Improving Photophysical Properties of Deazaflavin Derivatives by Acrylaldehyde Bridging: A Theoretical Investigation

The electronic structure and photophysical properties of several acrylaldehyde-bridged deazaflavin derivatives (cFLs) were investigated theoretically. The impact of acrylaldehyde bridging on photophysical properties of deazaflavin (cFL) is strongly site-dependent. Specifically, the change of adiabatic energy of electronic transitions(ΔE_ad) and vibronic coupling promote fluorescent emission to be comparable to internal conversion of cFL and cFL4 (both C5−C6 and C9−N10 bridged, but C9−N10 bridged by propene), turning them eligible as fluorescent sensors. As El-Sayed's rule is satisfied in cFL1(C5−C6 bridged), cFL2(C9−N10 bridged) and cFL3(both C5−C6 and C9−N10 bridged), intersystem crossing from first singlet excited state to triplet excited states (T_n) become dominant and the evolution of excited cFLs from T₁ appears vital. The rate constants of photophysical processes indicate these cFLs are of dominantly high steady state T₁ concentration and are potential triplet sensitizers. We expect the findings would pave the way for rational design of novel cFLs with extraordinary photophysical properties.