Photoluminescence spectrum of tetracyanoquinodimethane crystals at 25 K: temperature and pressure dependence

Using 488.0 and 514.5 nm laser radiation, we have obtained the luminescence spectrum of excited tetracyanoquinodimethane (TCNQ) crystals from 10 to 36 K at a pressure of 0 GPa. At 10 K, the luminescent peaks occur at 14 100, 15 000, 15 700 (±100), 16 300, 16 600, 16 800, and 17 200 (±50) cm−1. The temperature shift of the spectrum is −16 ± 7 cm−1∙K−1. At a constant temperature of 25 K, we followed the luminescent peaks as a function of pressure up to 8.5 GPa. The pressure shift of the luminescence, at low pressures, is −100 ± 50 cm−1∙GPa−1. We estimate that the volume-dilation contribution to the total temperature dependence is at most +2 cm−1∙K−1, in which case the electron–phonon-interaction contribution is −18 cm−1∙K−1. We have also examined the Raman spectrum at 25 K and have found the 1389 cm−1 ν4 (C=C) wing-stretching vibration of TCNQ−; its pressure behaviour is similar to that observed in TCNQ salts. The measurements were made on crystals grown very differently in two separate laboratories, and the effects induced by temperature or pressure were reversible. We attribute the luminescence to the 2B2g → 2B1μ transition of the TCNQ− anion radical, whose presence is due to impurities having a concentration of less than 0.01%.