Nonadiabatic Couplings Facilitate Excitation Wavelength–Dependent Fluorescence in a Cyclooctatetraene-Based Polymorph

Achieving wide-range tunable emission colors in the crystalline state of single fluorophores is a challenging task. Here, we report the vibronic coupling in the crystalline state of a hitherto unexplored class of nonaromatic tetrabrominated dibenzocyclooctatetraene in two distinct polymorphic forms. The thermodynamically stable polymorph (P1) revealed a single-emission maximum upon excitation at different wavelengths in the crystalline state adhering to Kasha’s rule and lacks luminescence from triplet states. In contrast, the kinetically stable form of polymorph (P2) exhibited excitation wavelength–dependent emission with a range of Δ𝜆maxemi$\mathrm{\Delta }{\lambda }_{\mathrm{emi}}^{\max }$$\mathrm{\Delta }{\lambda }_{\mathrm{emi}}^{\max }$= 100 nm and room temperature phosphorescence. Prominent nonadiabatic couplings between the bright state and several lower singlet states in the dimeric dibenzocyclooctatetraene polymorph elucidate the excitation wavelength–dependent emission. The variable fluorescence behavior highlights the possibility of cyclooctatetraene derivatives as a novel class of multicolor luminescent materials for future optoelectronic applications.