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

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.

A phenomenological scheme for reversed quartet mechanism of electron spin polarization in covalently linked systems of chromophore and free radical: Determination of magnitude of polarization and application to pyrene-TEMPO linked molecules.

Generation of electron spin polarization (ESP) during the bimolecular quenching of an excited chromophore by a free radical is generally explained by the radical-triplet pair mechanism, which is capable of giving the magnitudes of ESP arising from the quenching of the singlet or the triplet excited chromophore. When the chromophore and the free radical are covalently linked, although there are several mechanisms to explain the observed spin-polarized electron paramagnetic resonance signals under a variety of experimental conditions and in different chromophore-radical systems, there are no schemes that allow quantitative determination of the magnitude of ESP. In this work, we present a phenomenological scheme with this objective. In this scheme, we have incorporated several concepts of the reversed quartet mechanism of Rozenshtein et al. [J. Phys. Chem. A 109, 11144 (2005)] to our phenomenological sequential quenching scheme [V. Rane and R. Das, J. Phys. Chem. A 119, 5515 (2015)] of ESP in covalently linked chromophore-radical systems. This phenomenological reversed quartet scheme is able to explain the observed inversion of ESP with time and can also give a quantitative measure of the absorptive and emissive ESP in such systems. We have applied this scheme to the photophysical quenching of a series of newly synthesized pyrene-TEMPO molecules, where a spacer group of different lengths covalently links the pyrene chromophore and the TEMPO free radical. Given the simplicity of our scheme, reasonable estimates of the magnitudes of the ESP have been obtained in all cases.

Distance-dependent formation of electronic charge-transfer states in the ground states of anthracene and pyrene covalently linked to a TEMPO free radical.

Charge-transfer (CT) electronic states are generally seen in molecules involving interactions between species of low ionization potential and high electron affinity. In this context, the 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO) free radical is not considered to be a typical molecule to form charge transfer states with aromatic hydrocarbons. Nevertheless, involvement of such CT states has been invoked in rationalising the spin-dependent photophysical quenching of excited states of aromatic systems by TEMPO during bimolecular collisions. Direct observation of such CT states, however, has been elusive until recently, with our first report on the observation of CT states involving naphthalene and TEMPO moieties covalently linked through a spacer group (Rane et al., J. Fluoresc., 2015, 25, 1351-1361). With a view to demonstrating more systems of CT states involving a TEMPO donor, and establishing a possible dependence on its distance from an acceptor chromophore, we have now extended our investigation to anthracene (An) and pyrene (Py) moieties linked to TEMPO, using two different spacer groups of different lengths. The molecules are An-CH2-O-TEMPO, Py-CH2-O-TEMPO, Py-(CH2)2-O-TEMPO, Py-(CH2)4-O-TEMPO, Py-CH2-CO-O-TEMPO and Py-(CH2)3-CO-O-TEMPO, where a linear alkyl chain containing an ether or an ester moiety constitutes the spacer group. We established the formation of CT states in their ground states by comparing their electronic absorption spectra, steady-state fluorescence spectra and time-resolved fluorescence signals with those of the parent molecules An-CH2-OH, Py-CH2-OH and Py-CH2-COOH. CT bands of appreciable intensity were seen only with An-CH2-O-TEMPO, Py-CH2-O-TEMPO and Py-CH2-CO-O-TEMPO, the molecules with the shortest spacer group. Approximate shapes of the absorption and emission bands of the CT states have been determined. For the rest, very weak bands were seen. Similar trends were seen in their fluorescence lifetimes also. Absorption intensities of the CT bands were found to decrease exponentially with the length of the spacer group. The presence of the ether or the ester moiety in the spacer groups showed little influence on the intensities of the CT bands. Our results are probably the first experimental demonstration of the expected exponential dependence of the efficiency of the formation of CT states on the length of the spacer groups of chromophore-TEMPO linked molecules.

Efficient Radical-Enhanced Intersystem Crossing in an NDI-TEMPO Dyad: Photophysics, Electron Spin Polarization, and Application in Photodynamic Therapy.

A compact naphthalenediimide (NDI)-2,2,6,6-tetramethylpiperidinyloxy (TEMPO) dyad has been prepared with the aim of studying radical-enhanced intersystem crossing (EISC) and the formation of high spin states as well as electron spin polarization (ESP) dynamics. Compared with the previously reported radical-chromophore dyads, the present system shows a very high triplet state quantum yield (ΦT =74 %), a long-lived triplet state (τT =8.7 µs), fast EISC (1/kEISC =338 ps), and absorption in the red spectral region. Time-resolved electron paramagnetic resonance (TREPR) spectroscopy showed that, upon photoexcitation in fluid solution at room temperature, the D0 state of the TEMPO moiety produces strong emissive (E) polarization owing to the quenching of the excited singlet state of NDI by the radical moiety (electron exchange J>0). The emissive polarization then inverts into absorptive (A) polarization within about 3 µs, and then relaxes to a thermal equilibrium while quenching the triplet state of NDI. The formation and decay of the quartet state were also observed. The dyad was used as a three-spin triplet photosensitizer for triplet-triplet annihilation upconversion (quantum yield ΦUC =2.6 %). Remarkably, when encapsulated into liposomes, the red-light-absorbing dyad-liposomes show good biocompatibility and excellent photodynamic therapy efficiency (phototoxicity EC50 =3.22 µm), and therefore is a promising candidate for future less toxic and multifunctional photodynamic therapeutic reagents.

Photophysical Studies on Covalently-linked Naphthalene and TEMPO Free Radical Systems: Observation of a Charge Transfer State in the Ground State.

A series of molecules containing a naphthalene chromophore and a stable free radical 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) covalently linked by a spacer group of different lengths have been synthesized. In n-hexane solution, their photophysical behavior was studied and compared with a system of freely moving naphthalene and the free radical TEMPO. The linked molecules showed strong quenching of the singlet and triplet states of the naphthalene moiety, compared to when naphthalene and TEMPO were not linked. The quenching efficiency decreased with increasing the length of the spacer group. In addition, new electronic absorption and emission bands, along with the usual bands of the individual moieties, were also seen. These news bands have been attributed to the formation of electron donor-acceptor charge-transfer complexes in the ground state, arising from the interaction between the two moieties in close proximity. The photophysical dynamics of the linked molecules has been rationalized by assuming the existence of two types of population of the linked molecules: folded and extended. The ground state complex formation is proposed to occur only in the folded conformation of the linked molecules. To our knowledge, this is possibly the first example of a ground state charge-transfer complex formation involving a TEMPO free radical and naphthalene.