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.

Charge recombination to the electronic ground state typically occurs nonradiatively. We report a rational design of donor−bridge−acceptor molecules that exhibit charge-transfer (CT) emission through conjugated bridges over distances of up to 24 Å. The emission is enhanced by intensity borrowing and extends into the near-IR region. Efficient charge recombination to the initial excited state results in recombination fluorescence. We have established the identity of CT emission by solvent dependence, sensitivity to temperature, femtosecond transient absorption spectroscopy, and unique emission polarization patterns. Large excited-state electronic couplings and small energy gaps enable the observation of intramolecular long-range CT emission over the unprecedented long distance. These results open new possibilities of using intramolecular long-range CT emission in molecular electronic and biomedical imaging probe applications.

Quantification of the intensity and linewidth of the ν(CN) IR band in a series of neutral and anionic nitrile-functionalized oligophenylenes reveals that the CN vibration is coupled to nuclear and electronic structural changes.

The nature of excess electrons has been studied in donor−acceptor (D−A) compounds based on substituted triarylamines and a nitrile-functionalized fluorene by changing the substituents on the triarylamines and also the solvent polarity. We observed that both electron push−pull capability at the distant location in the amine donor unit and solvation in solvents of varying polarity significantly affect the nitrile ν(CN) vibrations of the fluorene acceptor unit in radical anions of these D−A compounds. Quantum calculations show that the push−pull capability translates the position of an excess electron while keeping its width relatively constant. On the other hand, solvation changes both, making an electron more compact in polar media. The current study points to the idea that solvation plays a more significant role in controlling the nature of excess electrons, while synthetic modification that influences electron push−pull capability enables further tuning.