The complete active space (CAS) SCF method and multiconfigurational second-order perturbation theory (CASPT2) have been used in a theoretical analysis of the electronic spectrum of indole. The calculations comprise a large number of singlet and triplet valence and Rydberg excited states with the aim to obtain a full understanding of the electronic spectrum. In addition to the gas-phase spectra, solvatochromic shifts have been computed at the CASPT2 level for the low-lying singlet valence states using a continuum representation of the solvent with the solute in a spherical cavity. The results support the assignments of two low-lying π → π* valence singlet excited states, 1Lb and 1La, computed at 4.43 and 4.73 eV in the gas phase. The location of the 1La band origin 0.23 eV above the 1Lb band origin is in agreement with the most recent experimental studies. The most intense feature of the spectrum is obtained at 5.84 eV as a π → π* singlet excited valence state. In total 25 singlet states have been computed...

Theoretical Study of the Absorption and Emission Spectra of Indole in the Gas Phase and in a Solvent

— Quite often the primary photochemical reaction of an excited state molecule is transfer of an electron to or from another molecule in its ground state. Rates of such reactions are closely dependent on differences between ground and excited state redox potentials of the reagents. The solvent also plays an important role in stabilizing ion pairs formed by the electron transfer. This Review discusses experimental data relating rates to electrochemical energy parameters in the context of a scheme which portrays the energy and electron transactions in a unified manner. Three consequences of reaction of a singlet excited state are distinguished: (S1) quenching without detectable products, (S2) exciplex fluorescence, (S3) transient radical ion production, and energetically necessary conditions are derived for each. Similarly, four kinds of reactions involving the triplet state are distinguished, which depend on the relation between the energy of the triplet state and that of the ion pair states: (TI) rapid quenching, (T2) slow quenching, (T3) accelerated intersystem crossing and (T4) generation by reaction between radical ions of like spin. The last may be followed by electrochemiluminescence. Classes of compounds for which data are available include chlorophylls, porphyrins and a few other molecules of biological interest, aromatic hydrocarbons and their derivatives, heterocyclic systems, carbonyl compounds, dyes, and complexes of Ru and U. A Table compiling median or selected values of ground and excited state electrochemical potentials of chlorophylls, some porphyrins, and a few other compounds is presented.

https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1751-1097.1978.tb07658.x

The energetics of electron-transfer reactions of chlorophyll and other compounds*

An efficient chemodivergent strategy for visible light photocatalysis is developed. In the presence of a dicyanopyrazine-derived chromophore (DPZ) photocatalyst, aerobic photooxygenation of indoles could produce either isatins or formylformanilides in satisfactory yields by judiciously selecting inorganic salts or modulating the reaction pH. The current chemodivergent method is also effective with 2-substituted indoles, opening straightforward synthetic routes to valuable 2,2-disubstituted 3-oxindoles, formylformanilide derivatives, and benzoxazinones. Mechanistic investigations involving cyclic voltammetry studies further confirm that reaction pH influences the electrochemical properties of DPZ, thus affecting the oxidative pathway by which indoles are being transformed.

Visible Light Photocatalytic Aerobic Oxygenation of Indoles and pH as a Chemoselective Switch

Publisher Summary This chapter focuses on the chemistry of excited complexes. The term exciplex (excited complex) is used to describe an electronically excited molecular complex of definite stoichiometry. Complexes falling into this broad classification include excimers (excited dimers) (electronically excited complexes formed between identical atoms or molecules), heteroexcimers (electronically excited complexes formed between two non identical atoms or molecules), and excited charge-transfer complexes (complexes produced by excitation of ground state molecular complexes for which there is conclusive evidence for association in the ground state). The formation of intermolecular complexes in fluid solution requires that after excitation of one of the partners, collision with the other partner must occur within the lifetime of the excited state. The maximum rate constant for the process is determined by the rate constant for diffusion control in the solvent employed.

The Chemistry of Excited Complexes: a Survey of Reactions

The photodegradation of tryptophan in oxygen saturated aqueous solution sensitized by riboflavin is accompanied by the generation of the following reactive oxygen species: 1O2, OH., H2O2 and O2.-. When parallel photodegradation experiments are run with 14C-riboflavin in one case and 14C-tryptophan in the other and the irradiation products are separated by Sephadex G-15 and C18-HPLC, the generation of the following species is detected: aggregate forms of riboflavin, indolic products associated to flavins, indolic products of molecular weight higher than tryptophan, formylkynurenine, and other tryptophan photodecomposition products. The significance of the riboflavin anion radical and tryptophan cation radical as intermediates in the photoproduct formation is discussed.

Riboflavin-sensitized photoprocesses of tryptophan

— Quenching of the triplet states of the aromatic ketones (KCO), benzophenone, acetophenone and xanthone, by indole and 3-methyl indole gives rise to the neutral radicals resulting from hydrogenatom transfer with variable efficiency (40–100%). Thus in addition to the reaction,



3KCO*+RH KCOH +R.



some other quenching path or paths occur. There is no evidence for any triplet energy transfer even when this is energetically favourable, and it is suggested therefore that quenching to give ground state species following favourable charge-transfer interactions accounts for the proportion of quenching without reaction.



The spectra of the indole radicals, R., were determined and the kinetics of their decay in aerated and deaerated solution were investigated and reaction schemes proposed to explain these observations.

Quenching of triplet states of aromatic ketones by indole and 3‐methyl indole in benzene solution. evidence for hydrogen‐atom abstraction, for quenching due to favourable charge transfer interaction and for the lack of electronic energy transfer

In laser photolysis experiments N-methyl indole quenches the triplet states of several aromatic carbonyl compounds including some with lower energy triplet states with rate constants in the range (0.02–10)× 109 dm3 mol–1 s–1. Quenching by N-methyl indole is shown to be either exclusively or partially due to electronic energy transfer from either triplet acetophenone or triplet xanthone respectively, and the sensitized spectrum assigned to the triplet state of N-methyl indole is given. Quenching of the other triplet carbonyls by N-methyl indole is shown to depend on the energy predicted for the charge transfer state of each interacting pair of compounds with N-methyl indole as the potential electron donor. In ethanol solution quenching of the triplet state of xanthone by N-methyl indole is demonstrated to occur via electron transfer leading to the formation of separate ions.Pulse radiolysis of indole, 3-methyl indole and N-methyl indole in benzene solution gives short lived triplet states of these compounds, the spectra of which have been determined. Quenching of triplet N-methyl indole by various compounds with higher and lower triplet states was investigated following pulse radiolysis in benzene solution. The rate constants for quenching of triplet N-methyl indole were also shown to depend on charge transfer interactions and evidence for energy transfer was obtained only in cases where the predicted charge transfer states lie at much higher energies than the locally excited triplet states. The reasons why charge transfer interactions cause rapid relaxation of triplet exciplexes are discussed.

Anthony Garner

Papers

Quenching of triplet states of aromatic ketones by indole and 3‐methyl indole in benzene solution. evidence for hydrogen‐atom abstraction, for quenching due to favourable charge transfer interaction and for the lack of electronic energy transfer

Triplet state quenching involving charge transfer interactions between N-methyl indole and aromatic carbonyl compounds in benzene solution