Missing Excitons: How Energy Transfer Competes with Free Charge Generation in Dilute-Donor/Acceptor Systems

Energy transfer across the donor–acceptor interface in organic photovoltaics is usually beneficial to device performance, as it assists energy transport to the site of free charge generation. Here, we present a case where the opposite is true: dilute donor molecules in an acceptor host matrix exhibit ultrafast excitation energy transfer (EET) to the host, which suppresses the free charge yield. We observe an optimal photochemical driving force for free charge generation, as detected via time-resolved microwave conductivity (TRMC), but with a low yield when the sensitizer is excited. Meanwhile, transient absorption shows that transferred excitons efficiently produce charge-transfer states. This behavior is well described by a competition for the excited state between long-range electron transfer that produces free charge and EET that ultimately produces only localized charge-transfer states. It cannot be explained if the most localized CT states are the intermediate between excitons and the free charge in this system

Photoinduced Electron Transfer in Naphthalene Diimide End-Capped Thiophene Oligomers

A series of linear thiophene oligomers containing 4, 6, 8, 10, and 12 thienylene units were synthesized and end-capped with naphthalene diimide (NDI) acceptors with the objective to study the effect of oligomer length on the dynamics of photoinduced electron transfer and charge recombination. The synthetic work afforded a series of nonacceptor-substituted thiophene oligomers, <b>T</b>_{<b><i>n</i></b>}, and corresponding NDI end-capped series, <b>T</b>_{<b><i>n</i></b>}<b>NDI</b>_<b>2</b> (where <i>n</i> is the number of thienylene repeat units). This paper reports a complete photophysical characterization study of the <b>T</b>_{<b><i>n</i></b>} and <b>T</b>_{<b><i>n</i></b>}<b>NDI</b>_<b>2</b> series by using steady-state absorption, fluorescence, singlet oxygen sensitized emission, two-photon absorption, and nanosecond–microsecond transient absorption spectroscopy. The thermodynamics of photoinduced electron transfer and charge recombination in the <b>T</b>_{<b><i>n</i></b>}<b>NDI</b>_<b>2</b> oligomers were determined by analysis of photophysical and electrochemical data. Excitation of the <b>T</b>_{<b><i>n</i></b>} oligomers gives rise to efficient fluorescence and intersystem crossing to a triplet excited state that is easily observed by nanosecond transient absorption spectroscopy. Bimolecular photoinduced electron transfer from the triplet states, ³<b>T</b>_{<b><i>n</i></b>}*, to <i>N</i>,<i>N</i>-dimethylviologen (MV²⁺) occurs, and by using microsecond transient absorption it is possible to assign the visible region absorption spectra for the one electron oxidized (polaron) states, <b>T</b>_{<b><i>n</i></b>}^+•. The fluorescence of the <b>T</b>_{<b><i>n</i></b>}<b>NDI</b>_<b>2</b> oligomers is quenched nearly quantitatively, and no long-lived transients are observed by nanosecond transient absorption. These findings suggest that rapid photoinduced electron transfer and charge recombination occurs, NDI-¹(T_<i>n</i>)*-NDI → NDI-(T_<i>n</i>)^+•-NDI^–• → NDI-T_<i>n</i>-NDI. Preliminary femtosecond–picosecond transient absorption studies on <b>T</b>_<b>4</b><b>NDI</b>_<b>2</b> reveal that both forward electron transfer and charge recombination occur with <i>k</i> > 10¹¹ s^–1, consistent with both reactions being nearly activationless. Analysis with semiclassical electron transfer theory suggests that both reactions occur at near the optimum driving force where −Δ<i>G</i> ∼ λ

π‑Conjugated Organometallic Isoindigo Oligomer and Polymer Chromophores: Singlet and Triplet Excited State Dynamics and Application in Polymer Solar Cells

An isoindigo based π-conjugated oligomer and polymer that contain cyclometalated platinum­(II) “auxochrome” units were subjected to photophysical characterization, and application of the polymer in bulk heterojunction polymer solar cells with PCBM acceptor was examined. The objective of the study was to explore the effect of the heavy metal centers on the excited state properties, in particular, intersystem crossing to a triplet (exciton) state, and further how this would influence the performance of the organometallic polymer in solar cells. The materials were characterized by electrochemistry, ground state absorption, emission, and picosecond–nanosecond transient absorption spectroscopy. Electrochemical measurements indicate that the cyclometalated units have a significant impact on the HOMO energy level of the chromophores, but little effect on the LUMO, which is consistent with localization of the LUMO on the isoindigo acceptor unit. Picosecond–nanosecond transient absorption spectroscopy reveals a transient with ∼100 ns lifetime that is assigned to a triplet excited state that is produced by intersystem crossing from a singlet state on a time scale of ∼130 ps. This is the first time that a triplet state has been observed for isoindigo π-conjugated chromophores. The performance of the polymer in bulk heterojunction solar cells was explored with PC₆₁BM as an acceptor. The performance of the cells was optimum at a relatively high PCBM loading (1:6, polymer:PCBM), but the overall efficiency was relatively low with power conversion efficiency (PCE) of 0.22%. Atomic force microscopy of blend films reveals that the length scale of the phase separation decreases with increasing PCBM content, suggesting a reason for the increase in PCE with acceptor loading. Energetic considerations show that the triplet state in the polymer is too low in energy to undergo charge separation with PCBM. Further, due to the relatively low LUMO energy of the polymer, charge transfer from the singlet to PCBM is only weakly exothermic, which is believed to be the reason that the photocurrent efficiency is relatively low

Photophysical Characterization of a Chromophore/Water Oxidation Catalyst Containing a Layer-by-Layer Assembly on Nanocrystalline TiO₂ Using Ultrafast Spectroscopy

Femtosecond transient absorption spectroscopy is used to characterize the first photoactivation step in a chromophore/water oxidation catalyst assembly formed through a “layer-by-layer” approach. Assemblies incorporating both chromophores and catalysts are central to the function of dye-sensitized photoelectrosynthesis cells (DSPECs) for generating solar fuels. The chromophore, [Ru_a^II]²⁺ = [Ru­(pbpy)₂(bpy)]²⁺, and water oxidation catalyst, [Ru_b^II-OH₂]²⁺ = [Ru­(4,4′-(CH₂PO₃H₂)₂bpy)­(Mebimpy)­(H₂O)]²⁺, where bpy = 2,2′-bipyridine, pbpy = 4,4′-(PO₃H₂)₂bpy, and Mebimpy = 2,6-bis­(1-methylbenzimidazol-2-yl)­pyridine), are arranged on nanocrystalline TiO₂ via phosphonate-Zr­(IV) coordination linkages. Analysis of the transient spectra of the assembly (denoted TiO₂-[Ru_a^II-Zr-Ru_b^II-OH₂]⁴⁺) reveal that photoexcitation initiates electron injection, which is then followed by the transfer of the oxidative equivalent from the chromophore to the catalyst with a rate of <i>k</i>_ET = 5.9 × 10⁹ s^–1 (τ = 170 ps). While the assembly, TiO₂-[Ru_a^II-Zr-Ru_b^II-OH₂]⁴⁺, has a near-unit efficiency for transfer of the oxidative equivalent to the catalyst, the overall efficiency of the system is only 43% due to nonproductive photoexcitation of the catalyst and nonunit efficiency for electron injection. The modular nature of the layer-by-layer system allows for variation of the light-harvesting chromophore and water oxidation catalyst for future studies to increase the overall efficiency

Cyclometalated Platinum-Containing Diketopyrrolopyrrole Complexes and Polymers: Photophysics and Photovoltaic Applications

A series of organometallic complexes and polymers has been synthesized with an objective of studying their fundamental photophysical properties together with their organic photovoltaic and organic field-effect transistor properties. The metal chromophores consist of a diketopyrrolopyrrole (DPP) core, end functionalized with cyclometalated platinum “auxochrome”. The photophysical properties of the metal complex and polymers are compared with the unmetalated chromophore <b>DPP-C8-Th-Py</b>. The polymers <b>Poly-DPP-Th-Pt</b> and <b>Poly-DPP-Ph-Pt</b> differ structurally in their cyclometallating ligands, where they consist of 2-thienylpyridine and 2-phenylpyridine, respectively. Efficient solar spectrum coverage was observed for all chromophores; specifically, the polymer <b>Poly-DPP-Th-Pt</b> has an onset of absorption at ∼900 nm with an optical band gap of 1.4 eV. The triplet excited state was detected for all chromophores and probed by both nanosecond and picosecond transient absorption spectroscopy. Both polymers were employed as donors in bulk-heterojunction solar cells with a polymer:<b>PC₇₁BM</b> ratio of 1:7. The thiophene-containing polymer <b>Poly-DPP-Th-Pt</b> shows a respectable power conversion efficiency (PCE) of 1.66% with a high fill factor (FF) of ∼66%. Higher charge carrier mobility was observed for <b>Poly-DPP-Th-Pt</b> when used in field-effect transistors compared to <b>Poly-DPP-Ph-Pt</b>

Cyclometalated Platinum-Containing Diketopyrrolopyrrole Complexes and Polymers: Photophysics and Photovoltaic Applications

A series of organometallic complexes and polymers has been synthesized with an objective of studying their fundamental photophysical properties together with their organic photovoltaic and organic field-effect transistor properties. The metal chromophores consist of a diketopyrrolopyrrole (DPP) core, end functionalized with cyclometalated platinum “auxochrome”. The photophysical properties of the metal complex and polymers are compared with the unmetalated chromophore <b>DPP-C8-Th-Py</b>. The polymers <b>Poly-DPP-Th-Pt</b> and <b>Poly-DPP-Ph-Pt</b> differ structurally in their cyclometallating ligands, where they consist of 2-thienylpyridine and 2-phenylpyridine, respectively. Efficient solar spectrum coverage was observed for all chromophores; specifically, the polymer <b>Poly-DPP-Th-Pt</b> has an onset of absorption at ∼900 nm with an optical band gap of 1.4 eV. The triplet excited state was detected for all chromophores and probed by both nanosecond and picosecond transient absorption spectroscopy. Both polymers were employed as donors in bulk-heterojunction solar cells with a polymer:<b>PC₇₁BM</b> ratio of 1:7. The thiophene-containing polymer <b>Poly-DPP-Th-Pt</b> shows a respectable power conversion efficiency (PCE) of 1.66% with a high fill factor (FF) of ∼66%. Higher charge carrier mobility was observed for <b>Poly-DPP-Th-Pt</b> when used in field-effect transistors compared to <b>Poly-DPP-Ph-Pt</b>

Mediating Photochemical Reaction Rates at Lewis Acidic Rare Earths by Selective Energy Loss to 4f-Electron States

Manifesting chemical differences in individual rare earth (RE) element complexes is challenging due to the similar sizes of the tripositive cations and the corelike 4f shell. We disclose a new strategy for differentiating between similarly sized Dy3+ and Y3+ ions through a tailored photochemical reaction of their isostructural complexes in which the f-electron states of Dy3+ act as an energy sink. Complexes RE(hfac)3(NMMO)2 (RE = Dy (2-Dy) and Y (2-Y), hfac = hexafluoroacetylacetonate, and NMMO = N-methylmorpholine-N-oxide) showed variable rates of oxygen atom transfer (OAT) to triphenylphosphine under ultraviolet (UV) irradiation, as monitored by 1H and 19F NMR spectroscopies. Ultrafast transient absorption spectroscopy (TAS) identified the excited state(s) responsible for the photochemical OAT reaction or lack thereof. Competing sensitization pathways leading to excited-state deactivation in 2-Dy through energy transfer to the 4f electron manifold ultimately slows the OAT reaction at this metal cation. The measured rate differences between the open-shell Dy3+ and closed-shell Y3+ complexes demonstrate that using established principles of 4f ion sensitization may deliver new, selective modalities for differentiating the RE elements that do not depend on cation size

Pathways Following Electron Injection: Medium Effects and Cross-Surface Electron Transfer in a Ruthenium-Based, Chromophore–Catalyst Assembly on TiO₂

Interfacial dynamics following photoexcitation of the water oxidation assembly [((PO₃H₂)₂bpy)₂Ru^II(bpy-bimpy)­Ru^II(tpy)­(OH₂)]⁴⁺, −[Ru_a^II–Ru_b^II–OH₂]⁴⁺, on nanocrystalline TiO₂ electrodes, starting from either −[Ru_a^II–Ru_b^II–OH₂]⁴⁺ or −[Ru_a^II–Ru_b^III–OH₂]⁵⁺, have been investigated. Transient absorption measurements for TiO₂–[Ru_a^II–Ru_b^II–OH₂]⁴⁺ in 0.1 M HPF₆ or neat trifluoroethanol reveal that electron injection occurs with high efficiency but that hole transfer to the catalyst, which occurs on the electrochemical time scale, is inhibited by local environmental effects. Back electron transfer occurs to the oxidized chromophore on the microsecond time scale. Photoexcitation of the once-oxidized assembly, TiO₂–[Ru_a^II–Ru_b^III–OH₂]⁵⁺, in a variety of media, generates −[Ru_a^III–Ru_b^III–OH₂]⁶⁺. The injected electron randomly migrates through the surface oxide structure reducing an unreacted −[Ru_a^II–Ru_b^III–OH₂]⁵⁺ assembly to −[Ru_a^II–Ru_b^II–OH₂]⁴⁺. In a parallel reaction, −[Ru_a^III–Ru_b^III–OH₂]⁶⁺ formed by electron injection undergoes proton loss giving −[Ru_a^II–Ru_b^IVO]⁴⁺ with possible conversion to −[Ru_a^II–Ru_b^II–OH₂]⁴⁺ by an electrolyte-mediated reaction. In the following slow step, re-equilibration on the surface occurs either by reaction with added Fe^III/II or by cross-surface electron transfer between spatially separated −[Ru_a^II–Ru_b^IVO]⁴⁺ and −[Ru_a^II–Ru_b^II–OH₂]⁴⁺ assemblies to give −[Ru_a^II–Ru_b^III–OH₂]⁵⁺ with a half-time of <i>t</i>_1/2 ∼ 68 μs. These results and analyses show that the transient surface behavior of the assembly and cross-surface reactions play important roles in producing and storing redox equivalents on the surface that are used for water oxidation

Disentangling the Physical Processes Responsible for the Kinetic Complexity in Interfacial Electron Transfer of Excited Ru(II) Polypyridyl Dyes on TiO₂

Interfacial electron transfer at titanium dioxide (TiO₂) is investigated for a series of surface bound ruthenium-polypyridyl dyes whose metal-to-ligand charge-transfer state (MLCT) energetics are tuned through chemical modification. The 12 complexes are of the form Ru^II(bpy-A)­(L)₂²⁺, where bpy-A is a bipyridine ligand functionalized with phosphonate groups for surface attachment to TiO₂. Functionalization of ancillary bipyridine ligands (L) enables the potential of the excited state Ru^III/* couple, <i>E</i>^<i>+</i>/*, in 0.1 M perchloric acid (HClO₄(aq)) to be tuned from −0.69 to −1.03 V vs NHE. Each dye is excited by a 200 fs pulse of light in the visible region of the spectrum and probed with a time-delayed supercontiuum pulse (350–800 nm). Decay of the MLCT excited-state absorption at 376 nm is observed without loss of the ground-state bleach, which is a clear signature of electron injection and formation of the oxidized dye. The dye-dependent decays are biphasic with time constants in the 3–30 and 30–500 ps range. The slower injection rate constant for each dye is exponentially distributed relative to <i>E</i>^<i>+</i>/*. The correlation between the exponentially diminishing density of TiO₂ sub-band acceptor levels and injection rate is well described using Marcus–Gerischer theory, with the slower decay components being assigned to injection from the thermally equilibrated state and the faster components corresponding to injection from higher energy states within the ³MLCT manifold. These results and detailed analyses incorporating molecular photophysics and semiconductor density of states measurements indicate that the multiexponential behavior that is often observed in interfacial injection studies is not due to sample heterogeneity. Rather, this work shows that the kinetic heterogeneity results from competition between excited-state relaxation and injection as the photoexcited dye relaxes through the ³MLCT manifold to the thermally equilibrated state, underscoring the potential for a simple kinetic model to reproduce the complex kinetic behavior often observed at the interface of mesoporous metal oxide materials

The Excited-State Lifetime of Poly(NDI2OD-T2) Is Intrinsically Short

Conjugated polymers composed of alternating electron donor and acceptor segments have come to dominate the materials being considered for organic photoelectrodes and solar cells, in large part because of their favorable near-infrared absorption. The prototypical electron-transporting push–pull polymer poly(NDI2OD-T2) (N2200) is one such material. While reasonably efficient organic solar cells can be fabricated with N2200 as the acceptor, it generally fails to contribute as much photocurrent from its absorption bands as the donor with which it is paired. Moreover, transient absorption studies have shown N2200 to have a consistently short excited-state lifetime (∼100 ps) that is dominated by a ground-state recovery. In this paper, we investigate whether these characteristics are intrinsic to the backbone structure of this polymer or if these are extrinsic effects from ubiquitous solution-phase and thin-film aggregates. We compare the solution-phase photophysics of N2200 with those of a pair of model compounds composed of alternating bithiophene (T2) donor and naphthalene diimide (NDI) acceptor units, NDI-T2-NDI and T2-NDI-T2, in a dilute solution. We find that the model compounds have even faster ground-state recovery dynamics (τ = 45, 27 ps) than the polymer (τ = 133 ps), despite remaining molecularly isolated in solution. In these molecules, as in the case of the N2200 polymer, the lowest excited state has a T2 to NDI charge-transfer (CT) character. Electronic-structure calculations indicate that the short lifetime of this state is due to fast nonradiative decay to the ground state (GS) promoted by strong CT–GS electronic coupling and strong electron-vibrational coupling with high-frequency (quantum) normal modes