"Symmetry Breaking"-Assisted Distinct Molecular Packing in Self-Assembled Oligothiophenes Leading to Emissive Trap States and Enhanced Photocatalytic Solar H2 Production.

Two different types of self-assemblies have been fabricated from 2,2'-bithiophene-5,5'-dicarboxaldehyde (BTDA) and 2,2':5',2''-terthiophene-5,5''-dicarboxaldehyde (TTDA) molecules through reprecipitation methods. Unlike the BTDA molecule, one thiophene unit remains out of plane with respect to the other two thiophene units in TTDA. This results in partial face-to-face intermolecular interactions during self-assembly, causing the formation of a defect-mediated emissive trap state at longer wavelengths. These trap states are charge-transfer types with a longer lifetime compared to the pure TTDA molecule. Detailed computational studies and femtosecond transient absorption spectroscopy support the findings. The appearance of the charge transfer type trap state in TTDA self-assembly facilitates the photoinduced charge separation and free carrier accumulation, which helps to boost the photocatalytic solar H2 production. However, highly defined head-to-tail molecular arrangements in BTDA self-assembly hinder the formation of such an emissive trap state, resulting in a decrease in the photocatalytic efficiency.