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.