Theoretical investigations on forward–backward ESIPT processes of three fluorophores deriving from 2-(2′-hydroxyphenyl)thiazole

Abstract

The photophysical properties and excited-state intramolecular proton transfer (ESIPT) processes for 2-(2′-hydroxyphenyl)-4-chloromethylthiazole (1), 2-(2′-hydroxyphenyl)-4-phenylthiazole (2), 2-(2′-hydroxyphenyl)-4-hydroxymethyl-thiazole (3) were studied at the TD-B3PW91/6-31 + G(d, p)/IEFPCM level. The structures of 1-3 were fully optimized and the corresponding structural parameters, infrared spectra and electron densities in the ground (S 0 ) and the first excited (S 1 ) states were analyzed. The calculated absorption and fluorescence wavelengths of 1-3 reproduced the experimental data. The potential energy curves of the S 0 and S 1 states were built and the ESIPT processes were clarified. Our results showed that the intramolecular H-bonds of 3 and 2 in the S 1 state were the strongest and the weakest, respectively, and then the ESIPT potential barriers of 3 and 2 were the lowest and highest, respectively. Among the three phenol-thiazole type probes, the compound 2 with phenyl ring group at the 4 position of the thiazole ring had the larger π-conjugation, and had the higher ESIPT potential barrier at the same time. The corresponding compound 1 and 3 with CH 2 Cl and CH 2 OH had the lower ESIPT barrier.

A DFT/TD-DFT study of effect of different substituent on ESIPT fluorescence features of 2-(2′-hydroxyphenyl)-4-chloromethylthiazole derivatives

“…The O 1 -H 2 stretching vibrational frequencies of HCT replaced by electron-donating group and electronwithdrawing group are slightly blue-shifted and red-shifted in the S 0 state, respectively, in comparison with the corresponding value of HCT. [47] These results confirm the electrondonating group (CH 3 and OH) and electron-withdrawing group (CF 3 , CHO) weaken and strengthen the intramolecular H-bond O 1 -H 2 . .…”

“…HCT [47] N 0.997 The changing of intramolecular H-bond of R-HCT (R: CH 3 , OH, CF 3 , CHO) can be confirmed by the infrared (IR) vibration spectra of the O 1 -H 2 bond (normal form) in the R-HCT compounds in acetonitrile (shown in Fig. 3).…”

“…For the normal forms of CH 3 -HCT and OH-HCT compound, it can be seen in Table 1 that the bond distances of O 1 -H 2 and H 2 -N 3 , and the bond angle O 1 -H 2 -N 3 in the S 0 state are 0.996 Å, 1.703 Å, 147.9 • , and 0.995 Å, 1.709 Å, 147.8 • respectively. Comparing with HCT, [47] the H 2 -N 3 bonds and the angles O 1 -H 2 -N 3 of CH 3 -HCT and OH-HCT in the S 0 state are elongated and shortened, respectively, which means that the intramolecular H-bonds O 1 -H 2 . .…”

“…Forward Reverse HCT [47] 0.53 These result suggest that the proton transfer from O-H group to thiazolic nitrogen atom is impossible for R-HCT (R: CH 3 , OH, CF 3 , CHO) in the ground state. However, it should be mentioned that there is a low reaction barrier less than 1 kcal/mol in the S 1 state for proton transfer process from N to T. Obviously, the low reaction barrier is prone to ESIPT process.…”

“….N 3 exists separately in the S 0 state and S 1 state, and the intramolecular H-bonds in the S 1 state are stronger than those in the S 0 state. In comparison with HCT, [47] the introduced electron-donating/electron-withdrawing group weakens/enhances and enhances/weakens the intramolelcular H-bond of HCT in the S 0 and S 1 states, respectively. Based on the non-covalent interaction analysis, the above-mentioned conclusion is affirmed again.…”