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

Abstract

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

Structure and dynamics of the triplet state of oligothiophenes in isotropic and partially oriented matrices

“…This approximation simplifies the calculation, and yields good results for planar p conjugated systems. 12 By retaining only the two-centers terms the equation becomes:…”

“…The much higher resolution of these techniques with respect to optical spectroscopies give clear cut information on the molecular structure in the excited state by comparing experimental and calculated principal values of the fine dipolar tensor [zero-field splitting (ZFS) parameters]. 12,13 Moreover, the three triplet sublevels are generated with non Boltzmann populations, and consequently the time resolved EPR spectra are spin polarized. The values of non equilibrium populations are the fingerprint of the photophysical path of triplet formation.…”

“…The values of non equilibrium populations are the fingerprint of the photophysical path of triplet formation. 12 However very few studies have dealt with the magnetic properties of the lowest triplet state investigated by means of EPR techniques. A series of end-capped (n ¼ 2-6, where n is the number of rings) and of dodecyl-linked a-oligothiophenes (n ¼ 3, 6 and 8) have been studied by Bennati et al with Echo-EPR in glassy toluene at 4 K. 14,15 The comparison with calculated values indicated a planar molecular geometry and all trans conformations for n 4 3.…”

Time-resolved EPR spectra of the triplet excited states of diaminoacridine guests in polar potassium hydrogen phthalate single crystals

“…The TR-EPR spectra are also spin polarized, and the values of non-equilibrium populations of the triplet sub-levels characterize the photophysical path of triplet formation, a unique advantage of this method. 5,6 Here, we describe, using TR-EPR spectroscopy, the properties of triplet excited states of an acridine derivative contained in three distinct growth sectors of the crystal potassium acid phthalate (KAP). The photophysics of dyes within crystalline hosts often differs from polymers, glasses or pure dye crystals.…”

“…For instance, we have shown by TR-EPR that the tight confinement of dye guests in molecular clathrates alters ground and excited state conformations thereby affecting the energies and lifetimes of triplet states. [3][4][5] KAP is distinct in that it does not have preformed or self-assembled pores or channels.…”