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

Abstract

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 (Φ_ISC), and triplet excited-state (T₁) 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 (k⁰_F), S₁ → S₀ nonradiative decay (k_nr), S₁ → T₁ ISC (k_ISC), and T₁ → S₀ 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 T₁ → S₀ 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 Φ_ISC at extraordinarily modest S₁-T₁ energy gaps (∼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 T₁ states.