Photoinduced intramolecular charge-transfer reactions in 4-amino-3-methyl benzoic acid methyl ester: A fluorescence study in condensed-phase and jet-cooled molecular beams

Abstract

Photoinduced intramolecular charge-transfer reactions in 4-amino-3-methyl benzoic acid methyl ester (AMBME) have been investigated spectroscopically. AMBME, with its weak charge donor primary amino group, shows dual emission in polar solvents. Absorption and emission measurements in the condensed phase support the premise that the short wavelength emission band corresponds to local emission and the long wavelength emission band to the charge transfer emission. Laser-induced fluorescence excitation spectra show the presence of two low-energy conformers in jet-cooled molecular beams. Theoretical calculations using density functional theory help to determine structure, vibrational modes, potential energy surface, transition energy and oscillator strength for correlating experimental findings with theoretical results.Keywords. 4-Amino-3-methyl benzoic acid methyl ester; fluorescence; quantum yield; laser-induced fluorescence; density functional theory; supersonic jet.

Spectroscopic study of excited state intramolecular charge transfer in ethyl ester of N,N′-Dimethylaminonaphthyl-(acrylic)-acid

“…The TDDFT method of calculation using Gaussian 03W does not implement analytical gradients within TDDFT and hence it fails to optimize the geometries for the excited states. However, the results obtained using this method correlate well with the experimental data and are reported in recent publications [9][10][11][12][45][46][47][48][49]. The PECs in acetonitrile solvent have been constructed using TDDFT-PCM model.…”

“…However, this dependence of absorption band maxima on solvent polarity is not linear, since in case of water, the position of emission maxima is blue-shifted to $352 nm. This exception to the trend observed in case of water which is a hydrogen bonding solvent with the highest value of dielectric constant (e = 81, a = 1.17) may be due to the formation of hydrogen bonded solvated cluster giving rise to a new absorption band at $352 nm [8][9][10][11][12] (Scheme 2). Very similar observation was found for some donor-acceptor molecules having benzene chromophore [8][9][10][11][12].…”

“…There are different ways for calculating the ground and excited states PECs for donor-acceptor CT systems [23,[40][41][42][43][44][45][46][47][48][49]. In our case the potential energy curves for the ground and first two excited singlet states have been explored following the twisted intramolecular charge transfer model as proposed in the literature to explain the fluorescence behaviour of several donor-acceptor charge transfer systems [8][9][10][11][12]. During the evaluation of the potential energy curves, both in vacuo and in acetonitrile solvent, the geometry has been kept frozen to that optimized for the ground state for all the points and all the states investigated; the only parameter varied is the twisting angle h 1 for donor group and h 2 for acceptor group (Scheme 1).…”

“…Over the past years, the photophysical study of donoracceptor systems linked to a chromophore has drawn considerable interest for basic and applied research [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16]. The first observation of dual fluorescence of the model compound 4-N,N 0 -dimethylaminobenzonitrile (DMABN) by Lippert et al [1] was a major breakthrough in the realm of photoinduced donor-acceptor charge transfer process.…”

“…According to this model, charge transfer from donor to the acceptor moiety occurs in the completely twisted state at the donor site and this is best achieved when the donor group is perpendicular to the rest of the molecule [28]. In this configuration, complete decoupling of the groups lead to full charge transfer, a large dipole moment and hence great stability is achieved in solvents of higher polarity due to reorientation of the solvent dipoles around the fluorophore to attain an energetically favourable conformation [28,[9][10][11][12]. The lower wavelength, higher energy band (L b ) arises due to normal fluorescence from the excited molecule whereas, the longer wavelength, lower energy band (L a ) is described as the ''anomalous'' fluorescence band from the CT state [29].…”