Excited-State Intramolecular Proton Transfer (ESIPT) Emission of Hydroxyphenylimidazopyridine: Computational Study on Enhanced and Polymorph-Dependent Luminescence in the Solid State

Although 2-(2'-hydroxyphenyl)imidazo[1,2-a]-pyridine (HPIP) is only weakly fluorescent in solution, two of its crystal polymorphs in which molecules are packed as stacked pairs and in nearly coplanar conformation exhibit bright excited-state intramolecular proton transfer (ESIPT) luminescence of different colors (blue-green and yellow). In order to clarify the enhanced and polymorph-dependent luminescence of HPIP in the solid state, the potential energy surfaces (PESs) of HPIP in the ground (S-0) and excited (S-1) states were analyzed computationally by means of ab initio quantum chemical calculations. The calculations reproduced the experimental photophysical properties of HPIP in solution, indicating that the coplanar keto form in the first excited (S-1) state smoothly approaches the S-0/S-1, conical intersection (CI) coupled with the twisting motion of the central C-C bond. The S-1-S-0 energy gap of the keto form became sufficiently small at the torsion angle of 60 degrees, and the corresponding CI point was found at 90 degrees. Since a minor role of the proximity effect was indicated experimentally and theoretically, the observed emission enhancement of the HPIP crystals was ascribed to the following two factors: (1) suppression of efficient radiationless decay via the CI by fixing the torsion angle at the nearly coplanar conformation of the molecules in the crystals and (2) inhibition of excimer formation resulting from the lower excited level of the S-1-keto state compared to the S-0-S-1 excitation energy in the enol form. However, the fluorescence color difference between the two crystal polymorphs having slightly different torsion angles was not successfully reproduced, even at the MS-CASPT2 level of theory.