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

We report 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 is 4-(<i>N</i>-piperidinyl)naphthalene-1,8-dicarboximide, the NDI acceptor is naphthalene-1,8:4,5-bis(dicarboximide), and xy is a 2,5-xylyl spacer. The results on these compounds are compared to the analogous derivatives having a <i>p</i>-methoxyaniline (MeOAn) as the donor. BDX^•<b>+</b> has no nitrogen atoms and only a single hydrogen atom coupled to its unpaired electron spin, and therefore has significantly decreased hyperfine interactions compared to MeOAn^<b>•+</b>. We use femtosecond transient absorption (fsTA) and nanosecond TA (nsTA) spectroscopies, the latter with an applied static magnetic field, to study the charge transfer dynamics and determine the spin-spin exchange interaction (<i>J</i>) for <b>BDX</b>^<b>•+</b><b>-ANI-NDI</b>^<b>•-</b> and <b>BDX</b>^<b>•+</b><b>-ANI-xy-NDI</b>^<b>•-</b> at both ambient and cryogenic temperatures. Time-resolved electron paramagnetic resonance (EPR) and pulse-EPR measurements on these spin-correlated radical pairs (SCRPs) were used to probe their spin dynamics. We demonstrate that <b>BDX</b>^<b>•+</b><b>-ANI-xy-NDI</b>^<b>•-</b> has an unusually long lifetime of ∼550 μs in glassy butyronitrile (PrCN) at 85 K, which makes it useful for pulse-EPR studies that target quantum information science (QIS) applications. We also show that rotation of the BDX group about the single bond linking it to the neighboring phenyl group has a significant impact on the spin dynamics, and in particular the magnitude of <i>J</i>. By comparing the results on these compounds to the analogous MeOAn series, insights into design principles for creating improved spin-correlated radical pair systems for QIS studies are obtained.