H. Koshima et al., PHOTOREACTIVITIES OF 2 KINDS OF BIMOLECULAR CRYSTALS FORMED FROM ACRIDINE AND PHENOTHIAZINE, Perkin transactions. 2, (10), 1997, pp. 2033-2038
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.