Dibenzoquinone Cyclopentadithiophene Diradicaloids Show Optical Transitions of Different Spin States and Anomalous Near-Infrared Emission.

We have prepared a series of bis(semiquinone) compounds with dithiophene bridges of different length that evolve from closed-shell (smaller compound) to full diradical (longer compound) for which the narrow singlet-triplet energy gap allows the triplet population at 298 K. The medium size system has a variety of photonic properties with absorptions and emission in the optical near-infrared region mediated by a unique case of anti-Kasha emission. A whole set of optical absorption/emission and vibrational steady state spectroscopies as well as picosecond transient absorption spectroscopy, all complemented with spectroelectrochemistry and theoretical calculations, is presented.

Self-Assembly-Directed Organization of a Fullerene-Bisporphyrin into Supramolecular Giant Donut Structures for Excited-State Charge Stabilization.

Functional materials composed of spontaneously self-assembled electron donor and acceptor entities capable of generating long-lived charge-separated states upon photoillumination are in great demand as they are key in building the next generation of light energy harvesting devices. However, creating such well-defined architectures is challenging due to the intricate molecular design, multistep synthesis, and issues associated in demonstrating long-lived electron transfer. In this study, we have accomplished these tasks and report the synthesis of a new fullerene-bis-Zn-porphyrin e-bisadduct by tether-directed functionalization of C60 via a multistep synthetic protocol. Supramolecular oligomers were subsequently formed involving the two porphyrin-bearing arms embracing a fullerene cage of the vicinal molecule as confirmed by MALDI-TOF spectrometry and variable temperature NMR. In addition, the initially formed worm-like oligomers are shown to evolve to generate donut-like aggregates by AFM monitoring that was also supported by theoretical calculations. The final supramolecular donuts revealed an inner cavity size estimated as 23 nm, close to that observed in photosynthetic antenna systems. Upon systematic spectral, computational, and electrochemical studies, an energy level diagram was established to visualize the thermodynamic feasibility of electron transfer in these donor-acceptor constructs. Subsequently, transient pump-probe spectral studies covering the wide femtosecond-to-millisecond time scale were performed to confirm the formation of long-lived charge-separated states. The lifetime of the final charge-separated state was about 40 µs, thus highlighting the significance of the current approach of building giant self-organized donor-acceptor assemblies for light energy harvesting applications.

Cyclopentadithiophene organic core in small molecule organic solar cells: morphological control of carrier recombination.

Two new planar and symmetrical A-D-A (electron acceptor-electron donor-electron acceptor) small molecules based on a commercial cyclopentadithiophene derivative have been synthesized for solution processed small molecule organic solar cells. The aim was to synthesise the molecules to be energetically identical (similar HOMO-LUMO energy levels) in order to assign the differences observed to changes in the film morphology or to differences in the interfacial recombination kinetics or both. Devices were electrically characterized under one sun simulated (1.5 AM G) conditions by determining current-voltage curves, light harvesting efficiencies and external quantum efficiencies. Moreover, time-resolved photo-induced techniques such as photo-induced charge extraction and photo-induced transient photo-voltage were also performed. The results demonstrate that, despite having the same core, i.e. cyclopentadithiophene, the use of one hexyl chain instead of two in the organic molecule leads to a greater control of the molecular ordering using solvent vapour annealing techniques and also to better solar cell efficiency.

Robust ethylenedioxythiophene-vinylene oligomers from fragile thiophene-vinylene cores: synthesis and optical, chemical and electrochemical properties of multicharged shapes.

Oligomers of ethylendioxythiophene-vinylene have been prepared. Their optical, electrochemical and chemical properties have been studied in detail by absorption and emission spectroscopy as well as cyclic voltammetry, Raman techniques and spectroelectrochemistry complemented with quantum chemical calculations. A comparison with their non-ethylendioxy and non-vinylene parents has been done. The inclusion of the EDO plus the vinylene function generates more robust electronic ground states regarding the largely flexible thiophene-vinylene (n TV) oligomeric homologues. The redox features of the new compounds are also rich of oxidative processes arising as an interesting stabilising balance effect between the oxygen fragment in the EDO groups (mesomeric effect) and the linear π-conjugated structure. The oxidised species have been characterised, which show the ability for the formation of mixed valence charge-transfer complexes and π dimers of different oxidation states, in particular, in the electrochemical medium, resulting that the electrochemical response is accounted for a succession of aggregation and electron-transfer steps. With this work, a full understanding of the optical and electronic properties of these new oligomers in the context of the oligomer approach has been proposed.

Role of the bridge in photoinduced electron transfer in porphyrin-fullerene dyads.

The role of π-conjugated molecular bridges in through-space and through-bond electron transfer is studied by comparing two porphyrin-fullerene donor-acceptor (D-A) dyads. One dyad, ZnP-Ph-C60 (ZnP = zinc porphyrin), incorporates a phenyl bridge between D and A and behaves very similarly to analogous dyads studied previously. The second dyad, ZnP-EDOTV-C60, introduces an additional 3,4-ethylenedioxythienylvinylene (EDOTV) unit into the conjugated bridge, which increases the distance between D and A, but, at the same time, provides increased electronic communication between them. Two essential outcomes that result from the introduction of the EDOTV unit in the bridge are as follows: 1) faster charge recombination, which indicates enhanced electronic coupling between the charge-separated and ground electronic states; and 2) the disappearance of the intramolecular exciplex, which mediates photoinduced charge separation in the ZnP-Ph-C60 dyad. The latter can be interpreted as a gradual decrease in electronic coupling between locally excited singlet states of D and A when introducing the EDOTV unit into the D-A bridge.

Photoinduced electron transfer of zinc porphyrin-oligo(thienylenevinylene)-fullerene[60] triads; thienylenevinylenes as efficient molecular wires.

Two novel donor-bridge-acceptor arrays (ZnP-nTV-C60) with zinc porphyrin (ZnP) and fullerene (C60), covalently connected by oligo(thienylenevinylene) (nTV) molecular wires (n = 3 and 8; ), have been prepared in a multistep convergent manner. The influence of the nTV-length on the electrochemical and electronic properties of the ZnP-nTV-C60 triads has been revealed. Interestingly, an efficient photoinduced electron transfer process occurs in both triads with formation of intermediate radical-ion pairs (namely, ZnP˙(+)-nTV-C60˙(-) and ZnP-nTV˙(+)-C60˙(-)) as confirmed by the nanosecond transient absorption measurements in the visible and NIR regions. In polar and nonpolar solvents, the rate constants of charge-separation processes (kCS) via(1)ZnP*-nTV-C60 were found to decrease from ca. 1.2 × 10(10) s(-1) for n = 3 (RDA = 20 Å) to (5-7) × 10(9) s(-1) for n = 8 (RDA = 60 Å) on the basis of fluorescence lifetime measurements of the ZnP moiety. From these data, together with those previously obtained ones for n = 4 in the related ZnP-nTV-C60 systems, a low attenuation coefficient was evaluated for the nTV molecular wires.

Organic Solar Cells Based on Non-Fullerene Low Molecular Weight Organic Semiconductor Molecules.

The development of narrow bandgap A-D-A- and ADA'DA-type non-fullerene small molecule acceptors (NFSMAs) along with small molecule donors (SMDs) have led to significant progress in all-small molecule organic solar cells. Remarkable power conversion efficiencies, nearing the range of 17-18 %, have been realized. These efficiency values are on par with those achieved in OSCs based on polymeric donors. The commercial application of organic photovoltaic technology requires the design of more efficient organic conjugated small molecule donors and acceptors. In recent years the precise tuning of optoelectronic properties in small molecule donors and acceptors has attracted considerable attention and has contributed greatly to the advancement of all-SM-OSCs. Several reviews have been published in this field, but the focus of this review concerns the advances in research on OSCs using SMDs and NFSMAs from 2018 to the present. The review covers the progress made in binary and ternary OSCs, the effects of solid additives on the performance of all-SM-OSCs, and the recently developed layer-by-layer deposition method for these OSCs. Finally, we present our perspectives and a concise outlook on further advances in all-SM-OSCs for their commercial application.

Effects of Halogenation on Cyclopentadithiophenevinylene-Based Acceptors with Excellent Responses in Binary Organic Solar Cells.

In recent years, non-fused non-fullerene acceptors (NFAs) have attracted increasing consideration due to several advantages, which include simple preparation, superior yield, and low cost. In the work reported here, we designed and synthesized three new NFAs with the same cyclopentadithiophenevinylene (CPDTV) trimer as the electron-donating unit and different terminal units (IC for FG10, IC-4F for FG8, and IC-4Cl for FG6). Both halogenated NFAs, i.e., FG6 and FG8, show red-shifted absorption spectra and higher electron mobilities (more pronounced for FG6) in comparison with FG10. Moreover, the dielectric constants of these materials also increased upon halogenation of the IC terminal units, thus leading to a reduction in the exciton binding energy, which is favorable for dissociation of excitons and subsequent charge transfer despite the driving force (highest occupied molecular orbital and lowest unoccupied molecular orbital offsets) being very small. The organic solar cells (OSCs) constructed using these acceptors and PBDB-T, as the donor, showed PCE values of 15.08, 12.56, and 9.04% for FG6, FG8, and FG10, respectively. The energy loss for the FG6-based device was the lowest (0.45 eV) of all the devices, and this may be attributed to it having the highest dielectric constant, which leads to a reduction in the binding energy of exciton and a small driving force for hole transfer from FG6 to PBDB-T. The results indicate that the NFA containing the CPDTV oligomer core and halogenated terminal units can efficiently spread the absorption spectrum to the NIR zone. Non-fused NFAs have a bright future in the quest to obtain efficient OSCs with low cost for marketable purposes.

Delocalization-to-localization charge transition in diferrocenyl-oligothienylene-vinylene molecular wires as a function of the size by Raman spectroscopy.

In going from short to large size thienylene-vinylene diferrocenyl cations, the transition from a charge delocalized to a localized state is addressed by resonance Raman spectroscopy and supported by theoretical model chemistry. The shorter members, dimer and tetramer, display conjugated structures near the cyanine limit of bond length equalization as a result of the strong interferrocene charge resonance, producing a full charge delocalized mixed valence system. In the longest octamer, charge resonance vanishes and the cation is localized at the bridge center (the mixed valence property disappears). The hexamer is at the delocalized-to-localized turning point. Solvent and variable-temperature Raman measurements highlight this borderline property. A detailed structure-property correlation of bond length alternation data and Raman frequencies is proposed to account for the whole set of spectroscopic properties, with emphasis on the changes observed with the size of the molecular wire.

Pyrazolinofullerenes: a less known type of highly versatile fullerene derivatives.

The cycloaddition of readily available nitrile imines to [60]fullerene affording 2-pyrazolino[60]fullerenes is a versatile procedure for the functionalization of fullerenes. In contrast to other functionalization methods these cycloadducts are obtained generally in good yields as single isomers, thus avoiding the formation of undesired stereoisomers. This tutorial review discusses these less known fullerene compounds that display, however, interesting electrochemical and photophysical properties. Owing to their outstanding electron acceptor character, similar to pristine C(60), and their remarkable thermal stability, these cycloadducts are good candidates for incorporation in photovoltaic devices. However, more work is needed in order to design better pyrazolinofullerenes exhibiting improved performances for PV applications.

Gold(III) Porphyrin Was Used as an Electron Acceptor for Efficient Organic Solar Cells.

The widespread use of nonfullerene-based electron-accepting materials has triggered a rapid increase in the performance of organic photovoltaic devices. However, the number of efficient acceptor compounds available is rather limited, which hinders the discovery of new, high-performing donor:acceptor combinations. Here, we present a new, efficient electron-accepting compound based on a hitherto unexplored family of well-known molecules: gold porphyrins. The electronic properties of our electron-accepting gold porphyrin, named VC10, were studied by UV-Vis spectroscopy and by cyclic voltammetry (CV) , revealing two intense optical absorption bands at 500-600 and 700-920 nm and an optical bandgap of 1.39 eV. Blending VC10 with PTB7-Th, a donor polymer, which gives rise to an absorption band at 550-780 nm complementary to that of VC10, enables the fabrication of organic solar cells (OSCs) featuring a power conversion efficiency of 9.24% and an energy loss of 0.52 eV. Hence, this work establishes a new approach in the search for efficient acceptor molecules for solar cells and new guidelines for future photovoltaic material design.

Synthesis and photoinduced energy- and electron-transfer processes of C60-oligothienylenevinylene-C70 dumbbell compounds.

Unsymmetric dumbbell molecules based on N-methylpyrrolidine[60]fullerene, oligothienylenevinylenes (nTV; n=2, 4), and N-methylpyrrolidine[70]fullerene, namely, C(60) -nTV-C(70) were synthesized and their photophysical properties were studied. In nonpolar solvents, photoinduced energy-transfer process predominantly takes place from the singlet excited state of nTV to C(60) and C(70) , as was confirmed by time-resolved emission and transient absorption spectroscopy. In polar solvent, charge-separation processes take place instead of energy transfer. The generated charge-separated radical-ion pairs decay to the neutral molecules by a fast charge-recombination process; for n=4, a rate constant of 2×10(7) s(-1) and lifetime of 50 ns were evaluated.

Triplication of the photocurrent in dye solar cells by increasing the elongation of the π-conjugation in Zn-porphyrin sensitizers.

Porphyrins are promising sensitizers for dye solar cells (DSCs) but narrow absorption bands at 400-450 and 500-650 nm limit their light-harvesting properties. Increasing elongation of the π-conjugation and loss of symmetry causes broadening and a red-shift of the absorption bands, which considerably improves the performance of the DSC. Herein we use an oligothienylenevinylene to bridge a Zn-porphyrin system and the anchoring group of the sensitizer. We separately study the performance of the two basic units: oligothienylenevinylene and Zn-porphyrin. The combined system provides a three-fold enhancement of the photocurrent with respect to parent dyes. This is caused by an additional strong absorption in the region 400-650 nm that leads to flat IPCE of 60%. Theoretical calculations support that the addition of the oligothienylenevinylene unit as a linking bridge creates a charge transfer band that transforms a Zn-porphyrin dye into a push-pull type system with highly efficient charge injection properties.

Reducing Energy Loss in Organic Solar Cells by Changing the Central Metal in Metalloporphyrins.

The effect of central donor core on the properties of A-π-D-π-A donors, where D is a porphyrin macrocycle, cyclopenta[2,1-b:3,4-b']dithiophene is the π bridge, and A is a dicyanorhodanine terminal unit, was investigated for the fabrication of the organic solar cells (OSCs), along [6,6]-phenyl-C71-butyric acid methyl ester (PC71 BM) as electron acceptor. A new molecule consisting of Ni-porphyrin central donor core (VC9) showed deep HOMO energy level and OSCs based on optimized VC9:PC71 BM realized overall power conversion efficiency (PCE) of 10.66 % [short-circuit current density (JSC )=15.48 mA/cm2 , fill factor (FF)=0.65] with high open circuit voltage (VOC ) of 1.06 V and very low energy loss of 0.49 eV, whereas the Zn-porphyrin analogue VC8:PC71 BM showed PCE of 9.69 % with VOC of 0.89 V, JSC of 16.25 mA/cm2 and FF of 0.67. Although the OSCs based on VC8 showed higher JSC in comparison to VC9, originating from the broader absorption profile of VC8 that led to more exciton generation, the higher value of PCE of VC9 is owing to the higher VOC and reduced energy loss.

Fullerene/Non-fullerene Alloy for High-Performance All-Small-Molecule Organic Solar Cells.

Organic solar cells (OSCs) that contain small molecules only were prepared with FG1 as the donor, a narrow band gap non-fullerene acceptor MPU4, and a wide band gap PC71BM. The OSCs based on optimized FG1:MPU4 (1:1.2) and FG1:PC71BM (1:1.5) active layers, respectively, gave power conversion efficiencies (PCEs) of 11.18% with a short circuit current (JSC) of 19.54 mA/cm2, open circuit voltage (VOC) of 0.97 V, and fill factor (FF) of 0.59, and 6.62% with a JSC of 12.50 mA/cm2, VOC of 0.84 V, and FF of 0.63%, respectively. A PCE of 13.26% was obtained from the optimized ternary FG1:PC71BM:MPU4 (1:0.3:0.9) OSCs and this arises because of the boost in a JSC of 21.91 mA/cm2 and FF of 0.68. The VOC of the ternary OSCs (0.89 V) lies between those for the OSCs based on FG1:MPU4 and FG1:PC71BM, which indicates the formation of an alloy of the two acceptors. The increase in JSC and FF in the ternary OSCs may result from the efficient energy transfer from PC71BM to MPU4 as well as more charge-transfer donor/acceptor interfaces, enhanced charge carrier mobilities resulting in better adjusted charge transport, and lower bimolecular and trap-assisted recombination. The appropriate phase separation, increased crystallinity, and reduced π-π stacking distance in the ternary active layer are consistent with the enhancement in the FF for OSCs based on a ternary active layer. The results of this work suggest the merging of the fullerene acceptor into the non-fullerene acceptor to form a fullerene/non-fullerene acceptor alloy, and this may be a viable approach to obtain high-performance OSCs.

Bandgap modulation in efficient n-thiophene absorbers for dye solar cell sensitization.

Five new sensitizers for dye sensitized solar cells have been designed consisting of conjugated thienylenevinylene units threaded with alkyl chains to improve solubility and cyanoacetic acid as anchoring group. The conjugation length was increased from 2 to 6 thienylenevinylene units, which resulted in a red-shift of the optical absorption of the dyes from 550 to 750 nm, improving the spectral overlap with the solar spectrum. The photovoltaic performance of these dyes as sensitizers in mesoporous TiO(2) solar cells shows a clear correlation of increasing photocurrent with the extension of the conjugation up to an optimal length. Further extension of the conjugation increases the absorption but additional effects like self-quenching or recombination processes reduce the photocurrent and photovoltages and consequently the overall efficiency of the DSC.

Synthesis and electronic properties of pyridine end-capped cyclopentadithiophene-vinylene oligomers.

A series of four oligomers of cyclopentadithiophene-vinylenes end capped with pyridine groups was prepared and their optical and electronic properties studied. Treatment with trifluoroacetic acid (TFA) leads to the bisprotonation of the nitrogens of the pyridine, which has an important impact on the optical properties. Excess treatment with TFA provokes the oxidation of the conjugated core, generating radical cations and dications. The ease of the TFA treatment in solution was extended to protonation in the solid-state where further characterization of the neutral and TFA-treated samples was carried out in electrically active substrates in organic field-effect transistors. Finally, the new molecules were found to be excellent conductors in single-molecule junctions thanks to strong electron delocalization and resonance orbital mediated transport. These studies show the opening of a spectrum of possibilities by suitable terminal substitution of π-cores.

Ferrocenyl-ended thieno-vinylene oligomers: donor-acceptor polarization and mixed-valence properties with emphasis on the raman mapping of localized-to-delocalized transitions.

What's your role? New oligothiophene-vinylene compounds have been synthesized to study the role of the conjugated chain in two different cases (see scheme; MV=mixed valence). The electronic and molecular structures were analyzed by means of electronic, X-ray photoelectron, and Raman spectroscopy, together with thermo spectroscopy, electrochemistry, and DFT calculations.New oligothiophene-vinylene compounds have been synthesized in order to study the role of the conjugated chain in two different cases: 1) when push-pull action operates between an electron-donor and an electron-acceptor group at the ends of the thiophene-vinylene conjugated chain, and 2) when mixed-valence action is induced by single oxidation of the same chain functionalized at both terminal positions with ferrocene groups leading to competition between the donor groups. The electronic and molecular structures are analyzed by means of electronic, X-ray photoelectron and Raman spectroscopies, together with thermospectroscopy, electrochemistry and density functional theory calculations. The cyclic voltammetry processes have been followed by spectrochemistry. It is shown that the radical cation of the diferrocenyl derivative is a class III mixed-valence system (i.e., fully delocalized) according to its Raman spectrum. Moreover, by Raman thermo-spectroscopy the thermal transition of this radical cation from a delocalized (class III, room temperature) to a localized (class II, -160 degrees C) state is scanned. In all cases the Raman study is paralleled by an electronic absorption spectroscopic analysis. Structure-property relationships are proposed for molecules of two important fields of very active research as that of the non-linear optics (i.e., organic optoelectronic) and that of the mixed-valence systems (i.e., charge-transfer processes).

Near-IR Absorbing D-A-D Zn-Porphyrin-Based Small-Molecule Donors for Organic Solar Cells with Low-Voltage Loss.

Two D-A-D small molecules with a DPP acceptor core and Zn-porphyrin donor with different electron-donating substituents, namely, 2,6-bis(dodecyloxy)phenyl and 5-hexylthieno[3,2- b]thiophen-2-yl at mesopositions, VC4 and VC5, were synthesized, and their optical and electrochemical properties were investigated. The results reveal that both molecules are suitable as donors for organic solar cells (OSCs) in which PC71BM is employed as the acceptor. Overall power conversion efficiencies of 8.05% ( Jsc = 13.83 mA/cm2, Voc = 0.91 V, and FF = 0.64) and 8.89% ( Jsc = 16.98 mA/cm2, Voc = 0.79 V, and FF = 0.663) were obtained, respectively. The high Voc value for the VC4-based OSC correlates with the deeper HOMO, whereas the high Jsc value for VC5 may be attributed to the extended absorption spectrum toward the longer wavelength region. Moreover, the relatively high FF value for VC5-based OSCs as compared to the VC4 counterparts may be related to the more balanced charge transport in the active layer, reduced charge recombination, and efficient charge collection. The energy loss for VC5 is smaller (0.52 eV) than that for VC4 (0.56 eV).

[60]Fullerene-based liquid crystals acting as acid-sensitive fluorescent probes.

Functionalization of [60]fullerene with liquid-crystalline dendrimers and a dibutylaniline-based phenylenevinylene moiety leads to supramolecular materials, the fluorescence of which responds to acid-base stimuli.