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Hydrogen-Bonded Water Clusters Confined in Nanocavity as Bright Color Emitters

Molecules confined in the nanocavity and nanointerface exhibits rich of
unique physicochemical properties, e.g., green fluorescent protein (GFP).
However, the physical origin of their photoluminescence (PL) emission
remains elusive. To mimic the microenvironment of GFP protein scaffold at the
molecule level, two groups of nanocavities were created by molecule
self-assembling using organic chromophores and by organic functionalization
of mesoporous silica, respectively. We provide strong evidence that, the
hydrogen-bonded water clusters (WCs) confined in these nanocavities are true
color emitters with a universal formula of {X+
∙(OH-
∙H2O)(H2O)n-1}, in which X is
hydrated protons (H3O+
) or protonated amino (NH3
+
) groups. Further controlled
experiments and combined characterizations by time-resolved steady-state
and ultra-fast transient optical spectroscopy showed that, the PL efficiency of
WCs strongly depends on the type and stability of WCs structure, which is
dominated by H-Bonds. Differing from the local excitation of organic
chromophores, the excited state of WCs has the characteristic of the
topological excitations due to the many-body nuclear quantum effect (MBNQE)
in confined nanocavity.
