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, T, and corresponding NDI end-capped series, TNDI (where n is the number of thienylene repeat units). This paper reports a complete photophysical characterization study of the T and TNDI 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 TNDI oligomers were determined by analysis of photophysical and electrochemical data. Excitation of the T 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, T*, to N,N-dimethylviologen (MV) occurs, and by using microsecond transient absorption it is possible to assign the visible region absorption spectra for the one electron oxidized (polaron) states, T. The fluorescence of the TNDI 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-(T)*-NDI → NDI-(T)-NDI → NDI-T-NDI. Preliminary femtosecond-picosecond transient absorption studies on TNDI reveal that both forward electron transfer and charge recombination occur with k > 10 s, 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 -ΔG ∼ λ.

The photophysics of a series of thiophene oligomers (T n ) with and without naphthalene diimide (NDI) acceptor end groups were investigated using femtosecond transient absorption spectroscopy. Photoexcited thiophene oligomers (n = 4, 6, 8, 10, and 12) exhibit complex length-dependent excited-state dynamics on the picosecond time scale due to rapid structural relaxation and intersystem crossing. The incorporation of NDI end groups leads to ultrafast charge separation after selective excitation of the thiophene donor. Initial location of photoexcitation dictates the time scale of charge separation and, therefore, recombination. Photoexcitations near the NDI acceptor result in fast charge separation in all T n -NDI 2 oligomers, whereas excitations near the center of the oligomer must undergo a length-dependent long-range electron-transfer or energy-transfer/electron-transfer step to create a charge-separated state. Oligomer structure plays a role in the charge separation and recombination processes, where T 4 -NDI 2 exhibits small-molecule behavior and longer oligomers approach polymeric behavior.

Homocoupling defects in conjugated polymers often go undetected but may cause significant batch-to-batch variations that ultimately give seemingly identical polymers different material properties. These defects may go easily unnoticed because conjugated polymers are commonly characterized via gelpermeation chromatography and elemental analysis, two techniques that are not able to provide information on monomer incorporation or end groups. Nuclear magnetic resonance spectroscopy has provided evidence of homocoupling defects, but is limited to polymeric repeat units with distinct chemical shifts and little spectral overlap, a luxury unavailable in polymeric dioxythiophenes. Here, matrix-assisted laser desorption/ionization timeof-flight (MALDI-TOF) mass spectrometry (MS) was used to characterize different dioxythiophene copolymer (PE 2 ) batches based on 3,4-propylenedioxythiophene (ProDOT) and 2,2′-bis-(3,4-ethylenedioxy)thiophene (biEDOT) to elucidate changes in structure within different polymer batches. It was determined through the analysis of MALDI-TOF mass spectra that excess biEDOT is incorporated into PE 2 when using standard direct (hetero)arylation polymerization (DHAP) conditions. It is hypothesized that the high nucleophilicity of biEDOT causes uncontrolled concerted metalation−deprotonation steps in the DHAP catalytic cycle at high temperatures. To improve control of the biEDOT incorporation, the reaction temperature was lowered from 140 to 80 °C, and a different polymerization procedure was used where the reaction temperature was ramped-up from room temperature. Ultimately, incorporation of excess biEDOT was advantageous to the conductivity of oxidatively doped polymer films, with values greater than 200 and 80 S/cm for the high-and low-temperature polymerizations, respectively. This work correlates small differences in polymer structure with solid-state conductivity to expose how batch-to-batch variations regarding homocouplings can produce different material properties.

Oligoether-functionalized dioxythiophene polymers are a promising class of materials for electrochemical applications requiring aqueous electrolytes with rapid, reversible redox behavior, high pseudocapacitance, and strong electrochromic contrast. By copolymerizing different monomers (EDOT, DMP, and PheDOT) with an oligoether-functionalized propylenedioxythiophene unit, we tune the redox properties, modulating the onsets of oxidation, redox kinetics, and conductance properties in an aqueous electrolyte (NaCl/H 2 O). Density functional theory calculations are subsequently employed to establish a theoretical basis for the observed differences in energy levels of the polymers. Polymer films demonstrate <1 s discharge rates, <1.5 s electrochromic switching times, and 90% charge retention after 1000 cycles. As these materials demonstrate rapid and reversible redox behavior, we test the utility of these materials as electrochromes and as active layers in type I aqueous supercapacitors. In both aqueous and organic electrolytes, these materials demonstrate high electrochromic contrasts, with comonomer selection altering the colors of the resultant polymers. As active layers in supercapacitors, all polymers show relatively constant current response as a function of cell voltage, and P(OE3)-E, in a test device, demonstrates high current retention after 15,000 charge/discharge cycles. This work demonstrates the broad utility of oligoether-functionalized dioxythiophenes for aqueous redox applications while detailing the tuning of optical, electrochemical, and conductance properties through comonomer selection.

The solution-state aggregation of conjugated polymers is critical to the morphology and device performance of bulk heterojunction (BHJ) organic solar cells (OSCs). However, the detailed structures of polymer solution-state aggregates and their impact on the morphology and device performance of OSCs remain largely unexplored. Herein, we utilize a benzodithiophenebased donor polymer (PM7) and its ester-functionalized derivatives (PM7 D1 and D2) with reduced backbone rigidity as our model systems to demonstrate how a polymer solution-state aggregate structure impacts the morphology and processing resiliency of OSCs. Using X-ray scattering and microscopic imaging techniques, we ascertain that PM7 solution forms a combination of semi-crystalline fiber aggregates and amorphous polymer chain network aggregates, whereas PM7 D1 and D2 solutions primarily form amorphous network aggregates through sidechain associations. Interestingly, when the solution temperature is increased, the fiber aggregates of PM7 break down while the polymer network aggregates remain stable. Due to this temperature-dependent behavior of the fiber aggregates, blade-coated devices fabricated from the PM7 donor polymer and non-fullerene acceptor, ITIC-4F, lead to highly processing temperature-sensitive performance, whereas PM7 D1 and D2 polymers exhibit improved processing temperature resiliency. More importantly, we report that amorphous, network-like aggregates are conducive to superior device performance in blade-coated OSCs owing to the formation of blend films with short π−π stacking distance, small domain spacing, and face-on preferred molecular orientation. In contrast, we find that fiber-like aggregates lead to large π−π stacking distance, large domain spacing, and isotropic molecular orientation in the blend film, which deteriorate the device performance.

This work exposes the importance of testing a polymers active layer thickness tolerance as small modifications to a polymers structure can radically change its ability to stack/pack in the BHJ which is reflected in thick active layer OSCs.

Donor−acceptor dyads consisting of octathiophene (T 8 ) paired with three (di)imide acceptors (naphthalene diimide (NDI), benzene diimide (BDI), and naphthalimide (NI)) were synthesized and probed for their photoinduced forward electron transfer (ET) and charge recombination kinetics by using ultrafast transient absorption (TA) spectroscopy. The three acceptors have different electron affinities, leading to variation in the energy of the charge-separated state and the driving force (ΔG) for forward ET and charge recombination. Analysis of the TA spectra and kinetics allows assignment of rates for forward ET and charge recombination for each of the oligomers. Electrochemistry and photoluminescence spectroscopy are used to determine the ΔG values for the ET processes. For two of the oligomers (T 8 NDI and T 8 BDI), the rates for forward ET and charge recombination are very rapid (k > 3 × 10 10 s −1 ). By contrast, for the third oligomer (T 8 NI), the rates for both processes are considerably slower (k < 5 × 10 9 s −1 ). Analysis of the rate/free energy correlation for the series of oligomers reveals generally good agreement with the Marcus semiclassical theory. In all of the oligomers, the ET reactions are nonadiabatic, in part, due to weak coupling caused by out-of-plane twisting of the phenylene spacer that lies between the T 8 segment and the (di)imide acceptors. The rapid ET dynamics for T 8 NDI and T 8 BDI are explained as arising due to the processes occurring near the barrierless region (−ΔG ≈ λ) or slightly into the Marcus inverted region (−ΔG > λ). The slower dynamics for T 8 NI are explained as arising because the forward ET is weakly exothermic, whereas charge recombination is deep into the inverted region. This study is the first to produce experimental results that match a full Marcus bell-shaped curve with ET rates in the normal, barrrierless, and inverted regions in dyads based on a π-conjugated oligomer donor.

conjugated polymers (CPs) with tunable electronic properties will remain a challenge without adequate solution processability due to the importance of techniques such as roll-to-roll manufacturing. Consequently, modifying CP backbones with polar side chains has recently resurged as an attractive structural design approach to improve polymer solubility and to provide CPs with the capability of transporting both electrons and ions, which is crucial for applications such as organic electrochemical transistors (OECTs). Here, a new dioxythiophene copolymer comprised of 2, 2'-bis-(3,4-ethylenedioxy)thiophene (biEDOT) and 3,4-propylenedioxythiophene (ProDOT) substituted with branched oligo(ether) side chains (PE 2 -biOE2OE3) is synthesized using two direct hereto(arylation) polymerization (DHAP) techniques. The typical DHAP technique results in a lower molecular weight polymer (PE 2 -biOE2OE3(L)), which is soluble in acetone and demonstrated a solid-state conductivity after oxidative doping of 55 ± 3 S cm −1 . Alternatively, a unique temperature ramp DHAP methodology results in a higher molecular weight polymer (PE 2 -biOE2OE3(H)) with an especially high solidstate conductivity of 430 ± 60 S cm −1 . Notably, the first OECT fabricated from an acetone-processed polymer is reported, which is stable up to 500 cycles and can provide a pathway for future material design aimed at eliminating the use of toxic chlorinated solvents in OECT active layer processing.