Photoinduced charge transfer in push-pull pyrazoline-based chromophores – Relationship between molecular structure and photophysical, photovoltaic properties

• Published in: Spectrochimica acta. Part A, Molecular and biomolecular spectroscopy Molecular and biomolecular spectroscopy, 2023-08, Vol.296, p.122643, Article 122643
• Main Authors: Uchacz, Tomasz, Maroń, Anna M., Szlachcic, Paweł, Danel, Andrzej, Pokladko-Kowar, Monika, Gondek, Ewa, Kolek, Przemysław, Zapotoczny, Szczepan, Stadnicka, Katarzyna M.
• Format: Article
• Language: English
• Subjects: DFT calculation; Photoinduced charge transfer; Photovoltaic cell; Pyrazoline; Ultrafast spectroscopy; Viscosity probe
• ISSN: 1386-1425
• DOI: 10.1016/j.saa.2023.122643

[Display omitted] •The relationship between the molecular structure and optical properties of pyrazoline-based push-pull dyes was discussed.•The photoinduced charge/electron transfer process over carbonyl bridge included in π-system was observed.•The thiophene-decorated pyrazoline presents good photovoltaic properties.•The phenyl end capped pyrazoline can be used as a viscosity sensor.•The deactivation pathway is mostly affected by conformational relaxation and back electron transfer process. The manuscript describes the effect of molecular structure on the photophysical and photovoltaic properties of the pyrazoline-based donor-branched-π-system-acceptor compounds decorated with two end groups: phenyl or thiophene. Although the absorption to the first singlet excited state is strongly allowed, the emission quantum yield is low in all studied solvents. This behaviour was explained by the existence of two non-radiative deactivation channels: the back electron transfer process, especially operated in polar solvents, and internal conversion realized as the rotation of flexible rotors (cyano, keto phenyl or thiophene). The feasibility of the photoinduced electron transfer process was corroborated by electrochemical, spectroelectrochemical measurements as well as DFT calculations. DFT calculations also support the existence of multiple conformations in the ground state, which differ from one another in terms of charge distribution and the values of ground state dipole moment. Finally, the mechanism of the singlet excited state deactivation of the studied compounds was determined by ultrafast pump-probe measurements. Our studies revealed that charge/electron transfer process may undergo over carbonyl bridge, included in branched π-system. Moreover, the thiophene decorated pyrazoline is characterized by a better photovoltaic power conversion efficiency, while the phenyl-ended pyrazoline can be applied as a viscosity sensor.