Molecular Road Map to Tuning Ground State Absorption and Excited State Dynamics of Long-Wavelength Absorbers
journal contributionposted on 18.10.2017, 00:00 by Yusong Bai, Jean-Hubert Olivier, Hyejin Yoo, Nicholas F. Polizzi, Jaehong Park, Jeff Rawson, Michael J. Therien
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 (k0F), S1 → S0 nonradiative decay (knr), S1 → T1 ISC (kISC), and T1 → S0 relaxation (kT1→S0) rate constants can be finely tuned in M-(PM′)n-M compounds and (ii) demonstrate designs in which the kISC 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.
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triplet excited-state850 nmMolecular Road Mapenergy gap lawdesign issuerate constantsM platformk ISC magnitudelong-wavelength absorberscoordination complexessinglet manifold relaxation dynamicsLong-Wavelength Absorbers Realizing chromophoresunit Φ ISCnear-unity intersystemT 1 statesT 1domain absorptivitiesPMphotophysical diversityone-electron excitation configurationstriplet excited-state wave functionsM systemsbenchmark materialsExcited State DynamicsM chromophoresTTA-UCtriplet-state formationoptoelectronic applicationsTuning Ground State Absorptionmicroseconds time scale triplet lifetimemicroseconds time scaleS 1high-oscillator-strength NIR absorptivityM compounds