It Is Good to Be Flexible: Energy Transport Facilitated by Conformational Fluctuations in Light-Harvesting Polymers.

We investigate the mechanism of energy transfer between ruthenium(II) (Ru) and osmium(II) (Os) polypyridyl complexes affixed to a polyfluorene backbone (PF-RuOs) using a combination of time-resolved emission spectroscopy and coarse-grained molecular dynamics (CG MD). Photoexcitation of a Ru chromophore initiates Dexter-style energy hopping along isoenergetic complexes followed by sensitization of a lower-energy Os trap. While we can determine the total energy transfer rate within an ensemble of solvated PF-RuOs from time-dependent Os* emission spectra, heterogeneity of the system and inherent polymer flexibility give rise to highly multiexponential kinetics. We developed a three-part computational kinetic model to supplement our spectroscopic results: (1) CG MD model of PF-RuOs that simulates molecular motions out to 700 ns, (2) energy transfer kinetic simulations in CG MD PF-RuOs that produce time-resolved Ru and Os excited-state populations, and (3) computational experiments that interrogate the mechanisms by which motion aids energy transfer. Good agreement between simulated and experimental emission transients reveals that our kinetic model accurately simulates the molecular motion of PF-RuOs during energy transfer. Simulated results indicate that pendant flexibility allows 81% of the excited state to sensitize an Os trap compared to a 48% occupation when we treat pendants statically. Our computational experiments show how static pendants are only able to engage in local energy transfer. The excited state equilibrates across a domain of complexes proximal to the initial excitation and becomes trapped within that unique, frozen locality. Side-chain flexibility enables pendants to swing in and out of the original domain spreading the excited state out to ±30 pendant complexes away from the initial excitation.

Tetrathiafulvalene-containing polymers for simultaneous non-covalent modification and electronic modulation of MoS2 nanomaterials.

Transition metal dichalcogenides (TMDCs) such as MoS2 comprise an important class of 2D semiconductors with numerous interesting electronic and mechanical features. Full utilization of TMDCs in materials and devices, however, necessitates robust functionalization methods. We report well-defined tetrathiafulvalene (TTF)-based polymers, exploiting synthetic routes that overcome challenges previously associated with these systems. These platforms enable basal plane coordinative interactions with MoS2, conceptually in parallel with pyrene-containing platforms for graphene and carbon nanotube modification. Not yet reported for TMDCs, these non-covalent interactions are universal and effective for MoS2 irrespective of the lattice structure, affording significantly enhanced solution stabilization of the nanosheets. Additionally, the TTF-functionalized polymers offer electronic structure modulation of MoS2 by ground state charge transfer and work function reduction, demonstrated using Kelvin probe force microscopy (KPFM). Notably, coordination and electronic effects are amplified for the TTF-polymers over TTF itself. Experiments are supported by first-principles density functional theory (DFT) calculations that probe polymer-TTF surface interactions with MoS2 and the resultant impact on electronic properties.

The Structural Origin of Electron Injection Enhancements with Fulleropyrrolidine Interlayers.

The orientation of the substituent groups in a new class of work function modification layers, based on functionalized fulleropyrrolidines, is measured and found to directly account for the sign of the work function change.

Ultrafast dynamics in multifunctional Ru(II)-loaded polymers for solar energy conversion.

The use of sunlight to make chemical fuels (i.e., solar fuels) is an attractive approach in the quest to develop sustainable energy sources. Using nature as a guide, assemblies for artificial photosynthesis will need to perform multiple functions. They will need to be able to harvest light across a broad region of the solar spectrum, transport excited-state energy to charge-separation sites, and then transport and store redox equivalents for use in the catalytic reactions that produce chemical fuels. This multifunctional behavior will require the assimilation of multiple components into a single macromolecular system. A wide variety of different architectures including porphyrin arrays, peptides, dendrimers, and polymers have been explored, with each design posing unique challenges. Polymer assemblies are attractive due to their relative ease of production and facile synthetic modification. However, their disordered nature gives rise to stochastic dynamics not present in more ordered assemblies. The rational design of assemblies requires a detailed understanding of the energy and electron transfer events that follow light absorption, which can occur on time scales ranging from femtoseconds to hundreds of microseconds, necessitating the use of sophisticated techniques. We have used a combination of time-resolved absorption and emission spectroscopies with observation times that span 9 orders of magnitude to follow the excited-state evolution within polymer-based molecular assemblies. We complement experimental observations with molecular dynamics simulations to develop a microscopic view of these dynamics. This Account provides an overview of our work on polymers decorated with pendant Ru(II) chromophores, both in solution and on surfaces. We have examined site-to-site energy transport among the Ru(II) complexes, and in systems incorporating π-conjugated polymers, we have observed ultrafast formation of a long-lived charge-separated state. When attached to TiO2, these assemblies exhibit multifunctional behavior in which photon absorption is followed by energy transport to the surface and electron injection to produce an oxidized metal complex. The oxidizing equivalent is then transferred to the conjugated polymer, giving rise to a long-lived charge-separated state.

Synthesis, optical properties, and electronic structures of nucleobase-containing π-conjugated oligomers.

The molecular recognition properties of the nucleobases instruct the formation of complex three-dimensional architectures in natural and synthetic systems; relatively unexplored is their use as building blocks for π-conjugated materials where they might mutually tune electronic and supramolecular structures. Toward this goal, an introductory set (1a-d and 2a-d) of six purine-terminated and two pyrimidine-terminated π-conjugated oligomers has been synthesized and used to develop experimental electronic and photophysical structure-property trends. Unlike 2,2':5',2″-terthiophene (TTT) derivatives 2a-d, intramolecular charge transfer dominates oligomers 1a-d bearing a 4,7-bisthienylbenzothiadiazole (TBT) spacer due to the strong electron-accepting ability of its benzothiadiazole (BTD) ring. The resulting donor-acceptor-donor systems feature lower HOMO-LUMO gaps than the terthiophene-linked nucleobases (ΔE(g) ∼ 1.8 eV vs 2.4 eV based on electrochemical measurements), and the lowest so far for π-conjugated molecules that include nucleobases within the π-framework. Experiments reveal a dependence of photophysical and electronic structure on the nature of the nucleobase and are in good agreement with theoretical calculations performed at the B3LYP/6-31+G** level. Overall, the results show how nucleobase heterocycles can be installed within π-systems to tune optical and electronic properties. Future work will evaluate the consequences of these information-rich components on supramolecular π-conjugated structure.

Poly(sulfobetaine methacrylate)s as electrode modifiers for inverted organic electronics.

We demonstrate the use of poly(sulfobetaine methacrylate) (PSBMA), and its pyrene-containing copolymer, as solution-processable work function reducers for inverted organic electronic devices. A notable feature of PSBMA is its orthogonal solubility relative to solvents typically employed in the processing of organic semiconductors. A strong permanent dipole moment on the sulfobetaine moiety was calculated by density functional theory. PSBMA interlayers reduced the work function of metals, graphene, and poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) by over 1 eV, and an ultrathin interlayer of PSBMA reduced the electron injection barrier between indium tin oxide (ITO) and C70 by 0.67 eV. As a result, the performance of organic photovoltaic devices with PSBMA interlayers is significantly improved, and enhanced electron injection is demonstrated in electron-only devices with ITO, PEDOT:PSS, and graphene electrodes. This work makes available a new class of dipole-rich, counterion-free, pH insensitive polymer interlayers with demonstrated effectiveness in inverted devices.

Intrinsic and extrinsic parameters for controlling the growth of organic single-crystalline nanopillars in photovoltaics.

The most efficient architecture for achieving high donor/acceptor interfacial area in organic photovoltaics (OPVs) would employ arrays of vertically interdigitated p- and n- type semiconductor nanopillars (NPs). Such morphology could have an advantage in bulk heterojunction systems; however, precise control of the dimension morphology in a crystalline, interpenetrating architecture has not yet been realized. Here we present a simple, yet facile, crystallization technique for the growth of vertically oriented NPs utilizing a modified thermal evaporation technique that hinges on a fast deposition rate, short substrate-source distance, and ballistic mass transport. A broad range of organic semiconductor materials is beneficial from the technique to generate NP geometries. Moreover, this technique can also be generalized to various substrates, namely, graphene, PEDOT-PSS, ZnO, CuI, MoO3, and MoS2. The advantage of the NP architecture over the conventional thin film counterpart is demonstrated with an increase of power conversion efficiency of 32% in photovoltaics. This technique will advance the knowledge of organic semiconductor crystallization and create opportunities for the fabrication and processing of NPs for applications that include solar cells, charge storage devices, sensors, and vertical transistors.

Azulene methacrylate polymers: synthesis, electronic properties, and solar cell fabrication.

We report the synthesis of novel azulene-substituted methacrylate polymers by free radical polymerization, in which the azulene moieties represent hydrophobic dipoles strung pendant to the polymer backbone and impart unique electronic properties to the polymers. Tunable optoelectronic properties were realized by adjusting the azulene density, ranging from homopolymers (having one azulene group per repeat unit) to copolymers in which the azulene density was diluted with other pendant groups. Treating these polymers with organic acids revealed optical and excitonic behavior that depended critically on the azulene density along the polymer chain. Copolymers of azulene with zwitterionic methacrylates proved useful as cathode modification layers in bulk-heterojunction solar cells, where the relative azulene content affected the device metrics and the power conversion efficiency reached 7.9%.

Establishing dual electrogenerated chemiluminescence and multicolor electrochromism in functional ionic transition-metal complexes.

A combination of electrochromism and electroluminescence in functional materials could lead to single-layer dual electrochromic/electroluminescent (EC/EL) display devices, capable of simultaneous operation in emissive and reflective modes. Whereas such next generation displays could provide optimal visibility in any ambient lighting situation, materials available that exhibit such characteristics in the active layer are limited due to the required intrinsic multifunctionality (i.e., redox activity, electroluminescence, electrochromism, and ion conductivity) and to date can only be achieved via the rational design of ionic transition-metal complexes. Reported herein is the synthesis and characterization of a new family of acrylate-containing ruthenium (tris)bipyridine-based coordination complexes with multifunctional characteristics. Potential use of the presented compounds in EC/EL devices is established, as they are applied as cross-linked electrochromic films and electrochemiluminescent layers in light-emitting electrochemical cell devices. Electrochromic switching of the polymeric networks between yellow, orange, green, brown and transmissive states is demonstrated, and electrochemiluminescent devices based on the complexes synthesized show red-orange to deep red emission with λ(max) ranging from 680 to 722 nm and luminance up to 135 cd/m(2). Additionally, a dual EC/EL device prototype is presented where light emission and multicolor electrochromism occur from the same pixel comprised of a single active layer, demonstrating a true combination of these properties in ionic transition-metal complexes.

Competition between Ultrafast Energy Flow and Electron Transfer in a Ru(II)-Loaded Polyfluorene Light-Harvesting Polymer.

This Letter describes the synthesis and photophysical characterization of a Ru(II) assembly consisting of metal polypyridyl complexes linked together by a polyfluorene scaffold. Unlike many scaffolds incorporating saturated linkages, the conjugated polymer in this system acts as a functional light-harvesting component. Conformational disorder breaks the conjugation in the polymer backbone, resulting in a chain composed of many chromophore units, whose relative energies depend on the segment lengths. Photoexcitation of the polyfluorene by a femtosecond laser pulse results in the excitation of polyfluorene, which then undergoes direct energy transfer to the pendant Ru(II) complexes, producing Ru(II)* excited states within 500 fs after photoexcitation. Femtosecond transient absorption data show the presence of electron transfer from PF* to Ru(II) to form charge-separated (CS) products within 1-2 ps. The decay of the oxidized and reduced products, PF(+•) and Ru(I), through back electron transfer are followed using picosecond transient absorption methods.