Improved light absorbance does not lead to better DSC performance: Studies on a ruthenium porphyrin-terpyridine conjugate

Article abstract of DOI:10.1039/c5ra27397h

The preparation and characterization of 7-(4-([2,2′:6′,2′′-terpyridin]-4′-yl)phenyl)-5,10,15,20-tetraphenylporphyrinatozinc(ii), 3, are reported, and the structure of 3 has been confirmed by a single crystal structure determination. Reaction of RuCl₃·3H₂O with diethyl (4-([2,2′:6′,2′′-terpyridin]-4′-yl)phenyl)phosphonate, 4, followed by 3 in reducing conditions gives [Ru(3)(4)][PF₆]₂. In solution, 3 and [Ru(3)(4)][PF₆]₂ undergo two, reversible porphyrin-centred oxidation processes at lower potential than the Ru²⁺/Ru³⁺ process in [Ru(3)(4)][PF₆]₂. In the solution absorption spectra, the Soret and Q bands in 3 are little perturbed upon complex formation; the MLCT band in [Ru(3)(4)][PF₆]₂ has λ_max = 492 nm. Spectroelectrochemical data for 3 and [Ru(3)(4)][PF₆]₂ are presented. [Ru(3)(4)]²⁺ binds to nanoparticulate TiO₂ and the solid-state absorption spectrum confirms enhanced light absorption with respect to the standard dye-sensitized solar cell (DSC) dye N719. However, the photoconversion efficiencies of DSCs sensitized with [Ru(3)(4)]²⁺ are disappointingly low. Transient absorption spectroscopic studies on this series of compounds indicate that triplet-triplet energy transfer processes are likely to be responsible for this poor performance.

Full text of DOI:10.1039/c5ra27397h

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PAPER
Improved light absorbance does not lead to better
DSC performance: studies on a ruthenium
porphyrin–terpyridine conjugate†
Cite this: RSC Adv., 2016, 6, 15370
a
b
b
a
Angelo Lanzilotto, Laura A. Buldt, Hauke C. Schmidt, Alessandro Prescimone,
¨
b
a
a
Oliver S. Wenger, Edwin C. Constable and Catherine E. Housecroft*
0
0
00
0
The preparation and characterization of 7-(4-([2,2 :6 ,2 -terpyridin]-4 -yl)phenyl)-5,10,15,20-tetraphenyl-
porphyrinatozinc(II), 3, are reported, and the structure of 3 has been confirmed by a single crystal
0
0
00
0
structure determination. Reaction of RuCl
phosphonate, 4, followed by 3 in reducing conditions gives [Ru(3)(4)][PF
PF undergo two, reversible porphyrin-centred oxidation processes at lower potential than the Ru /
3
$3H
2
O with diethyl (4-([2,2 :6 ,2 -terpyridin]-4 -yl)phenyl)
6 2
]
. In solution, 3 and [Ru(3)(4)]
2+
[
6
]
2
3
+
Ru process in [Ru(3)(4)][PF
perturbed upon complex formation; the MLCT band in [Ru(3)(4)][PF
Spectroelectrochemical data for 3 and [Ru(3)(4)][PF
TiO
standard dye-sensitized solar cell (DSC) dye N719. However, the photoconversion efficiencies of DSCs
6 2
] . In the solution absorption spectra, the Soret and Q bands in 3 are little
6
]
2
2
has
l
max
¼
492 nm.
+
6
]
2
are presented. [Ru(3)(4)] binds to nanoparticulate
2
and the solid-state absorption spectrum confirms enhanced light absorption with respect to the
Received 21st December 2015
Accepted 27th January 2016
2+
sensitized with [Ru(3)(4)] are disappointingly low. Transient absorption spectroscopic studies on this
series of compounds indicate that triplet–triplet energy transfer processes are likely to be responsible for
this poor performance.
DOI: 10.1039/c5ra27397h
www.rsc.org/advances
sensitizers have combined polypyridylruthenium(II) and
Introduction
20–22
porphyrin domains in a single molecular species.
Our
Ever since the rst development of Gr ¨a tzel n-type dye-sensitized approach to ruthenium(II) dyes containing light-harvesting
solar cells (DSCs) employing sintered semiconductor nano- porphyrin domains is predicated upon heteroleptic
II
1–3
2
particles functionalized with ruthenium(II)-based dyes, efforts {Ru (tpy) } domains in which the two tpy ligands bear
have been concentrated on the improvement of the photo- anchoring and porphyrin substituents, respectively. The
conversion efficiency. At present, state-of-the-art photo- majority of oligopyridine–porphyrin conjugates are character-
conversion efficiencies approach ꢀ13% in DSCs using ized by the attachment of the metal-binding domains directly or
ruthenium(II)-based, metal-free organic and zinc(II) porphyrin- with a spacer to the phenyl-substituents of 5,10,15,20-tetra-
4
–16
23
based dyes.
system II has lead to signicant interest in bioinspired devices
Nature's reliance on porphyrins in photo- phenyl-21H,23H-porphyrins.
We have now developed new approaches for the function-
utilizing porphyrin or metalloporphyrin-based sensitizers in alization of porphyrins in which oligopyridines and their metal
4,17–19
DSCs,
reported by Gr ¨a tzel and coworkers for a porphyrin dye incor- porphyrin core. The selective monobromination of H
and a power conversion efficiency of 13% has been complexes are attached directly to a pyrrole ring of the
TPP in the
porating a donor–p–bridge–acceptor domain combined with 7-position developed by Zhang and coworkers, provided an
2
24
1
4
a cobalt(II)/(III)redox shuttle.
attractive opening for our synthetic investigations into mono-
0
0
00
While both ruthenium- and porphyrin-based sensitizers are functionalization of a porphyrin core with a 2,2 :6 ,2 -terpyridine
popular choices in DSCs, to the best of our knowledge, few (tpy) domain.
Experimental
a
Department of Chemistry, University of Basel, Spitalstrasse 51, CH-4056 Basel,
General
Switzerland. E-mail: catherine.housecro@unibas.ch
b
Department of Chemistry, University of Basel, St. Johanns-Ring 19, CH-4056 Basel,
Microwave reactions were carried out in a Biotage Initiator 8
Switzerland
1
13
31
reactor. H, C and P NMR spectra were recorded at room
temperature using a Bruker Avance III-500 NMR spectrometer.
†
Electronic supplementary information (ESI) available: Fig. S1 and S2: HMQC
and HMBC spectra of 3; Fig. S3 and S4: additional CV and
spectroelectrochemical data for 3. CCDC 1442417. For ESI and crystallographic
data in CIF or other electronic format see DOI: 10.1039/c5ra27397h
1
13
H and C NMR chemical shis were referenced to residual
3
1
solvent peaks with respect to d(TMS) ¼ 0 ppm and P NMR
15370 | RSC Adv., 2016, 6, 15370–15381
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A6
A3
chemical shis with respect to d(85% aqueous H
Solution absorption and emission spectra were measured using 8.28–8.23 (m, 6H, H
an Agilent 8453 spectrophotometer and a Shimadzu RF-5301PC (m, 11H, H
spectrouorometer, respectively. Spectroelectrochemical and 7.23–7.29 (m, 3H, H
solid-state absorption spectroscopic measurements used a Var- 156.6 (C
3
PO
4
) ¼ 0 ppm. J ¼ 4.7, 1.8, 0.9 Hz, 2H, H ), 8.71 (dt, J ¼ 8.0, 1.1 Hz, 2H, H ),
E2+G2 A4+D2
), 7.93–7.88 (m, 4H, H
), 7.79–7.71
E3+E4+G3+G4+C2
C3
A5
), 7.50 (m, 2H, H ), 7.37 (m, 2H, H ),
D3+D4 13
). C NMR (126 MHz, CDCl ) d/ppm
3
A2/B2
A2/B2
Q
Q
Q
Q
), 156.1 (C
), 151.4 (C ), 150.7 (C ), 150.49 (C ),
ian-Cary 5000 spectrophotometer. Electrospray (ESI) mass 150.47 (C ), 150.43 (C ), 150.42 (C ), 150.4 (C ), 149.3 (C ),
Q
Q
B4
A6
Q
Q
Q
Q
Q
spectra and high resolution ESI-MS were measured on Bruker 148.0 (C ), 146.85 (C ), 146.8 (C ), 143.0 (C ), 142.91 (C ),
plus
Q
Q
Q
D2/A4
D2/A4
Esquire 3000
and Bruker maXis 4G instruments, respec- 142.89 (C ), 141.5 (C ), 140.8 (C ), 137.0 (C
), 135.8 (C
),
H3/Q
H3/Q
E2/G2
tively. Nanosecond transient absorption spectra were measured 135.4 (C
on an LP-920KS spectrometer from Edinburgh Instruments (C
using a frequency-doubled Quantel Brilliant B laser as a pump (C
source. Transient absorption measurements with picosecond 131.6 (C
time resolution were performed with the TRASS instrument (C
), 135.35 (C
), 134.6 (C
), 134.55
0
0
0
0
F3/F4/F3 /F4 /I3/I4
E2/G2
F3/F4/F3 /F4 /I3/I4
), 132.9 (C
), 132.3 (C
), 132.25
0 0
0
0
0
0
F3/F4/F3 /F4 /I3/I4
F3/F4/F3 /F4 /I3/I4
C3
F3/F4/F3 /F4 /I3/I4
), 132.2 (C
), 132.1 (C
),
), 127.65
0
0
F3/F4/F3 /F4 /I3/I4
E3/E4/G3/G4
), 131.0 (C ), 127.7 (C
E3/E4/G3/G4
D4
E3/E4/G3/G4
D3+C2
), 127.3 (C ), 126.8 (C
), 126.2 (C
),
A5
Q
A3
Q
Q
from Hamamatsu, equipped with a C7701-01 streak camera. 123.9 (C ), 122.6, 121.7 (C ), 121.6 (C ), 121.2 (C ), 120.8 (C ),
B3
Q
Excitation occurred with a picosecond mode-locked Nd:YVO
4
/
118.8 (C ) (C ¼ quaternary C, not unambiguously assigned).
+
YAG laser (PL2251B-20-SH/TH/FH) with PRETRIG option from ESI MS m/z (positive mode) 984.7 [M + H] (calc. 984.3). UV-Vis
ꢁ
7
ꢁ3
3
ꢁ1
ꢁ1
Ekspla.
(EtOH, 9.7 ꢃ 10 mol dm ) l/nm (3/dm mol cm ) 285
Electrochemical measurements were carried out using a CH (56 000), 315 (41 000), 427 (520 000), 560 (26 000), 599 (9500).
n
Instruments 900B potentiostat with [ Bu N][PF ] (0.1 M) as Found C 75.89, H 4.41, N 9.49; C H N Zn$3H O: C 75.10, H
4
6
65 41
7
2
ꢁ1
supporting electrolyte and at a scan rate of 0.1 V s . The 4.56, N 9.43.
working electrode was glassy carbon, pseudo-reference elec-
trode silver wire and counter-electrode platinum wire; poten-
tials were referenced with respect to the Fc/Fc couple.
Spectroelectrochemical measurements were performed using
[
Ru(4)Cl3]
+
Compound 4 (27 mg, 0.061 mmol, 1 eq.) and RuCl $3H O
3
2
(
16 mg, 0.061 mmol, 1 eq.) were suspended in EtOH (10 mL) and
heated at reux for 3.5 h. A brown precipitate formed, which
was centrifuged, then washed with EtOH and Et O. Brown
Ru(4)Cl ] (28 mg, 0.043 mmol, 70%) was used for the next step
without further characterization.
a CH
2
Cl
2
solution of 3 (1 mM) and an MeCN solution of
n
[
(
Ru(3)(4)][PF ] (0.6 mM) at room temperature with [ Bu N][PF ]
z0.1 M) as the supporting electrolyte. The solution was added
6
2
4
6
2
[
3
to an optically transparent thin-layer electrochemical (OTTLE)
cell with two Pt minigrid electrodes (working and auxiliary),
a silver wire pseudoreference electrode, and a path length of
z0.2 mm. The potential was controlled using a VersaSTAT 3
potentiostat from Princeton Applied Research.
[
6 2
Ru(3)(4)][PF ]
3
[Ru(4)Cl ] (28 mg, 0.043 mmol, 1 eq.), 3 (42.4 mg, 0.043 mmol,
RuCl
3 2
$3H O was purchased from Oxkem, and 5,10,15,20- 1 eq.) and N-ethylmorpholine (2 drops) were suspended in dry
tetraphenyl-21H,23H-porphyrin (H TPP), NBS and [Pd(PPh ) ] EtOH (2 mL) in a microwave reactor vial. The mixture was
2
3 4
ꢂ
from Sigma-Aldrich and used as received. 7-Bromo-5,10,15,20- heated in a microwave reactor at 140 C for 20 min and then the
24
25
tetraphenyl-21H,23H-porphyrin, 1, 2 (ref. 26) and 4 (ref. 27) dark red solution was poured into saturated aqueous NH PF6
4
were prepared as previously reported and spectroscopic data (50 mL). The precipitate that formed was centrifuged down,
matched those in the literature.
collected and washed with water and Et O (3 ꢃ 5 mL). The
2
residue was dissolved in MeCN and puried by column chro-
matography (SiO
volume). The rst orange fraction (R
(72 mg, concentrated to 3 mL and poured in saturated aqueous NH
2
, MeCN/saturated aq. KNO
¼ 0.67) was collected, then
PF
3
/water 7 : 1 : 0.5 by
Compound 3
f
Compound 1 (100 mg, 0.13 mmol, 1 eq.), K
2
CO
3
4
6
0
.52 mmol, 4 eq.) and 2 (53 mg, 0.15 mmol, 1.2 eq.) were (25 mL) to give a precipitate which was washed with water and
combined in a 20 mL microwave vial and dissolved in a mixture EtOH (3 ꢃ 5 mL). [Ru(3)(4)][PF was isolated as a purple powder
was (20 mg, 0.011 mmol, 26%). H NMR (500 MHz, CD CN) d/ppm
6 2
]
1
of toluene (9.5 mL) and deionized water (0.6 mL). N
bubbled through the solution for 30 min and then [Pd(PPh ) ] 9.06 (s, 2H, H ), 9.01 (s, 2H, H ), 8.89 (m, 2H, H
2
3
0
0
0
0
B3
B3
F3+F4/F3 +F4
),
3
4
H3
I3+I4
(
15 mg, 0.013 mmol, 0.1 eq.) was added and the vial sealed. The 8.86 (s, 1H, H ), 8.85 (s, 2H, H
), 8.82 (d, J ¼ 4.5 Hz, 2H,
0 0
F3/F4/F3 /F4
0
0
ꢂ
F3/F4/F3 /F4
reaction mixture was heated for 4 h at 120 C in a microwave
H
H
), 8.79 (d, J ¼ 4.5 Hz, 2H, H
), 8.75 (m, 2H,
00
00
A3
A3
C2
G2
reactor, aer which time the solvent was removed under
), 8.69 (m, 2H, H ), 8.33 (m, 2H, H ), 8.30 (m, 2H, H ),
00
C3
E2
reduced pressure. Column chromatography (SiO
2 3
, CHCl ) was 8.24 (m, 4H, H ), 8.09 (m, 2H, H ), 7.98–8.05 (m, 6H,
00
A4 +A4+D2
C2
performed on the crude material. The fraction containing
H
H
), 7.95 (d, J ¼ 8.5 Hz, 2H, H ), 7.77–7.86 (m, 9H,
00
A6
E3+E4+G3+G4
C3
[Zn(TPP)] eluted rst and compound 2 was collected as the
), 7.70 (d, J ¼ 8.5 Hz, 2H, H ), 7.53 (m, 2H, H ),
A6 D4 D3
second (dark red-purple) fraction (R
f
¼ 0.1). 3 was isolated as 7.47 (m, 2H, H ), 7.43–7.46 (m, 1H, H ), 7.38 (m, 2H, H ), 7.27
00
1 A5 A5 Et
a dark purple solid (89 mg, 0.090 mmol, 69%). H NMR (500 (m, 2H, H ), 7.22 (m, 2H, H ), 4.14 (m, 4H, H ), 1.35 (t, J ¼ 7
0
0
00
00
F3+F4/F3 +F4
Et 13
A2 /B2
MHz, CDCl ) d/ppm 8.95–8.99 (m, 2H, H
), 8.96 (s, 1H, Hz, 6H, H ). C NMR (126 MHz, CD CN) d/ppm 159.3 (C
),
),
3
3
0
0
00
00
00
A6+A6
H3
I3+I4
F3+F4/F3 +F4
A2/B2
A2 /B2
A2/B2
B4
H
), 8.95 (s, 2H, H
), 8.89 (d, J ¼ 4.5 Hz, 2H, H
), 159.0 (C
), 156.8 (C
), 156.3 (C
), 153.4 (C
0
0
00
F3/F4/F3 /F4
B3
Q
Q
Q
B4
8.84 (d, J ¼ 4.5 Hz, 2H, H
), 8.79 (s, 2H, H ), 8.76 (ddd, 151.9 (C ), 151.0 (C ), 148.5 (C ), 147.9 (C ), 147.1 (C ), 147.0
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Q
Q
Q
Q
Q
Q
(
(
(
1
(
(
(
7
C ), 144.0 (C ), 143.8 (C ), 143.2 (C ), 143.0 (C ), 141.5 (C ), 139.0 DSC measurements
00
A4+A4
D2
H3
E2
G2
C
), 136.9 (C ), 135.7 (C ), 135.3 (C ), 135.2 (C ), 134.4
0
0
0
0
0
0
Masks for the DSCs were made from a black-coloured copper
sheet with an aperture of average area 0.06012 cm (1% stan-
Q
C3
F3/F4/F3 /F4
C3
I3+I4+two of F3/F4/F3 /F4
C ), 133.4 (C ), 133.1 (C
), 132.7 (C
),
2
0
0
00
F3/F4/F3 /F4
C4
32.2 (H
), 132.1 (C ), 131.8 (C , J ¼ 189 Hz), 129.0
PC
00
00
dard deviation) placed over the active area of the DSC. The area
C2
A5+A5
E3+E4+G3+G4+D4
B3
C2
C
), 128.5 (H
), 127.8 (C
), 127.4 (C ), 127.0
00
00
of the mask hole was less than the surface area of TiO
2
D3
A3+A3
B3
Et
Et
C
), 125.5 (C
), 123.0 (C ), 122.2 (C ), 63.2 (C ), 16.6 (C ),
2
(0.36 cm ). Black tape was used to complete the masking of the
Q
C ¼ quaternary C, not all resolved). ESI MS m/z (positive mode)
cell. Performance measurements were made by irradiating the
DSC from behind with a LOT Quantum Design LS0811 instru-
2+
ꢁ
6
65.6 [M] (calc. 765.7), negative mode 144.8 [PF ] (calc. 145.0).
ꢁ6
ꢁ3
3
ꢁ1
ꢁ1
UV-Vis (MeCN, 1 ꢃ 10 mol dm ) l/nm (3/dm mol cm ) 284
ꢁ2
ment (100 mW cm ¼ 1 sun), and the simulated light power
(
(
79 000), 312 (87 000), 425 (370 000), 492 (56 000), 559 (28 000), 599
was calibrated with a silicon reference cell.
10 000). Found: C 57.38, H 4.07, N 8.01; C H F N O P -
9
0 65 12 10 3 3
RuZn$2H O requires C 58.18, H 3.74, N 7.54.
2
Results and discussion
Synthesis and characterization of compound 3
Crystallography
Data were collected on a Bruker Kappa Apex2 diffractometer
0
0
00
The synthetic route to the porphyrin-functionalized 2,2 :6 ,2 -
28
with data reduction, solution and renement using APEX and
terpyridine 3 is summarized in Scheme 1. The selective bromi-
2
TPP in the 7-position was carried out using NBS as
described by Zhang and coworkers, and an excess of zinc(II)
acetate was added to yield zinc(II) complex 1. When the met-
allation of H TPP with zinc(II) was carried out prior to reaction
29
30,31
CRYSTALS. The program Mercury v. 3.7
structural analysis.
was used for
nation of H
24
25
Compound 3$Me CO
2
2
with NBS, selective halogenation was no longer observed and
a mixture of brominated derivatives was obtained. The reaction
of 1 with boronic acid 2 under Suzuki–Miyaura cross-coupling
conditions led to 3 in 69% yield aer workup. Metallation of the
porphyrin core before the coupling reaction is essential.
Although 7-bromo-5,10,15,20-tetraphenyl-21H,23H-porphyrin
undergoes coupling with 2 to give the zinc-free analogue of 3,
C
68
H
47
N
7
OZn, M ¼ 1043.55, red block, monoclinic, space group
ꢀ
P2 /c, a ¼ 13.1741(13), b ¼ 20.850(3), c ¼ 19.578(2) A, b ¼
1
26
ꢂ
ꢀ
3
ꢁ3
1
02.558(6) , U ¼ 5249.2(10) A , Z ¼ 4, D ¼ 1.320 Mg m , m(Cu-
c
ꢁ1
Ka) ¼ 1.063 mm , T ¼ 123 K. Total 66 771 reections, 9749
unique, Rint ¼ 0.078. Renement of 7565 reections (694
parameters) with I > 2s(I) converged at nal R
1
¼ 0.0894 (R
1
all
data ¼ 0.2457), wR
2
¼ 0.1059 (wR
2
all data ¼ 0.2583), gof ¼
2 2
subsequent reaction with Zn(OAc) $4H O leads to competition
1.0000. CCDC 1442417.
between the porphyrin and tpy metal-binding domains for
coordination to zinc(II). Thus, the sequence of steps presented
in Scheme 1 is the optimal route to 3.
DSC fabrication
ꢂ
The highest mass peak envelope in the electrospray mass
spectrum of 3 came at m/z 984.7 and exhibited a characteristic
isotope pattern for zinc. The H NMR spectrum of 3 is shown in
Fig. 1a and was assigned using COSY and NOESY methods. The
spectrum is consistent with the desymmetrization of the
Solaronix Test Cell Titania Electrodes were heated to 450 C for
3
ꢂ
0 min and then cooled to 80 C, when they were dipped into
the dye-baths. N719 (Solaronix) reference electrodes and elec-
trodes with adsorbed [Ru(3)(4)] were made by dipping in an
EtOH solution of N719 (0.1 mM) or MeCN solution of [Ru(3)(4)]
6 2
PF ] (0.1 mM), respectively, for 3 days. The electrodes were
taken out of the dye-baths, washed with EtOH or MeCN,
respectively, and dried using a heatgun (60 C). Commercial
counter electrodes (Solaronix Test Cell Platinum Electrodes)
were washed with EtOH, and then heated on a hot plate at
1
2
+
[
Zn(TPP)] domain. This most noticeably affects the ortho-
[
D2
E2
G2
protons (H , H and H ) of the phenyl rings and the
ꢂ
remaining protons in phenyl ring D (Fig. 1a). The shi to lower
D3
D4
frequency of the signals for H and H compared to the meta-
and para-protons in rings E and G is attributed to the proximity
D3
D4
13
ꢂ
of H and H to the arene ring C. The C NMR spectrum of 3
was assigned using HMQC and HMBC methods (Fig. S1 and
S2†).
4
50 C for 30 min to remove volatile organic impurities.
The working and counter-electrodes were joined using
thermoplast hot-melt sealing foil (Solaronix Test Cell Gaskets,
0 mm) by heating while pressing together. The electrolyte (LiI
0.1 M), I (0.05 M), 1-methylbenzimidazole (0.5 M), 1-butyl-3-
methylimidazolinium iodide (0.6 M) in 3-methoxypropioni-
trile) was inserted between the electrodes by vacuum backlling
through a hole in the counter electrode; this was sealed
Single crystals of 3$Me CO were grown by slow evaporation
6
2
of solvent from an acetone solution of 3. The acetone adduct of
(
2
3
1
(Fig. 2) crystallizes in the monoclinic space group P2 /c. The
{
Zn(TPP)} unit is structurally as expected with the atom Zn1
ꢀ
lying only 0.14 A out of the mean plane of the porphyrin
4
N -donor set; Zn–N bond distances and N–Zn–N bond angles
are given in the caption to Fig. 2. The acetone molecule is axially
(
Solaronix Test Cell Sealings) and capped (Solaronix Test Cell
3
2,33
Caps). All DSCs were fully masked for measurements.
ꢀ
coordinated and the Zn–O bond distance of 2.345(4) A is within
the range of observed for axial ketones in porphyrinato zinc(II)
complexes. The twist angles between the planes of the phenyl
Electrodes for solid-state absorption spectroscopy
34
Dye-functionalized electrodes were assembled as above but rings with C3, C14, C25 and C36 and the porphyrin core are in
ꢂ
using Solaronix Test Cell Titania Electrodes Transparent.
the range 59.8 and 67.4 ; for the phenyl ring containing C45, the
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Fig. 1 Aromatic region of the 500 MHz NMR spectra of (a) 3 (in CDCl
and (b) [Ru(3)(4)][PF (in CD . Chemical shifts
CN). * ¼ residual CHCl
in d/ppm. See Schemes 1 and 2 for atom labels.
3
)
6
]
2
3
3
2
Fig. 2 Structure of 3$Me CO with H atoms omitted for clarity; thermal
ellipsoids plotted at 30% probability level. Selected bond metrics: Zn1–
N1 ¼ 2.034(3), Zn1–N2 ¼ 2.062(3), Zn1–N3 ¼ 2.042(3), Zn1–N4 ¼
ꢀ
2
8
.062(3), Zn1–O1 ¼ 2.345(4), C66–O1 ¼ 1.215(8) A; N1–Zn1–N2 ¼
9.69(14), N2–Zn1–N3 ¼ 89.27(13), N1–Zn1–N4 ¼ 89.52(13), N3–
Zn1–N4 ¼ 90.51(13), N1–Zn1–O1 ¼ 82.10(16), N2–Zn1–O1 ¼
ꢂ
8
8.43(16), N3–Zn1–O1 ¼ 106.45(16), N4–Zn1–O1 ¼ 98.11(16) .
6 2
Synthesis and characterization of [Ru(3)(4)]PF ]
2
+
Heteroleptic [Ru(tpy-I)(tpy-II)] complexes are most conve-
niently made by treating RuCl $3H O sequentially with the two
reflux, ligands, the second step in the presence of N-ethylmorpholine
3
3
2
Scheme 1 Synthetic route to 3. Conditions: (i) NBS, CHCl
4
.5 h; (ii) 2 equivalents Zn(OAc)
CHCl ; (iii) K CO , 2 in toluene/H
microwave conditions.
2
in MeOH; room temperature, 17 h, for the reduction of ruthenium(III) to ruthenium(II). In principle,
ꢂ
3
2
3
2
O, [Pd(PPh
)
3 4
], 4 h, 120 C under
two approaches could be used to prepare [Ru(3)(4)][PF
6 2
] : (i)
reaction of RuCl $3H O with 3, followed by treatment with 4 in
3
2
the presence of N-ethylmorpholine, or (ii) reaction of RuCl $3H O
3
2
with 4, followed by treatment with 3 in the presence of N-eth-
ylmorpholine. Both methods were tried but the former resulted
in cleavage of the porphyrin and tpy domains in 3. Thus,
ꢂ
ꢂ
corresponding angle is 61.9 , and the pyridine ring with N6 is
then twisted through 32.5 with respect to the plane of the
phenyl spacer. The similar twist angles for the arene rings
containing C36 and C45 permit the rings to engage in
a p-stacking interaction although the 16.2 angle between their
planes is not ideal; the centroid/centroid distance is 3.51 A.
The tpy unit is virtually planar (angles between planes of adja-
cent pyridine rings are 2.7 and 6.2 ). The planarity is associated
with a face-to-face p-interaction between centrosymmetric pairs
of tpy units (Fig. 3a). The centrosymmetric pairing of the
[
Ru(3)(4)][PF
ment of RuCl
Cl ] which was further reacted with 3 in the presence of N-eth-
ylmorpholine to give (aer anion exchange and workup)
Ru(3)(4)][PF ] as a purple powder in 26% yield.
6
]
2
was prepared as shown in Scheme 2 by treat-
3
$3H O with 4 to give the insoluble species [Ru(4)
2
ꢂ
3
ꢀ
[
6
2
ꢂ
The ESI mass spectrum of [Ru(3)(4)][PF ] showed a peak
6
2
envelope at m/z 765.6 with peaks at half-mass intervals consis-
2+
tent with assignment to [Ru(3)(4)] . In addition to signals for
{
Zn(TPP)} units (Fig. 3b) is typical and has been extensively
1
aromatic protons, the H NMR spectrum of a CD
3
CN solution of
3
5
discussed in the literature.
[Ru(3)(4)][PF
6
]
2
exhibited multiplets at d 4.14 and 1.35 ppm,
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Fig. 3 (a) Stacking of centrosymmetric pairs of tpy domains in
ꢀ
3
$Me
2
CO; separation of central pyridine rings ¼ 3.30 A and centroid/
ꢀ
centroid distance ¼ 3.71 A. (b) Stacking of centrosymmetric pairs of
i
ꢀ
{
Zn(TPP)} units; Zn(1)/Zn(1) ¼ 6.903(1) A (symmetry code i ¼ 2 ꢁ x, 1 ꢁ
y, 2 ꢁ z).
arising from the ethyl groups of the phosphonate ester group in
ligand 4. The relative integrals of these resonances compared to
the aromatic region conrmed that no ester hydrolysis occurred
during the complex formation; partial hydrolysis of PO(OEt)2-
functionalized tpy ligands has been observed during the
36–38
formation of some ruthenium(II) complexes.
The aromatic
region of the [Ru(3)(4)][PF is shown in Fig. 1b and its signa-
6 2
]
ture is consistent with the presence of two different tpy
domains. COSY, NOESY, HMQC and HMBC methods were used
1
13
to assign the signals in the H and C NMR spectra, although
not all quaternary signals could be unambiguously ascribed (see
Experimental section). A starting point for distinguishing
between the two tpy ligands was assignment of the ipso-C atom
of the arene ring attached to the phosphonate group; the reso-
0
0
C4
nance for C (Scheme 2) was a doublet (JPC ¼ 189 Hz) at d 131.8
2+
ꢁ
Scheme
Conditions: RuCl
2
Synthesis of [Ru(3)(4)] , isolated as the [PF
$3H
6
]
salt.
ppm. A comparison of Fig. 1a with Fig. 1b shows that formation
of the {Ru(tpy)
3
ꢂ
2
O, EtOH, reflux, 4.5 h; (ii) 2, EtOH, N-ethyl-
PF
2
+
2
}
domain leads to the characteristic shi of the morpholine, 140 C, 20 min under microwave conditions; (iii) NH
Atom labels for NMR spectra of [Ru(3)(4)][PF are shown.
4
6
.
A6
6 2
]
H
signal to lower frequency (d 8.76 ppm in 3 to d 7.47 ppm in
00
A6
[
6 2
Ru(3)(4)][PF ] ); the signal for H (d 7.53 ppm) appears close
A6
to that for H , consistent with these protons lying over the ring
2
+
reduction and oxidation processes provide reference data for
the interpretation of the electrochemical behaviour of the new
conjugate species. Data are summarized in Table 1 along with
the electrochemical band-gaps, DE1/2. The electrochemistry of 3
current of the adjacent tpy ligand in the octahedral {Ru(tpy)2}
unit.
Cyclic voltammetry
6 2
and [Ru(3)(4)][PF ] was studied using cyclic voltammetry. Like
[
Zn(TPP)], compound 3 exhibits two oxidative processes (Table 1
The redox behaviour of [Zn(TPP)] has previously been
described, and the potentials for the reversible one-electron
39
and Fig. 4a), reversible and reproducible over three scans, which
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Table 1 Redox potentials for 3 in CH
solution compared to [Zn(TPP)] in CH
2
Cl
Cl
tammetry. Potentials are referenced to Fc/Fc with 0.1 M [ Bu
2
and [Ru(3)(4)][PF
6
]
2
in MeCN going to [Ru(3)(4)][PF
]
, the potential of the [Zn(TPP)]-centred
6
2
2
2
measured using cyclic vol-
oxidative couples are little affected (Table 1). An additional
+
n
4
6
][PF ] as
ꢁ1
oxidation process at +0.88 V (Table 1 and Fig. 4b) is assigned to
supporting electrolyte and a scan rate of 0.1 V s (ir ¼ irreversible, qr ¼
2
+
3+
a
the Ru /Ru couple, and compares well with +0.895 V reported
quasi-reversible)
4
0
for [Ru(Phtpy)
2
][PF
6
]
2
.
6 2
[Ru(3)(4)][PF ] undergoes three irre-
ox
red
b
Compound
Zn(TPP)]
E
1/2/V
E1/2/V
DE1/2/V
Reference versible reduction processes.
[
+0.42 (82)
0.71 (81)
+0.34 (57)
0.66 (84)
ꢁ1.79 (79)
2.21
39
+
Absorption spectra and spectroelectrochemistry
ir
3
ꢁ1.76
2.10
This work
ir
Fig. 5 compares the solution absorption spectra of 1, 3 and
+
ꢁ1.97
ir
[Ru(3)(4)][PF
6 2
] . The spectrum of compound 1 shows the typical
ꢁ
2.09
2.27
ir
ir
ir
ir
39,41
ꢁ
features of a metallated porphyrin,
the intense Soret band at
3
ꢁ1
ꢁ1
[
Ru(3)(4)][PF
6
]
2
+0.39 (56)
0.67 (56)
0.88 (65)
ꢁ1.56
ꢁ1.87
ꢁ2.21
1.95
This work
4
25 nm (3 ¼ 660 000 dm mol cm ) arising from the S ) S0
2
+
+
transition, and weaker Q bands resulting from the vibrational
structure ((0, 0) and (0, 1)) from the S ) S transition (558 and
1
ꢁ1
0
ꢁ1
a
b
ox
red
3
Values in parenthesis ¼ Epc ꢁ Epa in mV. DE1/2 ¼ E1/2 ꢁ E1/2
5
97 nm, 3 ¼ 22 000 and 7600 dm mol cm , respectively). On
going to compound 3, an additional p-conjugated system in the
b-pyrrolyl position is introduced. The Soret band moves slightly
3
ꢁ1
and decreases in intensity (427 nm, 3 ¼ 520 000 dm mol
ꢁ
1
cm ), while the Q bands change little (Fig. 5, 560 and 599 nm,
3
ꢁ1
ꢁ1
3
¼ 26 000, 9500 dm mol cm , respectively). The high-
energy bands (around 285 nm, green trace in Fig. 4) are attrib-
uted to p* ) p transitions localized on the phenyltpy domain.
Upon formation of the ruthenium(II) complex [Ru(3)(4)]
6 2
[PF ] , the high-energy bands (below 350 nm) approximately
doubled in intensity with respect to the absorptions in 3 (blue
trace in Fig. 5), consistent with the presence of two tpy domains.
3
The Soret band again decreases in intensity (3 ¼ 370 000 dm
ꢁ
1
ꢁ1
mol
cm ) but is little shied from the free ligand 3
(425 versus 427 nm), providing evidence for electronic
communication between the porphyrin and tpy domains. Fig. 5
shows that there is also little difference in the Q bands
6 2
comparing 3 with [Ru(3)(4)][PF ] , (in the complex, 599 and
3
ꢁ1
ꢁ1
5
59 nm, 3 ¼ 28 000, 10 000 dm mol cm , respectively).
2+
Conrmation of the presence of the {Ru(tpy)
2
}
chromophore
comes from the appearance of the broad band at 492 nm arising
1
42
from the MLCT absorption of this chromophore.
We commence the spectroelectrochemical discussion by
presenting the results obtained from a study of a CH Cl solu-
2
2
tion of compound 3. Fig. 6a shows a superimposition of the
Fig. 4 Oxidative processes in the cyclic voltammograms of (a) 3 and
(
b) [Ru(3)(4)][PF
6
]
2
. For conditions, see Table 1.
+
+
2+
are assigned to [Zn(TPP)]/[Zn(TPP)] and [Zn(TPP)] /[Zn(TPP)]
couples. Reduction processes were poorly dened in the CV
Fig. S3†) but could be distinguished using differential pulse
(
red
voltammetry; E1/2 values are given in Table 1. By comparison
with data for [Zn(TPP)], the rst reduction at ꢁ1.76 V is
presumably centred on the {Zn(TPP)} domain in 3 and the
processes at more negative potential are phenyltpy-centred. On
Fig. 5 Absorption spectra of 1 (red line, EtOH, lmax ¼ 425 nm), 3
(
green line, EtOH, lmax ¼ 427 nm) and [Ru(3)(4)][PF
6 2
] (blue line, MeCN,
ꢁ6
ꢁ3
l
max ¼ 425 nm). Concentration ¼ 1 ꢃ 10 mol dm
.
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absorption spectra of 3 recorded before and aer an oxidative
cycle. The complete set of scans for the oxidative cycle are
shown in Fig. 6b. The irreversible changes are consistent with
43
the literature data for [Zn(TPP)]. Initial oxidation leads to
a p-radical cation, the Soret band of which is about half as
intense as the original band, and the Q bands are replaced with
an absorption with lmax z 650 nm. The second oxidation leads
to a dication which is unstable on the experimental timescale.
The results of the spectroelectrochemical reductive cycle for 3
are depicted in Fig. S4† and are again consistent with the
literature data for [Zn(TPP)].
The oxidative and reductive cycles of the spectroelec-
trochemical measurements carried out on an MeCN solution of
[Ru(3)(4)][PF
6 2
] are shown in Fig. 7 and 8. At the end of the
oxidative cycle, the regeneration of the absorptions associated
2
+
with the {Ru(tpy)
2
}
domain (the MLCT band at 492 nm, and
the bands at 284 and 310 arising from the phenyltpy p* ) p
transitions) conrms the reversibility of these processes. In
contrast, the oxidation of the [Zn(TPP)] moiety within the
complex is irreversible, the processes mimicking those of
compound 3 with the exception that the band at 650 nm is now
transient. This may be due to over-oxidation of the porphyrin
core. During the reductive cycle (Fig. 8), the absorptions arising
Fig. 7 Spectroelectrochemical data for the oxidative cycle of [Ru(3)(4)]
n
[PF
6
]
2
(z1 mM in MeCN, [ Bu
4
6
N][PF ] supporting electrolyte). (a)
Absorption spectra before (blue line) and after (red line) the oxidative
cycle. (b) A spectrum was recorded every 0.1 V, starting from 0 V (first
blue line at the front) to +1.5 V (last blue line) and back from +1.5 V (first
red line) to 0 V (last red line). See caption to Fig. 6 for referencing to
+
Fc/Fc .
2
+
from both the porphyrin and {Ru(tpy)
2
}
domains are irre-
versibly transformed. The irreversible changes to the Soret and
43
Q bands are consistent with those observed for 3 and [Zn(TPP)]
while irreversible reduction processes centred on the phenyltpy
units are responsible for the loss of the bands associated with
the p* ) p and MLCT transitions. Fig. 9b provides evidence for
a transient band between 800 and 900 nm, which can be
ꢁ
44
assigned to the [3]c radical anion.
[
Ru(3)(4)][PF ] as a dye in DSCs
6 2
45
Although phosphonic acid anchors bind more strongly than
46
phosphonate esters, it is has been demonstrated that TiO2
47,48
surfaces can be functionalized using phosphonate esters
with immobilization of the anchor taking place by hydrolysis of
49
POR groups by surface-OH groups. We therefore investigated
2
+
the use of [Ru(3)(4)] as a dye in DSCs. First, we conrmed that
the dye bound to a TiO surface. TiO electrodes (without
] supporting electrolyte). (a) Absorption a scattering layer) were soaked in an MeCN solution of [Ru(3)(4)]
Fig. 6 Spectroelectrochemical data for the oxidative cycle of 3
2
2
n
(
z1 mM in CH
2
Cl
2
, [ Bu
4
N][PF
6
spectra before (blue line) and after (red line) the oxidative cycle. (b) A
spectrum was recorded every 0.1 V, starting from 0 V (first blue line at
6 2
[PF ] for 3 days, and were then washed and dried. The electrode
retained a red colour similar to that of reference electrodes with
adsorbed N719. Compared to N719, the additional spectral
the front) to +1.8 V (last blue line) and back from +1.8 V (first red line) to
+
0
V (last red line). The potential is referenced with respect to the Fc/Fc
2
+
response that the Soret band imparts to [Ru(3)(4)] is clear
redox couple with the same cell under the same experimental
conditions.
from the solid-state absorption spectra Fig. 9. Adsorbed
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2+
Table 2 Performance parameters of duplicate DSCs with [Ru(3)(4)]
and N719. Measurements were made on the day of DSC fabrication
ꢁ
2
Dye
J
SC/mA cm
VOC/mV
ff/%
h/%
N719 (cell 1)
N719 (cell 2)
12.29
11.21
0.10
657
655
333
334
67
65
52
53
5.37
4.74
0.02
0.02
2+
[
Ru(3)(4)]2 (cell 1)
+
[
Ru(3)(4)] (cell 2)
0.11
3
days, and reference electrodes were made similarly using an
ꢁ
ꢁ
EtOH solution of N719. DSCs were fabricated using an I /I
electrolyte (see Experimental section) and were fully masked.
3
32,33
The reproducibility of performance parameters (Table 2) was
conrmed using duplicate DSCs for each dye. Despite the
2
+
enhanced light absorption of [Ru(3)(4)] with respect to N719,
the conversion efficiency (h) is poor; the main contributing
factor is the extremely low short-circuit current density (JSC
)
electron injection. The open-circuit voltage (VOC) is about half
that of N719. In order to understand the poor performance of
2
+
[
Ru(3)(4)] in DSCs, we have carried out a detailed investigation
of the energy-transfer processes that follow excitation.
Emission properties
The solution emission behaviour of 3 and [Ru(3)(4)][PF ] were
investigated and compared to those of 1. As discussed earlier, in
6
2
Fig.
8 Spectroelectrochemical data for the reductive cycle of
n
[
Ru(3)(4)][PF
6 2 4 6
] (z1 mM in MeCN, [ Bu N][PF ] supporting electrolyte).
the absorption spectrum of the latter, bands arising from S )
2
(
a) Absorption spectra before (blue line) and after (red line) the
S
0
and S
V (first blue line at the front) to ꢁ1.8 V (last blue line) and back from organic molecule, population of the S
1.8 V (first red line) to 0 V (last red line). See caption to Fig. 6 for
1
) S
0
transitions are observed. Normally, for an
excited state is followed
and the emission spectrum
can be related to the radiative decay of the lowest excited state of
same multiplicity. [Zn(TPP)] is emissive from both the S and S
reductive cycle. (b) A spectrum was recorded every 0.1 V, starting from
0
2
ꢁ
50
1
by fast internal conversion to S ,
+
referencing to Fc/Fc .
2
1
states, although the uorescence originating from the S state
2
2
+
[
Ru(3)(4)] exhibits lmax at 432, 500, 564 and 641 nm. The Soret
has a much lower quantum yield and only a picosecond life-
band at 432 nm is red-shied with respect to solution (425 nm)
and absorptions at 500, 564 and 641 nm compare with bands in
the solution spectrum (Fig. 5) at 492 nm (MLCT) and 560 and
00 nm (Q bands).
Photoanodes for n-type DSCs were made by immersion of
51
time. Excitation of 1 at 400 nm (into the Soret shoulder) results
in the emission spectrum shown in Fig. 10 with uorescence
from both the S (431 and 453 nm) and S (607 and 659 nm)
2 1
excited states. The assignments were conrmed from the exci-
tation spectra. Note that lmax of the Soret band (425 nm) is too
6
FTO/TiO electrodes in an MeCN solution of [Ru(3)(4)][PF ] for
2
6 2
ꢁ6
Fig. 9 Solid-state absorption spectra of transparent TiO
with dyes N719 (red) and [Ru(3)(4)][PF ] (blue).
6 2
2
electrodes Fig. 10 Solution emission spectrum of 1 (EtOH, 1 ꢃ 10 M, room
temperature). lexc ¼ 400 nm.
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6 2
Fig. 13 NIR transient absorption spectra of [Ru(3)(4)][PF ] (MeCN,
Fig. 11 Solution emission spectrum of [Ru(3)(4)][PF
6
]
2
(MeCN, 1 ꢃ
ꢁ6
ꢁ6
z2 ꢃ 10 M, room temperature). lexc ¼ 532 nm. The spectra were
1
0
M, room temperature). lexc ¼ 492 nm.
reconstructed from decay curves recorded every 10 nm.
max
close to the lem of 431 and 453 nm from the S uorescence to
2
Excitation spectra monitored at 560 and 600 nm conrm the
presence of a broad peak centred at 285 nm, in agreement with
the involvement of a tpy absorption in the population of the S1
state. The ruthenium(II) complex [Ru(3)(4)][PF ] exhibits an
6 2
emission behaviour similar to that of 3. Excitation into the tpy
observe these emissions using lexc ¼ 425 nm. Excitation into the
Q bands of 1 leads to the S emissions at 607 and 659 nm.
1
The emission spectrum of compound 3 does not display an
S uorescence. Excitation into either the Soret or Q bands leads
2
to emission at 613 and 660 nm. Interestingly it is possible to
detect porphyrin uorescence even upon exciting into the tpy
bands (lexc ¼ 285 and 320 nm). Since the tpy absorption is well
separated from the porphyrin absorption bands, this is a clear
indication of intramolecular energy transfer. The energetics of
the system are favourable for an energy transfer from the (p–p*)
absorption bands (lexc ¼ 284 and 310 nm) results in porphyrin
max

uorescence (lem ¼ 613 and 661 nm). Moreover Fig. 11 shows
that if the excitation is in the MLCT band (lexc ¼ 492 nm),
emission is again observed from the S state of the porphyrin. It
1
was not possible to detect S uorescence by exciting into the
2
shoulder of the Soret band.
tpy excited states (upper lying levels) to the S
2
state (lower level),
followed by internal conversion to S and radiative decay.
1
Transient absorption spectra of [Ru(3)(4)][PF
In order to further probe the emission behaviour of
Ru(3)(4)][PF , the transient absorption spectrum of the
6 2
]
[
6 2
]
Fig. 12 (a) Transient absorption spectrum of [Ru(3)(4)][PF
6
]
2
(MeCN,
z2 ꢃ 10 M, room temperature). lexc ¼ 532 nm. Acquisition time
00 ns, 5 acquisitions without time delay. (b) Temporal evolution of the
optical density between 440 and 463 nm after excitation at 532 nm Scheme 3 Triads and dyads related to [Ru(3)(4)][PF
ꢁ6
2
6 2
]
60
and reported by
55,57
with laser pulses of 30 ps duration.
(a) and (b) Benniston et al.,
and (c) Flamigni et al.
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53
8
2 2 6 2
32 nm for [Zn(TPP)] in CH Cl . For [Ru(3)(4)][PF ] , the NIR
absorption was detected only in deaerated solution. The
recovery of the ground state was monitored at 385, 425 and
470 nm for the aerated solution and at 385, 425, 470 and 820 nm
for the deaerated one. As expected for a triplet state, the life-
times ranged from hundreds of nanoseconds for the aerated
solution to tens of microseconds for the deaerated one, due to
the exclusion of a non-radiative deactivation pathway through
reaction with triplet O . The decay curves are consistent with
2
a mono-exponential decay in all cases; the lifetimes for ground
state recovery are: s385 ¼ 441 ꢄ 44 ns and 59 ꢄ 6 ms, s ¼ 418 ꢄ
425
4
4
2 ns and 52 ꢄ 5 ms, s ¼ 435 ꢄ 44 ns and 48 ꢄ 5 ms, s
¼
4
70
820
9 ꢄ 5 ms. Since all the lifetimes are consistent within experi-
2+
Fig. 14 Energetic level diagram for [Ru(3)(4)]
.
mental error, it is reasonable to assume that all observed tran-
sitions originate from a single chemical species which we
1
propose to be the porphyrin T state. Furthermore, we conclude
complex was recorded at room temperature. Upon excitation in
the MLCT band (lexc ¼ 532 nm), the transient absorption
spectrum obtained resembles the characteristic triplet–triplet
that upon MLCT excitation a triplet-to-triplet energy transfer
3
occurs from the MLCT level to T1, the latter being the lowest
accessible level for [Ru(3)(4)][PF ] .
6
2
52
spectrum of [Zn(TPP)], rst predicted by Gouterman and later
The literature contains a number of molecular triads and
53
reported by Holten and coworkers. Gouterman predicted two
possible transitions from the porphyrin T1 state: an intense
allowed transition that would result in a doubly excited
conguration and consist of two absorption peaks to lower
energy of the Soret band, and a weak, forbidden transition in
the near infrared (IR) leading to a highly excited singlet
conguration. In the transient absorption spectrum of
54
6 2
dyads related to [Ru(3)(4)][PF ] (Scheme 3). When the triad
55
(Scheme 3a) reported by Benniston et al. is excited in the
3
MLCT band, the MLCT emission is quenched in favour of
a triplet-to-triplet energy transfer to the porphyrin T
1
state. The
lifetime of T
1
was determined to be 65 ꢄ 5 ms with a triplet-to-
10
ꢁ1
triplet energy transfer rate constant of 8 ꢃ 10
s . A Dexter
56
type mechanism of energy transfer was proposed, and it is
6 2
[Ru(3)(4)][PF ] (Fig. 12), the loss of the ground state porphyrin
signicant that excitation into the Q band resulted in
is clearly indicated by bleaching of the Soret band (425 nm) and
of the Q(0, 1) and Q(0, 0) bands (560 and 600 nm). The broad
bands at 470 and 500 nm are associated with absorption of the
55
a decreased S
1
uorescence. Benniston et al. rationalize this in
3
terms of singlet (S
1
) to triplet ( MLCT) energy transfer (k ¼ 4 ꢃ
8
ꢁ1
10
s ), involving an endergonic Dexter type mechanism
porphyrin T state and creation of the doubly excited congu-
1
(a process that is spin-forbidden). In the osmium-containing
dyad in Scheme 3b, excitation in the Soret band leads to
ration. A broad absorption is present at lower energies,
extending from 600 nm to the NIR, in agreement with the
3
57,58
direct transfer to the MLCT state
followed by triplet-to-
53
literature spectrum.
The near-IR (NIR) transient absorption spectra of [Ru(3)(4)]
PF are shown in Fig. 13. The band at 820 nm is associated
triplet energy transfer to T . A high rate constant for
1
porphyrin uorescence quenching again accounts for complete
[
6 2
]
energy transfer. In oligopyridine complexes, the excited MLCT
with the forbidden, higher energy singlet which appears at
59
state is localized on one of the ligands, and Benniston et al.
1
Scheme 4 Energy-transfer scheme upon MLCT excitation at 532 nm in [Ru(3)(4)][PF
charge could be on either tpy domain. ISC ¼ intersystem crossing.
6
]
2
. For the middle species in the scheme, the negative
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2
+
1
argue that the energy ows from S to the tpy domain directly [Ru(3)(4)] and solid-state absorption spectra demonstrate
connected to the porphyrin. In addition, if the second tpy in the enhanced light absorption with respect to the standard DSC dye
complex lacks an extended p-system in the 4-position of the N719. However, the photoconversion efficiencies of dye-
2
+
central pyridine ring, the electronic energy can be considered to sensitized solar cells (DSCs) sensitized with [Ru(3)(4)] are
reside on the porphyrin-bearing tpy because the intra-ligand disappointingly low. Transient absorption spectroscopic
energy transfer would not be as fast as that of energy transfer studies indicate that triplet–triplet energy transfer processes are
to the porphyrin triplet state. The overall effect is an intersystem the most likely reason for the poor DSC photoconversion
2
+
crossing involving the porphyrin unit, involving the {Ru(tpy)}
efficiencies.
6
0
domain. For the triad reported by Flamigni et al. (Scheme 3c),
the energy transfer rate constant for the triplet-to-triplet trans-
Acknowledgements
1
0
ꢁ1
fer is >5 ꢃ 10
s , consistent with a fast and quantitative
quenching of the ruthenium-containing manifold. Once again We acknowledge the Swiss National Science Foundation as part
the uorescence originating from S
population of the MLCT.
1
in quenched in favour of of the NCCR Molecular Systems Engineering, and the University
3
of Basel for nancial support. The SNF R'Equip program is
Fig. 14 shows the relative energies of the tpy, MLCT and acknowledged for grant number 206021_157687/1. Nathalie
2
+
{
Zn(TPP)} singlet and triplet states of [Ru(3)(4)] . The energy of Marinakis and Sarah Keller are thanked for providing ligand 4
the tpy domain was obtained by plotting the normalized and recording NMR spectra, respectively.
absorption and emission spectra (lexc ¼ 284 and 310 nm) of
[
Ru(3)(4)][PF ] , expressed in wavenumbers and searching for
6 2
Notes and references
a crossing point which corresponds to the (0, 0) transition. The
values obtained were 345 and 353 nm, corresponding to
1 B. O'Regan and M. Gr ¨a tzel, Nature, 1991, 353, 737.
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3
.59 and 3.51 eV. An average value of 3.55 eV has been adopted
for the tpy level. The {Zn(TPP)} S level was obtained with the
aforementioned procedure using spectra of 1. A value of 427 nm
2.90 eV) was obtained. The {Zn(TPP)} S level was calculated
from spectra of [Ru(3)(4)][PF ] , and searching for the crossing
2
(
1
6
2
point between the Q(0, 0) bands. This gave a value of 596 nm
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1
(
2.08 eV). The MLCT energy level was derived from the UV-Vis
3
maximum of [Ru(3)(4)][PF
emission maximum of the analogous [Ru(pytpy)
6
]
2
, and the MLCT level from the
][PF (pytpy
4 -(4-pyridyl)-2,2 :6 ,2 -terpyridine) in MeCN (655 nm,
2
6
]
2
0
0
0
00
61
¼
3
1
.89 eV) since the MLCT state of [Ru(3)(4)][PF
6 2
] is not emissive
at room temperature in solution. Finally, the {Zn(TPP)} T
1
62
energy (770 nm corresponding to 1.61 eV) was obtained from
the literature data. The relative energies of the states allow us to
propose the energy transfer process shown in Scheme 4. Upon
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1
3
MLCT excitation, fast intersystem crossing to MLCT occurs. 10 A. Yella, H.-W. Lee, H. N. Tsao, C. Yi, A. K. Chandiran,
Triplet-to-triplet energy transfer takes place with a rate constant
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ACS Nano, 2009, 3, 3103.
1
0
ꢁ1
>
2 ꢃ 10
s
1
(Fig. 14b), leading to the {Zn(TPP)} T state. By
deactivation of this state the ground state is recovered.
Conclusions
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functionalized tpy ligand 3, and its single crystal structure has
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2
, the n-type
B. F. E. Curchod, N. Ashari-Astani, I. Tavernelli,
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]
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15380 | RSC Adv., 2016, 6, 15370–15381
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RSC Adv., 2016, 6, 15370–15381 | 15381