Paramagnetic Relaxation of Triplet Molecules Randomly Oriented in a Rigid-Glass Matrix

Investigations have been made of the paramagnetic relaxation of the triplet molecules zinc 2,2′-biquinoline randomly oriented in a rigid-glass matrix of 2-methyltetrahydrofuran. The saturation and relaxation behavior of these molecules is qualitatively consistent with the inhomogeneous nature of the ΔM=1 line-shape. The relaxation times are such that a hole burned in the line can recover by spin—lattice relaxation and by cross relaxation to adjacent spin packets. The temperature dependence of the spin—lattice relaxation time is given by T1−1=1.37×105 exp(−22/T)+2.94T2. We interpret the exponential as indicating an Orbach process between the ground-state triplet and the first excited singlet. The Orbach exponent implies that the triplet—singlet separation, which is determined by the value of the intramolecular exchange integral, is 15.5±1 cm−1. An expression for T1 similar to that derived by Scott and Jeffries has been used to identify the dominant interaction participating in this process. From crude estimates of the spin-vibronic matrix elements it seems unlikely that this interaction is large enough to account for the experimental value of T1=1.3 msec at 4.2°K. The only interactions which are large enough to explain this value are second-order vibronic effects which involve singlet—triplet mixing through static spin—orbit coupling. At this time we can offer no explanation of the T2 dependence which predominates below 2°K.