Ab initio configuration interaction study on electronically excited 4-dimethylamino-4′-cyanostilbene

Abstract

Ab initio con®guration interaction calculations have been performed to examine the electronic structures of both trans-4-dimethylamino-4¢-cyanostilbene (DCS) and four types of perpendicularly twisted DCSs, trans-DCS is predominantly excited into the S 1 state out of low-lying excited states. The S 1 state is an intramolecular charge-transfer (ICT) state in which the dipole moment is about twice as large as that in S 0 . The excited DCS at the 4-dimethylanilino twisted conformation, which becomes S 1 in polar solvents, has a very much larger dipole moment than that in S 1 to trans-DCS. This means that the geometrical structure of the twisted ICT (TICT) is the 4-dimethylanilino twisted form, not the dimethylamino twisted one which is well know from the TICT structure of 4-dimethylaminobenzonitrile.

Dual Fluorescence and Excited-State Structural Relaxations in Donor−Acceptor Stilbenes

“…In contrast to the behavior of DMABN, we can therefore conclude that the twisting moiety is not the amino or dialkylamino group but the anilino or dialkylanilino moiety. This conclusion was reached previously 15 and is supported by recent ab initio calculations. , Although in previous studies 10-17 the behavior of bridged compounds indirectly suggested the involvement of a twisting process, in this study, the twisting is more directly demonstrated by the viscosity and rotator size effect, and dual fluorescence is observed even in the case of ACS.…”

Isolated Building Blocks of Photonic Materials: High-Resolution Spectroscopy of Excited States of Jet-Cooled Push-Pull Stilbenes

“…Femtosecond fluorescence spectroscopy on DCS in polar solutions has shown a fast and efficient ICT process occurring on a time scale of the order of one picosecond. , Analysis of the solvatochromic shift of the absorption maxima as a function of the solvent polarity provided experimental evidence that in DCS the locally excited (LE) state has a dipole moment of 13 D, giving reason to believe that even this state has already to a large extent CT character. , This was supported by a recent ab initio CI study on DCS, which showed that the LE state is mainly described by a single excitation from the highest occupied molecular orbital (HOMO), localized to a large extent on the dimethylanilino group and the linkage ethylenic part, to the lowest unoccupied molecular orbital (LUMO), localized primarily on the 4-cyanostyryl group. In polar solvents, a second state of CT character becomes sufficiently stabilized to be accessible, giving rise to the sudden polarization effect. − ,,,, Experimental studies investigating selectively bridged derivatives of DCS suggest that this CT state, also referred to as a twisted intramolecular CT (TICT) state, is coupled to the initially prepared LE state via a normal coordinate associated with a rotation around the bond linking the dimethyl-anilino group to the double bond. − ,,, The dependence of the solvatochromic shift of the emission maxima on the solvent polarity 24 has been interpreted to indicate that this twisting motion, which leads to strongly enhanced charge-transfer, is responsible for the sudden polarization phenomenon and results in a further increase of the dipole moment of DCS in this TICT state by roughly 9 D. Recent ab initio configuration interaction (CI) calculations on the electronic properties of the lowest excited singlet state of DCS under both isolated conditions and in a polar environment corroborate these experimental observations …”

“…Primary targets of these studies have been 4-(dimethylamino)-4‘-cyanostilbene (DCS) − and 4-(dimethylamino)-4‘-nitrostilbene (DANS) − (see Figure ), with as a goal to investigate and understand their photophysics and photochemistry, in particular in relation to the intramolecular charge transfer (ICT) process that leads to sudden polarization − upon excitation from the electronic ground state to the equilibrated singlet excited state. Various theoretical and experimental studies have given evidence for and corroborated the presence of sudden polarization in DCS and DANS upon excitation. ,,,,−

1 Molecular structures of the targeted push−pull stilbenes: 4-(dimethylamino)-4‘-cyanostilbene (DCS), 4-(dimethylamino)-4‘-nitrostilbene (DANS), and 4-di(hydroxy-ethyl)amino-4‘-nitrostilbene (DANS-diol).

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Skeletal Relaxation Effect on the Charge Transfer State Formation of 4-Dimethylamino,4‘-cyanostilbene

“…In case of the rigidified p -dimethylamino, p ‘ ‘-cyano,1- 1‘-bi-indanylidene which is prevented from both τ 2 - and τ 4 -torsional motions defined in Figure , the more red-shifted emission around 560 nm is not observed irrespective of solvent polarity. ,, On the other hand, two types of emissions similar to that of DCS are observed in another rigidified (3- p -dimethylanilino,7-cyano,1,2-dihydro)-naphthalene which is bridged by a six-membered ring between the ethylenic portion and the benzonitrile group to hinder the τ 3 - and τ 4 -torsional motion. , Also two types of red-shifted emissions are observed in another rigidified (3-(5‘- N -methylindolinyl), 7-cyano,1,2-dihydro)-naphthalene which is prevented from τ 1 -, τ 3 - and τ 4 -torsional motions . From a theoretical side, we examined the potential energy surfaces with respect to the four torsional angles τ i 's (refer to Figure ) so as to give a reasonable interpretation on the experimental findings mentioned above. , That is, the nontwisted CT state forms very fast after electronic excitation, and then the nontwisted CT changes into the TICT state by the dimethylanilino (i.e., τ 2 ) torsional motion in a polar solvent. To the best of our knowledge, however, there are no experimental and theoretical realizations that an additional factor is important for the CT formation process in a solvent besides the torsional motion of the dimethylanilino group and solvent polarity.…”