Controlling the fluorescence quantum yields of benzothiazole-difluoroborates by optimal substitution

• Published in: Chemical science (Cambridge) Vol. 13; no. 45; pp. 13347 - 1336
• Main Authors: Rybczy ski, Patryk, Bousquet, Manon H. E, Kaczmarek-K dziera, Anna, J drzejewska, Beata, Jacquemin, Denis, O mia owski, Borys
• Format: Journal Article
• Language: English
• Published: CAMBRIDGE Royal Soc Chemistry 23.11.2022; Royal Society of Chemistry; The Royal Society of Chemistry
• Subjects: Chemical compounds; Chemistry; Chemistry, Multidisciplinary; Controllability; Energy dissipation; Extreme values; Fluorescence; Internal conversion; Mathematical analysis; Physical Sciences; Quantum chemistry; Rate constants; Science & Technology; Substitutes; BODIPY DYES; THIOFLAVIN-T BINDING; DESIGN; AGGREGATION-INDUCED EMISSION; NONRADIATIVE DECAY; VISCOSITY; CHARGE-TRANSFER; DERIVATIVES
• ISSN: 2041-6520; 2041-6539
• DOI: 10.1039/d2sc05044g

Precise tuning of the fluorescence quantum yield, vital for countless applications of fluorophores, remains exceptionally challenging due to numerous factors affecting energy dissipation phenomena often leading to its counterintuitive behavior. In contrast to the absorption and emission wavelength which can be precisely shifted to the desired range by simple structural changes, no general strategy exists for controllable modification of the fluorescence quantum yield. The rigidification of the molecular skeleton is known to usually enhance the emission and can be practically realized via the limiting molecular vibrations by aggregation. However, the subtle balance between the abundant possible radiative and non-radiative decay pathways makes the final picture exceptionally sophisticated. In the present study, a series of nine fluorophores obtained by peripheral substitution with two relatively mild electron donating and electron withdrawing groups are reported. The obtained fluorescence quantum yields range from dark to ultra-bright and the extreme values are obtained for the isomeric molecules. These severe changes in emission efficiency have been shown to arise from the complex relationship between the Franck-Condon excited state and conical intersection position. The experimental findings are rationalized by the advanced quantum chemical calculations delivering good correlation between the measured emission parameters and theoretical radiative and internal conversion rate constants. Therefore, the described substituent exchange provides a method to rigorously adjust the properties of molecular probes structurally similar to thioflavin T. A full palette of FQY (form ca. 0 to 98%) was covered by exchanging two groups in a series of nine compounds. The darkest (OMe/CF 3 ) and brightest (CF 3 /OMe) are isomers. All experimental data are supported by TD-DFT calculations.