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Bioelectromagnetism

FIGURE 4.3 Chemical structure, photocycle, and time dependence of the magnetic feld efect (MFE) of CPF

moieties (Kerpal et al., 2019). (a) Structure of the molecular CPF triad. (b) Simplifed photo scheme including all

processes of relevance for this study. For simplicity, the secondary radical pair C^•+ − P − F^•− is shown as created in a

pure singlet state. For more detailed photochemical information (Kodis et al., 2004; Maeda et al., 2011). (c) Transient

absorption subtraction signal ΔΔA of the radical pair, probed at 980 nm, obtained upon application of a magnetic

feld of B0 = 100 μT. Te dotted lines indicate delay times of 0.08, 0.28, and 0.71 μs afer laser excitation, respectively.

(d) Magnetic feld dependence of the MFE averaged for a time window of 20 ns centered around the indicated delay

times. Te dotted black line indicates the feld position of 100 μT and the feld region approximately corresponding

to the low feld region is highlighted in gray. (Reproduced with permission from Kerpal et al., 2019, Copyright 2019,

Springer Nature.) It is licensed under the Creative Commons Attribution 4.0 International.

the concentration profle of the carotenoid radical cation C⁺ via its absorbance in the near-infrared,

following radical pair creation by a 532 nm laser pulse. Application of a magnetic feld, B0, is expected to

change the rate of singlet-triplet (S-T) interconversion and consequently the overall radical pair kinetics,

concentration, and absorbance. Te efect of the feld is typically quantifed via ΔΔA(t, B0) = ΔA(t, B0) –

ΔA(t, B0 = 0), where ΔA(t, B0) and ΔA(t, B0 = 0) refer to the absorbance of the transient species at 980 nm

(predominantly C^•+), in the presence and absence of the feld, respectively. t defnes the time afer the 532

nm pump laser pulse. It can also be instructive to calculate the so-called percentage feld efect, defned

as magnetic feld efect (t, B0) = ΔΔA(t, B0)/ΔA(t, B0 = 0) × 100%.

Figure 4.3c demonstrates that a 100 μT feld confers a pronounced efect on the recombination kinet­

ics of the radical pair. In the presence of the magnetic feld, the concentration of radicals immediately

following the laser pulse is enhanced, ΔΔA(t < 220 ns, 100 μT) > 0, but fewer radicals survive to micro­

second timescales, ΔΔA(t > 220 ns, 100 μT) < 0. Tis biphasic behavior has been noted before and seems

to be characteristic of singlet-born radical pairs with kS > kT undergoing spin-lattice relaxation at a rate

comparable to recombination (van Dijk et al., 1998; Maeda et al., 2008). Te mixed initial spin state (93%

singlet: 7% triplet) in C^•+−P−F^•− further enhances this efect. Moreover, semiclassical spin dynamics

simulations have recently reproduced some of the complex feld- and time-dependent transient absorp­

tion characteristics of C^•+−P−F^•− without implicit consideration of relaxation processes or mixed initial

spin states (Lewis et al., 2018).

As shown in Figure 4.3d, the percentage of magnetic feld efect (t, B0) is presented at diferent times

(t) afer laser excitation. Te initial discussion will concentrate on the data obtained at early and late