Two-photon absorption of dipolar and quadrupolar oligothiophene-cored chromophore derivatives containing terminal dimesitylboryl moieties: a theoretical (DFT) structure–property investigation

Abstract

A series of dipolar and quadrupolar dimesitylboryl (BMes₂) derivatives containing different thiophene oligomers as the central conjugated bridge to which BMes₂ substituents are appended at both ends have been investigated theoretically via density functional theory (DFT) and time-dependent (TD) DFT calculations. The results indicate that for all quadrupolar compounds studied the excited state reached by the two-photon absorption (2PA) is the S₂ state which is one-photon forbidden and which corresponds to the HOMO to LUMO+1 electronic transition. When the terminal alkene (double) bonds in the nV species, containing n thiophene rings in the conjugated bridge, are replaced by alkyne (triple) bonds, giving the nT compounds, an increase of the 2PA cross-sections is always stated. Modification of the conjugated linker and addition of trimethylammonium (NMe₃⁺) substituents to the phenyl rings of the terminal BMes₂ moieties show two 2PA peaks at different wavelengths with the low energy one corresponding to the population of the S₂ excited state and the second one corresponding to the population of a higher excited state. Substitution of the NMe₃⁺ groups by neutral electron-attracting cyano (CN) groups shows lower 2PA cross-sections.