Photophysical Processes in Rhenium(I) Diiminetricarbonyl Arylisocyanides Featuring Three Interacting Triplet Excited States

Favale, Joseph M.; Danilov, Evgeny O.; Yarnell, James E.; Castellano, Felix N.

doi:10.1021/acs.inorgchem.9b01155

Title: Photophysical Processes in Rhenium(I) Diiminetricarbonyl Arylisocyanides Featuring Three Interacting Triplet Excited States

Journal Article · Wed Jun 19 00:00:00 EDT 2019 · Inorganic Chemistry

DOI:https://doi.org/10.1021/acs.inorgchem.9b01155· OSTI ID:1633637

Favale, Joseph M. ^[1]; Danilov, Evgeny O. ^[1]; Yarnell, James E. ^[1];

^[1]

North Carolina State Univ., Raleigh, NC (United States)

We present here a series of four transition-metal complexes based on the rhenium(I) tricarbonyl 1,10-phenanthroline (phen) template, with a lone ancillary arylisocyanide (CNAr) ligand to yield metal–organic chromophores of the generic molecular formula [Re(phen)(CO)₃(CNAr)]⁺ [CNAr = 2,6-diisopropylphenyl isocyanide (1), 4-phenyl-2,6-diisopropylphenyl isocyanide (2), 4-phenylethynyl-2,6-diisopropylphenyl isocyanide (3), and 4-biphenyl-2,6-diisopropylphenyl isocyanide (4)]. This particular series features varied degrees of π-conjugation length in the CNAr moiety, resulting in significant modulation in the resultant photophysical properties. All molecules possess long-lived [8–700 μs at room temperature (RT)], strongly blue-green photoluminescent and highly energetic excited states (λ_max,em = 500–518 nm; Φ = 14–64%). Each of these chromophores has been photophysically investigated using static and dynamic spectroscopic techniques, the latter probed from ultrafast to supra-nanosecond time scales using transient absorption and photoluminescence (PL). Time-resolved PL intensity decays recorded as a function of the temperature were consistent with the presence of at least two emissive states lying closely spaced in energy with a third nonemissive state lying much higher in energy and likely ligand-field in character. The combined experimental evidence, along with the aid of electronic structure calculations (density functional theory and time-dependent density functional theory performed at the M06/Def2-SVP/SDD level), illustrates that the CNAr ligand is actively engaged in manipulating the excited-state decay in three of these molecules (2–4), wherein the triplet metal-to-ligand charge-transfer (³MLCT) state along with two distinct triplet ligand-centered (³LC) excited-state configurations (phen and CNAr) conspire to produce the resultant photophysical properties. Because the π conjugation within the CNAr ligand was extended, an interesting shift in the dominant photophysical processes was observed. When the CNAr conjugation length is short, as in 1, the phenanthroline ³LC state dominates, resulting in a configurationally mixed triplet excited state of both LC and MLCT character. With more extended π conjugation in the CNAr subunit (2–4), the initially generated ³LC(phen)/³MLCT excited state ultimately migrates to the CNAr ³LC state on the order of tens of picoseconds. Molecules 3 and 4 in this series also feature unique examples of inorganic excimer formation, as demonstrated by dynamic self-quenching in the corresponding PL intensity decays accompanied by the observation of a short-lived low-energy emission feature.