Directing energy transfer in Pt(bodipy)(mercaptopyrene) dyads

Article abstract of DOI:10.1039/c9dt01737b

We report on the photophysical properties of three dyads that combine a 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (bodipy, BDP) and a mercaptopyrene (SPyr) dye ligand at a Pt(PEt₃)₂ fragment. σ-Bonding of the dyes to the Pt ion promotes intersystem crossing (ISC) via the external heavy atom effect. The coupling of efficient ISC with charge-transfer from the electron-rich mercaptopyrene to the electron-accepting BDP ligand (PB-CT) gives rise to a multitude of (potentially) emissive states. This culminates in the presence of four different emissions for the mono- and dinuclear complexes BPtSPyr and BPtSPyrSPtB with an unsubstituted BDP ligand and either a terminal 1-mercaptopyrene or a bridging pyrene-1,6-dithiolate ligand. Thus, in fluid solution, near IR emission at 724 nm from the ³PB-CT state is observed with a quantum yield of up to 15%. Excitation into the BDP-based ¹ππ? or the pyrene-based ¹ππ? band additionally trigger fluorescence and phosphorescence emissions from the BDP-centred ¹ππ? and ³ππ? states. In frozen solution, at 77 K, phosphorescence from the pyrene ligand becomes the prominent emission channel, while PB-CT emission is absent. Alkylation of the BDP ligand in KBPtSPyr funnels all excitation energy into fluorescence and phosphorescence emissions from the KBDP ligand. The assignments of the various excited states and the deactivation cascades were probed by absorption and emission spectroscopy, transient absorption spectroscopy, electrochemical and UV/Vis/NIR spectroelectrochemical measurements, and by quantum chemical calculations. Our conclusions are further corroborated with the aid of suitable reference compounds comprising of just one chromophore. All dyads are triplet sensitizers and are able to generate singlet oxygen.

Full text of DOI:10.1039/c9dt01737b

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Directing Energy Transfer in Pt(bodipy)(mercaptopyrene)
Dyads
†
†
†
‡
Peter Irmler, Franciska S. Gogesch, André Mang, Michael Bodensteiner, Christopher B.
§
§
,†
Larsen, Oliver S. Wenger, and Rainer F. Winter*
†Fachbereich Chemie, Universitꢀt Konstanz, Universitꢀtsstraße 10, D-78457 Konstanz, Germany
‡Fakultät für Chemie und Pharmazie, Universität Regensburg, Universitätsstraße 31, D-93053
Regensburg, Germany
§Department of Chemistry, University of Basel, St.-Johanns-Ring 19, CH-4056 Basel,
Switzerland.
NIR Phosphorescence, Panchromatic Absorber, Platinum, Singlet Oxygen, Bodipy, Pyrene, Dyad.
Abstract: We report on the photophysical properties of three dyads that combine a 4,4-difluoro-
4
-bora-3a,4a-diaza-s-indacene (bodipy, BDP) and a mercaptopyrene (SPyr) dye ligand at a
Pt(PEt fragment. -Bonding of the dyes to the Pt ion promotes intersystem crossing (ISC) via
3 2
)
the external heavy atom effect. The coupling of efficient ISC with charge-transfer from the
electron-rich mercaptopyrene to the electron-accepting BDP ligand (PB-CT) gives rise to a
multitude of (potentially) emissive states. This culminates in the presence of four different
emissions for the mono- and dinuclear complexes BPtSPyr and BPtSPyrSPtB with an
unsubstituted BDP ligand and either a terminal 1-mercaptopyrene or a bridging pyrene-1,6-
3
dithiolate ligand. Thus, in fluid solution, near IR emission at 724 nm from the PB-CT state is
1

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1
observed with a quantum yield of up to 15%. Excitation into the BDP-based * or the pyrene-
1
based * band additionally trigger fluorescence and phosphorescence emissions from the BDP-
1
3
centred * and * states. In frozen solution, at 77 K, phosphorescence from the pyrene ligand
becomes the prominent emission channel, while PB-CT emission is absent. Alkylation of the BDP
ligand in KBPtSPyr funnels all excitation energy into fluorescence and phosphorescence
emissions from the KBDP ligand. The assignments of the various excited states and the
deactivation cascades were probed by absorption and emission spectroscopy, transient absorption
spectroscopy, electrochemical and UV/Vis/NIR spectroelectrochemical measurements, and by
quantum chemical calculations. Our conclusions are further corroborated with the aid of suitable
reference compounds comprising of just one chromophore. All dyads are triplet sensitizers and are
able to generate singlet oxygen.
Introduction
Luminescent molecules capable of displaying multiple emissions are of great interest, e. g. as one-
component white-light emitters,1-4 and, in the case of dual fluorescence and phosphorescence
emitters, as one-component sensors for triplet molecules. The latter quench selectively the longer-
lived phosphorescence emission, while leaving the fluorescence unperturbed.5-9 An observation of
multiple emissions from the same compound obviously necessitates the presence and competitive
population of two or more emissive states. One possible strategy to achieve dual fluorescence and
phosphorescence emissions is to match the intersystem crossing (ISC) rate constant kISC to the
fluorescence time constant k
. According to El-Sayed’s rule,10, 11 this can be achieved by
f
capitalizing on the large spin orbit coupling constant of a proximal heavy atom12-15 or by admixing
sufficient charge-transfer character to the underlying absorption. Both effects are capable of
2

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changing the angular momentum of the excited electron.11, 16-18 Alternatively, multiple emissions
are also known for dyads with different electronically decoupled or weakly coupled
chromophores.19, 20 Examples of such a multichromophore approach with an ensuing competition
between dye-based fluorescence and MLCT- or spin-orbit coupling- (SOC-) triggered
phosphorescence emissions come from iridium, ruthenium, rhenium or platinum complexes that
contain dye-modified cyclometalating,21, 22 2,2’-bipyridine or phenanthroline,23-25 or alkynyl
ligands.18, 26-28 Phosphorescence quantum yields of such dyads are, however, usually lower than
2
4
1
0%. Particularly impressive are Ir(ppy) (Q) complexes with a 2-hydroxyquinoline ligand Q and
2
two mutually orthogonally aligned, cyclometalated phenylpyridine (ppy) ligands, which show
quadruple emissions comprising of ligand-localized fluorescence from the 1nπ* and
3
phosphorescence from MLCT (MLCT = metal-ligand charge-transfer) states of the different
ligands as a consequence of inefficient Förster resonance energy transfer.29
Platination of a dye, i. e. metal attachment via a direct Pt-carbon -bond, has been shown to be a
viable means of accelerating ISC via the remote heavy atom effect,15, 30, 31 in particular when using
3
2
8, 9, 33
compact thioxanthone or 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BDP) dyes
(note that
-6
the remote heavy-atom effect scales with r , where r denotes the distance between the heavy atom
1
2
and the centre of gravity of the dye). In this manner, quantum yields of up to 41% for dye-based
phosphorescence emission in fluid solution at r.t. could be achieved. Complexes trans-
X(PR
3
2
) Pt(BDP) (R = Ph, Et) were also found to be efficient sensors for the detection of triplet
molecules and efficient photosensitizers for the production of singlet oxygen.8, 9, 33, 34 An example
of a panchromatically absorbing dyad of such a complex with an ethynylnaphthalimide (NI) dye
as the anionic ligand X was also reported. The latter shows complete energy transfer from the
1
1
locally excited NI or NIBDP charge-transfer states to the lower-lying excited states of the BDP
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ligand.34 One drawback of this system, however, was its only limited stability under laser
irradiation due to the mutual trans-arrangement of two carbyl ligands, which both exert a strong
9

-trans influence. We therefore sought to improve on this general architecture by attaching the
second dye via a more robust Pt-thiolate bond. In doing so, we have chosen mercaptopyrenes as
the second dye.
Pyrenes usually absorb in a spectral region where BDP dyes are almost transparent. Most pyrenes
are highly fluorescent and show rather inefficient intersystem crossing with negligible
phosphorescence quantum yields.13, 35 The latter is a result of their long triplet-state lifetimes,
which increases the probability of excited state decay by radiationless deactivation channels.13
Pyrenes were already introduced into the coordination sphere of platinum ions, either by direct
platination, or as ethynylpyrene ligands. In both cases, phosphorescence from the pyrene ligand
could be obtained due to the proximity of the pyrene dye to the platinum ion, however with only
modest quantum yields, even in frozen, glassy matrices.15, 18, 21, 31, 36
We note in this vein that pyrene-BDP dyads with a direct connection between the two different
dyes have been known for more than ten years.37, 38 More recently it was observed that these
compounds, due to a near orthogonal arrangement of the pyrene donor and the BDP acceptor, are
capable of forming a charge-separated (CS) excited state, which promotes efficient ISC to the
3
corresponding CS state. Such dyads are characterized by a strong dependence of their absorption
and emission properties and their associated emission quantum yields on solvent polarity. They
are capable of photogenerating singlet oxygen, sometimes with high quantum yields  

particularly in polar solvents. These CS states are usually non-emissive though.39-41
3
4

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Very recently we communicated our results on the complex BPtSPyr, which combines a meso-
entity.42
platinated BDP acceptor and a mercaptopyrene donor ligand at a common trans-Pt(PEt
)
3 2
The most remarkable features of BPtSPyr are its multiple emissions and the dependence of its
emission profile on the excitation wavelength, the solvent polarity, and the temperature. This
behaviour indicates the presence of several competing pathways for the decay of the individual
3
excited states to the ground state. Most prominent is the long-lived emission from the CS state in
the near infrared (NIR), which becomes observable due to an SOC-accelerated ISC rate for
radiative decay from the corresponding T
1
to the S state. We have now further elaborated on this
0
3
3
complex by selectively manipulating the relative energies of the CS and the BDP excited states.
This was done through either alkylation of the BDP ligand or through interlinking two trans-
Pt(PEt
3
2
) (BDP) fragments with a bridging 1,6-pyrenedithiolate ligand. These alterations have the
effect of (re)directing the deactivation cascade to either BDP-localized or CS excited states.
Suitable reference compounds, which contain only one of these two chromophores, were also
included to aid in the understanding of their complex emissive behaviour. The results of this study
are described herein.
Results and discussion
Synthesis. The general synthesis route is depicted in Scheme 1 (for detailed syntheses and NMR
characterization and a brief discussion of the NMR spectroscopic trends see the Supporting
Information (SI) and Figures S1-S21 as well as Table S1 therein). The meso-Pt-BDP complexes
were synthesized according to our previously reported protocol by oxidative addition of
0
8
corresponding 8-chloro-BDP dye to the Pt (ethene)(PEt
3
) precursor. The latter is generated in
2
situ by thermally induced reductive elimination of ethane from the corresponding bis(ethyl)
complex.43 In the case of the peralkylated 8-chloro-krypto-BDP ligand KBDP, an elevated
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temperature and longer reaction time were required in order for the oxidative addition to proceed,
most probably as the result of a larger steric hindrance. Subsequent abstraction of the chlorido
ligand with AgOTf followed by addition of the triflato complex to a solution of deprotonated 1-
mercaptopyrene, or respectively, pyrene-1,6-dithiole, provided the target dyads in good to
moderate yields.
Scheme 1. Exemplary synthesis of KBPtSPyr and the structures of all relevant compounds.
The general pattern of resonance signals and, in particular, the observation of Pt-satellites for
the appropriate proton and carbon resonances in the NMR spectra confirm the constitution of the
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complexes and the presence of the Pt-C and the Pt-S bonds to the bodipy and mercaptopyrene
ligands. Single crystals of KBPtSPyr and its precursor KBPtI for molecular structure
determination by X-ray diffraction analysis were obtained by slow diffusion of n-pentane into
saturated solutions of the complexes in CH
2
Cl . Relevant crystal and refinement data are collected
2
in Table S2 of the Supporting Information. Figure 1 depicts their molecular structures (only one
of the two independent molecules in the unit cell of KBPtSPyr is shown), while Table S3 of the
Supporting Information summarizes the most relevant bond parameters together with those of
BPtI.9
Figure 1. ORTEPs of a) KBPtSPyr (only one of the two independent molecules of the units cell
is shown) and of b) KBPtI. Ellipsoids are drawn on a 40 % probability level. Solvent molecules
and protons are omitted for reasons of clarity.
In all complexes, the Pt(II) ion adopts the expected square planar coordination geometry with all
cis-bond angles close to 90° and the trans-P-Pt-P and I-Pt-C angles close to 180°, whereas the C8-
Pt1-S1 angle of 172.7(3)° or 167.9(3)° in KBPtSPyr shows larger deviations from ideality.
Substitution of the simple BDP ligand of BPtI by peralkylated KBDP in KBPtI lengthens the Pt-
C
bodipy and the Pt-I bonds by ca. 0.02 Å, the former probably as the result of increased steric
interactions between the alkyl substituents of the PEt and the KBDP ligands, and the latter as the
3
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9
result of the increased -trans-influence of the KBDP ligand. Substitution of the iodo by the 1-
mercaptopyrene ligand lengthens the Pt1-C8 bond further by 0.03 Å, which is expected on the
basis of the -accepting character of an S donor ligand and the associated, larger -trans-influence.
The BDP chromophore is aligned orthogonally to the Pt coordination plane with an interplanar
angle of 87.0° to 90°. In contrast, the pyrenyl substituent of KBPtSPyr forms an interplanar angle
of 70.0° or 73.2° with the Pt coordination plane, such that the interplanar angle between the two
chromophores is 19.7° or 18.2° for the two independent molecules of the unit cell. The centroids
of the pyrene and BDP dye ligands are 9.96 or 9.80 Å apart. Several intermolecular contacts are
found in the crystal packing of KBPtSPyr. Details are provided in the Supporting Information.
Photophysical properties.
A common feature of all BDP-Pt-SPyr dyads of this study is that their HOMO is centred on the
mercapto-substituted pyrene ligand while their LUMO is essentially bodipy-based. DFT-computed
contour diagrams of the relevant MOs of complexes BPtSPyr, BPtSPyrSPtB and KBPtSPyr and
the spin densities for their oxidized and reduced forms are displayed as Figures S23 to S28 of the
Supporting Information; the respective contributions and spin densities of the individual fragments
according to Mulliken analysis are collected in Tables S4 to S11. According to these calculations,
both frontier MOs (HOMO, LUMO) of the dyads receive only minor contributions (3% - 7%)
from the Pt(PEt
3
2
) fragment(s), such that its (their) influence is of a primarily inductive nature.
Substitution of the BDP by the more electron donating, peralkylated KBDP and of the terminal
mercaptopyrene by the pyrene-1,6-dithiolate ligand induce sizable energy shifts of the LUMO or
the HOMO, respectively, as revealed by our electrochemical measurements. Representative
voltammograms can be found as Figures S29 to S33 of the Supporting Information, while all
pertinent data are collected in Table S12 therein. In brief, substitution of the BDP by the KBDP
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ligand shifts the half-wave potential for the bodipy-centred reduction negative (cathodically) by
40 mV, while causing an only minor cathodic shift of 50 mV of the pyrene-based oxidation. Vice
3
versa, comparing BPtSPyr with BPtSPyrSPtB reveals a 190 mV cathodic shift of the oxidation
-
potential, resulting from the attachment of a second S-Pt(PEt
3
2
) BDP-type donor, while leaving
the potential of the BDP-based reduction essentially unaltered.
Figure 2 depicts the UV/Vis spectra of all dyad molecules along with those of suitable reference
compounds, which contain only the Pt-BDP or the mercatopyrene moiety. Tabulated data and the
corresponding TD-DFT-derived band assignments can be found in Table 1.
Figure 2. UV/Vis spectra of a) BPtSPyr, BPtSPyrSPtB, BPtI and MesPtSPyr, and b) of
KBPtSPyr, KBPtI, BPtSPyr and MesPtSPyr in toluene.
Complexes BPtSPyr, BPtSPyrSPtB and BPtI with the unsubstituted BDP dye as a -bonded
1
ligand exhibit the characteristic BDP-based * absorption band at 470 nm to 490 nm with an
3
-1
-1
3
-1
-1
extinction coefficient  of ca. 5010 M cm for the mononuclear, and of 9010 M cm for
the dinuclear complex with twice the number of BDP chromophores. The electron-donating
Pt(PEt
3
2
) -BDP moieties shift the pyrene-based * absorption bands bathochromically to 375 -
4
50 nm with a main peak at 418 nm for BPtSPyr. They are even more red-shifted in
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BPtSPyrSPtB, where they overlap with the BDP absorption. The even lower energy of the main
mercaptopyrene * absorption at max = 431 nm in the mesityl complex is consistent with this
trend and relates to the larger donor capacity of the mesityl ligand. The spectra of BPtSPyr and
BPtSPyrSPtB also feature a broad, solvatochromic (see Figure S34 of the Supporting
Information) SPyr-to-BDP charge transfer (PB-CT) band, which is located at ca. 500 nm in toluene
solution for BPtSPyr.42 Attachment of a dinegatively charged bridging dithiolate donor and
doubling the number of the bodipy acceptors has the effect of shifting the PB-CT band further red
to 550 nm and increasing its extinction coefficient (Table 1). The presence of the solvatochromic
PB-CT band provides first evidence for optical charge transfer on excitation to generate a charge-
separated excited state.
1
In the complexes KBPtSPyr and KBPtI with the peralkylated KBDP ligand, the BDP *
3
-1
absorption band is red-shifted to 500 nm, and its extinction coefficient is increased to 7010 M
-1
cm . In agreement with the better electron donating properties of the more electron-rich KBDP
-1
ligand, the SPyr-based absorption in KBPtSPyr is red-shifted by 5 nm (283 cm ) with respect to
BPtSPyr, but still blue of that in MesPtSPyr. More importantly however, KBPtSPyr completely
lacks the PB-CT absorption. TD-DFT calculations suggest that such a transition should also be
present in KBPtSPyr, but is associated with a small oscillator strength and blue shifted by 73 nm
-1
(
2600 cm ) compared to BPtSPyr (see Figure S25 of the Supporting Information), such that it
1
overlaps with the much more intense KBDP * absorption. The blue-shift and the lower
oscillator strength of the PB-CT band are immediate consequences of the inferior acceptor
properties of the peralkylated BDP ligand. The latter notion is also supported by our
electrochemistry data (vide supra). Neither of the two major absorption bands of KBPtSPyr shows
a significant dependence on solvent polarity as is evident from Figure S35 of the Supporting
1
0

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1
Information. Importantly, the clear separation of the SPyr and the BDP * transitions in the
present dyad systems suggests that the two chromophores are electronically decoupled from each
other in the ground state.
Table 1. Absorption data for BPtI, KBPtI, MesPtSPyr, BPtSPyr, BPtSPyrSPtB, and
KBPtSPyr as well as TD-DFT-calculated band energies and assignments
absorption data
TD-DFT calculationsa
max (εc)
b
λb
fd
m.c.e
assignment
solvent
λ
BPtI from ref. 9
CH
2
Cl
2
322 (12.5), 352 (4.3), 471 (52.6)
295
0.17
0.07
0.39
H−6 → L (90%)
H−5 → L (81%)
H → L (97%)
ππ* BDP
3
3
09
92
3 2
Pt(PEt ) to BDP CT
ππ* BDP
KBPtI
toluene
THF
353 (12), 502 (80)
351 (9), 500 (74)
347 (10), 498 (72)
n.d.
n.d.
n.d.
n.d.
acetone
MesPtSPyr from ref. 42
n.d. n.d.
toluene
THF
307 (14), 431 (25)
306 (12), 431 (22)
430 (23)
n.d.
n.d.
acetone
BPtSPyr from ref. 42
toluene
THF
418 (28), 469 (51), 499 (4)
419 (29), 467 (51), 493 (4)
416 (29), 465 (50), 483 (4)
392
402
567
0.37
0.66
0.05
H-1→L (97%)
H→L+1 (95%)
H→L (99%)
ππ* BDP
ππ* SPyr
PB-CT
acetone
BPtSPyrSPtB
toluene
THF
425 (34), 450 (77), 469 (90), 540 (6)
425 (41), 449 (84), 467 (91), 515 (8)
424 (38), 448 (76), 465 (88), 475 (8)
391
426
433
0.71
0.52
0.68
0.08
H-3→L+1 (48%),
H-2→L (49%)
ππ* BDP
H-1→L+1 (41%),
H→L+2 (55%)
ππ* SPyr
PB-CT
acetone
H-1→L+1 (53%),
H→L+2 (41%)
ππ* SPyr
PB-CT
6
18
H→L (97 %)
PB-CT
KBPtSPyr
toluene
THF
423 (30), 501 (72)
423 (31), 499 (69)
421 (24), 498 (58)
406
423
494
0.68
0.47
0.03
H→L+1 (95%)
H-1→L (97%)
ππ* SPyr
ππ* BDP
PB-CT
acetone
H→L (98%)
a
b
c
3
-1
-1 d
DFT calculations on the pbe1pbe/6-31G(d) level of therory; PCM model for CH
2
Cl
2
. λ in nm. ε10 M cm . f = oscillator
e
strength. m.c. = major contribution. H refers to HOMO and L to LUMO.
1
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Let us now turn to the emission properties of the dyads. In order to aid in the interpretation of their
complex, multiple emission features, we will first discuss the corresponding reference compounds
1
with only one type of chromophore. Upon excitation into the SPyr-based * band, MesPtSPyr
exhibits dual fluorescence and phosphorescence emissions, both at r.t. and at 77 K. Both emissions
emanate from the mercaptopyrene ligand as follows from the typical shapes of the emission bands
and their associated lifetimes of 1.11 ns and 448 µs to 1 ms (see Figures S36–S42 of the Supporting
4
2
Information). The observation of phosphorescence indicates that the heavy-metal effect of the
attached Pt ion is operative in promoting ISC, similar to Pt complexes with a σ-bonded pyrenyl
ligand.15, 21, 31
9
BPtI and KBPtI are also dual fluorescence and phosphorescence emitters (see Table 2 and Figure
S43 of the Supporting Information). While both exhibit significantly larger phosphorescence
quantum yields than MesPtSPyr, there are major differences between them. Thus, KBPtI is
associated with significantly larger fluorescence and lower phosphorescence quantum yields,
which attests to less efficient ISC. This is also evident from the increased fluorescence lifetime of
9
0
.96 ns as compared to 0.48 ns for BPtI and the derived rate constants kISC of 2.110 for BPtI and
8
-1
of 9.410 s for KBPtI. When considering that the two complexes share a common site of
platinum attachment to the BDP chromophore and show only small differences in their Pt-
C8(bodipy) bond lengths, a reduced external heavy atom effect seems a rather unlikely explanation
for the decreased kISC. We are therefore drawn to the hypothesis that these differences may be due
to the more locked, orthogonal conformation of the complexes with the KBDP ligand due to
strongly hindered rotation around the Pt-C8(bodipy) bond.
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Table 2. Emission data of BPtSPyr, KBPtSPyr and BPtSPyrSPtB
a
b
c
d
b
c
d
solvent
λ
exc
λ
fl (assignment)
Φ
fl
τ
fl
λ
ph (assignment)
Φ
ph
τ
ph
BPtI from ref. 9
1
CH
Cl
2 2
467 481 ( BDP)
0.002 0.48 ± 0.02 ns
641
0.364 297 ± 2 μs
KBPtI
toluene
Me-THFe 485 506
485 512
0.10
n.d.
0.959 ± 0.007 ns 684
1.171 ± 0.007 ns 673
MesPtSPyr from ref. 42
0.04
n.d.
239 ± 2 μs
436 ± 2 μs
1
3
toluene
430 460 ( SPyr)
0.24
1.113 ± 0.005 ns 664 ( pyrS)
<0.01 448 ± 6 μs
Me-THFe 430 450 ( SPyr)
1
658 ( pyrS)
3
n.d.
1.04 ± 0.06 ms
BPtSPyr
3
toluene
520
724 ( PB-CT)
0.08
0.15
0.15
6.7 ± 0.4 μs
6.7 ± 0.5 μs
6.4 ± 0.3 μs
1
3
4
4
70 484 ( BDP)
20
n.d.
n.d.
724 ( PB-CT)
3
724 ( PB-CT)
Me-THFe 530
3
6
6
35 ( BDP)
78 ± 0.4 μs
1.12 ± 0.02 ms
4
4
50 n.o.
15
n.d.
3
49 ( SPyr)
KBPtSPyr from ref. 42
1
3
toluene
485 512 ( BDP)
05 n.o.
<0.01 4.42 ± 0.07 ns
682 ( BDP)
0.03
0.05
n.d.
n.d.
204 ± 1 μs
203.7 ± 0.9 μs
423 ± 2 μs
n.d.
3
4
682 ( BDP)
Me-THFe 485 510 ( BDP)
1
n.d.
n.d.
2.34 ± 0.05 ns
n.d.
674 ( BDP)
3
1
3
4
05 510 ( BDP )
674 ( BDP)
BPtSPyrSPtB
3
toluene
550
770 ( PB-CT)
<0.01 32 ± 5 μs
1
3
4
60 482 ( BDP)
<0.01
635 ( BDP)
7
0.01
178 ± 1 μs
31 ± 2 μs
3
60 ( PB-CT)
1
3
4
20 482 ( BDP)
<0.01 5.0 ± 0.2 ns
653 ( BDP)
7
<0.01 164 ± 12 μs
3
60 ( PB-CT)
31 ± 1 μs
Me-THFe 550
696 ( SpyrS)
3
n.d.
n.d.
369 ± 6 μs
1
3
4
60 480 ( BDP)
n.d.
n.d.
n.d.
626 ( BDP)
499 ± 6 μs
369 ± 3 μs
3
6
96 ( SpyrS)
1
3
4
20 480 ( BDP)
4.1 ± 0.1 ns
626 ( BDP)
n.d.
479 ± 12 μs
348 ± 2 μs
3
6
96( SpyrS)
a
c
b
Excitation wavelength in nm. Wavelength of the fluorescence (fl) or phosphorescence (ph) emissions in nm.
d
Fluorescence (fl) or phosphorescence (ph) quantum yield. Lifetime of the fluorescence (fl) or phosphorescence
ph) emissions. Measured in a 2-MeTHF glass at 77 K.
e
(
Importantly, and in contrast to MesPtSPyr and BPtI, dyad BPtSPyr exhibits three different
emissions in fluid solution at r.t., depending on the excitation wavelength and solvent polarity.
Excitation into the PB-CT absorption band at 520 nm gives rise to a broad and unstructured NIR
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emission (see Figure S44a of the Supporting Information) with a quantum yield of 8% in toluene.
On increasing the solvent polarity, the emission peak shifts red with a concomitant, drastic
decrease of its associated lifetime (see Table S13 of the Supporting Information). When excited
1
into the BDP * absorption band at 470 nm, fluorescence and phosphorescence emissions from
1
3
the BDP-based * and * excited states add to the emission profile. The latter emissions are
hardly influenced by solvent polarity and hence become the dominant features in polar solvents,
3
1
where the PB-CT emission is very weak (Figure 3a). On excitation into the SPyr-based *
3
transition, again solely the PB-CT emission can be observed, but with an increased quantum yield
of up to 15% at r.t. in toluene solution (Figure S44c of the Supporting Information). This renders
3
BPtSPyr an unusually powerful NIR emitter. In a glassy Me-THF matrix at 77 K, only ππ* BDP
3
18, 36
and pyrene-based ππ*
Figure 3b).
emissions are observed, irrespective of the excitation wavelength
(
Dinuclear BPtSPyrSPyr has very similar properties to BPtSPyr, but shows also subtle
3
differences. As BPtSPyr, it emits solely from the PB-CT state, when excited into the PB-CT band
at 550 nm. In toluene solution, the associated emission wavelength is 770 nm with an associated
lifetime of 32 µs (see Figure 3c and S45 of the Supporting Information). Mirroring the trends in
the absorption spectra and cyclic voltammetry, the higher electron richness (and hence the higher
HOMO energy, vide supra) of the bridging pyrene-1,6-dithiolate ligand lowers the energy of the
3
corresponding CT-state with respect to mononuclear BPtSPyr. As a consequence, the PB-CT
-1
emission is red-shifted by 825 cm with a concomitant decrease of the quantum yield from 8% to
a value of ca. 1%. While one may be drawn to ascribe this decrease in quantum yield to the smaller
energy gap to the ground state, in line with the predications of the energy gap law,45-47 the strongly
3
-1
5 -1
decreased rate constant for radiationless decay of knr = 3.110 s as compared to knr = 110 s for
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4

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3
BPtSPyr does not support this view and points to less efficient ISC from the PB-CT to the ground
3
state as the underlying reason. No emission from the PB-CT state was observed down to 1270 nm
in solvents of higher polarity like THF, o-difluorobenzene or acetone.
Figure 3. Emission spectra of BPtSPyr a) in toluene, THF and acetone at r.t., and b) at 77 K in 2-
MeTHF. Emission spectra of BPtSPyrSPtB c) at r.t. in toluene, and d) at 77 K in 2-MeTHF.
Emission spectra of KBPtSPyr e) at r.t. in toluene, and f) at 77 K in 2-MeTHF.
3
As in BPtSPyr, the PB-CT emission is complemented by fluorescence and phosphorescence
1
3
emissions from the BDP-localized * and * excited states, when irradiated into the BDP
1
* band at 460 nm in toluene solution; these are the only emissions in more polar solvents.
Associated lifetimes are 5.0 ns and 178 s, respectively (see Figure S46 for emission spectra and
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1
lifetime decay traces in o-C
6
F
2
H
4
). At odds with BPtSPyr, excitation into the SPyr * band at
4
20 nm also gives rise to all three emissions. This indicates subtle differences in excited state
decay pathways between the two compounds (vide infra).
As in fluid solution, emission spectra of BPtSPyrSPtB in frozen, glassy matrices at 77 K strongly
3
depend on the excitation wavelength (Figure 3d). Selective irradiation into the PB-CT band (exc
=
550 nm) gives rise to a richly structured emission with a main peak at 696 nm and several
vibrational subbands with an associated lifetime of 396 µs. This feature is again identified as
3
15, 21, 31
phosphorescence from the pyrene-based * state.
Mirroring its influence on the energy
of the pyrene-localized excited singlet state, the stronger donor substitution of the bridging pyrene-
3
-1
1
,6-dithiolate ligand reduces the energy of the * state of the pyrene constituent by 1040 cm
compared to BPtSPyr. As in the fluid solution spectra, excitation into the BDP-based or, to a lesser
1
1
3
degree, into the pyrene-based * absorption bands at 460 or 420 nm adds the * and *
BDP emissions to the emission profile of BPtSPyrSPtB with associated lifetimes of 4.1 ns and
4
90 µs, respectively (for emission decay traces see Figure S47 of the Supporting Information). All
these observations match with the 77 K excitation spectra. Thus, when detecting at the BDP
1
phosphorescence emission at 626 nm, the PB-CT band is absent and the intensity of the *
pyrene band is strongly reduced, while all three excitations contribute to the emission from the
3
* state of the pyrene (see Figure S48 of the Supporting Information).
At odds with BPtSPyr and BPtSPyrSPtB, KBPtSPyr shows only the same two emissions as the
iodo complexes BPtI and KBPtI, independent of the excitation wavelength and temperature
(compare Figure 3e,f with Figure S43 of the Supporting Information and see Table 2; for emission
3
decay traces see Figures S49-S51 of the Supporting Information). In particular, neither the PB-
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6

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3
CT nor the * pyrene emissions are observed. This striking dichotomy is due to the different
energies of the corresponding excited states that are involved in the deactivation cascade and
energy differences between them. The inductive effect of the alkyl substituents on the KBDP
ligand decreases its electron-accepting capability and the energies of the BDP-localized excited
-1
-1
singlet and triplet states, which is reflected by their respective red shifts of 1260 cm or 980 cm .
3
On the other hand, the energy of the * state of the mercaptopyrene ligand is much less affected
by substitution of the BDP ligand, such that it remains at an only marginally lower energy than in
BPtSPyr (Table 2). In this case, the mercaptopyrene ligand acts as an efficient antenna for
populating the BPD-localized * state. Concomitantly, the phosphorescence quantum yield
increases to 5% as compared to 3% on direct excitation into the BDP * band. The absence of
any fluorescence emission from the mercaptopyrene and the BDP ligands on excitation at 420 nm
1
3
likewise attest to efficient ISC from the higher-lying * SPyr to the * KBDP state and the
1
1
absence of detectable energy transfer from the * SPyr to the * KBDP excited state. Other
examples of a direct singlet-to-triplet electron transfer by spin-orbit coupling were already reported
in the literature.48-50 The above conclusions are fully supported by the excitation spectra (see Figure
S52 of the Supporting Information). The same overall behaviour also prevails at 77 K.
Our results thus clearly show that the energy levels of the excited singlet and triplet states of either
the BDP or the mercaptopyrene ligands as well as those of the PB-CT excited states of both
manifolds can be selectively manipulated. This has drastic consequences for the energy cascade
underlying the interconversion between the different excited states and their deactivation into the
ground state.
An intriguing feature of Pt-BDP complexes is their ability to generate singlet oxygen, sometimes
with exceptionally high quantum yields.9, 33, 34 The amount of singlet oxygen formed in aerated
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1
solutions of the complexes was quantified by comparing the intensity of the O
2
emission at
at r.t. Figure
1
4
275 nm to that in the presence of tetraphenylporphyrin (TPP) in air-saturated C
6
H
6
compares the results of this study while Table S14 of the Supporting Information lists the
. Quite intriguingly, all complexes of this study produce singlet oxygen. This is
particularly true for the simple iodo complex BPtI, which, upon excitation into the principal
bodipy absorption band, achieves a quantum yield of 94%. In KBPtI, the value of  is reduced
quantum yields 


to 0.65, most probably due to a lower ISC rate constant (vide supra). Mononuclear dyads BPtSPyr
and KBPtSPyr also perform well when excited in either the PB-CT or the BDP * absorption
bands, and their quantum yields of 74% are also superior to that of TPP. In contrast to our
3
observation of enhanced quantum yields for PB-CT emission, excitation into the * band of the
mercaptopyrene chromophore reduces 
chromophore, has an even lower quantum yield of 28%. BPtSPyrSPtB has the smallest value of
(7-18%, depending on the excitation wavelength). Reasons are the lower ISC rate and a rapid
degradation under continuous irradiation of this complex in aerated solutions.

 to 33 - 40%. MesPtSPyr, which lacks a bodipy


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8

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Figure 4. a) Scheme for the generation of singlet oxygen with a triplet sensitizer. b)-d) Singlet
oxygen emission upon excitation of the given sensitizers at the stated excitation wavelength at an
absorbance of the sensitizer of 0.05 at exc
.
The chemical reversibility of the oxidations and reductions allowed us to determine the
spectroscopic characteristics of the oxidized mercaptopyrene and reduced bodipy ligands, which
are both present in the charge-separated states, and to further substantiate our above conclusions.
For doing so we resorted to in situ UV/Vis/NIR spectroelectrochemistry inside an optically
5
2
transparent thin-layer electrolysis cell. Figure 5 displays the results of such measurements for the
complexes BPtSPyr, BPtSPyrSPtB and KBPtSPyr, while the corresponding data are
summarized in Table S15 of the Supporting Information. Multiple isosbestic points attest to clean
interconversions between the different redox forms.
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Figure 5. Changes of UV/Vis/NIR spectra of BPtSPyr a) on oxidation and b) on reduction; of
KBPtSPyr c) on oxidation and d) on reduction; as well as that of BPtSPyrSPtB e) on oxidation
and f) on reduction in THF/0.1 M NBu
4
PF
6
or, in the case of 5e) CH
2
Cl
2
/0.1 M NBu
4
PF as the
6
supporting electrolyte at T = 293 K.
Oxidation of these complexes is accompanied by the bleach of the SPyr-based ππ* absorption
features while the BDP ππ* band experiences an only minor red-shift. Based on literature data53,
5
4
and the results of our TD-DFT calculations on BPtSPyr+ (Figure S53 of the Supporting
2
0

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
+
Information), the new absorption bands at ca. 520, 550, 750, and 1000 nm of BPtSPyr are safely

+
assigned to ππ* transitions of the oxidized mercatopyrene ligand. The results for KBPtSPyr are
very similar with only minor shifts between these two complexes. The new bands appearing during
oxidation of BPtSPyrSPtB have the same general appearance but are shifted further red due to the
presence of two auxochromic thiolate donors. Reduction of BPtSPyr and KBPtSPyr in turn
bleaches the BDP * band and shifts the SPyr * transition slightly red (Figure 5b and 5d; for
a TD-DFT assignment of the bands see Figure S54 of the Supporting Information). Spectral
changes upon reduction of BPtSPyrSPtB are similar but less characteristic as the BDP and the
SPyrS * bands overlap (Figure 5f).
Knowledge of the characteristic absorption features of the Pt-bonded SPyr+ and BDP-
chromophores allows us to identify them in the transient absorption (TA) spectra of the dyads.
Figure 6 compares the TA spectra of BPtI, MesPtSPyr, BPtSPyr and KBPtSPyr in THF.
Associated kinetic traces can be found in Figures S55-S59 of the Supporting Information. BPtI
shows two TA features when excited at 470 nm (Figure 6a): the bleached BDP signal at 470 nm
and, at ca. 400 nm, an excited state absorption (ESA) signal, which we assign to the triplet state of
the BDP chromophore.55-57 The bleach and the ESA features share the same kinetics with two time
constants. The longer one of about 149 μs microseconds fits with the corresponding emission data
(Table 2 and Figure S55). The shorter lifetime of about 30 μs is most likely due to triplet-triplet
annihilation of two molecules of BPtI because of the high optical densities required in the TA
measurements. On exciting MesPtSPyr into its SPyr band at 430 nm, the ground state bleach of
the SPyr band and a distinct ESA signal with peaks at 520, 600 and 760 nm are observed. The
3
latter are assigned to the * states of the SPyr chromophore and closely resemble those reported
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3
6
58
by Pomestchenko et al. and Wu et al. Again, the kinetics of ground-bleach recovery and ESA
decay match with the phosphorescence lifetime (Figure S56 of the Supporting Information).
TA spectra of BPtSPyr show three common features, irrespective of the excitation wavelength.
These features are the bleaches of the SPyr and BDP ground state absorption signals at ca. 400 nm
or 470 nm, and an ESA signal at 550 nm, which we assign to the SPyr+ radical cation (see
Figure 5a for comparison). All these features show identical monoexponential decay kinetics with
a common time constant of 50 ns when excited at 532 nm (for single wavelength kinetics for each
TA feature see Fig. S57 and S58 of the Supporting Information). Excitation into the BDP
absorption band at 470 nm leaves the time constants for the features at 415 nm and 550 nm of the
SPyr chromophore unchanged, but alters the time constant for recovery of the ground state bleach
1
3
of the bodipy * band at 475 nm to 16.5 μs, in accordance with the long-lived BDP ππ* excited
state in that solvent (red trace in Fig. S57a of the Supporting Information). BPtSPyrSPtB was
unfortunately not sufficiently photostable to allow for the recording of TA spectra.
Figure 6. Transient absorption spectra of a) BPtI, b) MesPtSPyr, c) BPtSPyr and d) KBPtSPyr
in ca. 20 M THF solutions at r.t. directly after laser excitation at the given wavelength. All spectra
are time-integrated over 200 ns.
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2

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The TA spectrum of KBPtSPyr, which solely emits from excited BDP * states, differs
strongly from that of BPtSPyr (compare Figures 6c and 6d), but shows close resemblance to that
of the parent complex BPtI. Following excitations at either 420 nm or 500 nm, an ESA signal at
4
25 nm is observed in addition to the ground state bleach at 500 nm. The TA spectra of KBPtSPyr
3
are thus completely devoid of the ESA signatures of an excited PB-CT or * state of the
mercaptopyrene ligand. Again, single wavelength kinetics for all TA features agree with the time
constant of the phosphorescence emission (see Figure S59 of the Supporting Information).
The differences in the TA spectra of complexes BPtSPyr and KBPtSPyr thus match with those
of their absorption and emission properties. The underlying reason for this dichotomy are the
different energies of the various excited states and the concomitant changes of their energy transfer
cascades. This is best illustrated with the aid of a schematic Jablonski diagram for BPtSPyr in
Figure 7. Given the close energetic proximity of three excited states within the singlet and triplet
manifolds each and the different degrees of structural reorganization from the initial Frack-Condon
states, there is no guarantee that the energy ordering of the structurally relaxed excited states
follows that of the corresponding transitions in their absorption or emission spectra. Indeed, test
1
1
calculations place the BDP-centered * state energetically below the PB-CT state, although the
absorption spectra suggest a different energy ordering. A thorough analysis of this issue, including
the identification of the crossing points between the different potential wells, is, however, beyond
our present capabilities.
We start our discussion with the emissive properties of BPtSPyr at 77 K. Excitation into any
3
absorption band triggers dual phosphorescence emission from the * states of the SPyr and the
3
BDP ligands. This signals that the PB-CT state is not directly populated from any of the excited
singlet states and that all excited states of the singlet manifold relax into these two emissive triplet
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1
3
states. One possible explanation for this odd behaviour is that the PB-CT and PB-CT states
3
possess very similar nuclear coordinates and the BDP and SPyr ππ* potential wells have more
1
easily accessible intersections with the PB-CT potential well. Similar accounts of a direct singlet-
to-triplet electron-transfer by spin–orbit coupling have been published on previous occasions.48-50
Figure 7. Schematic Jablonski diagrams for BPtSPyr. The insert provides electron density
difference maps of the individual electronic transitions with decreasing electron density in blue
and increasing electron density in red color.
3
The observed room-temperature emission from the PB-CT state is therefore likely due to thermal
3
equilibration of that state with the longer lived BDP- and SPyr-based * states (c. f. the short
lifetime of the CT- phosphorescence emission of less than 7 µs). The presence of a competitive
3
deactivation channel for the BDP ππ* state of BPtSPyr is also indicated by the significant
decrease of its associated lifetime to 78 µs as compared to 279 µs in BPtI and 204 µs in KBPtSPyr.
3
The latter scenario also explains the further boosting of the quantum yield of the PB-CT emission
1
on excitation into the * band of the SPyr ligand and the complete quenching of the associated
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4

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3
pyrene-based phosphorescence at r. t. All these observations strongly suggest that the PB-CT state
does not constitute the energetically lowest state of the triplet manifold.
The overall behaviour of BPtSPyrSPtB is very similar with the exception that the ratio of the time
1
1
constants for internal conversion from the pyrene-based * state to the * BDP state to that
3
of the ISC to the * state of the pyrenedithiolate ligand is somewhat increased. This allows the
BDP-based fluorescence and phosphorescence bands to be observed on irradiation at 420 nm.
Compared to the rather complex characteristics of BPtSPyr, that of its peralkylated counterpart
3
KBPtSPyr is much simpler. Here, the * BDP state is so much lowered in energy that it is
3
populated from all excited triplet states and no thermal equilibration with the PB-CT state occurs.
Summary
We present three dyads of the type trans-Pt(-BDP)(PEt
3
2
) (SPyr), where -BDP denotes a
meso-bonded 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (bodipy) dye and SPyr is a
mercaptopyrene ligand. Variations over this basic motif are made by exchanging the unsubstituted
BDP ligand of BPtSPyr by its peralkylated krypto-BDP derivative in KBPtSPyr, or by employing
either a terminal 1-mercaptopyrene ligand in the mononuclear complexes or a bridging pyrene-
1
,6-dithiolate ligand in dinuclear BPtSPyrSPtB. Our results show that BDP peralkylation has
drastic consequences for the energy ordering of the different excited states involved in the
deactivation cascades and hence the emissive properties. Thus, BPtSPyr and BPtSPyrSPtB emit
3
via four different radiative channels, featuring the charge-separated PB-CT state and the singlet
3
and triplet BDP-centred * states at r. t., or the BDP and SPyr-centred * states in a frozen
matrix at 77 K. Peralkylation of the BDP ligand in KBPtSPyr renders the BDP ligand a less strong
acceptor. This makes pyrene-to-BDP charge transfer much less exergonic. As a consequence,
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1
KBPtSPyr is a dual fluorescence and phosphorescence emitter from the BDP-based * and
3
* states. In this constellation, the SPyr ligand acts as an antenna for the BDP-based excited
states. The multitude of potentially emissive states and their various interconnections render the
emission profiles of these complexes sensitive to the excitation wavelength, the polarity of the
solvent, and the temperature. The interrelations between the various deactivation channels were
further probed by transient absorption spectroscopy. UV/Vis/NIR spectroelectrochemistry and
comparison to reference compounds with either only the BDP or the SPyr chromophore aid in the
assignment of excited state absorption signals.
All complexes act as panchromatic sensitizers for singlet oxygen generation. The mononuclear
complexes BPtI, KBPtSPyr and BPtSPyr outperform the tetraphenylporphyrin standard in this
respect.
1
13
1
31
1
Electronic supporting information available: H, C{ H}, and, if applicable, P{ H},
1
95
1
Pt{ H} NMR spectra of all compounds; Data pertaining to the structure solution and refinement
as well as packing diagrams; UV/Vis spectra in different solvents, MO diagrams, TD-DFT
computed transitions and electron density difference maps for individual transitions; cyclic
voltamnmograms and electrochemistry data; luminescence spectra in different solvents and at
different excitation wavelengths as well as time decay traces, computed energies of the frontier
MOs; absorption data, MO calculations of relevant MOs, spin densities and MO compositions of
the oxidized and reduced forms of the complexes; decay traces for transient spectra of the
individual complexes; atomic coordinates of geometry-optimized structures.
Experimental section
Methods. Oxidative addition reactions and ligand manipulations were performed under a N
2
atmosphere using standard Schlenk techniques or inside a glove box. C , CD Cl , CDCl and
6
D
6
2
2
3
2
6

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THF-d8 were supplied from Eurisotop and stored under N
2
-atomosphere over molecular sieves.
All other solvents were used as received. Complex trans-Pt(bodipy)I(PEt
3
)
2
(BPtI), trans-
Pt(bodipy)(PEt
3
)
2
(S-1-thiopyrene) (BPtSPyr) and trans-Pt(mesityl)(PEt
)
3 2
(S-1-thiopyrene)
(
MesPtSPyr) were synthesized according to the literature protocols.9, 42
NMR experiments were carried out on a Varian Unity Inova 400, a Bruker Avance III DRX 400
1
13
or a Bruker Avance DRX 600 spectrometer. H and C spectra were referenced to the solvent
signal, while P and 195Pt spectra were referenced to external standards (85% H
31
PO or saturated
3
4
K
2
[PtCl ] in D O, respectively). NMR data are given as follows: chemical shift (δ in ppm),
6
2
multiplicity (br, broad; d, doublet; dd, doublet of doublets; m, multiplet; s, singlet; t, triplet, vt
virtual triplet), integration, coupling constant (Hz). Unequivocal signal assignments were achieved
by 2D NMR experiments. The numbering of the nuclei follows that of the chemical structures
displayed at the synthesis protocols.
Combustion analysis was conducted with an Elementar vario MICRO cube CHN-analyzer from
Heraeus.
Luminescence spectra and lifetimes as well as quantum yields were measured in acetone,
CH
2
Cl , 2-MeTHF, THF, or toluene solutions of the complex on a PicoQuant FluoTime 300
2
spectrometer. Unless stated otherwise, absolute quantum yields were determined with an
integrating sphere within the FluoTime 300 spectrometer. Emission spectroscopy under inert gas
atmosphere was conducted in a quartz cuvette modified with an angle valve from Normag. The
emission data of BPtSPyr in acetone with excitation at 405 nm are not reported due to sample

decomposition upon laser excitation. The singlet oxygen generation quantum yields  were
determined by detecting the luminescence signal of singlet oxygen at 1275 nm in air-saturated
benzene solution containing the corresponding sensitizer at a concentration to render its
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absorbance as 0.05 at ex. The integrated intensity of the phosphorescence signal of O
2
was
compared that obtained for a solution containing the reference sensitizer tetraphenylporphyrin
5
1
(

(TPP) = 0.66), with an absorbance of 0.05 at ex
.
UV/Vis/NIR spectra were recorded on a TIDAS fiber optic diode array spectrometer (combined
MCS UV/NIR and PGS NIR instrumentation) from J&M in HELLMA quartz cuvettes with 0.1
cm optical path lengths.
Transient absorption spectroscopy was performed using an LP920‐KS instrument from
Edinburgh Instruments. 410 nm and 470 nm excitation were obtained from pulsed third‐harmonic
radiation from a Quantel Brilliant b Nd:YAG laser equipped with a Rainbow optical parameter
oscillator (OPO). 532 nm excitation was obtained from pulsed second-harmonic generation from
a Quantel Brilliant b Nd:YAG laser. The laser pulse duration was ~10 ns and the pulse frequency
1
0 Hz with a typical pulse energy of 7 mJ. Detection of transient absorption spectra occurred on
an iCCD camera from Andor. Single‐wavelength kinetics were recorded using a photomultiplier
tube. All optical spectroscopic experiments were performed under deaerated conditions using
quartz cuvettes in which oxygen can be removed by the freeze-pump‐thaw technique. Solutions
were typically 20 μM in the compound and deaerated through three cycles of freeze-pump‐thaw.
All electrochemical experiments were executed in a home-built cylindrical vacuum-tight one-
compartment cell. A spiral shaped Pt wire and an Ag wire as the counter and pseudoreference
electrodes are sealed into glass capillaries and fixed by Quickfit screws via standard joints. A
platinum electrode is introduced as the working electrode through the top port via a Teflon screw
cap with a suitable fitting. It is polished with first 1 μm and then 0.25 μm diamond paste before
measurements. The cell may be attached to a conventional Schlenk line via a side arm equipped
with a Teflon screw valve, allowing experiments to be performed under an argon atmosphere with
2
8

Page 29 of 40
Dalton Transactions
View Article Online
DOI: 10.1039/C9DT01737B
approximately 5-7 mL of analyte solution. NBu
4
PF
6
(0.1 mM) was used as the supporting
electrolyte. Referencing was done with addition of an appropriate amount of decamethylferrocene
Cp*
Fe) as an internal standard to the analyte solution after all data of interest had been acquired.
(
2
Representative sets of scans were repeated with the added standard. Electrochemical data were
acquired with a computer-controlled BASi CV50 potentiostat. The optically transparent thin-layer
electrochemical (OTTLE) cell was home-built and followed the design of Hartl et al.52 It
comprised a Pt working and counter electrode and a thin silver wire as a pseudoreference electrode
sandwiched between two CaF windows of a conventional liquid IR cell with the working
2
electrode positioned in the centre of the spectrometer beam.
Computational Details. The ground-state electronic structures were calculated by density
5
9
functional theory (DFT) methods using the Gaussian 09 program package. Quantum-chemical
studies were performed without any symmetry constraints. Open-shell systems were calculated by
6
0
the unrestricted Kohn−Sham approach. Geometry optimizations followed by vibrational analysis
were performed either in a vacuum or in solvent media. The quasi-relativistic Wood−Boring small-
core pseudopotentials (MWB),61, 62 the corresponding optimized set of basis functions63 for
6
4
platinum and the halogen atoms, and the 6-31G(d)-polarized double-ζ basis set for the remaining
atoms were employed together with the Perdew−Burke−Ernzerhof exchange and correlation
functional (PBE0).65, 66 Solvent effects were accounted for by the polarizable conductor continuum
model (PCM)67-70 with standard parameters for dichloromethane. Absorption spectra and orbital
7
1
energies were calculated using time-dependent DFT (TD-DFT) with the same functional/basis
set combinations as those mentioned above. For an easier comparison with the experiment, the
obtained absorption and emission energies were converted into wavelengths and broadened by a
2
9