Photoionization of N,N,N',N'-tetramethylphenylenediamine studied by Q-band time-resolved EPR spectroscopy. Separation of singlet and triplet ionization channels

The photoionization of N,N,N,N'-tetramethylphenylenediamine amine (TMPD) in alcohols at room temperature was studied by time-resolved electron paramagnetic resonance (TREPR) spectroscopy at Q-band (35 GHz). Direct photolysis of TMPD in 2-propanol led to spectra of the solvated electron (e(solv)(-)) and the cation radical of TMPD (TMPD(.+)). The spectra show emission due to the triplet mechanism of chemically induced dynamic electron spin polarization, along with an E/A pattern due to the radical pair mechanism, consistent with the existence of a triplet precursor. It is found that the exchange interaction (J) in this radical pair has a negative sign. The light intensity dependence of the e(solv)(-) signal shows that the triplet-state ionization pathway is biphotonic. Photoionization through the singlet state, considered to be dominant in alcohol solution from flash photolysis studies, does not appear in the TREPR spectra without an acceptor for e(solv)(-). By use of 2-bromo-2-methylpropionic acid as electron accepters and 1,3-cyclohexadiene as a triplet quencher, the photoionization pathway through the excited singlet state was isolated. The TREPR signal from singlet photoionization is found to increase approximately linearly with increasing light intensity until it saturates due to biphotonic ionization processes. The light intensity dependence for both ionization channels is simulated and discussed using a kinetic model. The consequences of exciplex formation in the singlet pathway are also discussed.