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 (TMP D) in alcohols at room temperature was studied by time-resolved electr on paramagnetic resonance (TREPR) spectroscopy at Q-band (35 GHz). Dir ect photolysis of TMPD in 2-propanol led to spectra of the solvated el ectron (e(solv)(-)) and the cation radical of TMPD (TMPD(.+)). The spe ctra show emission due to the triplet mechanism of chemically induced dynamic electron spin polarization, along with an E/A pattern due to t he radical pair mechanism, consistent with the existence of a triplet precursor. It is found that the exchange interaction (J) in this radic al pair has a negative sign. The light intensity dependence of the e(s olv)(-) signal shows that the triplet-state ionization pathway is biph otonic. Photoionization through the singlet state, considered to be do minant in alcohol solution from flash photolysis studies, does not app ear in the TREPR spectra without an acceptor for e(solv)(-). By use of 2-bromo-2-methylpropionic acid as electron accepters and 1,3-cyclohex adiene as a triplet quencher, the photoionization pathway through the excited singlet state was isolated. The TREPR signal from singlet phot oionization is found to increase approximately linearly with increasin g light intensity until it saturates due to biphotonic ionization proc esses. The light intensity dependence for both ionization channels is simulated and discussed using a kinetic model. The consequences of exc iplex formation in the singlet pathway are also discussed.