Intersystem Crossing Mediated by Photoinduced Intramolecular Charge Transfer:  Julolidine−Anthracene Molecules with Perpendicular π Systems

Abstract

Time-resolved electron paramagnetic resonance studies show that the primary mechanism of triplet formation following photoexcitation of julolidine-anthracene molecules linked by a single bond and having perpendicular pi systems is a spin-orbit, charge-transfer intersystem crossing mechanism (SOCT-ISC). This mechanism depends on the degree of charge transfer from julolidine to anthracene, the dihedral angle (theta1) between their pi systems, and the magnitude of the electronic coupling between julolidine and anthracene. We compare 4-(9-anthracenyl)-julolidine with the more sterically encumbered 4-(9-anthracenyl)-3,5-dimethyljulolidine and find that fixing theta1 congruent with 90 degrees serves to enhance SOCT-ISC by increasing the change in orbital angular momentum accompanying charge transfer. Given that the requirements for the SOCT-ISC mechanism are quite general, we expect it to occur in a variety of electron donor-acceptor systems.

The Role of Local Triplet Excited States and D‐A Relative Orientation in Thermally Activated Delayed Fluorescence: Photophysics and Devices

“…This process is thus formerly forbidden for the HOMO perpendicular to LUMO and no change in spatial orbital would occur upon interconverting between these two states, and so no orbital angular momentum change, as required for a spin flip, could occur 14. However, orthogonal D and A do allow efficient crossing between the 1 CT state and an energetically close local triplet state, the 3 LE state in this case, via a spin orbit charge transfer ISC process, (SOCT), as this spin flip transition couples to a change in orbital angular momentum (much like an n‐ π * transition), concomitant with the measured high ISC rate 15. Therefore in DPTZ‐DBTO2 , the ISC due to SOCT involves the 1 CT and 3 LE states, and thus achieving a very small energy gap between 3 LE and 1 CT is key for efficient thermally activated RISC.…”

Engineering the singlet–triplet energy splitting in a TADF molecule

“…The molecule has no crystallographic symmetry, and an approximate non-crystallographic mirror plane through S(1), O(1), O(2), C(7), C (14) and C (15) Please do not adjust margins…”

“…Spin orbit coupling (SOC) for example is formally forbidden between singlet and triplet CT states for the case of near orthogonal D and A units, and thus the 1 CT state can only couple strongly to a close lying local triplet state. 13,14 The gap between these two states is thus the real ∆EST that requires thermal energy for triplets to cross over to the singlet state. These means that 1 CT can be either above or below the coupling triplet state for TADF to occur, but needs to be close to a local triplet state, so efficient SOC can occur.…”

Selective Triplet‐State Formation during Charge Recombination in a Fullerene/Bodipy Molecular Dyad (Bodipy=Borondipyrromethene)

“…A common feature of all these systems is the close positioning of donor and acceptor units. Recently, Wasielewski et al [16] used time-resolved EPR spectroscopy to monitor intersystem crossing within intramolecular charge-transfer states of several anthracene-julolidine dyads, and confirmed that the efficiency is at a maximum when the reactants are held in a perpendicular geometry. These authors have probed [16] the mechanism in some detail, and conclude that the overall effect is likely to be applicable to a wide variety of electron donor-acceptor systems, provided the reactants are nearby.…”