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

• Published in: Journal of the American Chemical Society Vol. 139; no. 46; pp. 16946 - 16958
• Main Authors: Bai, Yusong, Olivier, Jean-Hubert, Yoo, Hyejin, Polizzi, Nicholas F, Park, Jaehong, Rawson, Jeff, Therien, Michael J
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 22-11-2017
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/jacs.7b09982

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 (ΦISC), 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 (k 0 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 ΦISC 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.