Articles written in Journal of Chemical Sciences
Volume 131 Issue 6 June 2019 Article ID 0052
Unconventional non-fluorescent J-aggregates of Tetracene (TC) and Naphtho[2,1,8-qra]tetracene (NT) were witnessed and their consequent dramatic quenching was unravelled by a steady state, time-resolved and transient spectroscopy in conjunctionwith excited state density functional calculations. The TC O-aggregate with slippage angle θ = 22.3◦ ˂ 54.7◦ exhibited substantial transition dipole moment (TDM) for both lower (2.79 D) and higher (1.59 D) energy singlet excitations, while, NT formed an ideal J-aggregate (polarizationangle, α ~ 0◦ ) with a predominant TDM to only a lower excitonic state (2.69 D). Subsequently, their unusual quenching was quantified with large drops in the photoluminescence quantum yields (PLQY) from 0.116 to 0.002 upon TC O-aggregation and from 0.478 to 0.038 upon NT J-aggregation. These intense PL drops were systematically investigated for possible occurrence of excimer-like emission quenching and/or photodegradation of the TC core unit. In view of the TC O-aggregates exhibiting a perfect energetic balance between the singlet (2.34 eV) and triplet (1.28 eV) energies for singlet fission (SF) and a concomitant delayed fluorescence signal, their S1 decay characteristics were attributed to SF followed by an inverse triplet-triplet recombination.In contrast, the energetic imbalance (E(S1) ˂ 2xE(T1)) in NT J-aggregates permitted only forward processof SF and the resulting long-lived triplet formation was traced with a positive transient absorption (T1 → Tn) band at 500 nm. Accordingly, the singlet excited state (S1) dynamics of TC O- and NT J-aggregates, being largely dominated by SF, depicted a depleted S1 population, accounting for the large deviation from aggregation induced enhanced emission, exhibited by classical dye J-aggregates.
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