Fluorescence of Trans-4-Cyano-4′-dimethylaminostilbene; No Evidence for a TICT State

Abstract

The fluorescence properties of trans-4-cyano-4'-dimethylaminostilbene (trans-1) and a rigid analogue, in which the dimethylaminophenyl group is replaced by an indoline ring, trans-1-(4-cyanophenyl)-2-(5-[2,3-dihydro]N-methylindolyl)ethylene (trans-2) were studied in 20 solvents. The quantum yields of fluorescence (tf> f ) and of trans-»eis photoisomerization (<2>t-c) as well as the fluorescence maximum (v f ) and the bandwidth (zlv f ) were examined. The activation energies (range: 3.5-7.8 kcal/mol) obtained from <2> f and <£ t _ c measurements are nearly the same for 1 and 2 in a given solvent. 0 f , v f and Jv f as a function of solvent polarity and temperature show similar trends for 1 and 2. No dual emission was observed for either of the cyanostilbenes. These findings indicate that a twisted intramolecular charge transfer (TICT) state is not involved in the relaxation process of the first excited trans singlet state ('t*). The main deactivation pathway of 't* competing with fluorescence is trans eis photoisomerization by twisting about the C = C double bond in the first excited singlet state for both.

Influence of charge transfer on the isomerisation of stilbene derivatives for application in cancer therapy

“…Functionalised charge transfer stilbenes, like 4-dimethylamino-4'-cyanostilbene (DCS), have been studied with experimental 5 and theoretical methods for more than forty years, but there is no general consent concerning the existence and nature of the TICT state. The dual fluorescence phenomenon of DCS can be partially explained by the existence of a TICT state formed by rotation around the C-C anilino bond [ [31][32][33] or the C-N amino bond [ [34][35][36][37], but no evidence for the existence of a TICT state and/or of dual fluorescence has been found in several studies [38][39][40]. Excited state re-absorption of fluorescence has been proposed as a potential alternative to the formation of a TICT state [ 41 ].…”

Sub-picosecond fluorescence evolution of amino-cyano-stilbenes in methanol: polar solvation obeys continuum theory without evidence of twisting

“…Like their trans-stilbene parent these molecules may photoisomerise to the cis configuration, depending on solvent viscosity, but now the isomerisation rates are strongly controlled by solvent polarity. [5][6][7] The dependence of fluorescence spectra on solvent polarity was already examined by Lippert who derived a dipole moment of 29 D for the emitting state of DCS. 8 A number of studies looked at the spectral dynamics on the ps time-scale before isomerisation.…”

“…8 A number of studies looked at the spectral dynamics on the ps time-scale before isomerisation. Conflicting views emerged as to how the CT state is reached and populated on the S 1 potential energy surface: (i) directly by optical excitation after which the solvent reorganises, 5,6,[9][10][11] or (ii) from the locally excited (LE) state by twisting the amino (t 1 in Scheme 1) or anilino group (t 2 ). [12][13][14] For a summary of experimental and theoretical work the reader is referred to ref.…”

The Excited State Behavior of Aminostilbenes. A New Example of the Meta Effect

“…The spectroscopy and photochemistry of donor−acceptor-substituted stilbenes such as 4-(dimethylamino)-4‘-cyanostilbene (DCS) have been extensively investigated. − The pronounced solvent dependence of the fluorescence behavior of such molecules has been attributed to rapid relaxation of the initially formed Franck−Condon excited state to a charge-transfer (CT) state. Trans → cis isomerization also originates from the CT state via twisting about the central double bond to form the perpendicular excited species which decays to a mixture of cis and trans isomers. , The singlet state behavior of 4-amino-4‘-(methoxycarbonyl)stilbene ( 1p ) is similar to that of DCS.…”