Excited-State Dynamics\nof a Bright Fluorescent\nDye with Precise Control of Emission Color Using Acid–Base\nEquilibrium, Intramolecular Charge Transfer, and Host–Guest\nChemistry

A fluorescent dye, a dithiophene-conjugated benzothiazole\nderivative\n(<b>DTBz</b>), was prepared to have high fluorescence emission\nquantum yields (Φ<sub>F</sub>) across various organic solvents.\nIts emission color modulation, from bright blue to deep red, was achieved\nthrough intramolecular charge transfer (ICT), acid–base equilibrium,\nand host–guest chemistry. Although it exhibits a weak solvatochromic\neffect, <b>DTBz</b> exhibited a bright fluorescence emission\naround 480 nm upon excitation at 390 nm in most solvents. In polar\nsolvents, such as MeOH (methanol), EtOH (ethanol), DMF (<i>N,N</i>-dimethylforamide), and DMSO (dimethyl sulfoxide), an additional\nICT emission band emerged around 640 nm, notably intense in DMSO,\nresulting in a bright greenish-white emission (Φ<sub>F</sub> = 0.67). The addition of 1,8-diazabicyclo[5,4.0]undec-7-ene (DBU)\naltered emission characteristics, reducing emission from the local\nexcited (LE) state and enhancing ICT state emission. The degree of\nemission spectral change saturation with DBU addition varied with\nthe solvent nature. Polar solvents with high dielectric constants,\nlike DMSO and DMF, saw a complete disappearance of LE state emission\nwith 5 equiv of DBU, resulting in a deep red emission (Φ<sub>F</sub>s of 0.53 and 0.48, respectively). Femtosecond transient absorption\nspectroscopy and time-resolved photoluminescence measurements elucidated\nthe excited-state dynamics, revealing a long-lived excited state (τ<sub>–H</sub> = 10.3 ns) at a lower energy emission (640 nm), identified\nas <b>DTBz</b><sup>–</sup>*, supported by transient absorption\nspectra analysis. Further analysis, including time-resolved fluorescence\ndecay measurements and time-dependent density-functional theory (TD-DFT)\ncalculations, underscored the role of deprotonation of <b>DTBz</b>’s hydroxyl group in promoting the ICT process. The CIE coordination\nplot demonstrated wide linear emission color changes upon successive\nDBU additions in all solvents, while emission color precision was\nachieved through host–guest chemistry. Emission changes induced\nby DBU were reverted to the original state upon beta-cyclodextrin\n(β-CD) addition, with the <sup>1</sup>H NMR study revealing\nthe competition between acid–base equilibrium and host–guest\ncomplex formation as the cause of emission color change.