Spin Dynamics of Photogenerated Triradicals in Fixed Distance Electron Donor−Chromophore−Acceptor−TEMPO Molecules

Abstract

The stable free radical 2,2,6,6-tetramethylpiperidinoxyl (TEMPO, T*) was covalently attached to the electron acceptor in a donor-chromophore-acceptor (D-C-A) system, MeOAn-6ANI-Phn-A-T*, having well-defined distances between each component, where MeOAn = p-methoxyaniline, 6ANI = 4-(N-piperidinyl)naphthalene-l,8-dicarboximide, Ph = 2,5-dimethylphenyl (n = 0,1), and A = naphthalene-1,8:4,5-bis(dicarboximide) (NI) or pyromellitimide (PI). Using both time-resolved optical and EPR spectroscopy, we show that T* influences the spin dynamics of the photogenerated triradical states 2,4(MeOAn+*-6ANI-Phn-A-*-T*), resulting in modulation of the charge recombination rate within the triradical compared with the corresponding biradical lacking T*. The observed spin-spin exchange interaction between the photogenerated radicals MeOAn+* and A-* is not altered by the presence of T*, which interacts most strongly with A-* and accelerates radical pair intersystem crossing. Charge recombination within the triradicals results in the formation of 2,4(MeOAn-6ANI-Phn-3*NI-T*) or 2,4(MeOAn-3*6ANI-Phn-PI-T*) in which T* is strongly spin polarized in emission. Normally, the spin dynamics of correlated radical pairs do not produce a net spin polarization; however, the rate at which the net spin polarization appears on T* closely follows the photogenerated radical ion pair decay rate. This effect is attributed to antiferromagnetic coupling between T* and the local triplet state 3NI, which is populated following charge recombination. These results are explained using a switch in the spin basis set between the triradical and the three-spin charge recombination product having both T* and 3*NI or 3*6ANI present.

Excited‐State Dynamics of Non‐Luminescent and Luminescent π‐Radicals

“…Wasielewski and co-workersi ntensively investigated the influence of appending ar adicals pin to ad onor-bridge-acceptor. [23][24][25][26][27][28][29][30][31] The excited-state dynamics of several types of molecules was studied by transient absorption and time-resolved ESR spectroscopy.I ti sw ell knownt hat the recombination dynamics of radical pairs is spin-selectivea nd affected by the externalm agnetic field. [10,52,105] Therefore, the presence of a stable radical as at hird spin interacting with the photogenerated radicali on pairs can modulate the excited-state dynamics.…”

“…The influence of at hird spin on the charge recombination (CR) lifetimes of photogenerated radicalp airs was first examined by using electron donor-bridge-acceptors ystemsa ppended to ssubstituted stable radicals (Figure 7b). [23,25,28] In these systems, the radical spin influenced the CR rate (lifetime) of the photogenerated radicalion pairs, which depended on the relative ordering of the energy levels of the radicali on pairs as well as the local neutralt riplet state. In addition, aD EP transfer occurred from the spin-correlated radical ion pairs to the stable radicalm oiety,l eadingt oanet electron spin polarization of the radical moiety,t he degree of which depended on the exchange betweent he donor( D) and acceptor (A).…”

“…[16][17][18] The appending of p-radical moieties to ac hromophore/functional moiety can be used to control or modify the excited-state dynamics. [19][20][21][22][23][24][25][26][27][28][29][30][31] Furthermore, luminescent organic p-radicalsh ave also been developed. [32][33][34][35][36][37][38][39][40][41][42][43][44][45][46][47][48] One of the straightforwarda pplications of luminescence is organic light-emitting devices (OLEDs).…”

Quantum Information and Computation for Chemistry

“…Potential molecular architectures could include Donor-Acceptor dyads appended with stable organic radicals. (Chernick et al, 2006;Mi et al, 2006;Colvin et al, 2013) New molecular architectures that combine electron delocalization and exchange coupling are needed. Such molecules, polymers or materials will be required to create important components of quantum information hardware.…”

Influence of structural dynamics on charge recombination rates in photogenerated radical ion pairs: Evidence from EPR spectroscopy and computation

“…The covalent donor-chromophore dyad D-C has been studied in a variety of contexts [10][11][12][13][14][15][16][17][18], one of the most significant is its incorporation into the donorchromophore-acceptor triad D-C-A (Chart 1). Here, the donor D is p-methoxyaniline, the chromophore C is 4-(N-piperidinyl)-naphthalene-l,8-dicarboximide, and the acceptor A is naphthalene-l,8:4,5-bis(dicarboximide).…”