Unveiling the Hidden, Dark, and Short Life of a Vibronic State in a Boron Difluoride Formazanate Dye

• Published in: Angewandte Chemie International Edition 2019-10, Vol.58 (43), p.15339-15343
• Main Authors: Melenbacher, Adyn, Dhindsa, Jasveer S., Gilroy, Joe B., Stillman, Martin J.
• Format: Article
• Language: English
• Subjects: BODIPY; Boron; boron difluoride dyes; Difluorides; Dyes; Emission; Encryption; Excitation spectra; fluorescence spectroscopy; Luminescence; optical encryption; Time dependence; vibronic structure; Wavelengths
• ISSN: 1433-7851; 1521-3773
• DOI: 10.1002/anie.201908999

Boron difluoride (BF2) formazanate dyes are contenders for molecular species that exhibit a large Stokes shift and bright red emission. Excitation of 3‐cyanoformazanate complexes with 10 μs wide pulses of specific wavelengths resulted in strong luminescence at 663 nm at both room temperature in solution and at 77 K in a frozen solution. Analysis of the short‐lived excitation spectrum from this luminescence shows that it arises from a vibronic manifold of a higher‐lying excited state. This dark state relaxes to the emitting state over 10 μs. TD‐DFT calculations of the two lowest‐energy excited states show that the relaxed geometries are planar for S1 but highly distorted in S2. The specific time‐ and wavelength‐dependence of the excitation profile provides a unique optical encryption capability through the comparison of emission intensities between adjacent vibronic bands only accessible in the 0–12 μs time domain. Hidden excitation: A novel microsecond delay of a high‐lying, vibronically accessed, significantly distorted excited state that pumps a planar emissive state is reported. The unusually long decay is associated with the significantly distorted geometry. An optically encryption method based on the wavelength‐ and time‐domain of the vibronic state is also described.