A 9.16% Power Conversion Efficiency Organic Solar Cell with a Porphyrin Conjugated Polymer Using a Nonfullerene Acceptor.

A new low-molecular-weight porphyrin-based polymer, PPPyDPP, with pyridine-capped diketopyrrolopyrrole (DPP) has been synthesized, and its optical and electrochemical properties were investigated. The polymer is prepared with a low content of homocoupling units and gives a widely spread absorption from 400 to 900 nm with a narrow optical band gap of 1.46 eV. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels are respectively located at -5.27 and -3.78 eV, respectively. PPPyDPP was used as the electron donor, whereas [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) and bis(rhodanine)indolo-[3,2-b]-carbazole (ICzRd2), a nonfullerene small molecule, were used as acceptors for the fabrication of solution-processed bulk heterojunction polymer solar cells. Overall power conversion efficiencies (PCEs) of 7.31 and 9.16% (record high for porphyrin-containing polymers) were obtained for PC71BM and ICzRd2, respectively. A high Voc of 1.01 V and a low Eloss of 0.45 eV may explain this new record.

Increasing the lifetimes of charge separated states in porphyrin-fullerene polyads.

Two linear polyads were designed using zinc(ii)porphyrin, [ZnP], and N-methyl-2-phenyl-3,4-fullero-pyrrolidine (C60) where C60 is dangling either at the terminal position of [ZnP]-C6H4-[triple bond, length as m-dash]-C6H4-[ZnP]-C60 (1) or at the central position of [ZnP]-C6H4-[triple bond, length as m-dash]-C6H4-[ZnP(C60)]-C6H4-[triple bond, length as m-dash]-C6H4-[ZnP] (2) in order to test whether the fact of having one or two side electron donors influences the rate of electron transfer, ket. These polyads were studied using cyclic voltammograms, DFT computations, steady state and time-resolved fluorescence spectroscopy, and femtosecond transient absorption spectroscopy (fs-TAS). Photo-induced electron transfer confirmed by the detection of the charge separated state [ZnP˙+]/C60˙- from fs-TAS occurs with rates (ket) of 3-4 × 1010 s-1 whereas the charge recombinations (CRs) are found to produce the [ZnP] ground state via two pathways (central [ZnP˙+]/C60˙- (ps) and terminal central [ZnP˙+]/C60˙- (ns) producing [1ZnP] (ground state) and [3ZnP*]). The formation of the T1 species is more predominant for 2.

Application of the boron center for the design of a covalently bonded closely spaced triad of porphyrin-fullerene mediated by dipyrromethane.

Two novel triads had been designed through covalent bond connection of the boron dipyrromethane (BODIPY), free base porphyrin (H2P) or zinc(ii) porphyrin (ZnP) and N-methyl-2-phenyl-3,4-fulleropyrrolidine (C60) mediated by BODIPY. This closely spaced triad arrangement where porphyrin and fullerene are placed apart is anticipated to stabilize charge separation by separating the two radicals from each other. Two model polyads were synthesized with BODIPY and H2P or ZnP to investigate interaction between the two chromophores. Photo-excitation of the BODIPY triggered an efficient singlet energy transfer where the rates are found to be ∼1010-1011 s-1. For triads with C60 fast electron transfer was confirmed by the detection of the C60˙- signature from femtosecond transient absorption (fs-TA) in ∼0.4-3 ps. The charge recombination is estimated to be in the nanosecond window. This indicates the convenience of this arrangement for stabilizing the charge-separated state.

Ultrafast energy and electron transfers in structurally well addressable BODIPY-porphyrin-fullerene polyads.

Two electron transfer polyads built upon [C60]-[ZnP]-[BODIPY] (1) and [ZnP]-[ZnP](-[BODIPY])(-[C60]) (2), where [C60] = N-methyl-2-phenyl-3,4-fulleropyrrolidine, [BODIPY] = boron dipyrromethane, and [ZnP] = zinc(ii) porphyrin, were synthesized along with their corresponding energy transfer polyads [ZnP]-[BODIPY] (1a) and [ZnP]-[ZnP]-[BODIPY] (2a) as well as relevant models. These polyads were studied using cyclic voltammetry, DFT computations, steady state and time-resolved fluorescence spectroscopy, and fs transient absorption spectroscopy. The rates for energy transfer, kET, [BODIPY]* → [ZnP] are ∼2.8 × 1010 s-1 for both 1a and 2a, with an efficiency of 99%. Concurrently, the fast appearance of the [C60]-˙ anion for 1 and 2 indicates that the charge separation occurs on the 20-30 ps timescale with the rates of electron transfer, ket, [ZnP]*/[C60] → [ZnP]+˙/[C60]-˙ of ∼0.9 × 1010 to ∼3.8 × 1010 s-1. The latter value is among the fastest for these types of polyads. Conversely, the charge recombination operates on the ns timescale. These rates are comparable to or faster than those reported for other more flexible [C60]-[ZnP]-[BODIPY] polyads, which can be rationalized by the donor-acceptor separations.

Electron-Transfer Kinetics within Supramolecular Assemblies of Donor Tetrapyrrolytic Dyes and an Acceptor Palladium Cluster.

9,18,27,36-Tetrakis[meso-(4-carboxyphenyl)]tetrabenzoporphyrinatozinc(II) (TCPBP, as a sodium salt) was prepared in order to compare its photoinduced electron-transfer behavior toward unsaturated cluster Pd3(dppm)3(CO)(2+) ([Pd3(2+)]; dppm = Ph2PCH2PPh2 as a PF6(-) salt) with that of 5,10,15,20-tetrakis[meso-(4-carboxyphenyl)]porphyrinatozinc(II) (TCPP) in nonluminescent assemblies of the type dye···[Pd3(2+)]x (x = 0-4; dye = TCPP and TCPBP) using femtosecond transient absorption spectroscopy. Binding constants extracted from UV-vis titration methods are the same as those extracted from fluorescence quenching measurements (static model), and both indicate that the TCPBP···[Pd3(2+)]x assemblies (K14 = 36000 M(-1)) are slightly more stable than those for TCPP···[Pd3(2+)]x (K14 = 27000 M(-1)). Density functional theory computations (B3LYP) corroborate this finding because the average ionic Pd···O distance is shorter in the TCPBP···[Pd3(2+)] assembly compared to that for TCPP···[Pd3(2+)]. Despite the difference in the binding constants and excited-state driving forces for the photoinduced electron transfer in dye*···[Pd3(2+)] → dye(•+)···[Pd3(•+)], the time scale for this process is ultrafast in both cases (<85 fs). The time scales for the back electron transfers (dye(•+)···[Pd3(•+)] → dye···[Pd3(2+)]) occurring in the various observed species (dye···[Pd3(2+)]x; x = 0-4) are the same for both series of assemblies. It is concluded that the structural modification on going from porphyrin to tetrabenzoporphyrin does not greatly affect the kinetic behavior in these processes.

Are the orientation and bond strength of the RCO2(-)···M link key factors for ultrafast electron transfers?

The photo-induced electron transfers in the "straight up" ionic assemblies [Pd3(2+)]···MCP and [Pd3(2+)]···DCP···[Pd3(2+)] ([Pd3(2+)]* → MCP or DCP) are ultrafast (<85 fs) indicating that it is not necessary to have a strong coordination bond or a bent geometry to obtain fast electron injection in porphyrin-containing DSSCs.

Drastic tuning of the photonic properties of "(push-pull)n" trans-bis(ethynyl)bis(tributylphosphine)platinum(II)-containing polymers.

The trans-Pt(PBu3)2 Cl2 complex reacts with 1 equiv. of 2,6-diethynyl-AQ and 2 equiv. of 2-ethynyl-AQ (AQ = anthraquinone) to form the polymer (trans-Pt(2,6-diethynyl-AQ)2 (PBu3)2)n, 1, and the model compounds, 2, trans-Pt(PBu3)2 (2-ethynyl-AQ)2 (in a 20:1 ratio as trans-(2a) and cis-(2b) rotational isomers), respectively. These redox-active and luminescent materials have been characterized by gel permeation chromatography, thermal gravimetric analysis, X-ray crystallography, electrochemistry, photophysics, and DFT computations (B3LYP). The typical π,π* T2 → S0 phosphorescence centered on the trans-Pt(PBu3)2 (aryl)2 chromophore, [Pt], generally encountered for the analogous polymers (trans-Pt(PBu3)2 (aryl)2-acceptor)n (acceptor = quinonediimine, QN2; anthraquinone diimine, AQN2), for which the CT T1 → S0 emission is silent, has been completely annihilated and replaced by a red-shifted T1 → S0 emission in 1 and 2a, which arise from a triplet charge transfer excited state [Pt]→ AQ.

Electronic communication across N-linked unconjugated polymers: important insight into the charge transfer processes of polyaniline.

The [C6H4C≡CPtL2C≡CC6H4] → quinone charge transfer bands in unconjugated ([Pt]-NMe-Q)n's are similar to the conjugated systems showing that the NMe secures communication.

The pt-organometallic version of perigraniline: going blue.

Four conjugated push-pull organometallic polymers ([Pt]-AQ)n ([Pt] = trans-bis(phenylacetylene)bis(tributylphosphine)platinum(II); AQ = 2-bromo-, 2,6-dibromo-, 2,6-diamino-, and unsubstituted anthraquinone diimine) were prepared and characterized by UV-vis spectroscopy and electrochemistry. A low-energy charge transfer, CT, band ([Pt]*→AQ; confirmed by density functional theory calculations), was found in the 445-500 nm window rather than the expected red-shifted range above 630 nm. X-ray structures of four model compounds reveal that steric hindrance induces large dihedral angles between the C6 H4 and NCC2 planes, rendering π-orbital overlap difficult between the [Pt] and AQ units. The position of the CT band is mainly driven the reduction potential of the anthraquinone diimine unit.

Reduced and oxidized forms of the Pt-organometallic version of polyaniline.

This work represents an effort to synthesize all four forms of polyaniline (PANI) in its organometallic versions. Polymers containing substituted 1,4-benzoquinone diimine or 1,4-diaminobenzene units in the backbone exhibiting the general structure (C≡CC(6)H(4)-N═C(6)X(4)═N-C(6)H(4)C≡C-PtL(2))(n) and (C≡CC(6)H(4)NH-C(6)X(4)-NHC(6)H(4)C≡C-PtL(2))(n) along with the corresponding model compounds (C≡CC(6)H(4)-N═C(6)X(4)═N-C(6)H(4)C≡C)(PtL(2)Cl)(2) and (C≡CC(6)H(4)NH-C(6)X(4)-NHC(6)H(4)C≡C)(PtL(2)Cl)(2) (L = PBu(3); X = H, F, Cl) were synthesized. The polymers and corresponding model compounds were characterized (including (1)H and (31)P NMR, IR, mass spectra, elemental analysis, and X-ray structure determinations) and investigated for their redox properties in the absence and in the presence of acid. Their optical properties, including ns transient spectroscopy were also investigated. These properties were interpreted through density functional theory (DFT) and time-dependent DFT (TDDFT) computations. These materials are found to be oligomers (GPC) with thermal stability (TGA) reaching 350 °C. The greatest stabilities were found in the cases with X = F. Using a data bank of 8 X-ray structures of diimine derivatives, a relationship between the C═N bond distance and the dihedral angle between the benzoquinone ring and the flanking phenyl planes is noted. As the size of the substituent X on the benzoquinone center increases, the degree of conjugation decreases as demonstrated by the C═N bond length. The largest dihedral angles are noted for X = Cl. These polymers exhibit drastic chemical differences when X is varied (X = H, F, Cl). The completely reduced polymer (C≡CC(6)H(4)NH-C(6)H(4)-NHC(6)H(4)C≡C-PtL(2))(n) (i.e., X = H) was not chemically accessible whereas in the cases of X = F, Cl, these materials were obtained and represent the first examples of fully reduced organometallic versions of PANI (i.e., leucoemaraldine). For the (C≡CC(6)H(4)-N═C(6)X(4)═N-C(6)H(4)C≡C-PtL(2))(n) polymers, the completely oxidized form for X = H was isolated (pernigraniline), but for X = F and Cl, only the largely reduced mixed-valence form (i.e., emaraldine) was obtained via chemical routes. In acidic solutions, the chemically accessible polymer for X = H, (C≡CC(6)H(4)-N═C(6)H(4)═N-C(6)H(4)C≡C-PtL(2))(n), exhibits two chemically reversible waves indicating that the reduced form (C≡CC(6)H(4)NH-C(6)H(4)-NHC(6)H(4)C≡C-PtL(2))(n) can be generated. The absorption spectra of the highly colored diimine-containing species exhibit a broad charge transfer band (assigned based on DFT calculations (B3LYP); C(6)H(4)C≡C-PtL(2)-C≡CC(6)H(4) → N═C(6)X(4)═N) in the 450-800 nm window red shifting according X = H → Cl → F, consistent with their relative inductive effect. The largest absorptivity is measured for X = H because this polymer is fully oxidized whereas for the cases where X = F and Cl, these polymers exists in the mixed valence form. The ns transient absorption spectra of two polymers (X = F; reduced and mixed-valence polymers) were measured. The triplet excited state in the mixed-valence polymer is dominated by the reduced diamine residue and the T(1)-T(n) absorption of the diimine is entirely quenched.