PHOTOREACTIVITIES OF 2 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 cry stals. 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 prod uct, whereas 4 shows almost no photoreactivity. The occurrence of phot oinduced electron transfer has been confirmed by the measurement of tr ansient absorption spectra of the crystals 3 and 4. However, the two-s tage 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 Angstrom betw een the preradical carbon and nitrogen atoms in the crystal lattice fo llowed by dehydrogenation to afford the product 5. In the case of the led crystal 4, the radical coupling is difficult because of the longer coupling distance of 5.6 Angstrom and the more restricted movement in the rigid crystal lattice. The results indicate that the observed pho tochemical behaviour of 3 and 4 are controlled by the molecular arrang ement in the crystal lattice.