Ultrafast spectral dynamics of structurally modified photochromic indolo[2,1-b][1,3]benzoxazines

Combined Picosecond Time-Resolved UV-Vis and NMR Techniques Used for Investigation of the Excited State Intramolecular Triplet-Triplet Energy Transfer

The phenomenon of the intramolecular triplet-triplet (T-T) energy transfer observed for spiro[9,10-dihydro-9-oxoanthracene-10,2'-5',6'-benzindan] (AN) molecule was investigated using stationary and time-resolved techniques in the UV/vis spectral region. The rate constant for energy transfer from anthrone chromophore to the triplet state localized on the naphthalene subunit of AN molecule is 2.8 × 10¹⁰ s^-1. NMR spectroscopy is rarely used for investigation of molecules in the electronically excited states. Here, we propose ¹H NMR combined with UV laser irradiation as a useful method for the recognition of an electron spin densities distribution in the excited triplet state that exists for tens of microseconds in the liquid phase. The direct registration of the ¹H NMR signals from molecules in the excited triplet state was not possible due to its short lifetime. However, even the short interaction between unpaired electrons and nuclear spins leads to the changes in the NMR spectrum. The analysis of difference NMR spectra delivers information about the electron spin densities distribution over the skeleton of the molecule in the excited triplet state. In order to understand the nature of the excited states involved in the triplet-triplet energy transfer process, quantum chemical calculations were performed.

Redefining the established understanding of excitation dynamics of photochromic oxazines

Since the introduction of photochromic indolo-benzoxazines, difference absorption of these photoexcited compounds has been assigned to the ground state of the ring-opened isomer. This assignment relies on the alleged resemblance of the spectra of photo- and chemically induced forms. In this paper, we expose the issue of the discrepancy between the absorption spectra of photoproducts and the corresponding chemically opened forms. As a result, a substantial change in the current explanation of photodynamics of photochromic oxazines is proposed. The spectral features earlier ascribed to the photoproduct are suggested to arise due to the absorption of the triplet state. This hypothesis was tested and confirmed in acetonitrile by measuring the effect of oxygen quenching of photoproduct states. In view of this interpretation, light-induced ring opening does not occur in indolo-benzoxazines dissolved in acetonitrile, and, consequently these molecules should no longer be regarded as molecular switches. On the other hand, we show that methanol solution under UV light does produce small amounts of ring-opened form of the molecule.