Molecular packing-dependent exciton dynamics in functionalized anthradithiophene derivatives: From solutions to crystals

Abstract

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.

Exciton Polariton-Enhanced Photodimerization of Functionalized Tetracene

“…Next, the first two allowed singlet excited-state (vertical) transition energies E S0‑Sn along with the corresponding oscillator strengths f S0‑Sn (for n = 1, 2) were calculated using time-dependent (TD) DFT methods at the same level of theory. The same calculations were repeated for the Tc molecule with the TIPS side groups interchanged for SiH 3 groups, which were also used for calculations on endoperoxides (EPOs) and photodimers discussed below . A visualization of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) for SiH 3 -Tc is given in Figure S2.…”

“…When oxygen is present ( B samples), acenes are known to form EPOs, and so the two possible EPOs were geometrically optimized and then TD DFT was used to predict the first two excited states using the same methodology as described above. Additionally, the natural transition orbitals were calculated, as described in our previous publication . Tc-EPO A is where the oxygen molecule bonds on the center ring without the side groups (see Figure S6), and Tc-EPO B is where it bonds across the same ring as the side groups.…”

“…The same calculations were repeated for the Tc molecule with the TIPS side groups interchanged for SiH 3 groups, which were also used for calculations on endoperoxides (EPOs) and photodimers discussed below. 26 A visualization of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) for SiH 3 -Tc is given in Figure S2. To explore the photodegradation pathways for TIPS-Tc, several candidate molecules were studied.…”

“…Additionally, the natural transition orbitals were calculated, as described in our previous publication. 26 Tc-EPO A is where the oxygen molecule bonds on the center ring without the side groups (see Figure S6), and Tc-EPO B is where it bonds across the same ring as the side groups. In T and C samples, a dimer-based alternative photodegradation pathway was explored, denoted (Tc + Tc) in Table 3.…”

Exciton Polaritons Reveal “Hidden” Populations in Functionalized Pentacene Films

“…Examples are photorefractive glasses such as a dicyanomethylene­dihydrofuran derivative (exhibiting ultrastrong coupling with Rabi splitting of >1 eV), OLED and OPV materials such as 4,4-cyclohexylidene­bis­[ N , N -bis­(4-methylphenyl)­benzenamine] (TAPC) and phthalocyanines, and OFET materials such as derivatives of acenes (e.g., anthracene (Ac), tetracene (Tc), pentacene (Pn)) − and of anthra­dithiophenes (ADTs) . In addition to their attractive electronic properties, acenes and ADTs have served as model systems for photophysical studies on all levels, from single molecules incorporated in solids, − which began from seminal demonstration of optical detection of single Pn molecules dispersed in a p -terphenyl crystal, to molecular crystals. − Many of these derivatives (including Tc, Pn, and ADT) exhibit singlet fission (SF), the process of creating two triplet (T 1 ) excitons upon excitation of a singlet (S 1 ) state (S 0 + S 1 → TT → T 1 + T 1 , where TT is a correlated triplet pair that serves as a precursor to the free triplets), which has generated a considerable amount of attention due to its potential to enhance the efficiency of organic solar cells …”