Effect of the Nitrogen Heteroatom on the Excited State Properties of 1,4-Distyrylbenzene

Abstract

The photophysical and photochemical properties of some aza-analogues of 1,4-distyrylbenzene, bearing a nitrogen heteroatom in the side arene rings, have been investigated. The kinetic competition between the radiative and reactive relaxation channels of the lowest excited singlet state has been compared with that of the parent hydrocarbon. The replacement of the side phenyl rings with pyridyl or quinolyl rings was found to markedly affect the photobehavior of the three stereoisomers (EE, ZE, and ZZ). Adiabatic formation of the excited 1 EE* by irradiation of the other isomers was evidenced by fluorescence measurements. The effect of the presence of the heteroatom in the central ring, while maintaining the linear conjugation, was investigated for EE-2,5-distyrylpyridine, and it was found to be less important than that of N in the side rings. Information about the conformational equilibria of the compounds bearing side quinolyl groups or the pyridyl central ring, due to internal rotation around the quasi single bonds with the ethenic carbons, was obtained by selective photoexcitation.

Time-resolved studies on the photoisomerization of a phenylene–silylene–vinylene type compound in its first singlet excited state

“…Using timeresolved spectroscopy techniques of observing transients in alkenes photoexcited to the S 1 state [14][15][16][17] it is possible to probe directly the cis-trans photoisomerization mechanisms elaborated from classical stationary-state photochemistry [12]. Photophysical and photochemical properties of the parent hydrocarbon, the E,E isomer of 1,4-distyrylbenzene (side 2-methylphenylsilyl groups in trans-ST substituted with phenyls), have been recently described [18,19]. The S 1 state of 1,4-distyrylbenzene is mainly deactivated by fluorescence (F F ¼0.84, t S1 ¼1.7 ns in methylcyclohexane/3-methylpentane), with some contribution from S 1 -S 0 internal conversion, while trans-cis photoisomerization (Fo0.001) and S 1 -T 1 intersystem crossing can be neglected [18].…”

“…Photophysical and photochemical properties of the parent hydrocarbon, the E,E isomer of 1,4-distyrylbenzene (side 2-methylphenylsilyl groups in trans-ST substituted with phenyls), have been recently described [18,19]. The S 1 state of 1,4-distyrylbenzene is mainly deactivated by fluorescence (F F ¼0.84, t S1 ¼1.7 ns in methylcyclohexane/3-methylpentane), with some contribution from S 1 -S 0 internal conversion, while trans-cis photoisomerization (Fo0.001) and S 1 -T 1 intersystem crossing can be neglected [18]. The current paper will show that trans-ST exhibits an extremely short lifetime in the S 1 state ($ 12 ps in hexane or acetonitrile), mainly due to a radiationless process repopulating trans-ST in the ground state.…”

Spectral and luminescent properties and photoisomerization of substituted styrylbenzoquinolines

“…To address this shortcoming, it is necessary to pass to het erodiarylethylenes, which possess pronounced fluo rescent properties (ϕ fl > 0.1), but it is important that the photoactivity of the compounds, that is, ability to photoisomerization, should persist. For example, when passing from 2SQ to the bis quinoline derivative of distyrylbenzene the ϕ fl value increases significantly (up to 0.63), but the quantum yield of the trans-cis photoisomerization reaction of the first double bond EE → ZE decreases steeply (down to 0.03), and the trans-cis photoi somerization reaction of the second double bond ZE → ZZ does not occur [7].It is known that the angular annulation of the quin oline nucleus, that is, passing from 2SQ to 3 styryl benzo[f]quinoline (3SBQ), leads to a significant increase in the fluorescence quantum yield up to 0.74, with the quantum yield of trans-cis photoisomeriza tion retaining a fairly high value (ϕ tc = 0.19), and the quantum yield of the reverse reaction of cis-trans pho toisomerization ϕ ct = 0.38 [8].Note that photoswitchable fluorophores are "real chemical basis" of one of the methods of ultrahigh res olution fluorescent microscopy ("nanoscopy") [9-11], for the development of which the Nobel Prize in Chemistry was awarded in 2014.To find out photochromes of the class of heterodi arylethylenes, being also fluorophores, that is, satisfy ing the conditions ϕ tc > 0.1 and ϕ fl > 0.1, we studied in this work a number of 3SBQ derivatives: trans 3 (3 methoxystyryl)benzo[f]quinoline (3MSBQ), trans 3 (4 methoxystyryl)benzo[f]quinoline (4MSBQ), and trans 3 (4 methoxycarbonylstyryl)benzo[f]quinoline (4MCSBQ) (Scheme 1): N N PHOTOCHEMISTRY Abstract-The spectral, luminescent, and photochemical properties of 3 styrylbenzo[f]quinoline (3SBQ)derivatives containing methoxy and methoxycarbonyl groups have been studied. All of these compounds luminesce in the region of 380-480 nm with quantum yields up to 0.74 and undergo reversible photoisomer ization with quantum yields of 0.2-0.5.…”

“…To address this shortcoming, it is necessary to pass to het erodiarylethylenes, which possess pronounced fluo rescent properties (ϕ fl > 0.1), but it is important that the photoactivity of the compounds, that is, ability to photoisomerization, should persist. For example, when passing from 2SQ to the bis quinoline derivative of distyrylbenzene the ϕ fl value increases significantly (up to 0.63), but the quantum yield of the trans-cis photoisomerization reaction of the first double bond EE → ZE decreases steeply (down to 0.03), and the trans-cis photoi somerization reaction of the second double bond ZE → ZZ does not occur [7].…”

“…For example, when passing from 2SQ to the bis quinoline derivative of distyrylbenzene the ϕ fl value increases significantly (up to 0.63), but the quantum yield of the trans-cis photoisomerization reaction of the first double bond EE → ZE decreases steeply (down to 0.03), and the trans-cis photoi somerization reaction of the second double bond ZE → ZZ does not occur [7].…”