The role of branching on the ultrafast dynamics and 2-photon absorption of two pyrimidine push-pull molecules

Abstract

The dynamics and 2-photon absorption (2PA) properties of two pyrimidine chromophores are studied by means of femtosecond time-resolved fluorescence and 2-photon excited fluorescence techniques. The pyrimidine is used as an electron withdrawing group and is substituted at the C2 position with phenylacridan fragment while at positions C4/6 diphenylaministyryl donor moieties are appended to afford the pseudo-dipolar and pseudo-quadrupolar molecules, respectively. The chromophore 2 shows more efficient fluorescence emission while 1 exhibits larger Stokes shifts. Their decay pathways are discussed through an emission from a locally excited (LE) and an intramolecular charge transfer (ICT) state. Ultrafast dynamics in tetrahydrofuran show an ICT population for 1 that is faster than solvation while for 2, due to its pseudo-quadrupolar nature, ICT is slower and takes place from the solvated LE state. Finally, molecule 2 shows better 2PA properties with cross sections reaching 560 GM at 820 nm.