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Dyad 4, comprising a triphenylamine (TPA) electron donor and 1,4-pentadien-3-one (pentadienone) electron acceptor tethered by a nonconjugated linker, displays solvatofluorochromism (SFC) and dual fluorescence associated with intramolecular charge transfer (ICT) in the excited state. While the fluorescence arises from a locally excited state of 4 (LE-4*) in saturated hydrocarbon solvents, the fluorescence from the ICT state of 4 (ICT-4*) occurs in aprotic solvents. ICT-4* has a much greater dipole moment than its corresponding ground state. The results of theoretical calculations suggest that the conversion of LE-4* to ICT-4* involves a unique structural change like a leaning of the pentadienone moiety. Two factors are responsible for the significant SFC displayed by 4, the first being the high electron-donating and -accepting abilities of the respective locally excited TPA and pentadienone moieties in LE-4* and the other being a rigid ethano bridge that links the two moieties in ICT-4*. The former property facilitates photoinduced electron-transfer (PET) and the latter prevents full single electron transfer (SET) by prohibiting direct π-conjugation and the spatial approach of the two dyad components. Consequently, these electronic and geometrical features lead to SFC arising from a large dipole moment change caused by ICT and partial intramolecular SET. [ABSTRACT FROM AUTHOR]

The photophysical properties of o-tolyl-, m-tolyl-, and p-xylyl-substituted asymmetric diaroylmethanatoboron difluorides in a mixture of CH2Cl2 and c-C6H12, and in the crystalline state were determined. In solution, the fluorescence (FL) properties of these substances are controlled by the position and number of methyl groups on the phenyl rings. An especially interesting finding is that FL from the p-xylyl derivative occurs from an excited state which possesses intramolecular charge-transfer character caused by the o- and m-methyl groups cooperatively. The results of X-ray crystallographic analysis reveal that these asymmetric diaroylmethanatoboron difluorides form dyads through orbital overlap of neighboring molecules. This phenomenon governs the unique FL properties of these substances in the solid state. [ABSTRACT FROM AUTHOR]

The thermally backward reaction involved in the mechanofluorochromism of dibenzoylmethanatoboron difluoride (BF2DBM) derivatives, accompanied by an amorphous–crystalline phase transition, was quantitatively evaluated based on kinetics and thermodynamics. The kinetics was discussed by evaluation of the effect of temperature on the time-dependent changes of the fluorescence intensity for amorphous samples obtained by mechanical grinding. The thermodynamics was discussed based on data for the amorphous–crystalline phase transition obtained by differential scanning calorimetry. The enthalpy of activation (ΔH‡) of BF2DBM derivatives with MeO groups (2aBF2) was larger than that of derivatives with alkyl groups (2b-dBF2), whereas the entropy of activation (ΔS‡) was smaller than that of the derivatives with alkyl groups. It is proposed that the reaction dynamics of 2aBF2 will be governed by rotational motion around the C(methyl)–O bond. Interestingly, the Gibbs energies of activation (ΔG‡) were comparable for the reactions of all members of the BF2DBM series, though ΔH‡ and ΔS‡ were strongly dependent on the identity of the substituent. It is proposed that the substituent-dependent ΔS‡ term is one of the key parameters for understanding the mechanofluorochromism of BF2DBM derivatives associated with the amorphous–crystalline phase transition. These findings will also provide important insights into the process of formation of crystal nuclei in moving from the melted to the crystalline state. [ABSTRACT FROM AUTHOR]