Luminescent Pt(2,6-bis(: N -methylbenzimidazol-2-yl)pyridine)X+: A comparison with the spectroscopic and electrochemical properties of Pt(tpy)X+(X = Cl, CCPh, Ph, or CH3)

Article abstract of DOI:10.1039/d0dt01496f

A series of platinum(ii) pincer complexes of the formula Pt(mbzimpy)X+, 1(a-d), (mbzimpy = 2,6-bis(N-methylbenzimidazol-2-yl)pyridine; X = Cl; (a), CCPh; (b), Ph; (c), or CH3; (d), CCPh = phenylacetylide, and Ph = Phenyl) have been synthesized and characterized. Electronic absorption and emission, as well as electrochemical properties of these compounds, have been investigated. Pt(tpy)X+ analogs (tpy = 2,2′;6′2′′-terpyridine), 2(a-d), have also been investigated and compared. Electrochemistry shows that 1 and 2 analogs undergo two chemically reversible one-electron reduction processes that are shifted cathodically along the a < b < c < d series. Notably, these reductions occur at slightly higher negative potentials in the case of 1. The absorption spectra of 1 and 2 in acetonitrile exhibit ligand-centered (1LC) transitions (? ≈ 104 M-1 cm-1) in the UV region and metal-to-ligand-charge transfer (1MLCT) transitions (? ≈ 103 M-1 cm-1) in the visible region. The corresponding visible bands of 1b and 2b have been assigned to 1(LLCT/MLCT) mixed state (LLCT: ligand-to-ligand-charge transfer). The preceding 1LC and 1MLCT transitions of 1 occur at lower energies than that of 2. These 1LC transitions have distinctly been blue-shifted along a < c < d in 2, but occur at nearly identical energies in 1. Conversely, 1MLCT transitions are red-shifted along a < c < d in both the analogs. The 77 K glassy solutions of 1 and 2 exhibit an intense vibronically-structured emission band at λmax(0-0) in the 470-560 nm range. This band is red-shifted along b < a ≤ c < d in 1 and along a ≤ d ≈ c ? b in 2. The main character of these emissions is assigned to 3LLCT emissive state in 1b and 2b, whereas to 3LC in the rest of the compounds. Relative stabilization of these spin-forbidden emissive states is discussed by invoking configuration mixing with the higher-lying 3MLCT state.

Full text of DOI:10.1039/d0dt01496f

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Luminescent Pt(2,6-bis(N-methylbenzimidazol-
2-yl)pyridine)X+: a comparison with the
spectroscopic and electrochemical properties of
Pt(tpy)X+ (X = Cl, CCPh, Ph, or CH3)†
Cite this: DOI: 10.1039/d0dt01496f
Vikas M. Shingade, * Levi J. Grove and William B. Connick‡
A series of platinum(II) pincer complexes of the formula Pt(mbzimpy)X+, 1(a–d), (mbzimpy = 2,6-bis(N-
methylbenzimidazol-2-yl)pyridine; X = Cl; (a), CCPh; (b), Ph; (c), or CH3; (d), CCPh = phenylacetylide, and
Ph = Phenyl) have been synthesized and characterized. Electronic absorption and emission, as well as
electrochemical properties of these compounds, have been investigated. Pt(tpy)X+ analogs (tpy = 2,2’;6’
2’’-terpyridine), 2(a–d), have also been investigated and compared. Electrochemistry shows that 1 and 2
analogs undergo two chemically reversible one-electron reduction processes that are shifted cathodically
along the a < b < c < d series. Notably, these reductions occur at slightly higher negative potentials in the
case of 1. The absorption spectra of 1 and 2 in acetonitrile exhibit ligand-centered (1LC) transitions (ε ≈
104 M−1 cm−1) in the UV region and metal-to-ligand-charge transfer (1MLCT) transitions (ε ≈ 103 M−1
1
cm−1) in the visible region. The corresponding visible bands of 1b and 2b have been assigned to (LLCT/
1
1
MLCT) mixed state (LLCT: ligand-to-ligand-charge transfer). The preceding LC and MLCT transitions of
1
1 occur at lower energies than that of 2. These LC transitions have distinctly been blue-shifted along a <
c < d in 2, but occur at nearly identical energies in 1. Conversely, 1MLCT transitions are red-shifted along a
< c < d in both the analogs. The 77 K glassy solutions of 1 and 2 exhibit an intense vibronically-structured
emission band at λmax(0–0) in the 470–560 nm range. This band is red-shifted along b < a ≤ c < d in 1
and along a ≤ d ≈ c ≪ b in 2. The main character of these emissions is assigned to 3LLCT emissive state in
1b and 2b, whereas to 3LC in the rest of the compounds. Relative stabilization of these spin-forbidden
emissive states is discussed by invoking configuration mixing with the higher-lying 3MLCT state.
Received 23rd April 2020,
Accepted 23rd June 2020
DOI: 10.1039/d0dt01496f
rsc.li/dalton
change in luminescence properties upon exposure to different vola-
tile organic compounds (VOCs).16,17,36,37 Grove et al.16 have pro-
Introduction
Square-planar platinum(II) complexes with 2,2′;6′,2″-terpyridine1–15 posed that the sorption of certain VOCs causes a decrease in Pt–Pt
and 2,6-bis(N-alkylbenzimidazol-2-yl)pyridine16–24 pincer ligands contacts, resulting in dramatic changes in colors and spectroscopic
have attracted widespread interest because of their intriguing spec- properties of these compounds. It is widely accepted that the
troscopic properties as well as their potential utility in applications luminescence from stacked platinum(II) polypyridyl complexes
ranging from chemical sensing to biomolecular interactions, with short Pt–Pt contacts (<3.5 Å) typically originates from a lowest
including DNA intercalation and biological labelling, to mixed-metal-to-ligand charge-transfer (MMLCT) state, involving an
catalysis.25–31 For example, simple solid salts (cf. double salts32–35
)
unoccupied π* level of the aromatic heterocyclic ligand and a filled
of 1a are vapochromic, undergoing a distinct color change and dσ* orbital, which derives primarily from the interaction of the dz2
orbitals of adjacent Pt atoms (Fig. S1 ESI†). Interestingly, simple
solid salts of 2a exhibit different colorimetric responses to VOCs
than their 1a counterparts.38,39 These differences are surely related
Department of Chemistry, University of Cincinnati, P.O. Box 210172, Cincinnati,
OH 45221-0172, USA. E-mail: vikas.shingade@gmail.com
to the shapes, steric properties, and intermolecular interactions of
the molecules, as well as differences in the electronic properties of
1a and 2a.
†Electronic supplementary information (ESI) available: Synthesis and character-
ization, NMR, absorption, excitation, and emission spectra, cyclic voltammo-
grams, and table of absorption maxima of platinum(II) and related compounds
Previous studies provide some insight into the potential
are provided in PDF format. See DOI: 10.1039/d0dt01496f
complexities of the less-studied Pt(mbzimpy)X+ system. For
‡Professor William B. Connick passed away in 2018.
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example, Hill et al.1,2 have noted that the electronic structure
of Pt(tpy) is perturbed by ancillary ligands and/or the sur-
rounding medium. Hill et al. further noted that the low-lying
Experimental section
Materials and methods
π* orbital (LUMO) in Pt(tpy) unit is significantly stabilized Compounds in Scheme 1 were synthesized in analytically pure
2+
compared to Ru(tpy)2 complex. The origin of this effect was form following the modifications of the published procedures
attributed to mixing of the π*(tpy) orbital with the unoccupied for 2(a–d).4,61–65 Details of the syntheses are provided in the
6pz(Pt) orbital, and an analogous effect involving
a π* ESI.† Characterization data of 1(a–b) are summarized below.
(mbzimpy) orbital is anticipated for 1a. On the other hand, For spectroscopic and electrochemical characterization, high
Che and others3,40–45 have shown that the spin-allowed tpy purity anhydrous dimethylformamide (DMF) and acetonitrile
ligand-centered absorption profile of Pt(tpy)Ln+ varies with the were obtained from Burdick and Jackson, whereas butyroni-
ancillary ligand (L: Cl, Br, I, SCN, or N3, n = 1; and NH3, n = 2) trile, ethanol, and methanol were obtained from Sigma-
in a manner suggestive of significant configuration interaction Aldrich. Tetrabutylammonium hexafluorophosphate (TBAH)
involving the low-lying, spin-allowed ligand-centered (1LC) and supporting electrolyte was obtained from Sigma-Aldrich. For
metal-to-ligand charge-transfer (1MLCT) states. Therefore, emission and excitation studies, ethanol : methanol : DMF
orbital mixing may play an important role in the spectroscopy (10 : 10 : 1 v/v) solvent mixture (abbreviated as EMD) was used.
of related mbzimpy complexes. A comparison of pKa values46 Deuterated solvents were purchased from Cambridge Isotope
of benzimidazoles and pyridine suggests that mbzimpy has a Laboratories. The 1D (1H, 13C, and 195Pt) and in several
stronger σ-donor capacity than tpy. On the other hand, a com- instances 2D (COSY, HSQC, and/or NOE) NMR spectra were
parison of the X-ray structures of the known transition metal recorded at room temperature (20–25 °C). In the case of 1b, a
1
mbzimpy,37,47–50 and tpy9,51–56 complexes shows that mbzimpy series of H NMR spectra were also recorded over the tempera-
has N–M–N bite angles that are 2–4° smaller than that of the ture range of 25–70 °C. 2D NOE experiments were run with
tpy, as expected for the geometric constraints of the five-mem- mixing time, τm, of 75 ms. Spectra are reported in parts per
bered imidazole group. Notably, the redox chemistry of ruthe- millions (ppm) relative to TMS (δ = 0 ppm), or the residual
nium mbzimpy and tpy complexes affords some insight into internal standard (∼ the protic solvent impurity) [(CD3)2SO,
the ligand donor properties. For example, Haga57 and δH = 2.50 ppm; and δC = 39.52 ppm for CD3SOCD2H], or rela-
others58–60 show that the oxidation of ruthenium from +2 to +3 tive to a saturated solution of Na2[PtCl6] in D2O in the case of
in Ru(mbzimpy)2 (+0.86 V vs. Ag/AgCl) is shifted cathodically 195Pt NMR. Elemental analyses were performed by Atlantic
2+
2+
by ∼0.5 V from that of the Ru(tpy)2 (+1.31 V vs. Ag/AgCl). Microlab, Norcross, GA.
This result is consistent with the notion that mbzimpy is a
Instrumentation
stronger electron donor. Provided that, it is unclear how the
similar effect compares in platinum(II) complexes with The 1H, 13C, COSY, and HSQC NMR spectra were recorded
mbzimpy and tpy ligands.
using Bruker AC 400 MHz instrument, whereas NOE and 195Pt
To better understand the influence of the mbzimpy and tpy NMR spectra were recorded using a Bruker DMX 500 MHz and
ligands, as well as the influence of the ancillary ligand, on a a Bruker AMX 400 MHz instruments, respectively. Mass
platinum(II) center, herein, we have undertaken experimental spectra were obtained by electrospray ionization using either
investigation of the electronic structures of a series of Pt an Ionspec HiRes ESI-FTICRMS instrument or a Micromass
(mbzimpy)X+ and Pt(tpy)X+ complexes in Scheme 1. Notably, Q-TOF-II instrument. The observed isotope patterns agreed
we anticipate that σ-donation by the ancillary ligand will well with those predicted based on natural isotopic abun-
increase along the Cl < CCPh < Ph < CH3 series, whereas dances (only monoisotopic masses are provided here). Infrared
π-donation should be increased along the Ph, CCPh < CH3 ≪ spectra were collected using a Nicolet 6700 FTIR spectrometer.
Cl series.
UV-visible absorption spectra were recorded using an HP8453
diode array spectrometer on samples contained in 1 cm and/or
1 mm pathlength quartz cuvettes. Cyclic voltammetry (CV)
measurements were performed at room temperature using a
BAS100b potentiostat (Bioanalytical Systems) and a standard
three-electrode cell consisting of either a 2.11 mm2 platinum
disk or a 7.07 mm2 glassy carbon disk working electrode; as
specified, Ag/AgCl (containing 3.0 M NaCl aqueous solution)
reference electrode, and a platinum wire auxiliary electrode.
Scans were recorded of ∼1 mM DMF solutions containing 0.1
M TBAH which was recrystallized at least twice from methanol
and dried under vacuum before use. Between scans, the
working electrode was polished with 0.05 mm alumina, rinsed
with distilled water and wiped dry using a Kimwipe. Reported
potentials are referenced versus Ag/AgCl (3.0 M NaCl) and are
not corrected for junction potential. Peak currents (ip) were
Scheme 1 Schematic representation and atom numbering for the
compounds in this study. (PF6)− is the anion.
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estimated with respect to the extrapolated baseline current, as
1d: Yield: 80%. MS-ESI (m/z): 549.14 (PtC22H20N5)+, calcd
549.14. Anal. Calcd for C22H20F6N5PPt: C, 38.05; H, 2.9; N,
described by Kissinger and Heineman.66 The values of (Epc
+
Epa)/2, which is an approximation of the formal potential of a 10.08%. Found: C, 38.28; H, 2.9; N, 10.22%. 1H NMR
3
redox couple, are referred to as E°′. Under these conditions, (400 MHz, 0.67 mM in (CD3)2SO, δ/ppm) δH 8.59 (2H, d, J =
3
the ferrocene/ferrocenium couple occurs at 0.54 V, ΔEp
=
7.8 Hz, H3 and H5), 8.52 (1H, t, J = 7.8 Hz, H4), 7.93 (4H, d,
80 mV.
3J = 8.2 Hz, H4′, H4″, H7′ and H7″), 7.59 (4H, m, H5′, H5″, H6′
2
For emission and excitation studies, the 77 K glassy solu- and H6″), 4.36 (6H, s, N-CH3), 1.98 (3H, s, JPt–H = 79.2 Hz, Pt-
tion was prepared by inserting a quartz EPR tube containing CH3). 13C NMR (400 MHz, 29.4 mM in (CD3)2SO, δ/ppm) δc
the solution into a quartz-tipped finger dewar filled with 154.1, 144.6, 140.8 (C4), 138.7, 134.0, 126.1; 125.3 (C5′; C5″, and
liquid nitrogen. The data were collected using
a SPEX C6′; C6″), 124.1 (C3 and H5), 115.4; 112.1 (C4′; C4″, and C7′; C7″),
Fluorolog-3 fluorimeter equipped with a double emission 32.3 (N-CH3), −25.6 (Pt-CH3). 195Pt NMR (400 MHz, 48 mM,
monochromator and a single excitation monochromator. The (CD3)2SO, δ/ppm) δ −3346.2.
emitted light was collected at 90° using an appropriate emis-
sion cutoff filter. The spectra were corrected for an instrumen-
tal response.
Results and discussion
Synthesis and characterization
Characterization of 1a. Yield: 85–90%. MS-ESI (m/z):
570.08 (PtC21H17N5Cl)+, calcd 569.93. Anal. Calcd for
C21H17N5F6ClPPt: C, 35.28; H, 2.40; N, 9.80%. Found: C, 34.98; The mbzimpy ligand was obtained in high yield and purity
1
H, 2.29; N, 9.63%. H NMR (400 MHz, 23.2 mM in (CD3)2SO, optimizing the synthetic procedure reported by Addison et al.
3
3
δ/ppm) δH 8.44 (1H, t, J = 8.0 Hz, H4), 8.25 (2H, d, J = 8.0 Hz, (see ESI†).67 Homologues of Pt(mbzimpy)X+ in Scheme 1, 1(a–
H3 and H5), 7.34 (2H, d, 3J = 8.0 Hz, H4′ and H4″), 7.22 (2H, dd, d), were synthesized following modifications of literature pro-
3J = 8.0 Hz, H5′ and H5″), 7.11 (2H, dd, 3J = 8.0 Hz, H6′ and cedures for the terpyridine analogs, 2(a–d);4,61–65 our rec-
H6″), 6.85 (2H, d, 3J = 8.0 Hz, H7′ and H7″), 3.74 (6H, s, N-CH3). ommended routes are outlined in Scheme 1S (ESI†). On
13C NMR (400 MHz, 23.2 mM in (CD3)2SO, δ/ppm) δc 152.4, occasion, we will refer to 1(a–d) and 2(a–d) analogs simply as 1
147.0, 142.3 (C4), 137.6, 133.3, 126.2 (C5′ and C5″), 125.5 (C6′ and 2, respectively. 1 and 2 form yellow to orange-red air-stable
and C6″), 124.7 (C3 and C5), 114.8 (C4′ and C4″), 112.0 (C7′ and solids, and in most cases, these colors are depending on
C7″), 32.17 (N-CH3). 195Pt NMR (26.2 mM, (CD3)2SO, δ/ppm) δ whether samples are wet or dry. For spectroscopic comparison,
−2596.
1b: Yield: 70–75%. MS-ESI (m/z): 635.15 (C29H22N5Pt)+, ESI†).47,68 The products were obtained in analytically pure
calcd 635.15. FT-IR, ν(CuC) = 2112 cm−1 1H NMR (400 MHz, form and characterized by elemental analysis, mass spec-
[Zn(mbzimpy)2](PF6)2 (3) complex also was synthesized (see
.
46 µM in (CD3)2SO, 60 °C, δ/ppm) δH 8.67 (2H, d, J = 8 Hz, H4′ trometry, multinuclear (1H, 13C, and 195Pt) NMR, and in a few
and H4″), 8.63 (2H, d, J = 8 Hz, H3 and H5), 8.58 (1H, t, J = instances with FTIR spectroscopy. NMR assignments were
8 Hz, H4), 8.02 (2H, d, J = 8 Hz, H7′ and H7″), 7.66 (4H, dd, J = made according to the numbering in Scheme 1 and by employ-
5.4 Hz, H5′, H5″, H6′, and H6″), 7.55 (2H, d, J = 7.6 Hz, H2′′′ and ing a combination of two or more 2D NMR techniques such as
H6′′′), 7.46 (2H, dd, J = 7.4 Hz, H3′′′ and H5′′′), 7.33 (1H, t, J = 7.2 COSY, NOESY, and HSQC, as required.
Hz, H4′′′), 4.48 (6H, s, N-CH3). 13C NMR (400 MHz, 54.6 mM in
1H NMR spectra of the compounds above exhibit expected
(CD3)2SO, δ/ppm) δc 155.6; 151.9; 146.6; 145.8 ((C2, C6), (C2′, patterns of resonances (Fig. S6–S24 ESI†). Notably, these pat-
C2″), (C7a′, C7a″), and (C3a′, C3a″)), 141.9 (C4), 137.2; 132.9; terns, excepting for 1c, 2c, 3, and free mbzimpy and tpy, are
126.03 (C1′′′, C1″″, and C2″″), 130.9, 128.9, 125.8, 125.3, 124.2, concentration and temperature-dependent, which is consistent
115.3, 114.4, 111.59 (C4′′′), 31.94 (N-CH3). 195Pt NMR (400 MHz, with
(CD3)2SO, δ/ppm) δ −3367.5.
a
tendency for these compounds to aggregate in
solution.5,32,44,69,70 For example, in the case of 1b, 1H NMR
1c: Yield: 75–80%. MS-ESI (m/z): 611.16 (PtC27H22N5)+, spectra recorded over the concentration range of 46 μM–
calcd 611.15. Anal. Calcd for C27H22F6N5PPt: C, 42.87; H, 4.8 mM in DMSO-d6 at 60 °C (Fig. S12 ESI†) show that all
2.93; N, 9.26%. Found: C, 42.62; H, 2.93; N, 9.23%. 1H NMR proton resonances are shifting monotonically upfield and
(400 MHz, 2.1 mM in (CD3)2SO, δ/ppm) δH 8.67 (2H, d, J = broadening with increasing concentration. The reverse effect
8.0 Hz, H3 and H5), 8.56 (1H, t, J = 8.0 Hz, H4), 8.00 (2H, d, was noted when the temperature was increased (Fig. S11 ESI†).
J = 8.4 Hz, H7′ and H7″), 7.70 (2H, d, J = 6.8 Hz, H2′′′ and H6′ These observations are consistent with a dynamic equilibrium
′′), 7.49 (2H, dd, J = 7.6 Hz, H6′ and H6″), 7.23 (2H, dd, J = between monomer and an aggregate, most likely supported by
7.8 Hz, H5′ and H5″), 7.22 (2H, dd, J = 7.8 Hz, H3′′′ and H5′′′), non-covalent Pt–Pt and/or mbzimpy–mbzimpy stacking
7.14 (1H, t, J = 7.2 Hz, H4′′′), 6.53 (2H, d, J = 8.4 Hz, H4′ and interactions.5,32,44,69–76 Interestingly, variations in the sensi-
H4″), 4.46 (6H, s, N-CH3). 13C NMR (400 MHz, (CD3)2SO, tivity of proton chemical shifts to concentration and tempera-
δ/ppm) δc 155.0, 146.4, 142.0 (C4), 138.9 (C2′′′ and C6′′′), ture suggest that changes in shielding are dependent on the
138.8, 136.5 (C1′′′), 135.0, 127.0 (C3′′′ and C5′′′), 125.8 (C5′ and specific stacking geometry of the aggregate. While understand-
C5″), 125.7 (C6′ and C6″), 124.6 (C3 and C5), 123.9 (C4′′′), 117.0 ing the nature of these aggregates is an interesting problem,
(C7′ and C7″), 32.84 (N-CH3). 195Pt NMR (400 MHz, 51.2 mM this study is beyond the scope of this article and will compre-
in (CD3)2SO, δ/ppm) δ −3390.4.
hensively be discussed in our forthcoming article.77 The only
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Table 1 Electrochemical potentials for 1 and 2 complexesa, as deter-
mined by cyclic voltammetry
thing we would like to point out in here is that the complexes
with mbzimpy show greater tendencies for aggregation in solu-
tions than their tpy counterparts (for example, 1b ≫ 2b;
Fig. S12 and S18 ESI†).
c
c
Complexes
Epa
E°′+/0, (ΔEp
)
E°′0/−, (ΔEp )
1a
1b
1c
1d
2a
2b
2c
2d
1bd
2bd
1cd
2cd
−0.80 (103)
−0.86(149)
−0.96 (75)
−1.01 (76)
−0.72(102)
−0.77 (88)
−0.89(102)
−0.93 (88)
−0.86 (156)
−0.77 (102)
−0.96 (80)
−0.89 (86)
−1.47 (76)
−1.51 (72)
−1.65 (77)
−1.67 (100)
−1.28 (86)
−1.30 (67)
−1.42 (86)
−1.49 (79)
−1.51 (78)
−1.30 (78)
−1.65 (102)
−1.42 (90)
FTIR spectra were recorded on solid samples of 1b and 2b
to gain insight into relative donor properties of the triimine
ligands. Direct comparison of the ν(CuC) stretching frequencies
to that of the free phenylacetylene is problematic because the
latter is solvent sensitive (e.g., CCl4, 2119 cm−1; CHCl3,
2109 cm−1).78,79 Nevertheless, the ν(CuC) of 1b (2112 cm−1) is
indubitably lower than that of 2b (2125 cm−1). Under the
assumption that electrostatic effects80 are similar in the two
complexes, these results are consistent with the notion that 1b
has greater π-electron density available for Pt–CuC π-back
bonding as might be expected for mbzimpy having weaker
π-accepting properties than tpy.
+1.30b
+1.31b
+1.54b
+1.57b
a (PF6)− salts of platinum complexes in 0.1 M TBAH/DMF; Pt electrode;
V vs. AgCl (3.0 M NaCl)/Ag. Couples are chemically reversible one-elec-
tron processes unless specified. b Chemically irreversible under experi-
mental conditions. E°′ = (Epa + Epc)/2, (V); Epa and Epc are the anodic
and cathodic peak potentials, respectively. c The difference between
the potentials of the forward (Epc) and reverse (Epa) peaks in mV. d On
a glassy carbon electrode.
Electrochemistry
To better understand the electronic structures of 1(a–d) and 2
(a–d), cyclic voltammograms (CVs) were recorded on samples
dissolved in 0.1 M TBAH/DMF solution (Fig. 1, and S2 (ESI†),
and Table 1). The electrochemistry of 2a and 2b have pre-
viously been described by the Gray and Yam groups,
respectively.2,65,81
1(a–d) and 2(a–d) undergo two chemically reversible one-
electron reduction processes in DMF. In the case of 1(a–d), the
1st reduction occurs in the −0.75 to −1.06 V range, depending
on the ancillary ligand. For each complex, the potential is cath-
odically shifted by 0.1 V from that of the mainly ligand-cen-
tered one-electron reduction2,18,63,65,81–83 of the corresponding
terpyridyl compound, 2(a–d). The results are consistent with
the conclusion that the LUMO of the 1(a–d) series is only
slightly destabilized with respect to that of the 2(a–d).
Compounds 1(a–d) undergo a second reduction process in the
−1.40 to −1.75 V range, which is cathodically shifted by ∼0.2 V
from that of the corresponding reduction process in 2(a–d)
analogs. There is some disagreement about the nature of the
second reduction (LUMO+1) of platinum(II) polypyridyl com-
plexes which has been suggested to involve addition of an elec-
tron to either predominantly metal-centered2,84,85 or ligand-
centered orbital.18,65,69,86
For 1 and 2, both reduction processes are slightly but
monotonically shifted to negative potentials along the Cl <
CCPh < Ph < CH3 series of ancillary ligands, which suggests
that electron donation by these ligands increases in the same
order. Within the uncertainties of the estimated potentials,
the correlation of the potentials of the mbzimpy and tpy series
is excellent with essentially unitary slope.
Related late 1st, 2nd and 3rd-row transition metals with
mbzimpy and tpy chelates display qualitatively similar
reduction processes.47,57,87 The reports by Haga57 and
others58–60 show that ligand-centered reductions in Ru
2+
2+
(mbzimpy)2 (−1.24, −1.55 vs. Ag/AgCl) and in Ru(tpy)2 (at
−1.22, −1.46 V vs. Ag/AgCl) occur at essentially same poten-
tials, which suggest that the ligand-centered low-lying π* orbi-
tals (LUMO) in these complexes have virtually same energies.
Notably, the metal-centered d5/d6-electron couple of Ru
3+/2+
(mbzimpy)2
(+0.86 V vs. Ag/AgCl) is shifted cathodically by
2+
∼0.5 V from that of Ru(tpy)2 (+1.31 V vs. Ag/AgCl), which is
consistent with the notion that mbzimpy is a stronger
electron donor than tpy. In comparison to these ruthenium
and other related47,88 metal complexes, the first reduction
wave of 1 and 2 has been substantially stabilized.89 This effect
in the case of 2a has been suggested to have resulted from the
coupling of the tpy π* orbital with the higher lying empty
6pz(Pt) orbital.2
Fig. 1 Cyclic voltammograms of 1 and 2 in 0.1 M TBAH/DMF at a scan
rate of 100 mV s−1. The working electrode was platinum, the auxiliary
electrode was a platinum wire, and the reference electrode was Ag/AgCl
in 3.0 M NaCl.
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In addition to the reductions processes (LUMO and transitions.47,88 Furthermore, vibronic spacings ranging from
LUMO+1) noted-above, 1b, 1c, 2b, and 2c also undergo 1337–1504 cm−1 are diagnostic of 1(π → π*) transitions.47,58
chemically irreversible oxidation on a glassy carbon electrode
Notably, the terpyridyl 2(a–d) complexes exhibit two dis-
within the experimental window (<+1.8 V) (Fig. S2 ESI†). tinctly separate absorption bands in the 250–375 nm region
Based on the electrochemistry of related platinum (Fig. 2B), which are relatively more structured and occur at
compounds,18,63,65,81–83,90–94 it seems probable that the oxi- relatively higher energies.3,4,65,98 The first vibronically struc-
dation process for 1c and 2c (at +1.54 for 1c and +1.57 V for tured and moderately intense (ε ≈ 104 M−1 cm−1) band orig-
2c) is associated with metal oxidation, whereas oxidation of 1b inating in 300–350 nm region is assigned to the low-lying 1LC1
and 2b (at +1.30 V for 1b and +1.31 V for 2b) has a substantial transitions, whereas the second band in 250–300 nm region
contribution coming from the acetylene group. Although irre- with relatively high intensity is assigned to the next low-lying
versible peak potentials cannot be considered as reliable esti- 1LC2 transitions. Between 1 and 2 analogs, some of the spec-
mates of redox potentials, it has not escaped our notice that tral variations, such as intensity and bandwidth are compar-
the difference between the oxidation peak potential and the able with those of the free ligands (Fig. S3 ESI†). Interestingly,
first reduction potential (2.16 eV for 1b and 2.08 eV for 2b) is the 1LC1 absorption band of 2(a–d) is slightly, but distinctly,
remarkably similar to the HOMO–LUMO energy gap for 1b shifted to longer wavelengths along the CH3 ≈ Ph < CCPh < Cl
and 2b, that has previously been predicted by computational series of ancillary ligands.3 Furthermore, the band profile (the
studies.95
shape and the vibronic intensities, including a number of
vibronic peaks) of the LC transitions is varied with the ancil-
lary ligand. In the case of 1(a–d), a comparable trend is not
apparent because of having a poorly structured LC band, but
the band shape is conspicuously varied with the changing
ancillary ligand.
Electronic absorption spectroscopy
The room-temperature absorption spectra of 1(a–d) and 2(a–d)
in acetonitrile are presented in Fig. 2 and S4 (ESI†). The
absorption spectra of 3 and free triimine ligands96,97 are
provided in Fig. S3 (ESI†) for comparison.47 Table S1 (ESI†)
summarizes the absorption maxima of the aforementioned
1
At the lower end of LC transitions, each of the 2 exhibits
relatively broad and weakly structured absorptions (ε ≈ 103
M−1 cm−1; 1200–1400 cm−1 spacings) in the 360–450 nm
range, which, excepting those from the 2b, have been assigned
to the metal-to-ligand-charge-transfer 1MLCT [dπ(Pt) → π*(tpy)]
transitions.3,4,65,98 In the case of 2b, these absorptions are rela-
tively more intense and suggested to be originated from the
1LLCT/1MLCT mixed state [LLCT: ligand-to-ligand-charge
transfer; π(CuC–Ph) → π*(tpy)].95 By analogy of 2 and related
mbzimpy16,18,37 complexes, the low energy (400–520 nm)
absorption band of 1(a, c, d) is assigned to 1MLCT transitions.
Similarly, the low energy band of 1b, which shows a striking
resemblance with that of the 2b, is tentatively assigned to the
1LLCT/1MLCT mixed state. In comparison to 2, the 1MLCT
bands of 1(a, c, d) are weak and, adding further to our dis-
compounds in acetonitrile, dimethyl sulfoxide or solvent as
specified. Data3 for Zn(tpy)2
comparison.
is also included for the
2+
The absorption spectra of 1(a–d) are characterized by a
broad, intense, and weakly structured band originating in the
280–380 nm region with the molar absorptivity in the order of
104 M−1 cm−1 (Fig. 2A). The absorption spectra of 3 and free
mbzimpy, although remain partially resolved, exhibit intense
absorption band(s) in the same region, which suggests a pres-
ence of spin allowed, mbzimpy ligand-centered 1LC
1
advantage, obscured by the extended tailing of the LC bands
and the poor spectroscopic resolution. As expected, the exci-
tation measurements in the frozen EMD solutions countered
the issue of low-resolution and yielded relatively well-resolved
spectra, especially in the MLCT region (Fig. 3 and S5 ESI†).
These excitation measurements reveal structured features
(1175–1550 cm−1 spacings) in the 1MLCT region (which,
however, look distinctly different from that of the 2). Notably,
the HOMO–LUMO transitions (at 457 nm) in analogous
complex, [Pt(R,R′-bzimpy)Cl](PF6) (R = C12H25, R′ = H), have
been attributed to the LC (R,R′-bzimpy → R,R′-bzimpy) tran-
sitions by computational study.18 At this point, our judgment
of assigning the above-structured features to MLCT transitions
comes purely from the perspective of intensities (relative
1
molar absorptivities, ε) of the characteristically similar MLCT
and 1LC bands in related platinum complexes. We also note
that similar features are absent from the excitation spectrum
of 3. Excitation measurements further reveal that the 1MLCT
band of each of the 1 occurs at lower energies than that of the
Fig. 2 Electronic absorption spectra of 0.1–0.3 mM solutions of 1 (A)
and 2 (B) in acetonitrile at room-temperature.
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Emission spectroscopy
The platinum compounds in this study are intensely lumines-
cent in the solid-state at room temperature and in dilute solu-
tions at 77 K. Fig. 3 illustrates emission and excitation spectra
of 1(a–d) in glassy EMD solution at 77 K. Emission spectra of 2
(a–b) were obtained under similar conditions and are provided
in Fig. S5 (see ESI†). Table 2 summarizes emission data for
these compounds.
Upon photoexcitation at λ 350 nm (and 400 nm), the low-
temperature glassy solutions of 1(a–d) give rise to one or more
characteristic emission bands in 450–850 nm region. The first
band beginning at in the range of λmax(0–0) 540–560 nm is
vibronically structured (1336–1450 cm−1 spacings), whereas
the second band (maxima > 650 nm), absent from 1c, is broad
and structureless and displays a concentration dependence.
The presence of any additional band(s) at longer wavelengths
is discussed below. The emission of the (PF6)− salt of 1a is
invariant from that of the Cl− salt that we reported previously
in frozen ethanol: methanol solution.16 With reference to this
Cl− salt and related complexes,16,47,88 the shortest wavelength
vibronic emission of 1(a–d), excepting 1b, is assigned to the
3(π* → π) emission originating from the lowest spin-forbidden
3LC (mbzimpy-centered) excited state (note: the shortest wave-
length emission of 1b, we note (vide infra), originates from the
significantly different excited state. Thus, to avoid confusion,
hereinafter, whenever possible, we will discuss 1b separately).
Fig. 3 Room temperature electronic absorption spectrum in dimethyl sulf-
oxide (—), and 77 K emission (λex = 400 nm, ) and excitation spectra (––,
)
in EMD glassy solution for (A) 1a (λem = 544 nm ––; λem = 670 nm, ), (B) 1b
(λem = 541 nm, ––; λem = 641 nm, ; λem = 709 nm, ), (C) 1c (λem = 547 nm,
––), (D) 1d (λem = 557 nm, ––; λem = 735 nm, ). Excepting for 1c, the emis-
sion spectra at various concentrations are normalized at λmax of the first vibro-
nic feature. Absorption spectra in Fig. 2 are duplicated in DMSO to facilitate
visual comparison.
corresponding terpyridyl compound, 2. For example, the
1MLCT band of 1a occurs at about ∼3300 cm−1 lower in energy
than that of the 2a. Furthermore, we note that the 1MLCT
band of both 1 and 2 show a bathochromic shift (or stabiliz-
ation) along the Cl < Ph < CH3 series. Notably, in the case of 2,
the 1MLCT band shifts along the Cl < Ph < CH3 series are
larger than that for the 1LC transitions. As noted earlier, a
similar trend is not immediately apparent in the case of 1
because of having a poorly structured 1LC band.
3
Notably, energies and vibronic intensities of the LC bands of
1(a, c, d) are distinctly different from that of 3 (λmax 462, 495,
530 nm; Huang–Rhys (HR) factor, (S) I(1,0)/I(0,0) = 2.5)47 and
free mbzimpy (weak features at λmax 457, 489, 520, 560, 608, S
= 1.2) that exhibit emissions decidedly from the pure 3LC
states. Speaking of energies and vibronic intensities, the 3LC
bands of 1(a, c, d) occur at relatively longer wavelengths than
that of 3 and show smaller HR factors. For example, in the
case of 1a, the 3LC band is red-shifted from that of the 3 by
∼3500 cm−1 and displays S about 0.70. Seemingly, these
The effect of ancillary ligand donor properties on the
1
energy and Frank-Condon factors of the LC1 band of Pt(tpy)
Ln+ complexes (L: Cl, Br, I, SCN, or N3, n = 1; and NH3, n = 2)
has previously been documented by Che and co-workers.40
This effect is attributed to the configuration interaction
1
1
1
between LC1 and MLCT states.40 In this model, MLCT state
undergoes stabilization (a shift to longer wavelengths) with an
increasing σ donor capacity of an ancillary ligand, leading to
widening of an energy gap between the low lying 1LC1 and
1MLCT states, as that, also noted for platinum(II) diimine
3
changes are consistent with the notion that the LC state of 1
(a, c, d) undergoes a perturbation upon interaction with the
closely lying CT state.47,103,104 Furthermore, the 3LC band is
complexes.99,100 This leads to the decreased mixing of the LC
1
Table 2 77 K EtOH-MeOH-DMF [10 : 10 : 1 (v/v)] glassy solution emis-
sion data
1
1
and MLCT states. As a result, the LC band maximum is also
expected to shift slightly to longer wavelengths. The energies
of a 1LC band of Pt(tpy)Ln+ including that of 2(a, c, d),
however, do not model well with the two-state perturbation
model, as used by De Armond et al. to describe mixing of
1MLCT and 1LC1 states in ruthenium(II) bipyridyl systems.101
The spectra show unexplained variations in the 1LC1 band
maxima with stabilization of the MLCT states. For example, as
noted above, the 1LC1 state of 2(a, c, d) is destabilized along
the Cl < Ph < CH3 series, while the 1MLCT state along the
same order is stabilized. We believe that sources of this com-
plexity include contamination of spectra by solution aggre-
gates,45 π*(tpy)/6pz(Pt) orbital mixing,102 and contributions
from higher-lying MLCT states.
λmax, nm
(λmax Conc. dependent
Compounds
)
S (S°)
1a
1b
1c
1d
2aa
2b
2ca
2da
544, 590; 588, (655)
541, 587, (641, 709)
547, 592, 644, 709
558, 604, 662, (734)
470, 506, 551, (578)
534, 570, (695)
0.70 (0.54)
0.59 (0.56)
0.70 (0.72)
0.62 (0.63)
0.71
0.71
1.20 (1.07)
0.68
472, 507, 541, 581
471, 506, 542, (728)
a 77 K glassy butyronitrile solution. S: Huang-Rhys factor, I(1,0)/I(0,0).
S°: Huang–Rhys factor, I(1,0)/I(0,0), obtained by deconvoluting the
emission band into Gaussian components using Origin (OriginLab,
Northampton, MA).
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bathochromically-shifted along the Cl < Ph < CH3 series of non-radiative decay increases exponentially with the decreas-
1
ancillary ligands. Since, the corresponding LC absorptions of ing excitation energy.106–108 The emission spectra of 1b also
1(a, c, d) occur at virtually identical energies, it is likely that show a shoulder at 709 nm, which is gaining intensity with the
the bathochromic-shift of 3LC state along this order resulted concentration. The corresponding excitation spectrum moni-
in from its mixing with the higher-lying 3MLCT state. This tored at this shoulder (λem 709 nm) shows a relatively weak
interpretation is in line with the absorption data (vide supra), absorption feature at 536 nm. Within the framework of plaus-
1
which presents that the MLCT state of 1(a, c, d) is bathochro- ible excited states and on energy grounds, this feature is
mically-shifted along the Cl < Ph < CH3 order as well.
attributable to 3[π(–CuCPh) → π*(mbzimpy)] state, and the
Surprisingly, the shortest wavelength emission band of 1b emission likely originates from the aggregated species that is
(at λmax, 541 nm) occurs at a relatively shorter wavelength than formed due mainly to ligand–ligand interactions.
that of the 1(a, c, d). On energy grounds, the band is assigned
Emissions of terpyridine 2(a–d) complexes have previously
to the 3LLCT state. This assignment is consistent with the been documented in various other rigid matrices.1,4,6,29,63,65
notion that the singlet–triplet (ΔEST) splitting is (generally) However, a dearth of the high-resolution spectral data in some
smaller for the CT states than that for the LC states; thus, the cases had made it difficult for us to formulate as well as
band will occur at relatively shorter wavelengths. The HR compare the electronic structural models. To counter this
factor for this band, ∼0.6 (Table 2), on the other hand, indica- difficulty and to also maintain the uniformity in our measure-
tive of perturbation of 3LLCT state, which is likely caused by ments, very refined emission measurements have been carried
3
the closely lying MLCT state. These observations are congru- out in EMD and butyronitrile matrices at 77 K (Fig. S5 ESI†).
ous with the computational study (by TDDFT/CPCM) by Yam Emission profiles of these complexes in both these matrices
et al.,95 that suggests the lowest energy triplet emissive state of are nearly identical. As illustrated in Fig. S5,† like 1(a–d), the
1b composed of transitions of 3LLCT/3MLCT mixed characters, 77 K butyronitrile glass of 2(a–d) upon photoexcitation gave
and it originates from the HOMO (π-orbital of the phenyl acety- rise to one or more emission bands. Similarly, like 1b, the
lide ligand mixed with the platinum dπ-(dxz/dyz) orbital) → emission of 2b found emanating from the significantly
LUMO (π*-orbital from mbzimpy) transitions.95 Notably, the different emitting state than that found in the rest of the tpy
energy estimate for 3LLCT/3MLCT excited state (2.16 eV, compounds, and it is thus presented here separately. The first
∼573 nm) by TDDFT/CPCM (in CH2Cl2 at the optimized and the shortest wavelength band of 2(a, c, d) is vibronically
ground state)95 is lower than 2.29 eV that we have estimated structured (1200–1600 cm−1 spacings), which is diagnostic of
experimentally (in EMD). At this point, it is, however, unclear 3LC transitions. Surprisingly, the 3LC band occurs at nearly
to what extent (consideration for) solvents will affect these esti- identical wavelengths, λmax(0–0) ∼ 470 nm, in all three com-
1
mates or this comparison for that matter.
plexes. It should be noted that the corresponding parent LC
As noted above, 1(a, b, d) also show a new band growing in state (vide supra), however, is shifted somewhat to the longer
with increasing concentration at longer wavelengths. The emis- wavelengths along the CH3 ≈ Ph < Cl series. For example, 1LC1
sion spectra of 1a (Fig. 3(A)) show that the band at λmax state of 2a is red-shifted by ∼900 cm−1 from that of the 2(c, d).
655 nm (FWHM = ∼1800 cm−1) gains intensity relative to the The 3LC emissions of 2(a, c, d) also occur at longer wave-
higher energy emission features over the concentration range lengths than that of the Zn(tpy)2+ at 77K,109 which upon photo-
of 9–22 μM. The band further shows a bathochromic-shift of excitation known to emit radiations from the pure 3LC state.
∼5 nm to 660 nm. As we noted earlier,16 these observations are These observations, similar to that made earlier in the case 1,
consistent with the formation of emissive aggregates, and the are consistent with the notion that the shortest wavelength
emission originates from the 3MMLCT state. The excitation emissions of 2(a, c, d) originate from the 3(LC/MLCT) mixed
spectrum of 1a monitored at λem 655 nm (Fig. 3(A)) exhibits a states4,110 rather than from the pure LC states. At this point, it
3
vibronically structured features at 556, 520, 485 nm seems coincidental to have configuration mixing between LC
(∼1300 cm−1 spacings) which are attributable to 1MMLCT and 3MLCT states being balanced in such a way that the short-
[(dσ*(Pt) → π*(mbzimpy)] excited state.16 Similarly, the emis- est wavelength emitting states of 2(a, c, d) occur at about the
sion bands at λmax 641 and 734 nm in 1b and 1d, respectively, same energies. Provided that, the 3(LC/MLCT) band of 2c in
are attributable to the similar type of the aggregation-induced contrast to that of the 2(a, d) found displaying variation in the
3MMLCT emissive state (Fig. 3). Surprisingly, even though the vibronic intensities, S = 1.2. But, on the other hand, to our sur-
3
emission maxima (for MMLCT) varied distinctly in all three 1 prise, the band also found displaying some similarities with
(a, b, d), their corresponding excitation spectra display longer that of the free terpyridine in frozen EMD glass,1 Zn(tpy)Cl2 in
wavelength vibronic features at virtually identical energies dichloromethane at room-temperature110 and Zn(4′-Ph-tpy)Cl2
(1MMLCT at 556, 520 nm for 1b and 558, 520 nm for 1d). We in ethylene glycol at 77 K.111 The emission from these latter
also note that, in the case of 1d, the emission from the aggre- species is assigned to the pure LC state. It is intriguing that
3
gated species is weaker, and it is bathochromically-shifted by despite the LC state in all three 1(a, c, d) has somewhat been
3
1528 cm−1 from that of 1a. The latter effect is consistent with perturbed by high-lying MLCT state, the emission profile (the
the notion that the relatively stronger σ-donor methyl group shape and the vibronic intensities) of 2c differs so significantly
enhances aggregation,105 whereas the weaker emission is in from that of the 2(a, d). While the source of this complexity is
accord with the energy-gap law which states that the rate of unclear, we believe it has likely resulted from the molecular
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distortion in the excited state. Given that the phenyl ring
forms a dihedral angle of ∼90° with the Pt coordination
plane,112–114 which in turn restricts a metal-tpy framework to
only a minor distortion, we think, 2(a, d) upon photoexcitation
might be undergoing a considerable molecular distortion than
that of 2c.
In the case of 2b, the shortest wavelength emission occurs
at relatively low energy, λmax = 534 nm (Fig. S5 ESI†).65
Although the band is poorly structured, it displays vibronic
spacings of ∼1200 cm−1 with S = 0.71, which is indicative of
ligand involvement. Similar to 1b, on energy grounds, this
band is attributable to the 3LLCT/3MLCT mixed state.
Consistent with this, the corresponding excitation spectrum
measured at λem 534 nm displays dominating vibronic features
at λmax 430 sh, 452 nm (1132 cm−1 spacing), that bear a mirror
image relationship with the emission band above. These exci-
tation features are assigned to metal perturbed 1LLCT states.
This assignment is congruous with the computational study
by Yam et al.,95 that suggests that the lowest energy excited/
emitting states of 2b arising from the admixture of LLCT/
MLCT transitions. Notably, the energy estimate for these tran-
sitions (2.08 eV ≈ 596 nm) by TDDFT/CPCM (in CH2Cl2 at the
Fig. 4 State energy level diagram for Pt(mbzimpy)X+ (in black) and Pt
(tpy)X+ (in red) complexes. Energies were derived from spectroscopic
data. Some mixed excited and emissive states are labelled only with the
main character (for details, see the text).
optimized ground-state geometries)95 is lower than the experi- enced/perturbed quite differently by the same group of ancil-
mentally estimated value (<534 nm). With that, we also note lary ligands. This behavior could principally be resulted from
that, for 1b and 2b, the order of the relative energies for LLCT/ (a) energies of the frontier orbitals of 1 being at different levels
MLCT transitions by experimental and computational calcu- from that of 2, which brings (b) variations in the extent of con-
lations stand in contrast to each other (computationally, 1b > figuration mixing between the frontier molecular orbitals, and
2b; whereas experimentally 1b < 2b). On the other hand, ΔEST hence between LC and MLCT states. Since the orbital parent-
(experimental) for LLCT/MLCT state is 3988 cm−1 for 1b and age of the lowest-energy excited state of phenylacetylene
3397 cm−1 for 2b, which is consistent with the notion that adducts (1b, 2b) differ significantly from the rest of the com-
MLCT character is relatively higher in 2b.
pounds, to avoid ambiguity, these compounds have been
Like mbzimpy analogs, the 2(a–d) complexes, excepting 2c, divided into two subgroups. Below, 1(a, c, d) and 2(a, c, d) are
display a broad and structureless emission in the 550–800 nm briefly mentioned as a group A complexes and 1b, 2b as a
region due to the formation of emissive aggregates. These fea- group B complexes.
tures are attributable to the MMLCT [(dσ*(Pt) → π*(tpy)] tran-
As can be seen from the diagram, the lowest energy 1LC
3
sitions, and display a red-shift along Cl < CCPh < CH3 series of excited states of group A compounds are destabilized along the
ancillary ligand which is indicative of enhancement of plati- Cl < Ph < CH3 series of ancillary ligands. In the case of
num–platinum interaction along the same order due mainly to mbzimpy compounds, the 1LC state energies, however, seem
an increased σ-donor capacity of ancillary ligand. The exci- to be unaltered (or less affected), although the electro-
tation spectra monitored at the λmax of these 3MMLCT emissive chemistry of both 1 and 2 indicates a perturbation of LUMO-π*
features reveal weak absorption features at 411, 439, 471 nm by somewhat identical energies. This unusual behavior
for 2a, and 458, 490sh nm (1426 cm−1 spacings) for 2b, and implies that a perturbation also likely occurs in the donor
440, 466 and 496 nm for 2d. On energy grounds, we tentatively π-MOs in a similar fashion, which leaves the energy of 1LC
assign these features to 1MMLCT transitions.
state somewhat unaffected along the Cl < Ph < CH3 series.
Whereas, in the case of terpyridyl compounds, the donor
π-MOs are presumably weakly perturbed. On the other hand,
Electronic structure: mixing between 3LC and 3MLCT states
Based on the spectroscopic data described above, an energy- both analogs display stabilization of the 1MLCT state along the
level diagram can be drawn. The diagram in Fig. 4 gives a Cl < Ph < CH3 series.
qualitative and quantitative overview of various low lying
In contrast to the 1LC state, the 3LC emitting state of
excited and emitting states of 1 and 2. The energy values mbzimpy compounds is stabilized along the Cl < Ph < CH3
shown are absolute values with respect to the ground state. series with ΔEST (single–triplet splitting energy) increasing lin-
The mixed states in the diagram are labelled only with the early from 8645 cm−1 to 9400 cm−1. Although, energies of the
main character of the state. It is conspicuous from the 3LC emitting states of terpyridyl compounds do not appear to
diagram that the excited states of mbzimpy and tpy complexes be sensitive to the σ-donation of ancillary ligands, the ΔEST
with similar orbital parentage differ distinctly in energies. found linearly increasing from 7450 cm−1 to 8440 cm−1 along
Furthermore, a given an excited state of 1 and 2 has been influ- the Cl < Ph < CH3 series as well. We believe that the stabiliz-
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ation of the 3LC state along these series likely resulted from its
mixing with the higher lying 3MLCT state. Furthermore, it is
conspicuous from the diagram that the energy gap between
5 K. W. Jennette, J. T. Gill, J. A. Sadownick and S. J. Lippard,
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1
3LC and MLCT states is narrowing along the Cl < Ph < CH3
order. Thus, the coupling between 3MLCT and 3LC states is
expected to increase in the same order.
Interestingly, in the case of group B complexes, the low-
lying 1(LLCT/MLCT) excited states occur at approximately the
same energies. In the case of 1b, the energy of this state,
however, is approximate because of not having a well-defined
emission-excitation spectrum. Notably, the 3(LLCT/MLCT)
emitting states of 1b and 2b also occur at approximately the
same energies with ΔEST being ∼3988 cm−1 for 1b and
∼3300 cm−1 for 2b. A relatively low ΔEST value for 2b indicates
higher MLCT character in 3(LLCT/MLCT) emitting state, which
we believe is the underlying cause for emission of 2b being
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3
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Conflicts of interest
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J. Am. Chem. Soc., 2004, 126, 1594–1595.
There are no conflicts to declare.
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Acknowledgements
We thank the NSF (Grants CHE-1152853 & CHE-1566438); and
the Department of Chemistry, University of Cincinnati (UC),
for NMR and Mass Spectrometry facilities. V. M. S. thanks Drs
S. Macha and L. Sallans (UC) for expert technical assistance
with mass spectrometry, and Drs K. Ding and N. Kaval (UC) for
assistance with NMR and FTIR, respectively. V. M. S also
thanks Dr J. A. Krause (Crystallographer at UC) for confirming
the structural identities of 1b and 1c by single X-ray
crystallography.
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Y. Sasaki, Inorg. Chem., 2000, 39, 4022–4028.
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