Solvent-Switched Photoinduced Charge and Energy Transfer in pi-Conjugated Diblock Oligomers

Diblock -conjugated oligomers feature two fully conjugated segments with distinct frontier molecular orbital energy levels. These oligomers are models for -conjugated polymers that feature donor-acceptor characteristics, and their study provides insight regarding the primary events involving exciton transfer and charge transfer that occur upon photoexcitation. This study is focused on a series of diblock -conjugated oligomers comprising two distinct segments consisting of oligo(phenylene ethynylene) (PEn, n = 3 and 5) and tetrathiophene (T4). The diblock oligomers are end-capped with a strongly coupled naphthalimide (NI) electron acceptor, which introduces a fluorescent intramolecular charge transfer state. Three oligomers were synthesized and studied, T4PE3-NI, T4PE5-NI and PE5T4-NI, where T4, PEn and NI refer to the individual units and the sequence corresponds to the oligomers’ structures. We report a detailed photophysical investigation of these diblock oligomers, including the UV-visible absorption, fluorescence and femtosecond transient absorption (TA) spectroscopy. The remarkable finding is that in a non-polar solvent environment, selective excitation of the T4PEn-NI oligomers at a wavelength corresponding to the PEn segment, leads to rapid exciton migration to the T4 segment. By contrast, in a more polar solvent photoexcitation T4PEn-NI at the same wavelength gives rise to rapid formation of a PEn to NI charge transfer state, which then evolves into a fully charge separated state where the hole is on the T4 and the electron is on the NI unit. The dynamics of the charge separation and charge recombination events are slower for T4PE5-NI than T4PE3-NI, indicating that the length of the oligo(phenylene ethynylene) segment plays a role in the electronic coupling across the oligomer. The results provide new insight concerning the structure and ultrafast excited state dynamics in fully -conjugated electronic systems, and they also demonstrate the ability to control energy and charge transfer in a molecular wire by the medium polarity.