Tuning the Rainbow: Systematic Modulation of Donor-Acceptor Systems through Donor Substituents and Solvent

Article abstract of DOI:10.1021/acs.inorgchem.6b01039

A series of donor-acceptor compounds is reported in which the energy of the triarylamine donor is systematically tuned through para substitution with electron-donating methoxy and electron-withdrawing cyano groups. The acceptor units investigated are benzothiadiazole (btd), dipyridophenazine (dppz), and its [ReCl(CO)₃(dppz)] complex. The effect of modulating donor energy on the electronic and photophysical properties is investigated using ¹H NMR spectroscopy, DFT calculations, electrochemistry, electronic absorption and emission spectroscopies, ground state and resonance Raman spectroscopy, and transient absorption spectroscopy. Qualitative correlations between the donor energy and the properties of interest are obtained using Hammett σ⁺ constants. Methoxy and cyano groups are shown to destabilize and stabilize, respectively, the frontier molecular orbitals, with the HOMO affected more significantly than the LUMO, narrowing the HOMO-LUMO band gap as the substituent becomes more electron-donating - observable as a bathochromic shift in low-energy charge-transfer absorption bands. Charge-transfer emission bands are also dependent on the electron-donating/withdrawing nature of the substituent, and in combination with the highly solvatochromic nature of charge-transfer states, emission can be tuned to span the entire visible region.

Full text of DOI:10.1021/acs.inorgchem.6b01039

Article
pubs.acs.org/IC
Tuning the Rainbow: Systematic Modulation of Donor−Acceptor
Systems through Donor Substituents and Solvent
Christopher B. Larsen,† Holly van der Salm,† Georgina E. Shillito, Nigel T. Lucas,* and Keith C. Gordon*
Department of Chemistry, University of Otago, P.O. Box 56, Dunedin, New Zealand
S
* Supporting Information
ABSTRACT: A series of donor−acceptor compounds is
reported in which the energy of the triarylamine donor is
systematically tuned through para substitution with electron-
donating methoxy and electron-withdrawing cyano groups.
The acceptor units investigated are benzothiadiazole (btd),
dipyridophenazine (dppz), and its [ReCl(CO)3(dppz)]
complex. The effect of modulating donor energy on the
electronic and photophysical properties is investigated using
1H NMR spectroscopy, DFT calculations, electrochemistry,
electronic absorption and emission spectroscopies, ground
state and resonance Raman spectroscopy, and transient
absorption spectroscopy. Qualitative correlations between the
donor energy and the properties of interest are obtained using
Hammett σ+ constants. Methoxy and cyano groups are shown to destabilize and stabilize, respectively, the frontier molecular
orbitals, with the HOMO affected more significantly than the LUMO, narrowing the HOMO−LUMO band gap as the
substituent becomes more electron-donatingobservable as a bathochromic shift in low-energy charge-transfer absorption
bands. Charge-transfer emission bands are also dependent on the electron-donating/withdrawing nature of the substituent, and
in combination with the highly solvatochromic nature of charge-transfer states, emission can be tuned to span the entire visible
region.
However, the use of peripheral substituents to systematically
tune donor energies, analogous to studies on modulating
acceptor energies,10,11 is underrepresented in the literature.
Re(I), Cu(I), and Ru(II) dipyridophenazine (dppz)
complexes appended with sulfur-based donors have also
exhibited interesting electronic properties with respect to
donor energy. [ReCl(CO)3(dppz)] and [Cu(bpy(Mes)2)-
(dppz)]+ complexes appended with weak thioether or
trithiocarbonate donors exhibit dual intraligand/metal-to-ligand
(IL/MLCT) transitions,22,23 while [Ru(bpy)2(dppz)]2+ com-
plexes appended with strong tetrathiafulvalene donors exhibit
independent ILCT and MLCT transitions of significantly
different energy.24 Systematic modulation of donor energy may
shed light on such phenomena.
The energy of MLCT and π,π* transitions has been
successfully tuned in a predictable manner using substitution
effects.25−31 These studies have utilized Hammett constants (σ)
for substituted benzene derivatives, typically used to estimate
the chemical reactivity of a site, to model the electronic
influence of the substituent. These constants are not universally
applicable; there are specific constants for meta- vs para-
substituted systems (σm and σp, respectively) as well as for
reactions in which the substituent is in resonance with the site
INTRODUCTION
■
Electron donor−acceptor (D−A) systems have been exten-
sively investigated in recent years for their potential use in a
wide range of molecular electronic,1,2 photovoltaic,3−5 and
biological imaging applications.6 Their utility in such systems is
a function of their frontier molecular orbitals, where occupied
orbitals are localized on the donor unit and unoccupied orbitals
on the acceptor unit.7−9 Upon photoexcitation with ultraviolet,
visible, or near-infrared irradiation, an electron is donated from
the donor to the acceptor in a charge-transfer (CT) process.
Due to the localized nature of the donor and acceptor
orbitals, the energy of a CT electronic transition depends on
the energy levels of each component and can be tuned through
alteration of either one or both of these components. Tuning
the energy of the CT transition has often involved substitution
of10,11 or replacing12−15 the acceptor unit. In order to allow
experimental results to guide the rational design of future
materials with predictable properties, studies must be system-
atic. Examples of energetics tuning are the modification of
benzo[c][1,2,5]thiadiazole to give periodic analoguesreplac-
ing S with Se results in a smaller electrochemical band gap and,
consequently, a bathochromic shift of both absorption and
emission CT bands16,17 and tuning of the acceptor energy
through extension of conjugation.18
In addition to replacing the donor unit,19 a number of studies
use π-conjugated bridges to tune the donor energy.20,21
Received: April 29, 2016
© XXXX American Chemical Society
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Figure 1. Compounds investigated in this study and naming convention used. Donor is depicted in red and acceptor in blue. Reactions described as
overnight typically had a duration of 15 h.
of reaction (σ− for when the reaction site becomes electron rich
and σ+ for when the reaction site becomes electron deficient).32
With respect to D−A systems, σ− is appropriate for describing
substitution effects on the acceptor (which receives an electron
upon photoexcitation) and σ+ for the donor (which loses an
electron upon photoexcitation).
reacted in a Buchwald−Hartwig coupling with di(4-
methoxyphenyl)amine to afford btd-dmpab in good yield.
The synthesis of btd-dcpab could not be realized in a manner
analogous to btd-dmpab, due to btd-dcpab and di(4-
cyanophenyl)amine being chromatographically inseparable.
However, it was identified that a key step in the preparation
of di(4-cyanophenyl)amine from diphenylamine was selective
iodination at the 4-positions. As the btd ring is electron poor, it
was believed that iodination of btd-dpab, obtained in one high-
yielding step from btd-Br, would occur selectively at the 4
positions of the TAA donor. This was proved correct,
selectively affording the desired product in excellent yield.
Rosenmund−von Braun cyanation affords btd-dcpab in good
yield.
btd-dpab and btd-dmpab were converted into dppz-dpab and
dppz-dmpab, respectively, using LiAlH4 reduction and sulfur
extrusion, followed by Schiff-base condensation with 1,10-
phenanthroline-5,6-dione. Cyano groups are sensitive to LiAlH4
reduction; CoCl2-catalyzed NaBH4 reduction, followed by
Schiff-base condensation successfully affords dppz-dcpab in
41% yield. Re(I) complexation was achieved in good to
excellent yields using standard methods.
In this study, the triarylamine (TAA) donor is substituted at
the para positions with electron-withdrawing cyano (R = CN,
dcpab, σ+ = 0.659) and electron-donating methoxy groups (R =
OMe, dmpab, σ+ = −0.778) and compared to the unsubstituted
TAA donor (R = H, dpab, σ+ = 0.000). Three acceptors
previously investigated are utilized in this study: benzothiadia-
zole (btd),1 the dipyridophenazine (dppz) ligand,9 and its
[ReCl(CO)3(dppz)] complex (Figure 1).33 The [ReCl-
(CO)3(dppz)] complex is of particular interest, as the presence
of the metal potentially allows competitive CT processes with
either the TAA donor or metal-donating electron density to the
dppz ligand, ILCT and MLCT, respectively.9,22,33−35 MLCT
transitions can be identified through the enhancement of
carbonyl stretching modes in resonance Raman spec-
tra.9,22,33,34,36,37 Electronic and photophysical properties are
qualitatively correlated to the σ+ Hammett constant and
1
characterized using H NMR spectroscopy, DFT calculations,
An X-ray crystal structure was obtained using a crystal
obtained from slow evaporation of a CHCl3 solution of dppz-
dmpab (Figure 2). dppz-dmpab crystallized in the P-1 space
group, cocrystallizing with one molecule of CHCl3 per
molecule of dppz-dmpab. The dppz skeleton is essentially
planar, with a maximum deviation from planarity at C18 of 0.14
Å, as is the donor amine, with N5 lying 0.09 Å above a plane
defined by the ipso carbons (C22, C25, and C31). As with
previous reports of dppz ligands,9,34 the cocrystallized CHCl3
hydrogen bonds with the phenanthroline-type nitrogens in a
bifurcated manner (2.29 Å to N1 and N2). The crystal
structure and DFT-optimized structure agree well, with an
average difference in bond length of 0.006 Å. The donor unit is
predicted to be rotated 31.4° from the dppz and is
electrochemistry, electronic absorption and emission spectros-
copies, ground state and resonance Raman spectroscopy, and
transient absorption spectroscopy.
RESULTS AND DISCUSSION
■
Synthesis and Crystallography. Synthetic routes toward
the target compounds are outlined in Scheme 1. btd-dmpab
was obtained in three steps from 5-bromobenzo[c][1,2,5]-
thiadiazole (btd-Br) using Suzuki−Miyaura coupling with 4-
trimethylsilylphenylboronic acid to afford a trimethylsilylphen-
yl-substituted btd; the trimethylsilyl groups acts as a masked
halide and is converted to an iodo group in good yield through
iododesilylation with ICl. The resulting iodo group is then
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a
Scheme 1. Synthesis of Target Compounds
a
(i) 4-Trimethylsilylphenylboronic acid, K2CO3, PdCl2(dppf), toluene, water, EtOH, reflux, 15 h, 87%; (ii) ICl, CH2Cl2, −78 °C, 2 h, 89%; (iii)
di(4-methoxyphenyl)amine, BuOK, [tBu3PH]BF4, Pd2(dba)3, toluene, reflux, 15 h, 61%; (iv) 4-diphenylaminophenylboronic acid, K2CO3,
t
PdCl2(dppf), toluene, water, EtOH, reflux, 15 h, 94%; (v) KI, KIO3, AcOH, water, 80 °C, 14 h, 93%; (vi) CuCN, DMF, reflux, 15 h, 84%; (vii) (a)
LiAlH4, THF, rt, 15 h; (b) 1,10-phenanthroline-5,6-dione, EtOH, reflux, 15 h, 39% (R = H), 32% (R = OMe); (viii) (a) NaBH4, CoCl2, EtOH,
reflux, 15 h; (b) 1,10-phenanthroline-5,6-dione, EtOH, reflux, 15 h, 41%; (ix) [ReCl(CO)5], EtOH, reflux, 15 h, 91% (R = H), 78% (R = OMe),
45% (R = CN).
Figure 2. X-ray crystal structure of dppz-dmpab. Ellipsoids are shown at 50% probability level.
experimentally observed as 32.3°. One terminal donor
phenylene shows good agreement between calculated and
experimental torsion angles (70.9° versus 71.9°, respectively).
The other terminal donor phenylene is predicted to be rotated
further out of plane than is observed experimentally (62.8°
versus 48.7°, respectively), likely due to solid state packing
interactions not modeled in the DFT-optimized structure. An
X-ray crystal structure of the trimethylsilylphenyl-substituted
btd intermediate was obtained from ethanol recrystallization
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and crystallized in the P-1 space group (see Supporting
Information).
1H NMR Spectroscopy. In the 1H NMR spectra, the
chemical shift (δ) of protons in resonance with the substituent
show significant shifts and may be related to the Hammett
constant,38,39 while protons not in resonance do not shift
significantly (Figure 3). Electron-donating substituents increase
Figure 4. FT-Raman spectra (solid sample, λex = 1064 nm, black) and
DFT-calculated spectra (in vacuo B3LYP, red) of [ReCl(CO)3(dppz-
dcpab)].
values of ≤10 cm−1 are considered a good fit.9,22,33,34,36,40−43
MAD values for the dppz ligands and complexes range from 4
to 7 cm−1 (Table S3).
Some strong similarities can be observed between all spectra:
all of the ligands show a strong band around 1410 cm−1 typical
of dppz, which is a vibrational mode of the phenazine (phz)
part of the molecule, and also show the strongest modes around
1600 cm−1 which are associated with the TAA;9 these modes
are also consistently calculated. The 1410 cm−1 phenazine
mode does not shift between the ligands, in either experimental
or calculated spectra, which is unsurprising due to the distance
between the phz part of the molecule and the substitution.
However, there are small shifts observed in the TAA-based
modes, although in both experimental and calculated spectra
the addition of either OMe or CN groups shifts the bands to
∼6 cm−1 higher in frequency. This appears to be a mass effect
as simulating a heavy mass (via a H atom with mass 12) gives a
predicted upshifted wavenumber also. A similar effect is
observed for the Re(I) complexes, with little shift in the phz
mode between compounds, and a shift to higher frequency of
∼6 cm−1 for the TAA-based modes around 1600 cm−1. Modes
around 1520 cm−1 also exhibit a relative increase in intensity for
complexes but only minimal shifting in frequency for
experimental spectra; this mode is predominantly associated
with the phenylene linker.
Electrochemistry. Cyclic voltammetry data may be used to
estimate the energies of frontier molecular orbitals (FMOs)
using the method of Li et al.44 Electrochemical data and
estimated FMO energies are presented in Table S6. DFT
calculations (B3LYP, in vacuo) predict the HOMO to be
localized on the TAA donor and the LUMO on the acceptor
for all investigated compounds (Tables S7−S9). The electro-
chemical band gap (ELUMO−HOMO) widens with increasing σ+
for each acceptor system, with the different acceptors offsetting
this trend (Figure S2). This is consistent with the substituent
significantly modulating donor energy and having a smaller
effect on acceptor energy, as implied by 1H NMR spectroscopy.
DFT calculations accurately predict these two trends (Figure
5), giving confidence in DFT-based assignments. It is
interesting to note that although the HOMO−1 for the dppz
complexes is predicted to be metal based, the energy of this
orbital is predicted to show a slight dependence on substituent.
1
Figure 3. Correlation between H NMR chemical shifts (500 MHz,
■
●
CDCl3) of donor group protons: btd ( ), dppz ( ), and
▲
[ReCl(CO)3(dppz)] ( ).
the electron density at positions in resonance with the
substitution, effectively shielding those protons and imbuing
an upfield shift. Conversely, electron-withdrawing substituents
decrease electron density at those positions, deshielding those
protons and eliciting a downfield shift. Thus, the overall effect
on resonant protons is a downfield shift with increasing σ+.
For the donor group protons, H3″ (numbering scheme
depicted in Figure 1) shows the greatest effect, while H3′ (with
a longer resonance pathway) shows a weaker effect, and H2″
(not in resonance) shows almost no effect. H2′ is not in
resonance with the substituent and is also affected by through-
space interaction with acceptor protons and therefore shows no
trend. Importantly, these trends are consistent across the range
of acceptors, indicating that only the substituent influence on
the donor energy is being probed.
Interestingly, acceptor group protons also exhibit some
sensitivity to the substituent (Tables S1 and S2), with the
protons positioned ortho to the imine nitrogens (H4 and H7 for
the btd systems, H10 and H13 for the dppz ligands and
complexes) shifting at the same magnitude, implying that
donor substitution also affects acceptor energy to a certain
extent. The proton position ortho to the donor but not to an
imine nitrogen (H6 for btd systems, H12 for dppz ligands and
complexes) shows no sensitivity to substituent, however. For
the dppz ligands and complexes, phen-type protons also show
slight shifts with substituent.
FT-Raman Spectroscopy. Experimental FT Raman and
calculated Raman spectra for dppz ligands and their respective
Re(I) complexes are presented in Figure S1; an example is
presented in Figure 4. Spectra could not be obtained for btd
compounds due to intense emission from these samples.
Calculated spectra are scaled in order to minimize the mean
absolute deviation (MAD) in frequency to the experimental
spectra. MAD values can be used to validate DFT calculations;
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the lowest energy absorption band (previously assigned as
ILCT in nature)9 hypsochromically shifts with increasing σ+.
For the btd compounds, this shift results in a lack of a clear low-
energy band for btd-dcpab. In addition to the electronic
influence of the substituent on the donor unit altering the
energy of absorption, the nature of the acceptor is also
significant, with btd compounds absorbing at higher energy
relative to dppz, while Re(I) complexation of dppz ligands
further lowers the energy of absorption. Higher energy
absorption bands do not show the same dependence on σ+
and are assigned as acceptor-localized π,π* bands. btd-dcpab
exhibits another low-energy band (∼350 nm) not present in
the other btd compounds; this band occurs at the same energy
and therefore is convoluted with the dppz-localized π,π* bands
in the dppz ligands and complexes. The lack of variation in the
energy of this band across the investigated acceptors implies
that this band is a TAA-localized π,π* or a TAA → CN CT
transition as recently reported by Easwaramoorthi et al.45
A plot of the energy of absorption (Eabs) against the
electrochemically estimated HOMO−LUMO band gap (ex-
cluding btd-dcpab for which Eabs could not be defined) shows a
linear correlation (Figure 7). This is consistent with a HOMO
Figure 5. Relationship between electrochemical band gap (recorded in
CH2Cl2) and DFT-predicted HOMO−LUMO band gap (CH2Cl2
solvent field). Points correspond to (1) [ReCl(CO)3(dppz-dmpab)],
(2) [ReCl(CO)3(dppz-dpab)], (3) dppz-dmpab, (4) btd-dmpab, (5)
dppz-dpab, (6) [ReCl(CO)3(dppz-dcpab)], (7) btd-dpab, (8) dppz-
dcpab, (9) btd-dcpab. Tabulated data are in Supporting Information
Tables S6 and S10.
Electronic Absorption Spectroscopy. Electronic absorp-
tion spectra are presented in Figure 6. For all acceptor systems,
Figure 7. Correlation between the experimental Eabs of the ILCT
absorption band and the electrochemical band gap, both recorded in
CH2Cl2. Points correspond to (1) [ReCl(CO)3(dppz-dmpab)], (2)
[ReCl(CO)3(dppz-dpab)], (3) dppz-dmpab, (4) btd-dmpab, (5)
dppz-dpab, (6) [ReCl(CO)3(dppz-dcpab)], (7) btd-dpab, (8) dppz-
dcpab. Tabulated data are in Supporting Information Tables S6 and
S11.
→ LUMO transition, as predicted by TD-DFT calculations
(vide infra), and shows that the electronic influence of the
substituent affects both Eabs and the electrochemical band gap
by altering the donor energy but not changing the nature of the
transition. This finding is closely analogous to early work on
tuning the energy of MLCT transitions in Re(I) and Ru(II)
polypyridyl complexes, in which the optical band gap was
related to changes in the reduction potential of the complex
upon substitution of the polypyridyl ligand.46,47
Both B3LYP and CAM-B3LYP TD-DFT calculations
successfully reproduce the experimental trends of bathochromi-
cally shifting absorption for the lowest energy absorption band
as the substituent becomes more electron donating and that for
a given substituent absorption also bathochromically shifts as
the acceptor is changed from dppz to [ReCl(CO)3(dppz)]
Figure 6. Electronic absorption spectra, recorded in CH2Cl2.
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(Figure 8). B3LYP and CAM-B3LYP calculations predict
HOMO → LUMO transitions (Figure 9) for the lowest energy
absorption band are observed at 1600 and 1539 cm−1, which
are assigned as TAA and dppz based modes, respectively. This
is consistent with an ILCT transition from the TAA-localized
HOMO to a dppz-based LUMO. This assignment is also
supported by TD-DFT calculations. The 1539 cm−1 vibration is
largely associated with the phz component of the dppz ligand;
however, the addition of the TAA substituent to dppz perturbs
the individual phen and phz molecular orbitals; hence, other
dppz-based modes at 1248, 1322, 1352, 1418, 1452, and 1578
cm−1 are also enhanced in the transition.
As the excitation wavelength shortens and starts to coincide
with the higher energy absorption band, these dppz-localized
vibrations become further enhanced, as does a TAA mode at
1614 cm−1, which is consistent with both dppz-localized and
TAA-localized π,π* transitions. With 351 nm excitation, two
additional modes are further enhanced at 1170 and 2224 cm−1,
assigned as TAA and CN vibrations, respectively, indicative of a
TAA-localized π,π* or TAA → CN transition as recently
reported by Easwaramoorthi et al.45 This is supported by TD-
DFT calculations which predict a significant contribution to the
transition from the HOMO to the LUMO+3, with the LUMO
+3 localized on the terminal phenylene rings and cyano
substituents of the donor.
For [ReCl(CO)3(dppz-dmpab)], similar enhancements to
[ReCl(CO)3(dppz-dcpab)] are observed at wavelengths
coinciding with the lowest energy absorption band. Key
enhancements at 1535 and 1165 cm−1 are associated with
predominantly phz-localized LUMO and TAA-localized
HOMO vibrations, respectively, and are therefore consistent
with a HOMO → LUMO transition. Analogous to [ReCl-
(CO)3(dppz-dcpab)], dppz-localized modes at 1245, 1322,
1349, 1405, 1450, and 1577 cm−1 are also enhanced. At 406
and 413 nm, very small enhancement of carbonyl modes at
2026 cm−1 is observed (also observed for [ReCl(CO)3(dppz-
dpab)] at 2027 cm−1), consistent with marginal contribution
from an MLCT transition. Interestingly, this is not observed for
[ReCl(CO)3(dppz-dcpab)]. Mulliken analysis of CAM-B3LYP
calculations shows a contribution from the metal to the lowest
energy transition of 6%, 0%, and 4% for [ReCl(CO)3(dppz-
dcpab)], [ReCl(CO)3(dppz-dpab)], and [ReCl(CO)3(dppz-
dmpab)], respectively.
Figure 8. Correlation between predicted (TD-DFT, CH2Cl2) and
experimental Eabs (recorded in CH2Cl2) for the ILCT absorption band.
Slopes are all the same within experimental error (1.0 0.2). Points
correspond to (1) [ReCl(CO)3(dppz-dmpab)], (2) [ReCl-
(CO)3(dppz-dpab)], (3) dppz-dmpab, (4) dppz-dpab, (5) [ReCl-
(CO)3(dppz-dcpab)], (6) dppz-dcpab. Tabulated data are in
Supporting Information Tables S11 and S12.
Figure 9. HOMO → LUMO transition for [ReCl(CO)3(dppz-dpab)]
predicted by CAM-B3LYP TD-DFT calculations (with CH2Cl2
solvent field).
absorption band (Tables S11 and S12). The Mulliken charge
density change modeled using CAM-B3LYP shows negligible
contribution from the metal to the lowest energy charge
transfer transition, with little change observed between the
complexes, supporting assignment of an ILCT transition (Table
S12).
The similarity between the resonance Raman spectra of the
complexes at excitation wavelengths coinciding with the lowest
energy absorption band implies that while the substituent tunes
the energy of the transition it has no effect on the electronic
nature of the transition.
Resonance Raman Spectroscopy. Resonance Raman
spectroscopy, in which the laser excitation wavelength is
coincident with an electronic transition and vibrational modes
enhanced are associated with the active chromophore,48 can be
used to qualitatively assign the nature of transitions by the
nature of bands enhanced. This may be used as an experimental
validation of TD-DFT calculations. Resonance Raman spectra
of the three dppz complexes are presented in Figure 10. We
previously characterized the behavior of [ReCl(CO)3(dppz-
dpab)] as ILCT in nature and by virtue of only subtle spectral
changes across excitation wavelengths concluded that the ILCT
transition was dominant across all excitation wavelengths.9 The
spectra of [ReCl(CO)3(dppz-dmpab)] show some similar
enhancement patterns to [ReCl(CO)3(dppz-dpab)], but for
[ReCl(CO)3(dppz-dcpab)] there are spectral differences, and
these become more obvious as the excitation wavelength is
changed. The key enhancements for [ReCl(CO)3(dppz-
dcpab)] at wavelengths coinciding with the lowest energy
Emission Spectroscopy. All compounds exhibit highly
solvatochromic emission that is insensitive to the presence of
3O2, consistent with 1ILCT emissive states,9 even for the
complexes, which often exhibit triplet-state emission due to
efficient intersystem crossing facilitated by the metal.34 As with
the CT absorption bands, λem shows a strong dependence on
the substituent, and in combination with the strong
solvatochromism of each compound, emission can be tuned
to span the entire visible region (Figure 11). It is clear that in
many cases there is more than one emissive state and that these
can show switching as illustrated in Figure S3. For the data
shown, the second emissive state can be seen in plot 9 of Figure
11. A plot of Eem for the CT emission bands against the
electrochemical band gap is linear (Figure 12), indicating that
emission is occurring from the absorbing state and λem can
therefore be rationally tuned using Hammett constants.
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Figure 10. Resonance Raman spectra of dppz complexes, recorded in CH2Cl2 (1 mM) at a range of excitation wavelengths, and FT-Raman spectra
(solid state, 1064 nm excitation).
Figure 12. Relationship between electrochemical band gap and energy
of emission for CT bands recorded in CH2Cl2. Points correspond to
(1) [ReCl(CO)3(dppz-dpab)], (2) dppz-dmpab, (3) btd-dmpab, (4)
dppz-dpab, (5) [ReCl(CO)3(dppz-dcpab)], (6) btd-dpab, (7) dppz-
dcpab, (8) btd-dcpab.
Figure 11. Emission of dppz-dcpab in toluene (1), 1,4-dioxane (2),
and CHCl3 (3), dppz-dpab in toluene (4), 1,4-dioxane (5), and CHCl3
(7), and dppz-dmpab in toluene (6), 1,4-dioxane (8), and CHCl3 (9).
methods by which solvent−solute polarization is more
accurately modeled.49−53 For instance, the state-specific (SS)
TD-DFT was used to calculate the emission energies of the
dppz ligands, presented in Table S13. The excited state
structures were optimized in different solvents using the
integral equation formalism (IEF) version of the polarizable
continuum model (PCM) with both B3LYP and CAM-B3LYP
functionals using the 6-31G(d) basis set. The default linear
response (LR) solvent parameter was employed due to its
computational efficiency and that a precise, quantitative analysis
of the emission energies of the compounds was not required for
this investigation. However, in recent years there has been
significant development of more sophisticated computational
formalism can be applied to the PCM model, in which the
solvent electrostatically responds to electron density of the
excited state, whereas with LR only the response of the solvent
with respect to the ground state electron density is considered.
The SS approach therefore provides a more reliable
representation of the polarization of the solvent in response
to formation of the excited state.50,51,54,55
A number of studies have compared LR and SS approaches
for calculating vertical excitation and emission energies of
different compounds in solution.50,51,54,56 Improta et al.
compared the emission energies of coumarin derivatives in
G
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Inorganic Chemistry
Article
ethanol calculated with PCM/TD-DFT using SS and LR
models.50 The study showed that the SS model gave predicted
values very close to the experimentally observed emission
energies; however, it also showed that the LR method was able
to provide qualitative estimates. Guido et al. compared the TD-
DFT-calculated absorption and emission spectra of Nile Red
using a range of functionals and polarization schemes.57 They
showed that due to error cancellation, accurate results were
obtained if LR was used with B3LYP and also when SS was
used with CAM-B3LYP. This is feasibly what is observed in this
case, as the emission energies predicted using B3LYP are a
closer match to the experimental data than those predicted
using CAM-B3LYP (Table S13).
A series of donor−acceptor dyes, also containing TAA donor
substituents, was studied by Bernini et al.58 The emission
energies were calculated using TD-DFT in conjunction with
wide variety of functionals, basis sets, and polarization schemes.
When range-separated hybrid functionals are used with LR, the
mean absolute error between the calculated and the
experimental emission energies is approximately 0.2 eV and
reduced to 0.15 eV when the SS approach is utilized. These
studies highlight the importance of the selection of functional,
basis set, and polarization scheme in regard to the type of
transition that is occurring if precise, quantitative analysis is
desired.
Figure 13. Transient absorption spectra of dppz complexes in CH2Cl2,
recorded 200 ns after 355 nm excitation.
likely to be similar in nature for all three, and the presence of
the broad absorption band at low energy is consistent with an
oxidized TAA unit,9,59,60 consistent with this being a ILCT
3
state. In addition, the reported spectra for phen•− show
absorption features in this region.61
For the purpose of our investigation the LR method was
satisfactory as the calculations successfully reproduce the trend
of increasing λem as the TAA substituent becomes more
electron donating and support assignment of an ILCT emissive
state. This trend is observed in both solvents modeled. In
general, the λem values modeled in chloroform were predicted at
longer wavelengths than those in toluene, as seen exper-
imentally. A reasonable correlation with experimental data was
observed from results obtained using B3LYP. Although the
trend in λem was correctly modeled using CAM-B3LYP, the
predicted values were significantly offset.
Transient Absorption Spectroscopy. Transient absorp-
tion (TA) spectroscopy provides the absorption spectrum of
the longer-lived (ns) excited states, and the excited state
lifetimes can also be determined by measuring the decay of the
signal. Emission lifetimes are too short to be measured on the
available instrumentation, indicating that two different states
are being probed: the 1ILCT for the emission and a 3ILCT dark
state for the transient absorption as previously seen for
[ReCl(CO)3(dppz-dpab)].9
The lifetimes of the three dppz complexes appear to be the
same within experimental error (Table 1). Consequently, the
differences in the electronic nature of the substituents on the
TAA donor are not enough to perturb the decay of the excited
state.
In addition, the transient absorption spectra are all very
similar (Figure 13) with bleaching of the ground state at ∼360
and 450−500 nm and positive absorption of the excited state at
∼420 nm and at wavelengths longer than 500 nm. This
indicates that the long-lived dark state in these compounds is
CONCLUSIONS
■
In order to systematically probe the effect of donor energy on
the D−A behavior, compounds which consist of a btd, dppz, or
[ReCl(CO)3(dppz)] acceptor and a triarylamine donor with
CN, H, or OMe substituents were studied. The electron-
withdrawing CN substituent is shown to lower the energy of
frontier molecular orbitals, while the electron-donating OMe
1
substituent increases the energy, as demonstrated using H
NMR spectroscopy, electrochemistry, and DFT calculations.
Unsurprisingly, the TAA-based HOMO is affected more
significantly than the acceptor-based LUMO, although this is
still affected to a certain degree; overall, the effect is to narrow
the HOMO−LUMO band gap as the substituent becomes
more electron donating (σ+ becomes more negative). This is
reflected in the electronic absorption spectra, for which there is
a bathochromic shift in the broad low-energy CT band as the
substituent becomes more electron donating; this behavior is
observed for the three different acceptors, which also
bathochromically shift going from btd to dppz to [ReCl-
(CO)3(dppz)]. TD-DFT calculations successfully predict these
trends and predict the lowest energy absorption band to be
predominantly ILCT in nature. Resonance Raman spectrosco-
py supports this assignment, indicating the substituent does not
alter the electronic nature, only the orbital energy. CT emission
bands show a similar trend in λem to the CT absorption bands
and combined with the solvatochromism of emission inherent
to such systems span the entire visible region. Substituents
appear to have little effect on the change in dipole moment
between ground and excited states or on the lifetime of a
3ILCT dark state observed through transient absorption
spectroscopy.
Table 1. Dark State Lifetimes
EXPERIMENTAL SECTION
■
compound
τ (μs)
General Experimental. btd-Br62 and 1,10-phenanthroline-5,6-
dione63 were prepared using literature procedures. Commercially
available reagents and solvents were used as received. Spectroscopic-
or HPLC-grade solvents were used for all spectroscopic measure-
[ReCl(CO)3(dppz-dcpab)]
[ReCl(CO)3(dppz-dpab)]
[ReCl(CO)3(dppz-dmpab)]
3.0 0.8
3.8 0.4
3.6 0.1
H
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Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
ments. Spectral data was analyzed using GRAMS A/I (ThermoScien-
tific) and OriginPro v9.0 (Origin Lab Corp.). Compound numbering
schemes are shown in Figure 1.
5-(4-Diphenylaminophenyl)benzo[c][1,2,5]thiadiazole (btd-
dpab). A mixture of btd-Br (0.516 g, 2.40 mmol), 4-diphenylamino-
phenylboronic acid (0.890 g, 3.08 mmol), and K2CO3 (1.84 g, 13.3
mmol) in toluene (15 mL), H2O (10 mL), and EtOH (5 mL) was
bubbled with argon for 15 min. PdCl2(dppf) (0.100 g, 0.122 mmol)
was added, and the reaction mixture was heated at reflux overnight
under an argon atmosphere. The reaction mixture was allowed to cool,
and the product was extracted into CHCl3 and washed with NH4Cl
solution (satd) and then water. The organic extract was dried over
MgSO4, and the solvent was removed under reduced pressure. The
residue was purified using preparative column chromatography (SiO2,
CH2Cl2) to afford btd-dpab (0.856 g, 94%) as a yellow crystalline
Materials. 5-(4-Trimethylsilylphenyl)benzo[c][1,2,5]thiadiazole.
A mixture of btd-Br (0.510 g, 2.37 mmol), 4-trimethylsilylphenylbor-
onic acid (0.524 g, 2.70 mmol), and K2CO3 (1.80 g, 13.0 mmol) in
toluene (15 mL), water (10 mL), and EtOH (5 mL) was bubbled with
argon for 15 min. PdCl2(dppf) (0.138 g, 0.168 mmol) was added and
the reaction mixture heated at reflux overnight under an argon
atmosphere. The reaction mixture was allowed to cool to rt, and the
product was extracted into CH2Cl2 and washed with aq. NH4Cl (satd)
and then water. The organic extract was dried over MgSO4, and the
solvent was removed under reduced pressure. The residue was purified
using preparative column chromatography (SiO2, CH2Cl2) to afford 5-
(4-trimethylsilylphenyl)benzo[c][1,2,5]thiadiazole (0.584 g, 87%) as a
1
solid. H NMR (500 MHz, CDCl3): δ 8.13 (dd, J = 1.7, 0.7 Hz, 1H,
H4), 8.03 (dd, J = 9.2, 0.7 Hz, 1H, H7), 7.89 (dd, J = 9.2, 1.8 Hz, 1H,
H6), 7.59 (d, J = 8.7 Hz, 2H, H2′), 7.30 (dd, J = 8.5, 7.4 Hz, 4H, H3″),
7.18 (d, J = 8.7 Hz, 2H, H3′), 7.17 (dd, J = 8.5, 1.0 Hz, 4H, H2″), 7.08
(tt, J = 7.4, 1.1 Hz, 2H, H4″) ppm. 13C NMR (126 MHz, CDCl3): δ
155.72 (C3a), 154.20 (C7a), 148.47 (C4′), 147.49 (C1″), 142.05 (C5),
132.79 (C1′), 130.10 (C6), 129.55 (C3″), 128.29 (C2′), 125.02 (C2″),
123.60 (C4″), 123.35 (C3′), 121.49 (C7), 117.50 (C4) ppm. HRMS
(ESI) calcd for C24H18N3S ([M + H]+): m/z 380.122. Found: m/z
380.121. Anal. Calcd for C24H17N3S: C, 75.69; H, 4.52; N, 11.07.
Found: C, 75.66; H, 4.37; N, 11.02.
1
white crystalline solid. H NMR (500 MHz, CDCl3): δ 8.18 (dd, J =
1.7, 0.8 Hz, 1H, H4), 8.05 (dd, J = 9.1, 0.7 Hz, 1H, H7), 7.89 (dd, J =
9.1, 1.7 Hz, 1H, H6), 7.71 (d, J = 8.3 Hz, 2H, H2′), 7.68 (d, J = 8.3 Hz,
2H, H3′), 0.35 (s, 9H, SiMe3) ppm. 13C NMR (126 MHz, CDCl3): δ
155.52 (C3a), 154.36 (C7a), 142.53 (C5), 140.98 (C4′), 139.95 (C1′),
134.21 (C3′), 130.22 (C6), 126.87 (C2′), 121.60 (C7), 118.65 (C4),
−1.00 (SiMe3) ppm. HRMS (ESI) calcd for C15H17N2SSi ([M + H]+):
m/z 285.088. Found: m/z 285.088. Anal. Calcd for C15H16N2SSi: C,
63.34; H, 5.67; N, 9.85. Found: C, 63.34; H, 5.65; N, 9.86.
5-(4-Di(4-iodophenyl)aminophenyl)benzo[c][1,2,5]thiadiazole. A
mixture of btd-dpab (0.881 g, 2.32 mmol), KI (0.786 g, 4.73 mmol),
and KIO3 (0.511 g, 2.39 mmol) in AcOH (70 mL) and water (10 mL)
was heated at 80 °C overnight. The reaction mixture was allowed to
cool, and water was added. The resultant precipitate was filtered and
washed with water. The residue was purified using preparative column
chromatography (SiO2, 10% EtOAc in CHCl3) to afford 5-(4-di(4-
iodophenyl)aminophenyl)benzo[c][1,2,5]thiadiazole (1.37 g, 93%) as
5-(4-Iodophenyl)benzo[c][1,2,5]thiadiazole. A solution of 5-(4-
trimethylsilylphenyl)benzo[c][1,2,5]thiadiazole (1.00 g, 3.52 mmol) in
CH2Cl2 (100 mL) under an argon atmosphere was cooled to −78 °C
using a dry ice/acetone bath. ICl (5.3 mL, 1 M in CH2Cl2) was added
dropwise and the resultant mixture stirred at −78 °C under an argon
atmosphere for 2 h. The dry ice/acetone bath was removed, excess ICl
quenched by dropwise addition of aqueous Na2S2O5 (satd, 20 mL),
and the mixture allowed to warm to rt. The product was extracted into
CH2Cl2 and washed with aqueous NH4Cl (satd) and then water. The
organic extract was dried over MgSO4, and the solvent was removed
under reduced pressure. The residue was purified using preparative
column chromatography (SiO2, CH2Cl2) to afford 5-(4-iodophenyl)-
1
a yellow solid. H NMR (500 MHz, CDCl3): δ 8.13 (dd, J = 1.8, 0.8
Hz, 1H, H4), 8.05 (dd, J = 9.2, 0.8 Hz, 1H, H7), 7.87 (dd, J = 9.2, 1.8
Hz, 1H, H6), 7.60 (d, J = 8.8 Hz, 2H, H2′), 7.58 (d, J = 8.9 Hz, 4H,
H3″), 7.17 (d, J = 8.7 Hz, 2H, H3′), 6.89 (d, J = 8.9 Hz, 4H, H2″) ppm.
13C NMR (126 MHz, CDCl3): δ 155.65 (C3a), 154.29 (C7a), 147.27
(C4′), 146.88 (C1″), 141.76 (C5), 138.65 (C3″), 134.37 (C1′), 130.01
(C6), 128.64 (C2′), 126.47 (C2″), 124.39 (C3′), 121.67 (C7), 117.92
(C4), 86.80 (C4″) ppm. MS (MALDI-TOF) calcd for C24H15I2N3S
([M]+): m/z 630.91. Found: m/z 630.88. Anal. Calcd for
C24H15I2N3S: C, 45.66; H, 2.40; N, 6.66. Found: C, 45.51; H, 2.43;
N, 6.55.
1
benzo[c][1,2,5]thiadiazole (1.07 g, 89%) as a white solid. H NMR
(500 MHz, CDCl3): δ 8.12 (dd, J = 1.7, 0.7 Hz, 1H, H4), 8.04 (dd, J =
9.1, 0.6 Hz, 1H, H7), 7.82 (d, J = 8.5 Hz, 2H, H3′), 7.80 (dd, J = 9.1,
1.8 Hz, 1H, H6), 7.41 (d, J = 8.5 Hz, 2H, H2′) ppm. 13C NMR (126
MHz, CDCl3): δ 155.38 (C3a), 154.37 (C7a), 141.41 (C5), 139.16
(C1′), 138.32 (C3′), 129.70 (C6), 129.31 (C2′), 121.86 (C7), 118.65
(C4), 94.60 (C4′) ppm. MS (MALDI-TOF) calcd for C12H7IN2S
([M]+): m/z 337.94. Found: m/z 337.93. Anal. Calcd for C12H7IN2S:
C, 42.62; H, 2.09; N, 8.28. Found: C, 42.98; H, 1.90; N, 8.14.
5-(4-Di(4-methoxyphenyl)aminophenyl)benzo[c][1,2,5]-
thiadiazole (btd-dmpab). A mixture of 5-(4-iodophenyl)benzo[c]-
[1,2,5]thiadiazole (0.410 g, 1.21 mmol), di(4-methoxyphenyl)amine
(0.426 g, 1.86 mmol), and tBuOK (0.475 g, 4.23 mmol) in toluene (30
mL) was bubbled with argon for 15 min. [tBu3PH]BF4 (0.050 g, 0.172
mmol) and Pd2(dba)3 (0.101 g, 0.110 mmol) were added, and the
reaction mixture was heated at reflux overnight under an argon
atmosphere. The reaction mixture was allowed to cool to rt, and the
product was extracted into CHCl3 and washed with aq. NH4Cl (satd)
and then water. The organic extract was dried over MgSO4, and the
solvent was removed under reduced pressure. The residue was purified
using preparative column chromatography (basic Al2O3, CH2Cl2) to
5-(4-Di(4-cyanophenyl)aminophenyl)benzo[c][1,2,5]thiadiazole
(btd-dcpab). A solution of 5-(4-di(4-iodophenyl)aminophenyl)benzo-
[c][1,2,5]thiadiazole (1.37 g, 2.17 mmol) in DMF (100 mL) was
bubbled with argon for 15 min. CuCN (1.69 g, 18.9 mmol) was added,
and the reaction mixture was heated at reflux overnight under an argon
atmosphere. The reaction mixture was allowed to cool, and water was
added. The resultant precipitate was filtered and washed with water.
The residue was purified using preparative column chromatography
(SiO2, 20% CH3CN in CHCl3) to afford btd-dcpab (0.782 g, 84%) as
1
a yellow solid. H NMR (500 MHz, CDCl3): δ 8.17 (dd, J = 1.7, 0.7
Hz, 1H, H4), 8.08 (dd, J = 9.1, 0.6 Hz, 1H, H7), 7.87 (dd, J = 9.1, 1.7
Hz, 1H, H6), 7.72 (d, J = 8.6 Hz, 2H, H2′), 7.57 (d, J = 8.8 Hz, 4H,
H3″), 7.26 (d, J = 8.6 Hz, 2H, H3′), 7.19 (d, J = 8.8 Hz, 4H, H2″) ppm.
13C NMR (126 MHz, CDCl3): δ 155.44 (C3a), 154.36 (C7a), 150.08
(C1″), 145.44 (C4′), 141.17 (C5), 137.36 (C1′), 133.80 (C3″), 129.76
(C6), 129.35 (C2′), 127.00 (C3′), 123.48 (C2″), 121.90 (C7), 118.87
(CN), 118.58 (C4), 106.51 (C4″) ppm. MS (MALDI-TOF) calcd for
C26H15N5S ([M]+): m/z 429.10. Found: m/z 429.08. Anal. Calcd for
C26H15N5S: C, 72.71; H, 3.52; N, 16.31. Found: C, 72.66; H, 3.81; N,
15.94.
11-(4-Diphenylaminophenyl)dipyrido[3,2-a:2′,3′-c]phenazine
(dppz-dpab). A mixture of btd-dpab (0.256 g, 0.675 mmol) and
LiAlH4 (0.273 g, 7.19 mmol) in dry THF (20 mL) was stirred at rt
overnight under an argon atmosphere. The reaction mixture was
cooled to 0 °C, then water (0.5 mL) was added dropwise, followed by
NaOH (1 mL, 10%), then more water (1.5 mL). Solids were removed
by filtration through Celite (CHCl3), and the product was extracted
into CHCl3 and washed with water. The organic extract was dried over
1
afford btd-dmpab (0.324 g, 61%) as an orange low-melting solid. H
NMR (500 MHz, CDCl3): δ 8.09 (dd, J = 1.7, 0.7 Hz, 1H, H4), 8.00
(dd, J = 9.2, 0.7 Hz, 1H, H7), 7.87 (dd, J = 9.2, 1.8 Hz, 1H, H6), 7.53
(d, J = 8.8 Hz, 2H, H2′), 7.13 (d, J = 9.0 Hz, 4H, H2″), 7.02 (d, J = 8.8
Hz, 2H, H3′), 6.87 (d, J = 9.0 Hz, 4H, H3″), 3.82 (s, 6H, OMe) ppm.
13C NMR (126 MHz, CDCl3): δ 156.41 (C4″), 155.81 (C3a), 154.12
(C7a), 149.36 (C4′), 142.22 (C5), 140.51 (C1″), 130.67 (C1′), 130.13
(C6), 128.08 (C2′), 127.16 (C2″), 121.38 (C7), 120.16 (C3′), 116.98
(C4), 114.97 (C3″), 55.64 (OMe) ppm. MS (MALDI-TOF) calcd for
C26H21N3O2S ([M]+): m/z 439.14. Found: m/z 439.13. Anal. Calcd
for C26H21N3O2S: C, 71.05; H, 4.82; N, 9.56. Found: C, 71.21; H,
4.69; N, 9.83.
I
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MgSO4, and the solvent was removed under reduced pressure. 1,10-
Phenanthroline-5,6-dione (0.134 g, 0.638 mmol) and EtOH (150 mL)
were then added, and the mixture was heated at reflux overnight. The
reaction mixture was allowed to cool, and the solvent was removed
under reduced pressure. The residue was purified using preparative
column chromatography (SiO2, 20% EtOAc in CHCl3) to afford dppz-
142.15 (C13a), 141.99 (C11), 141.95 (C8b/14a), 141.38 (C8b/14a), 137.34
(C1′), 133.93 (C1/8), 133.91 (C1/8), 133.88 (C3″), 130.32 (C13),
130.22 (C12), 129.50 (C2′), 127.75 (C8a/14b), 127.66 (C8a/14b), 127.09
(C3′), 126.67 (C10), 124.39 (C2/7), 124.36 (C2/7), 123.58 (C2″),
118.91 (CN), 106.64 (C4″) ppm. HRMS (ESI) calcd for C38H22N7
([M + H]+): m/z 576.193. Found: m/z 576.189. Anal. Calcd for
C38H21N7·H2O: C, 76.88; H, 3.91; N, 16.52. Found: C, 76.56; H, 3.87;
N, 16.37.
fac-Chlorotricarbonyl(11-(4-diphenylaminophenyl)dipyrido[3,2-
a:2′,3′-c]phenazine)rhenium(I) ([ReCl(CO)3(dppz-dpab)]). A mixture
of dppz-dpab (0.298 g, 0.568 mmol) and [ReCl(CO)5] (0.272 g, 0.751
mmol) in EtOH (100 mL) was heated at reflux overnight. The
reaction mixture was allowed to cool, and the precipitate was filtered
and washed with EtOH to afford [ReCl(CO)3(dppz-dpab)] (0.429 g,
91%) as a purple solid. 1H NMR (500 MHz, CDCl3): δ 9.81 (m, 2H,
H1,8), 9.45 (m, 2H, H3,6), 8.53 (s, 1H, H10), 8.42 (d, J = 8.9 Hz, 1H,
H13), 8.32 (d, J = 9.0 Hz, 1H, H12), 8.00 (m, 2H, H2,7), 7.76 (d, J = 8.4
Hz, 2H, H2′), 7.34 (t, J = 7.7 Hz, 4H, H3″), 7.25 (d, J = 8.6 Hz, 2H,
H3′), 7.21 (d, J = 7.9 Hz, 4H, H2″), 7.13 (t, J = 7.4 Hz, 2H, H4″) ppm.
13C NMR (126 MHz, CDCl3): δ 196.98 (COeq), 189.44 (COax),
154.36 (C3/6), 154.18 (C3/6), 149.48 (C4a/4b), 149.23 (C4a/4b), 149.17
(C4′), 147.30 (C1″), 144.52 (C11), 143.63 (C9a), 142.41 (C13a), 139.62
(C8b/14a), 138.57 (C8b/14a), 135.89 (C1/8), 135.74 (C1/8), 132.08 (C12),
131.64 (C1′), 130.81 (C8a/14b), 130.71 (C8a/14b), 130.14 (C13), 129.67
(C3″), 128.51 (C2′), 127.01 (C2/7), 126.97 (C2/7), 125.36 (C2″), 125.26
(C10), 124.01 (C4″), 122.97 (C3′) ppm. HRMS (ESI) calcd for
C39H23ClN5NaO3Re ([M + Na]+): m/z 854.094. Found: m/z 854.081.
Anal. Calcd for C39H23ClN5O3Re·H2O: C, 55.15; H, 2.97; N, 8.25.
Found: C, 54.89; H, 3.01; N, 8.23.
1
dpab (0.204 g, 39%) as an orange crystalline solid. H NMR (500
MHz, CDCl3): δ 9.64 (m, 2H, H1,8), 9.28 (m, 2H, H3,6), 8.50 (d, J =
1.9 Hz, 1H, H10), 8.37 (d, J = 8.8 Hz, 1H, H13), 8.20 (dd, J = 8.9, 2.1
Hz, 1H, H12), 7.80 (m, 2H, H2,7), 7.75 (d, J = 8.7 Hz, 2H, H2′), 7.33
(dd, J = 8.5, 7.4 Hz, 4H, H3″), 7.24 (d, J = 8.7 Hz, 2H, H3′), 7.20 (d, J
= 8.6, 1.1 Hz, 4H, H2″), 7.10 (tt, J = 7.3, 1.2 Hz, 2H, H4″) ppm. 13C
NMR (126 MHz, CDCl3): δ 152.62 (C3/6), 152.47 (C3/6), 148.65
(C4′), 148.40 (C4a/4b), 148.00 (C4a/4b), 147.49 (C1″), 143.09 (C9a),
142.93 (C11), 141.95 (C13a), 141.59 (C8b/14a), 140.72 (C8b/14a), 133.97
(C1/8), 133.89 (C1/8), 132.64 (C1′), 130.48 (C12), 129.90 (C13),
129.60 (C3″), 128.41 (C2′), 127.89 (C8a/14b), 127.80 (C8a/14b), 125.46
(C10), 125.13 (C2″), 124.32 (C2/7), 124.30 (C2/7), 123.71 (C4″),
123.36 (C3′) ppm. HRMS (ESI) calcd for C36H24N5 ([M + H]+): m/z
526.203. Found: m/z 526.204. Anal. Calcd for C36H23N5·CHCl3: C,
68.90; H, 3.75; N, 10.86. Found: C, 69.01; H, 3.71; N, 10.93.
11-(4-Di(4-methoxyphenyl)aminophenyl)dipyrido[3,2-a:2′,3′-c]-
phenazine (dppz-dmpab). A mixture of btd-dmpab (0.720 g, 1.64
mmol) and LiAlH4 (0.621 g, 16.4 mmol) in dry THF (20 mL) was
stirred at rt overnight under an argon atmosphere. The reaction
mixture was cooled to 0 °C using an ice bath and excess LiAlH4
quenched by dropwise addition of water (0.5 mL), aqueous NaOH
(0.5 mL, 15%), and more water (1.5 mL). Solids were removed by
filtration through Celite (CHCl3), and the product was extracted into
CHCl3 and washed with water. The organic extract was dried over
MgSO4, and the solvent was removed under reduced pressure. 1,10-
Phenanthroline-5,6-dione (0.346 g, 1.65 mmol) and EtOH (200 mL)
were added, and the mixture was heated at reflux overnight. The
reaction mixture was allowed to cool to rt, and the resultant precipitate
was filtered and washed with EtOH. The residue was purified using
preparative column chromatography (basic Al2O3, 1% MeOH in
CHCl3) to afford dppz-dmpab (0.309 g, 32%) as a red solid. 1H NMR
(500 MHz, CDCl3): δ 9.56 (m, 2H, H1,8), 9.23 (m, 2H, H3,6), 8.40 (d,
J = 1.9 Hz, 1H, H10), 8.27 (d, J = 8.9 Hz, 1H, H13), 8.14 (dd, J = 8.9,
2.0 Hz, 1H, H12), 7.74 (m, 2H, H2,7), 7.68 (d, J = 8.7 Hz, 2H, H2′),
7.16 (d, J = 8.9 Hz, 4H, H2″), 7.07 (d, J = 8.7 Hz, 2H, H3′), 6.89 (d, J =
8.9 Hz, 4H, H3″), 3.83 (s, 6H, OMe) ppm. 13C NMR (126 MHz,
CDCl3): δ 156.47 (C4″), 152.56 (C3/6), 152.39 (C3/6), 149.49 (C4′),
148.50 (C4a/4b), 148.29 (C4a/4b), 143.06 (C9a), 142.97 (C11), 141.77
(C13a), 141.45 (C8b/14a), 140.48 (C1″), 140.46 (C8b/14a), 133.78 (C1/8),
133.66 (C1/8), 130.51 (C1′), 130.34 (C12), 129.73 (C13), 128.18 (C2′),
127.80 (C8a/14b), 127.69 (C8a/14b), 127.23 (C2″), 124.84 (C10), 124.18
(C2/7), 124.15 (C2/7), 120.17 (C3′), 115.00 (C3″), 55.66 (OMe) ppm.
HRMS (ESI) calcd for C38H28N5O2 ([M + H]+): m/z 586.224.
Found: m/z 586.219. Anal. Calcd for C38H27N5O2·CHCl3: C, 66.44;
H, 4.00; N, 9.93. Found: C, 66.45; H, 3.97; N, 10.01.
fac-Chlorotricarbonyl(11-(4-di(4-methoxyphenyl)aminophenyl)-
dipyrido[3,2-a:2′,3′-c]phenazine)rhenium(I) ([ReCl(CO)3(dppz-
dmpab)]). A mixture of dppz-dmpab (0.068 g, 0.116 mmol) and
[ReCl(CO)5] (0.049 g, 0.134 mmol) in EtOH (100 mL) was heated at
reflux overnight. The reaction was allowed to cool to rt, and the
resultant precipitate was filtered and washed with EtOH to afford
1
[ReCl(CO)3(dppz-dmpab)] (0.080 g, 78%) as a dark red solid. H
NMR (CDCl3): δ 9.78 (m, 2H, H1,8), 9.44 (m, 2H, H3,6), 8.47 (d, J =
1.5 Hz, 1H, H10), 8.37 (d, J = 8.9 Hz, 1H, H13), 8.29 (dd, J = 9.0, 1.9
Hz, 1H, H12), 7.98 (m, 2H, H2,7), 7.71 (d, J = 8.8 Hz, 2H, H2′), 7.17
(d, J = 8.9 Hz, 4H, H2″), 7.08 (d, J = 8.0 Hz, 2H, H3′), 6.91 (d, J = 9.0
Hz, 4H, H3″), 3.84 (s, 6H, OMe) ppm. 13C NMR (126 MHz, CDCl3):
δ 196.85 (COeq), 189.32 (COax), 156.55 (C4″), 154.15 (C3/6), 153.93
(C3/6), 149.88 (C4′), 149.30 (C4a/4b), 149.00 (C4a/4b), 144.54 (C11),
143.59 (C9a), 142.22 (C13a), 140.05 (C1″), 139.43 (C8b/14a), 138.22
(C8b/14a), 135.71 (C1/8), 135.53 (C1/8), 131.91 (C12), 130.70 (C8a/14b),
130.58 (C8a/14b), 129.88 (C13), 129.41 (C1′), 128.20 (C2′), 127.30
(C2″), 126.81 (C2/7), 126.76 (C2/7), 124.49 (C10), 119.70 (C3′),
114.92 (C3″), 55.53 (OMe) ppm. HRMS (ESI) calcd for
C41H29N5O6Re ([M-Cl+H2O]+): m/z 874.167. Found: m/z =
874.171. Anal. Calcd for C41H27N5ClO5·0.5H2O: C, 54.69; H, 3.13;
N, 7.78. Found: C, 54.59; H, 3.16; N, 7.95.
11-(4-Di(4-cyanophenyl)aminophenyl)dipyrido[3,2-a:2′,3′-c]-
phenazine (dppz-dcpab). A mixture of btd-dcpab (0.683 g, 1.59
mmol), CoCl2 (0.052 g, 0.401 mmol), and NaBH4 (0.538 g, 14.2
mmol) in EtOH (200 mL) was heated at reflux overnight under an
argon atmosphere. The solvent was removed under reduced pressure,
and the resultant precipitate was extracted into CHCl3 and washed
with aqueous NH4Cl (satd) and water. The organic extract was dried
over MgSO4, and the solvent was removed under reduced pressure.
1,10-Phenanthroline-5,6-dione (0.299 g, 1.42 mmol) and EtOH (200
mL) were added, and the mixture was heated at reflux overnight. The
reaction mixture was allowed to cool to rt, and the resultant precipitate
was filtered and washed with EtOH to afford dppz-dcpab (0.337 g,
fac-Chlorotricarbonyl(11-(4-di(4-cyanophenyl)aminophenyl)-
dipyrido[3,2-a:2′,3′-c]phenazine)rhenium(I) ([ReCl(CO)3(dppz-
dcpab)]). A mixture of dppz-dcpab (0.050 g, 0.087 mmol) and
[ReCl(CO)5] (0.038 g, 0.105 mmol) in EtOH (100 mL) was heated at
reflux overnight. The reaction was allowed to cool to rt, and the
resultant precipitate was filtered and washed with EtOH to afford
1
[ReCl(CO)3(dppz-dcpab)] (0.039 g, 45%) as a dark yellow solid. H
NMR (500 MHz, CDCl3): δ 9.86 (m, 2H, H1,8), 9.47 (m, 2H, H3,6),
8.62 (d, J = 2.0 Hz, 1H, H10), 8.52 (d, J = 8.9 Hz, 1H, H13), 8.32 (dd, J
= 8.9, 2.1 Hz, 1H, H12), 8.04 (m, 2H, H2,7), 7.90 (d, J = 8.6 Hz, 2H,
H2′), 7.61 (d, J = 8.9 Hz, 4H, H3″), 7.34 (d, J = 8.6 Hz, 2H, H3′), 7.23
(d, J = 8.9 Hz, 4H, H2″) ppm. 13C NMR (126 MHz, CDCl3): δ 196.90
(COeq), 189.38 (COax), 154.58 (C3/6), 154.47 (C3/6), 150.07 (C1″),
149.64 (C4a/4b), 149.46 (C4a/4b), 146.15 (C4′), 143.64 (C9a), 143.38
(C13a), 142.60 (C11), 139.93 (C8b/14a), 139.25 (C8b/14a), 136.53 (C1/8),
135.87 (C1/8,1′), 133.93 (C3″), 131.88 (C13), 130.72 (C12), 130.64
(C8a/14b), 130.58 (C8a/14b), 129.58 (C2′), 127.11 (C2/7), 127.08 (C2/7),
126.99 (C3′), 126.66 (C10), 123.74 (C2″), 118.84 (CN), 106.87 (C4″)
ppm. HRMS (ESI) calcd for C41H23N7O4Re ([M-Cl+H2O]+): m/z
1
41%) as a red solid. H NMR (500 MHz, CDCl3): δ 9.67 (m, 2H,
H1,8), 9.29 (m, 2H, H3,6), 8.57 (dd, J = 2.0, 0.4 Hz, 1H, H10), 8.45 (dd,
J = 8.9, 0.4 Hz, 1H, H13), 8.20 (dd, J = 8.9, 2.1 Hz, 1H, H12), 7.89 (d, J
= 8.7 Hz, 2H, H2′), 7.82 (m, 2H, H2,7), 7.60 (d, J = 8.9 Hz, 4H, H3″),
7.32 (d, J = 8.6 Hz, 2H, H3′), 7.22 (d, J = 8.9 Hz, 4H, H2″) ppm. 13
C
NMR (126 MHz, CDCl3): δ 152.90 (C3/6), 152.82 (C3/6), 150.15
(C1″), 148.71 (C4a/4b), 148.59 (C4a/4b), 145.61 (C4′), 142.86 (C9a),
J
DOI: 10.1021/acs.inorgchem.6b01039
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
864.136. Found: m/z 864.134. Anal. Calcd for C41H21ClN7O3Re: C,
Steady-state absorption spectra were recorded as solutions on a
PerkinElmer Lambda 950 UV/vis/NIR Spectrometer. Extinction
coefficients were obtained from serial dilution measurements.
Steady-state emission spectra were recorded on a Princeton
Instruments SP2150i spectrograph with a 300 grooves mm−1 grating
and a Pixis 100B CCD and controlled using Winspec/32 v2.6.80
software using 355 nm excitation from a Cobalt Zouk solid state diode
laser.
55.88; H, 2.40; N, 11.13. Found: C, 55.72; H, 2.30; N, 11.35.
1
Physical Measurements. H NMR spectra were recorded at 500
MHz and 13C at 126 MHz on a Varian 500AR spectrometer. All
samples were recorded at 25 °C in 5 mm diameter tubes. Chemical
shifts were referenced internally to residual nonperdeuterated solvent
using δ values as reported by Gottlieb et al.64 Coupling constants are
rounded to the nearest 0.1 Hz. Assignment of signals is assisted
1
through the use of 2D NMR techniques (COSY, NOESY, H−13C
FT-Raman spectra were collected on powder samples using a
Bruker IFS-55 interferometer with an FRA/106S attachment. The
excitation source was a Nd:YAG laser with an excitation wavelength of
1064 nm. Raman photons were detected with a liquid nitrogen-cooled
D418T germanium diode. Spectra were measured with 256 scans, with
a laser power of 100 mW and spectral resolution of 4 cm−1.
Resonance Raman spectra were recorded using a previously
described setup.69−72 Excitation wavelengths 351, 406, and 413 nm
were provided by an I-302 krypton ion laser (Coherent Inc.), 457, 488,
and 514 nm were provided by an Innova Sabre argon ion laser
(Coherent Inc.), and 448 and 532 nm were provided by crystal diode
lasers (CrystaLaser). Concentrations were typically 1 mM in CH2Cl2.
Transient absorption and emission spectra were recorded on
CH2Cl2 solutions with concentrations typically 1 × 10−5 M, which
were bubbled with argon for 10 min prior to measurement. Transients
were acquired using an LP920 K TA systems (Edinburgh Instru-
ments), with excitation at 355 nm from pulsed third-harmonic
radiation from a Brilliant (Quantel) Nd:YAG laser at 1 Hz, and a
Xe900 450 W xenon arc lamp controlled by an XP920 pulser as the
probe source in TA mode. Photons were dispersed using a TMS300-A
Czerney-Turner monochromator with 1800 grooves mm−1 grating,
recorded on a R928 (Hamamatsu) photomultiplier, and transcribed on
a TDS3012C (Tektronix) digital oscilloscope.
Calculations were performed using the Gaussian09 package.
Geometry optimization and harmonic vibrational frequency calcu-
lations were performed in vacuo using density functional theory
(DFT) employing the B3LYP functional.73−75 A LANL2DZ effective
core potential and associated basis set was used for the rhenium
atoms,76−78 while the 6-31G(d) basis set was used for all other
atoms.79,80 Time-dependent density functional theory (TD-DFT)
calculations using the CAM-B3LYP functional81were run both in
vacuo and with a SCRF solvent field.82−85 Calculated vibrational
spectra were generated using GaussSum v2.2.5 software86 and scaled
to give the lowest value for the mean absolute deviation for band
position from experimental data, with scale factors typically around
0.975. Raman activities calculated by G09 were converted to Raman
scattering cross sections using an intensity correction.87 Vibrational
modes were illustrated using Molden.88
HSQC, and 1H−13C HMBC), recorded on a Varian 500AR
spectrometer using standard pulse sequences. HR-ESI-MS was
performed on a Bruker MicrOTOF-Q mass spectrometer operating
in positive mode. Values are quoted as m/z ratio, with an instrumental
uncertainty of m/z 0.003. MALDI-TOF MS was performed on an
Applied Biosystems 4800 Tandem TOF mass spectrometer calibrated
externally over the range m/z 300−3500. The analyte was
mechanically blended with the matrix (TCNQ) using a mixer mill
and applied as a suspension to the sample plate. Values are reported as
m/z values, with an instrumental uncertainty of m/z 0.08. Analysis
of elemental composition was made by the Campbell Microanalytical
Laboratory at the University of Otago using a Carlo Erba 1108 CHNS
combustion analyzer. The estimated error is the measurements in
0.4%.
For X-ray crystallography, single crystals were attached with
Paratone N to a fiber loop supported in a copper mounting pin and
then quenched in a cold nitrogen stream. Data were collected at 100 K
using Cu Kα radiation (microsource, mirror monochromated) using
an Agilent Supernova diffractometer with an Atlas detector. Data
processing was undertaken with CrysAlisPro.65 A multiscan or face-
indexed absorption correction was applied to the data. Structures were
solved by direct methods with SHELXS-97 and extended and refined
with SHELXL-97 using the X-Seed interface.66,67 The non-hydrogen
atoms in the asymmetric unit were modeled with anisotropic
displacement parameters, and a riding atom model with group
displacement parameters was used for the hydrogen atoms. X-ray
crystallographic data is available in CIF format (Supporting
Information). CCDC 1435218 and 1475303 contain the supple-
mentary crystallographic data for this article. These data can be
obtained free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif.
Crystal Data for dppz-dmpab·CHCl3. C39H28Cl3N5O2, M = 705.01,
orange plate, 0.62 × 0.11 × 0.03 mm3, triclinic, a = 5.9904(1) Å, b =
10.3974(3) Å, c = 27.3036(6) Å, α = 92.437(2)°, β = 96.088(2)°, γ =
100.467(2)°, V = 1659.54(7) Å3, space group P-1 (#2), Z = 2, μ(Cu
Kα) = 2.858 mm−1, 2θmax = 148.46°, 18 128 reflections measured,
6559 independent reflections (Rint = 0.0460). The final R1(F) = 0.0455
(I > 2σ(I)); 0.0491 (all data). The final wR2(F2) = 0.1265 (I > 2σ(I));
0.1311 (all data). GoF = 1.045. CCDC 1475303.
Crystal Data for 5-(4-trimethylsilylphenyl)benzo[c][1,2,5]-
thiadiazole. C15H16N2SSi, M = 284.45, colorless plate, 0.52 × 0.22
× 0.02 mm3, triclinic, a = 7.4105(2) Å, b = 12.6297(4) Å, c =
17.1588(7) Å, α = 110.283(3)°, β = 94.904(3)°, γ = 99.001(3)°, V =
1470.78(9) Å3, space group P-1 (#2), Z = 4, μ(Cu Kα) = 2.625
mm−1,2θmax = 153.18°, 15 320 reflections measured, 6106 independent
reflections (Rint = 0.0366). The final R1(F) = 0.0440 (I > 2σ(I));
0.0484 (all data). The final wR2(F2) = 0.1214 (I > 2σ(I)); 0.1269 (all
data). GoF = 1.050. CCDC 1435218.
The electrochemical cell for cyclic voltammetry was made up of a 1
mm diameter platinum rod working electrode embedded in a KeL-F
cylinder with a platinum auxiliary electrode and a Ag/AgCl reference
electrode. The potential of the cell was controlled by an ADI Powerlab
4SP potentiostat. Solutions were typically about 10−3 M in CH2Cl2
with 0.1 M tetrabutylammonium hexafluorophosphate (NBu4PF6) as a
supporting electrolyte and were purged with argon for approximately 5
min prior to measurement. The scanning rate was 100 mV s−1, and the
cyclic voltammograms were calibrated against the decamethylferroce-
nium/decamethylferrocene (Fc*+/Fc*) couple (−0.012 V in CH2Cl2)
and are reported relative to the saturated calomel electrode (SCE) for
comparison with other data by subtracting 0.045 V.68
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.6b01039.
Tabulated 1H NMR data, experimental vs calculated FT-
Raman spectra, tabulated mean absolute deviations,
tabulated Raman vibrational modes, tabulated electro-
chemical data, DFT-predicted frontier molecular orbitals,
tabulated TD-DFT data, calculated emission energies,
resonance-enhanced vibrational mode representations,
emission spectra, and Lippert−Mataga plots, and crystal
structure plot (PDF)
Crystallographic data in CIF format (CCDC 1435218,
1475303) (CIF)
AUTHOR INFORMATION
Corresponding Authors
*E-mail: nlucas@chemistry.otago.ac.nz. Phone: +64 3 479
5377. Fax: +64 3 479 7906.
■
K
DOI: 10.1021/acs.inorgchem.6b01039
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
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Z.-i. Org. Lett. 2006, 8, 717.
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Eur. J. 2009, 15, 10603.
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*E-mail: kgordon@chemistry.otago.ac.nz. Phone: +64 3 479
7599. Fax: +64 3 479 7906.
Author Contributions
†C.B.L. and H.v.d.S. contributed equally.
(33) van der Salm, H.; Larsen, C. B.; McLay, J. R. W.; Huff, G. S.;
Gordon, K. C. Inorg. Chim. Acta 2015, 428, 1.
Funding
The authors would like to acknowledge the MacDiarmid
Institute for Advanced Materials and Nanotechnology for
funding.
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Notes
The authors declare no competing financial interest.
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