Fluorescence, fluorescence-excitation and absorption spectra of trans-1-(2-anthryl)-2-phenylethene conformers

Temperature Dependence of the Photoisomerization of cis-1-(2-Anthryl)-2-phenylethene. Conformer-Specificity, Torsional Energetics and Mechanism

“…Plots for the λ exc = 350 and 390 nm data, based on eq 24, Figure , are parallel and linear (| r | = 0.9953 and 0.9997, respectively), and their slopes give Δ H AB = 0.91 ± 0.06 and 0.93 ± 0.02 kcal/mol, respectively. It follows that c -APE B is 0.92 ± 0.04 kcal/mol more stable than c -APE A , consistent with the conclusion that the more extended B conformer is similarly more stable in t -APE. ,, Neglecting entropy differences, this energy difference predicts a small variation in the percent [ c -APE B ] in the temperature range employed (from 84.0% to 80.0% as the temperature is increased from 4.3 to 59.3 °C).

7 Transition state plot for .

8 The temperature dependence of K AB , the c -APE B ⇌ c -APE A equilibrium constant based on λ exc = 350 (·) and 390 nm (▪).

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“…Substitution of FN for O 2 as the quencher avoids possible uncertainties in O 2 concentrations at different temperatures under our flow cell conditions. Earlier reports had shown that FN is a highly efficient quencher of the fluorescence of trans -stilbene 17 and of related 1,2-diarylethylenes, and our work has established that FN and O 2 quench t -APE fluorescence at diffusion-controlled rates, showing no selectivity between t -APE A and t -APE B . , Furthermore, the absence of fluorescence from potential t -APE/FN or c -APE/FN exciplexes renders the FN choice ideal for our resolutions as no additional fluorescent components are introduced.…”

“…The blue shift, clearly evident at the onset of the 59.3 °C spectrum, may reflect the temperature-induced decrease in medium polarizability as discussed for the resolved conformer spectra of t -APE . By analogy with t -APE, we expect the c -APE A UV absorption spectrum to be somewhat red-shifted relative to the absorption spectrum of c -APE B . This accounts for the sharp drop of φ fc when excitation is at the onset of the absorption spectrum of c -APE, from which an upper limit of φ fc -A ≤ 0.025 at 19.3 °C was estimated 8e.…”

“…The origins of t -APE and c -APE S 1 ← S 0 transitions had been assigned values of 70.2 and 71.3 kcal/mol, respectively, on the basis of unresolved spectra 5f. We assign values of 67.6 and 73.3 kcal/mol for t -APE B and c -APE B , respectively, on the basis of our resolved fluorescence and absorption spectra. 8e,, The resulting potential energy curve for the 1 c -APE B * → 1 t -APE B * torsional coordinate is shown in Figure . It accounts nicely for the absence of trans → cis photoisomerization in S 1 of APE B as this process faces an insurmountable 15 kcal/mol activation barrier.…”

Temperature and solvent effects on rotamer-specific photobehaviour of the cis and trans isomers of 2-styrylanthracene

“…The anomaly has been shown to be due to the contribution of two rotamers (A and B conÐgurations), which exist in Ñuid solutions at room temperature in as a mixture of almost S 0 isoenergetic species (*H about 1.2 kJ mol~1) of similar concentration.10,13 Upon excitation, the composition in is S 1 assumed to reÑect that of the ground state equilibrium since no equilibration is expected during the short lifetime S 1 ("" non-equilibration of excited rotamers ÏÏ, NEER, principle18) on the basis of the dependence of the emission spectrum shape on and considering the sizeable barrier between j exc 6,10,15 rotamers in Furthermore, a third Ñuorescence com-S 1 .9,17d,19 ponent, described in our previous work,20 was assigned to an anti-Kasha Ñuorescence from a thermally populated state S 2 of the long-lived s-trans conformation (B rotamer) of the trans isomer, trans-2-StAn(B). 15,20,21 Nevertheless, some aspects of the photobehaviour of the two 2-StAn geometrical isomers (such as the photoreactive processes and the surprisingly large decrease of the radiative rate constant of cis with decreasing temperature4,5a) must still be clariÐed.…”