On the possibility of observation of single quadrupoles by fluorescence resonance energy transfer scanning near-field optical microscopy

The possibility to observe single quadrupoles by the recently proposed fluorescence resonance energy transfer scanning near-field optical microscopy (FRET SNOM) is analyzed. When an excited dipole donor center of the SNOM tip is scanned close to a quadrupole molecule of the sample (at a distance of 1-2 nm), the probability of donor-quadrupole FRET becomes close to unity. Thus a single quadrupole can be imaged as a donor fluorescence quenching or by observing dielectric tip-enhanced fluorescence of the quadrupole. To the best of our knowledge, this is the only existing possibility to visualize single quadrupoles.