signals for the TAA radical cation and PDI
radical anion between 13,000 and 16,000 cm−^1
along with the ground state bleaching (GSB)
between 18,000 and 23,000 cm−^1 (blue spectrum
in Fig. 1B). The formation of the CSS [quantum
yield (QY) = 0.76] caused a substantial quench-
ing of the^1 PDI fluorescence (QY = 0.24; green
spectrum in Fig. 1B).
TheCSScanbeconceivedasaspin-
correlated RP that was born in the singlet
state as it derived from the singlet^1 PDI. As
outlined above, this^1 CSS underwent a time-
dependent spin interconversion with the
almost-degenerate^3 CSS. The tiny energy
difference between both CSS states is indi-
cated by the small value of ~1 mT of the
resonance fieldBmax(figs. S5 to S7), which is
usually assigned to twice the exchange energy
J. The decay of the TA spectra was multi-
exponential on a time scale of ~1ms (fig. S4),
which is a consequence of the complex decay
kinetics. Although the^1 CSS could directly
undergo CR to the ground state S 0 , the^3 CSS
decayed to a local^3 PDI* triplet state, whose
energy (1.2 eV) was below that of the CSS (1.4 eV;
for the evaluation of the state energies, see
supplementary text, section I). At later times
(>1.5ms), where all CSS has decayed, only
the broad^3 PDI* ESA between 17,000 and
24,000 cm−^1 was visible (red spectrum inFig. 1B), again superimposed by GSB. The(^3) PDI* then slowly decayed to S
0 by intersys-
tem crossing (ISC). This decay scenario has
also been observed and analyzed by us in de-
tail for analogous dyads with the same donor
and acceptor moieties but other bridging units
( 29 ). In these dyads, we also demonstrated
that the spin interconversion was magnetic
field dependent, which influenced the over-
all decay kinetics of the CSS substantially. On
the other hand, analysis of the magnetic field
effect allowed us to extract all relevant kinetic
parameters in detail. Notably,^1 CSS and^3 CSS,
although differing in their reactivity, are com-
pletely identical in their optical properties.
SCIENCEscience.org 17 DECEMBER 2021¥VOL 374 ISSUE 6574 1471
Fig. 1. Scheme of pump-push experiment.(A) Molecular structure
of dyad TAA-An-PDI. (B) Absorption spectrum (black), emission spectrum
(green), TA spectrum at 40 ns (blue), TA spectrum at 1000 ns (red),
and spectral position of push pulse (red vertical line) corresponding to a
relative excitation ratio of 4 to 1 favoring the radical anion over the cation.
(C) Energy-level scheme with state energies and photophysical processes,
including their rate constants. The processes depicted in the brown
box show transitions between the spin levels under Zeeman splitting of the
triplet CSS. Magnetic fieldÐdependent processes are marked in brown.
The inset in the blue frame shows the final^3 PDI yield (atB= 500 mT),
reflecting the^3 CSS amplitude at the instant of the delayed push
pulse. a.u., arbitrary units.
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