Picosecond Control of Photogenerated Radical Pair Lifetimes Using a Stable Third Radical

Abstract

Photoinduced electron transfer reactions in organic donor-acceptor systems leading to long-lived radical ion pairs (RPs) have attracted broad interest for their potential applications in fields as diverse as solar energy conversion and spintronics. We present the photophysics and spin dynamics of an electron donor - electron acceptor - stable radical system consisting of a meta-phenylenediamine (mPD) donor covalently linked to a 4-aminonaphthalene-1,8-dicarboximide (ANI) electron-accepting chromophore as well as an α,γ-bisdiphenylene-β-phenylallyl (BDPA) stable radical. Selective photoexcitation of ANI produces the BDPA-mPD(+•)-ANI(-•) triradical in which the mPD(+•)-ANI(-•) RP spins are strongly exchange coupled. The presence of BDPA is found to greatly increase the RP intersystem crossing rate from the initially photogenerated BDPA-(1)(mPD(+•)-ANI(-•)) to BDPA-(3)(mPD(+•)-ANI(-•)), resulting in accelerated RP recombination via the triplet channel to produce BDPA-mPD-(3*)ANI as compared to a reference molecule lacking the BDPA radical. The RP recombination rates observed are much faster than those previously reported for weakly coupled triradical systems. Time-resolved EPR spectroscopy shows that this process is also associated with strong spin polarization of the stable radical. Overall, these results show that RP intersystem crossing rates can be strongly influenced by stable radicals nearby strongly coupled RP systems, making it possible to use a third spin to control RP lifetimes down to a picosecond time scale.

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

“…[30] The third spin accelerated the radical pair recombination lifetimes down to the picosecond timescale via at riplet channel. [29] The polarization transfer from an excited high-spin state to the ground state was also observed for as trongly exchangecoupled p-conjugated pyrene-based biradical system,n amely 4,5,9,10-tetramethoxypyrene-2,7-bis(tert-butylnitroxide). [106] This biradical was found to exist in as emi-quinoid form with an unprecedented strong intramolecular antiferromagnetic magnetic exchange interaction of 2J/k B = À1185 K. Thus, the ground state was as inglet state (S 0 )w ith al ow-lying triplet state (T 0 ), which were located at af airly large energy separation of 2J/k B .…”

“…–D–C–A system shown in Figure d. The polarization transfer rate was found to be strongly solvent dependent . The third spin accelerated the radical pair recombination lifetimes down to the picosecond timescale via a triplet channel …”

“…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.…”

“…[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).…”

Photoinduced electron transfer from rylenediimide radical anions and dianions to Re(bpy)(CO)3 using red and near-infrared light

“…25,26 The output of the amplifier was frequency-doubled to 414 nm using a BBO crystal and that light was used to pump a laboratory-built collinear optical parametric (OPA) amplifier 27 for visible-light excitation or a commercial non-collinear optical parametric amplifier (TOPAS-White, Light-Conversion, LLC) for NIR excitation. Approximately 1–3 mW of the fundamental was focused onto a sapphire disk to generate the visible white-light probe spanning 430–850 nm, or into a 5 mm quartz cuvette containing a 1 : 1 mixture of H 2 O : D 2 O to generate a UV/visible white light probe spanning 385–750 nm, or onto a proprietary medium (Ultrafast Systems, LLC) to generate the NIR white-light probe spanning 850–1620 nm.…”