Excited state intramolecular proton transfer via different size of hydrogen bond ring: a theoretical insight

Modulating excited‐state intramolecular proton transfer of 2‐(5‐(4‐carboxyphenyl)‐2‐hydroxyphenyl)benzothiazole depending on substituents: A DFT/TD‐DFT study

“…The calculated absorption and fluorescence spectra by using TD‐M06‐2X/6‐31+G(d,p) method are displayed in Figure 5. Our previous studies [ 89 ] have shown that our theoretical fluorescence emission peaks and absorption peak of CHBT obtained at TD‐M06‐2X/6‐31+G(d,p) level are consistent with the experimental values. [ 82 ] For 4(CH 3 )‐CHBT and 4(OH)‐CHBT, the absorption peaks are at 311.6 and 314.8 nm and the fluorescent emission peaks of the enol/keto form are at 366.2/469.6 and 368.8/473.4 nm, respectively.…”

“…Such changes demonstrate that the intramolecular H‐bond (O 1 ‐H 2 … N 3 ) of 4R‐CHBT (R: CF 3 , CN, COOH) is also reinforced in the S 1 state in their enol forms. With comparison with the structural parameters of CHBT, [ 89 ] the S 1 state O 1 ‐H 2 and H 2 ‐N 3 distances of 4R‐CHBT (R: OH, CH 3 , COOH, CF 3 , CN) averagely decrease 0.011 Å and increase 0.019 Å, respectively. It is obvious that all the chosen substituent R (R: OH, CH 3 , COOH, CN, CF 3 ) weakens the intramolecular H‐bond O 1 ‐H 2 … N 3 in the S 1 state.…”

“…Those results demonstrate that the intramolecular H‐bonds in the 4R‐CHBT (R: OH, CH 3 , COOH, CF 3 , CN) are enhanced in the S 1 state. In the S 1 state, an average red‐shift of 1,167 cm −1 in the 4R‐CHBT (R: ‐OH, ‐CH 3 ) and 1,159 cm −1 in the 4R‐CHBT (R: COOH, CF 3 , CN) are smaller than a red‐shift of 1,220 cm −1 in CHBT, [ 89 ] which verifies the substituted groups (OH, CH 3 , COOH, CF 3 , CN) weaken the H‐bond. The electron‐withdrawing group (COOH, CF 3 , CN) plays greater parts in weakening the strength of intramolecular H‐bond than the electron‐donating group (OH, CH 3 ) does.…”

“…The introducing electron‐donating group (CH 3 , OH) makes the Stokes shift average 1.4 nm smaller than the corresponding value for CHBT. [ 89 ] In addition, the keto forms of 4(CH 3 )‐CHBT and 4(OH)‐CHBT molecule have a large red shift of 158.0 and 158.6 nm between the fluorescent emission peak and the absorption peak, which is average 2.6 nm smaller than that value in CHBT. [ 89 ] When the electron‐withdrawing group is introduced in CHBT, the Stokes shift are 56.6 nm (COOH), 57.1 nm (CF 3 ), and 57.6 nm (CN), respectively, which is average 1.4 nm bigger than that value in CHBT.…”

“…[ 89 ] In addition, the keto forms of 4(CH 3 )‐CHBT and 4(OH)‐CHBT molecule have a large red shift of 158.0 and 158.6 nm between the fluorescent emission peak and the absorption peak, which is average 2.6 nm smaller than that value in CHBT. [ 89 ] When the electron‐withdrawing group is introduced in CHBT, the Stokes shift are 56.6 nm (COOH), 57.1 nm (CF 3 ), and 57.6 nm (CN), respectively, which is average 1.4 nm bigger than that value in CHBT. [ 89 ] At the same time, the keto forms of 4R‐CHBT (R: COOH, CF 3 , CN) have a large red shift of 160.3 to 164.2 nm between the absorption peak and fluorescent emission peak, which is only 1.7 nm in average bigger than that value in CHBT.…”