Role of Intermolecular Interactions in the Excited-State Photophysics of Tetracene and 2,2′-Ditetracene

Abstract

We carried out a comparative study on the excited-state properties of tetracene, which is prone to singlet fission, and its 2,2′ditetracene derivative to analyze the dependence of such loss channels on molecular packing and, thus, intermolecular interactions. In neat single crystals, the absorption and emission spectra of 2,2′-ditetracene are significantly red-shifted by about 150 meV from those of tetracene, while in the case of isolated molecules dispersed in crystalline anthracene matrixes, both dyes show virtually identical emission patterns. Unlike tetracene, the absence of delayed fluorescence in 2,2′-ditetracene single-crystal photoluminescence (PL) and timedependent density functional theory (DFT) calculations based on experimental single-crystal structures indicate a decline in the excited singlet-state energy below the E(S 1 ) = 2E(T 1 ) threshold. Simultaneously, time-resolved temperature-dependent PL data reveal an interplay of the molecular S 1 state and an excited dimer state, which is efficiently populated above 10 K. Upon cooling, the photoluminescence of 2,2′-ditetracene crystals outperforms that of tetracene by more than an order of magnitude and highlights the potential of molecular design, here the covalent linking of two tetracenes, to conserve the optical properties of the individual chromophores while decisively improving their photophysical properties in the crystalline aggregate.

Distinguishing between Triplet-Pair State and Excimer Emission in Singlet Fission Chromophores Using Mixed Thin Films

“…However, one complication that arises in the investigation of SF materials by PL spectroscopy is the fact that the intermediate 1 (TT) state is a dark state that becomes only emissive via Herzberg–Teller coupling. ,,− The resulting red shift of the corresponding emission compared to the energy of the 1 (TT) state leads to an overlap of its spectral signature with other contributions at low photon energies such as excimers ,,,,− or trap-state emission. ,− This poses the experimental challenge to disentangle the various contributions leading to emission features at low photon energies (in the following referred to as red-shifted luminescence features (RSL)) and to clarify whether the related states mediate SF ,,,,− or constitute a loss channel. ,,,,,,− This question directly relates to an ongoing debate in the literature, which concerns the role of excimer formation in the SF process. ,,,− ,, Excimers as a competing channel for SF have been observed experimentally in tetracene (TET) ,,, and antradithiophene (ADT) derivatives, , but have also been proposed as an intermediate state for SF. ,,, …”

“…11,14,15,22,23,25,30−34 This question directly relates to an ongoing debate in the literature, which concerns the role of excimer formation in the SF process. 11,12,14,[21][22][23][24][25][26]31,34 Excimers as a competing channel for SF have been observed experimentally in tetracene (TET) 11,14,34,35 and antradithiophene (ADT) derivatives, 20,30 but have also been proposed as an intermediate state for SF. 10,27,36,37 The challenges preventing a general elucidation of the nature of the many possible states which can cause RSL (X RSL ) in thin films of SF materials arise from the above-mentioned ambiguity in the assignment of spectral signatures 10,13,16,20,34 and the lack of a tunable parameter that affects the energetic position of trap state emission, 1 (TT) state emission, and excimer emission differently.…”

Dispersion Interactions in Exciton-Localized States. Theory and Applications to π–π* and n−π* Excited States

“…Modeling of intermolecular forces is therefore useful in designing nanostructures with high phosphorescence quantum yields , or optoelectronic materials. , For instance, the efficiency of organic light emitting diodes can be increased by exploiting the spin fission process, in which a highly energetic singlet exciton is converted into two triplet excitons . Spin fission may be driven by molecular interactions, − but the mechanism of this process is not fully understood . Interactions between an excited molecule and its environment is also an active field of research.…”

Modulating Singlet Fission by Scanning through Vibronic Resonances in Pentacene-Based Blends

“…SF describes the photophysical process in which a singlet exciton (S 1 ) splits up into two triplet excitons (2 × T 1 ) via 1 (TT), a multiexcitonic state of singlet character. − There is an ongoing debate about what controls the first step of SF, namely, the conversion from S 1 to 1 (TT). Multiple experimental − and theoretical − studies have emphasized the role of relative molecular orientation ,− , and contributions from real or virtual charge-transfer states. − ,,,− In addition, motivated by theoretical modeling, ,− the importance of vibrations in mediating the electronic coupling between S 1 and 1 (TT) ,,− has been investigated. While pioneering studies employed sophisticated experimental techniques ,,− to probe vibronic coherences, precisely controlling the energetic resonance between the involved states was not a focus.…”