Effects of multibranching on 3-hydroxyflavone-based chromophores and the excited-state intramolecular proton transfer dynamics.
A series of one-, two-, and three-branched chromophores based on 3-hydroxyflavones (1-3) have been synthesized as the first example of multibranched chromophores demonstrating excited-state intramolecular proton transfer (ESIPT). Coupling between the 3-hydroxyflavone branches connected by an electron-donating triphenylamine core is manifested in the red-shifted and asymmetric absorption band of 2, whereas the absorption of 3 is governed by the divided donor strength. Their excited-state charge-transfer (ESCT)-coupled ESIPT dynamics is investigated via femtosecond fluorescence upconversion and is proved to be well correlated with the ratio of normal/tautomer emission in the fluorescence spectra. For 1 and 2, with increased donor strength compared with the 4'-N,N-dialkylamino-3-hydroxyflavone analogue, ESIPT appears to cease in the more polar solvent of acetonitrile. Nevertheless, similar dependence of 1-3 on solvent polarity signifies resembling charge-transfer character at the normal excited states (N*), despite their varying structures. As evidenced by the theoretical approach, the frontier orbitals of vibrationally relaxed (geometry-optimized) N*, from which fluorescence and ESIPT should take place, are localized on one specific branch, leading to similar emission patterns and dynamics, whereas the orbitals contributing to Franck-Condon excitation (absorption) spread over the entire molecule. The localization is found to be facilitated by rotation of a specific branch pivoting on the central nitrogen atom, while planarity is maintained within each 3-hydroxyflavone chromophore.
Lin
Chao-Chen
CC
Department of Chemistry, National Taiwan University, No. 1, Section 4, Roosevelt Road, Taipei 10617, Taiwan, ROC.
Chen
Chyi-Lin
CL
Chung
Min-Wen
MW
Chen
Yi-Ju
YJ
Chou
Pi-Tai
PT
D000814:Aniline Compounds / Q000737:chemistry; D005419:Flavonoids / Q000138:chemical synthesis / Q000737:chemistry*; D056004:Molecular Dynamics Simulation; D015394:Molecular Structure; D011522:Protons; D011789:Quantum Theory; D013237:Stereoisomerism
D000814:Aniline Compounds; D005419:Flavonoids; D011522:Protons; C041477:3-hydroxyflavone