Understanding the origin of phosphorescence in bismoles: A synthetic and computational study

Article abstract of DOI:10.1021/acs.inorgchem.8b00149

A series of bismuth heterocycles, termed bismoles, were synthesized via the efficient metallacycle transfer (Bi/Zr exchange) involving readily accessible zirconacycles. The luminescence properties of three structurally distinct bismoles were explored in detail via time-integrated and time-resolved photoluminescence spectroscopy using ultrafast laser excitation. Moreover, time-dependent density functional theory computations were used to interpret the nature of fluorescence versus phosphorescence in these bismuth-containing heterocycles and to guide the future preparation of luminescent materials containing heavy inorganic elements. Specifically, orbital character at bismuth within excited states is an important factor for achieving enhanced spin-orbit coupling and to promote phosphorescence. The low aromaticity of the bismole rings was demonstrated by formation of a CuCl π-complex, and the nature of the alkene-CuCl interaction was probed by real-space bonding indicators derived from Atoms-In-Molecules, the Electron Localizability Indicator, and the Non-Covalent Interaction index; such tools are of great value in interpreting nonstandard bonding environments within inorganic compounds.

Full text of DOI:10.1021/acs.inorgchem.8b00149

Article
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Understanding the Origin of Phosphorescence in Bismoles: A
Synthetic and Computational Study
Sarah M. Parke,† Mary A. B. Narreto,‡ Emanuel Hupf,† Robert McDonald,† Michael J. Ferguson,†
Frank A. Hegmann,*,‡ and Eric Rivard*,†
†Department of Chemistry, University of Alberta, 11227 Saskatchewan Drive, Edmonton, Alberta T6G 2G2, Canada
‡Department of Physics, University of Alberta, Edmonton, Alberta T6G 2E1, Canada
S
* Supporting Information
ABSTRACT: A series of bismuth heterocycles, termed
bismoles, were synthesized via the efficient metallacycle transfer
(Bi/Zr exchange) involving readily accessible zirconacycles. The
luminescence properties of three structurally distinct bismoles
were explored in detail via time-integrated and time-resolved
photoluminescence spectroscopy using ultrafast laser excitation.
Moreover, time-dependent density functional theory computa-
tions were used to interpret the nature of fluorescence versus
phosphorescence in these bismuth-containing heterocycles and
to guide the future preparation of luminescent materials
containing heavy inorganic elements. Specifically, orbital
character at bismuth within excited states is an important factor
for achieving enhanced spin−orbit coupling and to promote
phosphorescence. The low aromaticity of the bismole rings was demonstrated by formation of a CuCl π-complex, and the nature
of the alkene-CuCl interaction was probed by real-space bonding indicators derived from Atoms-In-Molecules, the Electron
Localizability Indicator, and the Non-Covalent Interaction index; such tools are of great value in interpreting nonstandard
bonding environments within inorganic compounds.
compounds are generally considered to have low toxicity9 and
cost, efficient phosphorescent bismuth-based emitters with
color tunability would be of significant value to the community.
Challenges that have limited progress in this regard are (1) a
lack of a general synthetic procedure to prepare air-stable
bismuth-containing heterocycles (especially bismoles) and (2)
difficulties in obtaining solid-state phosphorescence in the
presence of oxygen due to self-quenching at high concen-
INTRODUCTION
■
Main-group element-containing heterocycles represent a
valuable and increasingly explored class of π-conjugated
material due to their often-luminescent nature and conductive
properties.1 To date, this field has been dominated by
optoelectronically active nitrogen or phosphorus-containing
heterocycles, namely, pyrroles2 and phospholes.3 Advances in
synthetic methods have opened the door for the wider
exploration of the heavier group 15 element analogues, as
evidenced by recent reports on luminescent arsenic-based
trations (via triplet−triplet annihilation) and quenching of
excited state triplet species by O2.10
Given that this article describes the preparation of bismuth-
containing five-membered heterocycles (bismoles) and the
determination of factors that control their photoluminescence,
some key prior studies in this area should be mentioned (Chart
heteroles that are less prone to oxidation than their phosphole
counterparts.4 Moreover, the use of antimony heterocycles
(stiboles) and their Sb(V) congeners in fluoride ion sensing can
be seen as a promising new direction for this field.5
1). Approximately 50 years ago Wittig and Hellwinkel
The incorporation of heavier p-block elements into π-
conjugated materials can also greatly enhance the probability of
accessing triplet excited states (and eventual phosphorescence)
by increasing the rate of intersystem crossing (ISC) between
published the syntheses of the Bi(III) and Bi(V) bismafluor-
enes [PhBi(biph)]11 (A) and [Ph3Bi(biph)] (B).12 Later Ashe
studied the aromaticity and coordinating properties of formally
anionic bismole analogues, including bismaferrocene sandwich
excited singlet and triplet states, commonly termed as the
complexes (e.g., C).13 The first photoluminescent bismuth-
containing conjugated polymer (D) was reported in 2006 by
Chujo and co-workers;14 although emission lifetime measure-
ments were not reported, the small Stokes shift found in this
“heavy element effect”.6 Stable phosphorescent emitters are
highly sought in organic light-emitting diodes (OLEDs) due to
a possible maximum device efficiency of 100% versus 25% for
traditional fluorescence-based emitters.7 Furthermore, phos-
phorescent compounds that can exhibit long-wavelength red or
IR emission are of great value for bioimaging, as there is less
interference with undesired background emission.8 As bismuth
Received: January 16, 2018
© XXXX American Chemical Society
A
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Chart 1. Selected Bismuth-Containing Heterocycles Exhibiting Diverse Coordination Modes and Oxidation States
blue luminescent material was indicative of fluorescence. More
recently luminescent dithienylbismoles (e.g., E and F) were
prepared by the Ohshita group and show broad photo-
luminescence (PL) bands at 600−640 nm attributed to weak
phosphorescence, as well as concurrent PL at 400 nm due to
fluorescence.15 While dilithiated butadiene analogues can be
used to gain access to Bi heterocycles via condensation
reactions with bismuth halides (RBiCl2; R = alkyl or aryl
groups),13,15 our current study takes advantage of the mild
Fagan-Nugent protocol, whereby zirconium-mediated alkyne
cyclization followed by Zr/Bi exchange is used to form bismole
rings (Scheme 1).16 This general procedure has been actively
Scheme 2. Synthesis of Bismoles 1 and 2
prepared in situ via the known ligand scrambling reaction
between 2 equiv of BiCl3 and the respective triarylbismuthines
BiAr3 in diethyl ether (Scheme 2).18,19 Compound 1 was
observed to undergo decomposition when stored under
ambient conditions (64% decomposition into unidentifiable
insoluble products when stored at room temperature in the
presence of air for 24 h) but to remain stable indefinitely in an
inert atmosphere. However, compound 2 is stable to water and
air, both in solution and in the solid state; this is in sharp
contrast to structurally related phospholes,2 which are readily
oxidized in air.
Scheme 1. Bismole Synthesis via the Cyclization of a
Dilithiated Diene and by Metallacycle Transfer
With the goal of placing reactive pinacolboronate (BPin)
groups about the periphery of a bismole (for future ring
functionalization via Suzuki-Miyaura cross-coupling),17e the
known zirconium reagent B-Cp2Zr-6-B (Scheme 3) was
combined with in situ derived MesBiCl2. Efficient Zr/Bi
exchange only transpired in the presence of 10 mol % CuCl as a
catalyst to give the mesityl-functionalized heterocycle B-MesBi-
6-B (3) (Scheme 3). The same CuCl-assisted metallacycle
transfer protocol was used to generate the 2,5-thiophene-
substituted bismole 4 as well as the tetraphenyl-substituted
bismoles PhBiC4Ph4 (5) and MesBiC4Ph4 (6) (Scheme 3). As
desired, compounds 3−6 are stable indefinitely in air at room
temperature, both in solution and in the solid state.
In addition to the bismoles discussed above, we targeted the
preparation of an aryl-substituted bismole containing a
proximal −CH2NMe2 group (7, Scheme 4) that could be
used to modulate luminescence in the final product via a
possible hypercoordinate Bi···NMe2 interaction. Accordingly,
the known bismuth dihalide ArNMe2BiCl2 (ArNMe2 = 2-
Me2NCH2C6H4)19,20 was combined with the pinacolboro-
nate-capped zirconacycle B-Cp2Zr-6-B in the presence of 10
mol % of CuCl as a promoter. While the formation of the
desired arylbismole 7 was confirmed by 1H NMR spectroscopy
(ca. 90%), there was another minor product formed, which was
later identified by X-ray crystallography as the CuCl adduct (8)
(Scheme 4). When a crude mixture containing compounds 7
and 8 (ca. 9:1 ratio) was treated with a stoichiometric amount
used in our group to gain access to phosphorescent
tellurophenes,17a−e and herein we report the first identified
examples of phosphorescence from bismoles, a key initial step
toward developing these potentially nontoxic emitters for
OLED and bioimaging applications.
RESULTS AND DISCUSSION
■
Synthesis of New Bismoles via Metallacycle Transfer.
To gain access to a wider scope of molecular emitters based on
the heavy inorganic element, bismuth, we explored metallacycle
transfer between the readily available zirconacycle
16
Cp2ZrC4Et4 and various ArBiCl2 species (Ar = Ph and
Mes; Mes = 2,4,6-Me3C6H2; Scheme 2). These reactions went
to completion under mild conditions to yield the perethylated
bismoles PhBiC4Et4 (1) and MesBiC4Et4 (2) in yields of 75%
and 78%, respectively, as orange and yellow oils, with insoluble
Cp2ZrCl2 as the common byproduct. Notably, Cp2ZrCl2 can be
recovered and recycled into the starting zirconacycle
Cp2ZrC4Et4 via a convenient one-pot procedure.16 The
requisite arylbismuthdihalides PhBiCl2 and MesBiCl2 were
B
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Scheme 3. Copper(I) Chloride-Promoted Metallacycle Transfer Synthesis of Bismoles 3−6
Scheme 4. Synthesis of the ArNMe2-Substituted Bismoles 7 and 8
of CuCl, compound 7 was fully converted into the CuCl adduct
8, which could then be isolated in pure form (59% yield) by
crystallization from toluene at −30 °C. Compound 7 could
then be regenerated cleanly by treatment of 8 with a
stoichiometric amount of triphenylphosphine to remove the
bismole-bound CuCl (Scheme 5).
Scheme 5. Synthetic Route Yielding Analytically Pure 7
Figure 1. Molecular structure of 3 with thermal ellipsoids plotted at
the 30% probability level. All hydrogen atoms were omitted for clarity.
Select bond lengths (Å) and angles (deg): Bi−C(10) 2.243(3), Bi−
C(20) 2.231(3), Bi−C(31) 2.293(3), C(10)−Bi−C(20) 78.48(11),
C(10)−Bi−C(31) 108.27(11), C(20)−Bi−C(31) 92.44(10).
Structural Characterization of the New Bismoles. We
investigated the solid-state structures of our new bismoles to
uncover possible intermolecular bismole−bismole interactions
and better interpret the luminescence data obtained (vide
infra). X-ray diffraction experiments were first performed on
the aryl-functionalized bismoles 3−6, and the refined structures
are presented in Figures 1−4. When the structure of 3 (Figure
Compound 4 (Figure 2) contains a similar overall structural
arrangement as 3 with slight canting of the flanking thiophene
rings away from being coplanar with the central bismole ring
(torsion angles: Bi−C(10)−C(11)−C(12A) = 31.9(4)°; Bi−
C(20)−C(21)−C(22)A = 18.9(3)°).
1) is compared to its phosphole congener B-PPh-6-B,17a
a
substantially pyramidalized geometry can be found about the
bismuth center in 3 with a bond angle sum [279.19(17)°] that
is much smaller than that found at the phosphorus center in B-
PPh-6-B [304.53(17)°]; this observation can be explained by
an increase in s-orbital character at the bismuth lone pair. In
addition, the hindered Mes group in 3 causes the rotation of
one BPin group away from being coplanar to the bismole ring
by 29.9(4)°, while the other BPin group remains coplanar, as is
commonly found in most BPin-functionalized tellurophenes.17
Pentaphenylbismole PhBiC4Ph4 (5) contains a planar BiC4
(bismole) ring corralled by phenyl groups arranged in a
propeller-like fashion (Figure 3); a related structural motif is
also present in the well-studied silole Ph(Me)SiC4Ph4, a
compound that exhibits pronounced aggregation-induced
emission (AIE).21 The mesityl-functionalized bismole Mes-
BiC4Ph4 (6) (Figure 4) adopts a similar overall structure as its
phenylated congener 5 but with a slightly wider angle sum at
the bismuth [276.6(3)° vs 264.9(3)° average in 5] due to the
C
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Figure 4. Molecular structure of 6 with thermal ellipsoids plotted at
the 30% probability level. All hydrogen atoms were omitted for clarity.
Select bond lengths (Å) and angles (deg): Bi−C(1) 2.244(2), Bi−
C(4) 2.234(2), Bi−C(51) 2.287(2), C(1)−Bi−C(4) 78.04(9), C(1)−
Bi−C(51) 102.49(8), C(4)−Bi−C(51) 96.08(8).
Figure 2. Molecular structure of 4 with thermal ellipsoids plotted at
the 30% probability level. All hydrogen atoms were omitted for clarity.
Select bond lengths (Å) and angles (deg): Bi−C(10) 2.236(2), Bi−
C(20) 2.240(2), Bi−C(31) 2.282(2), C(10)−Bi−C(20) 77.93(7),
C(10)−Bi−C(31) 99.21(7), C(20)−Bi−C(31) 104.60(7).
phosphorescence17 by limiting luminescence-quenching trip-
let−triplet annihilation (vide infra).
Single crystals of the CuCl−bismole complex 8 were
analyzed by X-ray diffraction analysis, and the refined molecular
structure is shown in Figure 5. The most salient structural
Figure 3. Molecular structure of 5 with thermal ellipsoids plotted at
the 30% probability level. All hydrogen atoms were omitted for clarity,
and only one molecule of the two in the asymmetric unit is shown.
Select bond lengths (Å) and angles (deg) with values belonging to a
second molecule of 5 shown in square brackets: Bi−C(1) 2.238(4)
[2.248(4)], Bi−C(4) 2.244(4) [2.224(3)], Bi−C(5) 2.266(5)
[2.265(5)], C(1)−Bi−C(4) 78.06(15) [77.65(16)], C(1)−Bi−C(5)
93.49(16) [91.97(16)], C(4)−Bi−C(5) 93.96(15) [91.63(15)].
Figure 5. Molecular structure of 8 with thermal ellipsoids plotted at
the 30% probability level. All hydrogen atoms were omitted for clarity.
Select bond lengths (Å) and angles (deg): Bi−C(10) 2.245(3), Bi−
C(20) 2.255(3), Bi−C(31) 2.275(3), C(10)−Bi−C(20) 79.16(10),
C(10)−Bi−C(31) 97.64(11), C(20)−Bi−C(31) 102.44(11).
feature of 8 is the coordination of a CuCl array to one of the
CC π-units within the bismole; this interaction reflects the
low degree of aromaticity that is inherent to the BiC4 ring in
the precursor 7 (vide infra). A distorted trigonal planar
geometry exists about the Cu center in 8, and the proximal C
C π-bond length (C(2)−C(20) = 1.406(4) Å) is, as expected,
longer than in the noncomplexed CC π-bond within the
bismole ring [C(1)−C(10) distance = 1.350(4) Å]. A C(2)−
Cu−C(20) angle of 40.53(11)° is present in 8, along with Cu−
C(2) and Cu−C(20) bond distances of 2.060(3) and 1.995(3)
Å, respectively, consistent with values in previously reported
olefin-Cu complexes.22
added steric bulk imposed by the mesityl group in 6. Overall,
the intraring C−Bi−C angles remain relatively similar between
compounds 3 to 6 (range from 77.93(7) to 79.16(10)°), and
the small angle sums at Bi indicate a high amount of s-character
in the bismuth lone pair. The combined steric influence of the
phenyl substituents in compounds 5 and 6 and the aryl groups
at the bismuth (in compounds 3 to 6) prevent close packing of
the bismuth centers in the solid state, leading to intermolecular
Bi···Bi distances greater than 4.5 Å. The absence of close
intermolecular contacts is often of importance in preserving
D
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Figure 6. (a) AIM bond paths motifs of bismole 8. (b) Electron density mapped on the AIM Cu atomic basin (given in e bohr−3); the blue region
indicates increased electron density positioned toward both C atoms (view from below the C4Bi unit).
There were only a handful of reports involving compounds
containing structurally authenticated bismuth−copper bonds.
For example, Fenske and co-workers23 prepared the bis(silyl)-
and 0.96 h e−1, respectively; interestingly, the ellipticity ε of the
Cu−C bond critical path (bcp) is very large (1.11) and
indicates that the bonding electron density is smeared away
from the Cu−C bond path, a sign that the Cu···bismole
interaction could involve two carbon atoms. The lack of
“structurally expected” AIM bcps in regions of low electron
density and high ε is a known feature of the AIM method, for
example, in the description of the metal−ligand interaction in
metallocenes.31 The coordination of the CuCl leads to slightly
weaker CC bonding in 8 as can be seen from smaller ρbcp
values (2.06 e Å−3) and less negative Laplacian (−21.8 e Å−5)
and H/ρbcp values (−1.11 h e−1), assuming participation of
both C atoms in the CuCl···bismole interaction. Furthermore,
the electron density is smeared toward both C atoms as can be
seen by mapping the electron density on the AIM Cu atomic
basin (Figure 6b). An isosurface (localization domain
representation) of the ELI-D of bismoles 7 and 8 is given in
Figures S34 and S35,26 respectively, and it shows a substantially
smaller CC bonding basin of the coordinated CC bond in
comparison to the uncoordinated CC bonding basins
(Figures S34a and S35a).26 The Cu interaction to both C
atoms is further substantiated by an NBO analysis, which yields
a Wiberg bond index of 0.26 and 0.25 for the Cu−C
interactions, respectively (Table S13).26 Furthermore, second-
order perturbation theory analysis reveals the presence of a
donor−acceptor interaction between the CC π bond to an
empty d-orbital of the Cu atom [60.01 kcal/mol] and from a
Cu d-orbital to the π* orbital of the CC bond [27.42 kcal/
mol]. The Nuclear Independent Chemical Shift (NICS)32 value
for bismole 7 was calculated at the ring critical point of the C4Bi
unit (NICS(0) = −1.8) and indicates only a minimal degree of
aromaticity, especially when compared to the lighter pyrroles
(e.g., C4H4NH = −14.0) and also to the parent bismole
C4H4BiH (−2.7).33 The resulting rather localized CC bonds
in bismole 7, in addition to the N lone pair, account for the
ability to form a stable complex with CuCl resulting in bismole
8.
bismuthide copper(I) complex (Me3Si)2BiCu(PMe3)3 featuring
24
̈
a Bi−Cu bond length of 2.744(1) Å, while Gabbai and Ke
generated a series of BiCu3 coordination complexes with Cu−
Bi bond lengths that average to 2.934(2) Å. The Bi···Cu
distance of 3.4765(7) Å in 8 is just within the sum of the van
der Waals radii (3.47 Å)25 and suggests that little to no bonding
interaction exists between these atoms.26
Real-Space Bonding Indicator and Orbital-Based
Analysis for the Cu···Bismole Interaction in 8. To
understand better the nature of the Cu−C bonding in the
CuCl-complexed bismole 8, we applied a set of real-space
bonding indicators (RSBIs) obtained from density functional
theory (DFT) calculations. The topological and integrated
bonding and atomic properties were derived from the Atoms-
In-Molecules (AIM)27 and Electron Localizability Indicator
(ELI-D)28 space-partitioning schemes, respectively, and they
are summarized in Tables S11−S13 of the Supporting
Information.26 Classic covalent interactions such as the C−C
σ-bond in ethane are characterized by negative electron density
Laplacians at the bond critical point (∇2ρ(r)bcp) and a negative
total energy (H) over ρ(r)bcp (H/ρ(r)bcp) values; in addition,
the kinetic energy (G) over ρ(r)bcp (G/ρ(r)bcp) values are close
to zero in such covalent bonds. In contrast, atom−atom
contacts that are dominated by electrostatic interactions are
characterized by substantially positive ∇2ρ(r)bcp and G/ρ(r)bcp
values as well as an H/ρ(r)bcp value close to zero. Furthermore,
the bonding in 8 was examined by computing Noncovalent
Interaction (NCI) indices29 and via Natural Bond Orbital
(NBO)30 analyses (see Table S13).26
The Bi−C bonds in both bismoles 7 and 8 as well as the
Cu−N and Cu−Cl contacts in 8 show characteristics of polar
covalent bonds, such as positive Laplacians and G/ρbcp ratios, as
well as slightly negative H/ρbcp ratios (Table S11).26
Interestingly, only one Cu−C bond critical point could be
found (Figure 6a), and this interaction appears to be highly
polar in nature with Laplacians and G/ρbcp ratios of 5.6 e Å−5
Ultrafast Time-Resolved and Time-Integrated Photo-
luminescence Measurements. The bismole heterocycles 3,
E
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Figure 7. UV−Vis absorbance spectra in THF at room temperature for compounds (a) 1−4 and (b) 5−8.
and 5−8 display UV−vis absorption profiles that extend up to
ca. 425 nm in tetrahydrofuran (THF), with bismole 4 showing
the most red-shifted absorption of the compound series (Figure
7). This afforded an opportunity to conduct both ultrafast time-
resolved (TRPL) and time-integrated photoluminescence
(TIPL) studies using a common excitation source at 400 nm;
it was hoped that such ultrafast measurements would enable us
to examine the emission behavior of these bismoles at various
temperatures under high vacuum and to probe for possible
competing emission pathways such as thermally activated
delayed fluorescence (TADF). For our studies, we focused on
measuring the PL of compounds 4, 7, and 8 due to the
presence of substantial light absorption at 400 nm as well as
each bismole being structurally distinct. The samples were
drop-cast as films from THF for each photoluminescence (PL)
measurement, thus ruling out possible quenching interactions
by solvent, and allowing suppression of molecular rotations
compared to when the samples are in solution. Notably, no PL
was detected from the ethylated bismoles 1 and 2 due a lack of
absorption at 400 nm, and attempts to measure the
luminescence of these species upon excitation at a wavelength
of ca. 290 nm (in the presence or absence of O2) did not yield
any discernible luminescence. One explanation for the lack of
visible emission in 1 and 2 is their oily nature, which should
encourage nonradiative decay facilitated by molecular motion.21
Ultrafast time-resolved and time-integrated photoluminescence
studies were not conducted on bismoles 5 and 6 due to a
combination of the lack of strong absorption at 400 nm and the
lack of emission observable by the human eye either in solution
or in the solid state in the presence or absence of oxygen (see
Figure S23 for compounds 1−8 in MeTHF at 77 K).
Upon excitation at 400 nm, compound 4 displayed green PL,
with weak emission noted at room temperature (Figure 8a),
which became much more intense upon cooling to 77 K
(Figure S24).26 The dominant emission peak arising at ∼530
nm and the resulting small Stokes shift is in line with
fluorescence, which was confirmed by nanosecond scale time-
resolved photoluminescence (TRPL) measurements as shown
in Figure 8b,c. A 75% reduction in PL intensity over 28 min of
irradiation at 400 nm was observed for compound 4, and
despite this slow degradation, we were able to complete the
TRPL measurements. The TRPL measurements at 540 10
nm follow a biexponential decay, I(t) = A exp(−t/τ1) + B
exp(−t/τ2) + C, where A and B are the intensities, τ1 and τ2 are
estimated lifetimes, and C is the offset. The “fast” component at
Figure 8. (a) TIPL spectra (left y-axis) and absorbance associated with
a drop-cast film of 4 (right y-axis) at 295 K. Excitation source is 400
nm, and a long pass filter was used to cutoff wavelengths below 435
nm. (b) TRPL of 4 taken at 540 10 nm at 295 K, which follows a
biexponential decay (red line), I(t). (c) Lifetimes τ1 and τ2 extracted
from the biexponential fits of TRPL at low temperatures.
t < 0.08 ns could be attributed to vibrational modes. These
modes are suppressed at low temperature; thus, τ1 becomes
F
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Figure 9. (a) TIPL intensity at 77 K (left y-axis) and absorbance (right y-axis) at T = 295 K for 7 (drop-cast film from THF). (b) Normalized TIPL
intensity (left y-axis) and absorbance (right y-axis) at various temperatures of the copper complex 8. (c) Variation of PL intensity in 8 with an
increase in temperature from 77 to 300 K. (d) TRPL in nanosecond scale (TRPL-ns) for 8 taken at 485 10 nm. (e) Lifetime in microsecond scale
for 8 taken from the TRPL fits to biexponential decay at 720 2 nm at various temperatures.
larger, as shown in Figure 7c, and fluorescence intensity is
enhanced. The “slow” component at t > 0.1 ns was assigned as
fluorescence from a low-lying singlet transition, which has a
consistent lifetime τ2 at different temperatures. According to
our TD-DFT calculations, the energy difference between the
excited S1 and the T2 states is only 0.035 eV (Table S6); thus,
one might expect intersystem crossing (ISC) to yield
phosphorescence (after rapid internal decay from T2 to an
emissive T1 state). However, the lack of phosphorescence in 4
is likely partly due to the absence of substantial orbital character
at Bi in these states;17e thus, spin−orbit coupling (which
facilitates ISC) arising from the presence of the heavy element
is minimized (vide infra).
Compound 7 remarkably shows two broad PL peaks upon
excitation at 400 nm at 77 K (Figure 9a). The first peak at ca.
485 nm comprises 32% of the total integrated emission
intensity compared to the second low-energy peak at ca. 720
nm. The two peaks are indicative of fluorescence and
phosphorescence, respectively; however, TRPL could not be
measured due to fast photodegradation of the sample in the
excitation beam. Note that the weak absorption at 400 nm is
responsible for the weak PL, which was only observed at 77 K,
where molecular motions are inactivated and phosphorescence
then becomes observable.6e
73% of the integrated PL intensity of 8 at room temperature. As
the temperature is lowered from 200 to 77 K, the overall
integrated PL intensity increases with a linear trend (Figure 9c),
leading to the long wavelength (phosphorescence) emission
accounting for 99.75% of the total PL at 77 K. This observation
implies improved ISC efficiency and pronounced participation
of the Bi atom in the excitation processes; our computations
support this explanation (vide infra). Additionally, it can be
noticed that as the intensity of the phosphorescence is
enhanced, the corresponding emission maximum becomes
blue-shifted by 16 nm (0.04 eV) from room temperature to 77
K with slightly narrowing bandwidth, suggestive of the
suppression of low-lying vibrational levels from the excited
triplet state (as nonradiative pathways).
Analysis of the emission data by time-resolved methods
(TRPL) confirmed the presence of dual fluorescence and
phosphorescence in 8, as evidenced by concurrent short-lived
(ns time scale) and long-lived (0.1−10 μs) emission, measured
at different energy states, thus ruling out thermally activated
delayed fluorescence. Biexponential curves are shown in Figure
9d for the short-lifetime emission (ns) with τ1 = 0.226 0.006
ns and τ2 = 1.1 0.3 ns that do not significantly change at low
temperature. It is also noticeable that at t ≤ 0.2 ns, another fast
exponential decay is observed, which may again represent
vibrational relaxations being thermally quenched, enabling rapid
ISC and the emergence of the phosphorescence long-lifetime
emission profile. For the phosphorescence peak, the dramatic
increase of intensity is accompanied by a longer TRPL event in
the microsecond time scale, and the emission again follows
biexponential decay; the lifetime values are shown in Figure 9e.
As the temperature is lowered to 77 K, enhanced phosphor-
escence is observed accompanied by a longer microsecond
lifetime up to ∼2 μs. The nature of the biexponential decay
The CuCl complex 8 does not yield luminescence that is
visible by eye when excited at 365 nm with a hand-held lamp;
however, under the stronger laser excitation at 400 nm, clear PL
is found at room temperature (Figure 9b−e); in addition, this
compound undergoes much less photodegradation compared
to 7, and becomes almost negligible when 8 is progressively
cooled to 77 K (Figure S26), thus enabling TRPL to be
measured. Relative to the first peak at ca. 485 nm, the second
red-shifted phosphorescence peak at ca. 720 nm accounts for
G
DOI: 10.1021/acs.inorgchem.8b00149
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Article
could be due to mixed metastable triplet states at ca. 720 nm or
possibly morphological effects within the cast films.
TD-DFT Computations on the Emissive Bismoles 4, 7,
and 8. Time-dependent density functional theory (TD-DFT)
computations were performed for the bismoles 4, 7, and 8
using the B3LYP functional and the cc-pVTZ(-PP) basis-sets.26
The calculated Bi−C bond distances of the optimized
geometries are systematically ca. 0.03 Å longer than the
experimentally observed Bi−C distances in 4 and 8. The largest
deviation from the experimental distance is observed for the
Cu−N bond in bismole 8, which is 0.08 Å longer in the
optimized gas-phase structure. The overestimation of bond
distances of polar interactions is a known effect of DFT
computations involving the B3LYP functional.34 However, as
all structural trends are fully maintained in the bismoles 4 and
8, the observed differences are considered to be rather
insignificant.
The predicted absorption maxima for compounds 4, 7, and 8
are within 25 nm of the experimentally observed maxima
(Figure S28).26 In bismole 4, the main transition can
exclusively be assigned to a HOMO to LUMO transition that
is primarily π−π* in character with little contribution from the
Bi atom (Figure 10a). In contrast, bismoles 7 and 8 show
considerable oscillator strength to low-lying singlet states that
can be attributed to the HOMO−1 to LUMO and the
HOMO−2 to LUMO transitions, respectively (Figure 10b,c).
In bismole 7, both the HOMO and HOMO−1 to LUMO
transitions are mainly π−π* character (and also showing the
highest oscillator strength), whereas the HOMO−2 to LUMO
transition shows significant contributions from Bi.
According to our TD-DFT studies, bismoles 4, 7, and 8 each
have low-lying singlet states that are energetically similar (<0.1
eV, Table S6 and Figure S29) to low-lying triplet excited
states.26 A possible mechanism for the observation of
phosphorescence is initial photoexcitation to an Sn state with
subsequent ISC to an energetically similar Tn state, followed by
relaxation to the lowest T1 triplet state, then phosphorescence
and relaxation to the S0 ground state. As all investigated
methods show the existence of close-lying S1,2,3 and Tn states,
there is a high probability for ISC to occur. As Bi is an element
strongly affected by relativistic effects, enhanced spin−orbit
coupling should lead to significant mixing of singlet and triplet
states, thus further increasing the probability of ISC.17e To
determine the degree of mixing, vertical excitation energies
including scalar relativistic (ZORA) and spin−orbit relativistic
(SO) methods were computed for bismoles 4, 7, and 8.
Bismoles 7 and 8 show a low-lying “singlet” state with
considerable mixing of singlet and triplet character (7: 55.8% S,
44.0% T and 8: 63.4% S, 36.0% T; see Tables S8 and S9).26 In
contrast, mixing of singlet and triplet states for low-lying excited
states in bismole 4 is dramatically lower (e.g., 91.9% S, 7.9% T;
Table S7).26 The presence of the CuCl unit also has an effect
on the rate of ISC, as bismole 8 shows more mixing of singlet
and triplet character (Table S9) when compared to bismole 7
(Table S8).
Figure 10. TD-DFT computed main transitions for 4 (a), 7 (b), and 8
(c) to low-lying singlet states at the B3LYP/cc-pVTZ(-PP) level of
theory and the associated molecular orbitals; isosurface values of
+0.02/−0.02 (red/green).
comparison to the experimentally observed 1.75 eV (709 nm).
The calculated adiabatic energies of 4 (Eadia = 1.27 eV, E0−0
=
1.22 eV) predict emission in the near-infrared region (976−
1020 nm) and are in line with the lack of experimentally
observed phosphorescence. However, the lack of substantial
mixing of excited singlet and triplet states in bismole 4, because
there is minimal orbital participation from Bi in the excitation
processes and thus reduced spin−orbit coupling, likely hinders
effective ISC to an excited triplet state. In contrast, bismoles 7
and 8 show considerable orbital character from Bi associated
with the excited states; thus, spin−orbit coupling becomes
more pronounced, and phosphorescence is observed exper-
imentally. While the enhancement of ISC via an external heavy
element effect remains a possibility,35 the lack of observed
phosphorescence in 4, in conjunction with findings from
previous studies conducted on tellurophenes in our group,17e
suggest that participation of the heteroatom in the excitation
process seems to be necessary for phosphorescence to occur.
The phosphorescence energy can be defined as the difference
in energy between the S0 ground state and the T1 triplet state
(Eadia) or the zero-point energy corrected difference between
these states (E0−0). The phosphorescence energy of 7 (Eadia
=
1.82 eV, E0−0 = 1.76 eV) matches well the observed
phosphorescence energy of 1.73 eV (720 nm) (Table S10).26
In contrast, the predicted phosphorescence energy of 8 (Eadia
=
1.39 eV, E0−0 = 1.33 eV) is underestimated by 0.36−0.42 eV in
H
DOI: 10.1021/acs.inorgchem.8b00149
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
solution of Cp2ZrC4Et4 (0.316 g, 0.820 mmol) in 10 mL of Et2O. The
reaction mixture was allowed to stir at room temperature in the
absence of light for 27 h before being evaporated to dryness. The
crude product was extracted with 15 mL of hexanes and filtered
through a 0.5 cm plug of silica. The volatiles were removed from the
CONCLUSIONS
■
A series of bismole compounds were synthesized via efficient
copper(I) chloride-catalyzed metallacycle transfer, and the
luminescence properties of three bismoles, namely, 4, 7, and 8,
were studied in detail. Compound 4 was found to exhibit only
fluorescence at low temperatures, and this is most likely due to
the lack of participation of the bismuth atom in the excitation
process leading to minimal singlet and triplet mixing in the
lower-energy excited states. Bismoles 7 and 8 were found to
exhibit both fluorescence and phosphorescence, and this can be
attributed to the increased orbital participation from bismuth in
the excitation processes leading to significant mixing of triplet
and singlet character in the lower-energy excited states. The
ClCu···CC interaction in 8 was probed using real-space bond
indicators derived from AIM, the ELI-D, and the NCI index as
well as NBO analysis. Ongoing work is focused on the synthesis
of phosphorescent bismoles with high quantum yields for
potential applications in OLEDs and bioimaging.
1
filtrate to give 2 as a yellow oil (0.317 g, 78%). H NMR (500 MHz,
2
C6D6): δ 6.92 (s, 2H, ArH), 2.75 (dq, JHH = 15.1 Hz, 3JHH = 7.6 Hz,
2
3
2H, CH2CH3), 2.66 (dq, JHH = 15.0 Hz, JHH = 7.5 Hz, 2H,
CH2CH3), 2.50 (s, 6H, CH3 in Mes), 2.12−2.23 (m, 4H, CH2CH3),
3
2.10 (s, 3H, CH3 in Mes), 1.06 (overlapping t, JHH = 7.5 Hz, 12H,
CH2CH3). 13C{1H} NMR (126 MHz, C6D6): δ 161.3 (ArC), 147.8
(ArC), 137.5 (ArC), 129.1 (ArC), 30.8 (CH2), 28.1 (CH3 in Mes),
26.5 (CH2), 21.1 (CH3 in Mes), 19.4 (CH3), 14.5 (CH3). Anal. Calcd
(%) for C21H31Bi: C, 51.22; H, 6.35. Found: C, 50.57; H, 6.69. UV−
vis (THF): 317 nm (shoulder). Elemental analyses consistently gave
slightly low values for carbon content (by ca. 0.6%), and attempts to
purify 2 via column chromatograhy led to decomposition; for copies of
the NMR spectra see the Supporting Information.26
Synthesis of B-MesBi-6-B (3). A suspension of BiCl3 (0.169 g, 0.536
mmol) in 5 mL of Et2O was added to a solution of BiMes3 (0.149 g,
0.264 mmol) in Et2O; this mixture was stirred at room temperature in
the absence of light for 16 h before being concentrated to 1 mL. The
resulting pale yellow suspension was dissolved in 3 mL of THF, and
the solution was added dropwise to a mixture of B-Cp2Zr-6-B (0.463
g, 0.798 mmol) and CuCl (0.0027 g, 0.027 mmol) in 12 mL of THF in
the absence of light. The reaction mixture was allowed to stir for 2 h in
the dark before being evaporated to dryness. The product was
extracted into 13 mL hexanes and filtered through a 0.5 cm silica plug
before being evaporated to dryness. The crude product was
recrystallized from Et2O at −30 °C to give three crystalline fractions,
which were combined to give pure 3 as a yellow crystalline solid (0.355
EXPERIMENTAL SECTION
■
General Considerations. All reactions were performed using
standard Schlenk and glovebox (MBraun) techniques under a nitrogen
atmosphere. Solvents were all dried and degassed using a Grubbs-type
solvent purification system manufactured by Innovative Technology,
Inc., and stored under an atmosphere of nitrogen prior to use.
Bismuth(III) chloride was purchased from TCI, and all other
chemicals were purchased from Sigma-Aldrich; all commercially
obtained chemicals were used as received. Mes3Bi,36
(ArNMe2)BiCl2,20 (2-thienyl)-Cp2Zr-6-(2-thienyl),37 B-Cp2Zr-6-B,37
1
g, 65%). H NMR (400 MHz, C6D6): δ 6.93 (s, 2H, ArH), 2.93 (m,
Cp2ZrC4Et4,38 and Cp2ZrC4Ph4 were prepared according to
39
4H, CCCH2CH2), 2.57 (s, 6H, CH3 in Mes), 2.09 (s, 3H, CH3 in
Mes), 1.52 (m, 4H, CCCH2CH2), 1.01 (two closely spaced singlets,
24H, CH3 in BPin). 13C{1H} NMR (126 MHz, C6D6): δ 174.6
(BiCC), 147.4 (o-MesC), 136.8 (p-MesC), 129.5 (MesCH), 83.1
(C(CH3)2), 40.6 (CCCH2CH2), 28.6 (CH3 in Mes), 24.9 (CH3 in
BPin), 24.8 (CH3 in BPin), 23.5 (CCCH2CH2), 21.1 (CH3 in Mes).
11B{1H} NMR (128 MHz, C6D6): δ 34.1 (br). Anal. Calcd (%) for
C29H43B2BiO4: C, 50.76; H, 6.32. Found: C, 50.84; H, 6.45. UV−vis
(THF): λmax = 327 nm (ε = 2.93 × 103 M−1 cm−1). mp: 155−160 °C
(decomp).
1
literature procedures. H, 11B{1H}, and 13C{1H} NMR spectra were
recorded on 400, 500, 600, or 700 MHz Varian Inova instruments and
were referenced externally to SiMe4 (1H, 13C{1H}), or F3B·Et2O
(11B{1H}). Elemental analyses were performed by the Analytical and
Instrumentation Laboratory at the University of Alberta. Melting point
values were obtained in sealed glass capillaries in nitrogen using a
MelTemp melting point apparatus. UV−vis spectroscopic data were
obtained using a Cary 400 Scan spectrophotometer.
Synthetic Procedures. Synthesis of PhBiC4Et4 (1). A suspension
of BiCl3 (0.0866 g, 0.275 mmol) in 5 mL of Et2O was added to a
suspension of BiPh3 (0.0596 g, 0.135 mmol) in 5 mL of Et2O. The
reaction mixture was allowed to stir for 1 h, after which the formation
of a pale yellow precipitate was observed. The entire reaction mixture
was added as a suspension to a solution of Cp2ZrC4Et4 (0.157 g, 0.406
mmol) in 5 mL of Et2O. The reaction mixture was then stirred at
room temperature in the absence of light for 21 h before being
evaporated to dryness. The crude mixture was extracted with 15 mL of
hexanes and filtered through a 0.5 cm plug of silica. The resulting
filtrate was evaporated to dryness in vacuo to yield 1 as an orange-red
oil (0.136 g, 75%). 1H NMR (400 MHz, C6D6): δ 8.06 (dd, 3JHH = 7.8
Hz, 4JHH = 1.4 Hz, 2H, ArH), 7.22 (m, 2H, ArH), 7.13 (m, 1H, ArH),
2.70 (dq, 2JHH = 15.1 Hz, 3JHH = 7.6 Hz, 2H, CH2CH3), 2.57 (dq, 2JHH
= 15.1 Hz, 3JHH = 7.6 Hz, 2H, CH2CH3), 2.13 (m, 4H, CH2CH3), 1.07
(t, JHH = 7.6 Hz, 6H, CH2CH3), 1.00 (t, JHH = 7.6 Hz, 6H,
CH2CH3). 13C{1H} NMR (126 MHz, C6D6): δ 171.4 (ArC), 163.2
(ArC), 137.7 (ArC), 130.5 (ArC), 127.6 (ArC), 30.1 (CH2), 26.9
(CH2), 19.2 (CH3), 15.1 (CH3). Anal. Calcd (%) for C18H25Bi: C,
48.00; H, 5.60. Found: C, 49.01; H, 5.93. UV−vis (THF): 312 nm
(shoulder). Elemental analyses consistently gave high values for carbon
content (by ca. 1%), and attempts to purify 1 via column
chromatograhy led to decomposition; for copies of the NMR spectra
see the Supporting Information.26
Synthesis of MesBiC4Et4 (2). A suspension of BiCl3 (0.173 g, 0.549
mmol) in 6 mL of Et2O was added to BiMes3 (0.157 g, 0.277 mmol)
in 6 mL of Et2O, and the reaction mixture was allowed to stir at room
temperature in the absence of light for 16 h. The resulting mixture
containing MesBiCl2 was transferred as a suspension in Et2O to a
Synthesis of (2-thienyl)-BiMes-6-(2-thienyl) (4). A suspension of
BiCl3 (0.207 g, 0.656 mmol) in 5 mL of Et2O was added to a solution
of BiMes3 (0.183 g, 0.322 mmol) in 5 mL of Et2O; this mixture was
stirred at room temperature in the absence of light for 16 h before
being concentrated to a volume of ca. 1 mL. The resulting pale yellow
suspension was dissolved in 5 mL of THF, added dropwise to a
mixture of (2-thienyl)-Cp2Zr-6-(2-thienyl) (0.481 g, 0.977 mmol)
and CuCl (14.4 mg, 0.145 mmol) in 10 mL of THF, and stirred at
room temperature in the absence of light for 3 h, before being filtered
through diatomaceous earth, and the filtrate evaporated to dryness.
The crude product was extracted with two 20 mL portions of hexanes;
for each extraction, the product was stirred for 3−4 h in the hexanes
and filtered through diatomaceous earth. The filtrate fractions were
combined and concentrated in vacuo to a total volume of ca. 15−20
mL and stored at −30 °C for 16 h. The first fraction of precipitate was
discarded (as it consisted of Cp2ZrCl2 and another unknown Cp-
containing byproduct), and the mother liquor was concentrated
further to ca. 5 mL and stored at −30 °C. Subsequent recrystallizations
yielded two crystalline fractions, which were collected and combined
to give 4 as an orange solid (0.184 g, 32%). Single crystals of suitable
quality for X-ray diffraction were obtained by recrystallization of 4
3
3
1
from Et2O at −30 °C. H NMR (400 MHz, C6D6): δ 6.88 (s, 2H,
ArH), 6.84−6.87 (m, 4H, thienyl-H), 6.63 (m, 2H, thienyl-H), 2.69−
2.79 (m, 2H, CH2), 2.55−2.65 (m, 2H, CH2), 2.51 (s, 6H, CH3 in
Mes), 2.02 (s, 3H, CH3 in Mes), 1.37−1.43 (m, 4H, CH2). 13C{1H}
NMR (126 MHz, C6D6): δ 157.8 (ArC), 149.6 (thienylC), 147.7
(ArC), 138.2 (ArC), 129.9 (ArC), 128.7 (thienylC), 127.4 (thienylC),
125.6 (thienylC), 36.1 (CCCH2CH2), 27.8 (CH3 in Mes), 23.9
I
DOI: 10.1021/acs.inorgchem.8b00149
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(CCCH2CH2), 21.2 (CH3 in Mes). Anal. Calcd (%) for C25H25BiS2:
C, 50.16; H, 4.21; S, 10.71. Found: C, 50.67; H, 4.52; S, 10.57. UV−vis
(THF): λmax = 421 nm (ε = 1.21 × 104 M−1 cm−1). mp: 103−105 °C.
Elemental analyses consistently gave slightly high values for carbon
content (by ca. 0.5%); for copies of the NMR spectra see the
Supporting Information.26
product will begin to decompose into an unidentified insoluble dark
green-gray precipitate. Compound 8 was then obtained as a light
yellow crystalline solid (0.157 g, 38%). 1H NMR (500 MHz, C6D6): δ
3
3
8.51 (d, JHH = 7.2 Hz, 1H, ArH), 7.27 (t, JHH = 7.2 Hz, 1H, ArH),
6.95−7.03 (m, 2H, ArH), 3.25−3.52 (br, 2H, CCCH2CH2), 2.98−
3.10 (br, 2H, CCCH2CH2), 2.71−3.03 (br, 2H, CCCH2CH2),
2.31−2.39 (br, 8H, CH2N(CH3)2 and N(CH3)2), 1.42−1.52 (m, 2H,
CCCH2CH2), 1.07 (s, 12H, CH3 in BPin), 1.04 (s, 12H, CH3 in
BPin). 13C{1H} NMR (126 MHz, C6D6): δ 139.3 (ArC), 129.6 (ArC),
127.1 (ArC), 83.4 (C(CH3)2), 70.0 (CH2N(CH3)2), 47.6 (CH2N-
(CH3)2), 42.3 (br, CCCH2CH2), 25.1 (CH3 in BPin), 24.9 (CH3 in
BPin), 24.7 (br, CCCH2CH2). Note: some ArC signals are missing
due to the instability of 8 in solution, which limited the data
acquisition time. 11B{1H} NMR (128 MHz, C6D6): δ 32.8. Anal. Calcd
(%) for C29H44B2BiClCuNO4: C, 43.52; H, 5.54; N, 1.75. Found: C,
44.01; H, 5.82; N, 1.66. UV−vis (THF): λmax = 386 nm (ε = 3.33 ×
103 M−1 cm−1). mp: 120 °C (decomp). Elemental analyses
consistently gave slightly high values for carbon content (by ca.
0.5%); for copies of the NMR spectra see the Supporting
Information.26
Synthesis of PhBiC4Ph4 (5). A suspension of BiCl3 (0.0488 g, 0.150
mmol) in 5 mL of Et2O was added to a solution of BiPh3 (0.333 g,
0.0756 mmol) in 5 mL of Et2O and allowed to stir at room
temperature for 1 h. This mixture was then concentrated in vacuo to
ca. 1 mL, and the pale yellow suspension was dissolved in 5 mL of
THF and added dropwise to a mixture of Cp2ZrC4Ph4 (0.128 g, 0.221
mmol) and CuCl (2.5 mg, 0.025 mmol) in 12 mL of THF. The
reaction mixture was allowed to stir at room temperature in the
absence of light for 4 h before being evaporated to dryness. The crude
product was extracted into 20 mL of hexanes and filtered through a
silica plug (0.5 cm), before the volatiles were removed from the filtrate.
The crude product was recrystallized from Et2O at −30 °C to yield 5
as a yellow powder (0.0473 g, 33%). Single crystals suitable for X-ray
diffraction were obtained by recrystallization of 5 from Et2O at −30
1
°C. H NMR (500 MHz, CDCl3): δ 8.29 (m, 2H, ortho-H of Bi-Ph),
Synthesis of 7. Triphenylphosphine (0.0250 g, 0.0953 mmol) in 5
mL of hexanes was added to 8 (0.0757 g, 0.0947 mmol) in 5 mL of
hexanes. The reaction mixture was stirred at room temperature for 5 h
before being filtered through a 1.5 cm plug of diatomaceous earth. The
filtrate was evaporated to dryness to give 7 as a spectroscopically pure
3
7.48 (t, JHH = 7.5 Hz, 2H, ArH), 7.37 (m, 1H, ArH), 6.91−7.03 (m,
12H, ArH), 6.82−6.87 (m, 8H, ArH). 13C{1H} NMR (126 MHz,
CDCl3): δ 164.8 (ArC), 145.4 (ArC), 144.9 (ArC), 137.1 (ArC), 131.2
(ArC), 130.3 (ArC), 129.2 (ArC), 128.0 (ArC), 127.9 (ArC), 127.6
(ArC), 126.0 (ArC), 125.7 (ArC). Anal. Calcd (%) for C34H25Bi: C,
63.55; H, 3.92. Found: C, 63.49; H, 4.30. UV−vis (THF): λmax = 358
nm (ε = 5.47 × 103 M−1 cm−1). mp: 130−135 °C (decomp).
1
yellow solid (0.0470 g, 71%). H NMR (400 MHz, C6D6): δ 8.43−
8.45 (m, 1H, ArH), 7.35−7.38 (m, 1H, ArH), 7.11−7.14 (m, 2H,
ArH), 3.64 (s, 2H, ArCH2N(CH3)2), 2.91−2.95 (m, 4H, C
CCH2CH2), 2.24 (s, 6H, N(CH3)2), 1.46−1.59 (m, 4H, C
CCH2CH2), 1.03 (s, 12H, CH3 in BPin), 1.02 (s, 12H, CH3 in
BPin). 13C{1H} NMR (126 MHz, C6D6): δ 177.0 (ArC), 145.4 (ArC),
138.8 (ArC), 130.3 (ArC), 130.0 (ArC), 127.3 (ArC), 82.9 (C(CH3)2),
68.7 (CH2N(CH3)2), 45.3 (CH2N(CH3)2), 41.6 (CCCH2CH2),
25.0 (CH3 in BPin), 24.9 (CH3 in BPin), 23.6 (CCCH2CH2).
11B{1H} NMR (128 MHz, C6D6): δ 34.6. Anal. Calcd (%) for
Synthesis of MesBiC4Ph4 (6). A suspension of BiCl3 (0.0556 g,
0.176 mmol) in 5 mL of Et2O was added to a solution of BiMes3
(0.0490 g, 0.0865 mmol) in 5 mL of Et2O, and the mixture was
allowed to stir at room temperature in the absence of light for 16 h
before being concentrated in vacuo to ca. 1 mL. The resulting mixture
containing MesBiCl2 was dissolved in 3 mL of THF and added to a
mixture of Cp2ZrC4Ph4 (0.152 g, 0.263 mmol) and copper(I) chloride
(0.0040 g, 0.040 mmol) in 15 mL of THF. The reaction mixture was
allowed to stir for 5 h in the absence of light before being evaporated
to dryness. The product was extracted into 20 mL of hexanes and
filtered through a 0.5 cm plug of silica. The resulting filtrate was
concentrated in vacuo to a volume of ca. 5 mL and stored at −30 °C
for 16 h, after which 6 was obtained as a pale yellow solid (0.104 g,
59%). Single crystals suitable for X-ray diffraction were obtained by
C29H44B2BiNO4: C, 49.67; H, 6.32; N, 2.00. Found: C, 48.87; H, 6.25;
N, 2.22. UV−vis (THF): λmax = 329 nm (ε = 4.25 × 103 M−1 cm−1).
mp: 105−108 °C (decomp). Elemental analyses consistently gave low
values for carbon content (by ca. 0.8%); for copies of the NMR spectra
see the Supporting Information.26
Experimental Methods Used to Acquire Time-Integrated
Photoluminescence (TIPL) and Time-Resolved Photolumines-
cence (TRPL) Data. A sample dissolved in toluene or THF (ca. 10
mg/mL) was drop-cast onto a 1 mm thick optical grade fused quartz
substrate (Starna Scientific Ltd.). Samples were then placed in an
optical microscopy cryostat (Cryo Industries) with level of vibrations
less than or equal to 15 nm. The chamber was evacuated to a pressure
of ∼2.2 × 10−7 mbar and then cooled by free-flowing liquid nitrogen
with temperatures above 77 K regulated with the aid of a temperature
controller (Lakeshore 335). An 800 nm Ti:sapphire ultrafast laser
(Coherent RegA 900) with 65 fs pulse width and 250 kHz repetition
rate was used to optically excite the samples at 400 nm via second
harmonic signal generation from a barium borate (BBO) crystal. All
1
slow recrystallization from Et2O at −30 °C. H NMR (700 MHz,
3
C6D6): δ 7.11 (m, 4H, ArH), 7.02 (d, JHH = 7.0 Hz, 4H, ArH), 6.94
(s, 2H, ArH in Mes), 6.86−6.89 (m, 8H, ArH), 6.81−6.83 (m, 2H,
ArH), 6.71−6.74 (m, 2H, ArH), 2.65 (s, 6H, CH3 in Mes), 2.05 (s,
3H, CH3 in Mes). 13C{1H} NMR (176 MHz, C6D6): δ 172.3 (ArC),
163.5 (ArC), 159.6 (ArC), 147.9 (ArC), 145.8 (ArC), 145.6 (ArC),
138.3 (ArC), 130.5 (ArC), 130.3 (ArC), 129.6 (ArC), 126.3 (ArC),
126.1 (ArC), 27.7 (CH3 in Mes), 21.2 (CH3 in Mes). Anal.Calcd (%)
for C37H31Bi: C, 64.91; H, 4.56. Found: C, 65.05; H, 4.62. UV−vis
(THF): λmax = 360 nm (ε = 7.60 × 103 M−1 cm−1). mp: 85−95 °C.
Synthesis of 8. Suspended (ArNMe2)BiCl2 (0.216 g, 0.524 mmol) in
5 mL of Et2O was added to B-Cp2Zr-6-B (0.301 g, 0.519 mmol) in 5
mL of Et2O. A catalytic amount of CuCl (0.0050 g, 0.050 mmol) was
added, and the mixture was allowed to stir at room temperature in the
absence of light for 16 h. The reaction mixture was filtered through a
1.5 cm plug of diatomaceous earth, evaporated to dryness, and
extracted with 20 mL of hexanes, and the extract was filtered through
another 1.5 cm plug of diatomaceous earth. The filtrate was then
concentrated in vacuo to a volume of 12 mL, before another filtration
through diatomaceous earth was performed. The filtrate was
evaporated to dryness to yield 0.328 g of crude 7. The crude sample
of 7 (ca. 0.468 mmol) was dissolved in 10 mL of Et2O and added to
CuCl (0.0462 g, 0.467 mmol), and the mixture was stirred at room
temperature in the absence of light for 5.5 h. The reaction mixture was
evaporated to dryness, extracted into 15 mL of toluene, and filtered
through a 1.5 cm plug of diatomaceous earth. The filtrate was then
concentrated in vacuo to a volume of 4 mL and stored at −30 °C for
16 h. If 8 is stored in solution, even at −30 °C, for more than 48 h the
measurements were performed at an average of 0.2
0.02 mW
excitation power. The time-integrated and time-resolved photo-
luminescence (TIPL and TRPL, respectively) were collected using a
confocal setup with a 435 nm long pass filter (Edmund Optics). The
TIPL spectra were measured by a CCD (Princeton Instruments Acton
Spectrometer) with a resolution of 6.4 nm with a 1000 μm entrance
slit. For the nanosecond time scale TRPL, a time-correlated single
photon counting (TCSPC) technique was used, which consists of a
single-photon avalanche photodiode connected to a TCSPC module
(PicoHarp 300, Picoquant), providing a time resolution of 54 1 ps.
As for the recording of the microsecond lifetime component, a
frequency-doubled 800 nm Ti:sapphire with 1 kHz repetition rate was
used for the excitation. A set of parabolic mirrors collected the
photoluminescence onto an optical fiber to the Si avalanche
photodetector (Thorlabs APD130A, 20 ns time resolution) with a
band-pass filter of 705 5 nm. Emission lifetimes were recorded using
a 200 MHz oscilloscope (Tektronix DPO 2024B).
J
DOI: 10.1021/acs.inorgchem.8b00149
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Computational Methodology. Geometry optimizations of the
gas-phase structure were performed using density functional theory
(DFT) with the B3LYP40 functional and the cc-pVTZ (for H, B, C, N,
O, Cl and S)41 as well as the cc-pVTZ-PP (for Cu and Bi)42 basis sets.
The initial structures were taken from the experimentally obtained X-
ray structures of 4 and 8. The initial structure of bismole 7 was taken
from the optimized geometry of 8 with manual removal of CuCl. The
use of the cc-pVTZ and cc-pVTZ-PP basis sets will thereafter be
referred to as cc-pVTZ(-PP). The basis sets as well as the effective core
potential (ECP) for the Cu and Bi atom were obtained from the Basis
Set Exchange Library.43 Subsequent frequency analysis confirmed the
obtained structures to be a local minimum on the potential energy
surface. To calculate the phosphorescence energy, the geometry of the
lowest-lying triplet states (T1) of 4, 7, and 8 was optimized by applying
the spin-unrestricted B3LYP (UB3LYP) functional with the same basis
sets as specified above. The vertical excitation energies of the first 10
singlet and triplet states of 4, 7, and 8 were predicted by TD-DFT
calculations using the B3LYP functional and the cc-pVTZ(-PP) basis
sets starting from the respective B3LYP optimized gas-phase S0
geometry. Phosphorescence energies were calculated by the difference
of the energies at the UB3LYP optimized T1 geometry and the B3LYP
optimized S0 geometry. All calculations were performed with the
Gaussian16 software.44 The wave function files were used for a
topological analysis of the electron density according to the Atoms-In-
Molecules space-partitioning scheme27 using AIM2000,45 whereas
DGRID46 was used to generate and analyze the ELI-D related real-
space bonding descriptors28 applying a grid step size of 0.05 au. The
NCI grids were computed with NCIplot.47 Bond paths are displayed
with AIM2000,45 AIM atomic basins, ELI-D and NCI figures are
displayed with MolIso48 and VMD,49 respectively. The molecular
orbitals (MOs) were extracted from the Gaussian16 checkpoint-files
and are visualized with GaussView 5.0.50 The final molecular
geometries were used to compute the natural bond orbitals (NBOs)
using the NBO6 program.51 Spin−orbit coupling was considered using
the TD-DFT framework52 with the Amsterdam Density Functional
(ADF) software.53 The S0 ground-state optimized geometries of
bismoles 4, 7, and 8 as determined by the Gaussian16 computations
were used as input geometries. TD-DFT calculations were determined
at the B3LYP/TZ2P level of theory40,54 using the “core none” option.
All calculations with the ADF software include scalar relativistic
(ZORA)55 and spin−orbit relativistic (SO) methods.49,56 The
NICS(0) values of bismole 7, C4H4BiH, and C4H4NH were calculated
at the AIM ring critical point at the same level of theory stated above
using the GIAO57 formalism as implemented in Gaussian16.
ORCID
Sarah M. Parke: 0000-0002-1221-5469
Robert McDonald: 0000-0002-4065-6181
Michael J. Ferguson: 0000-0002-5221-4401
Eric Rivard: 0000-0002-0360-0090
Author Contributions
The manuscript was written through contributions of all
authors. All authors have given approval to the final version of
the manuscript.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We gratefully acknowledge Dr. S. Mebs for invaluable help and
discussion of the real-space bonding indicator analysis. The
computational studies in this work were made possible by the
facilities of the Shared Hierarchical Academic Computing
Network (SHARCNET: www.sharcnet.ca), WestGrid (www.
westgrid.ca), and Compute/Calcul Canada (www.
computecanada.ca). This work was supported by the Natural
Sciences and Engineering Research Council of Canada
(Discovery Grant to E.R. and F.A.H., CREATE grants/stipends
to E.R., F.A.H., M.A.B.N., and S.M.P., and PGS-D to S.M.P.),
the Canada Foundation for Innovation, Alberta Innovates−
Technology Futures (AITF Graduate Student Scholarship to
M.A.B.N.), and the Faculty of Science at the Univ. of Alberta
(Research Fellowship to E.R.). E.H. thanks the German
Research Foundation (Deutsche Forschungsgemeinschaft,
DFG) for a research fellowship (HU 2512/1-1). We also
thank P. Choi and B. Furlong for performing initial studies in
this project.
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ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.8b00149.
Full crystallographic, computational, and luminescence
measurement details, along with copies of NMR spectra
for all newly reported compounds, (PDF)
Illustrated 3D molecular structure (XYZ)
Accession Codes
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AUTHOR INFORMATION
Corresponding Authors
*E-mail: erivard@ualberta.ca.
*E-mail: hegmann@ualberta.ca.
■
K
DOI: 10.1021/acs.inorgchem.8b00149
Inorg. Chem. XXXX, XXX, XXX−XXX

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