The Photophysical Properties of Triisopropylsilyl-ethynylpentacene—A Molecule with an Unusually Large Singlet-Triplet Energy Gap—In Solution and Solid Phases

Abstract

The process of singlet-exciton fission (SEF) has attracted much attention of late. One of the most popular SEF compounds is TIPS-pentacene (TIPS-P, where TIPS = triisopropylsilylethynyl) but, despite its extensive use as both a reference and building block, its photophysical properties are not so well established. In particular, the triplet state excitation energy remains uncertain. Here, we report quantitative data and spectral characterization for excited-singlet and -triplet states in dilute solution. The triplet energy is determined to be 7940 ± 1200 cm−1 on the basis of sensitization studies using time-resolved photoacoustic calorimetry. The triplet quantum yield at the limit of low concentration and low laser intensity is only ca. 1%. Self-quenching occurs at high solute concentration where the fluorescence yield and lifetime decrease markedly relative to dilute solution but we were unable to detect excimer emission by steady-state spectroscopy. Short-lived fluorescence, free from excimer emission or phosphorescence, occurs for crystals of TIPS-P, most likely from amorphous domains.

Singlet Fission in Concentrated TIPS-Pentacene Solutions: The Role of Excimers and Aggregates

“…Our results accord with those of Schaberle et al, who report essentially unchanged emission spectra for TPn in toluene up to a concentration of 50 mM. 22 The lack of concentration-dependent emission spectra at longer wavelengths also accords with the results of Walker et al 14 and Grieco et al 20 Absorption spectra are also unchanged up to 30 mM (see SI), also demonstrated by Schaberle et al in toluene. 22 Immediate evidence of singlet fission in this experiment comes from the concentration-dependent photoluminescence quantum yield (Φ P L ).…”

“…Total-internal reflection excitation showed that the emission spectrum is unchanged over a range up to 28 mM, and as such there are no observable, emissive excimers present. This contrasts strongly with the findings of Walker et al whose emission spectra suffered severe reabsorption, 14 but accords with results obtained in toluene by Schaberle et al 22 NMR experiments confirmed that aggregation occurs in TPn solutions. At the highest concentrations employed spectroscopically, only a few percent of chromophores are present as dimers, and there are hints of this in the observed transient absorption.…”

“…22 The lack of concentration-dependent emission spectra at longer wavelengths also accords with the results of Walker et al 14 and Grieco et al 20 Absorption spectra are also unchanged up to 30 mM (see SI), also demonstrated by Schaberle et al in toluene. 22 Immediate evidence of singlet fission in this experiment comes from the concentration-dependent photoluminescence quantum yield (Φ P L ). If the rate of dynamic quenching is given by k q [T P n],…”

“…The triplet state has intense ESAs at 2.47 and 2.64 eV, and the weaker features at 1.28 and 1.43 eV, as previously identified. 14,22 At 28 mM the ps spectrum is resolved into the same components as above, but with time constants of 5.06 ns and 15 µs (Figure S2).…”

Exploitation of Baird Aromaticity and Clar’s Rule for Tuning the Triplet Energies of Polycyclic Aromatic Hydrocarbons

“…Due to their high tunability, PAHs and their derivatives are also particularly suitable for more intricate applications such as logic gates [7], single molecule conductance [8,9], thermoelectrics [10], and multi-stage redox systems [11]. Ever more intricate PAH structures are being synthesised [12,13] and ever more detailed spectroscopic experiments performed [14][15][16].…”