Spin-lattice relaxation and ODMR linenarrowing of the photoexcited triplet state of pyrene in polycrystalline Shpolʹskii hosts and glassy matrices Original Research Article

Phosphorescence and ODMR linewidths, and spin-lattice relaxation (SLR) rates were measured at pumped helium temperatures for the triplet excited state of pyrene doped in several n-alkane polycrystalline hosts as well as in 3-methylpentane (3-MP) glass, in order to test the expectation that the optimum Shpolʹskii matrix for pyrene should lead to the slowest SLR rates. Among the series of n-alkanes, n-hexane showed the best Shpolʹskii effect for pyrene in terms of the narrowest phosphorescence and weakest guest–host phonon coupling. The 2|E| and |D+E| ODMR linewidths for pyrene were minimized in n-hexane as well. Microwave saturated phosphorescence decay and fast passage methods were used to measure the SLR rates in the range 1.75–4.2 K. SLR appeared to be slowest in n-octane rather than n-hexane, but was significantly faster in n-decane and 3-MP. A marked anisotropy was observed in all hosts, in which the in-plane (x ⇄ y) relaxation rate was 20–40 times the other rates. While this was suggestive of a mechanism in which SLR occurs by means of thermal promotion to a local phonon state with rotated spin axes, the observed activation energy was too small (2–3 cm−1) to be consistent with such a mechanism. In n-hexane, the relaxation appears to proceed by means of a direct process (rate αT1) in this temperature range. In the 3-MP glass, relaxation was faster than in the polycrystalline hosts, and followed a power law temperature dependence with an exponent of 2.4±0.2, in agreement with earlier studies of naphthalene derivatives in the same host, indicating that a direct two-level-system phonon mechanism is important in this glassy host.