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Photophysical Processes in Rhenium(I) Diiminetricarbonyl Arylisocyanides Featuring Three Interacting Triplet Excited States
journal contribution
posted on 2019-06-19, 17:39 authored by Joseph M. Favale, Evgeny O. Danilov, James E. Yarnell, Felix N. CastellanoWe present 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 (λmax,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 evidenced by dynamic self-quenching
in the corresponding PL intensity decays accompanied by the observation
of a short-lived low-energy emission feature.
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Keywords
triplet metal-to-ligand charge-transferRTCNAr ligandlow-energy emission featureisocyanideInteracting Triplet Excited Statesphotophysical propertiesphenanthroline 3 LC stateCNAr conjugation lengthπ conjugationsupra-nanosecond time scalesTime-resolved PL intensityCNAr 3 LC stateMLCTπ- conjugation length
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