Design and Photo-Induced Dynamics of Radical-Chromophore Adducts with One- or Two-Atom Separation: Toward Potential Probes for High Field Optical DNP Experiments

Abstract

Breaking the maximum enhancement barrier of 660 at room temperature in a conventional dynamic nuclear polarization (DNP) experiment has the immense potential of practical applications. Optical DNP experiments with radical-chromophore (RC) adducts, which harnesses hyperpolarized radicals, instead of thermalized radicals, offers a powerful way to achieve this. Typical DNP and NMR experiments, however, are carried out at high magnetic fields of about 5−10 T, whereas the large electron spin hyperpolarization (ESP) demonstrated in the RC adducts so far are at a much low field of 0.3 T. Thus, in order to realize a successful optical DNP experiment, it is imperative to ask whether the RC adducts, which are currently available, can achieve a large ESP even at high fields. The present work poses this question and shows that the current RC adducts would not generate a large ESP at high fields unless the separation between the chromophore and nitroxyl moiety is reduced to less than four bonds. Two serious bottlenecks in this direction are the near impossibility of synthesizing such RC adducts using the common nitroxyl radicals and the absence of any photophysical studies on RC adducts with such short spacer groups. In this regard, the present work exploits the spin trapping methodology to synthesize one-and two-atom separated naphthalene−nitroxyl RC adducts. Good yields and excellent stability of the adducts have been demonstrated. Furthermore, the present work presents their detailed photophysical and photochemical studies by transient optical and time-resolved EPR studies. On the basis of the present results, a potential RC adduct is proposed for the high field optical DNP experiments. Finally, the prospect of exploiting the large EPR signal enhancement due to ESP in the field of spin trapping studies has been discussed.

Enhanced Intersystem Crossing and Transient Electron Spin Polarization in a Photoexcited Pentacene–Trityl Radical

“…Stable radical–chromophore open-shell hybrids have emerged as a versatile class of molecules for light emission, − chemical sensing, information storage, and studying the fundamental photophysics of processes such as spin catalysis. , In particular, the ability to spin polarize radicals by optical pumping of the coupled chromophore has allowed one to considerably increase the sensitivity of select electron spin resonance experiments. − In the last few decades there has been considerable progress in the understanding of optically induced spin polarization transfer and charge transfer mechanisms in photoactive molecules , using model systems such as C 60 –TEMPO, ,, naphthalene–TEMPO, NDI–TEMPO, , ZnTPP–BDPA, NDI–BDPA, and pentacene–TEMPO . In parallel, an increasing number of these studies have focused on closed- and open-shell pentacene derivatives with the goal of increasing their stability, solubility, and triplet yield. − An increased triplet yield has, for example, implications for improving charge carrier mobilities in pentacene-doped solar cell devices and applications in molecular spintronics, artificial photosynthesis, and signal enhancement methods for magnetic resonance. − The triplet yield can be enhanced by increasing the intersystem crossing rate, which may be accomplished by addition of free-radical substituents …”

Spinning-Driven Dynamic Nuclear Polarization with Optical Pumping

“…The polarization generated by the RQM is determined by the rates k DQ m , n (see eq , Scheme and Appendix 2, eq (S11)), which depends on the ZFS and exchange interaction ( J CR ) between the chromophore and radical in the excited D 1 –Q 1 state. , Because of the solid-state nature of the sample, k DQ m , n would also depend on the orientation of the ZFS tensor with respect to the magnetic field. However, the ZFS (∼0.3 cm –1 ) is relatively small compared to the electron Zeeman term at high fields (>5 cm –1 ), and the exchange interaction.…”

Harnessing Electron Spin Hyperpolarization in Chromophore–Radical Spin Probes for Subcellular Resolution in Electron Paramagnetic Resonance Imaging: Concept and Feasibility

“…To this end, I performed numerical simulations based on the time-dependent Bloch equations (TDBE), which have been very successful in rationalizing the spin dynamics under the rapid scan conditions − as well as for the hyperpolarization generation. ,,− The Bloch equations, as applicable for the RSEPR, have terms that account for the fast scan of the Zeeman magnetic field (characterized by the scan frequency, f m , and the peak-to-peak scan amplitude B m ) . The ESH generation via the radical triplet pair mechanism (RTPM) (or the reverse quartet mechanism for the linked C–R molecules) is included by adding two extra terms in the Bloch equations, ,, which account for the generation of hyperpolarization by singlet and triplet state quenching of the chromophore. In the present case, I adopted a similar strategy and added those extra terms in the RSEPR Bloch equations, eq (see the Materials and Methods section).…”

“…The former problem has not been addressed yet. However, given the possibility of finetuning the structural parameters of spin probe molecules to adapt for different ν EPR , as has been done earlier to synthesize potential molecules for high-field dynamic nuclear polarization (DNP) applications, 56 it should be possible to design efficient EHP-generating spin probe molecules even at lower ν EPR .…”