A combined computational and spectroelectrochemical study of platinum-bridged bis-triarylamine systems

Article abstract of DOI:10.1021/ic402538e

The character of the electronic transitions in the ultraviolet-visible-near infrared (UV-vis-NIR) spectra of platinum-bis(alkynyl) bridged, bis-triarylamine mixed-valence systems trans-[Pt(Ci CC₆H ₄NAr₂)₂ (PR₃)₂] ⁿ⁺ (R = ethyl, Ar = C₆H₄CH₃-4 (1) or C₆H₄OCH₃-4 (2); R = Ph, Ar = C ₆H₄CH₃-4 (3) or C₆H ₄OCH₃-4 (4), n = 0, 1, 2) has been determined from a combination of spectroscopic measurement and density functional theory calculations. The hybrid functional BLYP35 in combination with a suitable solvent model (i.e., conductor-like screening model (COSMO)) has been used to model the UV-vis-NIR and IR spectroscopic properties of [1-4]⁺, to confirm the description of [1-4]⁺ as examples of metal-bridged organic mixed-valence compounds, and to assign the principal features of the electronic spectra, including the triarylamine-based intervalence charge transfer transition located in the NIR region. The successful modeling of the charge distribution within the system demonstrates the utility of the BLYP35-COSMO protocol as a tool for use in the study of intramolecular charge transfer properties in mixed-valence complexes.

Full text of DOI:10.1021/ic402538e

Article
pubs.acs.org/IC
A Combined Computational and Spectroelectrochemical Study of
Platinum-Bridged Bis-Triarylamine Systems
Matthias Parthey,† Kevin B. Vincent,‡ Manuel Renz,† Phil A. Schauer,‡ Dmitry S. Yufit,‡
Judith A. K. Howard,‡ Martin Kaupp,*,† and Paul J. Low*,‡,§
†Institut fur Chemie, Theoretische Chemie, Technische Universitat Berlin, Sekretariate C7, Straße des 17. Juni 135, 10623 Berlin,
̈
̈
Germany
‡Department of Chemistry, Durham University, South Road, Durham DH1 3LE, United Kingdom
§School of Chemistry and Biochemistry, University of Western Australia, Crawley, Perth, WA 6009, Australia
S
* Supporting Information
ABSTRACT: The character of the electronic transitions in the ultraviolet−
visible−near infrared (UV−vis−NIR) spectra of platinum-bis(alkynyl)
bridged, bis-triarylamine mixed-valence systems trans-[Pt(C
CC6H4NAr2)2 (PR3)2]n+ (R = ethyl, Ar = C6H4CH3-4 (1) or
C6H4OCH3-4 (2); R = Ph, Ar = C6H4CH3-4 (3) or C6H4OCH3-4 (4), n
= 0, 1, 2) has been determined from a combination of spectroscopic
measurement and density functional theory calculations. The hybrid
functional BLYP35 in combination with a suitable solvent model (i.e.,
conductor-like screening model (COSMO)) has been used to model the
UV−vis−NIR and IR spectroscopic properties of [1−4]+, to confirm the
description of [1−4]+ as examples of metal-bridged organic mixed-valence
compounds, and to assign the principal features of the electronic spectra,
including the triarylamine-based intervalence charge transfer transition
located in the NIR region. The successful modeling of the charge distribution within the system demonstrates the utility of the
BLYP35-COSMO protocol as a tool for use in the study of intramolecular charge transfer properties in mixed-valence complexes.
transfer (MLCT)/ligand−metal charge transfer (LMCT)
transitions that usually fall in the visible region of the
spectrum,4,28−30 the lower-energy intervalence charge transfer
(IVCT) band typically found in the near infrared (NIR) region is
usually the primary source of information concerning the
electronic character of a MV system.3−5,27,31 The experimental
bandwidth at half height of the NIR band envelope is often used
as the main criterion to classify MV systems following the Robin-
Day scheme32 by comparison with the bandwidth for weakly
coupled systems calculated from the Hush model.33 However,
the observation of a simple, Gaussian-shaped IVCT band is rare
in organometallic MV systems,34,35 and more often the near-
degeneracy of the d orbitals leads to a set of transitions close in
energy and thus to a series of overlapping NIR bands.5 Although
typically complicated by solvent-induced band broadening,
excitation energies and bandwidth are often extracted from the
experimental spectral envelopes by Gaussian deconvolution.
However, a unique analytical solution to the deconvolution is
often not possible, leaving considerable ambiguity in the
spectroscopic assignments and much space for individual
interpretation. Various fits of near-equivalent quality can be
obtained by varying the number, width, and/or position of the
INTRODUCTION
■
The study of redox-active systems that exhibit facile intra-
molecular electron transfer (ET) processes continues to attract
attention due to potential applications in electrochromic devices1
and as components in future molecular electronics technology.2
Mixed-valence (MV) complexes represent perhaps the most
widely exploited test beds for such studies, due in no small part to
the models developed by Hush and others that link
experimentally observable spectroscopic parameters to under-
lying details of electronic coupling between the remote sites.3−5
While the great majority of MV molecular compounds feature
metal-complex electrophores and conjugated organic bridging
ligands6−10 borrowing from the designs established by the
Creutz-Taube ion,11,12 there is a growing interest in organic MV
compounds, in which nonmetal-based electrophores serve as the
sites for redox reactions.13 The combination of these concepts
has led to a small number of “inverted” MV complexes being
designed, in which organic redox systems are bridged by a metal-
based complex.14−19 In turn, these complexes serve as models to
study the transmission of electronic effects through metal
complex-based bridges.20−26
Regardless of the constituents and composition of the system,
electronic absorption data are crucial to the characterization of
MV complexes.4,27 While there is a wealth of information
contained in the (often overlooked) metal−ligand charge
Received: October 8, 2013
Published: January 22, 2014
© 2014 American Chemical Society
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Figure 1. Compounds studied in this work.
Gaussian-shaped sub-bands used to model the NIR band
envelope. Another complication to Gaussian-based deconvolu-
tions and spectral fitting occurs in strongly coupled systems,
which often exhibit bands with a pronounced “cutoff” on the low-
energy side of the band, leading to an asymmetric shape of the
NIR band,4,13 making the Gaussian shape of sub-bands an invalid
approximation. Considering these fundamental difficulties in
analytical treatment of the spectroscopic band envelopes, a
quantum-chemical perspective is highly desirable.36,37
pound trans-[Pt{CCC6H4N (C6H4OCH3-4)2}2(PEt3)2] (Et
= ethyl) undergoes amine-centered oxidations, giving rise to an
unusual metal-bridged organic MV compound, the Pt center
serving to limit extensive delocalization of the radical and
engineering Class II MV character in the one-electron oxidized
species.15 In this earlier work, TDDFT calculations (B3LYP/
LANL2DZ) were performed on the crystallographically
determined structure of trans-[Pt{CCC6H4N (C6H4OCH3-
4)2}2(PEt3)2] to support the spectroscopic observations made
on the family of neutral, mono-, and dicationic complexes, the
latter two being obtained by stoichiometric chemical oxidation.
The key localized orbitals thought to be involved in the IVCT
transition in the MV monocation were derived from a linear
combination of the HOMO and HOMO−1 of the neutral
compound following from a Koopmans’ Theorem-based treat-
ment of the electronic coupling. As a step toward the ultimate
goal of achieving general and universal quantum-chemical
protocols for describing and understanding intramolecular
charge transfer (CT) events, we report here a combined
experimental and computational study of the spectroscopic
properties of the family of platinum-bis(alkynyl) bridged bis-
triarylamine complexes trans-[Pt(CCC6H4NAr2)2 (PR3)2]n+
(R = Et or Ph, Ar = C6H4CH3-4 or C6H4OCH3-4, 1−4; n = 0,
1, 2) and the model monoamine complex trans-[Pt{C
CC6H4N (C6H4Me-4)2} (C6H4CH3-4)(PPh3)2] (5).
To achieve a proper description of the electron-transfer
transitions, it is important to account for effects from both
dynamic and nondynamic correlation, keeping in mind that there
is no clear-cut separation between these extremes. With large
configuration−interaction or coupled-cluster calculations within
an ab initio framework this problem could, in principle, be
addressed. Unfortunately, because of the size of typical MV
systems those methods are computationally too demanding.
Hartree−Fock calculations generally give solutions that are
biased toward localized descriptions, and standard density
functional theory (DFT) calculations generally overemphasize
the delocalized descriptions. In addition, almost all experimental
data are obtained in a polar solvent environment, which
inevitably stabilizes a localized charge-separated situation, raising
further challenges for the computational models, which must also
account for the effects of solvation on the relative stability of
different electronic states. On the basis of extensive benchmark-
ing, originally for the ground- and excited-state properties of bis-
triarylamine radical cations containing two N,N-di(4-anisyl)-
moieties as redox centers bridged by various organic units,36,37
later also for transition-metal complexes,38 a quantum-chemical
protocol36 has recently been developed. It employs the BLYP35
(global) hybrid functional with 35% exact exchange, together
with continuum-solvent models, and it has been shown to give a
near-optimum compromise for both class II and class III systems.
While the protocol tends to underestimate transition energies for
localized (Class II) systems and modestly overestimates them for
molecules exhibiting delocalized (Class III) behavior, the time-
dependent DFT (TDDFT) results may be considered in
reasonable agreement with experimental UV−vis−NIR data
(the different treatment of nonequilibrium solvation may lead to
somewhat different results for various program implementa-
tions).
RESULTS AND DISCUSSION
■
Syntheses and Characterization. Compounds 1−5
(Figure 1) were synthesized via CuI-catalyzed dehydrohaloge-
nation reactions of trans-PtCl2(PEt3)2, cis-PtCl2(PPh3)2, or trans-
PtI(C6H4Me-4) (PPh3)2 and the corresponding ethynyl-sub-
stituted triarylamine.15,39 The compounds were isolated in good
yield after purification by preparative thin-layer chromatography
(silica), with all details provided in the Supporting Information.
Compounds 1−5 are each characterized with a single
ν(CC) absorption near 2100 cm−1. Matrix-assisted laser
desorption ionization or atmospheric solids analysis probe
(ASAP) mass spectrometry (MS) for compounds 1−4 gave
signals corresponding to the [M]+ or [M + H]+ molecular ions,
while ASAP MS for compound 5 contained [Pt(C6H4Me)
(PPh3)2 + H]+ as the highest molecular mass fragment. The
31P{1H} NMR spectrum for each compound shows a singlet
resonance with platinum satellites: 1 11.0 (s, 1JP−Pt = 2385 Hz); 2
9.93 (s, 1JP−Pt = 2373 Hz); 3 18.73 (s, 1JP−Pt = 2663 Hz); 4 17.69,
A previous report by Jones et al. has shown that the closely
related platinum-bis(alkynyl) bridged bis-triarylamine com-
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Figure 2. A plot of a molecule of compound 1 showing the atom-labeling scheme, with thermal ellipsoids plotted at 50%. The atom Pt1 is located in a
special position on a center of symmetry. Hydrogen atoms have been omitted, and only one component of a disordered ethyl chain is shown for clarity.
Selected bond lengths (Å) and angles (deg): Pt1−P1 2.2667(19); Pt1−C1 1.999(6); C1−C2 1.194(8); C2−C3 1.453(8); N1−C6 1.423(7); N1−C9
1.423(7); N1−C16 1.409(8); Pt1−C1−C2 175.9(6); C1−C2−C3 178.5(7); C1−Pt1−P1 87.82(18); C1′−Pt1−P1 92.17(18).
(s, 1JP−Pt = 2671 Hz); 5 20.25 (s, 1JP−Pt = 3003 Hz). The 13C{1H}
Table 1. Oxidation Potentials for Platinum Complexes 1−5
a
NMR spectra of compounds 1, 2, and 4 show characteristic
resonances for alkynyl carbons at 109.05 and 106.13 (1), 108.90
and 105.40 (2), and 113.11 and 108.81 (4). However, due to
poor solubility not all of the quaternary carbons of 3 or 5 could be
observed.
and Related Organic Bis-Triarylamines
compound
[X]−
E1/V E2/V ΔE/V
Kc
{C2C6H4N (C6H4OMe-
[PF6]−
0.29
0.39
0.10
50
55
4)2}2
[C6H4{C2C6H4N (C6H4OMe- [PF6]−
0.28
0.34
0.06
10
4)2}2]55
Crystals of compound 1 were grown by slow diffusion of
EtOH into a CH2Cl2 solution of the complex to give pale yellow
needles that were suitable for X-ray diffraction (Figure 2). The
molecule is located in the inversion center and therefore has
trans-configuration of the ligands. The coordination geometry at
the platinum center is square planar. The extended rigid rod
structure is evidenced by the approximately linear C3−C2−C1−
Pt1− C1′−C2′−C3′ chain (Figure 2). The structure also shows
the characteristic propeller arrangement of the phenyl moieties
and the trigonal planar nitrogen geometry associated with the
triarylamine moiety.40−42 The bond lengths and angles are
common with the other known examples of trans-Pt(PR3)2
bis(acetylide) systems,43−48 such as trans-[Pt{CCC6H4N
(C6H4OMe-4)2}2(PEt3)2] (2)15 and trans-[Pt{C
CC6H4NPh2}2 (PBu3)2].49
1
2
3
4
5
[PF6]−
0.27
0.25
0.17
0.20
0.24
0.25
0.21
0.22
0.37
0.40
0.39
0.23
0.26
0.36
0.36
0.27
0.29
0.13
0.14
0.06
0.06
0.12
0.11
0.06
0.07
160
[BArF ]−
4
[PF6]−
10
[BArF ]−
4
[PF6]−
108
10
[BArF ]−
4
[PF6]−
[BArF ]−
4
[PF6]−
a
CV in CH2Cl2 with 0.1 M NBu4[X] at a scan rate of 100 mV/s and
referenced against DMFc at −0.48 V vs FcH (FcH = +0.0 V). Peaks
for the Pt complexes were resolved using differential pulse
voltammetry.
(Spectro)Electrochemical Investigations. Electrochemi-
cal methods are potentially useful tools with which to assess the
electronic interactions between two or more redox-active entities
mediated by a common bridging ligand, although the extraction
of electronic information from the myriad of medium effects is
not possible without considerable attention to detail and often
requires comparisons of data from a series of closely related
complexes.6,7,50−53 Nevertheless, electrochemical measurements
provide a simple method for the determination of comproportio-
nation constants and hence of the thermodynamic stability of
individual redox states with respect to disproportionation.54 Bis-
triarylamine compounds featuring various π-conjugated organic-
and organometallic-based bridges have been shown to exhibit
two reversible, one-electron oxidation processes centered on the
amine cores,40−42,55 with the separation of these processes often
large enough to permit study of the organic MV species.13,15 In
line with these general trends, the cyclic voltammograms of the
bis(alkynyl)platinum-bridged compounds 1−4 each exhibit two
reversible triarylamine-localized one-electron redox processes
(Table 1). The moderate degree of separation (ΔE) between the
amine-based oxidation processes in each of the compounds 1−4
in NBu4PF6, measured by both cyclic and differential-pulse
voltammetry, gives rise to small comproportionation constants,
Kc of 10−160 (Table 1). However, these values are consistent in
both 0.1 M NBu4PF6 and the “Geiger electrolyte” 0.1 M
NBu4[B{C6H3-3,5-(CF3)2}4], which features the very weakly
coordinating [B{C6H3-3,5-(CF3)2}4]− ([BArF ]−) anion.52,53
4
Given the likely similar energies of solvation, ion pairing, and
electronic factors across the series of complexes in Table 1, the
moderate ΔE values offered by compounds 1−4 are consistent
with a modest degree of through-bond electronic interaction
between the amine centers.7 This suggestion is also in agreement
with arguments presented earlier based on the observation of a
low-energy IVCT band in [2]+.15 In contrast, the monoamine
compound 5 exhibits a single reversible amine-centered
oxidation due to the presence of a single redox-active amine
moiety.
Spectroscopic data permit a more detailed assessment of the
physical and electronic structures of the redox products than
electrochemical measurements used in isolation. While the UV−
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vis−NIR spectrum of [2]+ has been determined from samples
prepared by chemical oxidation of 2,15 spectroscopic information
for the series [1−4]n+ was conveniently obtained here using
spectroelectrochemical methods.56 The IR spectra of the neutral
complexes 1−4 are characterized by a single weak ν(CC) band
(Table 2), consistent with the highly symmetric distribution of
electron density over the molecular backbone (Figure 3). On
oxidation, the IR band profile evolves through a unique ν(CC)
band pattern associated with the comproportionated equilibrium
mixture of the neutral, monocationic, and dicationic states to a
strong absorption feature characteristic of the dicationic state
(Figure 3). The observation of multiple ν(CC) bands
associated with the monocation, perhaps due to the effects of
Fermi coupling,57 in combination with the low comproportio-
nation constant, which ensures solutions of the monocations also
contain appreciable amounts of the neutral and dicationic forms,
makes definitive assignment of the individual vibrational modes
associated with the monocationic state difficult. The assignment
is assisted, however, as the unique features of the IR spectra
associated with the monocationic states are coupled with a rise
and fall in a unique low-energy (NIR) electronic absorption band
near 5000 cm−1 (the character of which is discussed in more
detail below). The observation of IR spectral band profiles for the
mono-oxidized complexes [1−4]+, which resemble a super-
position of the spectra of the associated neutral and dicationic
forms, suggests a description of these species in terms of a
localized MV structure. This suggestion is also consistent with
the IR spectra of the monoamine compounds [5]n+ (n = 0, 1) that
show a clear shift of the ν(CC) band on oxidation to the
monocationic species from 2103 to 2013 cm−1 (Figure 3b).
The UV−vis−NIR spectra of [1−4]n+ (n = 0, 1, 2) and [5]n+ (n
= 0, 1) were collected using spectroelectrochemical methods
from ca. 1 mM solutions in 0.1 M NBu4PF6/CH2Cl2, and data
were corrected for the comproportionation equilibria.60
Complexes 1−5 exhibit roughly the same overall electronic
absorption pattern in the UV−vis−NIR region in their different
oxidation states (Figure 4). The neutral complexes each feature
two pronounced bands near 33 000 cm−1 and 25 000 cm−1,
arising from the two N→π* transitions commonly observed in
triarylamine complexes of general form NArAr′2.40−42 No
transitions are evident below 22 500 cm−1 in the electronic
spectra of 1−5.
Table 2. Experimental and Calculated CC Stretching
Frequencies/cm−1 for Complexes [1−4]n+ (n = 0, 1, 2) and
a
[5]n+ (n = 0, 1)
complex
0
+1
+2
1
exp.
2100 (s)
2018 (s)
2046 (sh)
2070 (sh)
2100 (s)
2031 (92)
2098 (100)
1994
2018 (s)
2046 (sh)
2070 (sh)
BLYP35
2119
2043
B3LYP
exp.
2067
2001
2
3
4
5
2100 (s)
2025 (s)
2046 (sh)
2065 (sh)
2100 (s)
2048 (100)
2110 (19)
2024 (s)
2049 (sh)
2079 (sh)
2106 (s)
2029 (100)
2111 (55)
2030 (s)
2050 (sh)
2070 (sh)
2106 (s)
2050 (100)
2126 (8)
2013 (s)
2047 (sh)
2076 (sh)
2021
2025 (s)
2046 (sh)
2065 (sh)
2073 (s)
2072
BLYP35
exp.
2119
2106 (s)
2024 (s)
2049 (sh)
2079 (sh)
BLYP35
exp.
2131
2043
2106 (s)
2030 (s)
2050 (sh)
2070 (sh)
BLYP35
exp.
2130
2075
Upon oxidation of 1−4 to [1−4]+, a broad, low-intensity
IVCT band near 6000 cm−1 and a distinct absorption band near
10 000 cm−1 appear in each case (Figure 4). However, the visible
spectra of the tolyl systems 1 (Figures 4a,b) and 3 (Figures 4e,f)
differ noticeably from those of the anisyl derivatives 2 (Figures
4c,d) and 4 (Figures 4g,h). Thus, while the spectra of 1 and 3
exhibit a series of unresolved transitions leading to a broad
featureless absorption band envelope between 15 000 and 20 000
2103 (w)
BLYP35
2129
a
Calculated IR frequencies were scaled by an empirical factor of
0.95.58,59 Theoretical relative intensities/% for the monocationic forms
in parentheses.
Figure 3. IR spectra of 1 (left) and 5 (right) showing the ν(CC) band in the various oxidation states generated by in situ electrochemical oxidation in
CH2Cl2/0.1 M NBu4PF6 in an OTTLE cell.
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Figure 4. UV−vis−NIR spectra of compounds 1 (a), 2 (c), 3 (e), 4 (g), and 5 (i) in the various oxidation states generated by in situ electrochemical
oxidation in CH2Cl2/0.1 M NBu4PF6 in an OTTLE cell. Expansions of the NIR region are given in (b), (d), (f), (h), and (j) for [1−5]+, respectively.
cm−1, for compounds 2 and 4 a much better-resolved band is
apparent below 15000 cm−1 in each case (Figure 4), in addition
to some unresolved bands at higher energy that are similar to
those seen in the tolyl derivatives. For all complexes the two
absorptions at 25 000 cm−1 and 33 000 cm−1 become less
intense, and new features arise between 20 000 cm−1 and 25 000
cm−1 upon oxidation. This change in shape for the localized
amine transitions is consistent with previous studies of the
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a
Table 3. Calculated Ground State Properties for Complexes 1′−4′ and [1′−4′]2+
1′
1
2′
215
3′
4′
[1′]2+
[2′]2+
[3′]2+
[4′]2+
μa/D
⟨S2⟩
0.1
0.0
0.1
0.0
0.1
97.0
1.1
0.2
0.1
0.1
0.0
0.0
0.0
1.1
1.1
1.1
d(N−C6)/Å
1.411
1.411
1.418
1.419
1.227
1.227
2.028
2.028
6.4
1.423(7)
1.407
1.406
1.420
1.421
1.227
1.227
2.029
2.028
3.2
1.399(6)
1.413
1.413
1.418
1.418
1.225
1.225
2.029
2.029
11.5
1.408
1.408
1.420
1.420
1.225
1.225
2.029
2.029
24.8
1.380
1.379
1.421
1.421
1.233
1.233
2.010
2.009
6.5
1.395
1.395
1.410
1.410
1.230
1.230
2.016
2.016
12.6
1.377
1.377
1.422
1.422
1.232
1.232
2.011
2.011
−29.0
28.9
1.392
1.392
1.412
1.412
1.229
1.229
2.015
2.016
−29.3
30.6
d(N−C9/16)/Å
d(CC)/Å
d(Pt−C1)/Å
1.423(7)
1.409(8)
1.194(8)
1.442(6)
1.432(6)
1.171(7)
1.999(6)
6.2(1)
2.008(5)
46.1(1)
59.7(3)
b
Ω/°
−1.1
44.6
−8.3
49.0
−10.7
44.4
−23.4
−48.0
48.2
−11.0
45.8
−12.3
−42.0
41.9
b
Θ/°
50.2(3)
−46.7
46.7
−42.7
42.6
−44.9
−49.4
−44.4
−45.9
a
Total dipole moment μa, spin expectation value ⟨S2⟩ (theoretical value for singlet systems is 0.0), distance between nitrogen and the neighboring
carbon atom of the bridge d(N−C6), average distance between the two terminal phenyl rings and nitrogen d(N−C9/16), CC bond length d(C
C), platinum−carbon distances d(Pt−C1), the average of dihedral angles ∠(P1−Pt−C3−C4) and ∠(P2−Pt−C3−C8) Ω, and the average of ∠(C6−
N−C9−C10) and ∠(C6−N−C16−C17) Θ. Experimental data from the crystallographically determined structures of 1 and 215 are included for
b
comparison. Magnitude of torsion is given by the absolute value; the opposite sign of the dihedral angles is due to the Ci-symmetric input structure
of the optimization.
electronic spectra of triarylamine radical cations.40−42,61,62 Upon
further oxidation to the dicationic forms, the low-energy bands
between 5000 and 7000 cm−1 collapse, supporting the IVCT
assignment made here and elsewhere for [2]+,15 and all the other
absorptions below 25 000 cm−1 gain intensity (Figure 4).
Oxidation of compound 5 (Figures 4i,j) gives rise to a spectrum
that shows the changes of all the high-energy features of the
spectra for [1−4]+, with the absence of the lowest-energy
transitions.
calculated using the Yamaguchi spin projection procedure,63,64 is
only 1.8 kJ/mol for compound [1′]2+, 0.8 kJ/mol for [2′]2+, 2.4
kJ/mol for [3′]2+, and 0.8 kJ/mol for [4′]2+. The IR frequencies
(e.g., [1′]2+: ν(CC) = 2046 cm−1 for the triplet vs 2043 cm−1
for the singlet) and ground-state properties (e.g., [1′]2+: d(N−
C6) = 1.383 for the triplet vs 1.380 for the BS singlet), which are
discussed in this work, do not appreciably differ for the two spin
states and thus only the singlet states are considered further. For
the very important monocationic (MV) compounds, a doublet
configuration is obtained, as expected, and calculations exhibit
almost no spin contamination with the spin expectation values of
0.78 being only slightly larger than the theoretical value for
doublet systems (0.75).
Quantum-Chemical Calculations and Electronic Spec-
troscopy. Previously, the electronic and spectroscopic proper-
ties of [2]+ were inferred from gas-phase calculations based on
the crystallographic structure of 2, reflecting the available
computational capacity at the time.15 To better understand the
underlying electronic structure of these prototypical metal-
bridged organic MV systems, we have also turned to quantum-
chemical methods to study the complete series of complexes [1−
4]n+ (n = 0, 1, 2) and the mononuclear system [5]n+ (n = 0, 1),
using the BLYP35 functional and the COSMO solvent model.
Because of the high amount of exact exchange and the inclusion
of a CH2Cl2 solvent model, this protocol is capable of describing
the charge localization/delocalization behavior in the ground
state for organic MV systems with reasonable accuracy, and the
corresponding TDDFT calculations were found to be in good
agreement with experimentally measured UV−vis−NIR spectral
features. Indeed, this approach was developed and validated
initially for the characterization of bis-triarylamine MV systems
close to the Class II/III borderline36,37 and thus should give a
reliable description for the present systems as well. This provides
an alternative to the simpler computational models employed
previously.15 Unrestricted structure optimization of [1′−4′]n+ (n
= 0, 1, 2) and [5′]n+ (n = 0, 1) gave stable minima evidenced by
the absence of imaginary frequencies (the prime notation is used
to distinguish the computational systems from the experimental
complexes). For all neutral and dicationic complexes the singlet
state gave the lowest energy. For the dications [1′−4′]2+ the spin
expectation value ⟨S2⟩ of 1.1 indicates a broken-symmetry (BS)
description for a system with one antiferromagnetically coupled
pair of spins. Indeed the singlet−triplet separation, which is
The neutral and dicationic structures [1−4]n+ (n = 0, 2) are
effectively symmetric, and some general trends can be observed
across the series. In general the distances between the amine
nitrogen atom and the first carbon atom of the bridge, d(N−C6),
are the most influenced by the oxidation state: they contract
upon oxidation (Table 3; for atom labeling, see Figure 2). The
d(N−C6) distances are slightly longer for the neutral tolyl
systems 1′ and 3′ (1.411 Å and 1.413 Å) than they are for the
anisyl complexes 2′ and 4′ (1.407 Å and 1.408 Å), and the d(N−
C6) bond lengths are significantly shorter in the dicationic state
for [2′]2+ and [4′]2+ by some 0.012−0.016 Å (1.395 Å and 1.392
Å) and shorter still for the tolyl derivatives [1′]2+ and [3′]2+ by
0.031−0.036 Å (1.380 Å and 1.377 Å). For 1′, 2′, 3′, [1′]2+, and
[2′]2+ the two phenyl rings of the bridge and the P−Pt−P axis are
in plane, as the averaged dihedral angle Ω (average of ∠(P1−Pt−
C3−C4) and ∠(P2−Pt−C3−C8)) reaches a maximum value of
12.6° for [2′]2+. In general this torsion angle is larger (>23°) for
complexes with triphenylphosphine ligands 4′, [3′]2+, and [4′]2+,
as expected, because of the increased steric interactions.
For the MV monocationic structures [1′−4′]+, minima with
clearly localized electronic structures were obtained. In each case
a significant asymmetry in the two halves of the molecule is
observed, and the corresponding d(N−C6) distances differ by at
least 0.16 Å (Table 4), clearly pointing toward a localization of
the redox event in [1′−4′]+ to one amine, consistent with the
large calculated total dipole moment (>25 D; Table 4) in each
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−5.35 eV) to give the HOMO. The HOMO and SOMO are
located on opposite halves of the complex (e.g., composition of
the α-part Ar2C6H4CC/Pt(PEt3)2/CCC6H4N+Ar2 for [1]+:
HOMO: 0/3/91%; SOMO: 67/9/13%) (Figure 5) and are
similar in topology to the description in reference 15.
Table 4. Calculated Ground State Properties for
a
Monocationic Complexes [1′−4′]+
[1′]+
[2′]+
[3′]+
[4′]+
31.0
0.78
μa/D
⟨S2⟩
25.1
0.78
30.9
0.78
25.3
0.78
The subsequent vibrational frequency calculations show one
ν(CC) band for the neutral and the dicationic complexes and
two bands for the monocationic forms (Table 2). The
frequencies obtained within a harmonic vibrational approach
match the experimentally observed ones reasonably well after
scaling by an empirical factor of 0.95,58,59 although the
calculations suggest a slightly larger intensity for the higher-
energy band in [1′]+, in contrast to the experimental data. For all
other cationic complexes, computations give the correct relative
intensities. Full optimization using the commonly employed
hybrid functional B3LYP leads to a delocalized Class III charge
distribution for [1′]+ and thus to a single ν(CC) band at 1994
cm−1. Note that the calculations do not reproduce the precise
details of the splitting of the various ν(CC) bands, possibly
due to neglect of Fermi coupling in the calculated results.
To gain more insight into the origin of the electronic
transitions observed in the UV−vis−NIR spectroelectrochemical
studies, and to both confirm the identity of the IVCT bands and
test the accuracy of the computational methods, TDDFT
calculations were performed for [5′]+ and the MV monocations
[1′−4′]+ using the same basis sets, functionals, and solvent
models (using nonequilibrium solvation as implemented in
TURBOMOLE 6.4) as described above (Table 5). The results
for [1′]+ and [2′]+ are illustrative for the entire series and will be
discussed by way of example. The assignment of the broad low-
energy NIR band of [1′]+ with its peak center around 5000 to
6000 cm−1 to an IVCT is straightforward on the basis of the
quantum-chemical protocol. TDDFT calculations give the
lowest excitation energy in [1′]+ at 5572 cm−1, arising from the
β-HOMO (231β) to β-SOMO (232β) transition (Figure 6).
This transition has appreciable N→N+ character, with significant
contribution from the ethynyl parts of the bridge (Figure 6, Table
5). As might be expected from other studies reported
elsewhere,15,44 the mixing of platinum d-orbital character with
the alkynyl π system in the ground state is relatively limited, and
there is only a small contribution from the Pt d orbitals to the β-
HOMO (2%) and β-SOMO (6%).
d(N−C6)/Å
1.403
1.373
1.422
1.426
1.228
1.238
2.017
1.998
5.7
1.401
1.385
1.422
1.416
1.227
1.234
2.021
2.009
4.6
1.405
1.371
1.421
1.427
1.226
1.237
2.017
2.002
12.7
1.404
1.383
1.422
1.417
1.225
1.233
2.021
2.011
32.5
d(N−C9/16)/Å
d(CC)/Å
d(Pt−C1)/Å
b
Ω/deg
−11.4
48.8
−7.0
51.7
−15.3
47.8
−29.5
50.7
b
Θ/deg
−50.1
−45.1
−50.7
−45.3
a
Total dipole moment μa, spin expectation value ⟨S2⟩ (theoretical
value for doublet systems is 0.75), distance between nitrogen and the
neighboring carbon atom of the bridge d(N−C6), average distance
between the two terminal phenyl rings and nitrogen d(N−C9/16), C
C bond length d(CC), the average of dihedral angles ∠(P1−Pt−
C3−C4) and ∠(P2−Pt−C3−C8) Ω, and the average of ∠(C6−N−C9−
C10) and ∠(C6−N−C16−C17) Θ. Magnitude of torsion is given by
the absolute value; the opposite sign of the dihedral angles is due to
the Ci-symmetric input structure of the optimization.
b
case, and with the conclusions reached above on the basis of the
IR data, and with the electronic spectroscopic data described by
Jones et al.15
The localized behavior is further verified by the partial
localization of the canonical orbitals and spin densities obtained
for each of the compounds [1′−4′]+ (cf. Figure 5 and Supporting
The TDDFT calculations also suggest that the intense
asymmetric band envelope observed in the experimental
spectrum of [1]+ at around 10 000 cm−1 arises from two
excitations, computed at 11 252 cm−1 and 12 046 cm−1 in [1′]+.
The transition at 11 252 cm−1 is calculated to have substantial β-
HOMO-1 (230β)→β-SOMO (69% contribution) and 228β→β-
SOMO (12% contribution) character. The β-HOMO-1 and the
228β orbital are both unevenly distributed over the molecular
backbone (Figure 6), and while the β-HOMO-1 exhibits
noticeable Pt(d)/CC (13/26%) character, there is substan-
tially less d-orbital character (2%) in 228β. Thus the transition at
11 252 cm−1 also exhibits CT character and is best described as a
π→N+ excitation, with considerably more bridge character in the
donor orbital than for the IVCT transition at 5572 cm−1. The
transition at 12 046 cm−1 arises from transitions between 227β
(41% contribution) and 229β (34% contribution) and the β-
SOMO. Both orbitals are mainly localized in the metal
coordination sphere (50% and 24%) and the ethynyl parts of
the bridge (34% and 67%), giving this transition significant
MLCT (Pt→N+) character. Four low-intensity excitations are
computed for [1′]+ between 15 000 and 20 000 cm−1, consistent
Figure 5. Spin density isosurface plot (top left,
0.002 au) and
isosurface plots ( 0.03 au) of the α-SOMO (top right), α -HOMO
(bottom left) and the β-SOMO (bottom right, virtual orbital) of [1′]+.
Small differences in the α-SOMO and β-SOMO arise from spin
polarization.
Information). The spin density is predominantly centered at one
of the triarylamine units, with only a very small contribution from
the platinum and supporting phosphine ligands. While
calculations exhibit almost no spin contamination, the onset of
spin polarization is observed: contrary to expectations a β-orbital
(not shown in Figure 5) represents the occupied orbital with the
highest energy ([1′]+: −5.55 eV; [2′]+: −5.39 eV; [3′]+: −5.50
eV; [4′]+: −5.34 eV). Taking the overlap as criterion this β
orbital can be matched with the energetically highest-occupied α-
orbital ([1′]+: −5.60 eV; [2′]+: −5.41 eV; [3′]+: −5.54 eV; [4′]+:
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a
Table 5. Calculated Excited-State Parameters for [1′]+ and [2′]+
[1′]+
[2′]+
number
Etrans/cm−1
(μt/D)
character
Etrans/cm−1
(μt/D)
character
1
2
3
4
5
6
7
5572
N→N+ intervalence CT
5628
N→N+ intervalence CT
(15.3)
11 252
(6.6)
(13.0)
11 062
(8.2)
π→N+ bridge to amine CT
π→N+ bridge to amine CT
12 046
(4.0)
Pt+→N+ metal to amine ligand
13 588
(1.2)
Pt+→N+ metal to amine ligand
CT
π→N+ bridge to amine CT
CT
15 790
(1.0)
Pt+→N+ metal to amine ligand
16 000
(1.6)
CT
16 162
(1.3)
π→N+ bridge to amine ligand
16 529
(4.9)
intraligand transition at
CT
triarylamine
Pt+→N+ metal to amine ligand
17 548
(0.2)
Pt+→N+ metal to amine ligand
17 108
(0.2)
CT
CT
19 679
intraligand transition at
triarylamine
19 112
Pt+→N+ metal to amine ligand
(1.7)
(0.2)
CT
a
UV−vis−NIR transition energies Etrans/cm−1, transition dipole moments μt/D, and character of the excitation.
may thus not be expected to reproduce the relative intensities
faithfully.
The first three transitions of [2′]+ exhibit very similar character
to that of the tolyl analogue [1′]+. Interestingly, the calculated
“IVCT” transition in [2′]+ at 5628 cm−1 (μt = 13.0 D) is blue-
shifted by only 56 cm−1 relative to the analogous transition in
[1′]+ (Table 5). The quantum-chemical treatment thus suggests
a less-pronounced influence of the methoxy groups on the IVCT
band than is found experimentally (Figure 4). The BLYP35/
COSMO combination has been found to moderately but
systematically underestimate the IVCT excitation energies for
related organic Class II mixed valence radical cations.36,37 It is
conceivable that this underestimation will be more pronounced
in excitations with greater “true” charge-transfer character. In
comparison to that of compound [1′]+, the second (π→N+)
transition of [2′]+ is red-shifted by 190 cm−1, while the calculated
third (MLCT) transition for [2′]+ is some 1500 cm−1 higher in
energy and of lower intensity than it is for [1′]+. This compares
well with the experimental observations (Figure 4) with [1]+
featuring the band envelope at 9750 cm−1, while in [2]+
comparable features are observed near 9950 cm−1 (Table 5).
Also consistent with experimental observation, the computed
transitions between 15 000 and 20 000 cm−1 change appreciably
when going from tolyl ([1′]+) to anisyl ([2′]+) substituents on
the amine moieties (Table 5). The experimental data for [2]+ in
this region is dominated by an intense band centered at 14 500
cm−1, which compares with a less-intense broad band in [1]+
between 14 000 and 20 000 cm−1. The TDDFT calculations for
[2′]+ feature two excitations of similar energy at 16 000 cm−1 and
16 529 cm−1. While the first exhibits a relatively low-transition
dipole moment (μt = 1.6 D), the second is more intense (μt = 4.9
D) and thus is likely the major contributor to the experimentally
observed absorption band. However, the character of these two
transitions differs appreciably. The lower-energy, lower-intensity
excitation at 16 000 cm−1 originates from delocalized orbitals
(244β and 246β) and goes to the β-SOMO (248β). It thus is best
described in terms of a π→N+ transition (Table 5). The higher-
energy, more-intense excitation at 16 529 cm−1 originates
essentially from a single orbital, which is distributed over the
same Ar2NC6H4CC moiety (94%) as the β-SOMO. The
excitation thus exhibits no CT character and is better described as
a localized transition arising from the amine radical cation (Table
Figure 6. Isosurface plots ( 0.03 au) of the orbitals involved in the first
three UV−vis−NIR transitions for monocationic radical [1′]+;
transition energies and transition dipole moments are given in
parentheses.
with the observation of multiple transitions in this range (Figure
4). The calculated transitions at 15 790 cm−1 (μt = 1.0 D) and 17
548 cm−1 (μt = 0.2 D) also have significant MLCT character,
while the excitation at 16 162 cm−1 (μt = 1.3 D) corresponds to a
π→N+ transition. The highest energy calculated excitation at 19
679 cm−1 (μt = 1.7 D) can be attributed to an intraligand
transition associated with the oxidized triarylamine moiety (see
Supporting Information).
On the basis of the computed transition dipole moments, the
calculations appear to overestimate the relative intensity of the
IVCT band significantly. (In fact, the computed transition dipole
moment of 15.3 D is larger than the sum of those for the second
(μt = 6.6 D) and third (μt = 4.0 D) transitions, while the
experimentally observed IVCT band at 5000 to 6000 cm−1
exhibits much lower intensity than the band envelope near 10000
cm−1.) The low intensity and the broadening of the IVCT band
in Class II systems is well-known to be caused by vibronic
coupling.4,13 The computations do not take into account and
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a
Table 6. Calculated Excited State Parameters for [3′−5′]+
[3′]+
[4′]+
[5′]+
Etrans/cm−1
(μt/D)
character
Etrans/cm−1
(μt/D)
character
Etrans/cm−1
(μt/D)
character
5716
N→N+ IVCT
5626
N→N+ IVCT
10 543
(7.0)
π-tolyl→N+ CT
(13.8)
11 295
(5.5)
(11.0)
10 928
(8.8)
mixed π→N+ and
Pt+→N+ MLCT
mixed π→N+ and
Pt+→N+ MLCT
mixed π→N+ and
Pt+→N+ MLCT
mixed π→N+ and
Pt+→N+ MLCT
11 532
(8.7)
intraligand transition
at triarylamine
11 960
12 798
13 892
mixed π-tolyl→N+
(5.8)
(2.5)
(1.9)
and Pt+→N+ MLCT
a
UV−vis−NIR transition energies Etrans/cm−1, transition dipole moments μt/D, and character for the first three excitations.
6). The TDDFT calculations also predict three more low-
intensity excitations at 17 108, 19 112, and 19 562 cm−1 (μt ≤ 0.6
D), which are unlikely to have a significant role in determining
the overall appearance of the absorption spectrum.
localized MV systems, the lowest-energy transition at around
5600 cm−1 corresponds to an IVCT excitation for all bis-
triarylamine complexes [1−4]+. Generally the agreement
between computed and experimentally observed IVCT
excitation energies is better for the tolyl complexes [1]+ and
[3]+ than it is for the anisyl-substituted compounds [2]+ and
[4]+. While the assignment of the second excitation to a CT
transition from a delocalized π orbital and of the third to a CT
from the platinum moiety to the cationic triarylamine unit is
straightforward for [1]+ and [2]+, those excitations start to mix
for the triphenylphosphine complexes [3]+ and [4]+. For
complex [5]+ no IVCT transition is observed due to the absence
of the second triarylamine unit. Here the first excitation
corresponds to a CT transition from a π orbital, which is mainly
tolyl-centered, to the cationic triarylamine unit.
The ability of computational methods based on appropriately
balanced hybrid functionals to successfully model, and hence
predict, the optoelectronic properties of even relatively large and
complex molecular materials promises much not only for the
interpretation of experimental data, but also for computational
screening and molecular design optimization in advance of
synthetic effort, expenditure of chemical resources, and waste
generation. When used sensibly, the predictive power of DFT
methods may now be seen as a viable tool alongside experimental
molecular design and feedback-driven optimization as a route to
molecular materials with designed optoelectronic properties.
Turning attention briefly to the triphenylphosphine complex
[3]+, a small blue shift for the two lowest-energy absorption
bands is observed in the experimental spectrum relative to those
in the PEt3 analogue [1]+ (Figure 4f). This trend is reproduced in
the TDDFT calculations with the computational models [3′]+
and [1′]+ (Tables 5 and 6) and likely reflects the lower electron-
donating ability of the PPh3 ligand, which consequently lowers
the energy of the various donor orbitals. The most obvious
difference in the TDDFT results from [3′]+ relative to [1′]+ is the
character of the calculated transition at 11 295 cm−1 (5.5 D),
which is of mixed π→N+ and MLCT character for [3′]+. The
IVCT excitation calculated at 5716 cm−1 in [3′]+ is at almost the
same energy as that calculated for the PEt3 derivative [1′]+, and
the limited influence of the phosphine substituents on the IVCT
band is a further indication of the limited role the metal center
plays in this transition. The largest energy shift between [1′]+ and
[3′]+ can be observed for the third transition (11 960 cm−1),
which is due to the coupling of π→N+-type and MLCT
transitions for the associated band. Similarly, the character of the
transitions for [4′]+ (e.g., 5626 cm−1, μt = 11.0 D) are nearly
unchanged compared to [2′]+.
As expected, TDDFT calculations from the monocation [5′]+
do not exhibit any transitions below 10 000 cm−1. The
absorption pattern above 10 000 cm−1 exhibits the same features
as [3′]+. The first excitation of [5′]+ is calculated at 10 543 cm−1,
which is red-shifted by about 700 cm−1 compared to [3′]+ (11
295 cm−1). This can be explained by the nature of the involved
orbital. For [3′]+ this transition occurs from a delocalized orbital,
which has significant contributions from the second triarylamine
group (Supporting Information, Tables S5 and S6). As this group
is absent in [5′]+, the orbital is destabilized (−6.41 eV compared
to −6.60 eV for [3′]+) in the ground state and is predominantly
localized at the tolyl ligand (78%) (Supporting Information,
Tables S9 and S10).
EXPERIMENTAL SECTION
■
Computational Details. Full structure optimizations and analysis
of all ground-state properties were performed using a locally modified
version of the TURBOMOLE 6.465 program code enabling the use of
the BLYP35 hybrid functional based on
EXC = 0.65(EXLSDA + ΔEXB88) + 0.35EXexact + ECLYP
which proved to give accurate results for organic MV systems.36,37,66−68
For comparison with previous studies,15 results obtained using the
B3LYP functional were considered in selected cases.69 Split-valence
def2-SVP basis sets were employed on all lighter atoms, together with
the corresponding def2-SVP effective-core potential and a correspond-
ing valence basis set for platinum.70−72 For molecules 1−4 Ci-symmetric
structures were used as starting points for optimizations, and no
constraints were employed. Computed harmonic vibrational frequen-
cies were scaled by an empirical factor of 0.95.58,59 To account for
solvent effects, the conductor-like-screening (COSMO) solvent model
was employed for ground state structure optimizations and analysis as
well as in subsequent TDDFT calculations of excitation energies and
transition dipole moments in TURBOMOLE.73 Dichloromethane (ε =
8.93) was used, as experimental data were collected in this solvent
(nonequilibrium solvation was assumed in the TDDFT calculations).
Spin density and molecular orbital isosurface plots were generated with
the Molekel program.74
CONCLUSIONS
■
This combined spectroscopic and computational study demon-
strates that the nature of the UV−vis−NIR transitions observed
upon oxidation of the platinum-bridged bis-triarylamine
compounds 1−4 are consistent across the series and that the
effect of the phosphine ligand on most of them is relatively
insignificant. These transitions have been accurately modeled
using a computational protocol based on the BLYP35 functional
and a suitable solvent model, allowing a more detailed
investigation than was possible a few years ago. As expected for
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Inorganic Chemistry
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Crystal Data for 1. C56H66N2P2Pt, M = 1024.14, triclinic, space
Rev. 2013, 257, 1493. (e) Haga, M.; Kobayashi, K.; Terada, K. Coord.
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30, 1.
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168.
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101, 2655.
(6) Low, P.; Brown, N. J. Cluster Sci. 2010, 21, 235.
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(8) Launay, J. P. Chem. Soc. Rev. 2001, 30, 386.
(9) Kaim, W.; Sarkar, B.; Lahiri, G. K. Mixed-Valence Intermediates as
Ideal Targets for Spectroelectrochemistry (SEC); RSC Publishing:
Cambridge, 2008.
(10) Kaim, W.; Klein, A.; Glockle, M. Acc. Chem. Res. 2000, 33, 755.
(11) Creutz, C.; Taube, H. J. Am. Chem. Soc. 1973, 95, 1086.
(12) Creutz, C.; Taube, H. J. Am. Chem. Soc. 1969, 91, 3988.
(13) Heckmann, A.; Lambert, C. Angew. Chem., Int. Ed. 2012, 51, 326.
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Bennett, B.; Gardinier, J. R. Inorg. Chem. 2012, 51, 12720.
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Timofeeva, T.; Bredas, J.-L.; Marder, S. R. J. Am. Chem. Soc. 2004, 126,
11782.
(16) Vacher, A.; Barriere, F.; Camerel, F.; Bergamini, J.-F.; Roisnel, T.;
Lorcy, D. Dalton Trans. 2013, 42, 383.
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Organometallics 2011, 30, 3570.
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Organometallics 2012, 31, 8591.
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2008, 27, 383.
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Geskin, V.; Cornil, J.; Hng, H. H.; Rigaut, S.; Chen, X. Chem. Sci. 2012, 3,
3113.
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C. D. J. Phys. Chem. C 2011, 115, 19955.
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7521.
group P1, a = 9.9533(5), b = 11.0652(7), c = 11.5908(7) Å, α =
̅
76.561(5), β = 88.071(4), γ = 85.292(5)°, U = 1237.3(1) Å3, F(000) =
524, Z = 1, Dc = 1.374 g m−3, μ = 2.937 mm−1, T = 120(1) K. 19 690
reflections (2.69 ≤ θ ≤ 29.5°) were collected on an Agilent Gemini S-
Ultra diffractometer (graphite monochromator, λ Mo Kα, λ = 0.71073
Å, ω scan, 1°/frame) equipped with a Cryostream (Oxford
Cryosystems) open-flow nitrogen cryostat, yielding 6893 unique
reflections (Rmerg = 0.0436). The structure was solved by direct methods
and refined by full-matrix least-squares on F2 for all data using
SHELXTL and OLEX2 software.75,76 All nondisordered non-hydrogen
atoms were refined with anisotropic displacement parameters;
disordered atoms of Et groups were refined isotropically with fixed
SOF = 0.5. H-atoms were located on the difference map and refined
isotropically except the H-atoms of disordered groups, which were
placed in calculated positions and refined in “riding” mode. Final
wR2(F2) = 0.0636 for all data (349 refined parameters), conventional
R(F) = 0.0292 for 6696 reflections with I ≥ 2σ, GOF = 1.013.
ASSOCIATED CONTENT
* Supporting Information
■
S
Full details of the syntheses and characterization of 4-
ethynylenephenylenedi-p-tolylamine and compounds 1−5.
Figures showing the IR spectra of [1]n+, [2]n+, and [4]n+ (n =
0, 1, 2). Figures showing the UV−vis−NIR spectra of [1−4]n+ (n
= 0, 1, 2) and [5]n+ (n = 0, 1). Tables of orbital energies, orbital
composition, and calculated excited-state parameters for [1′−
5′]+. Plots of the spin density isosurfaces of [1′−5′]+. Plots of the
orbitals involved in the first 10 electronic transitions of [1′]+ and
[2′]+. Plots of the orbitals involved in the first five electronic
transitions of [3′]+ and [4′]+. Plots of the β-HOMO and β-
SOMO of [5′]+. This material is available free of charge via the
Internet at http://pubs.acs.org. Crystallographic data for the
structure have been deposited with the Cambridge Crystallo-
graphic Data Centre as supplementary publication CCDC-
961163.
AUTHOR INFORMATION
Corresponding Author
*E-mail: martin.kaupp@tu-berlin.de (M.K.), paul.low@uwa.edu.
au (P.J.L.).
■
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work has been supported by the Berlin DFG cluster of
excellence on “Unifying Concepts in Catalysis” (UniCat), by
DFG project KA1187/13-1, and by the Engineering and Physical
Sciences Research Council (EPSRC). M.P. is grateful to UniCat,
the Berlin International Graduate School of Natural Sciences and
Engineering (BIG-NSE), and the German Academic Exchange
Service (DAAD) for funding. P.J.L. held an EPSRC Leadership
Fellowship, and gratefully acknowledges funding from the ARC
in the form of a Future Fellowship (FTFT120100073). K.B.V.
gratefully acknowledges funding for Ph.D. studies from the
EPSRC. Dr. Hilke Bahmann is gratefully acknowledged for
modifying the TURBOMOLE 6.4 code.
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