R. Aaron Vogt

On the origin of ultrafast nonradiative transitions in nitro-polycyclic aromatic hydrocarbons: Excited-state dynamics in 1-nitronaphthalene

The electronic energy relaxation of 1-nitronaphthalene was studied in nonpolar, aprotic, and protic solvents in the time window from femtoseconds to microseconds. Excitation at 340 or 360 nm populates the Franck-Condon S(1)(pipi( *)) state, which is proposed to bifurcate into two essentially barrierless nonradiative decay channels with sub-200 fs lifetimes. The first main decay channel connects the S(1) state with a receiver T(n) state that has considerable npi( *) character. The receiver T(n) state undergoes internal conversion to populate the vibrationally excited T(1)(pipi( *)) state in 2-4 ps. It is shown that vibrational cooling dynamics in the T(1) state depends on the solvent used, with average lifetimes in the range from 6 to 12 ps. Furthermore, solvation dynamics competes effectively with vibrational cooling in the triplet manifold in primary alcohols. The relaxed T(1) state undergoes intersystem crossing back to the ground state within a few microseconds in N(2)-saturated solutions in all the solvents studied. The second minor channel involves conformational relaxation of the bright S(1) state (primarily rotation of the NO(2)-group) to populate a dissociative singlet state with significant charge-transfer character and negligible oscillator strength. This dissociative channel is proposed to be responsible for the observed photochemistry in 1-nitronaphthalene. Ground- and excited-state calculations at the density functional level of theory that include bulk and explicit solvent effects lend support to the proposed mechanism where the fluorescent S(1) state decays rapidly and irreversibly to dark excited states. A four-state kinetic model is proposed that satisfactorily explains the origin of the nonradiative electronic relaxation pathways in 1-nitronaphthalene.

Subpicosecond intersystem crossing in mono- and di(organophosphine)gold(I) naphthalene derivatives in solution.

Femtosecond-to-microsecond broadband transient absorption experiments are reported for Cy(3)PAu(2-naphthyl) (1), (Cy(3)PAu)(2)(2,6-naphthalenediyl) (2), and (Cy(3)PAu)(2)(2,7-naphthalenediyl) (3), where Cy = cyclohexyl. Global and target analyses of the data, based on a sequential kinetic model, reveal four spectral components. These components are assigned to (1) excited state absorption (ESA) of the ligand-centered S(1) state; (2) ESA of a receiver ligand-to-metal or metal-to-ligand charge transfer triplet state (τ(1) ≤ 300 fs); (3) ESA of the vibrationally excited, ligand-centered T(1) state (τ(3) = 7-10 ps); and (4) ESA of the relaxed T(1) state. Intersystem crossing (ISC) occurs in hundreds of femtoseconds, while internal conversion (IC) in the triplet manifold is slow (τ(2) ≈ 2 ps). The relaxed T(1) state shows biphasic decay kinetics in 2 and 3 with lifetimes of hundreds of picoseconds and hundreds of nanoseconds in air-saturated conditions, while only monophasic decay is observed in 1 under identical conditions. The primary decay pathway of the T(1) state is assigned to quenching by O(2), while the secondary channel is tentatively assigned to self-quenching or triplet-triplet annihilation. The ISC rate in 1 is not modulated significantly by the incorporation of a second heavy-atom group effecter. Instead, the position at which the second Au(I)-phosphine group is attached plays a noticeable role in the ISC rate, showing a 3-fold decrease in that of 2 compared to that of 3. The results challenge the conventional view that the rate of IC is larger than that of ISC, lending further support to the emerging kinetic model proposed for other transition-metal complexes. Gold(I) now joins the exclusive group of transition metals known to form organometallic complexes exhibiting excited-state nonequilibrium dynamics.

Excited-State Dynamics in Nitro-Naphthalene Derivatives: Intersystem Crossing to the Triplet Manifold in Hundreds of Femtoseconds

Femtosecond transient absorption experiments and density functional calculations are presented for 2-methyl-1-nitronaphthalene, 2-nitronaphthalene, and 1-nitronaphthalene in cyclohexane and acetonitrile solutions. Excitation of 2-methyl-1-nitronaphthalene at 340 nm populates the Franck-Condon singlet state, which bifurcates into two barrierless decay channels with sub-200-fs lifetimes. The primary decay channel connects the Franck-Condon singlet excited state with a receiver triplet state, whereas the second, minor channel involves conformational relaxation to populate an intramolecular charge-transfer state, as previously reported for 1-nitronaphthalene (J. Chem. Phys. 2009, 113, 224518). Conversely, the experimental and computational data for 2-nitronaphthalene shows that almost the entire Franck-Condon singlet excited-state population intersystem crosses to the triplet state in less than 200 fs due to a sizable energy barrier of ca. 5 kcal/mol that must be surmounted to access the intramolecular charge-transfer state. Our results lend support to the idea that the probability of population transfer to the triplet manifold in these nitronaphthalene derivatives is controlled not only by the small energy gap between the Franck-Condon singlet excited state and the receiver triplet state but also by the region of configuration space sampled in the singlet excited-state potential energy surface at the time of excitation. It is proposed that the ultrafast intersystem crossing dynamics in these nitronaphthalene molecules most likely occurs between nonequilibrated excited states in the strongly nonadiabatic regime.

Photophysical and photochemical properties of the pharmaceutical compound salbutamol in aqueous solutions.

Salbutamol is a potent β(2)-adrenergic receptor agonist widely used in the treatment of bronchial asthma and chronic obstructive pulmonary disease. An increasing number of studies have detected salbutamol in natural water systems worldwide. Studies have shown that sunlight degrades salbutamol resulting in the formation of products; some showing higher toxicity to bacteria Vibrio fischeri than the parent compound. In this contribution, steady-state absorption and emission techniques, high-performance liquid chromatography, and transient absorption spectroscopy are used to investigate the photochemistry of salbutamol in aqueous buffer solutions at controlled pH values. Ground- and excited-state calculations that include solvent effects are performed to guide the interpretation of the experimental results. Salbutamol is sensitive to UVB light absorption in the pH range from 3 to 12, forming products that absorb light at longer wavelengths than the parent compound. Quantum yields of degradation reveal that the deprotonated species is 10-fold more photo-active than the protonated species. In line with this result, the fluorescence quantum yield of the protonated species is more than an order of magnitude higher than that of the deprotonated species. Transient absorption spectroscopy shows that population of the triplet state occurs with a rate constant of 7.1×10(8)s(-1) in the protonated species, while a rate constant of 1.7×10(10)s(-1) is measured for the deprotonated species. While degradation of the deprotonated species is not affected by the presence of molecular oxygen, a twofold increase in the photodegradation yield of the protonated species in air-saturated conditions is observed.