Photoinduced processes in a dyad made of a linear and an angular perylene bisimide

A dyad (PI0-PIa) made of a linear (PI0) and an angular (PIa) perylene biscarboximide is synthesized and its spectroscopic, electrochemical and photophysical properties investigated in solvents of various polarity. PIa is characterized by a high intersystem crossing. The spectroscopy and electrochemistry data point to a modest electronic coupling. LUMO-LUMO electron transfer from the singlet excited state PI0-(1)PIa is thermodynamically feasible in polar solvents but its occurrence is precluded by a very fast energy transfer to yield (1)PI0-PIa, k(en) >= 10(11) s(-1). A HOMO-HOMO electron transfer in the latter state in polar solvents is precluded by the poor driving force, the reaction being unable to compete with the radiative deactivation of the excited state. The efficient energy transfer process is quantitatively examined in the frame of current theories and ascribed to a dipole-dipole (Forster) mechanism