When a fluorescent compound shows unique optical properties, an elucidation of the mechanism may lead to an important development of novel sensing strategies. A helical 3,3′-di-tert-butylsalen-zinc(II) complex, [Zn 2 L 1 2 ], has a red-shifted fluorescence as compared to that of [ZnL 2 2 ], a half-structured mononuclear complex of [Zn 2 L 1 2 ]; in addition, [Zn 2 L 1 2 ] exhibits a fluorescence color change from green to light blue under external stimulations. We investigated the origins of these phenomena by spectroscopy, fluorescence lifetime measurement, fluorescence microscopy, X-ray powder diffraction, and X-ray singlecrystal analysis. From the experimental data, we concluded that intramolecular and intermolecular π-π interactions are critical elements that determine the shifts of the fluorescence to a longer wavelength.

We develop a hybrid quantum mechanical/molecular mechanical-configuration interaction (QM/MM-CI) method for calculating the absorption maxima of photoreceptor proteins such as bacteriorhodopsin. A unique point of our method, discriminating it from usual QM/MM methods, is that the ground-state electronic structure of the whole protein is first evaluated by a linear scaling-molecular orbital calculation. The resultant electronic distribution is utilized to construct a modified Fock matrix for subsequent CI calculation. In the excitation energy calculation, only the chromophore located at the photoactive center of a protein is treated quantum mechanically and the surrounding environment is approximated by classical electrostatics. Another feature of the method is that the classical region is instantaneously polarized in response to the excitation of the chromophore. This corresponds to the incorporation of electronic polarization effects of the protein part. To allow the polarization of amino acid residues, each bond of them is approximated by a cylindrical dielectric with a given polarizability. The polarization in the classical part is determined self-consistently. Here, the above method is applied to the wild type of bacteriorhodopsin (bR568) and its mutants. It is revealed that their absorption maxima are not reproduced without taking into account the effect of electronic polarization of the protein part. In particular, the polarization of Trp86, Trp182, and Tyr185 plays a predominant role in causing a bathochromic shift in the absorption band of bR568.

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.