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

Abstract

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°, and the corresponding CI point was found at 90°. 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.

Exploring Potential Energy Surfaces for Aggregation‐Induced Emission—From Solution to Crystal

“…The model provides information on the main non‐radiative deactivation pathways connecting the FC region with the CI, and it does not carry any initial assumptions of the shape of the PES. As shown in Figure , the RACI model has been used to explain AIE in different kinds of systems including conjugated organic molecules, boranes, and excited‐state proton transfer (ESIPT) molecules . A similar picture is provided by excited‐state dynamics calculations with trajectory surface hopping (TSH), which directly simulate the decay to the ground state.…”

“…The luminescence of HPIP crystals comes from the keto form, which is formed after ESIPT from the enol ground state. HPIP is the first AIEgen where the involvement of a CI was invoked to explain the lack of fluorescence in solution, combining TD‐DFT and CASSCF calculations (Figure ) . After excitation of the enol ground state, the molecule relaxes on S 1 and yields the keto form.…”

Electronically‐tuned 2‐(2′‐Hydroxyphenyl)‐4‐pyrenylthiazole through Bond Energy Transfer Donor–Acceptor Couples: Sensing and Biological Applications

“…Such a phenomenon enhances intersystem crossing (ISC) and leads to weak emission. The unusually strong emission even in the twisted form confirmed the existence of nano‐aggregated zwitterionic states in the excited state in DMSO …”

Photoinduced excited state intramolecular proton transfer and spectral behaviors of Aloesaponarin 1

“…The ESIPT process through hydrogen bond interaction has been used to interpret the spectral behaviors of molecular system due to its important role in practical application [26][27][28]. The molecules undergoing ESIPT process can be candidates for molecular systems, such as optical memory, photolabeling, proton-transfer laser and an information storage device at the molecular level [29][30][31][32][33][34][35][36][37][38][39][40]. The ESIPT processes of some molecular systems often occur through intermolecular or intramolecular hydrogen bond, which are central to understanding their spectral behaviors and microscopic structure [12][13][14][15][16][17][18][19][20][21][22][23][24][25].…”