Although numerous studies have been devoted to the charge transfer through double-stranded DNA (dsDNA), one of the major problems that hinder their potential applications in molecular electronics is the fast deprotonation of guanine cation (G(+•)) to form a neutral radical that can cause the termination of hole transfer. It is thus of critical importance to explore other DNA structures, among which G-quadruplexes are an emerging topic. By nanosecond laser flash photolysis, we report here the direct observation and findings of the unusual deprotonation behavior (loss of amino proton N2-H instead of imino proton N1-H) and slower (1-2 orders of magnitude) deprotonation rate of G(+•) within G-quadruplexes, compared to the case in the free base dG or dsDNA. Four G-quadruplexes AG3(T2AG3)3, (G4T4G4)2, (TG4T)4, and G2T2G2TGTG2T2G2 (TBA) are measured systematically to examine the relationship of deprotonation with the hydrogen-bonding surroundings. Combined with in depth kinetic isotope experiments and pKa analysis, mechanistic insights have been further achieved, showing that it should be the non-hydrogen-bonded free proton to be released during deprotonation in G-quadruplexes, which is the N2-H exposed to solvent for G bases in G-quartets or the free N1-H for G base in the loop. The slower N2-H deprotonation rate can thus ensure less interruption of the hole transfer. The unique deprotonation features observed here for G-quadruplexes open possibilities for their interesting applications as molecular electronic devices, while the elucidated mechanisms can provide illuminations for the rational design of G-quadruplex structures toward such applications and enrich the fundamental understandings of DNA radical chemistry.

We report the excited-state intramolecular charge transfer (ICT) characteristics of four tetrahydro[5] helicene-based imide (THHBI) derivatives with various electron-donating substitutes in different polarity of solvents using steady-state, time-resolved transient absorption (TA) spectroscopy. It is found that, the small bathochromic-shift of the absorption spectra but large red shift of the emission spectra for all dyes with increasing solvent polarity indicates the larger dipole moment of the excited state compared to ground state. The results of theoretical calculations exhibit the charge transfer from the terminal donors to helical backbone, which accounts for the degrees of red shift of the emission spectra from different extent of ICT nature. Time-resolved TA spectra recorded as a function of electron-donating substitutes and solvent polarity show the dye with stronger donors (THHBI-PhNPh2) in more polar solvent behaves faster excited-state ICT relaxation, leading to the formation of solvent-stabilized ICT state (ICT’ state) from the excited ICT state; The dyes (THHBI-Ph, THHBI-PhCF3 and THHBI-PhOMe) with relative weaker donors show weaker dependence on solvent polarity, and instead of that intersystem crossing (ISC) becomes possible from ICT state to triplet state.