Photophysics and intramolecular photochemistry of adamantanethione, thiocamphor, and thiofenchone excited to their second excited singlet states: evidence for subpicosecond photoprocesses
“…This value is much higher (about 30 times) than the corresponding rate constant determined for XT (34,37) and much lower (about 10 times) than k H determined for aliphatic thioketones in the excited S 2 state (20,35,36). These differences correlate with the increasing order of the S 2 state energies of these thioketones [E(S 2 ) XT Ͻ E(S 2 ) DMTU Ͻ E(S 2 ) aliphatic thioketones].…”
Section: The Pp Properties Of Dmtumentioning
confidence: 99%
“…The value of k is much smaller than the 1/τ value determined for DMTU in PFDMCH (Table 2), which implies that other process must be responsible for the very fast, intramolecular radiationless deactivation of the excited S 2 state of DMTU. It has been shown that for XT (34) and acyclic thioketones (20,35,36) having a C–H bond sufficiently close to the thiocarbonyl group, a very fast process of hydrogen abstraction (partly or totally reversible) takes place in the S 2 state, leading to its deactivation to the S 1 state. An analogous process involving the thiocarbonyl group and the methyl group in ortho position, taking place with a rate constant k H , can be responsible for the unexpectedly short τ of DMTU in PFDMCH.…”
Intramolecular processes of deactivation of 1,3‐dimethyl‐4‐thiouracil (DMTU) from the second excited singlet (S2) (π, π*) and the lowest excited triplet (T1) (π, π*) states have been studied using perfluoro‐1,3‐dimethylcyclohexane (PFDMCH) as a solvent. The spectral and photophysical (PP) properties of DMTU in CCl4, hexane and water have also been described. For the first time, the fluorescence from S2 state DMTU has been observed. The picosecond lifetime of DMTU in the S2 state (τ S 2) in PFDMCH has been proposed to be determined by a very fast intramolecular reversible process of hydrogen abstraction from the ortho methyl group by the thiocarbonyl group. The shortening of τ S 2 in CCl4 is interpreted to be caused by the intermolecular interactions between DMTU (S2) and the solvent. Results of the phosphorescence decay as a function of DMTU concentration were analyzed using the Stern–Volmer formalism, which enabled determination of the intrinsic lifetime of the T1 state (τ0 T 1) and rate constants of self‐quenching (ksq). The lifetimes, τ0 T 1, of DMTU in PFDMCH and CCl4 are much longer than the values hitherto obtained in more reactive solvents. The PP properties of DMTU both in the S2 and T1 states have been shown to be determined by the thiocarbonyl group.
“…This value is much higher (about 30 times) than the corresponding rate constant determined for XT (34,37) and much lower (about 10 times) than k H determined for aliphatic thioketones in the excited S 2 state (20,35,36). These differences correlate with the increasing order of the S 2 state energies of these thioketones [E(S 2 ) XT Ͻ E(S 2 ) DMTU Ͻ E(S 2 ) aliphatic thioketones].…”
Section: The Pp Properties Of Dmtumentioning
confidence: 99%
“…The value of k is much smaller than the 1/τ value determined for DMTU in PFDMCH (Table 2), which implies that other process must be responsible for the very fast, intramolecular radiationless deactivation of the excited S 2 state of DMTU. It has been shown that for XT (34) and acyclic thioketones (20,35,36) having a C–H bond sufficiently close to the thiocarbonyl group, a very fast process of hydrogen abstraction (partly or totally reversible) takes place in the S 2 state, leading to its deactivation to the S 1 state. An analogous process involving the thiocarbonyl group and the methyl group in ortho position, taking place with a rate constant k H , can be responsible for the unexpectedly short τ of DMTU in PFDMCH.…”
Intramolecular processes of deactivation of 1,3‐dimethyl‐4‐thiouracil (DMTU) from the second excited singlet (S2) (π, π*) and the lowest excited triplet (T1) (π, π*) states have been studied using perfluoro‐1,3‐dimethylcyclohexane (PFDMCH) as a solvent. The spectral and photophysical (PP) properties of DMTU in CCl4, hexane and water have also been described. For the first time, the fluorescence from S2 state DMTU has been observed. The picosecond lifetime of DMTU in the S2 state (τ S 2) in PFDMCH has been proposed to be determined by a very fast intramolecular reversible process of hydrogen abstraction from the ortho methyl group by the thiocarbonyl group. The shortening of τ S 2 in CCl4 is interpreted to be caused by the intermolecular interactions between DMTU (S2) and the solvent. Results of the phosphorescence decay as a function of DMTU concentration were analyzed using the Stern–Volmer formalism, which enabled determination of the intrinsic lifetime of the T1 state (τ0 T 1) and rate constants of self‐quenching (ksq). The lifetimes, τ0 T 1, of DMTU in PFDMCH and CCl4 are much longer than the values hitherto obtained in more reactive solvents. The PP properties of DMTU both in the S2 and T1 states have been shown to be determined by the thiocarbonyl group.
“…This is likely due to the weaker P−S bond in 2 compared to the P−O bond in 1 and the concurrent lower energy of the excited state (S 1 ). With this lower energy S 1 state, the S 1 to S 0 transition is lower in energy, as cited for thiocarbonyls vs. their parent carbonyls [20] . Furthermore, all derivatives of 2 exhibited decreased quantum yields (3–8 %) compared to their 1 counterparts (11–52 %).…”
Lawesson's reagent is one of the most common thionating reagents that has found use for several decades on a variety of carbonyl systems. Herein, we report the transformation of the 2‐λ5‐phosphaquinolin‐2‐one structure to its respective 2‐λ5‐phosphaquinolin‐2‐thione derivative. Solution‐state characterization of both P−OPh and P−Ph thio analogues, as well as solid‐state structures for the latter system, are reported.
“…Excitation of the thioketones into S 2 (260 nm), at room temperature, gives rise to both S 2 fluorescence (300−450 nm in solution, 265−320 nm in the gas phase) and phosphorescence (500−800 nm in both media). The latter includes emission from two triplets, with thermally activated phosphorescence appearing as a shoulder on the high-energy portion of the “normal” phosphorescence band. 8b, The data are given in Table , and the full spectrum for 2 is shown in Figure . 2 Total emission spectra of thiocamphor in the gas phase (solid line) and in perfluoromethylcyclohexane (dotted line): “a” = Fluorescence; “b” = phosphorescence.…”
Section: Resultsmentioning
confidence: 99%
“…The S 2 states of larger aliphatic thioketones are weakly emissive and have lifetimes that are too short to be measured. Steer and co-workers have estimated that the upper state lifetimes are less than 1 ps, based on the small quantum yields of S 2 → S 0 fluorescence in perfluoroalkanes and the radiative lifetimes derived from the fluorescence using the Strickler−Berg equation 1 Spectroscopic Data for the Aliphatic Thioketone and Ketone Groups λ max (nm)ε max (L/mol·cm)λ max (nm)ε max (L/mol·cm) 190 ∼1000 270−300 16−25 240 ∼10000 490−555 2−10 …”
The solution and gas-phase photophysical and photochemical properties of a series of bicyclic and alicyclic
thioketones (apothiocamphor (1), thiocamphor (2), thiofenchone (3), endo-5,6-trimethylene-2-norborneanthione
(4), 3,3-diethylbicyclo[3.2.1]octane-2-thione (5), 2,2-diethyl-5,5-dimethylcyclopentanethione (6), 2-ethyl-2,6,6-trimethylcyclohexanethione (7), and 2,4,4-trimethyl-3-hexanethione (8)) are reported. Photolysis in solution
typically gives rise to products arising from insertion into β, γ, and, in one case (4), δ carbons to form cyclic
thiols. This chemistry is analogous to that observed in earlier studies. Novel photochemistry is found in the
gas phase where Norrish type II products are also isolated from several substrates (1, 2,
5, 6, and 7). The
effect of the quencher gas, butane, on both the spectral and photochemical properties of 2 in the gas phase
provide evidence to support the proposal that the Norrish type II chemistry arises from initially populated
vibrationally excited levels of S2.
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