Long-Range π-Conjugation in Phenothiazine-containing Donor-Acceptor Dyes for Application in Dye-Sensitized Solar Cells.

Four organic donor-π-bridge-acceptor dyes containing phenothiazine as a spacer and cyanoacrylic acid as an acceptor were synthesized and tested as sensitizers in dye-sensitized solar cells (DSCs). The influence of iodide- and cobalt-based redox electrolytes on the photovoltaic device performance was investigated. In these new dyes, systematic π-conjugation was extended by inserting one or two phenothiazine moieties and investigated within the context of the resulting photoinduced charge-transfer properties. A detailed investigation, including transient absorption spectroscopy and quantum chemical methods, provided important information on the role of extended π-conjugation on the photophysical properties and photovoltaic device performance. Overall, the results showed that the extension of π-conjugation by one phenothiazine unit resulted in the best device performance owing to reduced recombination rates, whereas extension by two phenothiazine units reduced dye adsorption on TiO2 probably owing to the increase in molecular size. The performance of the dyes in DSCs was found to be a complex interaction between dye structure and size.

Harvesting UV photons for solar energy conversion applications.

We report the synthesis and characterization of five new donor­π­spacer­acceptor dye molecules with a diphenylamine donor, fluorene­1,2,5-oxadiazole spacers and a range of acceptor/anchor groups (carboxylic acid 1, cyanoacrylic acid 2 and 3, alcohol 4 and cyano 5) to facilitate electron injection from the excited dye into the TiO2 photoanode in dye-sensitized solar cells (DSSCs). Detailed photophysical studies have probed the dyes' excited state properties and revealed structure­property relationships within the series. Density functional theory (DFT) and time dependent DFT (TDDFT) calculations provide further insights into how the molecular geometry and electronic properties impact on the photovoltaic performance. A special feature of these dyes is that their absorption features are located predominantly in the UV region, which means the dye-sensitized TiO2 is essentially colorless. Nevertheless, DSSCs assembled from 1 and 2 exhibit photovoltaic power conversion efficiencies of η = 1.3 and 2.2%, respectively, which makes the dyes viable candidates for low-power solar cells that need to be transparent and colorless and for applications that require enhanced harvesting of UV photons.

Blending through-space and through-bond π-π-coupling in [2,2']-paracyclophane-oligophenylenevinylene molecular wires.

A series of ZnP-pCp-oPPV-C60 conjugates covalently connected through [2,2']-paracyclophane-oligophenylenevinylene (pCp-oPPV) bridges containing one, two, and three [2,2']-paracyclophanes (pCps) has been prepared in multistep synthetic procedures involving Horner-Wadsworth-Emmons olefination reactions and/or Heck type Pd-catalyzed reactions. Molecular modeling suggests that charge transfer is effectively mediated by the pCp-oPPVs through a predominant hole-transfer mechanism. Photophysical investigation supports molecular modeling and reveals two major trends. On one hand, C60 excitation of 1, 2, and 3 leads exclusively to charge transfer between pCp and C60 to afford a ZnP-(pCp-oPPV)(•+)-C60(•-) radical ion pair state without giving rise to a subsequent charge shift to yield the ZnP(•+)-pCp-oPPV-C60(•-) radical ion pair state. On the other hand, ZnP excitation of 1, 2, and 3 results in a rather slow charge transfer between ZnP and C60, after which the ZnP(•+)-pCp-oPPV-C60(•-) radical ion pair state evolves. In temperature-dependent ZnP fluorescence experiments, which were performed in the temperature range from 273 to 338 K, two domains are discernible: low and high temperature behaviors. In the low temperature range (i.e., below 30 °C) the rate constants do not change, suggesting that a superexchange mechanism is the modus operandi. In the high temperature range (i.e., >30 °C) the rate constants increase. Moreover, we find rather strong distance dependence for 1 and 2 and weak distance dependence for 2 and 3. A damping factor of 0.145 Å(-1) is derived for the former pair and 0.012 Å(-1) for the latter.

Precise control of intramolecular charge-transport: the interplay of distance and conformational effects.

A new series of donor-bridge-acceptor (D-B-A) compounds consisting of π-conjugated oligofluorene (oFL) bridges between a ferrocene (Fc) electron-donor and a fullerene (C60 ) electron-acceptor have been synthesized. In addition to varying the length of the bridge (i.e., mono- and bi-fluorene derivatives), four different ways of linking ferrocene to the bridge have been examined. The Fc moiety is linked to oFL: 1) directly without any spacer, 2) by an ethynyl linkage, 3) by a vinylene linkage, and 4) by a p-phenylene unit. The electronic interactions between the electroactive species have been characterized by cyclic voltammetry, absorption, fluorescence, and transient absorption spectroscopy in combination with quantum chemical calculations. The calculations reveal exceptionally close energy-matching between the Fc and the oFL units, which results in strong electronic-coupling. Hence, intramolecular charge-transfer may easily occur upon exciting either the oFLs or Fcs. Photoexcitation of Fc-oFL-C60 conjugates results in transient radical-ion-pair states. The mode of linkage of the Fc and FL bridge has a profound effect on the photophysical properties. Whereas intramolecular charge-separation is found to occur rather independently of the distance, the linker between Fc and oFL acts (at least in oFL) as a bottleneck and significantly impacts the intramolecular charge-separation rates, resulting in beta values between ßCS 0.08 and 0.19 Å(-1). In contrast, charge recombination depends strongly on the electron-donor-acceptor distance, but not at all on the linker. A value of ßCR (0.35±0.01 Å(-1)) was found for all the systems studied. Oligofluorenes prove, therefore, to be excellent bridges for probing how small structural variations affect charge transport in D-B-A systems.

Dynamics and mechanisms of interfacial photoinduced electron transfer processes of third generation photovoltaics and photocatalysis.

Photoinduced electron transfer (PET) across molecular/bulk interfaces has gained attention only recently and is still poorly understood. These interfaces offer an excellent case study, pertinent to a variety of photovoltaic systems, photo- and electrochemistry, molecular electronics, analytical detection, photography, and quantum confinement devices. They play in particular a key role in the emerging fields of third-generation photovoltaic energy converters and artificial photosynthetic systems aimed at the production of solar fuels, creating a need for a better understanding and theoretical treatment of the dynamics and mechanisms of interfacial PET processes. We aim to achieve a fundamental understanding of these phenomena by designing experiments that can be used to test and alter modern theory and computational modeling. One example illustrating recent investigations into the details of the ultrafast processes that form the basis for photoinduced charge separation at a molecular/bulk interface relevant to dye-sensitized solar cells is briefly presented here: Kinetics of interfacial PET and charge recombination processes were measured by fs and ns transient spectroscopy in a heterogeneous donor-bridge-acceptor (D-B-A) system, where D is a Ru(II)(terpyridyl-PO3)(NCS)3 complex, B an oligo-p-phenylene bridge, and A nanocrystalline TiO2. The forward ET reaction was found to be faster than vibrational relaxation of the vibronic excited state of the donor. Instead, the back ET occurred on the micros time scale and involved fully thermalized species. The D-A distance dependence of the electron transfer rate was studied by varying the number of p-phenylene units contained in the bridge moiety. The remarkably low damping factor beta = 0.16 angstroms(-1) observed for the ultrafast charge injection from the dye excited state into the conduction band of TiO2 is attributed to the coupling of electron tunneling with nonequilibrium vibrations redistributed on the bridge, giving rise to polaronic transport of charges from the donor ligand to the acceptor solid oxide surface.

Stable radical anions inside fullerene cages: formation of reversible electron transfer systems.

The complexation behavior of La(2)@C(80) dimetallofullerenes with organic donor molecules (D) in solution has been investigated. It has been shown that La(2)@C(80) and D form 1:1 complexes as a result of electron transfer interactions in the ground state. The equilibrium between La(2)@C(80)/D and [La(2)@C(80)](•-)/[D](+) in solution is readily controlled by changing the temperature and solvent.

Triazole bridges as versatile linkers in electron donor-acceptor conjugates.

Aromatic triazoles have been frequently used as π-conjugated linkers in intramolecular electron transfer processes. To gain a deeper understanding of the electron-mediating function of triazoles, we have synthesized a family of new triazole-based electron donor-acceptor conjugates. We have connected zinc(II)porphyrins and fullerenes through a central triazole moiety--(ZnP-Tri-C(60))--each with a single change in their connection through the linker. An extensive photophysical and computational investigation reveals that the electron transfer dynamics--charge separation and charge recombination--in the different ZnP-Tri-C(60) conjugates reflect a significant influence of the connectivity at the triazole linker. Except for the m4m-ZnP-Tri-C(60)17, the conjugates exhibit through-bond photoinduced electron transfer with varying rate constants. Since the through-bond distance is nearly the same for all the synthesized ZnP-Tri-C(60) conjugates, the variation in charge separation and charge recombination dynamics is mainly associated with the electronic properties of the conjugates, including orbital energies, electron affinity, and the energies of the excited states. The changes of the electronic couplings are, in turn, a consequence of the different connectivity patterns at the triazole moieties.

[2,2']paracyclophane-based π-conjugated molecular wires reveal molecular-junction behavior.

The electronic coupling as well as the attenuation factor (ß), which depends primarily on the nature of the molecular bridge and is used as a benchmark to test the molecular wire behavior, have been determined in a systematic study carried out on a series of ZnP/C(60) conjugates connected through a [2,2']paracyclophane-oligophenylenevinylene (pCp-oPPV). The convergent synthesis involves a series of Horner-Emmons olefination reactions or double palladium-catalized Heck-type reactions. ZnP-pCp-C(60) conjugates were finally obtained by the 1,3-dipolar cycloaddition reaction of the in situ-generated azomethyne ylide containing the ZnP-pCp moiety to the [60]fullerene using Prato conditions. Experimental (UV-vis, fluorescence, transient absorption spectroscopy, and solution electrochemistry) and theoretical studies revealed that the pCps act as molecular junctions. If hole transfer is assumed to be the dominant charge transfer (CT) mechanism, CT is facilitated in one direction (from C(60) to ZnP via pCp) but disfavored in the other direction (from ZnP to C(60) via pCp).

Control over charge transfer through molecular wires by temperature and chemical structure modifications.

A series of electron donor-acceptor arrays containing π-conjugated oligofluorenes (oFL) of variable length between a zinc porphyrin (ZnP) as electron donor and fullerene (C(60)) as electron acceptor have been prepared by following a convergent synthesis. The electronic interactions between the electroactive species were determined by cyclic voltammetry, UV-visible, fluorescence, and femto/nanosecond transient absorption spectroscopy. Our studies clearly confirm that, although the C(60) units are connected to the ZnP donor through π-conjugated oFL frameworks, no significant electronic interactions prevail in the ground state. Theoretical calculations predict that a long-range electron transfer occurs primarily due to a maximized π-conjugated pathway from the donor to the acceptor. Photoexcitation of ZnP-oFL(n)-C(60) results in transient absorption maxima at 715 and 1010 nm, which are unambiguously attributed to the photolytically generated radical ion pair state, [ZnP(•+)-oFL(n)-C(60)(•-)], with lifetimes in the microsecond time regime. Temperature-dependent photophysical experiments have shown that the charge-transfer mechanism is controllable by temperature. Both charge separation and charge recombination processes give rise to a molecular wire behavior of the oFL moiety with an attenuation factor (ß) of 0.097 Å(-1). The correlation ß to the connection pattern between the ZnP donor and the oFL linker revealed that even small alterations of the linker π-electron system break the homogeneous π-conjugation pattern, leading to higher values of ß.

Dendronizing and metalating trans-2 C60 tetraaryl porphyrins--a versatile approach toward water-soluble donor-acceptor conjugates.

We have realized for the first time a series of truly water-soluble and tightly coupled porphyrin/C(60) electron-donor-acceptor conjugates in which the charge separation and charge recombination dynamics are controlled by modifying the nature of the dendrimer and/or the choice of the central metal atom.

Electron transfer through exTTF bridges in electron donor-acceptor conjugates.

Rigid and soluble electron donor-acceptor conjugates combining exTTF and/or TTF as donors and C60 as acceptor have been synthesized; fluorescence and transient absorption measurements confirm the generation of charge-separated radical-ion pairs with lifetimes in the micros timescale.

Self-association and electron transfer in donor-acceptor dyads connected by meta-substituted oligomers.

The synthesis of a new series of electron donor-acceptor conjugates (5, 10, 13, and 16) in which the electron acceptor--C(60)--and the electron donor--pi-extended tetrathiafulvalene (exTTF)--are bridged by means of m-phenyleneethynylene spacers of variable length is reported. The unexpected self-association of these hybrids was first detected to occur in the gas phase by means of MALDI-TOF spectrometry and subsequently corroborated in solution by utilizing concentration-dependent and variable-temperature (1)H NMR experiments. Furthermore, the ability of these new conjugates to form wirelike structures upon deposition onto a mica surface has been demonstrated by AFM spectroscopy. In light of their photoactivity and redoxactivity, 5, 10, 13, and 16 were probed in concentration-dependent photophysical experiments. Importantly, absorption and fluorescence revealed subtle dissimilarities for the association constants, that is, a dependence on the length of the m-phenylene spacers. The binding strength is in 5 greatly reduced when compared with those in 10, 13, and 16. Not only that, the spacer length also plays a decisive role in governing excited-state interactions in the corresponding electron donor-acceptor conjugates (5, 10, 13, and 16). To this end, 5, in which the photo- and electroactive constituents are bridged by just one aromatic ring, displays--exclusively and independent of the concentration (10(-6) to 10(-4) M)--efficient intramolecular electron transfer events on the basis of a "through-bond" mechanism. On the contrary, the lack of conjugation throughout the bridges in 10 (two m-phenyleneethynylene rings), 13 (three m-phenyleneethynylene rings), and 16 (four m-phenyleneethynylene rings) favors at low concentration (10(-6) M) "through space" intramolecular electron transfer events. These are, however, quite ineffective and, in turn, lead to excited-state deactivations that are at high concentrations (10(-4) M) dominated by intracomplex electron transfer events, namely, between exTTF of one molecule and C(60) of another molecule, and that stabilize the resulting radical ion pair state with lifetimes reaching 4.0 micros.

Fullerene for organic electronics.

This tutorial review surveys and highlights the integration of different molecular wires-in combination with chromophores that exhibit (i) significant absorption cross section throughout the visible part of the solar spectrum and (ii) good electron donating power-into novel electron donor-acceptor conjugates. The focus is predominantly on charge transfer and charge transport features of the most promising systems.

Porphyrin-beta-oligo-ethynylenephenylene-[60]fullerene triads: synthesis and electrochemical and photophysical characterization of the new porphyrin-oligo-PPE-[60]fullerene systems.

The synthesis and electrochemical and photophysical studies of new electron donor-acceptor arrays, bearing porphyrins covalently linked to fullerene, are described. In the reported investigation, phenyleneethynylene subunits were chosen as a linking bridge to guarantee a high conjugation degree between the donor (i.e., porphyrin), the molecular bridge (i.e., oligo-phenyleneethynylenes), and the acceptor (i.e., fullerene). To enhance the electronic interactions through the extended pi-system, the molecular bridge has been directly linked to the beta-pyrrole position of the porphyrin ring, generating a new example of donor-bridge-acceptor systems where, for the first time, the meso-phenyl ring of the macrocycle is not used to hold the "bridge" between porphyrin and fullerene moieties. This modification allows altering the chemical and physical properties of the tetrapyrrole ring. Steady-state and time-resolved fluorescence studies together with transient absorption measurements reveal that in nonpolar media (i.e., toluene) transduction of singlet excited-state energy governs the excited-state deactivation, whereas in polar media (i.e., tetrahydrofuran) charge transfer prevails generating a long-lived radical ion pair state. The lifetimes hereof range from 300 to 700 ns. The study also sheds light onto the wirelike behavior of the oligo-phenyleneethynylene bridges, for which a damping factor (beta) of 0.11 +/- 0.05 A(-1) has been determined in the current study.

Uniquely shaped double-decker buckyferrocenes--distinct electron donor-acceptor interactions.

Quantum chemical calculations and various photophysical techniques, ranging from steady-state absorption and steady-state as well as time-resolved fluorescence to femtosecond pump-probe experiments, were employed to examine ground- and excited-state interactions in a set of novel double-decker buckyferrocenes (i.e., Fe(2)(C(60)Me(10))Cp(2)): C(2v) and D(5d) isomers. When compared to the individual reference systems, the intimate fullerene/ferrocene contacts reflect appreciable ground-state interactions, namely, substantial redistribution of charge density between the two electron donors (i.e., ferrocenes) and the electron acceptor (i.e., fullerene). Furthermore, an intervalence charge-transfer transition (i.e., ferrocene-ferrocenium interaction) was established, but only in the C(2v) isomer. The first insight into the electron donor-acceptor interactions came from inspecting the fullerene-centered fluorescence. Relative to the reference compounds that contain no ferrocene, which exhibit quantum yields of up to 0.1, and knowing that the fluorescence of the investigated double-decker type conjugates is quenched to 10(-3), transient absorption measurements prove unequivocally the rapid formation of the radical ion-pair states as the dominant products of excited-state deactivation in the double-decker buckyferrocenes. Despite these products having much higher lying radical ion-pair states relative to the corresponding single-decker buckyferrocene, their lifetimes, which vary between 12 and 39 ps, are slightly shorter.

Cooperativity and tunable excited state deactivation: modular self-assembly of depsipeptide dendrons on a Hamilton receptor modified porphyrin platform.

A series of novel supramolecular architectures were built around a tin tetraphenyl porphyrin platform 6--functionalized by a 2-fold 1-ethyl-3-3-(3-dimethylaminopropyl)carbodiimide (EDC) promoted condensation reaction--and chiral depsipeptide dendrons of different generations 1-4. Here, implementation of a Hamilton receptor provided the necessary means to keep the constituents together via strong hydrogen bonding. Characterization of all architectures has been performed, including 4 which is the fourth generation, on the basis of NMR and photophysical methods. In particular, several titration experiments were conducted suggesting positive cooperativity, an assessment that is based on association constants that tend to be higher for the second binding step than for the first step. Importantly, molecular modeling calculations reveal a significant deaggregation of the intermolecular network of 6 during the course of the first binding step. As a consequence, an improved accessibility of the second Hamilton receptor unit in 6 emerges and, in turn, facilitates the higher association constants. The features of the equilibrium, that is, the dynamic exchange of depsipeptide dendrons 1-4 with fullerene 5, was tested in photophysical reference experiments. These steady-state and time-resolved measurements showed the tunable excited-state deactivations of these complexes upon photoexcitation.

p-Phenyleneethynylene molecular wires: influence of structure on photoinduced electron-transfer properties.

A series of donor-acceptor arrays (exTTF-oPPE-C60) containing pi-conjugated oligo(phenyleneethynylene) wires (oPPE) of different length between pi-extended tetrathiafulvalene (exTTF) as electron donor and fullerene (C60) as electron acceptor has been prepared by following a convergent synthesis. The key reaction in these approaches is the bromo-iodo selectivity of the Hagihara-Sonogashira reaction and the deprotecting of acetylenes with different silyl groups to afford the corresponding donor-acceptor conjugates in moderate yields. The electronic interactions between the three electroactive species were determined by using UV-visible spectroscopy and cyclic voltammetry. Our studies clearly confirm that, although the C60 units are connected to the exTTF donor through pi-conjugated oPPE frameworks, no significant electronic interactions are observed in the ground state. Theoretical calculations predict how a simple exchange from C=C double bonds (i.e., oligo(p-phenylenevinylene) to C triple chemical bond C triple bonds (i.e., oPPE) in the electron donor-acceptor conjugates considerably alters long-range electron transfer. Photoexcitation of exTTF-oPPE-C60 leads to the following features: a transient photoproduct with maxima at 660 and 1000 nm, which are unambiguously attributed to the photolytically generated radical-ion-pair state, [exTTF*+-oPPE-C60*]. Both charge-separation and charge-recombination processes give rise to a molecular-wire behaviour of the oPPE moiety with an attenuation factor (beta) of (0.2+/-0.05) A(-1).

Determination of the attenuation factor in fluorene-based molecular wires.

Fluorene-based bridges exhibit a molecular wire-like behaviour in C(60)-wire-exTTF systems with a very low attenuation factor (beta = 0.09 A(-1)).

Energy transfer in oligofluorene-C60 and C60-oligofluorene-C60 donor-acceptor conjugates.

Two complementary series of C(60)-(Fl)(n) and C(60)-(Fl)(n)-C60 (Fl = 9,9-dihexylfluorene-2,7-diyl; n = 1-5) derivatives with terminal N-methylfulleropyrrolidine units have been synthesized from CHO-(Fl)(n) and CHO-(Fl)(n)-CHO precursors via 1,3-dipolar cycloaddition reaction of in situ generated azomethine ylides with an excess of C60. In solution electrochemical experiments, these conjugates give rise to amphoteric redox behavior. Three consecutive quasireversible reduction waves have been observed at the expected potentials for the N-methylfulleropyrrolidine cores. For the C(60)-(Fl)(n)-C(60) series, each reduction wave is a two-electron process with no observable interaction between the C(60) units. Two or, in some cases, three oxidation waves--most of them irreversible--are ascribed to the oligofluorene system. These waves are cathodically shifted with an increasing number of fluorene units and anodically shifted by the conjugated terminal aldehyde units, compared to the N-methylfulleropyrrolidine termini. Steady-state and time-resolved photolytic techniques show that an efficient transduction of singlet excited-state energy transfer prevails from the photoexcited oligofluorene to the energy accepting fullerene.