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

Journal Article · · Inorganic Chemistry
 [1];  [2];  [2];  [2]
  1. North Carolina State Univ., Raleigh, NC (United States); NC State University
  2. North Carolina State Univ., Raleigh, NC (United States)
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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 (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 demonstrated by dynamic self-quenching in the corresponding PL intensity decays accompanied by the observation of a short-lived low-energy emission feature.
Research Organization:
North Carolina State Univ., Raleigh, NC (United States)
Sponsoring Organization:
USDOE Office of Science (SC), Basic Energy Sciences (BES) (SC-22)
Contributing Organization:
Air Force Institute of Technology (AFIT)
Grant/Contract Number:
SC0011979
OSTI ID:
1633637
Journal Information:
Inorganic Chemistry, Journal Name: Inorganic Chemistry Journal Issue: 13 Vol. 58; ISSN 0020-1669
Publisher:
American Chemical Society (ACS)Copyright Statement
Country of Publication:
United States
Language:
English

Figures / Tables (12)

Scheme 1(p. 2)figure Scheme 1
Figure 1(p. 3)figure Figure 1
Figure 2(p. 3)figure Figure 2
Figure 3(p. 4)figure Figure 3
Table 1(p. 4)table Table 1
Figure 4(p. 5)figure Figure 4
Table 2(p. 5)table Table 2
Table 3(p. 6)table Table 3
Figure 5(p. 7)figure Figure 5
Figure 6(p. 7)figure Figure 6
Figure 7(p. 8)figure Figure 7
Figure 8(p. 8)figure Figure 8

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