Spin-Allowed Transitions Control the Formation of Triplet Excited States in Orthogonal Donor-Acceptor Dyads

Abstract

Triplet excited states (triplets) serve as key intermediates in critical technologies and processes ranging from organic synthesis to biomedicine to molecular electronics. Production of triplets of p-conjugated organic molecules without heavy atoms remains challenging. Spin-orbit, charge-transfer intersystem crossing (SOCT-ISC) directly converts singlet charge-separated states to triplets in an electron donor-acceptor (D-A) pair. Here, using a series of orthogonal D-A type boron dipyrromethene (BODIPY) derivatives as a model system, we show that the formation of triplets is largely controlled by the spin-allowed transitions rather than by SOCT-ISC. Yet, the SOCT-ISC process can still proceed much faster than ordinary ISC between (p, p*) states because the spin-orbit coupling of SOCT-ISC is 2 orders of magnitude stronger. We further show that such a process can produce triplets in a non-triplet-forming molecule, perylene. Our findings reveal a clear physical basis for this spin-forbidden process and provide guidelines for future molecular designs exploiting the process.

Heavy-atom-free BODIPY photosensitizers with intersystem crossing mediated by intramolecular photoinduced electron transfer

“…Negligible triplet state yield (T = 0.005) observed for this dyad in acetonitrile was attributed to a strong stabilization of CT state in highly polar media and a reduced energy gap between 1 CT and S0 states, which promotes ground state recombination. 44 The rate of CRS was found to be two orders of magnitude higher than for CRT (1.9×10 10 s -1 and 0.97×10 8 s -1 , respectively) in this solvent. The triplet state yield in 42 greatly increases in less polar toluene (T = 0.2), due to the increased CT state energy level and 1 CT-S0 energy gap, unfavourable for the CRS process.…”

“…The mechanism of the BODIPY triplet state formation in several BODIPY-anthracene dyads, including 3, was later investigated by Mani and co-workers. 44 The absence of RP-ISC mechanism contribution to the formation of triplets was confirmed by conducting TA in the presence of an external magnetic field of up to 3000 gauss, which showed no effect on T values. The lack of a magnetic field effect (MFE) proves that the triplets are formed via SOCT-ISC mechanism and RP-ISC is not involved.…”

“…GPET = -0.031 eV in acetonitrile). 44 However, the rate of ground state recombination was found to be substantially higher than for recombination into triplet state (3.1×10 8 s -1 and 2.0×10 8 s -1 , respectively). This accounts for the modest triplet state yield (T = 0.33) in this dyad.…”

“…As was demonstrated for various dyads and dimers, the triplet state yield from 1 CT state depends on the rates of two competitive relaxation pathways: charge recombination into the local triplet state (CRT) and recombination into the ground singlet state (CRS), kCRT and kCRS, respectively (Figure 3b). 44 High triplet state yields can be achieved if kCRS is substantially lower than kCRT. This condition is met, for example, if the driving force of the CRS process (GCRS) has large negative values and falls within the Marcus inverted region.…”

“…Mani and co-workers were able to demonstrate that the presence of methyl groups in the BODIPY core makes electron transfer from the pyrene subunit in 37 thermodynamically unfavourable even in acetonitrile (GPET = 0.083 eV). 44 The presence of two additional ethyl groups in 38 further reduces the driving force of the process (GPET = 0.27 eV). As a result, the S1 state of the BODIPY in 38 decays predominantly via fluorescence emission, resulting in a rather low triplet state yield (T = 0.35).…”