The photochromic dyad 1c bears two DHP units in the axial positions of the central NDI moiety. Conjugate 1c shows intense absorption bands at 350 nm ( = 156,700 M -1 .cm -1 ), 390 nm ( = 128100 M -1 .cm -1 ), 484 nm ( = 18400 M -1 .cm -1 ) and 649 nm ( = 2800 M -1 .cm -1 ). Theoretical investigation by Boggio-Pasqua et al. has shown that the DHP absorption band in the region of 500 is responsible for its isomerization to the CPD form. The CPDphotoisomer does not absorb in this region of the spectra; therefore, no reverse isomerization (CPD → DHP) is possible under exposure at >500 nm. Our compound, that has extended 𝝿 conjugation because of the phenyl group has an absorption band from 418 nm extending its tail up to 540 nm. The tail portion of the spectra of 1c at 525 nm has a weak absorbance with an extinction coefficient of 140 M -1 .cm -1 (Figure ). Indeed, an intense green solution of 1c (10M in CHCl3:CH3CN 1:1, degassed with N2) turned pale yellowish-green when exposed to the light (525 nm, 43 W) suggesting the formation of the ring open isomer 1o in a 62:38 ratio of 1o:1c (Figure ). The successive decay of four absorption bands at 350 nm, 390 nm, 484 nm, and 649 nm and simultaneous enhancement of the absorption at 265 nm in the presence of an isosbestic point at 312 nm indicated the isomerization from planar DHP to the open benzenoid form (Figure ).