Reversible photoisomerization of 2,3,6,7-tetracyano-1,4,5,8-tetraazanaphthalene (TTN) in low-temperature rare-gas matrices was investigated by electronic absorption and Fourier transform infrared (FTIR) spectroscopies aided by-density-functional-theory (DFT) calculations. The reactant, TTN, was found to change to 4,5,9,10-tetracyano-1,3,6,8-tetraazacyclodeca-1,2,4,6,7,9-hexaene (TTH) upon ultraviolet-light irradiation, while the product, TTH, returns to the original reactant, TTN, upon visible-light irradiation. The time-dependent DFT calculation led to the conclusion that the forward and backward photoisomerizations occur by way of the S-3-S-0 transition of TTN and the SI-SO transition of TTH, respectively. (C) 2003 Elsevier Science B.V. All rights reserved.