Article pubs.acs.org/JPCA
Coherent and Homogeneous Intramolecular Charge-Transfer Dynamics of 1-tert-Butyl-6-cyano-1,2,3,4-tetrahydroquinoline (NTC6), a Rigid Analogue of DMABN Myeongkee Park, Donghong Im, Young Ho Rhee,* and Taiha Joo* Department of Chemistry, Pohang University of Science and Technology (POSTECH), Pohang 790-784, Korea S Supporting Information *
ABSTRACT: We report the intramolecular charge-transfer (ICT) dynamics of 1-tert-butyl-6cyano-1,2,3,4-tetrahydroquinoline (NTC6), a planar analogue of 4-(dimethylamino)benzonitrile (DMABN), by using time-resolved fluorescence (TRF) and TRF spectra (TRFS). TRFS allow accurate determination of the ICT dynamics free from the spectral relaxation caused by the solvation and vibronic relaxation. For NTC6 in tetrahydrofuran (THF), the locally excited (LE) state is populated exclusively presumably via a conical intersection from the initial photoexcited S2 (La) state, and the LE state undergoes ICT single exponentially with a time constant of 1.8 ± 0.2 ps. In acetonitrile, however, both LE (22%) and ICT (78%) states are populated from the S2 state, and the population in the LE state undergoes ICT in 800 ± 100 fs. The ICT state undergoes further relaxation in 1.2 ps along the solvation and the intramolecular nuclear coordinates involving the rotation of the amino group to form a twisted ICT state. Coherent nuclear wave packet motions of 130 cm−1, which can be assigned to the −CN group bending mode, were observed in the TRF of the reactant (LE) and product (ICT) states, indicating that the ICT reaction is partially coherent. Compared with DMABN, the ICT dynamics of NTC6 are quite homogeneous, and we speculated on the narrow conformational distribution of NTC6 in the ground state along the rotation of the amino group due to its rigid structure.
1. INTRODUCTION Since Lippert et al. reported the dual fluorescence from the locally excited (LE) and intramolecular charge-transfer (ICT) states of 4-(dimethylamino)benzonitrile (DMABN) in polar solvents,1 extensive theoretical and experimental studies have been performed to unravel their origin.2−10 Although DMABN has a deceptively simple molecular structure, there have been controversies over the structure of the ICT state, especially on the twist angle of the dimethylamino group leading to the twisted ICT (TICT)11−13 and planar ICT (PICT) models,14,15 and the ICT reaction dynamics. One piece of strong evidence for the PICT model comes from the ICT dynamics of 1-tertbutyl-6-cyano-1,2,3,4-tetrahydroquinoline (NTC6), a planar analogue of DMABN (Scheme 1) where the twisting of the dimethylamino group is prohibited.16,17 Druzhinin et al. reported that NTC6 shows the dual fluorescence, and its
ICT reaction time is about 1 ps determined by picosecond time-resolved fluorescence (TRF) and pump/probe transient absorption (TA).16 An ab initio study by Gómez et al. partially supports the PICT concept by introducing ultrafast internal conversion via conical intersection (CI) to the LE and TICT states.18 Interestingly, however, Hättig et al. showed through quantum mechanical calculations that NTC6 is somewhat flexible and that the electron-donor moiety is twisted to some extent for the ICT state.19 Recent time-resolved spectroscopies including femtosecond TRF and TA revealed that the TICT and PICT models involve several ICT states, and they are not strictly limited to either TICT or PICT.20,21 Rappoport and Furche argued that the existing ICT models are oversimplified and unable to describe the entire ICT dynamics of DMABN.22 Lim and co-workers introduced the πσ*-mediated ICT state and dark TICT state.23,24 Our recent study employing TRF spectra (TRFS) indicated that the ICT dynamics of DMABN is quite involved, consisting of the partially twisted and TICT conformations, which cannot be accounted for by the current ICT models.25 Herein, we report the TRF and TRFS of NTC6 in THF and acetonitrile solutions, and the ICT dynamics are discussed in comparison with the dispersive (nonexponential) ICT dynamics of flexible DMABN. The rigid structure of NTC6
Scheme 1. Molecular Structures of DMABN and NTC6
Received: November 15, 2013 Revised: June 27, 2014 Published: June 27, 2014 © 2014 American Chemical Society
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dx.doi.org/10.1021/jp411227r | J. Phys. Chem. A 2014, 118, 5125−5134
The Journal of Physical Chemistry A
Article
makes the ICT dynamics more homogeneous than those of DMABN. Here, we use the word homogeneous in a sense that molecules in the ground state adopt only one conformation so that all of them exhibit the same dynamics. We have also resolved coherent nuclear wave packet motions of NTC6 in solution for the first time, which gives valuable insight for the ICT dynamics of NTC6.
2. EXPERIMENTAL SECTION DMABN, acetonitrile, and THF were purchased from SigmaAldrich Co. Ltd. with the highest available purity and used without further purification. NTC6 was synthesized according to the method reported by Zachariasse et al.17 NTC6 was purified by several recrystallization steps and checked for impurity by NMR (Figure S1, Supporting Information). The TRF and TRFS apparatus utilizing fluorescence upconversion in a noncollinear sum frequency generation (SFG) scheme has been described in detail elsewhere.25−27 The femtosecond light source was based on a home-built cavitydumped Ti:sapphire oscillator. Generation of the excitation pulses shorter than 50 fs in the UV region has also been described in our previous report.25 The schematic of the whole setup is given in Figure S2 (Supporting Information). The time resolution of the fluorescence upconversion was estimated by using the difference frequency generation (DFG) between the scattered pump pulses at 280 nm and the gate pulses at 840 nm, and they are 70 and 100 fs for the TRF and TRFS measurements, respectively. The response of the TRF apparatus was calibrated by comparing the TRFS of 2-(4biphenylyl)-5-(4-t-butylphenyl)-1,3,4-oxadiazol (PBD) dye in toluene at 500 ps with its stationary fluorescence spectrum because the spectral relaxation by solvation process is supposed to be complete at 500 ps. We used a peristaltic pump to flow the samples for the fluorescence upconversion experiments to avoid photodamage. We also measured the absorption spectra after the fluorescence upconversion measurements to confirm that photodegradation was negligible. All measurements were performed at ambient temperature. Geometry optimization and vibrational normal-mode calculation for the LE state of NTC6 were carried out by the timedependent density functional theory (TDDFT) method at the B3LYP/6-3111+G(d,p) level using the Gaussian 09 quantum chemistry package.28 Vibrational frequencies were scaled by a scaling factor of 0.9613.29
Figure 1. Stationary absorption and emission spectra of (a) NTC6 and (b) DMABN in THF (solid line) and acetonitrile (dashed line).
summarized in Tables S2 and S3 (Supporting Information). It should be noted that TRF measured at a single wavelength reflects both the population dynamics and the spectral relaxation that arises from the solvation and vibronic relaxation. Because the ICT reaction involves a large change in dipole moment, large-amplitude solvation dynamics is expected. Therefore, accurate ICT dynamics can be obtained only from the full TRFS measurements. Inspection of the TRF signals shows that the ICT dynamics of NTC6 are only slightly sensitive to the solvent polarity, whereas the ICT dynamics of DMABN change drastically depending on the solvent polarity. We have suggested in our previous report that the flexible structure of DMABN results in the ICT(T) state, where T stands for the twisted, through the solvation and intramolecular relaxation involving primarily the twisting of the dimethylamino group.25 Here, we denote the relaxed state as ICT(T) to differentiate it from the conventional TICT state where the dimethylamino group is twisted nearly 90°. Cooling to the TICT state was invoked in a femtosecond stimulated Raman study by Rhinehart et al.30 For NTC6, the nearly insensitive time constants to the solvent polarity suggest that the relaxation is mostly intramolecular. That is, rigid NTC6 cannot undergo the large twisting motions as in flexible DMABN even in polar solvents such as acetonitrile. The rigid molecular structure of NTC6 results in another prominent distinction between the TRFs of NTC6 and DMABN. In our previous report for DMABN, an instrumentlimited (
