  Concentration quenching is a well-known challenge in many fluorescence
imaging applications. Here we show that the optical emission from hundreds of
chromophores confined onto the surface of a virus particle 28 nm diameter can
be recovered under pulsed irradiation. We have found that, as one increases the
number of chromophores tightly-bound to the virus surface, fluorescence
quenching ensues at first, but when the number of chromophores per particle is
nearing the maximum number of surface sites allowable, a sudden brightening of
the emitted light and a shortening of the excited state lifetime are observed.
This radiation brightening occurs only under short pulse excitation;
steady-state excitation is characterized by conventional concentration
quenching for any number of chromophores per particle. The observed suppression
of fluorescence quenching is consistent with efficient, collective relaxation
at room temperature. Interestingly, radiation brightening disappears when the
emitters' spatial and/or dynamic heterogeneity is increased, suggesting that
the template structural properties may play a role and opening a way towards
novel, virus-enabled imaging vectors that have qualitatively different optical
properties than state-of-the-art biophotonic agents.
