IntroductionThe tautomerism of benzotriazoleBenzotriazole can exist in two tautomeric forms, 1H(1) and 2H(2) (Scheme 1), and the relative stability of these two forms has been extensively studied both experimentally and theoretically.1–10Also, the photochemical and thermal transformations of benzotriazole exhibit several interesting reaction patterns yet to be fully understood.11–16Early experimental and computational studies indicated the 1Htautomer (1) to be the more stable.1Crystallographic measurements showed that solid benzotriazole exists almost exclusively as the 1Htautomer.2Mass spectrometry investigations,3spectroscopic studies in various solvents,4as well as analysis of microwave 5and electronic absorption 6spectra, confirmed the predominance of the 1Htautomeric form.A few of the more recent studies, however, have suggested that the 2Htautomer (2) may be the more stable.7–10Investigations of the temperature effect on the gas-phase electronic absorption spectra of benzotriazole, as well as 1- and 2-methylbenzotriazoles, indicated that the equilibrium favours the 2Htautomer.7Spectroscopic studies of benzotriazole in jet-cooled molecular beams showed the presence of either only the 2Htautomer 8,9or both tautomeric forms of benzotriazole.10Finally, a recent analysis of rotational band contours of the N–H stretching vibration at six different temperatures 17has led to the conclusion that the 1Htautomer is stabilized with respect to the 2Htautomer, but only by 5 kJ mol−1.Failure to detect the 2Htautomer of benzotriazole in the microwave spectrum 5was very probably due to its possessing a much lower dipole moment than the 1Htautomer. Dipole moments for 1H- and 2H-benzotriazole have been estimated theoretically, and are predicted to lie in the ranges 4.09–4.65 and 0.30–0.77 D, respectively.7Comparison can also be made with the non-benzannelated analogue, 1,2,3-triazole, for which the experimentally determined dipole moments are 4.38 and 0.218 D for the 1Hand 2Htautomers, respectively.18The photochemistry of benzotriazoleWhile the 2Htautomer of benzotriazole is considered to be relatively stable photochemically, the 1Htautomer certainly can undergo light-induced transformations.11The potential applications of benzotriazole photoreactivity are well illustrated by the recent development of a new class of light-activatable DNA cleaving agents containing the benzotriazole group.12The cleaving action is based on photoinducible nitrogen–nitrogen bond breaking in the triazole ring, followed by nitrogen loss. The resulting carbenes or radicals are then capable of hydrogen abstraction and thereby serve as potential agents for DNA cleavage. It has been pointed out, however, that photolysis of benzotriazole in solution tends to give poor yields of isolable products from nitrogen elimination and that this application is therefore remarkable.16The primary intermediate produced by the N–NH bond scission is expected to be the diazoimine compound3, as shown inScheme 1. This intermediate was first recognized experimentally by an electronic absorption withλmaxat 423 nm and a characteristic, broadν(CNN) absorption in the IR, with a maximum at 2070 cm−1, both of which arose during 254 nm photolysis of benzotriazole in an EPA glass at 77 K (EPA is a mixture of diethyl ether, isopentane and ethanol).13The UV and IR absorptions were bleached by subsequent irradiation of the glass with 420 nm light, but no other IR bands belonging to3were recorded at that time, no doubt because of strong solvent and cell-window absorptions. In contrast, broad band photolysis (λ > 300 nm) of benzotriazole in an argon matrix at 10 K apparently did not give rise to detectable amounts of3,19presumably owing to secondary photolysis.Although the primary intermediate generated in the course of the photodecomposition of benzotriazole has been identified, the final products of the transformations strongly depend on experimental conditions and, in particular, the reaction medium. In the previous study utilizing Ar matrices,19the main product was ketenimine5(ν(CCN) at 2044 cm−1); while, in the gas phase,20the main product was found to be 1-cyanocyclopentadiene (6). On the other hand, in reactive condensed media, such as solutions 21or low temperature glasses (e.g.MeOH–EtOH or EPA at 77 K),13various products have been observed, arising eitherviainsertion reactions (e.g.methoxy- or ethoxyaniline) orviahydrogen-atom abstraction (aniline).The various products obtained from photolysis of benzotriazole can all be rationalized on the basis ofScheme 1. The primary intermediate, diazoimine3, is not stable and, depending on the conditions in which it is generated, can undergo facile thermal or photochemical decomposition to give the transient carbene4and a nitrogen molecule. This loss of N2is an example of the general case observed for the decomposition of diazo compounds.14,15Triplet4has been detected by means of EPR spectroscopy,22but has so far eluded detection by other spectroscopic techniques. In the presence of reactive species, such as protic solvents,4can be trapped as insertion or H-abstraction products. Alternatively,4may undergo a photochemical or thermal rearrangement, analogous to the Wolff rearrangement, to give ketenimine5and ultimately its tautomer6.Very recently it has been shown in one of our laboratories that, at ambient temperature in solution, diazoimine3can undergo thermal recyclization to benzotriazole, a process which takes place within a few nanoseconds at room temperature in aprotic solvents.16Under these conditions, loss of N2is a minor side reaction.Thus, in the previous studies of the photolysis of benzotriazole, all of the intermediates and products shown inScheme 1have been detected spectroscopically, but only in separate experiments under widely differing conditions. Moreover, the diazoimine intermediate (3) has been identified solely on the basis of an electronic absorption and itsν(CNN) band in an EPA glass, and a fuller characterization would be desirable.Matrix isolation as a technique for studying benzotriazole and its photolysisThe issue of the equilibrium between the two tautomeric forms of benzotriazole is an interesting problem for investigation by matrix isolation, a technique from which some advantages are expected. Firstly, owing to the lack of solvent absorptions in typical matrices such as Ar and N2, more complete IR spectra of the various intermediates and products should be obtainable than in,e.g., organic glasses. Secondly, very rapid condensation of benzotriazole vapour with an excess of an inert gas on the low temperature surface may preserve the tautomer ratio of the equilibrium at, or near, room temperature. In addition, because of the narrow bandwidths and lack of rotational fine structure typical in matrix IR spectra, careful analysis of the spectra of benzotriazole prior to and after photolytic treatment may resolve the characteristic absorption bands which belong to particular tautomeric forms of benzotriazole. Finally, the matrix-isolation technique in conjunction with selective photolysis can be used to study the reactions of isolated benzotriazole molecules in a ‘step-by-step’ fashion, and thus help to establish the reaction pathways and the ultimate products.In this paper we report the results of matrix-isolation IR and UV–visible studies of the photolysis of benzotriazole, in which some of the expected advantages of the matrix-isolation technique have been realized.