AUTHOR=Cortés-Villena Alejandro , Soriano-Díaz Iván , Domínguez Moisés , Vidal Matías , Rojas Pablo , Aliaga Carolina , Giussani Angelo , Doménech-Carbó Antonio , Ortí Enrique , Galian Raquel E. , Pérez-Prieto Julia 

TITLE=Governing the emissive properties of 4-aminobiphenyl-2-pyrimidine push–pull systems via the restricted torsion of N,N-disubstituted amino groups

JOURNAL=Frontiers in Chemistry

VOLUME=11

YEAR=2023

URL=https://www.frontiersin.org/journals/chemistry/articles/10.3389/fchem.2023.1292541

DOI=10.3389/fchem.2023.1292541

ISSN=2296-2646

ABSTRACT=<p>Donor–acceptor-substituted biphenyl derivatives are particularly interesting model compounds, which exhibit intramolecular charge transfer because of the extent of charge transfer between both substituents. The connection of a 4-[1,1′-biphenyl]-4-yl-2-pyrimidinyl) moiety to differently disubstituted amino groups at the biphenyl terminal can offer push–pull compounds with distinctive photophysical properties. Herein, we report a comprehensive study of the influence of the torsion angle of the disubstituted amino group on the emissive properties of two pull–push systems: 4-[4-(4-<italic>N</italic>,<italic>N</italic>-dimethylaminophenyl)phenyl]-2,6-diphenylpyrimidine (<bold>D1</bold>) and 4-[4-(4-<italic>N</italic>,<italic>N</italic>-diphenylaminophenyl)phenyl]-2,6-diphenylpyrimidine (<bold>D2</bold>). The torsion angle of the disubstituted amino group, either <italic>N</italic>,<italic>N</italic>-dimethyl-amine or <italic>N</italic>,<italic>N</italic>-diphenyl-amine, at the biphenyl end governs their emissive properties. A drastic fluorescence quenching occurs in <bold>D1</bold> as the solvent polarity increases, whereas <bold>D2</bold> maintains its emission independently of the solvent polarity. Theoretical calculations on <bold>D1</bold> support the presence of a twisted geometry for the lowest energy, charge-transfer excited state (S<sub>1,90</sub>), which corresponds to the minimum energy structure in polar solvents and presents a small energy barrier to move from the excited to the ground state, thereby favoring the non-radiative pathway and reducing the fluorescence efficiency. In contrast, this twisted structure is absent in <bold>D2</bold> due to the steric hindrance of the phenyl groups attached to the amine group, making the non-radiative decay less favorable. Our findings provide insights into the crucial role of the substituent in the donor moiety of donor–acceptor systems on both the singlet excited state and the intramolecular charge-transfer process.</p>