“…Plots of τ s vs T for 1 , 4 − 9 , and 11 are shown in Figure and lifetime data reported in the Supporting Information. The decay times (τ s ) at or near 77 and 298 K are reported in Table along with the fluorescence quantum yields measured at or near 298 K. Also reported in Table are the reported quantum yields and decay times for the cyclic phenylalkenes 12 − 16 . , 4 Temperature-dependent lifetimes for styrenes 1 , 4 − 9 , and 11 .…”
Section: Resultsmentioning
confidence: 99%
“…This unexpected behavior is attributed to breaking conjugation, which results in enhanced spin−orbit coupling in the highly nonplanar styrenes. This model for styrene singlet state decay can also account for the singlet state behavior of two families of cyclic styrene derivatives, the benzocycloalkadienes 10 − 13 and 1-phenylcycloalkenes 14 − 16 , and that of the anti-rotamers of the 2-vinylbiphenyls 17 and 18 1 …”
The ground state conformation, spectroscopy, and photochemical behavior of styrene and eight of its vinyl- and ring-methylated derivatives have been investigated. Introduction of methyl groups at the alpha-position of the vinyl group or the ortho positions of the phenyl results in increased phenyl-vinyl dihedral angles. Styrenes possessing both alpha-methyl and ortho-methyl groups adopt orthogonal geometries. Decreased planarity results in a progressive blue-shift in the lowest energy allowed pi,pi transition and a decrease in the singlet lifetime. Kinetic modeling of the temperature-dependent singlet lifetimes provides activation parameters for the activated decay pathway, which is assigned to C=C torsion for styrenes with phenyl-vinyl dihedral angles, phi, < 60 degrees. Planar styrenes have large torsional barriers (7 +/- 1 kcal/mol) and decay predominantly via intersystem crossing and fluorescence at room temperature. Styrenes with values of 30 degrees < phi < 60 degrees have smaller torsional barriers and decay predominantly via C=C torsion at room temperature. Highly nonplanar styrenes decay predominantly via relatively rapid, weakly activated intersystem crossing.
“…Plots of τ s vs T for 1 , 4 − 9 , and 11 are shown in Figure and lifetime data reported in the Supporting Information. The decay times (τ s ) at or near 77 and 298 K are reported in Table along with the fluorescence quantum yields measured at or near 298 K. Also reported in Table are the reported quantum yields and decay times for the cyclic phenylalkenes 12 − 16 . , 4 Temperature-dependent lifetimes for styrenes 1 , 4 − 9 , and 11 .…”
Section: Resultsmentioning
confidence: 99%
“…This unexpected behavior is attributed to breaking conjugation, which results in enhanced spin−orbit coupling in the highly nonplanar styrenes. This model for styrene singlet state decay can also account for the singlet state behavior of two families of cyclic styrene derivatives, the benzocycloalkadienes 10 − 13 and 1-phenylcycloalkenes 14 − 16 , and that of the anti-rotamers of the 2-vinylbiphenyls 17 and 18 1 …”
The ground state conformation, spectroscopy, and photochemical behavior of styrene and eight of its vinyl- and ring-methylated derivatives have been investigated. Introduction of methyl groups at the alpha-position of the vinyl group or the ortho positions of the phenyl results in increased phenyl-vinyl dihedral angles. Styrenes possessing both alpha-methyl and ortho-methyl groups adopt orthogonal geometries. Decreased planarity results in a progressive blue-shift in the lowest energy allowed pi,pi transition and a decrease in the singlet lifetime. Kinetic modeling of the temperature-dependent singlet lifetimes provides activation parameters for the activated decay pathway, which is assigned to C=C torsion for styrenes with phenyl-vinyl dihedral angles, phi, < 60 degrees. Planar styrenes have large torsional barriers (7 +/- 1 kcal/mol) and decay predominantly via intersystem crossing and fluorescence at room temperature. Styrenes with values of 30 degrees < phi < 60 degrees have smaller torsional barriers and decay predominantly via C=C torsion at room temperature. Highly nonplanar styrenes decay predominantly via relatively rapid, weakly activated intersystem crossing.
“…This is one of the motivations of the present study. Of course, styrene itself also has been extensively studied as a prototype of aryl olefin,3–25 but there seems to be also no widely accepted picture of the photochemical behavior of styrene. This is another motivation to propose a new picture of the photochemical behavior of styrene in the low‐lying excited states.…”
Section: Introductionmentioning
confidence: 99%
“…Hui and Rice4 reported that the lifetime of the radiationless decay decreases with increasing the excitation energy, though they made a wrong interpretation of the stable geometrical structure in the excited state, as mentioned above. Turro and Lyons5 measured the quantum yields of styrene and several benzocycloalkadienes in which the styryl double bond is fixed by steric constraints to be out‐of‐plane with the benzene ring. They found that the quantum yields of benzocycloalkadienes are much smaller than those of styrene (ϕ F = 0.22).…”
The electronic structures of styrene in the Franck-Condon region have been theoretically examined by means of ab initio complete active space self-consistent field (CASSCF) and the second order multireference Møller-Plesset calculations. The optimized structure of styrene in S(0) is planar but the torsional motion of the phenyl group is very floppy. The S(1) state is assigned to the local pi-pi* excitation within the benzene ring. On the other hand, S(2), above S(1) by 0.561 eV, is assigned to a state that resembles the so-called V-state of ethylene. The transition intensity of S(0)-S(1) is weak, while that of S(0)-S(2) is strong. This is in good agreement with the experimental absorption spectrum where the S(0)-S(1) and S(0)-S(2) transitions are in the energy range of 290-220 nm. The optimized geometry of S(1), characterized by an enlarged benzene ring and its vibrational analyses, further justifies the assignment of the S(1) state.
“…Despite the importance of cis–trans isomerization, simple 1,2-biradicals have been studied sporadically because the direct irradiation of simple alkenes does not result in intersystem crossing to their triplet surface. , In comparison, the cis–trans isomerization of styrene on its singlet and triplet surface has been studied extensively. − For example, Caldwell et al have shown that triplet-sensitized photolysis of styrene derivatives results in triplet 1,2-biradicals. The lifetimes of acyclic flexible 1,2-biradicals were found to be within ∼100 ns, whereas for rigid or cyclic styrene derivatives, which have limited rotation, the lifetimes increased to a few microseconds …”
The irradiation of trans-vinylketones 1a-c yields the corresponding cis isomers 2a-c. Laser flash photolysis of 1a and 1b with 308 and 355 nm lasers results in their triplet ketones (T1K of 1), which rearrange to form triplet 1,2-biradicals 3a and 3b, respectively, whereas irradiation with a 266 nm laser produces their cis-isomers through singlet reactivity. Time-resolved IR spectroscopy of 1a with 266 nm irradiation confirmed that 2a is formed within the laser pulse. In comparison, laser flash photolysis of 1c with a 308 nm laser showed only the formation of 2c through singlet reactivity. At cryogenic temperatures, the irradiation of 1 also resulted in 2. DFT calculations were used to aid in the characterization of the excited states and biradicals involved in the cis-trans isomerization and to support the mechanism for the cis-trans isomerization on the triplet surface.
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