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Revealing the Dynamic of Excited State Proton Transfer of a π‑Conjugated Salicylidene Compound: An Experimental and Theoretical Study M. G. Vivas,†,‡,* J. C. Germino,§ C. A. Barboza,§ Deborah de A. Simoni,§ P. A. M. Vazquez,§ L. De Boni,† T. D. Z. Atvars,§ and C. R. Mendonça† †

Instituto de Física de São Carlos, Universidade de São Paulo, São Carlos, SP Brazil Instituto de Ciência e Tecnologia, Universidade Federal de Alfenas, Poços de Caldas, MG Brazil § Chemistry Institute, University of Campinas, Campinas, SP Brazil ‡

S Supporting Information *

ABSTRACT: Excited state intramolecular proton transfer (ESIPT) in a novel salicylidene sal-3,4-benzophen chromophore is studied by white-light femtosecond pump−probe and time-resolved fluorescence techniques, as well as by theoretical calculations under the time dependent density functional theory framework. We show that when the sal-3,4-benzophen chromophore (in enol form) is excited (at 390 nm) to the cisenol* form, it is quickly converted to the hot cis-keto* (hK*) form due to the fast ESIPT process (τESIPT = 150 fs). Subsequently, cooling from the hot cis-keto* (hK*) to cold cisketo* (cK*) state takes place with a characteristic time constant of ∼600 fs, with following relaxation decay time of 37 ps related with the photodynamic from sal-3,4-benzophen due to the occurrence of the cis → trans photoisomerization and intersystem crossing mechanisms. Finally, we observed the fluorescence emission from the cis-keto* at 575 nm. The dynamics of the optical process was modeled using rate equations with the proper energy level diagram and supported by theoretical calculations.

I. INTRODUCTION Excited state intramolecular proton transfer (ESIPT) is the initial event of numerous photophysical processes found in nature, and is crucial in chemistry.1−5 Furthermore, due to the transient character of its ground state, ESIPT has been used in several applications, from the white-light generation to molecular logical gates.3,6 The basic photophysical principle of the ESIPT involves a hydrogen transfer from the nitrogen to the oxygen atoms of the iminic group, leading to a balance between the enol (E*) and keto (K*) tautomers, stimulating a dual fluorescence emission and large Stokes shifts.7−9 Upon photoexcitation, electrons are promoted to the singlet excited state of the enol form. Subsequently, the ultrafast ESIPT process occurs and the keto form at the singlet excited state is produced, which is stabilized by the intramolecular hydrogen bond.7−9 Moreover, due to their molecular structure, the K* species emits at a lower energy than the E* tautomer. Given the ultrafast nature of the ESIPT process (from 50 to 400 fs),10−15 the fluorescence observed for the ESIPT chromophores is preferentially due to the K* tautomer.16−18 Hence, two emission bands are observed and a broader range of the steady-state fluorescence can be covered, making these molecules suitable for white emitting OLEDs,19,20 material chemistry,21 optical chemosensors22 and photonics,23−25 among other applications.26−29 Salicylidenes molecules, among several others, can undergo ESIPT.30,31 For example, we recently studied the possibility of © XXXX American Chemical Society

the ESIPT processes for (N,N′-bis(salicylidene)-1,2-phenylenediamino, N,N′-bis(salicylidene)-4,5-diaminopyrimidine, and salicylidene-5-chloroaminopyridine (salophen, sal-4,5-pym, sal5Cl-py, respectively)26,31 and observed that it does not occur for the symmetric salophen ligand. Calculations performed under the time dependent density functional theory (TD-DFT) framework indicated that the distance between the nitrogen and the proton controls the ESIPT process.26 The radiative decay for the K* species can occur through three different deactivation channels: (i) intersystem crossing (ISC), which leads to the triplet excited state of the cis-K* tautomer; (ii) photoisomerization producing the trans-K* form; and (iii) tautomerization generating the keto form, which is a slower process. Such processes can be mapped by time-resolved transient absorption spectroscopy.32 In this context, here we report a complete study on the excited state dynamics of the ESIPT for the ligand N,N′-bis(salicylidene)-3,4-benzophenone (sal-3,4-benzophen) through time-resolved techniques and theoretical calculations by using PBE0/cc-pVTZ, which allowed explaining pathways for the deactivation processes. Received: June 23, 2016 Revised: December 19, 2016 Published: December 19, 2016 A

DOI: 10.1021/acs.jpcc.6b06366 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

II. EXPERIMENTAL PROCEDURES II.A. Materials. 3,4-Diaminobenzophenone and salicylaldehyde were purchased from Sigma-Aldrich, São Paulo, Brazil. Dimethyl sulfoxide (DMSO) solvent in HPLC/Spectrum grades was purchased from Tédia, Brazil. The salicylidene derivative was synthesized and characterized according to the procedure described in ref 30. II.B. Synthesis of Sal-3,4-benzophen. The salicylidene derivative shown in Figure 1 was obtained by dissolving the 3,4-

elemental analysis (%): C, 77.13; H, 4.79; N, 6.66; O, 11.42. MS (ESI+): m/z 421.20 g mol−1. Mp: 111 °C. 1H NMR (300 MHz, DMSO-d6), δ (ppm): 6.95 (m, 2H), 7.41 (m, 4H), 7.57 (t, J = 9.0 Hz, 2H), 7.69 (m, 6H), 7.79 (d, J = 6.6 Hz, 2H) 8.96 (s, 1H), and 8.97 (s, 1H).13C NMR (75 MHz, DMSO-d6), δ (ppm): 117.37, 117.46, 119.86, 119.90, 120.13, 120.21, 120.70, 121.40, 129.37, 129.99, 130.39, 133.09, 133.22, 133.54, 134.42, 134.72, 136.60, 137.60, 142.93, 146.76, 161.05, 161.05, 165,54, 165.65, and 195.44. The principal infrared bands at ATR mode are (cm−1) νCN = 1610, νO−H = 3337 and 3460, νCO = 1648, νC−O = 1277 and 1270, νAr(C−H) = 3053, and νAr = 747 and 706. II.C. Methods. The crystal structure of sal-3,4-ben was determined by single crystal X-ray diffraction (X-ray diffractometer Bruker Apex Duo with a monochromatic source using Mo (Kα = 0.71073 Å) radiation) with the following computer programs: APEX233 for data collection, SAINT33 for cell refinement and data reduction, SHELXS-9734 for structure solution, SHELXL-2014/735 for structure refinement, and Mercury36 for molecular graphics. Melting temperatures were determined using the Fisatom 430 melting point equipment. 13 C and 1H NMR 1D spectra were measured using a DMSOd6 solution (33 mg mL−1) in a Varian spectrometer Mercury 300 MHz. The FTIR spectra were recorded in an Agilent Technologies Cary 630 FTIR spectrometer at attenuated total reflectance (ATR) mode. Elemental analysis of the C, H, and N atoms was performed using a PerkinElmer microanalyzer model PE 2400.

Figure 1. Schematic synthesis reaction for the sal-3,4-benzophen.

diaminobenzophenone (1 mmol; 212 mg) in ethylene glycol (15 mL) by sonication. After 5 min, the salicylaldehyde (244 mg; 211 μL; 2 mmol) was slowly dropped into this solution, and the mixture was then sonicated for 1 h. The precipitate was filtered, recrystallized in hot ethanol, it kept in refrigerator (10 °C) for 2 days to furnish plate-like yellow single crystals and washed with cold ethanol. The reaction yield was 71%. The sal3,4-ben was an yellow crystalline solid with the following

Figure 2. (a) Molecular structure of the sal-3,4-benzophen using 50% probability displacement ellipsoids. (b) Molecular structures obtained by X-ray diffraction of the cis-sal-3,4-benzophen, and optimized at ground (S0) and first singlet excited state (S1) at the PBE0/cc-pVTZ level. Dotted lines indicate hydrogen bonding. B


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involving the fragments that contain the imine groups are very close (C1−N1−C14−C15 = −179.49(18)° and C2−N2− C21−C22 = −179.84(18)°), while those for C−C−C−O are slightly different from each other (C14−C15−C20−O2 = 0.3(3)° and C21−C22−C23−O3 = 1.5(3)°). Both C14N1 and C21N2 of the imine groups assume an (E) configuration that favors the O2H2A···N1 and O3H3A···N2 intramolecular hydrogen bonds (Table 1). These intramolecular interactions

The electronic absorption spectrum was measured using a Hewlett-Packard 8452A diode array spectrophotometer. Steady-state fluorescence spectra were obtained using a PerkinElmer LS55 spectrofluorometer (λexc = 375 nm). Solutions with concentrations of 10 μmol L−1 in an 1 cm quartz cuvette were used. Fluorescence decays were recorded using time-correlated single photon counting and an Edinburgh Analytical Instruments FL 900 spectrofluorimeter with an MCP-PMT detector (Hamamatsu R3809U-50). The excitation wavelength for sal3,4-benzophen DMSO solution (10 μmol L−1) was λexc = 375 nm (Edinburg model EPL-375, with a 10 nm bandwidth, 77.0 ps). The decay signals for these samples were collected at λem = 430 and 575 nm for E* and K* species, respectively. The instrument response was recorded using Ludox samples. At least 10 000 counts in the peak channel were accumulated for lifetime measurements. The emission decays were analyzed using exponential functions.26,31 The time-resolved emission spectra (TRES) was recorded using the same TCSPC setup and laser source. The intervals between emission wavelengths were 5 nm. Delays between 0 to 2.880 ns were used to generate the TRES plots. Femtosecond time-resolved ESA spectra were recorded using 150 fs pulses (775 nm) from a Ti:sapphire chirped pulse amplified system operating at 1 kHz repetition rate. This beam was divided in two using a beam splitter (90% - 10%). The stronger beam was doubled (387.5 nm) using a BBO crystal and employed as the pump pulse. A small portion of the weaker beam was used to generate the white-light continuum (WLC) probe, using a sapphire window (2 mm of thickness). The time delay between pump and probe pulses was carefully varied by a computer controlled translation stage, providing a maximum resolution of 37.5 fs. The intensity of the relative spectral components of WLC probe pulse, at each specific time, that characterize the time-resolved transient absorption (ΔA), was monitored by means of a spectrometer. The chirp of WLC probe pulse was measured to be 0 and is associated with the transient absorption of hot cis-keto* state. Another ESA band also is observed between 550 and 680 nm, with ΔA < 0 (saturable absorption). This negative part of the ESA spectrum (for wavelength greater than 550 nm) is due to the stimulated emission of cis-keto* state.16 As we compare this

conformation. Selected structural parameters are given in Table 2, whose labels are shown in Figure 2a. Neither ground state or Table 2. Selected Bond Lengths (Å) and Dihedral Angles (deg) for the Ground and First Singlet State Structures of the Ligand Calculated at the PBE0/cc-pVTZ Level C2−N2 N2−C21 C21−C22 C22−C27 C27−C26 C26−C25 C25−C24 C24−C23 C23−O3 O3−H O2−N1−N2−O3

S0

S1

1.394 1.284 1.438 1.400 1.375 1.395 1.380 1.396 1.328 0.993 41.7

1.439 1.276 1.440 1.374 1.404 1.395 1.372 1.446 1.238 1.895 29.0

crystal structures are planar, however, as previously observed for this type of ligand, the planarity of the ligand framework increases upon excitation,26,31 favoring the appearing of the absorption bands in the 400 nm region, observed in the absorption spectrum. In addition, as shown in ref 30 for relative ligands salophen and sal-4,5-pym, the ground state N−H bond distance of the sal-3,4-benzophen is short enough (∼0.7 Å) to allow proton transfer upon excitation. The possibility of the ESIPT was evaluated through a structure relaxed scan, performed by optimizing the structure corresponding to the ground and first excited singlet states (S0 and S1), varying the O−H distance, as shown in Figure 4. As it

Figure 4. Relative energy profiles along the O−H coordinate for both the electronic ground state and the lower-lying excited state of the cissal-3,4-benzophen obtained at the PBE0/cc-pVTZ level.

can be seen, in the ground state the enol conformation is more stable than keto by ∼5 kcal mol−1. The opposite trend is observed for S1, suggesting the occurrence of the proton transfer from the phenolic oxygen to the iminic nitrogen upon excitation. The small energy barrier observed between enol* and keto* species in the S1 state (∼10 kcal mol−1) suggests that the single proton transfer process can occur in this state. This data suggests that the cis-keto tautomer is significantly more stable than cis-enol in the first active singlet state, in agreement with the large Stokes shift and dual fluorescence decay profile observed experimentally, supporting ESIPT process for the sal3,4-benzophen. In addition, calculated emission energy for the D


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Figure 5. Time-resolved fluorescence for the sal-3,4-benzophen dissolved in DMSO (10 mmol L−1). The excitation wavelength used was 375 nm. The fluorescence signal was collected at (a) 430 nm and (b) 575 nm.

hundreds of femtoseconds to picoseconds, and, then, there is the fluorescence relaxation time from the cis-keto* tautomer to the ground cis-keto state (τcK*→K) with a maximum emission at 575 nm (measured by time-resolved fluorescence spectroscopysee Figure 5τcK*→K = 2.17 ns). This simplified kinetic model can be described through the following rate equations: dnE(t ) 1 = −nE(t )WE → E + nE (t ) * dt (τE → E) * *

dnE (t ) n (t ) 1 * = +nE(t )WE → E − E * − nE (t ) * τESIPT dt (τE → E) * *

(1)

(2)

dnhK (t ) n (t ) n (t ) * = + E* − hK * − nhK (t )WhK → K * * ** τESIPT τhK → c K dt * *

Figure 6. Normalized time-resolved emission spectra (TRES) of sal3,4-benzophen in DMSO solution: λexc = 375 nm, delay: 0 to 2.880 ns (step between the spectra was of 320 ps). Concentration: 10 μmol L−1.

(3)

part of the ESA spectrum with the cis-keto* emission spectrum (Figure 3), we observed that the shape and the maximum are quite different. Such aspect can be explained by the strong attenuation in WLC intensity for wavelengths longer than 620 nm, caused by the broadband filter used to remove the pump pulse (775 nm) from the WLC. Adding to the fact that the stimulated emission signal is low, we obtained a poor signal/ noise rate for the stimulated emission (>620 nm). However, this drawback does not affect the interpretation of the ESIPT dynamics.50 On the basis of these results, we proposed a kinetic model to the ESA dynamics of sal-3,4-benzophen, illustrated by the energy-diagram presented in Figure S6 of the Supporting Information. In such model, electrons are promoted from ground-state (cis-enol form, S0) to the first excited state (cisenol*, S1) by the pump pulse (390 nm). After that, the cis-enol* form is quickly converted to the hot cis-keto* (hK*) form due to the fast ESIPT process (τESIPT), which is stabilized by the intramolecular hydrogen bond. Subsequently, there is a cooling process from the hot cis-keto* (hK*) to cold cis-keto* (cK*) state (τhK*→cK*), which has a characteristic time constant from

dnK (t ) ** = +nhK (t )WhK → K * * ** dt

(4)

dnc K (t ) n (t ) n (t ) nc K (t ) * * = + hK * − cK* − τhK → c K τc K → K dt (τc K → phD) * * * *

(5)

dn phD(t ) dt

=+

nc K (t ) * (τc K → phD) *

(6)

n (t ) dnK (t ) = + cK* τc K → K dt (7) * in which, n(t) describe the population in each state. The transition rate is given by WE→E* = σE→E*Ipump/(hνpump) and WhK*→K** = σK*→K**Iprobe/(hνprobe). Here, σE→E* corresponds to the one-photon absorption cross-section obtained through the linear absorption data (σE→E* = 4.8 × 10−17 cm2 at 390 nm) and σK*→K** = 7.0 × 10−17 cm2 is the cross-section between the first and the nth excited state in cis-keto form, which is associated with the amplitude of pump−probe signal. Ipump and Iprobe are the irradiance for the pump and probe pulses, h is the Planck constant, and ν is the photon frequency. τE*→E = 0.89 ns is the E


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Figure 7. (a) ESA colormap representing the time- and wavelength-resolved dynamics of transient absorption spectrum. (b) ESA spectra for different times (0 ps, 2.25 ps, 8.25 and 50 ps). (c) Decay curves for probe pulse tuning at 468 nm.

fluorescence decay lifetime from the cis-enol* form (emission at 430 nm, see Figure 3). In this approach, ΔA(t) = N0 log(e) L{nS0(t)σE→E* + [nS1(t) + nShK*(t) + nScK*(t)]σK*→K**} describes the transient absorption change, in which N 0 is the concentration and L is the optical path length. The solid line in Figure 7c corresponds to the fit obtained using the kinetic model described by eqs 1 to 7. The faster components of 150 and 600 fs were assigned to ESIPT (τESIPT) and cooling (τhK*→cK*) times, respectively. It is worth mentioning that the ESIPT time of about 150 fs is related with the DMSO solvent used in our measurements. DMSO is a polar aprotic solvent with high viscosity and, therefore, it is expected longer times for the excited state dynamics for the chromophore in solution. The slow decay, 37 ± 3 ps, is assigned to photodynamic from sal-3,4-benzophen ((τcK*→phD)). It is known that the benzophenone group, present in sal-3,4-benzophen, have a very efficient intersystem crossing with lifetime S1 → T1 of about 10−20 ps (depending of the solvent) at 530 nm (peak of the ESA spectrum).51 The hot cis-keto* state for the sal-3,4-benzophen is located at 475 nm in the ESA spectrum, favoring the ISC process. Therefore, we associated the decay time of 37 ps with a nonemissive state (phD) related with the photodynamic from sal-3,4-benzophen due to the occurrence of the cis → trans photoisomerization (iso) and ISC mechanisms, after the cooling process 1 ((τc K → phD) = k + k in the rate equation model). We * iso ISC discarded the reverse photodynamic process because their time is too long. The longer decay time, on the scale of nanoseconds, is related with the lifetime of the keto* tautomer.

IV. FINAL REMARKS In summary, the entire excited state dynamics of a novel salicylidene ligand sal-3,4-benzophen were evaluated. Upon excitation of the sal-3,4-benzophen chromophore (in the enol form) to the cis-enol* form (pump at 390 nm), a fast conversion to the hot cis-keto* (hK*) form occurs due to the ESIPT process (τESIPT = 150 fs), which is stabilized by the intramolecular hydrogen bond, as can be seen in the optimized structures for the first excited state obtained at PBE0/cc-pVTZ level. The occurrence of ESIPT leads to an enol* ⇋ keto* balance in the excited state, hence a broader emission range spectrum is obtained, allowing its applications in photonics and organic electronics devices such as white OLEDs.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b06366. More computational details and experimental procedures, 1 H NMR, 13 C NMR, and FTIR spectra, determination of molar absorptivity coefficient, and Xray single-crystal structure for sal-3,4-benzophen (PDF)

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AUTHOR INFORMATION

Corresponding Author

*(M.G.V.) E-mail: [email protected]. ORCID

M. G. Vivas: 0000-0003-4777-1323 Notes

The authors declare no competing financial interest. F


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ACKNOWLEDGMENTS The authors acknowledge Fundaçaõ de Amparo à Pesquisa do Estado de São Paulo (FAPESP-Grants 2009/51602-5, 2011/ 12399-0 and 2013/16245-2), Fundaçaõ de Amparo à Pesquisa do Estado de Minas Gerais (FAPEMIG), the Conselho ́ Nacional de Desenvolvimento Cientifico e Tecnológico (CNPq 470529/2012-1), Coordenaçaõ de Aperfeiçoamento ́ Superior (CAPES), the Air Force Office of de Pessoal de Nivel Scientific Research (FA9550-12-1-0028), the National Institute of Organic Electronics (INEO) (MCT/CNPq/FAPESP), and UNICAMP/FAEPEX for financial support and fellowships and GRID/UNESP, LCCA/USP and CENAPAD/SP for providing computational time.

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