TD-DFT study of the excited state intramolecular proton transfer (ESIPT) mechanism and photophysical properties in coumarin-benzothiazole derivatives: substitution and solvent effects

The detailed excited state intramolecular proton transfer (ESIPT) mechanism of coumarin-benzothiazole fluorescent dyes (BT-Cou-R1-8; R1-8 = -H, -NH2, -OH, -OCH3, -CH3, -CF3, -CN, -NO2) with potential application in drug delivery systems was investigated at the TD-PBE0/6-311++G(d,p) level of theory in the gas phase and three solvent media. The potential energy curves in the ground (S-0) and first excited states (S-1) were constructed to demonstrate the enol -> keto ESIPT mechanism. The results revealed that the ESIPT in BT-Cou-R1-5 is an energy barrier-less process while there is an energy barrier for BT-Cou-R6-8 having electron-withdrawing substituents. It was found that BT-Cou-R1-5 exhibited a single keto fluorescence emission while BT-Cou-R6-8 showed double enol and keto fluorescence emissions with the dominance of keto emission. Both the enol and keto emission wavelengths of BT-Cou-R-8 (R = NO2) are larger than those of BT-Cou-R6-7, and in the solvent media, they are close to the near-infrared region with a red shift value of 236-259 nm for the keto form and 326-339 nm for the enol one. However, the intensity of enol emission in BT-Cou-R-8 is lower than the keto one and the S-1(E) state can be considered as a dark state. Because S-1(K) emission possesses an extremely large Stokes shift, thereby this molecule can be an attractive material for chemosensors, fluorescent probes, laser dyes, and optoelectronic devices. The natural bond orbital (NBO) and atoms-in-molecules (AIM) population analyses were carried out to calculate the atomic charges and electron density properties as well as to characterize the nature of the hydrogen bonding interaction along the proton transfer.