  Nanoplasmonics has been used to enhance molecular spectroscopic signals, with
exquisite spatial resolution down to the sub-molecular scale. By means of a
rigorous, state-of-the-art multiscale model based on a quantum chemical
description, here we show that optimally tuned tip-shaped metal nanoparticles
can selectively excite localized regions of typically coherent systems,
eventually narrowing down to probing one single pigment. The well-known major
light-harvesting complex LH2 of purple bacteria has been investigated because
of its unique properties, as it presents both high and weak delocalization
among subclusters of pigments. This finding opens the way to the direct
spectroscopic investigation of quantum-based processes, such as the quantum
diffusion of the excitation among the chromophores, and their external
manipulation
