10710
J. Phys. Chem. 1994,98, 10710-10719
Complexes of Methyl 4-(NjV-Dimethylamino)benzoate: Spectroscopy and Dynamics of the Charge Transfer State Robert A. Weersink and Stephen C. Wallace* Department of Chemistry, University of Toronto, 80 St. George Street, Toronto, Ontario, Canada M5S IAI Received: May 30, 1994; In Final Form: July 21, 1994@
The excited states of methyl 4-(N,N-dimethylamino)benzoate isolated in a free jet expansion and complexed with polar, aprotic solvents were studied using mass-resolved, two-color resonance-enhanced multiphoton ionization and picosecond fluorescence emission spectroscopy. In the isolated molecule, the excited electronic state observed is the 'Lb state rather than the energetically more stable 'L, state. The large influence of solvent-induced nonadiabatic interactions on the twisted intramolecular charge transfer process is manifested in the study of methyl 4-(N,N-dimethylamino)benzoatecomplexes with polar, aprotic solvents. Large red shifts are observed in REMPI spectra of the methyl 4-(N,N-dimethylamino)benzoatecomplexes, with the extent of the shift proportional to the dipole moment of the solvent. Besides normal Stokes-shifted emission in the fluorescence spectra of the complexes, a second emission band is observed at much lower energies than the excited energy. This band is assigned to the charge transfer state, which is observed because of solvent-induced nonadiabatic interactions.
1. Introduction
The large class of molecules that exhibit what has been called the twisted intramolecular charge transfer (TICT)'q2 phenomenon are interesting for two reasons: because they provide a means of studying the general theory of charge transfer processes and because they have proven to be useful as a probe of solvation dynamics. The fundamental spectroscopic characteristic of the TICT process is the observation of two distinct fluorescence bands: a normal Stokes-shifted band and a second band for which the fluorescence transitions are much lower in energy, resulting in a significantly red-shifted band. The normal Stokesshifted band is labeled the b-band, while the substantially redshifted band is labeled the a-band.' Previous work has implicated the TICT process in condensed phase studies of alkylaniline derivatives, such as 4-(N,Ndimethy1amino)benzonitrile (DMABN)3 and methyl 4-(N,Ndimethy1amino)benzoate (DMABM)4 (Figure 1). These molecules consist of two planar chromophores, the dialkylamino substituent and the para-substituted benzene ring, joined by a a-bond between the nitrogen atom of the amino group and a carbon atom of the benzene ring. Steric interactions favor a configuration in which the two groups are twisted relative to each other by 90". However, conjugation between the n orbitals of the benzene ring and the lone pair of electrons on the nitrogen tends to favor a coplanar configuration. In the ground electronic state, conjugation is the dominant factor and the molecule adopts a copolanar configuration. Upon excitation, a fractional charge separation occurs between the two chromophores with the dialkylaminogroup as the donor and the substituted benzene ring as the acceptor. In this excited state, conjugation also occurs between the donor and acceptor groups, and hence the system retains the flat conformer. Fluorescence originating from this state, referred to as the locally excited state, results in the b-band observed in the emission spectrum. The molecule can be further stabilized relative to this locally excited state by the acceptor and donor groups twisting about the C,l-N bond until they are perpendicular to each other. @
Abstract published in Advance ACS Abstracts, September 15, 1994.
0022-365419412098-107 10$04.50/0
N=c-@N'
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4-N,N-Dimethylaminobenzonitrile (DMABN)
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C ' -@N
