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Structural Correlations between Luminescent Properties and Excited State Internal Proton Transfer in some Zinc(II) N,N’-bis(Salicylidenes) Cristina Aparecida Barboza, José Carlos Germino, Anderson Martinez Santana, Fernando Júnior Quites, Pedro Antônio Muniz Vazquez, and Teresa Dib Zambon Atvars J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp510476h • Publication Date (Web): 16 Feb 2015 Downloaded from http://pubs.acs.org on February 18, 2015

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The Journal of Physical Chemistry

Structural Correlations between Luminescent Properties and Excited State Internal Proton Transfer in some Zinc(II) N,N’-bis(Salicylidenes)

Cristina Aparecida Barbozaa†*, José Carlos Germinoa†, Anderson Martinez Santanab Fernando Junior Quites††a, Pedro Antônio Muniz Vazqueza and Teresa Dib Zambon Atvarsa a

Chemistry Institute, State University of Campinas - UNICAMP, Campinas, Brazil b Department of Chemistry, Institute of Exact and Earth Sciences, Federal University of Mato Grosso – UFMT, Cuiabá, Brazil

*

corresponding author: [email protected]

†

authors contributed equally to this work.

††

permanent address Department of Chemistry, Institute of Exact and Earth Sciences, Federal University of Mato Grosso – UFMT, Cuiabá, Brazil

Abstract

In this study two salicylidene ligands, N,N'-bis(salicylidene)-1,2-phenylenediamine and N,N'bis(salicylidene)-4,5-diaminopyrimidine, and their respective aquo-zinc(II) coordination compounds were synthesized. Their characterization was performed by FTIR, proton and carbon NMR, elemental analysis,mass spectroscopy and cyclic voltammetry. Crystal structures of the ligands were determined by monocrystal X-ray diffraction. The photoluminescent properties under photostationary conditions indicate that the ligand emission predominates in both the pristine materials and their zinc(II)complexes. For both ligands, the coordination of a metal atom leads to a redshift of their emission bands in both solvent and solid state. Molecular structures and excitation energies of ligands and complexes were evaluated at DFT level using PBE0/aug-cc-pVDZ. Their ligand and complex electronic transitions can be assigned mainly to the intraligand π→π* type, mainly involving frontier molecular orbitals, with a small participation of the metal. According to our calculations, there is an increasing in the planarity of the ligand structure in the complex, which could explain the redshifting observed in the absorption and emission spectra. The dynamic photoluminescence suggests the occurrence of excited state intramolecular proton transfer from the oxygen to the nitrogen atoms in the coordination site of the sal-4,5-pym.Moreover, they are able to predict the occurrence of the excited state internal proton transfer for the sal-4,5-pym. The dynamic of this proton transfer is demonstrated by both, time resolved emission spectra (TRES) and studies in protic solvent (ethanol).

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Introduction

Organic electronics is a field that has attracted increasing attention due to their practical applications1-6as light emitting diodes,5,6 solar cells,4 transistors,9,10 super capacitors,11 sensors and actuators,12-14 photoswitching,15,16 etc. Many materials can be used as organic semiconductors, including coordination compounds, polymers, hybrids and nanocomposites.1-7 Aromatic molecules with extended π frameworks are particularly interesting for this aim.7Among the several types of semiconductors used in these devices is the class of salicylidenes.17 Salicylidenes are aromatic compounds that have been the subject of extensive research for many years due to their applications in different fields such as catalysis,18-21 molecular architectures,2225

magnetism,26,27 materials chemistry,28 biomarkers,29and optical chemosensors,30 among others.31-34

These tetradentate chelating Schiff bases can coordinate transition metals,allowing to tune their luminescent properties by structural modification in the ligand framework and by changing the metal in their coordination site.12,13,15,35-39 Electroluminescent compounds can be obtained through the coordination of zinc(II) ions by salicylidenes.35-37 Metal to ligand and intraligand charge transfer processes can be observed in the absorption spectrum of zinc(II) salicylidene compounds.40 Some of these ligands might also undergone excited-state intramolecular proton transfer (ESIPT).41-48This process involves a keto (K*) ↔enol (E*) balance in the electronic excited state of molecules containing intramolecular H-bonds.49,50 The electronic absorption of energy by N,N’bis(salicylidenes) in the ground state leads to an excited electronic state in the enol form. This excited form can undergo radiative decay from the E* state or might undergo ESIPT to generate a K* species, which radiatively decays by emitting at lower energy than from E*.47,48 Faster decays and larger Stokes shifts (SS) between the absorption and emission bands are the fingerprints of this process.50-53 Because compounds exhibiting ESIPT may depict two emission bands covering a wide spectral range, they can be interesting compounds for white light emitting diodes.49,54 In this work, the synthesis and photophysical properties of two salicylidene derivatives and their respective aquo-Zn(II) complexes, the N,N’-bis(salicylidene)-1,2-phenylenediamine (salophen) and its analogue N,N’-bis(salicylidene)-4,2-diaminopyrimidine (sal-4,5-pym)(Figure 1),were studied experimentally and theoretically as an attempt to evaluate the possibility of ESIPT occurrence. To analyze the structural aspects of the intramolecular enol-keto process, experimental measurements were performed in DMSO (non-protic solvent) and in protic solvent (ethanol) (only for the ligands) and also in solid-state. The experimental measurements included the electronic absorption and emission


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spectroscopies under steady-state and dynamic conditions. In particular, the time resolved emission spectroscopy (TRES) gave complementary information about the time evolution of the K* ↔ E* balance of the (sal-4,5-pym) compound. Theoretical calculations were performed to explain the optical properties, giving insights into the molecular orbitals involved in the electronic transitions, the excited state ligand structures and the conformational changes associated with the observed spectral shifts. Comparing the structural characteristics determined by X-ray diffraction and calculated results, the occurrence of ESIPT process was discussed.

Figure 1. Molecular structure of a. the ligands salophen (A = carbon atom) and sal-4,5-pym (A = nitrogen atoms) and b. the complexes [Zn(salophen)(OH2)] and [Zn(sal-4,5-pym)(OH2)], showing the labels used in Table 1.

Experimental Section

Synthesis of the Ligands and the Zn-coordination Compounds All of the reagents were purchased from commercial suppliers in analytical reagent grade. The solvent DMSO HPLC/Spectrum grade was purchased from Tédia, Brazil. The o-phenylenediamino, 4,5-diaminopyrimidine and sodium hydroxide were purchased from Sigma-Aldrich. The zinc(II) acetate dihydrate was purchased from Fisher Scientific Company. The synthesis reaction scheme is shown in Figure 2. The Schiff bases were obtained by dissolving 1 mmol of the 1,2-diaminophenylene in ethyleneglycol by sonication. For the compound a, after 5 minutes, the salicylaldehyde was slowly dropped into this solution, and the mixture was then stirred for 1 hour. The precipitate was filtered, recrystallized at hot ethanol and washed with cold ethanol. The reaction yield was 95%. The salophen was an orange crystalline solid with the following elemental analysis (%): Calculated: C 75.93, H 5.10 and N 8.86; found: C 76.90, H 5.10 and N 8.85. m.p.: 168 °C. MS (ESI +) m/z 317.40 (calcd for M+ 317.36). 1HNMR (300 MHz, CDCl3) δ(ppm): 6,92



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(dt, J = 7.8 Hz, 2H), 7.04 (d, J = 0.9 Hz, 2H), 7.06 (m, 2H), 7.23 (m, 2H), 7.33 (m, 2H), 7.38 (dd, J = 7.65 Hz, 2H) and 8.62 (s, 2H).13C NMR (75 Hz, CHCl3) δ(ppm): 117.2,119.22, 119.5, 119.5, 127.9, 132.6, 133.6, 142.8, 161.6 and 169.9. The infrared bands measured in KBr pellet are (cm-1) νC=N = 1615, νO-H = 3052 and 2920, νC-O = 1276, νC-H= 3052, and νAr = 1192 and 760. The ligand sal-4,5-pym was obtained in a similar way but using 4,5-diaminopyrimidine instead of 1,2-diaminophenylene. The reaction yield was 85%. A yellow crystalline solid with elemental analysis (%): Calculated: C 67.91, H 4.43 and N 17.60; found: C 67.92, H 4.43 and N 17.60. m.p.: 180 °C. MS (ESI +) m/z 319.40 (calcd for M+ 319.34). 1HNMR (300 MHz, DMSO-d6) δ (ppm): 6.93 (m, 2-H), 6.96 (m, 2-H), 7.40 (m, 2-H), 7.71 (m, 2-H), 8.00 (s, 1-H), 8.25 (s, 2-H), 8.82 (s, 1-H), 11.75 (s, 2-H). 13C NMR (75 Hz, DMSO-d6) δ (ppm): 117.7, 119.9, 120.6, 129.7, 132.8, 134.2, 143.1, 156.7, 158.7, 160.2 and 163.9. The infrared bands measured in KBr pellet are (cm-1) νC=N= 1637, νO-H = 3313 and 2188,νC-O = 1286, νC-H = 3056, and νAr= 752. The aquo zinc(II) salicylidene was produced by dissolving 0.316 mmol of the ligand in 20 mL of ethanol under constant stirring. A solution of 0.632 mmol of NaOH in 5 mL of ethanol was then slowly added to the salicylidene solution. An ethanolic solution of a zinc(II) acetate dihydrate (0.316 mmol) was slowly dropped into the ligand mixture. The precipitate was filtered and washed adequately in hot deionized water and ethanol. The synthesis of [Zn(salophen)(OH2)] results in a yellow polycrystalline powder obtained with 95% yield. Its elemental analysis (%) gives: Calculated: C 60.4, H 4.1 and N 7.0; found: C 60.4, H 3.9 and N 7.1. MS (ESI-) m/z 396.70 (calcd for M- 396.75). 1HNMR (400 MHz, DMSO-d6) δ (ppm): 6.53 (dt, J = 7.30 Hz, 2H), 6.73 (dd, J = 8.18 Hz, 2H), 7.26 (m, 2H), 7.40 (q, J = 3.06 Hz, 2H), 7.43 (dd, J = 7.92, 2H), 7.91 (dd, J = 6.16, 2H) and 9.03 (s, 2H).

13

C NMR (125 Hz, DMSO-d6) δ (ppm): 113.5,

117.0, 119.9, 123.5, 127.8, 134.58, 136.7, 139.9, 163.3 and 172.7. The infrared bands measured in KBr pellet are (cm-1) νZn-N= 431, νZn-O= 534, νC=N= 1614, ν(O-H)Zn= 3428, νC-O= 1251,νC-H= 3048,and νAr= 1180 and 750. TGA weight loss (m%,in parenthesis calculated values): 5.0 (4.53) (150 - 230 ºC) – coordination water and 0.5 of hydration water, 74.5 (75.0) (300 - 600 ºC) ligand pyrolysis and residual 20.5 (20.47) –ZnO. An orange polycrystalline powder is obtained from the synthesis of the [Zn(sal-4,5-pym)(OH2)] with 90% reaction yield. Its elemental analysis (%) gives: Calculated: C 54.1%, H 3.5% and N 14.0%; found: C 54.1%, H 3.5% and N 14.0%. MS (ESI-) m/z 398.70 (calcd for M-398.73). 1HNMR (300 MHz, DMSO-d6) δ (ppm): 6.53 (t, 2-H), 6.73 (dd, 2-H), 7.27 (m, 2-H), 7.34 (m, 2-H) 7.58 (dd, 2-H), 8.90 (s, 1-H), 9.19, (s, 1-H), 9.28 (s, 1-H), 9.46 (s, 1-H).

13

C NMR (75 Hz, DMSO-d6) δ (ppm): 114.1, 114.8,


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119.6, 120.2, 124.3, 124.7, 132.5, 135.8, 136.9, 137.5, 138.3, 146.7, 155.7, 156.1, 165.6, 165.8, 173.5 and 175.8. The infrared bands measured in KBr pellet are (cm-1) νZn-N= 431, νZn-O= 534, νC=N= 1614, ν(O-H)Zn= 3428, νC-O= 1251, νC-H = 3048, and νAr= 1180 and 750. The TGA weight loss is similar to the loss measured for [Zn(salophen)(OH2)].

Figure 2. Synthesis scheme for salicylidenes and their Zn(II) coordination compounds: i. ultrasonic bath in ethyleneglycol (25 mL); ii. ethanol (20 mL), heating and stirring, 30 min.

Methods The crystal structures of the ligands were determined by X-ray diffraction using andiffractometer Bruker Apex Duo with a monochromatic source and resolved by the Patterson method.55-57 Attempts to crystallize the complexes failed. Fusion temperatures were determined using the Fisatom 430 melting point equipment. 13C and 1H NMR 1D spectra were measured using deuterated solvents in a Varian spectrometer Mercury 300 MHz and a Bruker 400 MHz. FTIR spectra were recorded in a Varian 660-IR spectrometer, with the solid dispersed in KBr pellet. The electrospray mass spectroscopies (ESI-MS) were measured on a QUATTRO MICROTMspectrometer with a mass work range of 50-500 m/z coupled on ultra-perform liquid chromatograph from WATERS. The ESI AcquityTM with mist flow of 100 L h-1in modes ESI+sampler was used for ligands and ESI- for coordination compounds. Thermal stability was evaluated using a thermobalance Shimadzu model DTG 60H. TG/TGA curves were recorded from T = 25 ºC to 1000 ºC with a heating rate of 25 ºC min-1 in argon atmosphere. Elemental analysis of the C, H and N atoms was performed using a PerkinElmermicroanalyzer, model PE 2400. The ground-state redox potentials of the ligands and Zn(II) coordination compounds were determined by cyclic voltammetry according to the literature procedure58 using a potentiostat model PAR 273A with a set of three electrodes: work and counter electrodes of platinum and a reference electrode of Ag/Ag+ in an acetonitrile solution of supporting electrolyte tetrabutylammoniumACS Paragon Plus Environment


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hexafluorophosphate (0.1 mol L-1). The energy correction was performed employing ferrocene (Fc+/Fc) as the internal standard with a scan rate of 50 mV s-1. The formal standard potential of the redox couple Fc+/Fc in acetonitrile is approximately -5.1 eV, and thus, the HOMO and LUMO energies were determined using the following equations: (1)

(2) The electronic absorption spectrum was acquired in a Hewlett-Packard 8452A diode array UVVis spectrophotometer. Steady-state fluorescence spectra were acquired using an ISS-PC1 spectrofluorometer. The excitation wavelengths correspond to the maximum of the absorption spectra and are thus different for every sample. These wavelengths are indicated in the text and figures. For solutions in DMSO and ethanol(10 µmol L-1), a 1 cm quartz cuvette was used. Films of every solid were deposited by casting of a DMSO solution on a glass slice, which was accommodated in a homemade sample holder within the fluorimeter. It was oriented in a back-face position between the excitation beam and the emission detector. Fluorescence decayswere recorded using time-correlated single photon counting in an Edinburgh Analytical Instruments FL 900 spectrofluorimeter with a MCP-PMT detector (Hamamatsu R3809U-50). Salophen and sal-4,5-pym ligands in DMSO and ethanol solutions were excited by a pulsed diode with λexc = 335 nm (model EPLED-340, band width of 14.4 nm, 81.5ps) and λexc = 375 nm (model EPL-375, with a 10 nm bandwidth, 77.0 ps), respectively. Their coordination compounds [Zn(salophen)(OH2)] and [Zn(sal-4,5-pym)(OH2)] in DMSOsolutions were excited using a pulsed diode laser with λexc = 405 nm (model EPL-405, with a 10 nm bandwidth, 46.3 ps). These compounds are not soluble in ethanol.The decay signals for these samples were collected at λem = 460, 425 and 545, 495 and 535 nm for salophen, sal-4,5-pym, [Zn(salophen)(OH2)] and [Zn(sal-4,5-pym)(OH2)], respectively. For samples at solid-state, a pulsed diode operating at λexc = 375 nm (model EPL-370, with a band width of 5 nm, 77 ps) was used. The instrument response was recorded using Ludox samples. At least 10.000 counts in the peak channel were accumulated for lifetime measurements. Emission decays were analyzed using exponential functions, as previously described.59-61 Time resolved emission spectra (TRES) were recorded using the same instrument. These spectra gave information on the spectra evolution with the time and provided additional evidences for the ESIPT process. The following wavelengths were used: for salophen λexc = 335 nm andλem = 400 to ACS Paragon Plus Environment

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640 nm, and sal-4,5-pym λexc = 375 nm; λem = 405 to 705 nm, in both DMSO and ethanol. Intervals between emission wavelengths were 10 nm. Delays between 0 to 15 ns were used to generate the TRES plots.

Computational Details

Ground and first singlet excited state S1 geometry optimizations were performed at the density functional theory (DFT) level. In the case of the ligands, the ground state molecular structure was not optimized; the X-ray structure was used for the calculations. Considering that the PBE0 density functional gives a reasonable ratio between the description of the excited states and the computational cost,62-64 it was chosen for this study. The basis set aug-cc-pVDZ65-67and the software Gaussian 09 were used.68Electronic transitions were calculated by time-dependent density functional theory (TDDFT) implemented in the Dalton program version 2011.69 Solvent effects were considered using the Polarizable Continuum Model - PCM.70-73 Results and Discussion Characterization of the Salicylidenes and their Aquo-Zinc(II) Complexes The molecular structures of salophen and sal-4,5-pym were determined by the solid-state X-ray diffraction technique for the ligands only. Using diffraction patterns, the ligand structures were resolved by the Patterson method.54-56 These geometrical data (Table 1) were used in the theoretical calculations. The electronic ground state structures for bothS0 and S1states of the Zn-coordination compounds were obtained at the PBE0/aug-cc-pVDZ level (Figure 3). According to our results (Table 1), the structures of the Zn(II)-coordination compounds correspond to a bent conformation with an average deviation from planarity of ~36º, with Cs and C1 symmetries for [Zn(salophen)(OH2)] and [Zn(sal-4,5-pym)(OH2)], respectively, in agreement with the structure of [Zn(salophen)] obtained by Girichev and coworkers74 at the B3LYP/cc-pVTZ (Stuttgart ECP for the metal atom) level. Optimized geometries for both complexes are given in Table 1.



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Figure 3. X-ray and optimized structures of the S1 state for salophen and sal-4,5-pym showing the Hbond observed between the N and H atoms, and optimized structures of complexes. ACS Paragon Plus Environment

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Table 1. Selected bond lengths (Å) and dihedral angles (degrees) for the structures of the ligands and the Zn-complexes obtained by the X-ray diffraction technique* and calculated using the PBE0/aug-ccpVDZ basis set for the electronic ground state. Salophen

Sal-4,5-pym

[Zn(salophen)(OH2)]

[Zn(sal-4,5-pym)(OH2)]

S 0*

S1

S 0*

S1

S0

S1

S0

S1

C1-A2

1.368

1.416

1.382

1.346

1.400

1.409

1.338

1.347

A2-C3

1.377

1.378

1.386

1.340

1.394

1.377

1.340

1.326

C3-C4

1.392

1.416

1.396

1.382

1.404

1.413

1.400

1.400

C4-C5

1.399

1.454

1.400

1.433

1.423

1.447

1.421

1.434

C4-N6

1.415

1.353

1.422

1.394

1.406

1.371

1.400

1.380

N6-C7

1.278

1.325

1.282

1.304

1.305

1.327

1.307

1.312

C7-C8

1.451

1.425

1.454

1.431

1.432

1.424

1.429

1.427

C8-C9

1.392

1.412

1.392

1.391

1.424

1.417

1.425

1.418

C9-C10

1.373

1.384

1.382

1.399

1.380

1.379

1.379

1.375

C10-C11

1.379

1.408

1.384

1.411

1.414

1.411

1.415

1.411

C11-C12

1.367

1.385

1.377

1.375

1.383

1.377

1.382

1.376

C12-C13

1.388

1.405

1.383

1.446

1.427

1.423

1.300

1.421

C13-O14

1.351

1.319

1.351

1.254

1.300

1.290

1.300

1.297

O14-H(Zn)15

0.821

1.026

0.820 (1.894)**

1.785 (1.037)**

1.987

1.995

1.988

1.989

O14--N16

3.549

3.549

4.234 (3.548)♦

3.848 (3.846)♦

4.046

4.033

4.040

4.024

N16-Zn15

-

-

-

-

2.104

2.071

2.090

2.053

Zn15-O14-C13-C12

-

-

-

-

9.7

0.4

6.7

17.7

In parenthesis are given the bond lengths of

**

N6···H17 and ♦N6···O18.

To obtain more information about the possibility of the ESIPT processes involving ligands both alone and in the coordination compounds, their structures corresponding to the low-lying electronic excited

singlet

state S1

were optimized at

the PBE0/aug-cc-pVDZ level.

Relative to

[Zn(salophen)(OH2)] the most significant difference between S0 and S1 is the increased planarity of the molecule observed in the excited state, which could be related to the significant redshift observed in the complexes absorption and emission bands (see optical properties below). The same trend is observed for [Zn(sal-4,5-pym)(OH2)], but since its unsymmetrical the dihedral angle Zn15-O14-C13-C12 for the



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S1structure is disturbed by the formation of an H-bond between the ligand phenyl oxygen and one of the hydrogen atoms of the coordinated water. A shortening of the N6···H15 distance by 16% in the optimized geometry of the excited state relative to the X-ray structure for salophen was obtained (Table 1). This trend is consistent with the possibility of proton transfer in the excited state, also predicted by experimental measurements. This effect is more pronounced for the sal-4,5-pym, an unsymmetrical ligand, where the N6···H15 and N16···H17 bond distances for the S1 structure are significantly shorter than the corresponding bond lengths in S0, respectively. For this ligand, the structure corresponding o the K* tautomer is about 5 kcalmol-1 more stable than E*, while for salophen only the E* tautomer is found stable. This result for sal-4,5-pym is also supported by the vertical energies calculated in DMSO (Table 2) using S1 optimized structures for E* and K*, showing a distance between λem about 100 nm, consistent to the experimental measures. This results reveal that the substitution of the phenyl for a pyrimidine ring in the ligand structure favours an increase in the electronic density in the iminic nitrogen atom when excited, leading to the ESIPT process, as previously observed by Jayabharathiet al. for imidazole derivatives.75 In conclusion, the theoretical calculations gave important structural information for these compounds: the molecular geometries of the ligands in the electronic ground and excited states are quite similar; the salophen is more planar than the sal-4,5-pym; the possibility of the ESIPT process generating a keto*↔enol* balance is higher for the sal-4,5-pym ligand due to the shortening of the N16···H17 bond distance; and when the coordination complexes are formed, there is planarization of the ligand geometries, but the complex is not completely planar. All of these geometrical characteristics may play important roles in their optical properties by potentially increasing the electronic conjugation (see below).

Photoluminescence of Salicylidenes and their Zinc(II) Coordination Compounds

Absorption and emission data for ligands and their Zn(II) coordination compounds measured in solid-state and DMSO and ethanol solutions (10 µmol L-1) are given in Figure 4 and summarized in Table 2. The absorption spectrum of salophen shows a band at ~ 335 nm (ε = 1.67×104 L mol-1 cm-1). When coordinated to Zn(II), this bandis significantly redshifted to 405 nm (ε = 2.49×104 L mol-1 cm-1), probably due to the increased ligand conjugation (optimized structures showed a remarkable planarization of the ligand framework upon coordination). The substitution of the phenyl by thepyrimidine group in the sal-4,5-pym does not significantly disturb the ligand (325 nm; ε = 9.68×103 ACS Paragon Plus Environment

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L mol-1 cm-1) or its Zn-complex (440 nm; ε = 1.12×104 L mol-1 cm-1) spectral profile. For both coordination compounds, these absorption bands can be assigned as intraligand charge transfer transitions - ILCT-type. The electronic absorption spectra of both the salophen (λabs = 335 nm, ε = 1.76×104 L mol-1 cm-1) and sal-4,5-pym (λabs = 350 nm, ε = 2.13×104 L mol-1 cm-1) were also obtained in ethanol solutions and the spectral profile is very similar to those in DMSO (Figures S23 and S24). Only the absorption of sal-4,5-pym in ethanol is redshifted. Measurements with the coordination compounds in ethanol were not done because their lack of solubility in this solvent. Table 2. Optical properties of ligands and Zn(II)-coordination compounds measured in DMSO, ethanol (10 µmol L-1) and in solid-state. In parenthesis are given emission wavelengths for ligands calculated at PBE0/aug-cc-pVDZ level using DMSO as solvent. λmax/nm

ε/L mol-1 cm-1

λem/nm

Φ%

τ/ns

B%

χ2

SS/cm-1

DMSO solution 10 µmol L-1 Salophen [Zn(salophen)(OH2)] Sal-4,5-pym

335

1.67×104

405

4

325

2.49×10

9.68×103

460 (463)

7.0

4.54 ± 0.01

100

1.096

8112

495

2.2

0.23 ± 0.04

94

1.115

4489

10

2.75 ± 0.01

6

2.75 ± 0.01

73

1.033

7240

8.52 ± 0.01

27

0.51 ± 0.03

48

3.54 ± 0.02

52

3.87 ± 0.01

425 (466) (E*) *

545 (566) (K ) [Zn(salophen)(OH2)]

425

1.12×10

4

535

2.0

12420 1.058

4837

-1

EtOH solution 10 µmol L Salophen Sal-4,5-pym

335

1.76×104

350

4

2.13×10

450

100

1.017

7629

425 (E )

0.90 ± 0.04

#

26

1.139

5042

535 (K*)

5.02 ± 0.07#

74

1.29 ± 0.05∇

40

∇

60

*

4,95 ± 0.13

11159 1.157

-

Solid-state Salophen

335

565

4.7

1.60 ± 0.01

100

1.148

12152

[Zn(salophen)(OH2)]

425

515

0.7

0.30 ± 0.01

83

1.001

4112

0.88 ± 0.01

17

0.25 ± 0.01

42

1.012

10293

0.77 ± 0.01

58

0.14 ± 0.04

70

1.049

4349

0.45 ± 0.01

30

Sal-4,5-pym

[Zn(salophen)(OH2)]

345

440

530

545

0.2

0.5

Fluorescence decays of sal-4,5-pym in ethanol monitored at λem = 425 nm# and λem = 535 nm∇.



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Figure 4. Normalized electronic absorption and photoluminescence spectra in DMSO solutions (10 µmol L-1): a. salophen (λexc = 335 nm; λem = 460 nm) and [Zn(salophen)(OH2)] (λexc = 405 nm; λem = 495 nm); b. sal-4,5-pym (λexc = 375 nm; λemE* = 425 nm and λemK* = 535 nm) and [Zn(sal-4,5pym)(OH2)] (λexc = 425 nm; λem = 535 nm).

The emission spectrum (λexc = 335 nm) of the salophen in DMSO shows a maximum at λem = 460 nm, characteristic of the emission of the enol speciesE* from its electronic excited state S1 (Figure 4a). The Stokes shift calculated using the emission and the absorption peaks is SS = 8112 cm-1 (Table 2). The fluorescence decay (λexc = 335 nm and λem = 460 nm) shown in Figure 5a can be fitted with a monoexponential function, yielding a lifetime of τ(salophen) =4.54 ± 0.01 ns. These results suggest that the ESIPT process is not observed for this molecule in agreement itsS1theoretically optimized structure. The emission spectrum for this compound in ethanol solution has the same band profile (Figure S25a), but is blue-shifted to λem = 450 nm (EtOH) with the SS as 7629 cm-1. The fluorescence decay of this compound in ethanol is monoexponential and faster than in DMSO, τ(salophen) = 3.87 ± 0.01 ns (Figure A10). In addition, the emission spectrum is time-independent (λem = 450 nm) as showed by the TRES experiments in DMSO (Figure S28a) as well as in ethanol solution (Figures S28b). From these experiments we concluded that salophen isnot undergoingESIPT processneither in DMSO nor in ethanol. The fluorescence spectrum of the [Zn(salophen)(OH2)] in DMSO solution (10 µmol L-1) showed a peak at λem = 495 nm (Figure 5a, Table 2) with a well-resolved vibronic band at 525 nm. The Stokes shift (4489 cm-1) is substantially reduced compared with the free ligand (8112 cm-1). Both results are expected for a more planar ligand geometry, as indicated by our calculations. The fluorescence decay (Figure 5b) (λexc = 405 nm and λem = 495 nm) is faster than for the free ligand and ACS Paragon Plus Environment

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can only be fitted by a biexponential function. The faster decay with τFL1 = 0.23 ± 0.04 ns predominates with a contribution of B1 = 94%, and a longer component is observed with τFL2 = 2.75 ± 0.01 ns (B2 = 6%). The faster decays indicate that the metal disturbs the electronic excited states of the ligand, as also confirmed by the decrease of the emission quantum yield from 7.0 (free ligand) to 2.2 % (Zn-complex) in DMSO solutions. The fluorescence spectrum of sal-4,5-pym in DMSO solution (10 µmol L-1) exhibits a peak in λem = 425 nm (Figure 4b) with a shoulder at 545 nm. Considering the peak of the absorption band (λabs = 325 nm), the Stokes shift can be determined as SS = 7240 cm-1which is larger than for salophen, indicating that this ligand has a remarkable change in its geometry in this electronic excited state compared to the electronic ground state. The fluorescence spectrum in ethanol showed two emission bands centered in the same maxima,atλE* = 425 nm, which we assigned to the E* species and at λK* = 535 nm, attributed to K*species emission. The SS in EtOH solution was 5042 cm-1 (in relation to λemE*) and 11159 cm-1 (in relation to λemK*) suggesting anE*↔K* balance occurs for protic and aprotic polar solvents for sal-4,5-pym. Additional evidences for this process was demonstrated by the TRES experiments (see below). The emission decay (Figure 5c) (λexc = 370 nm and λem = 520 nm) can only be fitted using biexponential functions, rendering two lifetimes, a faster component τFL1 = 2.75 ± 0.04 ns with a contribution of B1 = 73% and a longer component with τFL2 = 8.52 ± 0.01 ns (B2 = 27%) (Table 1).The emission decay of this compounds in ethanol monitored at λem = 535 nm (which excites preferentially K*) is also biexponential with the components of τFL1 = 1.29 ± 0.05 ns (B1 = 40%) and a longer component with τFL2 = 4.95 ± 0.07 ns (B2 = 60%) (Figure S27b).When the emission decay is monitored at λem = 425 nm (which preferentially excites E*), the former decay become faster τFL1 = 0.90 ± 0.04 ns with a smaller contribution of B1 = 26% and the later component is practically unchanged λem= 5.02 ± 0.07 ns (B2 = 74%) (Figure S27a). Furthermore, the TRES profile of the sal4,5-pym showed a time dependence:in DMSO the relative intensity of the lower laying band increases for longer times as expected for the K*(Figure S29b) whereas the relative intensity of the K* decreases for longer timein ethanol (FigureS29b). Therefore, we may concluded that two emissive species are present, with a time evolution correlated to the changes of the relative intensities of the emission bands. The amount of K* decreases at longer times after the excitation pulse and for longer delays, only the E* species is observed (Figure S29). Then, according the TRES analyses in solutions and the TD-DFT calculations at gas phase, the enol species dominates in the electronic excited state E* for salophen in both protic and aprotic polar solvents, whereas, for sal-4,5-pym there is a balance between theE*and K*in the excited state for protic and aprotic polar solvents.



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The fluorescence spectrum of the [Zn(sal-4,5-pym)(OH2)] (λexc = 425 nm and λem = 535 nm) (Figure 4b) in DMSO solution (10 µmol L-1) has a maximum at λem =535 nm and also exhibits a biexponential decay (Figure 5d). Similar to [Zn(salophen)(OH2)], the Stokes shift SS = 4837 cm-1 decreases substantially, indicating that a more rigid structure is formed in the complex. The faster lifetime of τFL1 = 0.51 ± 0.03 ns has almost the same contribution as the longer lifetime (B1 = 48%). Again, the presence of the faster decay indicates that the metal is influencing the deactivation of the electronic excited state of the sal-4,5-pym ligand, which is also confirmed by the decrease of the emission quantum yield from 10 to 2.0 % from the free ligand to the Zn(II)-complex in DMSO solutions. This behavior is observed for both complexes.


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Figure 5. Fluorescence decays in DMSO solution (10 µmol L-1): a. salophen (λexc = 335 nm; λem = 460 nm), b. [Zn(salophen)(OH2)] (λexc = 405 nm; λem = 495 nm), c. sal-4,5-pym (λexc = 375 nm; λem = 425 nm) and d. [Zn(sal-4,5-pym)(OH2)] (λexc = 405 nm; λem = 535 nm). Additionally, the absorption and emission spectra of both free ligands and their complexes were obtained in the solid-state (Figures 6). The spectral profiles are very similar to those obtained in DMSO solutions. For the salophen (Figure 6a), the low-lying absorption band is very broad and centered at approximately λabs = 350 nm. The ligand emission (λexc = 375 nm) band is also broad, being centered at ACS Paragon Plus Environment


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λem = 565 nm, with a fluorescence quantum yield of 4.7% and giving a very large Stokes shift (12152 cm-1) (Table 2). The fluorescence decay can be fitted by a monoexponential function, giving a lifetime of τK* = 1.60 ± 0.01 ns. When coordinated to the zinc(II), an intraligand charge-transfer broader band is observed in the electronic absorption spectrum at λabs = 425~450 nm, with a larger red-edge tail, indicating the presence of solid state aggregates.The emission (λexc = 375 nm) is centered at λem =515 nm, with a fluorescence quantum yield of 0.7 %. A smaller Stokes shift is obtained (SS = 4112 cm-1). The fluorescence decay is also biexponential with two lifetimes: τFL1 = 0.30 ± 0.01 ns (BFL1 = 83 %) and τFL2 = 0.88 ± 0.01 ns (BFL2 = 17 %). The decrease observed in the ligand fluorescence lifetime and quantum yield, due to the ligand coordination of the zinc(II) is in agreement to the possibility of fluorescence quenching by heavy atom in the complex. The absorption spectrum of sal-4,5-pymmeasured in the solid-stateshows a band peak inλabs = 345 nm. The fluorescence spectrum exhibits a band at λem = 535 nm (λexc = 375 nm) (Figure 7a), leading to a large Stokes shift (10293 cm-1). Its fluorescence decay is biexponential with the lifetimes of τFL1 = 0.25± 0.01 ns (BFL1 = 42 %) and τFL2 = 0.77 ± 0.01 ns (BFL2 = 58 %) (Figure 7b, Table 2). The difference between the monoexponential decay for the salophen and the biexponential for sal-4,5-pym may be correlated with the distribution of the molecules within the unit cell (Figure 3). Hence, there are two types of relative molecular orientations in the sal-4,5-pym unit cell and only one type in salophen. For the [Zn(sal-4,5-pym)(OH2)] in the solid state, the absorption band peak is λabs = 440 nm. The maximum emission occurs at λem = 545 nm (λexc = 375 nm) and is very similar to the emission of the free ligand (Figure 7a, Table 2). The fluorescence decay is biexponential with lifetimes of τFL1 = 0.14± 0.01 ns (BFL1 = 70 %) and τFL2 = 0.45± 0.01 ns (BFL2 = 30 %) (Figure 7c), both faster than for the free ligand.The Stokes shift is again smaller than for the free ligand (SS = 4349 cm-1), indicating that the geometrical relaxation is smaller in the solid state than in the DMSO. The quantum yield of the sal4,5-pym coordinated to Zn(II) increased from 0.2 % (free ligand) to 0.5%, in contrast to the observed values for the analogous salophen/[Zn(salophen)(OH2)]. In summary, optical properties of both the ligands are partially influenced by the coordination with the zinc(II)ion in the complexes. In addition, because of the increased basicity of the iminic nitrogen caused by the presence of the pyrimidine ring, the ESIPT is probably occurring in the sal-4,5pym compound. Planarization of ligands structure upon metal coordination can explain the differences of the optical properties of ligands and complexes. To get some additional insights to these conclusions, theoretical calculations were performed and the frontier orbital energies and charge densities were


Page 17 of 32

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analyzed.

Figure 6. a. Normalized electronic absorption and photoluminescence (λexc = 375 nm) spectra of salophen and [Zn(salophen)(OH2)] in the solidstate; fluorescence decays of b. salophen (λexc = 375 nm λPL = 565 nm) and c. [Zn(salophen)(OH2)] (λexc = 375 nm; λem = 515 nm).



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Figure 7.a. Normalized electronic absorption and photoluminescence (λexc = 375 nm) spectra of sal4,5-pym and [Zn(sal-4,5-pym)(OH2)] in the solid-state; fluorescence decays of b. sal-4,5-pym (λexc = 375 nm; λem = 535 nm) and c. [Zn(sal-4,5-pym)(OH2)] (λexc = 375 nm; λem = 545 nm).


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Electronic Transitions The electronic transitions and the charge density of the frontier orbitals were calculated as an attempt to explain the geometrical changes observed experimentally. Table 3 lists the main vertical excitation energies calculated at the aug-cc-pVDZ/PBE0 level in gas phase and DMSO. The consideration of solvent effects improves the agreement between computed and experimental electronic transitionsof both ligands and monoaquozinc(II) complexes, within a deviation of less than 4%. The Zn–(OH2) coordination by the ligand site leads to chromophore bands redshifted to ~300 and 420 nm, mainly due to the increased planarity and consequent electronic delocalization in the complexes with respect to the ligands. According to our calculations and cyclic voltammetry measures the replacement of the phenyl by pyrimidine leads to EH destabilization. The same trend is observed for calculated molecular orbital energies, however EHL is considerably overestimated. For both ligands, the coordination of the salicylidene by the metal ion decreases both EL and EHL, as reflected in their band positions, because their electronic transitions involve mainly frontier molecular orbitals, particularly of the HOMO → LUMO type. Molecular orbital densities can be used to qualitatively describe the main orbitals participating in low-lying transitions (Figure 8). They are mainly delocalized over the entire π framework of both ligands and complexes. These bands can be assigned as π→π∗ transitions, with a small contribution to the orbitals from the Zn(II) and without participation of the aquo ligand.



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Table 3. Emission andexcitation energies calculated at the PBE0/ aug-cc-pVDZ level in gas phase and DMSO (given in parenthesis). Molecular orbital energies were obtained by cyclic voltammetry. E/eV

λabs/nm

f

Assignment

EHL/eV λabs/nm

EH/eV

EL/eV

EHL/eV

Experimental

PBE0/aug-cc-pVDZ salophen

3.48 (3.59)

356 (345)

0.23 (0.36)

77% H→L§ 23% H→L+1

4.21

335

-5.72

-2.02

3.70

[Zn(salophen)(OH2)]

3.02 (3.18)

411 (390)

0.18 (0.27)

74% H→L 26% H-1→L+1

3.76

425

-5.59

-2.71

2.88

3.93 (4.05)

315 (306)

0.33 (0.37)

61% H-2→L 27% H-3→L+1 12% H-4→L+1

sal-4,5-pym

3.37 (3.46)

368 (358)

0.19 (0.20)

47% H→L 31% H-1→L 22% H-3→L

4.02

345

-5.63

-2.04

3.59

[Zn(sal-4,5-pym)(OH2)]

2.93 (3.07)

423 (404)

0.15 (0.34)

48% H→L 28% H-1→L 24% H→L+1

3.62

440

-5.61

-2.79

2.82

3.85 (3.98)

322 (312)

0.31 (0.34)

74% H-2→L 13% H-3→L 13% H-5→L+1

*

300

320

calculated in DMSO.

Figure 8. Molecular orbital energy diagram obtained at the PBE0/aug-cc-pVDZ level, showing HOMOs and LUMOs for ligands and complexes.


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Conclusions

The synthesis and characterization of the fluorescent ligands salophen and sal-4,5-pym and their coordination compounds with zinc(II) were successfully achieved with good reaction yields. Spectral data obtained by DMSO and in ethanol solution and solid-state absorption and steady state and time resolved fluorescence spectroscopy strongly suggest the occurrence of ESIPT only for the sal-4,5-pym ligand. No time evolution of the emission spectrum of the salophen is observed by TRES whose spectrum is similar to that under steady-state conditions. On the contrary, there is a time evolution of the emission spectra of the sal-4,5-pym for which the E* specie showed a fastest decay, appearing in shorter delays which supports the presence of the ESIPT process in both DMSO and ethanol solutions. Since the relative intensities evolute with the time, the E*↔K* balance of this compound is confirmed. This trend is also supported by excited state calculations performed at the PBE0/aug-cc-pVDZ level. For ligands and complexes, low-lying absorption bands can be assigned as π →π* type, with a small contribution to the orbitals from the zinc(II) atom. The metal presence in the complexes significantly disturbs their electronic structure, mainly due to an increase in the planarity and consequently the ligand electron delocalization when coordinated to the metal, which could be responsible for the observed redshift of the chromophore bands. The significant reduction of the lifetime and fluorescence quantum yield in the complex with respect to the free ligandmay be related to the quenching of the photoluminescence due to the presence of the metallic cation.

Acknowledgments

The authors acknowledge FAPESP (grant 2013/16245-2) CNPq, the National Institute of Organic Electronics (INEO) (MCT/CNPq/FAPESP), CNPq, and UNICAMP/FAEPEX for financial support and fellowships.



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For Table of Contents Only

In this study, two N,N'-bis(salicylidene) ligands and their respective aquo-zinc(II) coordinated compounds were synthesized and characterized by FTIR, proton and carbon NMR, elemental analysis, and cyclic voltammetry measurements in solid-state and solutions of DMSO and ethanol. In order to obtain more information about ground and excited states, in special to the excited state proton transfers observed for these systems theoretical calculations using TD-DFT were also done.


Structural Correlations between Luminescent ... - ACS Publications

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