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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 conjunction with 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 (polarization angle, 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 photo-degradation 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 S 1 decay characteristics were attributed to SF followed by an inverse triplet-triplet recombination. In contrast, the energetic imbalance ( E (S 1) < 2xE (T 1) ) in NT J-aggregates permitted only forward process of SF and the resulting long-lived triplet formation was traced with a positive transient absorption ( T 1 → T n) band at 500 nm. Accordingly, the singlet excited state (S 1) dynamics of TC O- and NT J-aggregates, being largely dominated by SF, depicted a depleted S 1 population, accounting for the large deviation from aggregation induced enhanced emission, exhibited by classical dye J-aggregates. Unconventional O- and J -aggregates of Tetracene and Naphtho[2,1,8-qra]tetracene showed non-fluorescent behaviour whose singlet excited state (S 1) dynamics was largely governed by fast Singlet Fission decay, accounting for atypical deviation from the well-established aggregation induced enhanced emission from classical J-aggregates. [ABSTRACT FROM AUTHOR]