Molecular Road Map to Tuning Ground State Absorption and Excited State Dynamics of Long-Wavelength Absorbers

Realizing chromophores that simultaneously possess substantial near-infrared (NIR) absorptivity and long-lived, high-yield triplet excited states is vital for many optoelectronic applications, such as optical power limiting and triplet-triplet annihilation photon upconversion (TTA-UC). However, the energy gap law ensures such chromophores are rare, and molecular engineering of absorbers having such properties has proven challenging. Here, we present a versatile methodology to tackle this design issue by exploiting the ethyne-bridged (polypyridyl)metal(II) (M; M = Ru, Os)-(porphinato)metal(II) (PM'; M' = Zn, Pt, Pd) molecular architecture (M-(PM')(n)-M), wherein high-oscillator-strength NIR absorptivity up to 850 nm, near-unity intersystem crossing (ISC) quantum yields (Phi(ISC)), and triplet excited-state (T-1) lifetimes on the microseconds time scale are simultaneously realized. By varying the extent to which the atomic coefficients of heavy metal d orbitals contribute to the one-electron excitation configurations describing the initially prepared singlet and triplet excited-state wave functions, we (i) show that the relative magnitudes of fluorescence (k(F)(0)), S-1 -> S-0 nonradiative decay (k(nr)), S-1 -> T-1 ISC (k(ISC)), and T-1 -> S-0 relaxation (k(T1 -> S0)) rate constants can be finely tuned in M-(PM')(n)-M compounds and (ii) demonstrate designs in which the k(ISC) magnitude dominates singlet manifold relaxation dynamics but does not give rise to T-1 -> S-0 conversion dynamics that short-circuit a microseconds time scale triplet lifetime. Notably, the NIR spectral domain absorptivities of M-(PM')(n)-M chromophores far exceed those of classic coordination complexes and organic materials possessing similarly high yields of triplet-state formation: in contrast to these benchmark materials, this work demonstrates that these M-(PM')(n)-M systems realize near unit Phi(ISC) at extraordinarily modest S-1-T-1 energy gaps (similar to 0.25 eV). This study underscores the photophysical diversity of the M-(PM')(n)-M platform and presents a new library of long-wavelength absorbers that efficiently populate long-lived T-1 states.