Near-IR excitation transfer and electron transfer in a BF2-chelated dipyrromethane-azadipyrromethane dyad and triad

Abstract

A molecular dyad and triad, comprised of a known photosensitizer, BF(2)-chelated dipyrromethane (BDP), covalently linked to its structural analog and near-IR emitting sensitizer, BF(2)-chelated tetraarylazadipyrromethane (ADP), have been newly synthesized and the photoinduced energy and electron transfer were examined by femtosecond and nanosecond laser flash photolysis. The structural integrity of the newly synthesized compounds has been established by spectroscopic, electrochemical, and computational methods. The DFT calculations revealed a molecular-clip-type structure for the triad, in which the BDP and ADP entities are separated by about 14 Å with a dihedral angle between the fluorophores of around 70°. Differential pulse voltammetry studies have revealed the redox states, allowing estimation of the energies of the charge-separated states. Such calculations revealed a charge separation from the singlet excited BDP ((1)BDP*) to ADP (BDP(.+)-ADP(.-)) to be energetically favorable in nonpolar toluene and in polar benzonitrile. In addition, the excitation transfer from the singlet BDP to ADP is also envisioned due to good spectral overlap of the BDP emission and ADP absorption spectra. Femtosecond laser flash photolysis studies provided concrete evidence for the occurrence of energy transfer from (1)BDP* to ADP (in benzonitrile and toluene) and electron transfer from BDP to (1)ADP* (in benzonitrile, but not in toluene). The kinetic study of energy transfer was measured by monitoring the rise of the ADP emission and revealed fast energy transfer (ca. 10(11) s(-1)) in these molecular systems. The kinetics of electron transfer via (1)ADP*, measured by monitoring the decay of the singlet ADP at λ=820 nm, revealed a relatively fast charge-separation process from BDP to (1)ADP*. These findings suggest the potential of the examined ADP-BDP molecules to be efficient photosynthetic antenna and reaction center models.