Three aryl and three naphthylmethylene derivatives containing thiadiazole ring have been synthesized. The structures of target compounds were characterized on the basis of spectral (FT-IR, 1H NMR, and MS). The optical properties were detected using UV-vis absorption spectroscopy and fluorescence spectroscopy. The absorption spectra of 1a, 2a and 3a substituted by naphthylmethylene are primarily characterized by a peak around 284 nm originating from naphthalene, which is different from that of compounds 1b, 2b and 3b. Compared to 1a and 2a (separated by a saturable atomic cluster −CH2−), the maximum absorption wavelength of 1b and 2b takes on obvious red-shifted, which is from thiadiazole and benzene with more large conjugated system. The fluorescence intensity of 2-(4-aminobenzoyl) amide-5-naphthylmethylene-1,3,4-thiadiazole (3a) was significantly higher than that of 5-(4-aminobenzoyl)-1,3,4-thiadiazole (3b) due to the presence of naphthalene.
A series of all-trans acceptor–π-donor (acceptor) compounds (BAQ, SFQ, BLQ, and XJQ) were conveniently synthesised and characterised by infrared, nuclear magnetic resonance, mass spectrometry, and elemental analysis. Their photophysical properties, including linear absorption, one-photon excited fluorescence, two-photon absorption, and two-photon excited fluorescence, were systematically investigated. All the compounds show obvious solvatochromic effects, such as significant bathochromic shifts of the emission spectra and larger Stokes shifts in more polar solvents. Under excitation from a femtosecond Ti : sapphire laser with a pulse width of 140 fs, they all exhibit strong two-photon excited fluorescence, and the two-photon absorption cross-sections in THF are 851 (BAQ), 216 (SFQ), 561 (BLQ), and 447 (XJQ) GM respectively. A combination of density functional theory (DFT) and time-dependent density functional theory (TDDFT) approaches was used to investigate the relationships between the structures and the photophysical properties of these compounds. The results show that they may have a potential application as polarity-sensitive two-photon fluorescent probes.
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