Frank C. Spano

Doping is a cornerstone strategy for enhancing charge transport in semiconducting polymers, important for their application in, for example, semi‐transparent electrode materials, thermoelectric devices, and antistatic coatings. Both chemical and electrochemical doping have, for this purpose, been the focus of extensive research resulting in considerable progress. However, the interactions between neutral excitons and doping‐induced charges to form multi‐particle states are largely unexplored in soft organic semiconductors, and their signatures remain poorly understood. Here, we demonstrate that coupling between excitons and polarons in doped polymers can lead to bound states such as trions (i.e., quasiparticles of an electron and two holes delocalized across three chromophores for p ‐doping), or bound exciton‐hole pairs. Combining spectroscopic evidence with theoretical insights, we hypothesize that polymer architecture, dopant chemistry, and charge delocalization govern the formation and stability of these multi‐particle states. More broadly, our findings reveal that trions and bound exciton‐hole pairs—that is, three‐body entities—are a key species in organic semiconductors that could open new pathways toward optoelectronic functionalities beyond conventional doping, including enhanced charge transport and quantum‐coherent excitations.

Analysis of the microcavity reflectivity spectrum of a thin poly(thiophene) (P3HT) film is presented, based on the Frenkel–Holstein–Tavis–Cummings Hamiltonian and a Lindblad formalism to describe relaxation through system–bath interactions. A partitioning scheme is employed to treat large, disordered ensembles of P3HT chain segments, which, based on analysis of the free-space absorption spectrum, is divided into aggregate (60%) and amorphous (40%) domains. The reflectivity spectrum is in excellent agreement with the measured spectrum when the ensemble light–matter coupling, NgS, is taken to be 0.9 eV, where N is the total number of P3HT chain segments (in the aggregate and amorphous domains) and gs is the light–matter coupling for a single segment. The spectrum exhibits a relatively narrow lower polariton (LP) feature with a much broader upper polariton (UP), with an approximate Rabi splitting of 1 eV. The two relatively weak middle polaritons are attributed to bright vibronic polaritons, which owe their spectral appearance to a Herzberg–Teller mechanism in which exciton–phonons with zero quasi-momentum borrow optical intensity from the LP and UP. The spectral feature attributed to the LP originates mainly from aggregate domains, while the UP originates from both aggregate and amorphous domains.

A theoretical study is conducted to unveil the nature of intermolecular interactions in orthorhombic 2,4-bis[4-(N,N-diisobutylamino)-2,6-dihydroxyphenyl]squaraine (SQIB) crystalline thin films. The orthorhombic polymorph of SQIB crystallizes in the Pbcn space group with four equivalent molecules per unit cell (Z = 4), leading to three orthogonally polarized Davydov components and a dark state. The remarkable agreement between experimental polarized absorbance spectra and the simulated spectra utilizing the Frenkel-Holstein Hamiltonian allows one to reliably assign the observed spectroscopic features to three Davydov components originating from intermolecular Coulomb interactions. Additionally, the energetic location of the dark state, which plays an important role in nonradiative relaxation, is predicted to lie roughly midway between the lower and upper Davydov components.

Dipyrrins form a group of versatile chromophores, which find use as laser dyes as well as in light harvesting and bioimaging applications. The mode of the central ion coordination and the ensuing molecular geometry play a key role in the photophysics of dipyrrins, whereby some complexes are brightly fluorescent and some completely lack emissivity. However, the relationship between the structure and excitation dynamics in dipyrrins is still poorly understood. Here, we used a range of spectroscopic methods to investigate the photophysics of Zn(II) complexes of meso-Ar-2,2'-di-tert-butoxycarbonyl-dibenzodipyrrins (BDP; Ar = 4-MeO₂C-C₆H₄). In particular, two-dimensional electronic spectroscopy (2DES) was used to characterize the initial excited states in a homoleptic bis-dipyrrinate Zn(BDP)₂, in which two dipyrrin ligands are oriented in a nonorthogonal geometry. From the position of the peaks in the 2DES spectra and spectral modeling, the initial excited states of Zn(BDP)₂ were assigned to excitonic states. The low oscillator strength, associated with excitation to the lower excitonic state, is responsible in part for the weak emissivity of Zn(BDP)₂, contrasting the bright fluorescence of mono-dippyrinate Zn(BDP)X. Femtosecond (fs-), nanosecond (ns-) transient absorption (TA), and time-resolved fluorescence spectroscopies were used to monitor the solvent-dependent evolution of the excitonic states, which appear to evolve into an intermediate state possibly with charge transfer character. Taken together, our findings reveal a significant impact of both structural and environmental factors on the photophysics of dipyrrins and present the first example of the application of 2DES to investigate excitonic states in a system where the interacting chromophores are held together via coordination of an optically neutral metal ion. On a broader scale, we demonstrate that nonorthogonal bis-dipyrrin complexes constitute a versatile model for studying exciton coupling and associated energy and charge dynamics.

A thorough understanding of the nature and pattern of intermolecular interactions in molecular aggregates and crystals is a prerequisite for the design of the next generation of functional materials. In systems with multiple, symmetrically equivalent molecules per unit cell, each excited state of the isolated molecule splits into several Davydov components that appear in the absorption spectra in up to three orthogonally polarized transitions. In this work, a Frenkel–Holstein Hamiltonian is adopted to simulate the vibronic structure of the Davydov components in aggregates and crystals with up to four molecules per unit cell, where electrostatic intermolecular interactions define either 1D or 2D structures. Analysis shows that vibronic signatures report directly on the electronic couplings that contribute to the Davydov splitting and the exciton band shapes. Specifically, the vibronic signature of a given Davydov component is solely determined by its free excitonic shift. For crystals with two molecules per unit cell, the lower and upper Davydov components can each exhibit J-like or H-like behavior, resulting in JJ, JH, and HH components in order of increasing energy, all with unique vibronic signatures. Under certain conditions, null points can exist in either band, leading to a monomer-like absorption spectrum for the corresponding Davydov component. In crystals with four symmetrically equivalent molecules per unit cell, the J- or H-nature of the three orthogonally polarized Davydov components results in four possible combinations, JJJ, JJH, JHH, and HHH in order of increasing energy, all readily identified through vibronic signatures.

In this work, we compare two structurally near-amorphous rigid-rod polymers─poly(indacenodithiophene-co-benzothiadiazole), p(IDT-BT), and poly(indacenodithiophene-co-benzopyrollodione), p(IDT-BPD)─with orders of magnitude different mobilities to understand the effect charge carrier intrachain delocalization has on electronic transport. Quantum chemical calculations show that p(IDT-BPD) has a barrier to torsion that is significantly lower than that of p(IDT-BT) and is thus more likely to have reduced conjugation lengths. We utilize absorption and photoluminescence spectroscopy to characterize energetic disorder and show that p(IDT-BPD) has higher energetic disorder. Charge modulation spectroscopy (CMS) and model calculations are used to show that charge carriers are substantially delocalized in p(IDT-BT) and occupy near-uniform energetic environments. We find that mobility activated hopping barriers are similar in these two materials. Electronic structure calculations show that both intrachain and interchain couplings of monomer units are poor enough in p(IDT-BPD) that charge carriers collapse to single IDT units and transport via a through-space tunneling mechanism. This work highlights the remarkable charge transport properties of p(IDT-BT) by showing that high mobilities are achievable on device-relevant length scales with only 1D carrier delocalization.

The impact of exciton-vibrational (EV) coupling involving low-energy ("slow") intermolecular vibrations and higher-energy ("fast") intramolecular vibrations on the absorption and emission spectra of H- and J-dimers is studied theoretically for a pair of chromophores with excitonic coupling dominated by transition dipole-dipole coupling, J_C. Exact quantum-mechanical solutions based on a Frenkel-Holstein-Peierls Hamiltonian reveal a fascinating interplay between the two coupling sources in determining the spectral line widths, Stoke shifts and radiative decay rates. It is shown that the ratio rules derived from the vibronic progression of the fast mode in molecular dimers remain valid under the influence of slow-mode EV coupling under most conditions. However, a highly unusual aggregate behavior occurs when the product of local and nonlocal couplings, |g_Lg_NL|, exceeds 2ℏω_s|J_C|, where ℏω_s is the energy of the slow mode. In this regime and when g_L and g_NL are in-phase, an H-dimer (J_C > 0) becomes strongly emissive and can even be super-radiant, while a J-dimer (J_C < 0) with out-of-phase g_L and g_NL values becomes subradiant. Such behaviors are in marked contrast to the predictions of Kasha theory and demonstrate the richness of the photophysical behavior resulting from EV coupling involving inter- and intramolecular vibrations.