An interesting triplet
excited-state absorption-emission conversion
of a fullerene-functionalized Pt(II) metallacycle (C
60
–Pt) caused by a concentration
effect was directly observed by nanosecond transient absorption (ns
TA) spectroscopy. In dilute solution, the triplet excited-state absorption
(TESA) band was observed at about 750 nm with a lifetime of ca. 10.7
μs. However, with increasing the concentration, the absorption
band converted to a triplet excited-state emission (TESE) band with
a longer lifetime of ca. 15.4 μs. Femtosecond transient absorption
experiments and quantum chemistry calculations were performed to reveal
the excited-state decay pathways of C
60
–Pt in concentrated solution. This conversion
was ascribed to the formation of a triplet excimer, which forms at
localized 3C60
* states. This work demonstrates that radiative excimers with
longer-lived triplet excited states can exist in concentrated solution,
and this finding will provide useful information for applications
of fullerene complexes, especially as photosensitizers.
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