Article pubs.acs.org/JPCB
Switching to a Reversible Proton Motion in a Charge-Transferred Dye Mario Gutierrez,†,§ Noemí Alarcos,†,§ Marta Liras,‡ Félix Sánchez,‡ and Abderrazzak Douhal*,† †
Departamento de Química Física, Facultad de Ciencias Ambientales y Bioquímica, and INAMOL, Universidad de Castilla-La Mancha, Avenida Carlos III, S.N., 45071 Toledo, Spain ‡ Instituto de Química Orgánica General, IQOG-CSIC, Juan de la Cierva, 3, 28006 Madrid, Spain S Supporting Information *
ABSTRACT: We report on the steady-state, pico- and femtosecond time-resolved emission studies of 6-amino-2-(2-methoxyphenyl)benzoxazole (6A-MBO) and 6-amino-2-(2-hydroxyphenyl)benzoxazole (6A-HBO) in different solvents. We observed an intramolecular charge transfer (ICT) reaction following by slow (relatively) solvent relaxation, which happened in the same time domain for both molecules. The ultrafast ICT reaction happens in 80−140 fs whereas the solvent relaxation occurs in 0.5−1.1 ps. In 6A-MBO the excited CT species has a lifetime of ∼2.5 ns. However, in 6A-HBO and after the ICT reaction, a reversible excited-state intramolecular proton transfer (ESIPT) reaction takes place in the formed enol charge transfer (ECT*) species producing a keto (K*) type tautomer. Depending on the solvent, the forward ESIPT reaction (ECT* → K*) happens in 40−175 ps while that of the reverse one (ECT* ← K*) occurs in 240−990 ps. Kinetic isotopic effect (OH/OD exchange) study in acetone shows that the reversible ESIPT reaction occurs via tunneling, while we suggest that in acetonitrile solution it evolves along the IHB and solvent coordinates. Our results show a reversible proton motion coupled to charge-transfer reactions opening the way to new explorations of charge- and proton-transfer dynamics and spectroscopy. example is 4′-dimethylaminoflavonol7,8 (DMAF) in which the slow proton motion is reversible in chloroform, dichloromethane (DCM), and acetonitrile (ACN), while it is irreversible in cyclohexane, dioxane, diethyl ether, and tetrahydrofuran (THF).7 The proton motion takes place in ∼10 ps when the reaction is irreversible, whereas it becomes much slower when the process is reversible (from ∼50 to ∼80 ps). 7 A similar behavior has been found in 4′-N,Ndiethylamino-3-hydroxyflavone.22 In the same way, 2-(2′hydroxy-4′-diethylaminophenyl)benzothiazole (HABT) exhibits an ultrafast ICT reaction (less than 150 fs), followed by a reversible proton motion in a few picoseconds.23 HABT has the amino group on the phenol (the proton-donating group) ring. However, the HABT parent molecule, HBT, does not exhibit a reversible ESIPT reaction, while it has been shown that it undergoes a proton-coupled electron transfer (PCET), from the hydroxyphenyl to the benzothiazole rings, and the formation of a double C−C bond linking the two rings.17,24 Thus, from the point of view of the electronic structure− dynamics relationship, it is of interest to elucidate the behavior of a classical (well-known) proton-transfer dye where the electronic change is on the proton acceptor moiety. Thus, we have recently reported on the stepwise ICT and ESIPT reactions in a new derivative of 2- (2′-hydroxyphenyl)benzoxazole (HBO).25 To examine the photodynamics of this dye in other solvents and the possibility of reversible protontransfer reaction and tunneling or not, we have performed
1. INTRODUCTION Proton- (or H atom) and charge-transfer reactions are key events in many natural and artificial systems.1−6 While great advances have been made in understanding the ultrafast dynamics of both processes when they happen separately, only a few ultrafast studies of coupled proton and charge transfers have been reported.7−13 The related systems are characterized by the presence of an electron/proton donor and acceptor groups in the molecular structure of the dye in which one can witness the occurrence of excited-state intramolecular proton transfer (ESIPT) and intramolecular charge transfer (ICT) reactions. Depending on the system under study, the ESIPT and ICT can occur separately or concomitantly (E*, CT*, and K* are the excited enol and intramolecular chargeand proton-transferred forms, respectively): (1) (E* → CT* → K*) when the rate of ICT is faster than that of ESIPT,8,14 (2) (E* → K* → CT*) when the ESIPT takes prior to ICT,15 and (3) (E* → K*/CT*) when the ESIPT and ICT events occur in similar times.10,11 It is very well-known that the ICT and ESIPT reactions are polarity solvent dependent. Reports on 3-hydroxyflavone and 2(2′-hydroxyphenyl)benzothizole (HBT) have shown the involvement of polar solvent molecules on the ESIPT reaction mechanism.16−18 Moreover, the ESIPT reaction can be kinetically controlled where the whole reaction is virtually irreversible or thermodynamically controlled whether or not the reverse motion is on the same time scale as the direct one.19,20 One factor that governs the irreversibility or reversibility of the ESIPT reactions is the solvent nature. This dependence has been demonstrated for the family of 4′-N,Ndialkylamino-3-hydroxyflavone.7,8,21,22 Thus, a representative © 2014 American Chemical Society
Received: November 24, 2014 Revised: December 18, 2014 Published: December 18, 2014 552
DOI: 10.1021/jp511345z J. Phys. Chem. B 2015, 119, 552−562
Article
The Journal of Physical Chemistry B Scheme 1. Possible Molecular Structures of 6A-MBO and 6A-HBO
The femtosecond (fs)-emission transients have been collected using the fluorescence up-conversion technique. The system consists of a femtosecond Ti:sapphire oscillator MaiTai HP (Spectra Physics) coupled to a second harmonic generation and up-conversion setups.28 The oscillator pulses (90 fs, 250 mW, 80 MHz) were centered at 700 nm and doubled in an optical setup through a 0.5 mm BBO crystal to generate a pumping beam at 350 nm (∼0.1 nJ/pulse). The polarization of the latter was set to the magic angle with respect to the fundamental beam. The sample has been placed in a 1 mm thick rotating cell. The fluorescence was focused with reflective optics into a 1 mm BBO crystal and gated with the fundamental femtosecond beam. The IRF of the full setup (measured as a Raman signal of pure solvent) was ∼220 fs. To analyze the decays, a multiexponential function convoluted with the IRF was used to fit the experimental transients. All experiments were performed at 293 K.
femtosecond−nanosecond as well as steady-state emission studies in different solvents. Here, we report on the steady-state and femtosecond− nanosecond behavior of 6-amino-2-(2-hydroxyphenyl)benzoxazole (6A-HBO) and 6-amino-2-(2-methoxyphenyl)benzoxazole (6A-MBO) emission in different solvents. We found the occurrence of an ICT reaction in 80−140 fs follow by the solvent dynamics in 0.5−1.1 ps. Moreover, 6A-HBO shows a reversible ESIPT reaction occurring in ∼40 to ∼180 ps for the direct proton motion in the enol charge-transferred (ECT) structures to give K tautomers, and in ∼240 to ∼990 ps for the reverse reaction. Furthermore, 6A-HBO in acetone solution shows, contrary to the behavior in ACN, a quantum tunneling effect. Our results give new insight into the behavior of reversible proton motion coupled to charge-transfer reactions and tunneling in the equilibrated structures.
2. EXPERIMENTAL SECTION The synthesis, purification and characterization of 6A-MBO and 6A-HBO are described in our previous report.25 The deuterated compound (6A-DBO) was obtained by heating 6AHBO in deuterated methanol and evaporating the solvent. The used solvents (anhydrous), dioxane (99.8%), acetonitrile (ACN, 99.8%), and THF (≤ 99.9%), were purchased from Sigma-Aldrich while acetone (99.8%) was from Scharlau. The deuterated solvents, acetonitrile-d3 (99.5%) and acetone-d6 (99.9%), were from Sigma-Aldrich. All of the solvents were used as received. The steady-state UV−visible absorption and fluorescence spectra have been recorded using JASCO V-670 and FluoroMax-4 (Jobin-Yvon) spectrophotometers, respectively. Fluorescence quantum yields were measured using quinine sulfate in a 0.1 N H2SO4 solution as a reference (φ = 0.51 at 293 K).26 Pico- to nanosecond-emission decays were measured using a time-correlated single-photon counting (TCSPC) system.27 The sample was excited by a 40 ps pulsed diode laser centered at 371 nm (
