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Experimental and theoretical studies on the thermodynamics and properties of 2-substituted 6(4)-methyl-1,4(1,6)-dihydropyrimidine-5-carboxylates were undertaken by 1 H NMR measurements and DFT (density functional theory) calculations. The ratios of tautomers a / b of dihydropyrimidines (DPs) 1 , 2 , and 3 were determined under various conditions to reveal the effects of temperature, solvent, and concentration on the thermodynamic data. The observed results, the free energy differences (Δ G ), enthalpy differences (Δ H ), and entropy differences (Δ S ), are discussed in terms of the molecular structures, dipole moments (DM), and the electrostatic potential maps calculated by the DFT to clarify the nature of the DPs. [ABSTRACT FROM AUTHOR]

Electron donor–acceptor–donor (D–A–D) π-conjugated molecules based on dipyrido[3,2-a:2′,3′-c]phenazine (dppz) were developed as photosensitizers for singlet oxygen generation. These dppz derivatives showed electronic absorption assigned to an intramolecular charge transfer transition in the red region at a wavelength of around 600 nm. Evaluation of singlet oxygen generation quantum yield (ΦΔ) using the trapping agent 1,3-diphenylisobenzofuran showed relatively efficient singlet oxygen generation (ΦΔ; 0.173–0.255) for the dppz derivatives with selenophene-containing π-conjugated side-arms. On the other hand, 2,7-dibromo substitution and Pt(II)Cl2 complexation of the dppz derivative with thiophene-containing side-arms did not facilitate photosensitized singlet oxygen generation. Density functional theory (DFT) and time-dependent DFT calculations, as well as evaluation of the kinetics of the photophysical processes, revealed that appropriate placement of heavy atoms on the frontier orbitals facilitates intersystem crossing from the lowest singlet excited state to the triplet excited state, thus improving ΦΔ. [ABSTRACT FROM AUTHOR]

In this study, we observed that methylenecyclopropane 1 c, possessing geminal phenyl and 1‐naphthyl groups at C‐2 of the three membered ring, emits green thermoluminescence (TL) in a low temperature matrix, associated with two sets of bands in the 480–600 and 600–700 nm regions. The bands are assigned to phosphorescence from 31 c* and triplet‐triplet (T−T) fluorescence (FL) from the corresponding trimethylenemethane biradical 32 c..*, respectively. In contrast, as previously reported, the analogous 2‐naphthyl derivative 1 b emits red TL associated with two bands in the 350–450 and 600–700 nm regions, which are assigned to FL from 11 b* and T−T FL from 32 b..*. The results of spectroscopic studies and density functional theory calculations show that the observed dissimilarity in the TL properties of 1 b and 1 c is a consequence of differences in the rate of methylenecyclopropane ring opening of 1 b.+ and 1 c.+ associated with stereoelectronic effects of the naphthyl groups. [ABSTRACT FROM AUTHOR]

The substituent effect on optical properties of 3,14-diphenyl-17,17-di( n -pentyl)tetrabenzofluorene derivatives bearing electron-withdrawing groups (NC, CH 3 OCO, CH 3 SO 2 , and OHC) at 4-position of the phenyl groups were investigated. These compounds were readily prepared by using Suzuki–Miyaura cross-coupling as a key step. The 4-cyano, 4-methoxycarbonyl, and 4-methylsulfonyl derivatives display intense blue to blue-green fluorescence in CH 2 Cl 2 and in the solid states. On the other hand, the title compound forms two polymorphs, and thus, exhibit crystallochromism in fluorescence; the prismatic crystal emits intense yellow fluorescence, and the other crystal form does blue-green fluorescence. Moreover, the compound shows large solvatofluorochromism (Δ λ EM = 67 nm). The crystallographic analysis of the prismatic crystal reveals that the tetrabenzofluorene moiety of the compound possesses a helical structure with a C 2 symmetry and the molecules form a herringbone-like structure. The density function theory calculations predict the remarkable intramolecular charge-transfer character of the compound. The prediction is in good agreement with the observed fluorescence properties. [ABSTRACT FROM AUTHOR]

[Display omitted] • A crystal of the [2.2]paracyclophane-containing organoboron complex shows PFC. • Inter- and intra-molecular π-stacking distances are reduced under high pressure. • The intermolecular π–π interaction is still the important factor leading to PFC. Piezofluorochromism (PFC) of crystals of a tert -butyl and [2.2]paracyclophane-containing diaroylmethanatoboron difluoride (p CP- t Bu), which is expected to possess both intermolecular and intra molecular π–π interactions, was investigated. p CP- t Bu crystals were found to exhibit remarkable PFC associated with an over 150-nm redshift of fluorescence (FL) wavelength occurring under increasing isotropic pressure applied by using a diamond anvil cell. X-ray crystallographic analysis showed that p CP- t Bu molecules in the crystal at atmospheric pressure exist in the form of an intermolecular π-stacking dimer through back-to-back π–π interactions. Moreover, the crystallographic data under high pressure showed that increasing isotropic pressure causes a reduction in both the intermolecular π-stacking distance of the dimer (D INTER) and the intra molecular π-stacking distance between two benzene rings in the [2.2]paracyclophane moiety (D INTRA). Density functional theory calculations, using the Cartesian coordinates obtained by X-ray analysis, suggest that the origin of PFC of p CP- t Bu crystals is the reduction of the LUMO energy of the π-stacking dimer and that the intermolecular π–π interaction is still the important factor leading to PFC of p CP- t Bu crystals. [ABSTRACT FROM AUTHOR]