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.

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6,6'-Diamino-2,2'-bipyridine (la) has been found to exhibit a strong fluorescence in the near-UV region. Some amino and/or chloro substituted bipyridines (bpys) have been synthesized and studied to show that only 6-amino-substituted derivatives exhibited a strong emission. The emission of 6-amino-6'-chloro-bpy (3a) was the strongest (A,,, = 429.0 nm; @ = 0.78 in ethanol) among them. On the other hand, little or no emission was observed for monochloro-, dichloro-and 4-amino-derivatives.The application of fluorescent organic compounds to optical devices and photo-functionalized materials is a topic of current interest.' For these uses, the chemical and thermal stability of the compounds are essential factors in addition to their colour and higher emission efficiency. But because of limitations in the modification of existing fluorophores to meet the conditions mentioned above, new series of fluorophores are actively studied.We have been studying the properties and functionality of 6,6'-diamino-2,2'-bipyridine (la) and its derivative^,^-^ and we recently found that l a displayed a relatively strong emission in the near-UV region. 2,2'-Bipyridine (bpy) is a n-electron deficient compound and needs much more drastic conditions than pyridine for both electrophilic and nucleophilic substitution. Though synthesis of its derivatives is rather complicated, the derivatives are inactive for ring-directed reactions and are thermally stable in general. However, relatively little has been known about the fluorescent property of bpy and its derivatives. The bpy derivatives are generally non-fluorescent,' though 3,3'-dimethyl and 3,3'-dihydroxy7 bpys have been reported to show fluorescence.In this report, we describe the fluorescent property of a series of amino-and chloro-substituted bpys at 4-and/or 6-position(s). Among the bpys tested in this study, asymmetrically substituted 6-amino-6'-chloro-bpy (3a) was found to exhibit a remarkably strong emission.