pople, GAUSSIAN DV, (2007) Gaussian, Inc., Wallingford, CT.

Dalton Trans., 2010, 39, 2070–2077 | 2077