Spin-orbit charge
transfer-induced intersystem crossing (SOCT-ISC) is of particular
interest for preparation of heavy atom-free triplet photosensitizers.
Up to now, examples for SOCT-ISC dyads are limited and electron donor/acceptor
SOCT-ISC dyads showing strong visible-light harvesting are rare. Herein,
we studied the photophysics of a series of Bodipy-anthracene (BDP-An)
compact dyads, especially the triplet state electron spin dynamics
with the time-resolved electron paramagnetic resonance (TREPR) spectroscopy.
The electronic coupling matrix elements (V
DA
*) between the 1CT (charge transfer) state and 1LE (locally excited)
state are in the range 773–1545 cm–1. For
one dyad, we observed three triplet states simultaneously with TREPR,
that is triplet states confined on the anthracene (3An)
and the Bodipy (3BDP) moieties as well as a 3CT state. Based on the electron spin polarization of these three
triplet states and the optical experiments, the SOCT-ISC mechanism
is confirmed and the radical pair-ISC mechanism as the main ISC channel
was excluded. Triplet–triplet annihilation-induced delayed
fluorescence was observed for the dyads, which is rare for Bodipy
fluorophores.
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 (Φ = 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 Φ = 6.7%).
Intersystem crossing (ISC) of triplet photosensitizers is a vital process for fundamental photochemistry and photodynamic therapy (PDT). Herein, we report the co‐existence of efficient ISC and long triplet excited lifetime in a heavy atom‐free bodipy helicene molecule. Via theoretical computation and time‐resolved EPR spectroscopy, we confirmed that the ISC of the bodipy results from its twisted molecular structure and reduced symmetry. The twisted bodipy shows intense long wavelength absorption (ϵ=1.76×105 m−1 cm−1 at 630 nm), satisfactory triplet quantum yield (ΦT=52 %), and long‐lived triplet state (τT=492 μs), leading to unprecedented performance as a triplet photosensitizer for PDT. Moreover, nanoparticles constructed with such helical bodipy show efficient PDT‐mediated antitumor immunity amplification with an ultra‐low dose (0.25 μg kg−1), which is several hundred times lower than that of the existing PDT reagents.
An efficient spin orbit charge transfer intersystem crossing (SOCT-ISC) was demonstrated in non-orthogonal compact carbazole-perylenebisimide electron donor/acceptor dyads.
A variety of host L-alanine homo-peptides (to the pentamer) containing one or two spin-labelled TOAC (2,2,6,6-tetramethylpiperidine-1-oxyl-4-amino-4-carboxylic acid) residues were synthesized by solution methods and fully characterized. The conformational features of the terminally blocked, doubly spin-labelled TOAC-(Ala)2-TOAC-Ala-pentapeptide were examined in the crystal state by X-ray diffraction and in solution using a combination of techniques (Fourier transform infrared, circular dichroism, cyclic voltammetry and electron spin resonance) in comparison with singly labelled shorter peptides. The 3(10)-helical structure of the pentapeptide, promoted by the two C alpha, alpha-disubstituted glycines under favourable experimental conditions, allows an interaction to take place between the two nitroxide TOAC side chains spaced by one turn of the helix. Taken together, these results suggest that TOAC is an excellent probe for exploring bends and helices in doubly labelled peptides.
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