Singlet fission (SF) in two or more electronically coupled organic chromophores converts a high-energy singlet exciton into two low-energy triplet excitons, which can be used to increase solar cell efficiency. Many known SF chromophores are unsuitable for device applications due to chemical instability and low triplet state energies. The results described here show that efficient SF occurs in polycrystalline thin films of 9,10bis(phenylethynyl)anthracene (BPEA), a commercial dye that has singlet and triplet energies of 2.40 and 1.11 eV, respectively, in the solid state. BPEA crystallizes into two polymorphs with space groups C2/c and Pbcn, which undergo SF with kSFA = (109 ± 4 ps) −1 and kSFB = (490 ± 10 ps) −1 , respectively. The high triplet energy and efficient SF evidenced from the 180 ± 20% triplet yield make BPEA a promising candidate for enhancing solar cell performance.

Singlet fission (SF) is a spin-allowed process that involves absorption of a photon by two electronically interacting chromophores to produce a singlet exciton state, (SS), followed by rapid formation of two triplet excitons if the singlet exciton energy is about twice that of the triplet exciton. The initial formation of the multiexciton correlated triplet pair state, (TT), is thought to involve the agency of charge transfer (CT) states. The dynamics of these electronic states were studied in a covalent slip-stacked terrylene-3,4:11,12-bis(dicarboximide) (TDI) dimer in which the conformation of two TDI molecules is determined by a xanthene spacer (XanTDI). Femtosecond mid-infrared (fsIR) spectroscopy shows that the multiexciton (TT) state has absorptions characteristic of the T state in the carbonyl stretch region of the IR spectrum, in addition to IR absorptions specific to the CT state in the C═C stretch region. The simultaneous presence of CT and triplet state features in both high dielectric constant CHCl and low dielectric constant 1,4-dioxane throughout the multiexciton state lifetime suggests that this state has both CT and triplet character.

We have designed and used four different spacers, denoted A–D, to connect two pentacenes and to probe the impact of intramolecular forces on the modulation of pentacene–pentacene interactions and, in turn, on the key steps in singlet fission (SF), that is, the 1(S1S0)-to-1(T1T1) as well as 1(T1T1)-to-5(T1T1) transitions by means of transient absorption and electron paramagnetic resonance measurements. In terms of the 1(S1S0)-to-1(T1T1) transition, a superexchange mechanism, that is, coupling to a higher-lying CT state to generate a virtual intermediate, enables rapid SF in A–D. Sizeable electronic coupling in A and B opens, on one hand, an additional pathway, that is, the population of a real intermediate, and changes, on the other hand, the mechanism to that of hopping. In turn, A and B feature much higher 1(T1T1) quantum yields than C and D, with a maximum value of 162% for A. In terms of the 1(T1T1)-to-5(T1T1) transition, the sizable electronic coupling in A and B is counterproductive, and C and D give rise to higher 5(T1T1)-to-(T1 + T1) quantum yields than A and B, with a maximum value of 85% for D.

Singlet fission (SF) is a photophysical process in which one of two adjacent organic molecules absorbs a single photon, resulting in rapid formation of a correlated triplet pair (T1T1) state whose spin dynamics influence the successful generation of uncorrelated triplets (T1). Femtosecond transient visible and near-infrared absorption spectroscopy of a linear terrylene-3,4:11,12-bis(dicarboximide) dimer (TDI2), in which the two TDI molecules are directly linked at one of their imide positions, reveals ultrafast formation of the (T1T1) state. The spin dynamics of the (T1T1) state and the processes leading to uncoupled triplets (T1) were studied at room temperature for TDI2aligned in 4-cyano-4′-pentylbiphenyl (5CB), a nematic liquid crystal. Time-resolved electron paramagnetic resonance spectroscopy shows that the (T1T1) state has mixed5(T1T1) and3(T1T1) character at room temperature. This mixing is magnetic field dependent, resulting in a maximum triplet yield at ∼200 mT. The accessibility of the3(T1T1) state opens a pathway for triplet–triplet annihilation that produces a single uncorrelated T1state. The presence of the5(T1T1) state at room temperature and its relationship with the1(T1T1) and3(T1T1) states emphasize that understanding the relationship among different (T1T1) spin states is critical for ensuring high-yield T1formation from singlet fission.

Photoexcitation of molecular chromophore aggregates can form excimer states that play a significant role in photophysical processes such as charge and energy transfer as well as singlet fission. An excimer state is commonly defined as a superposition of Frenkel exciton and charge transfer states. In this work, we investigate the dynamics of excimer formation and decay in π-stacked 9,10-bis(phenylethynyl)anthracene (BPEA) covalent dimers appended to a xanthene spacer, where the electronic coupling between the two BPEA molecules is adjusted by changing their longitudinal molecular slip distances. Using exciton coupling calculations, we quantify the relative contributions of Frenkel excitons and charge transfer states and find that there is an upper and lower threshold of the charge transfer contribution for efficient excimer formation to occur. Knowing these thresholds can aid the design of molecular aggregates that optimize singlet fission.

Dimerisation of a wide range of fluorophores through multiple CB[8] clampings leads to constrained intracomplex motion and distinct photophysical properties.

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We demonstrate adsorption and depolymerization of long-chain β-glu strands derived from cellulose, within the microporous confines of a zeolite-templated carbon (ZTC) material. The ZTC adsorbs β-glu strands that have a radius of gyration several-fold larger than the ZTC micropore diameter and do so rapidly (less than 2 min) and in adsorbed β-glu coverages of up to 80% of the ZTC mass. Principles of supramolecular chemistry predict that such adsorption occurs inside of the ZTC based on its micropore size as host being nearly ideal for glucan guest. A comparative study of partially etched materials and nitrogen physisorption at −196 °C indeed demonstrates β-glu adsorption to occur within internal ZTC micropores rather than on the external surface. Such adsorption under micropore confinement is expected to place significant mechanical strain on the β-glu strand, and this strain can be in principle relieved by depolymerization via hydrolysis. This hypothesis motivated us to investigate depolymerization of adsorbed β-glu strands in ZTC, where the ZTC serves as a catalyst for adsorbed β-glu hydrolysis. After a 3 h treatment in water at 180 °C, adsorbed β-glu was converted to soluble glucose in 73% yield. This represents the highest glucose yield observed to date for a carbon catalyst without postsynthetic surface functionalization and speaks to the effectiveness of weak-acid sites for β-glu hydrolysis within a constrained micropore environment.