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.

To elucidate the origin of the opsin shift of bacteriorhodopsin (bR), a self-consistent reaction field method combined with configuration interaction calculation is employed. In addition, the absorption maxima of all-trans-retinal and its Schiff bases are measured in a variety of aprotic solvents. It is shown that the calculation reproduces well the observed solvatochromic shifts. From regression analysis, we obtain an empirical relationship between the absorption maximum of protonated retinal Schiff base and physical parameters of solvent, including dielectric constant and refractive index. On the other hand, based on the crystal structure of bR, we estimate the effective values of such parameters for the retinal-binding pocket. Combining these results, it is shown that the opsin shifts of bR568 and M412 can be quantitatively reproduced if the protein matrix acts as a polarizable medium with a high refractive index. From decomposition analysis of the calculated opsin shift, the contributions of (i) ring/chain coplanarization, (ii) separation of a counterion, and (iii) medium effects of the protein are shown to be 2500, 1200, and 1000 cm-1, respectively. It is revealed that the effects (i) and (ii) are independent of each other, but the effects (ii) and (iii) are significantly correlated. In a polarizable medium, a shift induced by a counterion is almost canceled out by an opposite shift induced by medium effects. In conclusion, the polarizable medium effects play a decisive role in the wavelength regulation of bR.

We apply a SCRF-PCM-CI calculation to elucidate the mechanism of spectral tuning in photoactive yellow protein (PYP). It is shown that the calculation well reproduces solvatochromic shifts observed for some model compounds of the PYP chromophore. By regression analysis, we obtain an empirical equation to predict solvatochromic shifts of these compounds for a given set of dielectric constant and refractive index. Next, using a classical electrostatic theory and the crystal structure of PYP, the value of refractive index is calculated for the chromophore-binding pocket. The value of the dielectric constant is estimated from the fact that the binding pocket is highly hydrophobic. On the basis of these results we predict the absorption maximum of PYP. In addition, the spectral tuning mechanism in PYP is divided into three factors, that is, counterion effect, hydrogen-bonding effect, medium effect of the protein matrix, and each contribution is quantitatively evaluated. It is shown that the electronic polarization effects of the protein matrix plays a nonnegligible role in tuning the absorption maximum of PYP as similar to the case of bacteriorhodopsin.

Chromism-color changes by external stimuli-has been intensively studied to develop smart materials because of easily detectability of the stimuli by eye or common spectroscopy as color changes. Luminescent chromism has particularly attracted research interest because of its high sensitivity. The color changes typically proceed in a one-way, two-state cycle, i.e. a stimulus-induced state will restore the initial state by another stimuli. Chromic systems showing instant, biphasic color switching and spontaneous reversibility will have wider practical applicability. Here we report luminescent chromism having such characteristics shown by mechanically controllable phase transitions in a luminescent organosuperelastic crystal. In mechanochromic luminescence, superelasticity-diffusion-less plastic deformation with spontaneous shape recoverability-enables real-time, reversible, and stepless control of the abundance ratio of biphasic color emissions via a single-crystal-to-single-crystal transformation by controlling a single stimulus, force stress. The unique chromic system, referred to as superelastochromism, holds potential for realizing informative molecule-based mechanical sensing.

We prepared and characterized a series of mono- and dicarbaldehydes of 2,6-dihydroxynaphthalene that bear potential resonance-assisted hydrogen bonding (RAHB) unit(s). X-ray crystal structures of selected compounds revealed that each salicylaldehyde moiety forms an intramolecular hydrogen bond and that the introduction of formyl groups into either the alpha- or beta-position causes a considerable difference in geometry, which was interpretative from a conventional scheme of resonance hybrids including ionic state. Analyses on NMR chemical shifts suggested that the compounds in solution are present as an equilibrium mixture between closed and open forms with respect to RAHB units. Ab initio calculations indicated that the formation of an intramolecular hydrogen bond strikingly influences the aromaticity of the individual local six-membered ring of naphthalene. The trend of the change in aromaticity was analyzed in connection with the extra stabilization energy of RAHB. In the UV-vis spectra, the beta-formyl derivatives specifically showed a substantial red shift compared to alpha-formyl derivatives. The absorption features were successfully reproduced by TD-DFT calculation, and those data were consistently explained from the effects of RAHB on electronic state of the naphthalene's pi-system. Finally, we pointed out a similarity in the electronic state between RAHB-bearing molecules and cata-condensed aromatic hydrocarbons.

The internal rotational barrier heights of phenol and anisole were calculated using several basis sets up to cc-pVQZ with MP2-level electron correlation correction to evaluate the basis set effects. The calculations showed that the effects of the further improvement of the basis set beyond the cc-pVTZ were very small. Although the electron correlation substantially increased the barrier heights of the two molecules, the effects of the electron correlation beyond the MP2 method were not large. The barrier heights calculated with the CCSD(T) method were close to those with the MP2 method. The internal rotational potentials of methoxy and hydroxyl groups of o-hydroxyanisole were calculated at the MP2/cc-pVTZ//HF/6-311G** level. The calculated potentials were compared with those of phenol and anisole. o-Hydroxyanisole preferred planar structure in which the hydroxyl group had an intramolecular hydrogen bond with the oxygen atom of the methoxy group. The calculated torsional potential of the methoxy group had the maximum (7.30 kcal/mol) when the methoxy group rotated 180° from the minimum energy structure, in which the hydroxyl group did not have the hydrogen bond. The barrier height of the methoxy group of o-hydroxyanisole was considerably larger than that of anisole (2.99 kcal/mol). The large internal rotational barrier height of o-hydroxyanisole showed that the intramolecular hydrogen bond greatly stabilized the energy minimum structure and that the hydrogen bond strictly restricted the conformational flexibility of the methoxy group.