Hoi Ling Luk

Photochemistry of 2-Naphthoyl Azide. An Ultrafast Time-Resolved UV―Vis and IR Spectroscopic and Computational Study

The photochemistry of 2-naphthoyl azide was studied in various solvents by femtosecond time-resolved transient absorption spectroscopy with IR and UV-vis detection. The experimental findings were interpreted with the aid of computational studies. Using polar and nonpolar solvents, the formation and decay of the first singlet excited state (S(1)) was observed by both time-resolved techniques. Three processes are involved in the decay of the S(1) excited state of 2-naphthoyl azide: intersystem crossing, singlet nitrene formation, and isocyanate formation. The lifetime of the S(1) state decreases significantly as the solvent polarity increases. In all solvents studied, isocyanate formation correlates with the decay of the azide S(1) state. Nitrene formation correlates with the decay of the relaxed S(1) state only upon 350 nm excitation (S(0) → S(1) excitation). When S(n) (n ≥ 2) states are populated upon excitation (λ(ex) = 270 nm), most nitrene formation takes place within a few picoseconds through the hot S(1) and higher singlet excited states (S(n)) of 2-naphthoyl azide. The data correlate with the results of electron density difference calculations that predict nitrene formation from the higher-energy singlet excited states, in addition to the S(1) state. For all of these experiments, no recovery of the ground state was observed up to 3 ns after photolysis, which indicates that both internal conversion and fluorescence have very low efficiencies.

Direct observation of a sulfonyl azide excited state and its decay processes by ultrafast time-resolved IR spectroscopy.

The photochemistry of 2-naphthylsulfonyl azide (2-NpSO(2)N(3)) was studied by femtosecond time-resolved infrared (TR-IR) spectroscopy and with quantum chemical calculations. Photolysis of 2-NpSO(2)N(3) with 330 nm light promotes 2-NpSO(2)N(3) to its S(1) state. The S(1) excited state has a prominent azide vibrational band. This is the first direct observation of the S(1) state of a sulfonyl azide, and this vibrational feature allows a mechanistic study of its decay processes. The S(1) state decays to produce the singlet nitrene. Evidence for the formation of the pseudo-Curtius rearrangement product (2-NpNSO(2)) was inconclusive. The singlet sulfonylnitrene (1)(2-NpSO(2)N) is a short-lived species (τ ≈ 700 ± 300 ps in CCl(4)) that decays to the lower-energy and longer-lived triplet nitrene (3)(2-NpSO(2)N). Internal conversion of the S(1) excited state to the ground state S(0) is an efficient deactivation process. Intersystem crossing of the S(1) excited state to the azide triplet state contributes only modestly to deactivation of the S(1) state of 2-NpSO(2)N(3).

Direct Observation of Acyl Azide Excited States and Their Decay Processes by Ultrafast Time Resolved Infrared Spectroscopy

The photochemistry of three carbonyl azides was studied by ultrafast time-resolved IR spectroscopy. Benzoyl, 2-naphthoyl, and pivavoyl azides are promoted to upper excited states S(n) with 270 nm excitation in chloroform. The S(n) states decay in 300 fs to form both the carbonylnitrenes and the S(1) excited states. The decay of the S(1) states of the carbonyl azides correlates with the growth of isocyanates. Formation of carbonylnitrene from S(1) is at most a minor process if it happens at all. The quantum yields of azide decomposition of these azides with 270 nm light are close to unity in chloroform.

Mechanistic aspects of ketene formation deduced from femtosecond photolysis of diazocyclohexadienone, o-phenylene thioxocarbonate, and 2-chlorophenol.

The photochemistry of diazocyclohexadienone (1), o-phenylene thioxocarbonate (2), and 2-chlorophenol (3) in solution was studied using time-resolved UV-vis and IR transient absorption spectroscopies. In these three cases, the same product cyclopentadienyl ketene (5) is formed, and two different mechanistic pathways leading to this product are discussed: (a) rearrangement in the excited state (RIES) and (b) a stepwise route involving the intermediacy of vibrationally excited or relaxed carbene. Femtosecond UV-vis detection allows observation of an absorption band assigned to singlet 2-oxocyclohexa-3,5-dienylidene (4), and this absorption feature decays with an ∼30 ps time constant in hexane and acetonitrile. The excess vibrational energy present in nascent carbenes results in the ultrafast Wolff rearrangement of the hot species. IR detection shows that photoexcited o-phenylene thioxocarbonate (2) and 2-chlorophenol (3) efficiently form the carbene species while diazocyclohexadienone (1) photochemistry proceeds mainly by a concerted process.

Direct observation of a carbene-alcohol ylide.

Carboethoxycarbene reacts with methanol-OD to form an ylide. The formation and decay of this ylide was monitored by ultrafast time-resolved IR spectroscopy. The formation and decay of the ylide is linearly dependent on the concentration of methanol-OD in acetonitrile with second-order rate constants of ylide formation (8.4 × 10(9) M(-1) s(-1)) and decay (1.4 × 10(9) M(-1) s(-1)). Similar results were obtained with 1-butanol.

Mechanistic Study of the Photochemical Hydroxide Ion Release from 9-Hydroxy-10-methyl-9-phenyl-9,10-dihydroacridine

The excited-state behavior of 9-hydroxy-10-methyl-9-phenyl-9,10-dihydroacridine and its derivative, 9-methoxy-10-methyl-9-phenyl-9,10-dihydroacridine (AcrOR, R = H, Me), was studied via femtosecond and nanosecond UV-vis transient absorption spectroscopy. The solvent effects on C-O bond cleavage were clearly identified: a fast heterolytic cleavage (τ = 108 ps) was observed in protic solvents, while intersystem crossing was observed in aprotic solvents. Fast heterolysis generates 10-methyl-9-phenylacridinium (Acr(+)) and (-)OH, which have a long recombination lifetime (no signal decay was observed within 100 μs). AcrOH exhibits the characteristic behavior needed for its utilization as a chromophore in the pOH jump experiment.

Toward Organic Photohydrides: Excited-State Behavior of 10-Methyl-9-phenyl-9,10-dihydroacridine

The excited-state hydride release from 10-methyl-9-phenyl-9,10-dihydroacridine (PhAcrH) was investigated using steady-state and time-resolved UV/vis absorption spectroscopy. Upon excitation, PhAcrH is oxidized to the corresponding iminium ion (PhAcr(+)), while the solvent (acetonitrile/water mixture) is reduced (52% of PhAcr(+) and 2.5% of hydrogen is formed). The hydride release occurs from the triplet excited state by a stepwise electron/hydrogen-atom transfer mechanism. To facilitate the search for improved organic photohydrides that exhibit a concerted mechanism, a computational methodology is presented that evaluates the thermodynamic parameters for the hydride ion, hydrogen atom, and electron release from organic hydrides.

The photochemistry of 4,5-carbomethoxy-1,2,3-thiadiazole: direct observation of thiirene formation and its decay in solution.

The photochemistry of 4,5-carbomethoxy-1,2,3-thiadiazole in solution was studied at room temperature with use of UV-vis and IR transient absorption spectroscopies (λ(ex) = 266 nm). Ultrafast time-resolved techniques demonstrate that there is a very fast rise (<0.4 ps) of bis(carbomethoxy)thiirene in acetonitrile, and that it is the only intermediate formed. The lifetime of the thiirene is limited by dimerization to eventually form tetra(carbomethoxy)thiophene.

Excited-state hydroxide ion transfer from a model xanthenol photobase.

This article reports a study of excited-state hydroxide ion release from a model xanthenol photobase, XanOH. The driving force for the reaction was tuned using solvent mixtures with varying water/acetonitrile ratios, and the kinetics of the reaction was monitored using ultrafast pump-probe spectroscopy. The intrinsic barrier for the heterolysis was evaluated using Marcus and bond-energy bond-order (BEBO) models. The obtained value (ΔG(o)(#) = 10.17-10.80 kcal/mol) is significantly higher than the intrinsic barriers found for the proton release from previously studied photoacids. These results were discussed in terms of the difference in structures of solvated H(+) and OH(-) ions.

Reactivity of Hydroxyl Radical in Nonaqueous Phases: Addition Reactions

The effect of ring substitution on the kinetics of reaction of arenes, heterocycles, and alkenes with hydroxyl radical is investigated in terms of reactivity and selectivity, using laser flash photolysis (LFP) in acetonitrile solution. The LFP data indicate that charge-transfer contributions in the transition state play an important role in dictating reactivity, and there is a correlation between the experimental and calculated ionization potentials of the arenes and alkenes and their respective reactivities. The reactivity observed for arenes in acetonitrile exhibits a much greater sensitivity toward substitution on the ring than in water, and therefore aqueous data cannot be used to predict reactivity in nonaqueous environments. Nonaqueous solution data may be predictable from gas phase data, and vice versa.