Photophysical characterization of a light-harvesting tetra naphthalene imide/perylene bisimide array

Photoinduced processes in a light-harvesting array PBI-NI(4), made of a central perylene bisimide (PBI) with four appended naphthalene imide (NI) units, have been studied in toluene by several spectroscopic techniques, and the steps of energy dissipation have been identified. The singlet excited states (1)PBI (lambda(max) = 960 nm, epsilon = 40 000 M(-1) cm(-1)) and (1)NI (lambda(max) = 810 nm, epsilon ca. 4 000 M(-1) cm(-1)) and the charge-separated state PBI--NI4+ (A a. = 775 run, E = 80 000 M(-1) cm(-1)) were identified as reaction intermediates. In the array PBI-NI(4), energy transfer from the peripheral (1)NI to the PBI core was shown to compete efficiently (90%) with electron transfer from the NI unit to the PBI unit. Subsequently, (1)PBI displays a slow electron abstraction (k = 4.6 x 10(8) s(-1)) from the NI unit to form the charge-separated state PBI(-)-NI(4)(+), which competes with emission of light energy from the PBI core. The CS state PBI(-)-NI(4)(+) decays faster (k = 2.6 x 10(9) s(-1)) than it is populated, preventing any back reaction to re-form the singlet excited state localized on PBI.