Effect of Environment on Pressure-Induced Emission of Benzophenone, 4,4‘-Dichlorobenzophenone, and 4-(Dimethylamino)benzaldehyde in Solid Media

Abstract

The pressure-induced emission of three aromatic carbonylsbenzophenone (BP), 4,4‘-dichlorobenzophenone (DCBP), and 4-(dimethylamino)benzaldehyde (DMABA)dissolved in solid polymers and also in their crystalline state has been studied. Under pressure all compounds dissolved in polymers reveal a significant enhancement of the luminescence emission in the low-pressure region. The degree of increase is strongly dependent on the type of polymer. DMABA exhibits both fluorescence and phosphorescence, whereas BP and DCBP molecules show only phosphorescence. For all these molecules a model based on the increase with increasing pressure of the amount of π,π* character in the initially predominately n,π* triplet state is proposed to account for the observed increase of the emission intensity. In the case of DMABA it is proposed that the increase of the π,π* character in the triplet state increases the emission intensity in two concurrent steps: by an increase of the radiative rate from the triplet state and by a decrease of the intersystem crossing rate between singlet and triplet states. The first factor increases phosphorescence intensity, and the second one effects the fluorescence. In the case of BP and DCBP, where the intersystem crossing process is very fast, the increase of pressure increases mainly the radiative rate of the lowest triplet state. In contrast to the polymer matrices, the emission intensity of these compounds strongly decreases under pressure in their crystalline environment. This is explained in terms of the increasing role of the nonradiative processes due to a strong coupling of the vibronic states of the triplet to the environment. The observed decrease in luminescence intensity in the polymers at high pressures has the same explanation.