Luigi Pasimeni

Efficient Charge Separation in Porphyrin-Fullerene-Ligand Complexes

Photoprocesses associated with the complexation of a pyridine-functionalized C60 fullerene derivative to ruthenium- and zinc-tetraphenylporphyrins (tpp) have been studied by time-resolved optical and transient EPR spectroscopies. It has been found that upon irradiation in toluene, a highly efficient triplet-triplet energy transfer governs the deactivation of the photoexcited [Ru(tpp)], while electron transfer (ET) from the porphyrin to the fullerene prevails in polar solvents. Complexation of [Zn(tpp)] by the fullerene derivative is reversible and, following excitation of the [Zn(tpp)], gives rise to very efficient charge separation. In fluid polar solvents such as THF and benzonitrile, radical-ion pairs (RPs) are generated both by intramolecular ET inside the complex and by intermolecular ET in the uncomplexed form. Charge-separated states have lifetimes of about 10 micros in THF and several hundred of microseconds in benzonitrile at room temperature.

Synthesis of N-acylated fulleropyrrolidines: New materials for the preparation of Langmuir-Blodgett films containing fullerenes

The synthesis of a series of new C60 derivatives of interest for their potential in the study of LB properties of fullerenes is reported. Besides that of compounds 4b and 4c, featuring fatty acid chains, the preparation of the well-characterized, perfluorinated fulleropyrrolidine 4d is described. A preliminary investigation of LB behavior of the N-acetyl compound 4a gives very promising results.

Structural and spectroscopic characterization of Cu(II) [tetrakis-(3,3-dimethyl-l-butoxycarbonyl)] phthalocyanine thin films deposited by the Langmuir—Blodgett technique

Abstract Langmuir-Blodgett (LB) films of Cu(II) [tetrakis(3,3-dimethyl-l-butoxycarbonyl]phthalocyanine were prepared and characterized by X-ray diffraction, and polarized UV-Vis and electron paramagnetic resonance (EPR) spectroscopies. It was found that the phthalocyanine molecules deposited on quartz and polyethylene terephthalate (pet) are arranged edge-on on the substrate with the normal to the molecular plane that lies close to the plane of the film. Polarized UV-Vis spectra of films show in-plane anisotropy in the orientation of molecules for LB films deposited on quartz and on pet. EPR spectra also, assigned to Cu 2+ interacting species, were detected for the LB films deposited on pet. The spectra show a marked in-plane anisotropy of the Zeeman interaction when the magnetic field lies in the support plane. The analysis of their angular dependence confirms that the plane of the phthalocyanine molecule is almost perpendicular to the film plane and gives evidence that the in-plane orientation anisotropy arises from two different distributions of orientations pinned around two average orientations having an angle of 68 ° between them.

EPR studies of the excited triplet states of C60O and C60C2H4N(CH3) fullerene derivatives and C70 in toluene and polymethylmethacrylate glasses and as films

Abstract The EPR spectra of excited triplet states of two fullerene derivatives, C60O and C60C2H4N(CH3), in the glassy matrices of toluene and of polymethylmethacrylate (PMMA) and as films were examined at low temperature and the magnetic parameters were measured. In films, species with zfs parameters smaller than those in toluene were found and they were assigned to triplet excitations visiting more than one molecule in the crystallites that form the film. When C60C2H4N(CH3) was excited in PMMA grown by in sity thermal polymerization a new triplet species was originated characterized by a dipolar splitting constant D about three times larger than that measured in toluene. Evidence was gained that such species participates to cross-linking in the polymer. The effect of PMMA matrix on the molecular dynamics of 3C70 was also investigated. It was found that a dynamical model holds based on the pseudorotation of the molecule around the axis of larger dipolar splitting. Motion is activated with activation energy of 210 ± 50 cm−1 a value comparable with that obtained for pseudorotation of 3C70 in cyclohexane.

A noncovalently linked, dynamic fullerene porphyrin dyad. Efficient formation of long-lived charge separated states through complex dissociation

Photoexcitation of a Zn–tetraphenyl porphyrin–fullerene complex leads to electron-transfer with very long lifetimes of the charge-separated pairs.

Spin correlated radical ion pairs generated by photoinduced electron transfer in composites of sexithiophene/fullerene derivatives: a transient EPR study

Abstract Photoinduced electron transfer was observed in a series of methylsulfanyl sexithiophene/fulleropyrrolidine composites deposited as films. Paramagnetic states were observed by transient EPR spectroscopy in the microseconds time domain. The spectra displayed polarized lines with characteristic antiphase emission/absorption pattern of spin polarization and were assigned to spin correlated radical ion pairs (SCRP) formed by intermolecular electron transfer from sexithiophene donor to C60 fullerene acceptor molecules. Also transient signals detected at selected magnetic fields showed phase effects that are typical of SCRPs. Spectrum simulation was obtained by allowing for a distribution of respective orientations of the dipolar axes and g-tensors of partners in a pair. Fitting parameters used for one composite were D/gβ=−135 μT and J/gβ=2.5 μ T for dipolar and spin exchange coupling constants, corresponding to an interradical mean distance of 27.4 A. Similar values were obtained for the other examined systems. Pair structure and dynamics suggest to ascribe the charge-separated state to a radical pair generated after a hopping of the electron–hole charges from a primary pair originated in neighbor molecular sites.

Electron spin polarisation effects of ISC into a superposition of spin states: Triplet excitons in biphenyl-tetracyanobenzene 1:1 complex

Abstract The behaviour in a magnetic field of the spin polarisation of triplet excitons in biphenyl-tetracyanobenzene is accounted for by the generation of the triplet state into a superposition of spin states. The molecular conformation in the excited singlet state is discussed.

Transient EPR emission spectra of a free radical trapped in a single crystal of chloranil

Abstract The EPR spectrum of a free trapped in single crystals of chloranil changes its phase from absorption to emission when the crystal is illuminated by visible light. The time evolution of the EPR signal is discussed in terms of the interaction between the doublet species and triplet excitons produced by light excitation.

Delayed-fluorescence ODMR and EPR studies of triplet excitons in charge-transfer molecular crystals☆

Abstract Triplet excitons in electron donor—acceptor charge-transfer (CT) molecular crystals are generated through the intersystem crossing process by excitation in the CT visible band and give rise to delayed fluorescence. Delayed-fluorescence optically detected magnetic resonance (DF ODMR) in magnetic field is analyzed in terms of microwave-induced transitions between energy levels of either the isolated triplet excitons or the annihilating triplet exciton pair. The spin polarization of the triplet excitons plays an important role in the described phenomena. A comparison between DF ODMR and EPR spectra of the anthracene—tetracyanobenzene and biphenyl—tetracyanobenzene systems is presented. In the former case the microwave transitions occurring between free exciton sublevels are predominantly responsible of the DF ODMR signal, whereas the transitions between energy levels of the exciton pair are the most important for biphenyl—TCNB.

Crystal Structure of Biphenyl-1,2,4,5-Tetracyanobenzene 1:1 CT Complex and EPR Investigation of Photoexcited Triplet Excitons

Abstract The crystal structure of biphenyl and 1,2,4,5-tetracyanobenzene 1:1 charge transfer complex has been determined at room temperature. The crystals belong to the triclinic system with space group CI, a = 9.592(6) A, b = 12.359(9) A, c = 7.335(5) A, α = 94.17(6)°, β = 96.50(8)° and γ = 89.07(7)° with two complexes per unit cell. The increase of the zero-field splitting tensor of photoexcited triplet excitons with increasing temperature has been discussed in terms of two early proposed models. In the former approach a thermal equilibrium between two triplet states was assumed. In the second the variation of the fine structure tensor elements was attributed to changes of the charge transfer character of the triplet state as a consequence of the temperature dependent molecular packing in the crystal. Experimental evidences are in favor of this latter model.