Geminate charge recombination in the photoionization of N,N,N′,N′-tetramethyl-p-phenylenediamine (TMPD) in various solvents

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Abstract
The photoionization of N,N,N′,N′‐tetramethyl‐p‐phenylenediamine (TMPD) dissolved in n‐hexane (HEX), 2,2,4‐trimethylpentane (TMP), and tetramethylsilane (TMS) is studied using measurements of single‐photon photoconductivity. Quantum yields for free carrier production—extrapolated to zero applied field—are reported for the energy range 4.4–7.5 eV. The applied electric field dependence of the quantum yields is measured up to fields approaching 2×107 V/m and interpreted via Onsager’s theory of geminate recombination. The low‐field, slope/intercept predictions of the theory are obeyed quantitatively for HEX and TMP as solvent but fail by 25% for TMS. Various distribution functions for the separations of geminate pairs are used to generate predicted field dependence curves, which are compared with the high‐field data. A broad distribution of thermalization lengths is definitely required to fit the TMP and TMS data, but the TMS result is suspect because of the observed failure of Onsager theory in that solvent. Thermalization lengths generally increase with increasing photon energy. Estimates of the photoionization (geminate pair) quantum yield are also made. For TMP, the geminate pair quantum yield exhibits maxima at 5.8 and 6.9 eV.