Photoinduced Single- versus Double-Bond Torsion in Donor−Acceptor−Substituted trans-Stilbenes

Abstract

The electronic absorption and fluorescence spectra, quantum yields for fluorescence (Phi(f)) and trans --> cis photoisomerization (Phi(tc)), and fluorescence lifetimes of trans-4-(N-arylamino)-4'-cyanostilbenes (2H, 2Me, 2OM, 2CN, and 2Xy with aryl = phenyl, 4-methylphenyl, 4-methoxyphenyl, 4-cyanophenyl, and 2,5-dimethylphenyl, respectively), trans-4-(N-methyl-N-phenylamino)-4'-cyanostilbene (2MP), trans-4-(N,N-diphenylamino)-4'-cyanostilbene (2PP), trans-4-(N-methyl-N-phenylamino)-4'-nitrostilbene (3MP), and three ring-bridged analogues 2OMB, 2MPB, and 3MPB are reported. Whereas fluorescence and torsion of the central double bond account for the excited decay of the majority of these donor-acceptor substituted stilbenes in both nonpolar and polar solvents (i.e., Phi(f) + 2Phi(tc) approximately 1), exceptions are observed for 2OM, 3MP, and 3MPB in solvents more polar than THF and for 2Me and 2MP in acetonitrile as a result of the formation of a weakly fluorescent and isomerization-free twisted intramolecular charge transfer (TICT) state (i.e., Phi(f) + 2Phi(tc) << 1). The TICT state for 2OM, 2Me, and 2MP results from the torsion of the stilbenyl-anilino C-N single bond, but the torsion of the styryl-anilino C-C bond is more likely to be responsible for the TICT state formation of 3MP and 3MPB. In conjunction with the behavior of aminostilbenes 1, a guideline based on the values of Phi(f) and Phi(tc) for judging the importance of a TICT state for trans-stilbenes is provided. Accordingly, the TICT state formation is unimportant for the excited decay of trans-4-(N,N-dimethylamino)-4'-cyanostilbene (DCS). In contrast, our results support the previously proposed TICT state for trans-4-(N,N-dimethylamino)-4'-nitrostilbene (DNS).

Fate of photoexcited trans-aminostilbenes

“…A similar value of 3.6 kcal/mol was also determined for the case of trans-4-aminostilbene (4AS) and its N,N-dimethyl derivative 4DS [54,55]. However, the C¼C torsion barriers are 4.9 and 7.4 kcal/mol for p1H and p1CN, respectively, in hexane [52,56]. The N-aryl substituent effect on raising the singlet-state C¼C torsional barrier could be interpreted with the resonance structures of the 1 t* and 1 p* states.…”

“…The first type is performed by the group of p1R with a methyl (p1Me), hydrogen (p1H), chloro (p1Cl), or trifluoromethyl (p1CF) substituent. This group conforms to the common behavior of trans-stilbenes of K f + 2K tc % 1.0 in both nonpolar and polar solvents (note that values in the range 0.80-1.20 were considered to be near unity to accommodate the experimental uncertainty of both the K f and K tc data) [51,52]. The other group consisting of p1R with a methoxy (p1OM), cyano (p1CN), or methoxycarbonyl (p1CO) substituent displays the behavior of K f + 2K tc % 1.0 only in nonpolar solvents such as hexane but not in more polar solvents such as THF and acetonitrile.…”

“…The cyano effect is also noticeable in the TICT-active p1OM system, as p3OM display a larger and nearly quantitative yield for forming the TICT state (K TICT > 0.97). The TICT state of p3OM is essentially non-fluorescent, as evidenced by the much smaller half-bandwidth ($4900 cm À1 ) for p3OM relative to that for p1OM ($6200 cm À1 ) in acetonitrile and by the opposite temperature effect on the fluorescence intensity of p3OM vs. p1OM (vide supra) [52]. The decreased fluorescing ability for p3OM vs. p1OM might be associated with the larger driving force toward the TICT state.…”

“…A good example for illustrating the relationship between the electron-accepting strength of the A moiety and the TICT-forming activity of trans-aminostilbenes comes from the cyano-and nitrosubstituted systems p3R (R = H, Me, OM, and CN, Chart 7), p4MP, 4DCS, and 4DNS [52]. The cyano substituent in p3R does enhance the TICT-forming propensity as compared to the parent systems p1R.…”

Synthesis of p‐Nitro‐stilbene Derivatives with Different Linking Bonds: An Attempt to Tune Spectroscopy of Dyes with Molecular Engineering

“…Photoisomerization and torsion of double bond might lead to decrease of the fluorescence emission of compounds 3 and 4 in very strong polar solvents such as acetonitrile and DMF. 27,28 Electrochemistry…”