Molecular Design of Heptazine-Based Photocatalysts: Effect of Substituents on Photocatalytic Efficiency and Photostability.

Recently, a derivative of the heptazine (tris-triazine) molecule, trianisole-heptazine (TAHz), was synthesized and was shown to catalyze the oxidation of water to hydroxyl radicals under 365 nm LED light in a homogeneous reaction (E. J. Rabe et al., J. Phys. Chem. Lett. 2018, 9, 6257-6261). The possibility of water photo-oxidation with a precisely defined molecular catalyst in neat solvents opens new perspectives for clarifying the fundamental reaction mechanisms involved in water oxidation photocatalysis. In the present work, the effects of chemical substituents on the three CH positions of Hz on the photocatalytic reactivity were explored with wave function-based ab initio electronic-structure calculations for hydrogen-bonded complexes of Hz and three selected Hz derivatives (TAHz, trichloro-Hz, and tricyano-Hz) with a water molecule. While anisole is an electron-donating substituent, Cl is a weakly electron-withdrawing substituent and CN is a strongly electron-withdrawing substituent. It is shown that the barrier for the photoinduced abstraction of an H atom from the water molecule is raised (lowered) by electron-donating (electron-withdrawing) substituents. The highly mobile and reactive hydroxyl radicals generated by water oxidation can recombine with the reduced chromophore radicals to yield photohydrates. The effect of substituents on the thermodynamics of the photohydration reaction was computed. Among the four chromophores studied, TAHz stands out on account of the metastability of its photohydrate, which suggests self-healing of the photocatalyst after oxidation of TAHzH radicals by OH radicals. In addition, the effect of substituents on the H atom photodetachment reaction from the reduced chromophores, which closes the catalytic cycle, has been investigated. The energy of the repulsive 2πσ* state, which drives the photodetachment reaction is lowered (raised) by electron-donating (electron withdrawing) substituents. All four chromophores exhibit inverted S1/T1 gaps. This feature eliminates long-lived triplet states and thus avoids the activation of molecular oxygen to highly reactive singlet oxygen.

Photooxidation of water with heptazine-based molecular photocatalysts: Insights from spectroscopy and computational chemistry.

We present a conspectus of recent joint spectroscopic and computational studies that provided novel insight into the photochemistry of hydrogen-bonded complexes of the heptazine (Hz) chromophore with hydroxylic substrate molecules (water and phenol). It was found that a functionalized derivative of Hz, tri-anisole-heptazine (TAHz), can photooxidize water and phenol in a homogeneous photochemical reaction. This allows the exploration of the basic mechanisms of the proton-coupled electron-transfer (PCET) process involved in the water photooxidation reaction in well-defined complexes of chemically tunable molecular chromophores with chemically tunable substrate molecules. The unique properties of the excited electronic states of the Hz molecule and derivatives thereof are highlighted. The potential energy landscape relevant for the PCET reaction has been characterized by judicious computational studies. These data provided the basis for the demonstration of rational laser control of PCET reactions in TAHz-phenol complexes by pump-push-probe spectroscopy, which sheds light on the branching mechanisms occurring by the interaction of nonreactive locally excited states of the chromophore with reactive intermolecular charge-transfer states. Extrapolating from these results, we propose a general scenario that unravels the complex photoinduced water-splitting reaction into simple sequential light-driven one-electron redox reactions followed by simple dark radical-radical recombination reactions.

Singlet-Triplet Inversion in Heptazine and in Polymeric Carbon Nitrides.

According to Hund's rule, the lowest triplet state (T1) is lower in energy than the lowest excited singlet state (S1) in closed-shell molecules. The exchange integral lowers the energy of the triplet state and raises the energy of the singlet state of the same orbital character, leading to a positive singlet-triplet energy gap (ΔST). Exceptions are known for biradicals and charge-transfer excited states of large molecules in which the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) are spatially separated, resulting in a small exchange integral. In the present work, we discovered with ADC(2), CC2, EOM-CCSD, and CASPT2 calculations that heptazine (1,3,4,6,7,9,9b-heptaazaphenalene or tri-s-triazine) exhibits an inverted S1/T1 energy gap (ΔST ≈ -0.25 eV). This appears to be the first example of a stable closed-shell organic molecule exhibiting S1/T1 inversion at its equilibrium geometry. The origins of this phenomenon are the nearly pure HOMO-LUMO excitation character of the S1 and T1 states and the lack of spatial overlap of HOMO and LUMO due to a unique structure of these orbitals of heptazine. The S1/T1 inversion is found to be extremely robust, being affected neither by substitution of heptazine nor by oligomerization of heptazine units. Using time-resolved photoluminescence and transient absorption spectroscopy, we investigated the excited-state dynamics of 2,5,8-tris(4-methoxyphenyl)-1,3,4,6,7,9,9b-heptaazaphenalene (TAHz), a chemically stable heptazine derivative, in the presence of external heavy atom sources as well as triplet-quenching oxygen. These spectroscopic data are consistent with TAHz singlet excited state decay in the absence of a low-energy triplet loss channel. The absence of intersystem crossing and an exceptionally low radiative rate result in unusually long S1 lifetimes (of the order of hundreds of nanoseconds in nonaqueous solvents). These features of the heptazine chromophore have profound implications for organic optoelectronics as well as for water-splitting photocatalysis with heptazine-based polymers (e.g., graphitic carbon nitride) which have yet to be systematically explored and exploited.

The role of spin in the kinetic control of recombination in organic photovoltaics.

In biological complexes, cascade structures promote the spatial separation of photogenerated electrons and holes, preventing their recombination. In contrast, the photogenerated excitons in organic photovoltaic cells are dissociated at a single donor-acceptor heterojunction formed within a de-mixed blend of the donor and acceptor semiconductors. The nanoscale morphology and high charge densities give a high rate of electron-hole encounters, which should in principle result in the formation of spin-triplet excitons, as in organic light-emitting diodes. Although organic photovoltaic cells would have poor quantum efficiencies if every encounter led to recombination, state-of-the-art examples nevertheless demonstrate near-unity quantum efficiency. Here we show that this suppression of recombination arises through the interplay between spin, energetics and delocalization of electronic excitations in organic semiconductors. We use time-resolved spectroscopy to study a series of model high-efficiency polymer-fullerene systems in which the lowest-energy molecular triplet exciton (T1) for the polymer is lower in energy than the intermolecular charge transfer state. We observe the formation of T1 states following bimolecular recombination, indicating that encounters of spin-uncorrelated electrons and holes generate charge transfer states with both spin-singlet ((1)CT) and spin-triplet ((3)CT) characters. We show that the formation of triplet excitons can be the main loss mechanism in organic photovoltaic cells. But we also find that, even when energetically favoured, the relaxation of (3)CT states to T1 states can be strongly suppressed by wavefunction delocalization, allowing for the dissociation of (3)CT states back to free charges, thereby reducing recombination and enhancing device performance. Our results point towards new design rules both for photoconversion systems, enabling the suppression of electron-hole recombination, and for organic light-emitting diodes, avoiding the formation of triplet excitons and enhancing fluorescence efficiency.

Activationless Multiple-Site Concerted Proton-Electron Tunneling.

The transfer of protons and electrons is key to energy conversion and storage, from photosynthesis to fuel cells. Increased understanding and control of these processes are needed. A new anthracene-phenol-pyridine molecular triad was designed to undergo fast photoinduced multiple-site concerted proton-electron transfer (MS-CPET), with the phenol moiety transferring an electron to the photoexcited anthracene and a proton to the pyridine. Fluorescence quenching and transient absorption experiments in solutions and glasses show rapid MS-CPET (3.2 × 1010 s-1 at 298 K). From 5.5 to 90 K, the reaction rate and kinetic isotope effect (KIE) are independent of temperature, with zero Arrhenius activation energy. From 145 to 350 K, there are only slight changes with temperature. This MS-CPET reaction thus occurs by tunneling of both the proton and electron, in different directions. Since the reaction proceeds without significant thermal activation energy, the rate constant indicates the magnitude of the electron/proton double tunneling probability.

Ultrafast Spectroscopy Reveals Electron-Transfer Cascade That Improves Hydrogen Evolution with Carbon Nitride Photocatalysts.

Solar hydrogen generation from water represents a compelling component of a future sustainable energy portfolio. Recently, chemically robust heptazine-based polymers known as graphitic carbon nitrides (g-C3N4) have emerged as promising photocatalysts for hydrogen evolution using visible light while withstanding harsh chemical environments. However, since g-C3N4 electron-transfer dynamics are poorly understood, rational design rules for improving activity remain unclear. Here, we use visible and near-infrared femtosecond transient absorption (TA) spectroscopy to reveal an electron-transfer cascade that correlates with a near-doubling in photocatalytic activity from 2050 to 3810 µmol h-1 g-1 when we infuse a suspension of bulk g-C3N4 with 10% mass loading of chemically exfoliated carbon nitride. TA spectroscopy indicates that exfoliated carbon nitride quenches photogenerated electrons on g-C3N4 at rates approaching the molecular diffusion limit. The TA signal for photogenerated electrons on g-C3N4 decays with a time constant of 1/ke' = 660 ps in the mixture versus 1/ke = 4.1 ns in g-C3N4 alone. Our TA measurements suggest that the charge generation efficiency in g-C3N4 is greater than 65%. Exfoliated carbon nitride, which liberates only trace hydrogen levels when photoexcited directly, does not appear to independently sustain appreciable long-lived charge generation. Thus, the activity enhancement in the two-component infusion evidently results from a cooperative effect in which charge is generated on g-C3N4, followed by electron transfer to exfoliated carbon nitride containing photocatalytic chain terminations. This correlation between electron transfer and photocatalytic activity provides new insight into structural modifications for controlling charge separation dynamics and activity of carbon-based photocatalysts.

Local Hydrogen Bonding Determines Branching Pathways in Intermolecular Heptazine Photochemistry.

Heptazine is the molecular core of the widely studied photocatalyst carbon nitride. By analyzing the excited-state intermolecular proton-coupled electron-transfer (PCET) reaction between a heptazine derivative and a hydrogen-atom donor substrate, we are able to spectroscopically identify the resultant heptazinyl reactive radical species on a picosecond time scale. We provide detailed spectroscopic characterization of the tri-anisole heptazine:4-methoxyphenol hydrogen-bonded intermolecular complex (TAHz:MeOPhOH), using femtosecond transient absorption spectroscopy and global analysis, to reveal distinct product absorption signatures at ∼520, 1250, and 1600 nm. We assign these product peaks to the hydrogenated TAHz radical (TAHzH•) based on control experiments utilizing 1,4-dimethoxybenzene (DMB), which initiates electron transfer without concomitant proton transfer, i.e., no excited-state PCET. Additional control experiments with radical quenchers, protonation agents, and UV-vis-NIR spectroelectrochemistry also corroborate our product peak assignments. These spectral assignments allowed us to monitor the influence of the local hydrogen-bonding environment on the resulting evolution of photochemical products from excited-state PCET of heptazines. We observe that the preassociation of heptazine with the substrate in solution is extremely sensitive to the hydrogen-bond-accepting character of the solvent. This sensitivity directly influences which product signatures we detect with time-resolved spectroscopy. The spectral signature of the TAHzH• radical assigned in this work will facilitate future in-depth analysis of heptazine and carbon nitride photochemistry. Our results may also be utilized for designing improved PCET-based photochemical systems that will require precise control over local molecular environments. Examples include applications such as preparative synthesis involving organic photoredox catalysis, on-site solar water purification, as well as photocatalytic water splitting and artificial photosynthesis.

Polymer triplet energy levels need not limit photocurrent collection in organic solar cells.

We study charge recombination via triplet excited states in donor/acceptor organic solar cells and find that, contrary to intuition, high internal quantum efficiency (IQE) can be obtained in polymer/fullerene blend devices even when the polymer triplet state is significantly lower in energy than the intermolecular charge transfer (CT) state. Our model donor system comprises the copolymer PIDT-PhanQ: poly(indacenodithiophene-co-phenanthro[9,10-b]quinoxaline), which when blended with phenyl-C(71)-butyric acid methyl ester (PC(71)BM) is capable of achieving power conversion efficiencies of 6.0% and IQE ≈ 90%, despite the fact that the polymer triplet state lies 300 meV below the interfacial CT state. However, as we push the open circuit voltage (V(OC)) higher by tailoring the fullerene reduction potential, we observe signatures of a new recombination loss process near V(OC) = 1.0 V that we do not observe for PCBM-based devices. Using photoinduced absorption and photoluminescence spectroscopy, we show that a new recombination path opens via the fullerene triplet manifold as the energy of the lowest CT state approaches the energy of the fullerene triplet. This pathway appears active even in cases where direct recombination via the polymer triplet remains thermodynamically accessible. These results suggest that kinetics, as opposed to thermodynamics, can dominate recombination via triplet excitons in these blends and that optimization of charge separation and kinetic suppression of charge recombination may be fruitful paths for the next generation of panchromatic organic solar cell materials with high V(OC) and J(SC).

Current challenges in organic photovoltaic solar energy conversion.

Over the last 10 years, significant interest in utilizing conjugated organic molecules for solid-state solar to electric conversion has produced rapid improvement in device efficiencies. Organic photovoltaic (OPV) devices are attractive for their compatibility with low-cost processing techniques and thin-film applicability to flexible and conformal applications. However, many of the processes that lead to power losses in these systems still remain poorly understood, posing a significant challenge for the future efficiency improvements required to make these devices an attractive solar technology. While semiconductor band models have been employed to describe OPV operation, a more appropriate molecular picture of the pertinent processes is beginning to emerge. This chapter presents mechanisms of OPV device operation, based on the bound molecular nature of the involved transient species. With the intention to underscore the importance of considering both thermodynamic and kinetic factors, recent progress in elucidating molecular characteristics that dictate photovoltage losses in heterojunction organic photovoltaics is also discussed.

Porphyrins fused with unactivated polycyclic aromatic hydrocarbons.

A systematic study of the preparation of porphyrins with extended conjugation by meso,ß-fusion with polycyclic aromatic hydrocarbons (PAHs) is reported. The meso-positions of 5,15-unsubstituted porphyrins were readily functionalized with PAHs. Ring fusion using standard Scholl reaction conditions (FeCl(3), dichloromethane) occurs for perylene-substituted porphyrins to give a porphyrin ß,meso annulated with perylene rings (0.7:1 ratio of syn and anti isomers). The naphthalene, pyrene, and coronene derivatives do not react under Scholl conditions but are fused using thermal cyclodehydrogenation at high temperatures, giving mixtures of syn and anti isomers of the meso,ß-fused porphyrins. For pyrenyl-substituted porphyrins, a thermal method gives synthetically acceptable yields (>30%). Absorption spectra of the fused porphyrins undergo a progressive bathochromic shift in a series of naphthyl (λ(max) = 730 nm), coronenyl (λ(max) = 780 nm), pyrenyl (λ(max) = 815 nm), and perylenyl (λ(max) = 900 nm) annulated porphyrins. Despite being conjugated with unsubstituted fused PAHs, the ß,meso-fused porphyrins are more soluble and processable than the parent nonfused precursors. Pyrenyl-fused porphyrins exhibit strong fluorescence in the near-infrared (NIR) spectral region, with a progressive improvement in luminescent efficiency (up to 13% with λ(max) = 829 nm) with increasing degree of fusion. Fused pyrenyl-porphyrins have been used as broadband absorption donor materials in photovoltaic cells, leading to devices that show comparatively high photovoltaic efficiencies.

Singlet and triplet excitation management in a bichromophoric near-infrared-phosphorescent BODIPY-benzoporphyrin platinum complex.

Multichromophoric arrays provide one strategy for assembling molecules with intense absorptions across the visible spectrum but are generally focused on systems that efficiently produce and manipulate singlet excitations and therefore are burdened by the restrictions of (a) unidirectional energy transfer and (b) limited tunability of the lowest molecular excited state. In contrast, we present here a multichromophoric array based on four boron dipyrrins (BODIPY) bound to a platinum benzoporphyrin scaffold that exhibits intense panchromatic absorption and efficiently generates triplets. The spectral complementarity of the BODIPY and porphryin units allows the direct observation of fast bidirectional singlet and triplet energy transfer processes (k(ST)((1)BDP→(1)Por) = 7.8 × 10(11) s(-1), k(TT)((3)Por→(3)BDP) = 1.0 × 10(10) s(-1), k(TT)((3)BDP→(3)Por) = 1.6 × 10(10) s(-1)), leading to a long-lived equilibrated [(3)BDP][Por]⇌[BDP][(3)Por] state. This equilibrated state contains approximately isoenergetic porphyrin and BODIPY triplets and exhibits efficient near-infrared phosphorescence (λ(em) = 772 nm, Φ = 0.26). Taken together, these studies show that appropriately designed triplet-utilizing arrays may overcome fundamental limitations typically associated with core-shell chromophores by tunable redistribution of energy from the core back onto the antennae.

Photochemistry of carbon nitrides and heptazine derivatives.

We explore the photochemistry of polymeric carbon nitride (C3N4), an archetypal organic photocatalyst, and derivatives of its structural monomer unit, heptazine (Hz). Through spectroscopic studies and computational analysis, we have observed that Hz derivatives can engage in non-innocent hydrogen bonding interactions with hydroxylic species. The photochemistry of these complexes is influenced by intermolecular nπ*/ππ* mixing of non-bonding orbitals of each component and the relative energy of intermolecular charge-transfer (CT) states. Coupling of the former to the latter appears to facilitate proton-coupled electron transfer (PCET), resulting in biradical products. We have also observed that Hz derivatives exhibit an extremely rare inverted singlet/triplet energy splitting (ΔEST). In violation of Hund's multiplicity rules, the lowest energy singlet (S1) is stabilized relative to the lowest triplet (T1) electronic excited state. Exploiting this unique inverted ΔEST character has obvious implications for transformational discoveries in solid-state OLED lighting and photovoltaics. Harnessing this inverted ΔEST, paired with light-driven intermolecular PCET reactions, may enable molecular transformations relevant for applications ranging from solar energy storage to new classes of non-triplet photoredox catalysts for pharmaceutical development. To this end, we have explored the possibility of optically controlling the photochemistry of Hz derivatives using ultrafast pump-push-probe spectroscopy. In this case, the excited state branching ratios among locally excited states of the chromophore and the reactive intermolecular CT state can be manipulated with an appropriate secondary "push" excitation pulse. These results indicate that we can predictively redirect chemical reactivity with light in this system, which is an avidly sought achievement in the field of photochemistry. Looking forward, we anticipate future opportunities for controlling heptazine photochemistry, including manipulating PCET reactivity with a diverse array of substrates and optically delivering reducing equivalents with, for example, water as a partial source of electrons and protons. Furthermore, we wholly expect that, over the next decade, materials such as Hz derivatives, that exhibit inverted ΔEST character, will spawn a significant new research effort in the field of thin-film optoelectronics, where controlling recombination via triplet excitonic states can play a critical role in determining device performance.

Ion-Pairing Dynamics Revealed by Kinetically Resolved In Situ FTIR Spectroelectrochemistry during Lithium-Ion Storage.

Understanding the kinetics of interfacial ion speciation could inform battery designs. However, this knowledge gap persists, largely due to the challenge of experimentally interrogating the evolution of ions near electrode interfaces in a sea of bulk signals. We report here the very first kinetically resolved correlation between interfacial ion speciation and lithium-ion storage in a model system, by applying global target analysis to in situ attenuated total reflectance (ATR) Fourier-Transform infrared (FTIR) spectroelectrochemical data. Our results suggest that it may be more kinetically viable for lithium to be extracted from contact ion pairs (CIPs) to contribute to faster electrode charging compared to fully solvated lithium. As the search for fast-charging lithium-ion batteries and supercapacitors wages on, this discovery suggests that manipulating the ion pairing within the electrolyte could be one effective strategy for promoting faster-charging kinetics.

Solution-phase synthesis of SnSe nanocrystals for use in solar cells.

Nanocrystals of phase-pure tin(II) selenide (SnSe) were synthesized via a solution-phase route employing stoichiometric amounts of di-tert-butyl diselenide as a novel and facile selenium source. The direct band gap of the resulting nanocrystals (E(g) = 1.71 eV) is significantly blue-shifted relative to the bulk value (E(g) = 1.30 eV), a likely consequence of quantum confinement resulting from the relatively small average diameter of the nanocrystals (mu(D) < 20 nm). Preliminary solar cell devices incorporating SnSe nanocrystals into a poly[2-methoxy-5-(3',7'-dimethyloctyloxy)-1,4-phenylenevinylene] matrix demonstrate a significant enhancement in quantum efficiency and short-circuit current density, suggesting that this earth-abundant material could be a valuable component in future photovoltaic devices.

Intermolecular Hydrogen Bonding Tunes Vibronic Coupling in Heptazine Complexes.

To better understand how hydrogen bonding influences the excited-state landscapes of aza-aromatic materials, we studied hydrogen-bonded complexes of 2,5,8-tris (4-methoxyphenyl)-1,3,4,6,7,9,9b-heptaazaphenalene (TAHz), a molecular photocatalyst related to graphitic carbon nitride, with a variety of phenol derivatives (R-PhOH). By varying the electron-withdrawing character of the para-substituent on the phenol, we can modulate the strength of the hydrogen bond. Using time-resolved photoluminescence, we extract a spectral component associated with the R-PhOH-TAHz hydrogen-bonded complex. Surprisingly, we noticed a striking change in the relative amplitude of vibronic peaks in the TAHz-centered emission as a function of R-group on phenol. To gain a physical understanding of these spectral changes, we employed a displaced-oscillator model of molecular emission to fit these spectra. This fit assumes that two vibrational modes are dominantly coupled to the emissive electronic transition and extracts their frequencies and relative nuclear displacements (related to the Huang-Rhys factor). With the aid of quantum chemical calculations, we found that heptazine ring-breathing and ring-puckering modes are likely responsible for the observed vibronic progression, and both modes indicate decreasing molecular distortion in the excited state with increasing hydrogen bond strength. This finding offers new insights into intermolecular excited-state hydrogen bonding, which is a crucial step toward controlling excited-state proton-coupled electron transfer and proton transfer reactions.

Proton-Coupled Electron Transfer from Water to a Model Heptazine-Based Molecular Photocatalyst.

To gain mechanistic understanding of heptazine-based photochemistry, we synthesized and studied 2,5,8-tris(4-methoxyphenyl)-1,3,4,6,7,9,9b-heptaazaphenalene (TAHz), a model molecular photocatalyst chemically related to carbon nitride. On the basis of time-resolved photoluminescence (TR-PL) spectroscopy, we kinetically reveal a new feature that emerges in aqueous dispersions of TAHz. Using global target analysis, we spectrally and kinetically resolve the new emission feature to be blue shifted from the steady-state luminescence, and observe a fast decay component exhibiting a kinetic isotope effect (KIE) of 2.9 in H2O versus D2O, not observed in the steady-state PL. From ab initio electronic-structure calculations, we attribute this new PL peak to the fluorescence of an upper excited state of mixed nπ*/ππ* character. In water, the KIE suggests the excited state is quenched by proton-coupled electron transfer, liberating hydroxyl radicals that we detect using terephthalic acid. Our findings are consistent with recent theoretical predictions that heptazine-based photocatalysts can participate in proton-coupled electron transfer with H2O.

Hole Transfer from Low Band Gap Quantum Dots to Conjugated Polymers in Organic/Inorganic Hybrid Photovoltaics.

We use photoinduced absorption (PIA) spectroscopy to investigate pathways for photocurrent generation in hybrid organic/inorganic quantum dot bulk heterojunction solar cells. We study blends of the conjugated polymer poly(2,3-bis(2-(hexyldecyl)quinoxaline-5,8-diyl-alt-N-(2-hexyldecyl)dithieno[3,2-b:2',3'-d]pyrrole) (PDTPQx-HD) with PbS quantum dots and find that positively charged polarons are formed on the conjugated polymer following selective photoexcitation of the PbS quantum dots. This result provides a direct spectroscopic fingerprint demonstrating that photoinduced hole transfer occurs from the photoexcited quantum dots to the host polymer. We compute the relative yields of long-lived holes following photoexcitation of both the polymer and quantum dot phases and estimate that more long-lived polarons are produced per photon absorbed by the polymer phase than by the quantum dot phase.

The molecular nature of photovoltage losses in organic solar cells.

Since the inception of heterojunction organic photovoltaic research the organic/organic interface has been thought to play a crucial role in determining the magnitude of the open-circuit voltage. Yet, the task of defining the molecular properties dictating the photovoltage delivered by these devices, that employ mixed or neat layers of different organic molecules to convert incident photons to electricity, is still an active area of research. This will likely be a key step in designing the new materials required for improving future device efficiencies. With the intent to underscore the importance of considering both thermodynamic and kinetic factors, this article highlights recent progress in elucidating molecular characteristics dictating photovoltage losses in heterojunction organic photovoltaics.

Acetylide-bridged tetracene dimers.

Alkyne linked tetracene dimers were synthesized from naphthacenequinone and terminal acetylenes. The silylethynyl tetracene dimers exhibit good solubility, high photostability, and broad absorbance leading to photocurrent generation in an organic photovoltaic device.