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 carbonylssbenzophenone (BP), 4,4′-dichlorobenzophenone (DCBP), and 4-(dimethylamino)benzaldehyde (DMABA)sdissolved 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.

Crystallization-Induced Phosphorescence of Pure Organic Luminogens at Room Temperature

“…In the DFBP-doped PMMA film, the luminogens are molecularly dispersed and are physically shielded and protected from the attacks from oxygen and water by the polymer matrix. However, the luminogen is still nonemissive at room temperature . Furthermore, its spots adsorbed on the TLC plates remain incapable of emitting light even after they have been placed under a nitrogen atmosphere.…”

Pressure-Induced Isomerization of 2-(2‘-Hydroxyphenyl)benzoxazole in Solid Media

“…It has been shown that, given the lifetime of triplet states, much of the quenching is via vibrational coupling to the matrix which coupling increases with compression. 11 If the emission efficiency from E 2 is less than that for Kt, the increasing fraction of E 2 at higher pressures could contribute to the modest decrease in I T observed from 30 to 60 kbar.…”

“…However, nothing is known about the nonradiative rate from E 1 , and the literature 4 indicates that the nonradiative rate from the excited state 1 Kt to 0 E 1 may be via 3 Kt and 3 E 1 . It has been shown that, given the lifetime of triplet states, much of the quenching is via vibrational coupling to the matrix which coupling increases with compression . If the emission efficiency from E 2 is less than that for Kt, the increasing fraction of E 2 at higher pressures could contribute to the modest decrease in I T observed from 30 to 60 kbar.…”

Continuous Irradiation Induced Luminescence from Benzophenone, 4,4‘-Dichlorobenzophenone, and 4-(Dimethylamino)benzaldehyde in Solid Environments and Its Pressure Dependence

“…The samples and their preparation have been described in the preceding paper . The light-induced experiments, either the emission or absorption, were performed in a Merrill−Bassett diamond anvil cell (DAC) at room temperature, utilizing low-fluorescence diamonds with a culet diameter of approximately 0.8 mm.…”

“…The present paper is a continuation of our investigations of pressure and light induced effects on luminescence of molecules with nonbonding electrons dissolved in solid media. − The general interest in this class of molecules has stemmed mainly from their prolific triplet state features providing a variety of excellent examples for that kind of photophysical and photochemical studies like singlet−triplet intersystem crossing, triplet−triplet annihilation, energy transfer and migration, triplet-state absorption, and triplet-state photoreactivity via the hydrogen abstraction and/or α-cleavage reactions. As we have shown, for some of these processes, they can be significantly perturbed under external hydrostatic pressure providing new information inaccessible at atmospheric pressure.…”