Journal of Physical Chemistry B, Vol.117, No.49, 15684-15694, 2013
Excited State Charge Redistribution and Dynamics in the Donor-pi-Acceptor Flavin Derivative ABFL
Chromophores containing a donor-pi-acceptor (D-pi-A) motif have been shown to exhibit many interesting photophysical properties. The lowest electronic transition of a flavin derivative containing this motif, azobenzylflavin (ABFL), has previously been shown to be highly sensitive to solvent environment and hydrogen bonding ligands. To better understand this sensitivity, we have investigated the excited state charge redistribution and dynamics of ABFL in a low-dielectric, non-hydrogen bonding solvent by steady-state Stark and femtosecond optical transient absorption spectroscopies. The Stark measurements reveal the difference dipole moment, Delta(mu) over right arrow (01), between the ground and first excited states to be 22.3 +/- 0.9 D. The direction of Delta(mu) over right arrow (01) in the molecular frame was assigned with the aid of TD-DFT and finite field calculations, verifying the hypothesis that electron density moves from the diethylaniline donor to the flavin acceptor in the excited state. The magnitude of the difference dipole moment was used to estimate the hyperpolarizability of ABFL, beta(0) = 720 x 10(-30) esu. Subsequent excited state decay via charge recombination was shown to take place in a few picoseconds. The data was best fit to a kinetic model composed of a sub-picosecond internal conversion step from S-2 -> S-1, followed by a 5 ps decay to the ground state. A competing process involving formation of an additional long-lived state from S-1 was also observed. Cyclic voltammetry shows one oxidation and two reduction waves and is completely reversible. This analysis lays the groundwork for developing new flavin dyads with the desired excited electronic state properties for applications such as nonlinear optical devices, molecular electronics applications, or dye-sensitized solar cells.