Enhancing Photogenerated Radical Pair Properties in\nDonor-Chromophore-Acceptor Systems for Quantum Information Applications

We\nreport on new donor-chromophore-acceptor triads <b>BDX-ANI-NDI</b> and <b>BDX-ANI-xy-NDI</b> where the BDX donor is 2,2,6,6-tetramethylbenzo[1,2-<i>d</i>;4,5-<i>d</i>]bis[1,3]dioxole, the ANI chromophore\nis 4-(<i>N</i>-piperidinyl)naphthalene-1,8-dicarboximide,\nthe NDI acceptor is naphthalene-1,8:4,5-bis(dicarboximide), and xy\nis a 2,5-xylyl spacer. The results on these compounds are compared\nto the analogous derivatives having a <i>p</i>-methoxyaniline\n(MeOAn) as the donor. BDX<sup>•<b>+</b></sup> has no\nnitrogen atoms and only a single hydrogen atom coupled to its unpaired\nelectron spin, and therefore has significantly decreased hyperfine\ninteractions compared to MeOAn<sup><b>•+</b></sup>. We\nuse femtosecond transient absorption (fsTA) and nanosecond TA (nsTA)\nspectroscopies, the latter with an applied static magnetic field,\nto study the charge transfer dynamics and determine the spin–spin\nexchange interaction (<i>J</i>) for <b>BDX</b><sup><b>•+</b></sup><b>-ANI-NDI</b><sup><b>•–</b></sup> and <b>BDX</b><sup><b>•+</b></sup><b>-ANI-xy-NDI</b><sup><b>•–</b></sup> at both\nambient and cryogenic temperatures. Time-resolved electron paramagnetic\nresonance (EPR) and pulse-EPR measurements on these spin-correlated\nradical pairs (SCRPs) were used to probe their spin dynamics. We demonstrate\nthat <b>BDX</b><sup><b>•+</b></sup><b>-ANI-xy-NDI</b><sup><b>•–</b></sup> has an unusually long lifetime\nof ∼550 μs in glassy butyronitrile (PrCN) at 85 K, which\nmakes it useful for pulse-EPR studies that target quantum information\nscience (QIS) applications. We also show that rotation of the BDX\ngroup about the single bond linking it to the neighboring phenyl group\nhas a significant impact on the spin dynamics, and in particular the\nmagnitude of <i>J</i>. By comparing the results on these\ncompounds to the analogous MeOAn series, insights into design principles\nfor creating improved spin-correlated radical pair systems for QIS\nstudies are obtained.