Radical-Enhanced Intersystem Crossing in New Bodipy Derivatives and Application for Efficient Triplet–Triplet Annihilation Upconversion

A long-lived
triplet excited state of the well-known fluorophore boron dipyrromethene
(Bodipy) was observed for the first time via efficient radical-enhanced
intersystem crossing (EISC). The triplet state has been obtained in
two dyads in which the Bodipy unit is linked to a nitroxide radical,
2,2,6,6-tetramethyl-1-piperidinyloxyl (TEMPO), with two different
length spacers. The photophysical properties were studied with steady-state
and time-resolved transient optical spectroscopies and electron spin
resonance (cw-ESR and TR-ESR). The fluorescence of Bodipy units is
significantly quenched in the dyads, and the spin-polarized TEMPO
signals were observed with TR-ESR, generated by a radical triplet
pair mechanism. Efficient EISC (ΦT = 80%) was observed
for the dyad with a shorter linker, and the triplet state lifetime
of the Bodipy chromophore is exceptionally long (62 μs). The
EISC takes 250 ps. Poor ISC was observed for the dyad with a longer
linker. The efficient ISC and long-lived triplet excited state in
this flexible system are in stark contrast to the previously studied
rigid EISC systems. The EISC effect was employed for the first time
to perform triplet–triplet annihilation (TTA) upconversion
(quantum yield ΦUC = 6.7%).