The effect of size on the optical properties of guanine nanostructures: a femtosecond to nanosecond study.

G-quadruplexes, whose building blocks are guanine tetrads, encounter increasing interest with respect to their potential applications in the field of molecular electronics. Here we study how the size of these nanostructures affects their fluorescence. We compare the properties of thymine capped G-quadruplexes, formed by association of four single DNA strands d(TG3T), d(TG4T) and d(TG5T) and stabilized by K(+) ions. We show that an increase in the number of tetrads induces a narrowing of the fluorescence spectrum, an increase in the fluorescence quantum yield, a lengthening of fluorescence lifetime and a decrease of the anisotropy detected on the femtosecond time-scale. The in-plane depolarization of the fluorescence, occurring in less than 1 ps, is attributed to population of Franck-Condon exciton states and ultrafast intraband scattering, leading to energy transfer. The persistence of excitons with partial J-aggregate character on the picosecond time-scale increases with the G-quadruplex size, which enhances the stiffness of the system.