Andreas Gouloumis

Long-lived photoinduced charge separation for solar cell applications in phthalocyanine-fulleropyrrolidine dyad thin films{

The photophysical properties of a new dyad molecule composed of a covalently linked Zn-phthalocyanine (antenna/donor) and a C60 derivative (acceptor) have been investigated. We report experimental evidence of long-lived charge separation in the solid state with a lifetime several orders of magnitude higher than in solution. Such a long lifetime, unusual for phthalocyanine–fullerene dyads, is the basis for possible photovoltaic applications. A first demonstration of a working solar cell using phthalocyanine–fullerene dyads as the active material is presented. Though the power conversion efficiency under simulated solar illumination of 80 mW cm−2 is found to be moderate (0.02%), it is an encouraging result for application of C60 dyad molecules to photovoltaics.

A Panchromatic Supramolecular Fullerene‐Based Donor–Acceptor Assembly Derived from a Peripherally Substituted Bodipy–Zinc Phthalocyanine Dyad

A panchromatic 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene-zinc phthalocyanine conjugate (Bodipy-ZnPc) 1 was synthesized starting from phthalocyanine aldehyde 4, via dipyrromethane 3 and dipyrromethene 2. Conjugate 1 represents the first example in which a Bodipy unit is tethered to the peripheral position of a phthalocyanine core. Electrochemical and optical measurements provided evidence for strong electronic interactions between the Bodipy and ZnPc constituents in the ground state of 1. When conjugate 1 is subjected to photoexcitation in the spectral region corresponding to the Bodipy absorption, the strong fluorescence characteristic of the latter subunit is effectively quenched (i.e., > or = 97%). Excitation spectral analysis confirmed that the photoexcited Bodipy and the tethered ZnPc subunits interact and that intraconjugate singlet energy transfer occurs with an efficiency of ca. 25%. Treatment of conjugate 1 with N-pyridylfulleropyrrolidine (8), an electron-acceptor system containing a nitrogen ligand, gives rise to the novel electron donor-acceptor hybrid 1<-->8 through ligation to the ZnPc center. Irradiation of the resulting supramolecular ensemble within the visible range leads to a charge-separated Bodipy-ZnPc(*+)-C(60)(*-) radical-ion-pair state, through a sequence of excited-state and charge transfers, characterized by a remarkably long lifetime of 39.9 ns in toluene.

Charge-transfer states in strongly coupled phthalocyanine fullerene ensembles

Fluorescence and transient absorption measurements show that in strongly coupled ZnPc-C60 and H2Pc-C60 dyads charge-separated states are formed; large -delta GCR degree and small lambda assist in stabilising ZnPc(.+)-C60.-/H2Pc(.+)-C60.-.

Photophysical characterization of a cytidine–guanosine tethered phthalocyanine–fullerene dyad

A new non-covalent electron transfer model system, based on the use of cytidine-guanosine hydrogen bonding interactions, is described that incorporates a phthalocyanine photodonor and a C60 fullerene acceptor.

Synthesis and Photophysical Properties of Fullerene–Phthalocyanine–Porphyrin Triads and Pentads

The synthesis and photophysical properties of several fullerene-phthalocyanine-porphyrin triads (1-3) and pentads (4-6) are described. The three photoactive moieties were covalently connected in an one-step synthesis through 1,3-dipolar cycloaddition to C(60) of the corresponding azomethine ylides generated in situ by condensation reaction of a substituted N-porphyrinylmethylglycine derivative and an appropriated formyl phthalocyanine or a diformyl phthalocyanine derivative, respectively. ZnP-C(60)-ZnPc (3), (ZnP)(2)-ZnPc-(C(60))(2) (6), and (H(2)P)(2)-ZnPc-(C(60))(2) (5) give rise upon excitation of their ZnP or H(2)P components to a sequence of energy and charge-transfer reactions with, however, fundamentally different outcomes. With (ZnP)(2)-ZnPc-(C(60))(2) (6) the major pathway is an highly exothermic charge transfer to afford (ZnP)(ZnP(.+))-ZnPc-(C(60)(.-))(C(60)). The lower singlet excited state energy of H(2)P (i.e., ca. 0.2 eV) and likewise its more anodic oxidation (i.e., ca. 0.2 V) renders the direct charge transfer in (H(2)P)(2)-ZnPc-(C(60))(2) (5) not competitive. Instead, a transduction of singlet excited state energy prevails to form the ZnPc singlet excited state. This triggers then an intramolecular charge transfer reaction to form exclusively (H(2)P)(2)-ZnPc(.+)-(C(60)(.-))(C(60)). A similar sequence is found for ZnP-C(60)-ZnPc (3).

A Fulleropyrrolidine-phthalocyanine dyad for photovoltaic applications

We report on photophysical properties of a novel dyad molecule having as antenna/donor a Zn-phthalocyanine derivative and as acceptor a C 60 derivative covalently attached. We found evidences for long living photoinduced electron transfer in solid state Photovoltate action of thin film devices of the dyad is demonstrated.

Second harmonic generation of a series of axially substituted titanium(IV) and gallium(III) tetra-tert-butylphthalocyanines

Electric Field Induced Second-Harmonic (EFISH) Generation and Hyper Raleigh Scattering (HRS) measurements were performed on a representative series of axially substituted titanium(IV) and gallium(III) tetra-tert-butylphthalocyanines. The comparison of γEFISH and βHRS values within the series and in particular in the case of the dimeric species shows the influence of both the reduction of symmetry of the phthalocyanine ring and the intramolecular stacking on the second order nonlinear optical properties.

Impact of concentration self-quenching on the charge generation yield of fullerene based donor-bridge-acceptor compounds in the solid state.

A fullerene based Donor-Bridge-Acceptor (DBA) compound, incorporating a π-extended tetrathiafulvalene electron donor, is investigated with respect to its photophysics in solution versus solid state. Solid films of neat DBA are compared with blend films where the DBA compound is diluted in the inert, low dielectric, polymer poly(styrene). It is found that the moderate intermolecular electronic coupling and donor-acceptor separation (22 Å) in this case leads to the generation of more dissociated, intermolecular charges than a mixture of the donor and acceptor reference compounds. However, the increased intermolecular interactions in the solid state lead to the excited state of the fullerene suffering from concentration self-quenching. This is found to severely affect the charge generation yield in solid films. The impact of competing intra and intermolecular interactions in the solid state upon the film photophysics is analysed in terms of a kinetic model which includes both the effects of concentration self-quenching and the impact of film composition upon the dielectric stabilisation of charge separated states. We conclude that both concentration self-quenching and dielectric stabilisation are critical in determining the photophysics of the blend films, and discuss strategies based upon our observations to enhance the charge photogeneration properties of organic films and photovoltaic devices based upon DBA compounds.

Synthesis and electrochemical switching of a dianthraquinone cryptand and related anthraquinone-diazacrown ether oligomers

Abstract Bis(diazacrown ether)-bisanthraquinones such as the cryptand represented in 5 , and related anthraquinone-diazacrown ether oligomers like 2 – 4 or 6 have been prepared and their potential for electrochemically-switched enhanced ion binding ability were evaluated. The presence of two or more quinone units close to the complexation site which act cooperatively in cation binding allows enhanced values for cation complexation. Thus, in compound 5 a binding enhancement ratio K 2 / K 1 of 6×10 6 for sodium cation has been found upon electrochemical reduction.

Flavin Core as Electron Acceptor Component in a Zinc(II)-Phthalocyanine-Based Dyad

A zinc(ii)-phthalocyanine-flavin dyad has been synthesized by Heck-type cross-coupling between a flavin that bears a p-iodophenyl group and a phthalocyanine functionalized with a vinyl moiety. Electrochemical experiments reveal that no significant interaction occurs at the ground state between the two electroactive subunits. However, the occurrence of a photoinduced electron transfer in this donor–acceptor conjugate is observed in transient absorption experiments. Charge-separation (i.e., 4.0 × 1011 s–1) and charge-recombination dynamics in benzonitrile (2.2 × 1010 s–1) reveal a remarkable stabilization of the radical ion pair in this solvent.