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We demonstrate that a single polycyclic π‐scaffold can undergo sequential multistep excited‐state structural evolution along the bent, planar, and twisted conformers, which coexist to produce intrinsic multiple fluorescence emissions in room‐temperature solution. By installing a methyl or trifluoromethyl group on the ortho‐site of N,N′‐diphenyl‐dihydrodibenzo[a,c]phenazine (DPAC), the enhanced steric effects change the fluorescence emission of DPAC from a dominant red band to well‐resolved triple bands. The ultra‐broadband triple emissions of ortho‐substituted DPACs range from ≈350 to ≈850 nm, which is unprecedented for small fluorophores with molecular weight of <500. Ultrafast spectroscopy and theoretical calculations clearly reveal that the above dramatic changes originate from the influence of steric hindrance on the shape of excited state potential energy surface (S1 PES). Compared to the steep S1 PES of parental DPAC, the introduction of ortho‐substituent is shown to make the path of structural evolution in S1 wider and flatter, so the ortho‐substituted derivatives exhibit slower structural transformations from bent to planar and then to twisted forms, yielding intrinsic triple emission. The results provide the proof of concept that the bent, planar, and twisted emissive states can coexist in the same S1 PES, which greatly expand the fundamental understanding of the excited‐state structural relaxation. [ABSTRACT FROM AUTHOR]

Abstract: ortho‐Methyl effects are exploited to tune steric hindrance between side‐chain N,N′‐diaryls and polycyclic dihydrodibenzo[a,c]phenazine, and in turn control the conformations of N,N′‐diphenyl‐dihydrodibenzo[a,c]phenazine (DPAC) and its ortho‐methyl derivatives Mx‐My (x=0, 1 or 2, y=1 or 2, x and y correlate with the number of methyl groups in the ortho‐positiond of N,N′‐diphenyl). The magnitude of steric hindrance increases as x and y increase, and the V‐shaped dihydrodibenzo[a,c]phenazine skeleton is gradually tuned from a bent (DPAC) to planar (M2‐M2) structure in the ground state. As a result, the relaxation of the excited‐state structure of DPAC and its numerous analogues could be mimicked by model structures Mx‐My, demonstrating for the first time the the conformation change from bent‐to‐planar and hence a large range of energy‐gap tuning of polycyclic conjugated structures controlled by the steric hindrance. [ABSTRACT FROM AUTHOR]