Two-photon fluorescence properties of novel quadrupolar and octupolar molecules

Using a wavelength-tunable optical parametric oscillation (OPO) laser, we investigate the two-photon fluorescence (TPF) properties of two quadrupolar (P2SS2Ph and PSS2PhNp) and three octupolar (P3SPh, P3SS2Ph and TRZ-Ph-Tol) donor-acceptor-donor (D-A-D) type molecules. TPF cross-section δ of all the samples lies at the wavelength between 740 nm and 990 nm. Even though P3SS2PH octupolar and P2SS2PH quadrupolar molecules have distinctly different structures and symmetries, their maximum δ are close. On the other hand, the maximum δ of octupolar TRZ-Ph-Tol is very much larger than that of the structurally different octupolar P3SPh The π-electron delocalization in TRZ-Ph-Tol is more extended than in P3SPh. It is thus tempting to conclude that the longer the conjugation length, the larger the maximum δ. Although the conjugation system of P3SS2Ph is no less extended, compared with TRZ-Ph-Tol, its δ is much smaller. This may be due to the weaker central acceptor of P3SS2Ph. Furthermore, we investigate the δ spectral difference between octupolar and quadrupolar molecules.