Laser flash photolysis of 2-methyl-1-phenylbut-3-en-1-one (1) conducted at irradiation wavelengths of 266 and 308 nm results in the formation of triplet 1,2-biradical 2 that has λmax at 370 and 480 nm. Biradical 2 is formed with a rate constant of 1.1 × 107 s–1 and decays with a rate constant of 2.3 × 105 s–1. Isoprene-quenching studies support the notion that biradical 2 is formed by energy transfer from the triplet-excited state of the ketone chromophore of 1. Density functional theory calculations were used to verify the characterization of triplet biradical 2 and validate the mechanism for its formation. Thus, it has been demonstrated that intramolecular sensitization of simple alkenes can be used to form triplet 1,2-biradicals with the two radical centres localized on the adjacent carbon atoms.

This chapter describes how intramolecular sensitization has been used to successfully form triplet vinylnitrene intermediates from vinyl azide, isoxazole, and azirine compounds. Triplet vinylnitrenes have been thoroughly characterized in cryogenic matrices using UV/vis absorption, infrared, and electron spin resonance spectroscopies. Electron spin resonance spectroscopy shows that vinylnitrenes have a significant 1,3‐biradical character, which is further supported by density functional theory calculations. Laser flash photolysis, which has allowed the direct detection of triplet vinylnitrenes in solution, reveals that they are short‐lived intermediates with lifetimes on the order of a few microseconds. Vinylnitrenes decay efficiently by intersystem crossing to form products because their 1,3‐biradical character renders their vinylic CC bond flexible, which enhances intersystem crossing. At cryogenic temperatures, flexible triplet vinylnitrenes are not stable and intersystem cross to form products. Nevertheless, triplet vinylnitrenes can be stabilized by limiting the flexibility of the vinylic CC bond, which renders them stabile in cryogenic matrices. Thus, they are promising building blocks for high‐spin assemblies. Furthermore, as stabilized vinylnitrenes can also be employed in bimolecular reactions, they have potential for use in various synthetical applications.

Solid‐state photolysis of 3‐azido‐1,3‐diphenyl‐isobutyrophenone 1 results in selective formation of isobutyrophenone 2 and benzonitrile 3. X‐ray structure analysis of azide 1 demonstrates that the conformer (1Cr) adopted for packing in the crystal lattice has the carbonyl moiety perpendicular to the α‐phenyl group (Ph‐C=O torsional angle of 85°). Laser flash photolysis of azide 1 nanocrystals allows direct detection of the lowest excited triplet ketone (TK) of 1Cr (λmax≈475 nm). Thus, in crystals, 1 is proposed to react via the TK state to cleave the Cβ−Cγ bond. Time‐dependent density functional theory (TD‐DFT) calculations verify that the energy of the TK of 1Cr is 79 kcal mol−1 above the ground state (S0), and thus TK has sufficient energy for cleavage of the Cβ−Cγ bond to form products 2 and 3, further supporting this solid‐state reaction mechanism.

Although alkyl azides are known to typically form imines under direct irradiation, the product formation mechanism remains ambiguous as some alkyl azides also yield the corresponding triplet alkylnitrenes at cryogenic temperatures. The photoreactivity of 3-azido-3-phenyl-3H-isobenzofuran-1-one (1) was investigated in solution and in cryogenic matrices. Irradiation (k = 254 nm) of azide 1 in acetonitrile yielded a mixture of imines 2 and 3. Monitoring of the reaction progress using UV-Vis absorption spectroscopy revealed an isosbestic point at 210 nm, indicating that the reaction proceeded cleanly. Similar results were observed for the photoreactivity of azide 1 in a frozen 2-methyltetrahydrofuran (mTHF) matrix. Irradiation of azide 1 in an argon matrix at 6 K resulted in the disappearance of its IR bands with the concurrent appearance of IR bands corresponding to imines 2 and 3. Thus, it was theorized that azide 1 forms imines 2 and 3 via a concerted mechanism from its singlet excited state or through singlet alkylnitrene 1 1N, which does not intersystem cross to its triplet configuration. This proposal was supported by CASPT2 calculations on a model system, which suggested that the energy gap between the singlet and triplet configurations of alkylnitrene 1N is 33 kcal/mol, thus making intersystem crossing inefficient.