Conjugated Donor Acceptor Research Articles

ChemInform Abstract: A Voyage into the Synthesis and Photophysics of Homo‐ and Heterobinuclear Ensembles of Phthalocyanines and Porphyrins

AbstractReview: 281 refs.

Rates and energetics of intramolecular electron transfer processes in conjugated metallofullerenes

In this paper, we report on the design, redox potentials, excited state energies and radical ion pair state energies in electron donor-acceptor conjugates comprising the electron-donating π-extended tetrathiafulvalene and several electron-accepting fullerenes. To this end, we contrast an empty fullerene, that is, C₆₀, with two endohedral metallofullerenes, that is, open-shell La@C₈₂ and closed-shell La₂@C₈₀, in terms of charge separation and charge recombination dynamics.

Probing Charge Transfer in Benzodifuran–C60 Dumbbell‐Type Electron Donor–Acceptor Conjugates: Ground‐ and Excited‐State Assays

Rigid electron donor-acceptor conjugates (1-3) that combine π-extended benzodifurans as electron donors and C60 molecules as electron acceptors with different linkers have been synthesized and investigated with respect to intramolecular charge-transfer events. Electrochemistry, fluorescence, and transient absorption measurements revealed tunable and structure-dependent charge-transfer processes in the ground and excited states. Our experimental findings are underpinned by density-functional theory calculations.

A Metallofullerene Electron Donor that Powers an Efficient Spin Flip in a Linear Electron Donor–Acceptor Conjugate

The dream target of artificial photosynthesis is the realization of long-lived radical ion pair states that power catalytic centers and, consequently, the production of solar fuels. Notably, magnetic field effects, especially internal magnetic field effects, are rarely employed in this context. Here, we report on a linear Lu3N@Ih-C80-PDI electron donor-acceptor conjugate, in which the presence of the Lu3N cluster exerts an appreciable electron nuclear hyperfine coupling on the charge transfer dynamics. As such, a fairly efficient radical ion pair intersystem crossing converts the initially formed singlet radical ion pair state, (1)[(Lu3N@Ih-C80)(•+)-PDI(•-)], to the corresponding triplet radical ion pair state, (3)[(Lu3N@Ih-C80)(•+)-PDI(•-)]. Most notably, the radical ion pair state lifetime of the latter is nearly 1000 times longer than that of the former.

Self-Assembled via Metal–Ligand Coordination AzaBODIPY–Zinc Phthalocyanine and AzaBODIPY–Zinc Naphthalocyanine Conjugates: Synthesis, Structure, and Photoinduced Electron Transfer

Using near-IR emitting photosensitizers, viz., zinc phthalocyanine (ZnPc), zinc naphthalocyanine (ZnNc), and BF2-chelated azadipyrromethene (azaBODIPY), donor–acceptor conjugates have been newly formed using the well-known metal–ligand axial coordination approach. To accomplish this task, the electron-deficient azaBODIPY was functionalized to possess either two pyridine or imidazole ligating entities. The X-ray crystal structure of one such derivative revealed the presence of axial binding capable pyridine entities on the azaBODIPY macrocycle. The structural integrity of the newly formed conjugates was established from optical absorption, emission, 1H NMR, electrochemical, and computational methods. The experimentally determined binding constants suggested moderately stable conjugates. The absence of excitation energy transfer from singlet excited azaBODIPY to either ZnPc or ZnNc in the conjugates was confirmed from the steady-state emission measurements. However, free-energy calculations suggested photoi...

β-pyrrole substituted porphyrin–pyrene dyads using vinylene spacer: Synthesis, characterization and photophysical properties

We have designed and synthesized donor–acceptor conjugates having donor pyrene at the pyrrole-β position of either free-base porphyrin or Zn(II) porphyrin using vinylene spacer. Both the dyads have been completely characterized by elemental analysis, MALDI-MS, UV-Vis., and fluorescence (steady state and time-resolved) spectroscopies as well as cyclic voltammetry. The absorption maxima of both dyads are red-shifted by 8–12 nm. The ground state properties showed that there exist minimum π–π interaction between the aromatic subunits of these D–A systems. Quenched emission was observed in both the dyads when excited at 290 nm. The quenched emission explained in terms of intramolecular excitation energy transfer competes with the photo-induced electron transfer reaction in these D–A system.

Femtosecond Spectroscopic Study of Photoinduced Charge Separation and Recombination in the Donor–Acceptor Co-Oligomers for Solar Cells

Femtosecond transient absorption and time-resolved fluorescence techniques are employed to study the formation and recombination processes of the charge transfer (CT) state in a series of liquid-crystalline conjugated donor–acceptor (D–A) co-oligomers F3T4-hP, F4T6-hP, and F5T8-hP. It is revealed that the CT state of the D–A co-oligomer film forms faster but recombines slower than that of its solution, which is beneficial for the charges to be extracted in the D–A co-oligomer-based photovoltaic cells. It is also found that all the films made from different D–A co-oligomers, different processing conditions, and the blend of the donor and acceptor moieties show exactly the same CT dynamics, even though the films show very different film morphologies and photovoltaic performances, which indicate that the CT state property is only decided by the intrinsic molecular nature but not the film morphologies. The film morphology only affects the charge transport process but does not affect the CT state formation and...

New Alkylfuranyl-Substituted Benzo[1,2-b:4,5-b′]dithiophene-Based Donor–Acceptor Polymers for Highly Efficient Solar Cells

Two new alkylfuranyl-substituted conjugated donor–acceptor polymers—PBDTF-DPP and PBDTF-DPPF—were designed and synthesized. To compare the properties of the new polymers, PBDT-DPP and PBDT-DPPF with alkoxy side chains were also synthesized. The photophysical and electrochemical measurement demonstrated that the alkylfuranyl-substituted polymers had smaller optical band gaps, broader absorption range, and lower HOMO energy levels, thus leading to a larger short current density (Jsc) and higher open circuit voltage (Voc) in photovoltaic devices. Under the same fabricating conditions, the efficiency of the polymer solar cells (PSCs) based on PBDTF-DPP and PBDTF-DPPF reached 3.5% and 5.1%, respectively, whereas PSCs based on PBDT-DPP and PBDT-DPPF only showed an efficiency of 1.0% and 2.9%. After thermal annealing, the efficiency of the PSCs based on PBDTF-DPPF:PC71BM further achieved as high as 6.1%. The results indicate a great potential for largely improving the efficiency of the PSCs by replacing alkoxy with alkyfuranyl group and the building block BDTF in creating exceptional performance materials for PSCs.

Conjugated NDI–Donor Polymers: Exploration of Donor Size and Electrostatic Complementarity

Conjugated donor–acceptor copolymers comprised of electron-deficient 1,4,5,8-naphthalenetetracarboxylic diimide (NDI) linked to a series of relatively electron-rich aromatics via ethynyl spacers were synthesized and characterized. While LUMO levels remained constant at −3.75 eV, HOMO levels were sensitive to the relatively electron-rich aromatic donors and systematically tuned from −5.68 to −5.17 eV. Regardless of the electron-rich comonomer, fluorescence and X-ray diffraction data were consistent with the polymer chains being assembled through the stacking of NDI moieties in an offset face-to-face fashion rather than alternating donor–acceptor stacks.

Conjugated donor–acceptor (D–A) copolymers in inverted organic solar cells – a combined experimental and modelling study

Quantum chemical methods are useful for materials design to improve the performance of organic bulk heterojunction (BHJ) solar cells. However, more integrated studies of quantum chemical modelling and experimental results need to be performed to further improve both the materials design and understanding of the related photo-induced processes and photocurrent generation. In this work we investigated the internal relationship between the molecular structures of four donor–acceptor (D–A) copolymers (P1–P4) and their photovoltaic performances. The effects of the molecular structures on the generation of photo-induced charge carriers, exciton diffusion, dissociation and carrier transmission were compared by combining density functional theory (DFT) calculations of intrinsic geometric, electronic and optical properties with the results of electrochemical, spectroscopic, thermal, AFM and solar cell measurements of the polymers. The quantum chemical methods, which provided a tool to assess the electronic properties and conjugation length in the polymers, highly support the experimental results and therefore the usefulness of quantum chemistry for solar cell materials design.

Modulation of singlet and triplet excited states through σ spacers in ternary 1,3,5-triazines

A series of ternary asymmetric triazines with tunable singlet and triplet excited states were successfully synthesized on the basis of σ-spacer methodology and a facile synthetic route. The optoelectronic properties are dominated by the conjugated donor–acceptor (D–A) backbone of peripheral carbazoles (D) and triazine core (A), and can be further modulated via the partially interrupted conjugation between the core and the third substituents by the σ spacers of N–CH3, NH, O, S and OSO, which offers new opportunities to fine tune the electronic structures and properties of triazines.

A voyage into the synthesis and photophysics of homo- and heterobinuclear ensembles of phthalocyanines and porphyrins

The remarkable properties of both phthalocyanines and porphyrins as individual building blocks have motivated the synthesis and study of homo- and heterobinuclear conjugates as light-harvesting systems. These planar chromophores share important electronic features such as high molar absorption coefficients, rich redox chemistry and interesting photoinduced energy and/or electron transfer abilities. In addition, some of these properties can be tuned by the introduction of different peripheral substituents and metal centres. In this review, we present relevant synthetic strategies for the preparation of covalent and supramolecular, homo- and heterobinuclear systems based on phthalocyanine and porphyrin chromophores, leading to a variety of architectures. In such systems, the degree of electronic interaction between the components is highly dependent on the electronic features of the two macrocycles, their linkage, and the molecular topology of the ensemble. In addition, incorporation of electroactive units into these binuclear systems has been pursued, affording multicomponent, donor-acceptor conjugates. In-depth photophysical characterization of the ground- and excited-state features of many of these homo- and heterobinuclear phthalocyanine and/or porphyrin ensembles has also been presented. Particular attention has been paid to understand the fundamental dynamics of the energy transfer and charge separation processes of these systems. This review intends to offer a general overview of the preparation of this class of compounds and the study of their photophysical properties which clearly show their potentiality as model compounds of light-harvesting complexes.

Adjustment of charge trap number and depth in molecular backbone to achieve tunable multilevel data storage performance

A series of conjugated donor–acceptor (D–A) molecules with the electron-rich unit triphenylamine (TPA) as donor and azobenzene chromophore and/or cyano group as acceptors were successfully synthesized. Acceptors varying in number and electron-withdrawing strength were incorporated into the molecular backbone to investigate the impact on device switching behavior. It is found that the data storage states and memory effects are highly dependent on the number and strength of the electron-deficient groups. The devices based on the D–π–A1–π–A2 molecule with two electron-withdrawing groups exhibited excellent ternary memory behavior, while those based on the D–π–A1 or D–π–A2 molecule containing one electron-accepting moiety showed binary memory characteristics. In addition, the bistable memory effects of TPAVC 2 and TPAAH 3-based data storage devices varied from WORM to flash when the acceptor changed from a cyano group to an azobenzene chromophore. Therefore, tunable multilevel memory device performance has been achieved by adjusting the number and electron-withdrawing strength of acceptor moieties in the molecular backbone, offering guidance for the rational design of superior D–A molecules for high density data storage (HDDS) applications.

Synthesis and efficient solid-state emission of conjugated donor–acceptor–donor triphenylamine chromophores

Two symmetrical donor–acceptor–donor chromophores built by connecting two triphenylamine donors to a central electron acceptor 2,5-diphenyl-1,3,4-oxadiazole or anthracene core have been synthesized and characterized. The aggregation-induced emission phenomenon of these compounds with bright and deep emission from orange to green in the solid state can be obsreved. The chromophores based on the triphenylamine unit exhibit efficient solid-state emission. The results indicate that D–A–D conjugation bridging provides a new opportunity for AIE-materials. The multiphoton induced fluorescence spectra of these chromophores were measured using a femtosecond Ti:sapphire. The three-photon excited fluorescence spectra are in the green region with peaks at 554 nm for An-BIPAS and 504 nm for Ox-BIASP in THF, respectively. These AIE-active compounds are potential materials to design sensitive and selective fluorescent sensors or bioprobes.

Enhanced performance of solution-processed solar cells based on porphyrin small molecules with a diketopyrrolopyrrole acceptor unit and a pyridine additive

A conjugated donor–acceptor porphyrin small molecule was designed and synthesized with diketopyrrolopyrrole as the acceptor unit. The new porphyrin-based small molecule exhibits broad and intense absorption in the visible and near IR regions, and the hole mobility and the power conversion efficiency of the bulk heterojunction devices based on the porphyrin : [6,6]-phenyl-C-61-butyric acid methyl ester (1 : 1, w/w) are increased up to 4.6 × 10−5 cm2 V−1 s−1 and 3.71%, respectively, which are further enhanced to 1.6 × 10−4 cm2 V−1 s−1 and 4.78%, respectively, upon the introduction of 3.0 vol% of pyridine additive. Further studies show that the performance of the solar cells based on other zinc porphyrins could also be improved by the pyridine additive.

Synthesis and two-photon absorption property of new π -conjugated donor–acceptor polymers carrying different heteroaromatics

In this communication, we report the synthesis of three newly designed fluorescent polymers P1–P3, starting from simple thiophene derivatives through precursor polyhydrazide route. The new polymers, carrying donor and acceptor heterocyclic moieties with different spacer groups were found to be thermally stable and good of nonlinear optical (NLO) materials with two photon absorption property. The structures of newly synthesized monomers and polymers were confirmed by FTIR, NMR spectral and elemental analyses. Further, polymers were characterized by GPC and TGA studies. Their linear optical and electrochemical properties were evaluated by UV-vis, fluorescence spectroscopic and cyclic voltammetric (CV) studies, respectively, whereas their NLO properties were studied by Z-scan technique using Nd: YAG laser at 532 nm with 7 ns pulse. The electrochemical band gap of P1–P3 was determined to be 1.98, 1.91 and 2.05 eV, respectively. The NLO results reveal that polymers P1–P3 show good optical limiting property with TPA coefficient values 2.9 × 10 − 11 m/W, 8.0 × 10 − 11 m/W and 1.4 × 10 − 11 m/W, respectively.

Low dimensional nanocarbons – chemistry and energy/electron transfer reactions

This Minireview sheds light onto the electronic communication between, on one hand, low dimensional nanocarbons – single and multiwalled 1D carbon nanotubes and 2D graphene – and, on the other hand, a variety of electroactive species en-route to novel electron donor–acceptor conjugates and hybrids in relation to their covalent and non-covalent chemistry, respectively. A common denominator to any of the highlighted conjugates/hybrids is charge transport across different scales, that is, from individual molecular conjugates/hybrids to morphologically controlled devices.

All-Polymer Solar Cells Based on Fully Conjugated Block Copolymers Composed of Poly(3-hexylthiophene) and Poly(naphthalene bisimide) Segments

Fully conjugated donor–acceptor block copolymers composed of poly(3-hexylthiophene) and poly(naphthalene bisimide) segments, P3HT-PNBI-P3HTs, were synthesized using quasi-living Grignard metathesis polymerization and the Yamamoto coupling reaction. Broad absorption in a range of 350–850 nm, which corresponded to the optical band gap of 1.46 eV, was observed for the P3HT-PNBI-P3HT thin films. In addition, the optical band gap decreased to 1.38 eV (the light absorption band extended to 893 nm) by thermal annealing, which was much smaller than those of previously reported donor–acceptor block copolymers (1.6–1.8 eV). The annealed P3HT:P3HT-PNBI-P3HT blend film (1:1 by weight) also exhibited broad absorption in the range of 350–950 nm. Cyclic voltammetry demonstrated that the P3HT-PNBI-P3HT thin films exhibited the oxidation and reduction properties derived from the P3HT and PNBI segments. The HOMO and LUMO levels were in the range of 5.57–5.60 and 4.22–4.27 eV, respectively. All-polymer solar cells using the...

Ultrafast Photoinduced Processes in Subphthalocyanine Electron Donor–Acceptor Conjugates Linked by a Single B–N Bond

We have prepared two different subphthalocyanine conjugates by linking these macrocycles either to an electron-accepting perylene diimide or to an electron-donating phenothiazine through a single B-N covalent bond. The short spacing between the two active building blocks results in ultrafast photoinduced electron-transfer reactions.

Diketopyrrolopyrrole‐based liquid crystalline conjugated donor–acceptor copolymers with reduced band gap for polymer solar cells

AbstractA new liquid crystalline (LC) acceptor monomer 2,5‐bis[4‐(4′‐cyanobiphenyloxy)dodecyl]‐3,6‐dithiophen‐2‐yl‐pyrrolo[3,4‐c]pyrrole‐1,4‐dione (TDPPcbp) was synthesized by incorporating cyanobiphenyl mesogens into diketopyrrolopyrrole (DPP). The monomer was copolymerized with bis(2‐ethylhexyloxy)benzo[1,2‐b:4,5‐b′] dithiophene (BDT) and N‐9′‐heptadecanylcarbazole (CB) donors to obtain donor–acceptor alternating copolymers poly[4,8‐bis(2‐ethylhexyloxy)benzo[1,2‐b:4,5‐b′]dithiophene‐alt‐3,6‐bis(thiophen‐5‐yl)‐2,5‐bis[4‐(4′‐cyanobiphenyloxy)dodecyl]‐2,5‐dihydropyrrolo[3,4‐c]pyrrole‐1,4‐dione] (PBDTDPPcbp) and poly[N‐9′‐heptadecanyl‐2,7‐carbazole‐alt‐3,6‐bis(thiophen‐5‐yl)‐2,5‐bis[4‐(4′‐cyano‐biphenyloxy)dodecyl]‐2,5‐dihydropyrrolo[3, 4‐c]pyrrole‐1,4‐dione] (PCBTDPPcpb) with reduced band gap, respectively. The LC properties of the copolymers, the effects of main chain variation on molecular packing, optical properties, and energy levels were analyzed. Incorporating the mesogen cyanobiphenyl units not only help polymer donors to pack well through mesogen self‐organization but also push the fullerene acceptor to form optimized phase separation. The bulk heterojunction photovoltaicdevicesshow enhanced performance of 1.3% for PBDTDPPcbp and 1.2% for PCBTDPPcbp after thermal annealing. The results indicate that mesogen‐controlled self‐organization is an efficient approach to develop well‐defined morphology and to improve the device performance. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2013