Naphthalene Diimide Acceptor Research Articles

Donor–Acceptor–Donor Modular Small Organic Molecules Based on the Naphthalene Diimide Acceptor Unit for Solution-Processable Photovoltaic Devices

Two novel solution-processable small organic molecules, 4,9-bis(4-(diphenylamino)phenyl)-2,7-dioctylbenzo[3,8]phenanthroline-1,3,6,8(2H,7H)-tetraone (S6) and 4,9-bis(benzo[b]thiophen-2-yl)-2,7-dioctylbenzo[3,8]phenanthroline-1,3,6,8 (2H,7H)-tetraone (S7), have been successfully designed, synthesized, characterized, and applied in solution-processable photovoltaic devices. S6 and S7 contain a common electron-accepting moiety, naphthalene diimide (NDI), with different electron-donating moieties, triphenylamine (S6) and benzothiophene (S7), and are based on a donor–acceptor–donor structure. S7 was isolated as black, rod-shaped crystals. Its triclinic structure was determined by single crystal x-ray diffraction (XRD): space group \(P\bar{1}\), Z = 2, a = 9.434(5) A, b = 14.460(7) A, c = 15.359(8) A, α = 67.256(9) degrees, β = 80.356(11) degrees, γ = 76.618(10) degrees, at 150 Kelvin (K), R = 0.073. Ultraviolet–visible absorption spectra revealed that use of triphenylamine donor functionality with the NDI acceptor unit resulted in an enhanced intramolecular charge transfer (ICT) transition and reduction of the optical band gap compared with the benzothiophene analogue. Solution-processable inverted bulk heterojunction devices with the structure indium tin oxide/zinc oxide (30 nm)/active layer/molybdenum trioxide (10 nm)/silver (100 nm) were fabricated with S6 and S7 as donors and (6,6)-phenyl C70-butyric acid methyl ester (PC70BM) as acceptor. Power conversion efficiencies of 0.22% for S6/PC70BM and 0.10% for S7/PC70BM were achieved for the preliminary photovoltaic devices under simulated AM 1.5 illumination (100 mW cm−2). This paper reports donor–acceptor–donor modular small organic molecules, with NDI as central accepting unit, that have been screened for use in solution-processable inverted photovoltaic devices.

Crystal Engineering of Dual Channel p/n Organic Semiconductors by Complementary Hydrogen Bonding

The supramolecular arrangement of organic semiconductors in the solid state is as critical for their device properties as the molecular structure, but is much more difficult to control. To enable supramolecular design of semiconducting materials, we introduced dipyrrolopyridine as a new donor semiconductor capable of complementary hydogen bonding with naphthalenediimide acceptors. Through a combination of solution, crystallographic, and device studies, we show that the self-assembly driven by H bonding a) modulates the charge-transfer interactions between the donor and acceptor, b) allows for precise control over the solid-state packing, and c) leads to a combination of the charge-transport properties of the individual components. The predictive power of this approach was demonstrated in the synthesis of three new coassembled materials which show both hole and electron transport in single-crystal field-effect transistors. These studies provide a foundation for advanced solid-state engineering in organic electronics, capitalizing on the complementary H bonding.

Crystal Engineering of Dual Channel p/n Organic Semiconductors by Complementary Hydrogen Bonding

AbstractThe supramolecular arrangement of organic semiconductors in the solid state is as critical for their device properties as the molecular structure, but is much more difficult to control. To enable supramolecular design of semiconducting materials, we introduced dipyrrolopyridine as a new donor semiconductor capable of complementary hydogen bonding with naphthalenediimide acceptors. Through a combination of solution, crystallographic, and device studies, we show that the self‐assembly driven by H bonding a) modulates the charge‐transfer interactions between the donor and acceptor, b) allows for precise control over the solid‐state packing, and c) leads to a combination of the charge‐transport properties of the individual components. The predictive power of this approach was demonstrated in the synthesis of three new coassembled materials which show both hole and electron transport in single‐crystal field‐effect transistors. These studies provide a foundation for advanced solid‐state engineering in organic electronics, capitalizing on the complementary H bonding.

Stepwise versus pseudo-concerted two-electron-transfer in a triarylamine–iridium dipyrrin–naphthalene diimide triad

A triad based on triarylamine donors, an iridium dipyrrin sensitiser and a naphthalene diimide acceptor is investigated using fs-pump-probe spectroscopy at two different pump wavelengths. Excitation of the naphthalene diimide induces a stepwise electron transfer process that yields within ca. 100 ps a charge separated state with 50-60% quantum yield in which one triarylamine is oxidised and the imide is reduced. Although being in the Marcus normal region, this state has a lifetime of 580 ns in MeCN. Excitation at the dipyrrin ligand centred absorption directly produces the charge separated state within ca. 1 ns however without formation of an intermediate. This pseudo-concerted two-electron transfer step was analysed with the help of a model dyad and a modified triad and is interpreted to be caused by inverted electron transfer rates. Although being much slower the latter process has a quantum yield of practically 100%.

Inverse Rectification in Donor–Acceptor Molecular Heterojunctions

The transport properties of a junction consisting of small donor-acceptor molecules bound to Au electrodes are studied and understood in terms of its hybrid donor-acceptor-electrode interfaces. A newly synthesized donor-acceptor molecule consisting of a bithiophene donor and a naphthalenediimide acceptor separated by a conjugated phenylacetylene bridge and a nonconjugated end group shows rectification in the reverse polarization, behavior opposite to that observed in mesoscopic p-n junctions. Solution-based spectroscopic measurements demonstrate that the molecule retains many of its original constituent properties, suggesting a weak hybridization between the wave functions of the donor and acceptor moieties, even in the presence of a conjugated bridge. Differential conductance measurements for biases as high as 1.5 V are reported and indicate a large asymmetry in the orbital contributions to transport arising from disproportionate electronic coupling at anode-donor and acceptor-cathode interfaces. A semi-empirical single Lorentzian coherent transport model, developed from experimental data and density functional theory based calculations, is found to explain the inverse rectification.

Inside Cover: Hydrogen‐Bonding Directed Assembly and Gelation of Donor–Acceptor Chromophores: Supramolecular Reorganization from a Charge‐Transfer State to a Self‐Sorted State (Chem. Eur. J. 22/2011)

In a mixed assembly of bis-amide-functionalized dialkoxynaphthalene donor and naphthalenediimide acceptor, when there was a mismatch in the relative distance of the amide groups with respect to the two chromophores, self-sorting was observed. On the other hand if the two distances almost matched, a stable charge-transfer gel was formed in tetrachloroethylene. For more details see the Communication by S. Ghosh et al. on page 6061 ff.

Photoinduced Charge Separation in Platinum Acetylide Oligomers

The series of three donor-spacer-acceptor complexes, DPAF-Ptn-NDI, has been synthesized and characterized using time-resolved absorption spectroscopy. In these complexes, the donor is a (diphenylamino)-2,7-fluorenylene (DPAF) unit, the acceptor is a naphthalene diimide (NDI), and the spacers are a series of platinum acetylides of varying lengths, [-Pt(PBu(3))(2)-C≡C-Ph-C≡C-](n) (where Bu = n-butyl, Ph = 1,4-phenylene and n = 1, 2, and 3). Electrochemistry indicates that the DPAF-Ptn-NDI system has a charge transfer state at ca. 1.5 eV above the ground state that is based on one electron transfer from the DPAF donor to the NDI acceptor. Transient absorption spectroscopy on time scales ranging from 0.2 ps to 1 μs reveals that excitation of all of the complexes leads to production of the charge transfer state with nearly unit quantum efficiency. The rates for charge separation and charge recombination are not strongly dependent upon the length of the platinum acetylide spacer, suggesting that the spacer is actively involved in the electron (hole) transport processes. Analysis of the experimental results leads to a model in which charge separation and charge recombination occur by hole-hopping via states localized on the [-Pt(PBu(3))(2)-C≡C-Ph-C≡C-](n) bridge.

Comparative PCET Study of a Donor−Acceptor Pair Linked by Ionized and Nonionized Asymmetric Hydrogen-Bonded Interfaces

A Zn(II) amidinium porphyrin is the excited-state electron donor (D) to a naphthalene diimide acceptor (A) appended with either a carboxylate or sulfonate functionality. The two-point hydrogen bond (...[H(+)]...) formed between the amidinium and carboxylate or sulfonate functionalities establishes a proton-coupled electron transfer (PCET) pathway for charge transfer. The two D...[H(+)]...A assemblies differ only by the proton configuration within the hydrogen-bonding interface. Specifically, the amidinium ion transfers a proton to the carboxylate to form a nonionized amidine-carboxylic acid two-point hydrogen network, whereas the amidinium retains both protons when bound to the sulfonate functionality, forming an ionized amidinium-sulfonate two-point hydrogen bond network. These two interface configurations within the dyads thus allow for a direct comparison of the PCET kinetics for the same donor and acceptor juxtaposed by ionized and nonionized hydrogen-bonded interfaces. Analysis of the PCET kinetics ascertained from transient absorption and transient emission spectroscopy reveals that the ionized interface is more strongly impacted by the local solvent environment, thus establishing that the initial static configuration of the proton interface is a critical determinant in the kinetics of PCET.

Electron Transfer Driven by Proton Fluctuations in a Hydrogen-Bonded Donor−Acceptor Assembly

The temperature-isotope dependence of proton-coupled electron transfer (PCET) for a noncovalent molecular dyad is reported. The system consists of an excited-state Zn(II) porphyrin that transfers an electron to a naphthalene diimide acceptor through an amidinium-carboxylate interface. Two different isotope effects are observed for variant temperature regimes. A reverse isotope effect (i.e., kH/kD < 1) is observed as T approaches 120 K (kH/kD = 0.9, 120 K), whereas a normal isotope effect (i.e., kH/kD > 1) is recovered as the temperature is increased (kH/kD = 1.2, 300 K). The transition between these limits is smooth, with a crossover temperature of T approximately 160 K. These observations are in accordance with charge-transfer dynamics that are susceptible to bath-induced fluctuations in the proton coordinate.

Progress in Charge Transfer Systems Utilizing PorphyrinDonors and Simple Aromatic Diimide Acceptor Units

An overview of the evolution of artificial photosynthetic charge transfer systems containing porphyrin donors and pyromellitic or naphthalene diimide acceptor units is presented. Progression in this area of research is highlighted by the complexity of the systems, the nature of the medium separating donor and acceptor as well as the progression in the lifetime of the charge-separated state upon photoexcitation. A number of supramolecular systems that utilize hydrogen bonding or axial ligation of zinc porphyrins as a means for spatial orientation are highlighted.

Synthetic Ion Channels with Rigid-Rod π-Stack Architecture that Open in Response to Charge-Transfer Complex Formation

We describe the design, synthesis, and evaluation of synthetic ion channels with a new rigid-rod pi-stack architecture that open in response to guest binding by aromatic electron donor-acceptor interactions. Highly cooperative and highly selective ligand gating is shown to yield anion selective, small ion channels that have the characteristic plum color of the charge-transfer complexes formed between the dialkoxynaphthalene ligands and the stacked naphthalenediimide acceptors of the channel.

Modulating Electron Transfer in a Simple Bichromophoric System Employing Axial-Ligation as an Organising Precept

A naphthalene diimide acceptor 1 bearing a pyridine group linked directly (a series) or through a —CH2CH2— spacer (b series) coordinated axially to a metallotetraarylporphyrin (MP) undergoes fast photoinduced electron transfer (in the case of MP = Zn(II)TTP) while the kinetically more stable ruthenium complexes (MP = Ru(CO)TPP) have been used to illustrate the correlation between the distance of probe protons from the porphyrin plane and the change in chemical shift (Δδ) upon coordination. Changes in the emission spectra at 650 nm (λex= 400 nm) upon the addition of Zn(II) ions and/or 1b to TTPH2 can be interpreted in a truth table to illustrate a NAND gate.

Observation of Proton-Coupled Electron Transfer by Transient Absorption Spectroscopy in a Hydrogen-Bonded, Porphyrin Donor−Acceptor Assembly

Proton-coupled electron transfer (PCET) kinetics of a Zn(II) porphyrin donor noncovalently bound to a naphthalene-diimide acceptor through an amidinium-carboxylate interface have been investigated by time-resolved spectroscopy. The S1 singlet excited-state of a Zn(II) 2-amidinium-5,10,15,20-tetramesitylporphyrin chloride (ZnP-beta-AmH+) donor is sufficiently energetic (2.04 eV) to reduce a carboxylate-diimide acceptor (DeltaG degrees = -460 mV, THF). Static quenching of the porphyrin fluorescence is observed and time-resolved measurements reveal more than a 3-fold reduction in the S1 lifetime of the porphyrin upon amidinium-carboxylate formation (THF, 298 K). Picosecond transient absorption spectra of the free ZnP-beta-AmH+ in THF reveal the existence of an excited-state isosbestic point between the S1 and T1 states at lambdaprobe = 650 nm, providing an effective 'zero-kinetics' background on which to observe the formation of PCET photoproducts. Distinct rise and decay kinetics are attributed to the build-up and subsequent loss of intermediates resulting from a forward and reverse PCET reaction, respectively (kPCET(fwd) = 9 x 108 s-1 and kPCET(rev) = 14 x 108 s-1). The forward rate constant is nearly 2 orders of magnitude slower than that measured for covalently linked Zn(II) porphyrin-acceptor dyads of comparable driving force and D-A distance, establishing the importance of a proximal proton network in controlling charge transport.

A Bipolar Charge Transport Molecule

Hole and electron mobilities have been measured in vapor deposited layers of N-(p-(di-p-tolylamino)phenyl)- N′-(1,2-dimethylpropyl)-1,4,5,8-naphthalenetetracarboxylic diimide (TAND). The TAND molecule contains a naphthalene diimide acceptor functionality and a triarylamine donor functionality. The mobilities are of comparable magnitude and show similar field and temperature dependencies. The results are described within the framework of a formalism based on disorder. The formalism is premised on the assumption that charge transport occurs by hopping through a manifold of localized states with superimposed energetic disorder. The key parameter of the formalism is the energy width of the hopping site manifold. For TAND, the widths are 0.110 and 0.091 eV for hole and electron transport, respectively. The result leads to the conclusion that the hole and electron transport manifolds are independent and not influenced by the transport states of the oppositely charged carriers.

Bipolar Transport in a Molecularly Doped Polymer Containing a Bifunctional Dopant Molecule

Hole and electron mobilities have been measured in N-[p-(di-ptolylamino) phenyl]-N'-(1,2-dimethylpropyl)-1,4,5,8-naphthalenetetracarboxylic diimide (TAND)- oped polycarbonate (PC). The TAND molecule contains a naphthalene diimide acceptor functionality and a triarylamine donor functionality. Both electron and hole transport are observed in TAND-doped PC with comparable mobilities. To our knowledge, this is the first literature report of bipolar transport in a doped polymer containing a bifunctional dopant molecule. The results are described by a model based on disorder, due to Bassler and coworkers. The model is premised on the assumption that charge transport occurs by hopping through a manifold of localized states with superimposed energetic and positional disorder. The key parameters of the formalism are &#x3C3;, the energy width of the hopping site manifold, and &#x3A3;, the degree of positional disorder. For TAND-doped PC, the widths are 0.130 and 0.137 eV for hole and electron transport, respectively. The corresponding values of the positional disorder are 3.7 and 3.4. The results suggest that the hole and electron transport manifolds are each independent and not influenced by the transport states of the oppositely charged carriers.