Tuning the Conformation and Color of Conjugated Polyheterocyclic Skeletons by Installing ortho‐Methyl Groups

: 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.