Influence of intermolecular interactions on solid state luminescence of imidazopyridines: theoretical interpretations using FMO-TDDFT and ONIOM approaches.

6-Cyano-2-(2'-hydroxyphenyl)imidazo[1,2-a]-pyridine (6CN-HPIP) shows polymorph-dependent luminescence with the three different crystal forms exhibiting the packing-controlled tuning of bright colors, orange, yellow, and red. The distinctive emission in aggregated states was treated with finite cluster models and analyzed by means of quantum chemistry calculations. The influence of structural displacements and intermolecular interactions in the crystalline state on solid state luminescence was examined in detail using the Fragment Molecular Orbital (FMO) scheme, suitable for studies of aggregated molecular systems. The FMO pair interaction analysis of the S1-S0 emission maxima indicated that the intermolecular side-to-side interactions cause hypsochromic shifts; facial interactions induce bathochromic shifts; and crystal packing effects in total induce hypsochromic shifts. The FMO predictions of the emission maxima offered qualitatively satisfactory agreements with the experiments. However, the small cluster models including up to 17 molecules did not reach quantitative convergence, i.e., the emission colour order among them was not well reproduced.

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°, 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.

Electronic spectra of cycl[3.3.2]azine and related compounds: solvent effect on vibronic couplings.

Quantitative ab initio calculations are presented for the ultraviolet-visible peaks of cycl[3.2.2]azine and its mono- and dibenzannulated polycyclic compounds at the multistate CASPT2 (MS-CASPT2) level of theory, with 11 nm deviation from the experimental S0 → S1 absorption. The electrophilic substitution reactions of cycl[3.2.2]azine, benzo[a]/[g]annulated cycl[3.2.2]azines, and 6-dimethylamino[2.2.3]cyclazine-1-carboxylates with 3-cyano-4-methylthiomaleimide gave the corresponding functionalized cycl[3.2.2]azine derivatives, which exhibited the absorption maxima around 510-630 nm. The first intense peaks were investigated by means of time-dependent density functional theory (TD-DFT). These peaks were systematically underevaluated by ∼50 nm, within the acceptable accuracies of TD-DFT. Furthermore, we calculated vibronic coupling constants of the electronic excited states of cycl[3.2.2]azine and simulated absorption spectra both in vacuo and in ethanol. The solvent effect is found to enhance oscillator strengths and vibronic couplings. This is because the solvent effect gives rise to changes in the electron density difference on the phenyl ring, and in turn, the intensified overlap between the electron density difference and the potential derivative in the phenyl ring leads to enhanced vibronic couplings in ethanol.

Synthesis, solid-state fluorescence properties, and computational analysis of novel 2-aminobenzo[4,5]thieno[3,2-d]pyrimidine 5,5-dioxides.

New fluorescent compounds, benzo[4,5]thieno[3,2-d]pyrimidine 5,5-dioxides (3a-g), 2-amino-4-methylsulfanylbenzo[4,5]thieno[3,2-d]pyrimidine (6), and 2-amino-4-methylsulfanyl-7-methoxybenzo[4,5]furo[3,2-d]pyrimidine (7), were synthesized in good yields from heterocyclic ketene dithioacetals (1a-c) and guanidine carbonate (2a) or (S)-methylisothiourea sulfate (2b) in pyridine under reflux. Among the fused pyrimidine derivatives, compound 3c, which has an amino group at the 2-position and a benzylamino group at the 4-position of the pyrimidine ring, showed the strongest solid-state fluorescence. The absorption and emission properties of the compounds were quantitatively reproduced by a series of ab initio quantum-chemical calculations.

Faradaic phase transition of dibenzyl viologen on an HOPG electrode surface studied by in situ electrochemical STM and electroreflectance spectroscopy.

Phase transitions of an adsorption layer of dibenzyl viologen (dBV) as a typical diaryl viologen on a basal plane of a highly oriented pyrolytic graphite (HOPG) electrode are described using voltammetry, in situ electrochemical scanning tunneling microscopy (EC-STM), and electroreflectance (ER) spectroscopy. A monolayer redox process at less negative potential than the bulk redox process was found to be the first-order faradaic phase transition between a gaslike adsorption layer of dication (dBV(2+)) and a 2D condensed monolayer of radical cation (dBV(•+)). Comparison of the results of cyclic voltammetry and potential step chronoamperometry was made with those of heptyl viologen (HV), which also undergoes a faradaic phase transition of the first order. It suggested that the contribution of intermolecular π-π interaction between benzyl groups of dBV to the phase transition is minor and apparently equivalent to interchain interaction between the heptyl chains of HV. In situ EC-STM images of the 2D condensed monolayer demonstrated stripe patterns of the rows of dBV(•+) molecules forming 3-fold rotationally symmetric domains. The results of the ER measurements also revealed that the orientation of the longitudinal molecular axis of the bipyridinium moiety of dBV(•+) molecules lying flat on the HOPG electrode surface, most likely with a side-on configuration.