Fluorescence Excitation Spectrum, Lifetimes and Photoisomerization of Jet-Cooled Conformers of 1,1´-bi(benzocyclobutylidene)

Abstract

Fluorescence Excitation Spectra / 1,1 -bi(benzocyclobutylidene) / PhotoisomerizationFirst measurements of fluorescence excitation spectra of the recently synthesized rigid stilbene analogue 1,1 -bi(benzocyclobutylidene) in a supersonic jet expansion show that, in contrast to the parent compound, both the trans-and the cis-conformer fluoresce under these conditions. The excitation energy dependence of fluorescence lifetimes indicates the onset of an efficient non-radiative decay channel above energy thresholds of 1340 cm −1 and 990 cm −1 for the trans-and cis-form, respectively, which is assigned to photoisomerization in the singlet state. From an RRKM analysis of the microcanonical rate coefficients an estimate of the high pressure limit of the thermal photoisomerization rate coefficient is obtained and compared with photoisomerization rate coefficients measured in low viscosity solution and in thermal vapor. There are strong indications that for this compound there are no dynamic or static solvent induced effects that lead to an anomalous acceleration of the reaction in solution.

Quantitative Representation of Specific Rate Constants k(E) for the Photoisomerization of Diphenylpolyenes: The Solution of a Longstanding Problem

“…Recently, we presented a similar comparison between measurements of k(E) and the corresponding calculated k ∞ with measured thermal rate coefficients k nr in low viscosity solutions for the trans- and cis -conformers of benzocyclobutenyliden-benzocyclobutene . While, for the cis -isomer, the two rate constants agreed to within better than 10% (1.2 and 1.3 × 10 10 s -1 , respectively), for the trans -isomer there was a factor of 2−3 discrepancy (1.8 versus 4.6 × 10 9 s -1 ) which may be considered as still close to the experimental uncertainty.…”

“…The ambiguity of the situation has led us to a new attempt to solve the dilemma. Improved possibilities to calculate vibrational frequencies for excited electronic states by ab initio methods allowed us to do such calculations for a series of diphenylpolyenes. The surprising accuracy of the calculated frequencies for those vibrations, where a comparison could be made, provides confidence that the calculations are essentially correct.…”

Stable Ion and Electrophilic Chemistry of the Sterically Crowded Stilbene 1,1′-Bi(benzocyclobutenylidene) and Its Derivatives

“…The X-ray structure of trans - 1 showed that the molecule is planar . Photophysical and photochemical properties of the trans isomer along with a series of other cyclic stilbenes were investivated. , …”

“…In 2002, Lenoir and associates reported on the synthesis of sterically hindered stilbene 1,1′-bi(benzocyclobutenylidene) 1 (Figure ) by McMurry coupling of benzocyclobuten-1-one, resulting in a 1:1 mixture of cis and trans isomers, along with the isomeric diols 2 , which were obtained as minor products. The pure trans isomer of 1 was isolated on small scale by semipreparative HPLC or by crystallization techniques . The X-ray structure of trans - 1 showed that the molecule is planar .…”

Reply to Comment by E. Pollak on “Photoisomerization of trans-Stilbene in Moderately Compressed Gases: Pressure-Dependent Effective Barriers” (J. Phys. Chem. A 1999, 103, 10528−10529)

“…On the contrary, other photochemical activation systems could be interpreted satisfactorily under the assumption that the thermal energy of the ground state in absorption together with the photon energy was carried up more or less undistorted into the excited electronic state. Examples of such systems are alkyl-substituted cycloheptatrienes and benzenes, 1,1‘-(benzocyclobutylidene), and also NO 2 . Why should trans -stilbene photoisomerization be governed by Franck−Condon cooling while all of these other systems do not show evidence for such effects?…”