Photophysics of Benzoxazole and Dicyano Functionalised Diketopyrrolopyrrole Derivatives: Insights into Ultrafast Processes and the Triplet State

Diketopyrrolopyrrole (DPP) functionalised with electron donating unit act as donor-acceptor molecules that have shown potential for dye application and photovoltaics. These molecules offer broad absorption/emission properties and structure-dependent dynamics. We utilised femtosecond pump-probe spectroscopy, to study the photo-initiated dynamics of thiophene linked DPP derivatives. The thio-DPPs are further functionalised by different electrons withdrawing terminal groups: benzoxazole and thiophene dicyanide. The strongly emissive benzoxazole derivative (chloroform solution) directly relaxes to the ground singlet state. Thiophene dicyanide derivative exhibit distinct spectral evolution in the first 10 ps associated with structural and vibronic process. Later it crosses over to the triplet state with a large yield (20%). In the solid-state thin film, we observed signal that reminiscences singlet fission. However, upon careful analysis of temperature-dependent steady state absorbance spectra, we conclude that these features are due to laser-induced thermal artefacts. We describe a simplified excited state evolution in the thin film which is devoid of any additional excited state. These findings bear significant implications for the analysis of triplet formation, which plays a major role in the photophysics of many organic materials.