Photoinduced Intramolecular Charge Transfer in a Series of Differently Twisted Donor−Acceptor Biphenyls As Revealed by Fluorescence

Abstract

This photophysical study addresses the general question of how electron transfer in bichromophoric molecules influences the conformational relaxation, which can be toward either more or less π-conjugation. The effects of photoinduced intramolecular charge transfer on the electronic and molecular properties of a series of differently twisted 4-N,N-dimethylamino-4‘-cyanobiphenyls are investigated by steady-state and time-resolved fluorescence. The dipole moments, radiative rates, and torsional relaxations in the excited state are analyzed by comparison with the absorption spectra and interannular twist angle (φ)-dependent CNDO/S calculations. Independent of the twist angle φ and solvent polarity, the first excited singlet state of these donor−acceptor (D−A) biphenyls (I − III) is an emissive intramolecular 1CT state of the 1La-type transferring charge from the dimethylaminobenzene (D) to the cyanobenzene (A) subunit. Similar to the planar restricted D−A fluorene I, the flexible D−A biphenyl II shows only a weak dependence of the fluorescence radiative rate constants k f (0.4−0.6 ns-1) on the solvent polarity, consistent with a planarization in the excited state of II. In contrast, the strongly pretwisted biphenyl III behaves similarly to I and II only in nonpolar solvents (〈k f〉 = 0.3 ns-1, indicating partial excited-state relaxation toward planarity), whereas with increasing polarity the mean radiative rate 〈k f〉 decreases down to 0.03 ns-1. A fast equilibrium between a more planar and a more twisted rotamer distribution in the 1CT state of III explains the appearance for III of additional photophysical effects such as (a) strong decrease of the radiative rates with increasing polarity, (b) two long (>200 ps) fluorescence lifetimes with precursor−successor relation, and (c) excited-state quenching by protic solvents.

Two heterocyclic merocyanine classes and their optical properties in relation to the donor-acceptor strength of end substituents

“…Although stilbenoids like the one shown in Scheme 1 do not form TICT-states because valence bond resonance stabilization leads to a planar geometry, the closely related biphenyl derivatives do form TICT-states with very high excited state dipole moments, if sterically hindering substituents favor the orthogonal conformation [23,24].…”

The Geometry of the Excited Charge Transfer States: Flattening or Twisting?

“…Because for 4-N,N-dimethylamino-4 -cyanobiphenyl the significant dependence of the fluorescence on the solvent polarity was observed [23], it is reasonable to ascribe the X coordinate to the transformation of the N(CH 3 ) 2 group. For aniline in gas phase it was experimentally established that in ground state the NH 2 group has pyramidal structure, whereas in the first excited state this group is coplanar with the aromatic ring [33].…”

“…The excited state dipole moment (µ e ) was determined from the relation (2) and was found as 27 ± 4 D. The values of the ground state dipole moment (µ g ) and the Onsager cavity radius (a 0 ) were assumed to be equal to 5.65 D and 6Å, respectively, as for the cyano analogue of ADAB [23].…”

Recent progress on intramolecular charge-transfer compounds as photoelectric active materials

“…However, only a few researches studied the ICT state based on the other structural properties of molecular frameworks such as substitution position, coplanarity and flexibility [27,28]. In fact, the properties of ICT compounds are influenced by both the intrinsic nature of D/A components and the relative orientation of the components [29].…”