Understanding the impact of inter-molecular orientation on the optical properties of organic semiconductors is important for designing next-generation organic (opto)electronic and photonic devices. However, fundamental aspects of how various features of molecular packing in crystalline systems determine the nature and dynamics of excitons have been a subject of debate. Toward this end, we present a systematic study of how various molecular crystal packing motifs affect the optical properties of a class of high-performance organic semiconductors: functionalized derivatives of fluorinated anthradithiophene. The absorptive and emissive species present in three such derivatives (exhibiting “brickwork,” “twisted-columnar,” and “sandwich-herringbone” motifs, controlled by the side group R) were analyzed both in solution and in single crystals, using various modalities of optical and photoluminescence spectroscopy, revealing the nature of these excited states. In solution, in the emission band, two states were identified: a Franck–Condon state present at all concentrations and an excimer that emerged at higher concentrations. In single crystal systems, together with ab initio calculations, it was found in the absorptive band that Frenkel and Charge Transfer (CT) excitons mixed due to nonvanishing CT integrals in all derivatives, but the amount of admixture and exciton delocalization depended on the packing, with the “sandwich-herringbone” packing motif least conducive to delocalization. Three emissive species in the crystal phase were also identified: Frenkel excitons, entangled triplet pairs 1(TT) (which are precursors to forming free triplet states via singlet fission), and self-trapped excitons (STEs, similar in origin to excimers present in concentrated solution). The “twisted-columnar” packing motif was most conducive to the formation of Frenkel excitons delocalized over 4–7 molecules depending on the temperature. These delocalized Frenkel states were dominant across the full temperature range (78 K–293 K), though at lower temperatures, the entangled triplet states and STEs were present. In the derivative with the “brickwork” packing, all three emissive species were observed across the full temperature range and, most notably, the 1(TT) state was present at room temperature. Finally, the derivative with the “sandwich-herringbone” packing exhibited localized Frenkel excitons and had a strong propensity for self-trapped exciton formation even at higher temperatures. In this derivative, no formation of the 1(TT) state was observed. The temperature-dependent dynamics of these emissive states are reported, as well as their origin in fundamental inter-molecular interactions.

Organic semiconductors have attracted increasing attention due to their low cost, solution processability, and tunable properties. Of special interest are molecules with enhanced environmental stability. We have recently reported on the (opto)electronic properties of a remarkably stable, naturally derived pigment xylindein. Here, we establish that one particular aspect of xylindein's molecular structure, namely the presence of hydroxyl (OH) groups, is critical for enabling its enhanced stability and relatively high electron mobility. In particular, we synthesized a methylated derivative of xylindein, dimethylxylindein, where the OH groups are replaced with OCH 3 groups, and compared photophysics and the (opto)electronic properties of dimethylxylindein and xylindein. We reveal the presence of a long-lived excited state in dimethylxylindein, in contrast to xylindein, which has an efficient fast nonradiative pathway to the ground state. This results in significantly reduced photostability of dimethylxylindein as compared to xylindein. The effective electron mobility, obtained from space-charge-limited currents, in amorphous xylindein films was found to be 4 orders of magnitude higher than that in amorphous and crystalline dimethylxylindein films. In contrast, the photosensitivity of dimethylxylindein is about 2 orders of magnitude higher than that of xylindein. The mechanism of charge transport in all films was thermally activated hopping, with the xylindein films characterized by considerably shallower charge traps than dimethylxylindein films, attributed to hydrogen bonding via hydroxyl groups promoting an efficient conductive network in xylindein.

Abstract

When considering the optimal molecular packing to realize charge multiplication in organic photovoltaic materials, subtle changes in intermolecular charge transfer (CT) coupling can strongly modulate singlet fission. To understand why certain packing arrangements are more conducive to charge multiplication by triplet pair (TT) formation, we measure the diffraction-limited transient absorption (TA) response from four single-crystal functionalized derivatives of fluorinated anthradithiophene: diF R-ADT (R = TES, TSBS, TDMS, TBDMS). diF TES-ADT and diF TDMS-ADT both exhibit 2D brickwork packing structures, diF TSBS-ADT adopts a 1D sandwich-herringbone packing structure, and diF TBDMS-ADT exhibits a 1D twisted-columnar packing structure. When brickwork or twisted-columnar single crystals are resonantly probed parallel to their charge transfer (CT)-axis projections, the TA signal is dominated by a rising component on the picosecond time scale (rate k TT), attributed to TT state population. When probed orthogonal to the CT-axis, we instead recover the falling TA kinetics of singlet state depletion at rate k A. The rising to falling rate ratio estimates the TT formation efficiency, εTT = k TT/k A relative to exciton self-trapping. εTT ranged from near 100% in diF TES-ADT to 84% in diF TDMS-ADT. Interestingly, diF TSBS-ADT crystals only manifest falling kinetics of CT-mediated self-trapping and singlet state depletion. Singlet fission is prohibitive in diF TSBS-ADT crystals owing to its lower symmetry sandwich-herringbone packing that leads to S1 to CT-state energy separation that is ∼3× larger than in other packings. Collectively, these results highlight optimal packing configurations that either enhance or completely suppress CT-mediated TT formation.

We explore the photodegradation mechanisms in functionalized tetracene (TIPS-Tc) films and how they are influenced by strong exciton−photon coupling in planar microcavities. We demonstrate that degradation of TIPS-Tc films exposed to air proceeds mainly through an oxygen-mediated pathway, assigned to endoperoxide (EPO) formation, whereas degradation in microcavities proceeds through oxygen-independent photodimerization. The aerobic and anaerobic decay mechanisms were found to differ in rate by more than two orders of magnitude. Both the EPO formation and photodimerization proceeded more efficiently in molecules in configurations favorable for the correlated triplet pair (TT) state formation (precursor to the singlet fission) and their immediate surroundings. For the photodimerization, an alkyne dimer is reported as one of the photoproducts, and its optical properties are presented. Strong coupling of TIPS-Tc to resonant microcavities enhanced the photodimerization quantum yield by a factor of 4.2, with the enhancement robust with respect to cavity detuning.