A study of acridine and acridinium-substituted bis(terpyridine)zinc(ii) and ruthenium(ii) complexes as photosensitizers for O2 ( 1Δg) generation

Article abstract of DOI:10.1039/c3pp50349f

The homoleptic zinc(ii) and ruthenium(ii) metal complexes of bis(tridentate) 9-acridine and 10-methyl-9-acridinium-substituted terpyridines were tested for their suitability as triplet photosensitizers (PS) using the photooxidation of 1,5-dihydroxynaphthalene (DHN) to Juglone as a model reaction. Singlet oxygen (O₂¹Δ_g) generation is superior or comparable to Ru(bpy)₃²⁺ for the acridine complexes, whereas the acridinium complexes are ineffective. The molecular structure of the bis(9-(5-([2,2′:6′,2′′-terpyridin]- 4′-yl)thien-2-yl)-10-methylacridinium)zinc(ii) complex ([Zn(MeATT) ₂][PF₆]₄) is determined by X-ray structure analysis, whereas for other complexes DFT calculations were performed for structural parameters to obtain insights into their electronic properties. The Royal Society of Chemistry and Owner Societies.

Full text of DOI:10.1039/c3pp50349f

Photochemical &
Photobiological Sciences
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PAPER
A study of acridine and acridinium-substituted
bis(terpyridine)zinc(II) and ruthenium(II) complexes
Cite this: Photochem. Photobiol.
Sci., 2014, 13, 380
1
as photosensitizers for O ( Δ ) generation†‡
2
g
a
b
a
c
Jens Eberhard,* Katrin Peuntinger, Susann Rath, Beate Neumann,
c
b
a
Hans-Georg Stammler, Dirk M. Guldi and Jochen Mattay*
The homoleptic zinc(II) and ruthenium(II) metal complexes of bis(tridentate) 9-acridine and 10-methyl-
-acridinium-substituted terpyridines were tested for their suitability as triplet photosensitizers (PS) using
the photooxidation of 1,5-dihydroxynaphthalene (DHN) to Juglone as a model reaction. Singlet oxygen
9
1
2+
3
(
O
2
Δ
g
) generation is superior or comparable to Ru(bpy)
dinium complexes are ineffective. The molecular structure of the bis(9-(5-([2,2’:6’,2’’-terpyridin]-4’-yl)-
thien-2-yl)-10-methylacridinium)zinc(II) complex ([Zn(MeATT) ][PF ) is determined by X-ray structure
for the acridine complexes, whereas the acri-
Received 7th October 2013,
Accepted 28th November 2013
2
6 4
]
DOI: 10.1039/c3pp50349f
analysis, whereas for other complexes DFT calculations were performed for structural parameters to
obtain insights into their electronic properties.
www.rsc.org/pps
multifarious research fields of their own: porphyrins and
phthalocyanines.
Introduction
Photosensitizers (PSs) are a common need in the field of
Typical metals employed in polypyridine complexes are
1
,2
9
photochemistry and -biology
physical and molecular mechanisms are well understood.
and the underlying photo- transition metals of the platinum group (e.g. Ru, Ir). Progress
3
–6
10
has also been made in employing less precious metals in
II 11
In the last few years the development of specifically polypyridine complexes for such applications (e.g. Zn
or
I 12,13
II/III
14
designed PS dyes has become particularly important for use in Cu
sensitizers, Co
electrolytes or a combination of
7
15
dye-sensitized solar cells (DSSC), light upconversion by them ), as most of the porphyrin and phthalocyanine dyes
8
triplet–triplet annihilation (TTA), or – more generally seen – show favorable properties with more abundant metals (e.g.
II 16
in the study of charge separation in order to mimic the natural Zn ). It should also be mentioned that metal ion free sensi-
9
photosynthetic reaction cascade. Metal complexes of the poly- tizers have been studied widely in the last decade due to DSSC
1
7
pyridine family are the backbone of such efforts next to two application.
Nevertheless “classic” triplet sensitizers and their appli-
cation remain an active research area in recent years and the
progress in this field has been summarized only recently.
a
18
Organische Chemie I, Fakultät für Chemie, Universität Bielefeld, Universitätsstr. 25,
3501 Bielefeld, Germany. E-mail: mattay@uni-bielefeld.de,
3
Advances in this field are on the one hand a “side-effect” due
to the need for phosphorescent dyes in materials chemistry
but also due to the need for biocompatible (water-soluble,
jens.eberhard@uni-bielefeld.de; Tel: +49 521 106 2072
b
Physikalische Chemie I, Department of Chemistry and Pharmacy and
Interdisciplinary Center for Molecular Materials, Friedrich-Alexander-Universität
Erlangen-Nürnberg, Egerlandstr. 3, 91058 Erlangen, Germany
1
8
non-toxic, etc.) far-red absorbing dyes for photodynamic
c
19
Abteilung für Röntgenstrukturanalyse, Fakultät für Chemie, Universität Bielefeld,
therapy (PDT).
Universitätsstr. 25, 33501 Bielefeld, Germany
1
The photosensitized production of O
2
( Δ
g
) by triplet
†
This article is dedicated to the memory of Professor Nicholas J. Turro
1938–2012).
Electronic supplementary information (ESI) available: Additional views on
2
0
energy transfer is unarguably one of the main applications.
However, just as it is a worthwhile research goal to achieve the
(
‡
X-ray crystal structure of 2, analytical data (full spectra) for all new compounds, best possible coverage of the incident solar electromagnetic
7
b
exemplary UV-Vis spectra of DHN photooxidations, data of 2-methylnaphthalene
spectrum in optimization of DSSCs, one can ask the same
for PSs in order to facilitate, for instance, an environmentally
friendly production of chemicals by sunlight (viz. “green”
1
and limonene irradiation experiments, O
2
( Δ
g
) NIR emission upon irradiation
of 4, transient absorption (TA) spectra of the ATT ligand and complex 2, and
computational details, such as optimized geometries (S
density plots. See DOI: 10.1039/c3pp50349f
0 1
, T ), MO and spin
2
1
22
photochemistry ) in large area solar collectors.
Such
380 | Photochem. Photobiol. Sci., 2014, 13, 380–396
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1
techniques can be easily adopted for O
2
( Δ
g
)-mediated photo-
More often the chromophore triplet state is lower in energy
oxidations, for example of (−)-citronellol in rose oxide syn- than the triplet state of interest and therefore an energy trans-
2
3
thesis or 1,5-dihydroxynaphthalene (DHN) to Juglone and fer may occur. In some cases the occurrence of this energy
2
4
subsequent photo-Friedel–Crafts acylation. Similarly, the use transfer might be beneficially employed, e.g. first principle
of such techniques in solar water disinfection by photogene- identification of the chromophore triplet state has been
1
25
2+
44
rated O
2
( Δ
g
) has been reported. In contrast, in terms of size reported for a BODIPY dye connected to a [Ru(tpy)
2
]
system
III
the advantages of microfluidic flow reactor photochemistry and a perylenebisimide (PBI) dye on C^N-cyclometalated Ir
4
3b
1
and the availability of efficient miniaturized light sources complexes.
LEDs, diode lasers, etc.) enable production processes without ation of the triplet state is of course less important than
the need to isolate the potentially dangerous peroxide overall light absorption, yield of triplet formation and triplet
For O ( Δ ) sensitization purposes the localiz-
2
g
(
2
6
1
intermediates.
lifetime itself, and consequently the O ( Δ ) quantum yield
2
g
4
3b
The generation, application and biological impact of (Φ
) have been reviewed,
emphasis on synthetic organic photochemistry.
Correspondingly, there is a need for new PSs as the typi- have been analyzed for their suitability as PS, as there is a
cally employed organic dyes, such as methylene blue (MB) or good background of data on the well-known O
rose bengal (RB), exhibit a lack of long-term stability and wave- [Ru(bpy)3]
length coverage. More recently, heavy atom-substituted could only find a few reports dealing with Φ
Δ
) was evaluated.
1
2,27
O
2
( Δ
g
as well as its use with an
One might ask why not more of the promising – multichro-
mophoric or otherwise improved – bis(terpyridine) systems
2
8–32
3
3
1
2
( Δ
g
) sensitizer
2
+
27,38,45,46
and similar complexes.
However, we
of [Ru(tpy) ]
2
2
+
Δ
3
3,34
47,48
BODIPY dyes
and other purely organic examples (e.g. cou- complexes.
3
5
marins ) were reported, mainly focused on PDT (no undesir-
able metal ions) or TTA upconversion applications.
In part the apparent absence of such reports can be attribu-
4
2a
49,50
ted to the popular choice of anthracene
and pyrene
dye
Recently, Murata et al. reported the use of neutral and in multichromophore approaches because of their suitable
III
cationic C^N-cyclometalated Ir complexes for the photooxi- matching triplet levels. An early seminal work on anthracene-
3
6
51
dation of DHN – stimulated by the seminal work of Selke substituted terpyridine ligands by Maestri et al. mentioned
3
7
1
1
and Thompson et al. about the efficient O
zation capability of platinum group metal complexes. In this of Ru , which in turn degraded the anthracene moiety due to
regard, Wilkinson and Abdel-Shafi et al.
and derivatives in detail for their capabilities employed as a switch for the [Ru(bpy)
over ten years ago and some Ir and Rh complexes more
recently.
Although polypyridine complexes of Ir
advantages over Ru
2
( Δ
g
) sensiti-
O
2
g
( Δ ) generation for a homoleptic bis(terpyridine) complex
II
3
8
examined endo-peroxide formation. Very recently, this reaction was
2
+
2+
52
[
Ru(bpy)
3
]
3
]
luminescence.
III
I
First principle studies for 9-acridine and 10-methyl-9-acridi-
nium-substituted homoleptic bis(terpyridine) complexes of
3
9
III
II
53
have certain Ru have been reported elsewhere. For these systems it
II 9
3
(i.e., no low-lying metal-centered turned out that a multichromophoric benefit, i.e. MLCT lumi-
energy levels), one should not leave aside the progress made nescence, could only be deduced at low temperature for both
II
on the parent Ru complexes in the last few years. Especially dye forms. While acridinium compounds featured short-lived
for terpyridine complexes, which lack on prolonged excited components only, the acridine analogues exhibited a long-
2
+
3
state lifetimes compared to [Ru(bpy)
3
] , rational design has lived dark state, which was quenchable by O
2
( Σ
although it is only appli-
but also by cable in the blue spectral region (lowest energy abs. at λmax
attachment of multiple chromophores with a matching triplet 375 nm), but more importantly it cannot be decomposed by
g
). Pristine
3
18,54
lead to lifetimes in the μs-regime. This was achieved by acridine is a known PS sensitizer
manipulation of the terpyridine ligand itself,
4
0,41
4
2
level.
endo-peroxide formation. In the present work, we were there-
1
The choice of a suitable chromophore for such purposes is fore interested to evaluate the O ( Δ ) sensitization efficiency
a priori difficult and certain limitations apply as summarized of the Ru complexes and their Zn counterparts (Scheme 1).
by Zhao et al., i.e., one or several chromophores tethered to The synthesis and photophysical properties of these new Zn
2
g
II
II
1
8
II
the metal-coordination side with an excited singlet state complexes will be reported here. For comparison, we included
higher in energy than the singlet state of interest may serve as also acridinium systems in this study because such dyes are
5
5
an additional photon absorber (“antenna effect”). However, if lively discussed
and may act as Type I or Type II
5
6
we focus on the T1 as state of interest and consider the sensitizers.
chromophore triplet state higher in energy but the metal ion
as main cause for the intersystem crossing (ISC) then no
photophysical benefit is gained in terms of triplet photophy-
sics. This approach is therefore only appealing for compounds
Experimental
Material and methods
with an intrinsic long triplet lifetime and high ISC rates like
2
+
[
Ru(bpy)3] where the singlet state is quickly depleted by ISC.
Very recently this concept has been applied to cyclometalated The manufacturer and model of the used instrumentation for
III
II
Ir and Pt complexes and was also used to evaluate photo- routine characterization (NMR, MS, IR, mp) as well as purity
4
3
oxidation of DHN.
and commercial sources for solvents and reagents are given in
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Femtosecond fluorescence upconversion experiments were
performed with the aforementioned Ti/sapphire fs laser
system (Clark-MXR) which is combined with an ultrafast
upconversion spectrometer system HALCYONE for detection.
Emission pump pulses of 387 nm (vide supra) were used to
stimulate the sample. With the use of a beam splitter, the
fundamental wavelength of 775 nm was used as a gate
pulse for sum frequency generation inside a BBO crystal.
Accordingly, fluorescence kinetics were monitored at 580 nm
with a time window of 0–3300 ps and a temporal resolution of
3
50 fs.
Zn(ATT)
Zn(OTF) (20 mg, 54 μmol) were added to a solvent mixture of
[
2
][PF
6
]
2
(1). ATT ligand (56 mg, 112 μmol) and
2
Scheme 1 Generalized structure of photocatalysts 1–5 under study
and labelling scheme for H NMR spectroscopic assignments.
1
EtOH and THF (30 mL, 2 : 1 v/v) under stirring. The reaction
mixture was heated to reflux for 15 min and was allowed to
cool to r.t. The almost clear, golden yellow solution was filtered
the ESI.‡ The synthesis of the ligands and complexes 3–5 has and an aqueous NH PF solution (301 mg in 20 mL) was
4
6
5
3
been reported elsewhere.
added to the filtrate. A bright yellow suspension was formed
Steady-state UV-Vis absorption spectroscopy was performed immediately. Precipitation was completed by the addition of
on a Lambda 2, Lambda 40 (both Perkin Elmer) or V-630 water (50 ml) and the suspension was filtered over a pad of
(
Jasco) double beam spectrometer using the neat solvent as a Celite (ca. 2 cm). The filter pad was washed with water
blank reference in standard PTFE-stoppered quartz cuvettes of (3 × 20 mL) and diethyl ether (3 × 20 mL) and was left to dry
0 mm path length (Hellma).
in air overnight. The yellow solid was dissolved and flushed
Steady state fluorescence spectra were carried out using a down by using MeCN (4 × 20 mL). The volatiles of the
1
FluoroMax 3 spectrometer (Horiba Yvon Jobin, HYJ) in the filtrate were removed in vacuo to give pure 1 (81%, 59 mg,
visible detection range and using a FluoroLog 3 spectrometer 44 μmol). The complex can be recrystallized from MeCN–CH Cl
2
2
(HYJ) with an InGaAs detector (512 × 1 × 1 μm, HYJ Symph- (1 : 1 v/v) by diethyl ether vapor diffusion (recovery 74%). Mp
ony®) in the NIR detection range with 180 s integration time 327 °C (sharp). Found: C, 55.15; H, 3.21; N, 8.00; S, 4.36.
and a slit width of 14.7 nm. Alternatively, fluorescence spectra C H ON S ZnP F ·3H O requires C, 55.12; H, 3.32; N, 8.04;
6
4
4
8
2
2
12
2
3
−1
−1
(
uncorrected) were measured with a Perkin Elmer LS 50B. S, 4.60. λmax(MeCN)/nm 387 sh. (ε × 10 /M cm , 32.4), 361
Emission and excitation monochromator slit widths were both sh. (44.5), 347 (54.4), 284 (49.0), 252 (216). IR (ATR) ˜v max/cm
−
1
typically set to 5 nm.
3065, 1609, 1600 (s), 1552, 1476, 1458, 1425, 1404, 1251, 1026,
−
1
Fluorescence lifetimes were determined by the time corre- 1015, 832 (vs, PF6 ), 789, 758, 691, 657, 555 (s). H NMR
lated single photon counting (TCSPC) technique using a Fluoro- (500 MHz, MeCN-d ) δ 9.04 (4 H, s, H
B3′,5′
3
), 8.70 (4 H, dt, J 8.0
A3
C4
E4
Log
3 emission spectrometer (HYJ) equipped with an 1.1, H ), 8.44 (2 H, d, J 3.7, H ), 8.31 (4 H, dt, J 8.8, 1.1, H ),
E1
A4
R3809U-58 MCP (Hamamatsu) and an N-403L laser diode (HYJ) 8.12 (4 H, dt, J 8.8, 1.1, H ), 7.97 (4 H, td, J 7.8, 1.5, H ), 7.93
with an excitation wavelength of 403 nm (<200 ps FWHM).
Femtosecond transient absorption (TA) experiments were
carried out with an amplified Ti:Sapphire laser system (4 H, ddd, J 7.2, 5.5, 1.2, H ). C{ H} NMR (151 MHz,
CPA-2101 fs laser (Clark MXR; output 775 nm, 1 kHz, and 150 CD NO –CD Cl 4 : 1 v/v) δ 152.5 (C
E3
(4 H, ddd, J 8.8, 6.5, 1.3, H ), 7.69 (4 H, ddd, J 8.8, 6.5, 1.3,
E2
C3
A6
H
), 7.66 (2 H, d, J 3.7, H ), 7.50 (4 H, dt, J 5.5 1.2, H ), 7.23
A5
13
1
A2o.B2′,6′
C2o.C5
3
2
2
2
), 151.4 (C
),
A2o.B2′,6′
A6
DE4′
A4
fs pulse width) using a transient absorption pump/probe 151.0 (C
detection system (TAPPS Helios, Ultrafast Systems). The (C ), 143.3 (C
), 150.2 (C ), 149.9 (C
), 143.6 (C ), 143.4
B4′
C2o.C5
D9
C3
E3
), 140.0 (C ), 135.0 (C ), 132.7 (C ),
C4
E4
A5
E2
A6
3
87 nm excitation wavelength was obtained by frequency-dou- 132.1 (C ), 131.9 (C ), 130.0 (C ), 129.1 (C ), 128.2 (C ),
DE1′
A3
B3′,5′ 19
bling of the fundamental wavelength in the frequency genera- 128.0 (C
), 125.3 (C ), 121.3 (C
). F NMR (282 MHz,
−
tor STORC (Clark-MXR). For excitation an energy of 200 nJ per MeCN-d ) δ 72.3 (d, J 708, PF6 ) MS (ESI) not stable, see
3
F,P
pulse was selected.
main text.
Nanosecond transient absorption experiments were carried
[Zn(MeATT) ][PF ] (2). [MeATT][PF6] ligand (109 mg,
2 6 4
out with a Nd:YAG laser. The 355 nm excitation wavelength 167 μmol) and Zn(OTf) (31 mg, 84 μmol) were suspended in
2
was formed by third harmonic generation. Moreover, pulse MeOH–MeCN (20 mL, 4 : 1 v/v). The orange-yellow reaction
widths of less than 5 ns with energies of up to 7 mJ were mixture was refluxed for 2 h to give a clear solution. After
selected. The optical detection was based on a pulsed Xenon being cooled down to r.t. an aqueous NH PF solution (314 mg
4
6
lamp, a monochromator, a photomultiplier tube or a fast silicon in 6 mL) was added and precipitation was completed by the
photodiode with 1 GHz amplification and a 500 MHz digital addition of water (20 mL). The bright yellow solid was isolated
oscilloscope. The laser power of every laser pulse was registered by filtration (G4 glass frit), washed with water (3 × 20 mL) and
using a bypath with a fast silicon photodiode. The experiments left to dry overnight in air to give pure 2 (88%, 123 mg,
were performed in a 5 mm × 10 mm quartz glass cuvette.
74 μmol). It can be recrystallized from a mixture of MeCN–
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2 2
CH Cl (30 mL 2 : 1 v/v) by diethyl ether vapor diffusion (recov- stoppered two-neck 50 ml round-bottom flask made from stan-
ery 61%). Mp >270 °C (darkens), 303 °C (decomp.). Found: C, dard borosilicate glass, in a vigorously stirred oxygen-saturated
4
7.47; H, 2.97; N, 6.93; S, 3.70. C66
46 8 2 4
H N S ZnP F24 requires C, (30 min bubbling) MeCN–MeOH solution (25 ml, 4 : 1 v/v) irra-
3
4
7.74; H, 2.79; N, 6.75; S, 3.86. λmax(MeCN)/nm 437 (ε × 10 / diated at a 30 cm distance by a 400 W high pressure Hg lamp
−
1
−1
M
cm , 22.5), 366 (46.2), 347 (67.2), 334 (67.5), 284 (57.5), (Helios Italquartz). An aqueous NaNO2 solution (0.72 M) of
−
1
2
62 (194). IR (ATR) ˜v max/cm 3115, 1612 (s), 1575, 1553, 1478, 5 cm path length directly in front of the reaction flask
−
1
6
426, 1273, 1250, 1028, 1015, 837 (vvs, PF ), 793, 764, 557 (s). served as >385 nm longpass and, particularly important, also
1
B3′,5′
H NMR (500 MHz, MeCN-d ) δ 9.01 (4 H, s, H
d, J 7.9, H ), 8.70 (4 H, d, J 9.1, H ), 8.56 (2 H, d, J 3.8, H ), 0.5 min (initial) to 5 min time intervals. Details about the pro-
), 8.77 (4 H, as a heat filter. Samples were withdrawn (by syringe) in
3
A3
E4
C4
E3
36
8
.48 (4 H, m, partially superimposed, H ), 8.47 (4 H, d, J 9.1, cedure can be found in the reports of Murata et al. and Zhao
E1
A4
43
partially superimposed, H ), 8.21 (4 H, td, J 7.9, 1.4, H ), et al., specifically, we adopted the semi-preparative mode of
E2
A6
8
.03 (4 H, dd, J 7.8, 1.7, H ), 7.91 (4 H, dt, J 4.9, 1.4, H ), 7.83 operation but without the stringent use of 10 mol% photo-
C3
A5
(2 H, d, J 3.8), H ), 7.46 (4 H, ddd, J 7.9, 4.9, 1.4, H ), 4.92 (6 catalyst.
D10 13
1
D9
H, s, H ). C{ H} NMR (126 MHz, MeCN-d ) δ 153.7 (C ),
Experiments were performed with the absorbance
3
A2o.B2′,6′
A6
B4′
A2o.B2′,6′
1
1
51.3 (C
44.8 (C
), 149.2 (C ), 149.1 (C ), 148.5 (C
), maximum of the sensitizer adjusted between 0.1 and
C2o.C5
DE4′
A4
E3
37c,43b
), 142.7 (C
), 142.3 (C ), 140.0 (C ), 137.4 0.3,
preferably lower concentrated and optically
C2o.C5
C3
C4
E1
E2
(C
), 135.8 (C ), 131.2 (C ), 130.5 (C ), 129.6 (C ), matched (vide infra). The intrinsically different absorption
A5
DE1′
A3
B3′,5′
1
28.7 (C ), 128.1 (C
), 124.4 (C ), 120.8 (C
), 119.8 profiles (and small differences in optical match) were cor-
E4
D10 19
(C
), 40.2 (C ), F NMR (282 MHz, MeCN-d ) δ 72.9 (d, JF,P rected by use of the intensity (also called light-harvest or
3
−
2+
−A(λ) 43,7b,59
7
07, PF6 ) HRMS (ESI) m/z 684.09337 ([M − 2 × PF ] , calc. for absorbance) correction factor Farea = 1 − 10
for
6
6
4
C
66
H
46
N
8
S
2
ZnP
2
F
12 684.09255, Δ 1.20 ppm, total of 7 385 nm (filter cut-off) to 800 nm. The quantum yields of O
2
1
resolved signals with 0.5 m/z spacing).
( Δ
g
)
generation (Φ
Δ
)
were calculated by the usual
6
0
equation for relative determinations. Concentrations of all
reagents and the product 5-hydroxy-1,4-naphthoquinone
X-ray structure elucidation
(
Juglone) were calculated from the known molar extinction
Single crystals of 2 were grown by diethyl ether vapor diffusion
into a MeCN solution of the complex. A suitable crystal
coefficients (ε) at λmax (DHN and Juglone, see ref. 36; own
sensitizers, see Table 2). The molar extinction coefficients of
(0.2968 mm × 0.051 mm × 0.0264 mm, orange needle) was
2
+
[Ru(bpy)
3
]
and MB were determined for the specific
selected and measured on an Agilent SuperNova Dual Cu K
α
−1
−1
5
7
solvent mixture and found to be 15 200 M
[Ru(bpy) ] , and 93 850 M cm for MB at λmax, respect-
ively, which are both in good agreement with the values
reported.
Irradiance was measured to 39 ± 5 mW cm at the front
cm
for
Atlas diffractometer. Using Olex2, the structure was solved
2
+
−1
−1
5
8
3
with the ShelXS structure solution program using direct
5
8
methods and refined with the ShelXL refinement package
using least squares minimisation (Table 1).
6
1,62
−2
−
2
side of the reaction vessel position and 28 ± 5 mW cm
Photochemical experiments
behind (average of a 30 s measurement interval) by aid of a
digital power meter (PM100, Thorlabs) equipped with a manu-
facturer calibrated thermal sensor (S212A, Thorlabs). Note that
the large standard deviation is due to fluctuations of the lamp
intensity itself as indicated by the voltmeter of the lamp power
Photochemical reactions with DHN as a substrate (1.58 ±
−4
0
.09 × 10 M, Abs. ≈ 1.2) were performed in a septum-
63
Table 1 Crystal data and structure refinements of 2
supply. Standard ferrioxalate actinometry (0.012 M) revealed
an average photon flux of 4.4 ± 0.1 × 10− Einstein s
7
−1
Composition
Formula
[Zn(MeATT)
2
][PF
6
]
4
·5MeCN
(
2 runs‡) in the range of the actinometer absorbance (here:
76 61
C H F
N
24 13
4
P S
2
Zn
385–500 nm). Based on some approximations (see ESI‡) and
correction for the normalized fraction of incident light not
covered by the actinometer, i.e., the 546, 577, and 579 nm Hg
−1
F.W./g mol
1865.75
Temperature/K−
Abs. coeff./mm
Crystal system
1
200.00 ± 0.10
2.630
Monoclinic
−2
lines, a similar minimum value (22 ± 4 mW cm ) was
6
3,64
Space group
Z
P2
4
1
/c
calculated.
Some irradiations, such as NMR runs with (+)-limonene
and 2-methylnaphthalene, were exemplary also performed
a/Å
b/Å
c/Å
β/°
38.6752 (7)
8.61835 (12)
24.8997 (4)
104.5408 (18)
8033.7 (2)
28 592
with
a Rayonet® photoreactor (RPR-100, Southern New
−
3
England Ultraviolet Co.) equipped with 16 × 35 W low-
pressure Hg lamps (RPR-4190A, λmax 419 ± 40 nm) and a
merry-go-round inset. Samples were also aerated with O2
and irradiated in standard 5 mm (o.d.) NMR tubes or
1 cm (o.d.) Pyrex® photoirradiation tubes with a threaded
septum cap.
Volume/Å
Reflections coll.
Indp. Reflns.
14 124
1.036
2
GoF on F
Final R indexes (I ≥ 2σ(I))
Final R indexes (all data)
Disordered parts
R
R
1
= 0.0743, wR
= 0.0950, wR
2
0.2044
0.2206
1
2
F atoms site disordered on 2PF
6
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Computational methods
The geometries were optimized to the convergence criteria of
−
5
−1
1
× 10
E
h
a
0
(opt = tight) by the use of a B3LYP hybrid func-
tional with an explicit combination of 6-31G(d) (C, H, N, S)
6
5
66
and LANL2DZ (Zn) basis sets (plus 18 electron ECP)
together with a CPCM solvent model for MeCN. All calcu-
6
7
lations were performed with the Gaussian 09 package
without symmetry constraints and were found to be stable
minima by normal mode analysis (under standard state con-
ditions). Time-dependent density functional theory calcu-
lations (TD-DFT) were performed on the same level of theory
and with the same solvent model for 150 singlet and 30 triplet
transitions calculated each.
1
Fig. 1 Partial H NMR spectra (aromatic region) and assignment of the
+
Results and discussion
Synthesis and characterization
terpyridine proton resonances from the ligands (MeATT , violet; ATT,
II
green) and the corresponding Zn complexes (1, red; 2, cyan) recorded
in MeCN-d
3
solution, except for the ATT ligand which had to be
II
recorded in CDCl
reasons.
3
(crossed out signal at 7.26 ppm) due to solubility
The newly synthesized Zn complexes have been prepared by
the reaction of two equivalents of the appropriate ligand with
Zn(OTf)
2
in water-miscible solvent mixtures and precipitated
PF solution as bright
by addition of an excess aqueous NH
4
6
could be obtained for 2 with a m/z peak pattern for the [M −
2
+
yellow solids. Further purification was usually not necessary
but the complexes were routinely recrystallized once by diethyl
ether vapor diffusion into a medium concentrated MeCN solu-
tion. By this technique suitable crystals could be obtained for
X-ray structure analysis of 2 (see next section). In contrast to
the bis(terpyridine)ruthenium(II) complexes, a color change
2
× PF
6
]
pseudo-molecular ion at m/z 684.09337 and a well-
resolved isotope pattern, but the m/z 507.2 ligand signal con-
stitutes usually the base peak in low resolution ESI spectra.
For complex 1 no pseudo-molecular peak could be observed.
Even with the MALDI-ToF technique and a non-protic matrix,
such as DCTB,‡ loss of a ligand and substitution by a fluoride
ion (or matrix molecule in case of DHB) was found as the
strongest signal (see ESI‡). The fluoride ion source originates
(by visual inspection) cannot serve as an indication for
complex formation as bis(terpyridine)zinc(II) complexes
1
0
51
−
possess no MLCT or LMCT transitions (d ion). Some photo-
physical key parameters of the studied compounds are given
in the corresponding section (vide infra).
presumably from the decomposition of the PF6 counterion
under MALDI conditions.
The complexes may be distinguished from the pure ligand
1
in the usual manner by H NMR spectroscopy due to character-
Crystallographic results
6
8
istic shifts of the proton resonances up on complexation, i.e.,
the 3′/5′-protons on the 4′-substituted tpy ligand, which thus
appear as a singlet resonance, undergo a downfield shift or
the pronounced upfield shift of the 6/6″-protons (Fig. 1). The
0-methylacridinium compounds of the series can also be
easily assigned due to the signal of the methyl group protons
at 4.74 ppm for the MeATT ligand and 4.92 ppm for the
3
corresponding complex in MeCN-d , respectively (omitted in
Fig. 1, see ESI‡). Full NMR spectroscopic assignment was
possible by means of 2D techniques (see ESI for spectra‡) and
comparison to the Ru analogs.
Furthermore, the success of the anion metathesis could be
4
+
The molecular structure of the complex cation [Zn(MeATT)2]
2) is shown in Fig. 2. Selected bond lengths and angles are
(
compiled in Table 3 together with selected references and DFT
calculated geometries for 1 and 2.
The bond lengths and angles for the bis(κ N,N′,N″)
coordinated metal center are in good agreement with
several structure reports for the pristine [Zn(tpy)2] complex,
i.e., the average distance (d) of the principal axis (davg.(Zn–N′)
1
3
+
2
+
72
2.084 ± 0.011 Å) or the corresponding angle which commonly
II
53
1
9
69
−
directly proven by F NMR spectroscopy where the PF
6
counterion gives rise to a doublet at −72.3 ppm (JP,F 708 Hz)
−
70
vs. the singlet resonance of CF SO at ca. −77 to −80 ppm.
3
3
The infrared spectra of 1 and 2 show only marginal differences
−
and are dominated by the very strong absorption of the PF
6
−1 69
ion at 834 ± 3 cm
.
4+
Fig. 2 Structure of the [Zn(MeATT)
2
]
6 4
cation (2) in [2][PF ] × 5 MeCN
Mass spectrometric (MS) analysis was particularly difficult
with thermal ellipsoids plotted at the 50% probability level. Important
atoms are labeled but hydrogen atoms, solvent molecules, and counter-
II
71
for the kinetic labile Zn complexes compared to the corres-
II
53
ponding Ru complexes. High resolution MS-ESI spectra ions are omitted for clarity.
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derives from the ideal 180° geometry (∠avg.(N′–Zn–N′) 172.9 ±
6
.1°).
II
Only a few 4′-π-extended bis(terpyridine) complexes of Zn
7
3
73b
have been reported to date, among them p-biphenyl, p-ter-
phenyl as well as p-phenyl-conjugates with 1,2,4-triazole
or carbazole
pyridin-4-yl)phenyl extended complex. However, to the best
of our knowledge, no thienyl-substituted Zn complex has
7
3c
73d
7
3e
moieties and recently a well-comparable 4′-(4-
7
3f
II
been reported yet, which limits the direct comparison to the
phenyl-spacer. Statistical evaluation of the conjugated com-
plexes shows a marginally shorter bond length of the principal
coordination axis (davg.(Zn–N′) 2.070 ± 0.014 Å) but a similar
non-linearity at this axis (∠avg.(N′–Zn–N′) 174.6 ± 2.0°). The
remaining Zn–N′/N″ bond lengths of the coordination sphere
are elongated (ca. 0.11 Å) and unequally distributed (ca.
0
.01 Å) between the two terpyridine ligands and can be aver-
aged to 2.183 ± 0.017 Å. The individual angle to the main axis
N′/N″–Zn–N′ corresponds to 75.5 ± 0.8°.
The structural parameters of 2 fall well within all of these
boundaries (see Table 3).
+
Fig. 3 UV/Vis absorption spectra (left scale) of the ATT and MeATT
ligands, and acridine (dotted lines in shades of blue), the corresponding
Zn complexes 1 and 2 (shades of green), and the Ru complexes 3, 4, 5
No remarkable packing features were found, i.e., in contrast
to the 4′-(4-pyridin-4-yl)phenyl extended bis(terpyridine)zinc(II)
II
II
2+
as well as [Ru(bpy)
3
]
(orange-red) in MeCN. Part of the AM 1.5 solar
7
3f
82
complex reported by Hayami et al. The terpyridine fragment spectra (right scale, 37° global tilt irradiance) is also shown in semi-
of 2 shows a weak interaction with the outer pyridyl-ring transparent. Note that the scaling was arbitrarily chosen.
A
A
among one another (d(py ctr.⋯py ctr.) 3.709 Å), which is typical
7
4
for terpyridine solid state assemblies. The acridinium units
face each other with a considerable offset (only partial overlap methylacridinium-substituted
bis(terpyridine)ruthenium(II)
+
at the outer ring) excluding any strong π–π interaction (d(acr - complexes and the ligands alone have been described separ-
+
53
plane⋯acr pl.) 3.529 Å).
ately. Briefly, acridines show local absorbance maxima at
7
6,77
For the structure description of the ligand fragment of 2 390, 360, and 345 nm
whereas the narrow absorption band
7
8
other thienyl–tpy metal ion complexes may also serve for com- at 365 nm and a broader second band at ca. 420–450 nm
−
1
−1
parison, i.e., 4-bromo- and 5-bromo-2-thienyl substituted bis- (with ∼20 000 and 7000 M cm , respectively) are typical for
II 75
79
(
terpyridine) complexes of Ru . For this series of complexes, acridinium ions (see Fig. 3).
II
the mean bond distance between the tpy unit and the thienyl
The spectral signature of the Zn complexes 1 and 2
spacer lies within 1.467 ± 0.004 Å. Therefore, complex 2 shows resemble largely the one of the pure ligands but with an
a slightly elongated bond on one ligand in the solid state approximately doubled molar extinction coefficient (ε) as sup-
assembly (see Table 3). On the other hand, the 4′-phenyl-tpy posed for the formation of a homoleptic bis(terpyridine)
II
derived Zn complexes show a mean bond distance for the complex. Differences are visible in the 300 to 350 nm region
inter-connecting bond of 1.475 ± 0.033 Å. The dihedral angle where pronounced absorption bands emerge in the case of 1
φ
tpy-ph averages to 26 ± 15°, for these complexes whereby the and 2 (see Fig. 3). These differences can be attributed to the
large standard deviation indicates a rather rotational flexible saturation, i.e. by coordination or protonation, of the terpyri-
phenyl unit. Similar indications were found for the thienyl dine nitrogen atoms and the thereby induced conformational
spacer by statistical X-ray structure evaluation (φ 8.6 ± 9.3°) changes. This phenomena was i.a. studied for the 4′-(9-anthra-
II
53
and DFT calculations (φ 20.4 ± 1.9°; both for Ru ) by us.
cenyl)-terpyridine ligand in the early seminal work of Maestri
II
51
However, in the case of the 4′-phenyl-tpy derived Zn com- et al. as well as for pyrenyl-substituted terpyridines more
plexes some examples with a nearly planar arrangement of recently.
φtpy-ph (1 ± 1°) on at least one ligand fragment have also been For the [Zn(MeATT)
4
9
4
+
2
]
complex (2) the local maximum in
7
3b,d
observed.
the visible range is slightly blue shifted (437 nm) in compari-
+
son to the MeATT ligand (454 nm) but the absorption bands
itself are very similar in shape (see Fig. 3). The ATT ligand and
its Zn complex 1 show only a slight absorbance in the visible
Photophysical properties
II
The molar absorption spectra of the ligands and the corres- region, viz. the ca. 385 nm bands tail towards 410 and 430 nm,
II
II
ponding Ru and Zn complexes are depicted in Fig. 3 respectively, due to the large ε values. The increase in ε is
together with the solar irradiance air mass (AM) 1.5 spectrum. caused by the increased conjugation as well as favorable pro-
Some photophysical key parameters are compiled in Table 2. perties of the thienyl linker, e.g. as shown in combination with
8
0
The photophysical properties of the acridine and 10- a triphenylamine (TPA) unit, which can be reasoned by the
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bathochromic shift comparing pristine acridine to the AT or
the ATT ligand. The slightly stronger increase in ε (more than
II
twice) for the Zn complexes may also be caused by changes in
4
9
intra-ligand (IL) transitions due to coordination – even for
1
0
“
innocent” d metal ions which were studied in detail by
8
1
Schubert et al.
II
The Ru complexes show the typical broad MLCT absorp-
tion band in the 450–550 nm region. As expected, the
thienyl-linker caused a large increase in ε for 4 and 5 in com-
8
0
2
+
parison to the linker-free [Ru(AT)
2
]
complex (3). The influ-
ence of the acridinium unit is discernible by the increased
absorption in the 410–470 nm region for 5 as deduced from
II
the Zn counterpart 2 (see Fig. 3).
The fluorescence quantum yields (Φ ) in MeCN, a typical
fl
example of a polar organic solvent, are low (∼0.02) in all cases
where fluorescence can be observed (see Table 2). This is
expected for rotational unhindered acridinium ions with a
7
9,83
donor-type arene-substituent
(e.g. 9-thienyl-10-methylacri-
7
9a
dinium Φfl < 0.005, τ ∼ 100 ps;
9-ph- Φfl 0.063–0.005,
7
9c,d
79c,d
τ 1.5 ns;
9-p-biph- Φfl 0.020, τ 1.2 ns
) and as well
expected for pristine and substituted acridines based on the
literature data for organic solvents (e.g. 9-phenylacridine Φfl
8
4
84–86
0
.080, τ 1.4 ns (for MeOH)).
For the ATT ligand and its
II
Zn complex fluorescence lifetimes of 0.73 and 0.22 ns were
determined, respectively, corroborating the aforementioned
literature.
Fig. 4 Upper part: differential absorption spectra (visible) obtained
To gain further insights into the photophysical behavior of
the compounds transient absorption (TA) measurements were
carried out. The signature of the TA spectrum for the
2+
upon fs flash photolysis (387 nm, 200 nJ per pulse) of [Zn(ATT)
2
]
(1) in
deaerated MeCN with several time delays between 0.1 and 7500 ps.
Lower part: differential absorption spectra (visible) obtained upon ns
2
+
[
Zn(ATT)
2
]
complex (1) is very similar to the spectra for the flash photolysis (355 nm, 6 mJ per pulse) of 1 in deaerated MeCN with
II
53
time delays of 1.6 μs, 7.5 μs, 29 μs and 88 μs.
Ru counterparts reported elsewhere, with the exception of
1
the MLCT bleaching (see Fig. 4). The major decay component
of complex 1 (225 ± 47 ps) agrees well with the fluorescence
decay dynamics as determined by TCSPC. However, in TA
spectra the formation of a signature with two weak but distinct
maxima at 440 and 645 nm at early times is visible (1–6 ps)
(see Fig. 4). At later times, i.e., >150 ps, a slightly different sig-
nature emerges with two well-defined maxima at 475 and
3
Table 2 Lowest energy absorption band maximum (λabs/nm, ε × 10 /
M
−1
−1
cm in parenthesis), highest energy emission maximum (λem/nm),
quantum yield of fluorescence (Φ ), and lifetime (τ/ns) in deaerated
fl
MeCN solution at r.t
Compound
ATT
λ
abs (ε)
λ
em
Φ
fl
τ/ns
625 nm (see Fig. 4). This transient does not decay within our
a
a
b
c
time resolution of 7.5 ns. The decay of this species was sub-
sequently determined by ns laser flash photolysis and was
387 (11.3)
589
0.017
0.73 ± 0.02
0.71 ± 0.06
+
—d
545
MeATT
454 (8.8)
387 (32.5)
—
0.019
n.d.
0.22 ± 0.01
II
shown to be quite slow (15.8 ± 0.6 μs), exceeding the Ru
2
2
+
b
c
Zn(ATT)
counterpart (1.09 μs) by an order of magnitude. It was readily
0
1
.22 ± 0.05
c
5 800 ± 600
2
quenched by O . Therefore, we assigned this to the formation
4
+
e
d
d
Zn(MeATT)
2
457 (20.3)
484 (21.2)
504 (50.6)
—
—
—
—
0.14 ± 0.02
80 ± 6
0.27 ± 0.03
3
of the intra-ligand triplet state IL.
2
+ f
d
c
Ru(AT)
Ru(ATT)
2
—
2
2
+ f
d
c
The signature on an early timescale observed for the ATT
complex 1 is attributed to the singlet excited state. This assign-
ment is based on a comparison with the TA spectra of the
uncomplexed ATT ligand (see ESI‡) which resembles that of 1
—
c
c
1
090 ± 140
4
2
+ f
d
Ru(MeATT)
510 (47.4)
—
—
0.17 ± 0.02
a
Determined by the relative method. Error ± 0.011; average of
independent determinations in both groups. Reference standard (all at early timescales, viz. a weak maximum at ca. 440 nm and a
aerated) were 9,10-diphenylanthracene in EtOH Φfl 0.95, quinine
second, broader one at 630 nm. This transient decays mono-
exponentially within 713 ± 58 ps which coincides with the fluo-
disulfate in 0.05 M H
2 4
SO Φfl 0.55, fluorescein in 0.1 M NaOH Φfl 0.92
b
(cross-checked and within 10% of the values reported in ref. 59). To
compare both methods, explicit errors for τ are given, viz. determined rescence decay (see Table 2).
c
by time correlated single photon counting (TCSPC). Determined by
4+
The [Zn(MeATT)2] complex (2) shows only a very weak
d
e
transient absorption spectroscopy. Not luminescent. Shoulder (weak
−
1
transient signal (ESI‡) with two distinct transient absorption
bands at ca. 495 and 670 nm, which transform into a broader
vibronic structure typical for acridinium ions), λmax 437 (22 500 M
−
1
f
cm ). Ref. 53.
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signal centered at 580 nm, which then later decays quickly of the N–Zn bond but further decrease of the dihedral angle ϕ
144 ± 17 ps). between the terpyridine and thienyl unit. The bond distance
Further insight came from fluorescence upconversion overestimation of 0.066 ± 0.007 Å is typical for the DFT
experiments following excitation at 387 nm and detection at approach at this level of theory. The higher dihedral angle ϕ
(
2
+
5
80 nm. For the [Zn(ATT)2] complex (1) two decay lifetimes found experimentally on one ligand fragment might not be
were derived. One, which is about 0.23 ± 0.08 ns, and is there- overrated as the rotational flexibility of the thienyl group is
fore assigned to the fluorescence lifetime, determined already high (vide supra). The second dihedral angle (ω) between
by TCSPC and fs-TA and the second one of about 85 ps, which the spacer and the acridine/acridinium moiety was correctly
might be attributed to the ISC into the triplet state (see ns predicted by this method. In general, the X-ray relaxation
laser flash results). A similar behavior was found for the ATT by DFT gave a structure for the complex cation within a
ligand. This revealed basically the same decay as derived from root-mean-square deviation (RMSD) of 0.59 Å to the experi-
TCSPC and fs-TA (0.70 ± 0.07 ns) but also a faster component mental data set. An independent computational run to find
of ca. 48 ps, which is attributed to the ISC due to the overall the S
0
geometry (of same conformation) resulted in a geometry
low fluorescence quantum yields. The [Zn(MeATT)2] complex with a RMSD of 0.64 Å but 3 meV lower in energy. We
2) exhibits markedly shorter decay components of 4.5 ps and are therefore confident that the DFT results are also acceptable
6 ps, whereby the shorter component agrees well with the rise for 1.
4
+
(
2
of the fs-TA signal at 580 nm (see ESI‡).
For both compounds the MO analysis showed that the
highest occupied molecule orbitals (HOMO 0/−1) as well as
the lowest unoccupied MO set (LUMO 0/+1) are not degener-
Computational studies
Because no structural data could be obtained experimentally ated (no perfect D2d symmetry) but nearly isoenergetic (within
for 1, density functional theory (DFT) calculations were per- 11 meV,‡ artificially degenerated in Fig. 5). For the 1 the fron-
formed by the aid of the popular B3LYP/6-31G(d)/LANLDZ tier orbital sets are localized mainly on the acridinium frag-
3
7,43b
approach (=LACVP*).
The optimized structural parameters ment with some contributions from the thienyl linker. More
are included in Table 3. The second purpose was to obtain interesting, in this regard, is the ATT complex 2 where the
insight in the electronic properties of the complexes and to LUMO set is mainly composed by terpyridine fragment contri-
clarify the localization (in terms of energy as well as structure) butions. As supposed, and in contrast to the DFT calculations
II
53
of the lowest lying triplet state (T1).
for Ru series, no metal ion influence is deducible for the
II
DFT relaxation without restraints of the experimentally first few occupied and unoccupied MOs in the Zn complexes.
determined structure of complex 2 resulted in the elongation The individual MO plots are shown in the ESI.‡
Table 3 Comparison of selected experimentally determined structural parameters (distances (d)/Å, angles/°) with B3LYP/6-31G(d)/LANLDZ DFT
calculations for complexes 1 and 2 (and ΔE(SCF) energies to S /eV) as well as experimental data of some reference complexes
0
2
+
2+
2+
[
Zn(MeATT)
2
]
(2)
[Zn(ATT)
2
]
(1)
[Zn(R-tpy)
2
]
, 4′-R=
p-biph
n.a.
2.077 (3)
2.066 (3)
2.185 (3)
2.174 (3)
2.192 (3)
2.195 (3)c
1.486 (5) ,
1.478 (5)d
1.483 (5) ,
1.484 (5)
n.a.
7
3a
73b
73f
Parameter
Exptl.
DFT relaxed
S
0
T
1
S
0
T
1
Ph
4-pyph
ΔE(SCF)
n.a.
0.003
2.154
0
1.741
2.153
2.151
2.254 (1)
0
1.627
2.152
2.138
2.256 (1)
n.a.
n.a.
a
d(N2–Zn)
2.088 (3)
2.078 (3)
2.180 (4)
2.196 (4)
2.197 (4)
2.189 (4)
1.461 (6),
2.153 (1)
2.255 (1)
2.150
2.256
2.069 (4)
2.091 (4)
2.192 (4)
2.193 (4)
2.178 (4)
2.217 (4)c
1.475 (7) ,
1.469 (7)
n.a.
2.084 (4)
2.067 (4)
2.181 (4)
2.177 (4)
2.168 (5)
2.168 (5)c
1.487 (6) ,
1.471 (7)
a
d(N6–Zn)
d(N1–Zn)
d(N3–Zn)
d(N5–Zn)
d(N7–Zn)
d(Ctpy–Cspacer
2.255 (1)
)
1.463
1.464
1.464,
1.459
1.479,
1.448
1.480,
1.475
1.461
1.483 (1)
n.a.
1.461,
1.441
1.483,
1.398
b
b
1
.479 (6)
1.479 (6),
.484 (6)
1.478 (6),
.493 (6)
d
d(Cspacer–Cacr
)
1.479 (1)
1.480
1.480,
1.478
1.480
1.478 (6) ,
b
b
1
1.482 (7)
n.a.
d(N4/8–CMe
)
n.a.
n.a.
b
1
a
∠
∠
∠
∠
∠
(N2–Zn–N6)
176.02 (15)
75.06 (13)
75.25 (13)
75.43 (14)
75.64 (14)
20.8, 30.9
66.7, 71.3
179.33
73.99 (1)
179.22
74.00
73.97
178.99
73.97
74.02 (1)
179.56
74.02 (1)
179.73
73.95
73.93
74.20
173.2 (2)
75.4 (1)
75.5 (1)
75.3 (1)
74.9 (1)
42.6, 31.3
n.a.
175.6 (1)
75.5 (1)
75.6 (1)
75.6 (1)
75.9 (1)
16.8, 0.3
n.a.
177.8 (2)
75.8 (2)
76.3 (2)
76.1 (2)
75.4 (2)
46.2, 9.4
n.a.
(N2–Zn–N1)
(N2–Zn–N3)
(N6–Zn–N5)
74.00 (1)
(N6–Zn–N7)
e
b
b
ϕ
tpy-spacer
18.3, 20.4
68.9, 71.4
18.4, 20.3
69.9, 71.4
17.1, 22.4
14.6, 17.4
74.9, 77.2
5.2, 15.6
e
b
b
ω
spacer-acr
52.5, 70.4
10.3, 77.3
a
Principal coordination axis. For comparison with reported X-ray structures which employed a different numbering scheme care was taken to
b
c
adopt the alignment accordingly, i.e., lowest number to the left. Ligand fragment with triplet localization. Bond distance of Ctpy–Cspacer=phenyl
.
d
e
Bond distance of Cspacer−Cphenyl instead of Cacr
.
Dihedral angle was measured as cis-torsion angle along the S–C–C–C bonds. For spacer where
no preferred side exists, e.g. phenyl, the lower angle is given.
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0 1
primary MOs involved in the S → T transition are the HOMO
set (0/−1) and the higher unoccupied S states (LUMO+2/3/4)
n
whereas for 2 only the HOMO to LUMO (0/±1) sets are
involved. Both findings are in agreement with the transient
3
absorption data for a IL-based but differently located T state.
1
In both cases bond and angle distortions on the triplet loca-
lized ligand fragment (for geometry and spin density plots see
ESI‡) are observed. This was found to be especially severe for
the acridine moiety in 1 which is bent out-of-plane in “butter-
fly”-like fashion along the C9–N10 atom axis (γ 158.8°, α 8.5°,
β 25.2°, Δ(C9) 0.34 Å, Δ(N10) 0.10 Å; see ref. 88 for explanation
II
of the parameters), as also found for the Ru counterparts by
5
3
DFT calculations, or experimentally for over-crowded acridi-
8
8
nium ions.
Photocatalytic studies
II
In polypyridine chemistry Zn complexes are commonly used
as references to characterize the coordinated ligand without
interference from MLCT transitions. As indicated in the Intro-
II
duction, bathochromically-shifted highly absorbing Zn com-
Fig. 5 Molecule orbital (MO) energy levels for 2 and 1 as calculated by
(
(
TD-)DFT. The lowest energy optical transitions are indicated by arrows plexes, i.e., by suitable dye substitution and π-conjugation,
grey shading symbolizes size of CI coefficient) with their corresponding may also be useful for sensitization applications such as
energy and oscillator strength (f). The individual contributions to the
MOs from the different units of the complex are color coded as follows:
tpy (blue), thienyl (orange), acridine/acridinium (green). Note that Zn
contributions (cyan) are virtually not present in the selected
energy range and that the degeneracy threshold was manually set to
11
DSSCs. In this regard, preliminary tests by the use of the
11
in situ capping technique with the ATT and MeATT ligand
were discouraging and showed no noteworthy activity.
From the transient absorption data, it is evident that the
2
5.6 meV = kT (298 K, D2d approximation) to visualize otherwise over- acridine chromophore acts as a triplet energy sink which
lapping MOs.
1
imposed the use as an O
interesting since the neat absorption data of the Zn com-
plexes are roughly comparable with typical C^N Ir
2
( Δ
g
) sensitizer. This is especially
II
III 89
com-
TD-DFT calculations predict the lowest energy transition for
at 473 nm with considerable oscillator strength ( f 0.574)
which is an overestimation of the transition energy by ca.
.1 eV only, compared to the experimentally found data
Table 2). The transition is composed from the frontier orbital
plexes or improved multichromophoric versions studied by the
2
3
6,43
groups of Murata et al. and Zhao et al.
In this regard, the
4
+
[
Zn(MeATT)2] complex (2) resembles the absorption profile
0
(
2
+
II
of [Ru(bpy)
3
]
quite well whereas the corresponding Ru com-
plexes of this work exceed all examples in solar spectra cover-
age (see Fig. 3).
sets (HOMO/LUMO±0/1). The second lowest absorption of sig-
nificant oscillator strength f is predicted at 350 nm ( f 0.642)
and corresponds to a transition from the HOMO set (0/−1) to
the unoccupied second lowest MOs (+2/3) – an underestima-
tion in energy of the typical second absorption band for acridi-
nium compounds at 365 nm.
In the present case, the photooxidation reaction of DHN to
2
3
36,91
1
Juglone is used as an assay
for the O
2
g
( Δ ) generation
efficiency by monitoring the change of the absorption of DHN
at 300 nm in a MeCN–MeOH (4 : 1) mixed solvent system
(
Scheme 2).
For 1 a pure HOMO to LUMO transition is calculated at
52 nm followed by a nearly degenerated transition at
96/394 nm from the HOMO set (0/−1) to higher unoccupied
We avoided using strictly 10 mol% due to the different
4
3
intrinsic molar absorptivities of the studied compounds and
highly absorbing dyes, such as MB, typically employed as a
reference. The kinetic parameters, i.e., the pseudo-first order
rate constant (kobs) and initial reaction velocity (vinitl), were cal-
MOs (viz. LUMO+2/3/4). The latter transition is matching well
to the experimental spectra whereas the aforementioned ILCT
(acr → tpy) transition is largely overestimated (0.46 eV).
3
6
culated as reported. The linearized data plots to determine
Further explicit data on the calculated transitions are compiled
in the ESI.‡
kobs (see Fig. 6) indicate valid pseudo-first order conditions in
Triplet TD-DFT calculations (in S geometry approximation)
0
8
7
suggested vertical S
(
0 1
→ T transitions at 689/687 nm
1.80 eV) for 2 and 684/680 nm (1.82 eV) for 1, both energeti-
8
7
cally well separated (>0.62 eV) from higher energy transitions.
The excitation energies are similar to the energies obtained
from ΔE(SCF) calculation for the optimized geometries (viz.
1
adiabatic, see Table 3). In the case of ATT complex 1 the Scheme 2 Photooxidation of DHN to Juglone by O
2
( Δ
g
).
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36
Murata et al. report non-linear behavior for the DHN consump-
tion after about 20 min for their data set. It should also be men-
tioned that the product, Juglone, absorbs in the visible range and
therefore is competing with incident photons. Therefore,
−
1
although its molar absorption coefficient is quite low (3811 M
−
1 36
cm
), the use of early kinetics is favorable. On the other side,
this limits the possibility to achieve total conversion, except for
non-overlapping sensitizers like MB.
Despite the different absorption profile, we chose MB as
6
0
sens
the primary standard for the calculation of Φ
Δ
. The value
MB
of ΦΔ (∼0.5) appears to be independent of the solvent and
9
0,93
concentration range.
0
We chose the recommended value of
5
4
MB
.50 and a good reproducibility of Φ
Δ
was achieved in the
given concentration range for our data (see Table 4, entry
set 1).
2
+
As indicated above, we used [Ru(bpy)
3
]
as a cross-reference
to account for the absorption profile differences in between MB
and the compounds under study. However, it should not
2
+
remain unmentioned here that [Ru(bpy)
3
]
is somehow pro-
blematic because a more diversified data background exists in
comparison to MB. This was i.a. summarized in the review of
t
Fig. 6 Plots of ln([DHN] /[DHN]t=0) vs. irradiation time (t) for the photo-
oxidation of DHN using different sensitizers (color coding as in Fig. 3).
The lines represent the least squares fit of the linear data regime. The
different qualitative effective sensitizer concentrations (see Table 4 for
explicit data) are indicated as follows: (—) < (---) < (⋯).
27
38
DeRosa et al., and by Wilkinson and Abdel-Shafi et al. as
Ru(bpy)3
well, who reported a value of 0.57 for ΦΔ
lana and Braun et al. suggested 0.73 (MeOD-d ), but Tanie-
3
lian et al. determined for Φ
MeOH). In the latter case, an incident light × Φ vs. concen-
in MeCN. Orel-
4
5
Ru(bpy)3
Δ
0.77 (MeCN) and 0.87
9
4
(
Δ
−5
the time-period of the experiments (30 min) by correlation coeffi- tration study was also performed for [sens] > 8 × 10 M.
cient R > 0.98 for the least squares fit. The results are summar- However, our sensitizer concentrations are below this
2
ized in Table 4. Deviations of the DHN consumption, viz. threshold. There is a clear trend of lower ΦΔ values at lower
violation of the pseudo-first order conditions, are visible for MB concentrations and light intensities as examined from the first
2+
94
and our second standard [Ru(bpy)
3
]
both exemplary also few data points depicted by Tanielian et al. It is not within
employed at a higher concentration (3.1 and 12 mol%) and viz. the scope of this work to evaluate such effects, but our (low
9
2
absorbance, respectively. This justifies the call for low absor- concentration) data set reflects
a
value of 0.57 for
3
7c,43b
Ru(bpy)3
bance measurements
instead of comparison by mol%.
Φ
Δ
quite well.
Table 4 Sensitizer concentration and light harvesting parameters (absorbance A at given wavelength and absorption area correction factor Farea),
1
pseudo-first-order kinetics parameters of DHN consumption (kobs, vinitl), quantum yield for O
2
( Δ
g
) generation (Φ
Δ
) and yields of Juglone using
54,90
2+
complexes 1–5, the ATT ligand, and MB (=reference standard
) as well as Ru(bpy)
3
as photo-catalysts
a
b
[
sens] [DHN]t=0
k
obs3×
×
v
initl
×
A (at λmax
a.u. (nm)
)
F
(%)
area
Yield
(%)
−
−1
−4
−6
−1
sens
Δ
Entry#
Compound
mol%
10 min
10
M
10 M min
Φ
5
4,90
1
1
1
2
2
3
4
4
5
6
7
7
8
8
.1
.2
.3
.1
.2
MB
MB
MB
Ru(bpy)
Ru(bpy)
Ru(AT)
Ru(ATT)
Ru(ATT)
Ru(MeATT)
ATT
Zn(ATT)
Zn(ATT)
Zn(MeATT)
Zn(MeATT)
0.71
1.4
3.1
4.3
12
−22.7
−34.5
−90.8
−30.0
−67.4
−12.4
−34.4
−114 ± 1
−6.44
−2.5
1.68
1.59
1.53
1.52
1.53
1.58
1.61
1.51 ± 0.02
1.64
1.59
1.69
1.63
1.68
1.57
−3.66
−5.53
−13.9
−4.57
−10.3
−1.96
−5.88
−17.4 ± 0.01
−1.05
−0.39
−1.46
−2.93
−1.66
−2.24
0.113 (655)
0.216 (655)
0.448 (655)
0.099 (450)
0.295 (450)
0.120 (484)
0.079 (504)
0.273 ± 0.05 (504)
0.108 (510)
0.106 (387)
0.112 (387)
0.216 (387)
0.061 (437)
0.270 (437)
100
174
358
112
271
172
91
282 ± 5
142
44
39
77
83
281
0.50
0.42
0.51
0.52
0.50
0.15
0.82
54
56
71
46
74
17
52
70 ± 3
10
12
16
47
8
26
2
2
+
+
3
3
2
+
2
3.5
2
+
.1
.2
2
0.97
3.5 ± 0.1
1.4
5.9
2.0
4.1
1.6
7.6
c
2+
2
0.81 ± 0.01
0.10
0.12
0.49
0.50
0.26
0.11
4
+
2
2
2
+
+
.1
.2
.1
.2
2
−8.5
2
−18
4
4
+
+
2
−9.8
2
−14
a
Absorption correction factor F = 1 − 10−A(λ) = 1 − 10−ε(λ)cl integrated over 385–800 nm and normalized to 100% for MB as a standard. Yield of
b
−
1
−1 36 c
Juglone after 30 min (extrapolated if necessary) calculated from absorbance change Δ(A − At=0) at λmax 427 nm (3811 M cm ).
values of an independent determination on two different days.
Average
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The observed rate constants (kobs) and initial reaction vel- not with O
Photochemical & Photobiological Sciences
1
2
( Δ
g
) and in our case, no reaction at all was
ocities (vinitl) of our data set are comparable to values reported noticed in MeCN saturated with O . However, with respect to
2
4
3
III
III
56
by Zhao et al. for (multichromophoric) Ir and Pt com- the study of Griesbeck et al., we were limited to (very) polar
2
+
plexes as well as one modified [Ru(bpy)
3
]
example studied by solvents due the two- or fourfold charge of the complexes.
them. However, in our case the DHN consumption velocities Exemplary studies with limonene, which is a product-specific
−
7
−5
−1
1
95
span a wider range (10 –10 M min ). Direct comparison is probe to O
2
( Δ
g
), showed the expected product distribution
only possible in a singular case where MB was employed as a of hydroperoxides as probed via NMR and GC (after reduction
9
5d,97
reference together with the well-known Ir(ppy) complex as the to alcohols with NaBH
) due to the Schenck ene reac-
3
4
3
2
secondary standard. For the latter the Φ
different from the value reported by Murata et al. (0.50).
Despite the different light sources used, and in this case also 1275 nm could be directly observed for the well-performing
Δ
was somehow tion of the endo-cyclic double bond.
3
6
1
Last but not least, the typical O
2
g
( Δ ) NIR emission at
9
8
2
+
solvent systems, the kinetics determined by Zhao et al. (kobs [Ru(ATT)
2
]
complex 4 exemplarily probed in oxygen-saturated
matches MeCN solution at r.t. (see ESI for spectra‡).
well to our data (Table 4, entry no. 1.2 and 1.3, respectively).
−
3
−1
−6
−1 43b
−41.6 × 10 min , vinitl −8.32 × 10 M min )
3
6
The kinetic data reported by Murata et al., among them also
data for MB, are almost all an order of magnitude larger (and
Conclusions
4
3
even more for MB) as also noticed previously. In this regard,
4
3
II
II
we concur with the argumentation of Zhao et al. that this In summary, ATT ligand-based systems 1 (Zn ) and 4 (Ru )
1
g
might be due to higher lamp power, concentrations, and also revealed singlet oxygen ( Δ ) generation with quantum yields
II
the different implementation method (1 cm cuvettes vs. batch) as high as 0.82. Despite the fact that 1 (Zn ) features only a
employed. Yields are generally hard to compare as different poor overlap with the solar spectrum, efficient sensitizing
end points were used across all reports. Furthermore, as indi- evolves due to a long lived triplet excited state. In stark con-
II
cated before, product inhibition occurs for sensitizers absorb- trast, the good spectral overlap of 4 (Ru ) with the solar spec-
ing in the wavelength region of Juglone.
trum assists in extending the sensitization capability of the
2
+
With respect to the present work, the [Ru(ATT)
2
]
complex acridine building block. The acridinium containing systems 2
II
II
(
4) clearly stands out (Φ 0.82), followed by the corresponding (Ru ) and 5 (Ru ) as well as the linker-free acridine-terpyridine
Δ
II
II
Zn complex (Φ
Δ
0.50). As indicated in the photophysical pro- system 3 (Ru ) are ineffective sensitizers, presumably due to
perties section the absorption profiles are quite different. Fur- their short lived excited states. From the aforementioned it
1
II
thermore, acridine itself is also known to be a good O ( Δ ) seems interesting to revisit some of the highly optimized Ru
2
g
5
4
sensitizer, i.e., 0.83 ± 0.01 in MeCN and benzene, and this conjugates with respect to their singlet oxygen production.
value is nicely reflected by 4. However, acridine does not Notably, the design of metal complex containing conjugates
−
1
absorb significantly above 385 nm (=filter cut-off; ε < 730 M
carrying organic building blocks en-route towards the long-
cm ). Therefore our strategy to use the metal fragment as lived triplet energy sinks bears great potential. Nevertheless,
antenna” and possible ISC-enhancer for the organic dye stability with respect to singlet oxygen and photo-degradation
−
1
“
served both purposes. In this regard, it is puzzling that the needs careful consideration. Such conjugates may not be of
II
Zn complex exhibits a lower efficiency. But this might be greatest interest for the area of PDT – due to the presence of
regarded to the cut-off filter employed which could have also metal ions – but may evolve as efficient photosensitizers in
affected the ATT ligand itself (ΦΔ 0.10). The latter may have a green photochemistry. Here, widely used “off-the-shelf”
low fluorescence quantum yield indicating a possibly efficient organic sensitizers such as MB, RB, and others lack the needed
ISC but exhibits only a short excited state lifetime and no long term stability.
visible triplet–triplet absorption as discussed above. The
II
efficiency of the Zn complex can therefore be solely attributed
to its long triplet state lifetime. Complex 4, on the other hand,
deactivates markedly faster (but still above 1.0 μs) which may
Acknowledgements
be correlated with the metal-centered states typically thermally This work was supported by the Deutsche Forschungs-
accessible in bis(terpyridine)ruthenium(II) complexes. All other gemeinschaft (DFG) and the Bayerische Staatsregierung as part
complexes feature only very short excited state lifetime. Even of the “Solar Technologies go Hybrid” initiative. The authors
2
+
the [Ru(AT)
2
]
complex (3) is short-lived, despite the decent are grateful to Liselotte Siegfried and Dr Biljana Bozic-Weber
absorption profile and the acridine moiety, indicating a crucial (University of Basel) for preliminary DSSC tests of the
II
role of the thienyl-linker.
ligands in Zn complexes, to PD Dr Tilman Kottke (Bielefeld
In order to gain further insight in possible reactions with University) for the use of spectroscopic equipment, and to
2
, in particular of the acridinium compounds as shown by Dr Yvonne Hertle and Prof. Dr Thomas Hellweg (Bielefeld
O
5
6
Griesbeck et al. for the lively discussed 9-mesityl-10-methyl- University) for the use of the 400 W high pressure Hg lamp.
5
5
acridinium system, we studied the reaction towards the well- We furthermore thank the regional computing center of the
9
5
96
known limonene and 2-methylnaphthalene probes as sub- University of Cologne (RRZK) for providing the CPU time on
strates. The latter can only react with the superoxide ion but the DFG-funded supercomputer ‘CHEOPS’ and support.
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5
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Paper
Photochemical & Photobiological Sciences
Selektivität der photosensibilisierten O
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