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 photo physical 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 (Phi(T) = 80%) was observed for the dyad with a shorter linker, and the triplet state lifetime of the Bodipy chromophore is exceptionally long (62 mu s). 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 Phi(UC) = 6.7%).