Dicyanostilbene-derived two-photon fluorescence dyes with large two-photon absorption cross sections

Four dicyanostilbene-derived two-photon fluorescence (TPF) dyes were synthesized as the model compounds to systematically study the effect of the dicyano and the terminal substituent on the two-photon absorption (TPA). These four compounds (DSO, DCY, DTO and DPH) exhibit very large two-photon absorption cross sections (δ). DCY (A–π–A) with the terminal cyano group has especially high fluorescence quantum yield (0.71) and relatively large δ (1480 GM), while DPH (D–π–A) with the substitutedamino group at its terminus possesses the largest δ (2800 GM) and the longest emission wavelength (620 nm). The idealest terminal substituent should not be the alkoxy group but the substitutedamino group. This class of dicyanostilbene dyes possess small molecule size, large δ (830–2800 GM), long-wavelength emission (459−620 nm) and large Stokes shift (80−206 nm), and are ideal chromophores for TPF labels and probes.