Role of Structure in Ultrafast Charge Separation and Recombination in Naphthalene Diimide End-Capped Thiophene Oligomers

The photophysics of a series of thiophene oligomers (T-n) with and without naphthalene diimide (NDI) acceptor end groups were investigated using femtosecond transient absorption spectroscopy. Photoexcited thiophene oligomers (n = 4, 6, 8, 10, and 12) exhibit complex length-dependent excited-state dynamics on the picosecond time scale due to rapid structural relaxation and intersystem crossing. The incorporation of NDI end groups leads to ultrafast charge separation after selective excitation of the thiophene donor. Initial location of photoexcitation dictates the time scale of charge separation and, therefore, recombination. Photoexcitations near the NDI acceptor result in fast charge separation in all T-n-NDI2 oligomers, whereas excitations near the center of the oligomer must undergo a length-dependent long-range electron-transfer or energy-transfer/electron-transfer step to create a charge-separated state. Oligomer structure plays a role in the charge separation and recombination processes, where T-4-NDI2 exhibits small-molecule behavior and longer oligomers approach polymeric behavior.