Two-Photon Excitation of Gold Nanorods Interrupted by Extremely Fast Solvent-to-Metal Electron Transfer

Gold nanorods (Au NRs) have been widely exploited for various biomedical applications due to their strong two-photon photoluminescence (2PPL). 2PPL of Au NRs were found significantly quenched in organic solvents, which was originally ascribed to reduced luminescence yields caused by electron transfer quenching. It was recently found that excitation of 2PPL of Au NRs involved two sequential one-photon absorption steps. Here various ultrafast spectroscopic techniques have been employed to demonstrate two different solvent-to-metal electron transfer pathways: electron transfer from organic solvents to sp and d band holes of excited Au NRs, which have different influences on 2PPL of Au NRs. Electron transfer to sp band holes occurs extremely fast (similar to 25 fs), which blocks absorption of the second photon and reduces overall two-photon excitation efficiency, which is the dominant mechanism for the observed 2PPL quenching. In contrast, electron transfer to d band holes hinders the radiative recombination process and results in reduced luminescence yield, which plays the minor contribution for the observed 2PPL quenching. This work has proposed and demonstrated an additional brand-new quenching mechanism of 2PPL emission of Au NRs: extremely fast electron transfer to the intermediate states within the laser pulse duration interrupts sequential absorption of two photons, which results in significantly reduced two-photon excitation efficiency. This study provides new insight on fundamental understanding of two-photon excitation processes and nonlinear optical properties of these materials.