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

• Published in: Chem Vol. 5; no. 1; pp. 138 - 155
• Main Authors: Buck, Jason T., Boudreau, Andrew M., DeCarmine, André, Wilson, Reid W., Hampsey, James, Mani, Tomoyasu
• Format: Journal Article
• Language: English
• Published: United States Elsevier Inc 10.01.2019; Elsevier
• Subjects: charge recombination; electron transfer; photophysics; SDG7: Affordable and clean energy; solar energy conversion; spin chemistry; triplet excited states; solar energy conversion; charge recombination; electron transfer; triplet excited states; photophysics; SDG7: Affordable and clean energy; spin chemistry
• ISSN: 2451-9294; 2451-9294
• DOI: 10.1016/j.chempr.2018.10.001

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. [Display omitted] •Spin-allowed electron-transfer reactions control the formation of triplets•The Marcus inverted region is used to suppress spin-allowed transitions•Determination of spin-orbit coupling constant 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.