Photoreactivities of two kinds of bimolecular crystals formed from acridine and phenothiazine

A yellow crystal 3 and a red crystal 4 simultaneously crystallize from a solution of acridine 1 and phenothiazine 2 in acetonitrile. Similar N · · · H–N hydrogen bonding pairs of 1 and 2 are formed in the two crystals. The 3∶4 crystal 3 has a slightly uncommon molecular arrangement of which four acridine molecules out of twelve are disordered in the unit cell. The 1∶1 crystal 4 has a more rigid packing of the hydrogen bonding pairs. Irradiation of 3 gives a photoadduct 5 as the sole product, whereas 4 shows almost no photoreactivity. The occurrence of photoinduced electron transfer has been confirmed by the measurement of transient absorption spectra of the crystals 3 and 4. However, the two-stage decay of the transient absorption of 3 with lifetimes of 2 and 50 ps is different from that of 4 with a single short lifetime of 2 ps. Upon irradiating 3, electron transfer, proton transfer and subsequent radical coupling occur over the shortest distance of 4.1 A between the preradical carbon and nitrogen atoms in the crystal lattice followed by dehydrogenation to afford the product 5. In the case of the red crystal 4, the radical coupling is difficult because of the longer coupling distance of 5.6 A and the more restricted movement in the rigid crystal lattice. The results indicate that the observed photochemical behaviour of 3 and 4 are controlled by the molecular arrangement in the crystal lattice.