Photoexcited triplets of nitrodiphenylpolyenes hosted in channel cavities. A time resolved EPR study

The photoexcited triplet states of the three nitro substituted diphenylpolyenes 4-nitrostilbene, 4-nitrodiphenylbutadiene and 4,4'-dinitrodiphenylbutadiene included in nanostructured channel-like cavities of a PHTP-based inclusion compound have been investigated by time-resolved electron paramagnetic resonance. The inclusion compounds were synthesized by using as host dihydrotriphenylene partially hydrogenated. The same dyes have been studied also in disordered solid matrices. The triplet states produced in the inclusion compounds originate only from the ground state trans conformer, since the constraint of the host channel prevents the trans-cis photoisomerization. The simulation of all spectra in disordered matrices shows the presence of the same triplet from the trans isomer as in inclusion compounds, but also of another state with the same ZFS parameters and different spin polarization. This derives from the cis isomer through a photophysical path to the cis triplet relaxing therefore conformationally to the trans triplet. For the diphenylpolyene derivatives the effect of irradiation at two wavelengths has been studied, giving further support to this interpretation of the spectra. Comparison of the experimental ZFS parameters with the calculated ones and with those of the corresponding unsubstituted compounds shows that the nitro groups are tilted out of the pi-system plane. The tilting angle is different in the inclusion compounds with respect to the disordered matrices as a consequence of the head-to-tail interactions between nitro groups in neighbour molecules packed in the channels.