Chem p. 138 - 155 (2019)
Update date:2022-08-10
Topics:
Buck, Jason T.
Boudreau, Andrew M.
DeCarmine, André
Wilson, Reid W.
Hampsey, James
Mani, Tomoyasu
Reliance on triplet excited states (triplets) of molecules with heavy atoms, such as precious metals, limits their potential in technological applications. We envision that triplets of π-conjugated organic molecules could play bigger roles; however, their production without heavy atoms remains challenging. The direct, spin-forbidden conversion of singlet charge-separated states to triplets in an electron donor-acceptor (D-A) pair is a promising approach. 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. Yet, this spin-forbidden process can still proceed much faster than ordinary intersystem crossing between (π π*) states under favorable conditions because of stronger spin-orbit coupling. Our findings reveal a clear physical basis for this spin-forbidden process and provide guidelines for future molecular designs exploiting the process. Production of triplet excited states of π-conjugated organic molecules in high yields without using heavy atoms remains challenging. The direct formation of triplet excited states from singlet charge-separated states is a promising approach. Here, we show that spin-allowed electron-transfer reactions largely control such a formation, yet the spin-forbidden transition can outcompete the spin-allowed transitions under favorable conditions because of stronger spin-orbit coupling. 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 π-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 (π π*) 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.
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