Photophysical properties of ligand localized excited state in ruthenium(ii) polypyridyl complexes: A combined effect of electron donor-acceptor ligand

Article abstract of DOI:10.1039/c1dt10266d

We have synthesized ruthenium(ii) polypyridyl complexes (1) Ru(ii)(bpy)₂(L₁), (2) Ru(ii)(bpy)₂(L ₂) and (3) Ru(ii)(bpy)(L₁)(L₂), where bpy = 2,2′-bipyridyl, L₁ = 4-[2-(4′-methyl-2,2′- bipyridinyl-4-yl)vinyl]benzene-1,2-diol) and L₂ = 4-(N,N-dimethylamino-phenyl)-(2,2′-bipyridine) and investigated the intra-ligand charge transfer (ILCT) and ligand-ligand charge transfer (LLCT) states by optical absorption and emission studies. Our studies show that the presence of electron donating -NMe₂ functionality in L₂ and electron withdrawing catechol fragment in L₁ ligands of complex 3 introduces low energy LLCT excited states to aboriginal MLCT states. The superimposed LLCT and MLCT state produces redshift and broadening in the optical absorption spectra of complex 3 in comparison to complexes 1 and 2. The emission quantum yield of complex 3 is observed to be extremely low in comparison to that of complex 1 and 2 at room temperature. This is attributed to quenching of the ³MLCT state by the low-emissive ³LLCT state. The emission due to ligand localized CT state (ILCT and LLCT) of complexes 2 and 3 is revealed at 77 K in the form of a new luminescence band which appeared in the 670-760 nm region. The LLCT excited state of complex 3 is populated either via direct photoexcitation in the LLCT absorption band (350-700 nm) or through internal conversion from the photoexcited ³MLCT (400-600 nm) states. The internal conversion rate is determined by quenching of the ³MLCT state in a time resolved emission study. The internal conversion to LLCT and ILCT excited states are observed to be as fast as ～200 ps and ～700 ps for complexes 3 and 2, respectively. The present study illustrates the photophysical property of the ligand localized excited state of newly synthesized heteroleptic ruthenium(ii) polypyridyl complexes.

Full text of DOI:10.1039/c1dt10266d

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PAPER
Photophysical properties of ligand localized excited state in ruthenium(II)
polypyridyl complexes: a combined effect of electron donor–acceptor ligand†
Sandeep Verma,a Prasenjit Kar,b Amitava Das*b and Hirendra Nath Ghosh*a
Received 18th February 2011, Accepted 8th July 2011
DOI: 10.1039/c1dt10266d
We have synthesized ruthenium(II) polypyridyl complexes (1) Ru(II)(bpy)2(L1), (2)
Ru(II)(bpy)2(L2) and (3) Ru(II)(bpy)(L1)(L2), where bpy = 2,2¢-bipyridyl, L1 =
4-[2-(4¢-methyl-2,2¢-bipyridinyl-4-yl)vinyl]benzene-1,2-diol) and L2 = 4-(N,N-dimethylamino-
phenyl)-(2,2¢-bipyridine) and investigated the intra-ligand charge transfer (ILCT) and ligand–ligand
charge transfer (LLCT) states by optical absorption and emission studies. Our studies show that the
presence of electron donating –NMe2 functionality in L2 and electron withdrawing catechol fragment in
L1 ligands of complex 3 introduces low energy LLCT excited states to aboriginal MLCT states. The
superimposed LLCT and MLCT state produces redshift and broadening in the optical absorption
spectra of complex 3 in comparison to complexes 1 and 2. The emission quantum yield of complex 3 is
observed to be extremely low in comparison to that of complex 1 and 2 at room temperature. This is
attributed to quenching of the 3MLCT state by the low-emissive 3LLCT state. The emission due to
ligand localized CT state (ILCT and LLCT) of complexes 2 and 3 is revealed at 77 K in the form of a
new luminescence band which appeared in the 670–760 nm region. The LLCT excited state of complex
3 is populated either via direct photoexcitation in the LLCT absorption band (350–700 nm) or through
internal conversion from the photoexcited 3MLCT (400–600 nm) states. The internal conversion rate is
determined by quenching of the 3MLCT state in a time resolved emission study. The internal
conversion to LLCT and ILCT excited states are observed to be as fast as ~200 ps and ~700 ps for
complexes 3 and 2, respectively. The present study illustrates the photophysical property of the ligand
localized excited state of newly synthesized heteroleptic ruthenium(II) polypyridyl complexes.
regard, diimine ligands (bpy/terpy) substituted with electron-
1. Introduction
donating and electron-withdrawing groups, have drawn special
Ruthenium(II) polypyridyl complexes have been studied exten-
attention as it introduces new ligand localized CT states to MLCT
sively for their rich optoelectronic and redox properties.1 Absorp-
tion and fluorescence spectra of these complexes are generally
dominated by dpRu(II) → p*bpy/terpy-based transitions. Energies of the
electronic transitions and the corresponding energy gap between
dpRu(II)-based highest occupied molecular orbitals (HOMO) and
p*bpy/terpy-based lowest unoccupied molecular orbitals (LUMO)
can be tuned with appropriate substitution of the polypyridyl
ligand (bpy or terpy) with suitable electron donor or acceptor
functionalities.2,3 Recent advancements in the area of synthetic
chemistry and vast number of literature reports draw an appropri-
ate correlation between the structure and redox activity/spectral
properties which helped in the design and synthesis of newer
molecules with attractive spectral and redox behaviors. In this
manifolds of metal–polypyridyl complexes.4 The optical response
of such complexes are significantly modified by incorporating
new intra-ligand charge-transfer (ILCT) or ligand–ligand charge-
transfer (LLCT) transitions to aboriginal MLCT transitions.5
The appropriate selection of the substituent in diimine ligands of
Ru(II) polypyridyl complexes may lower the energy of the ligand
localized CT excited state in comparison to that of the dpRu(II)
→
p*bpy/terpy-based MLCT excited state. Thus, ligand localized CT
states offer the possibility of a new deactivation channel of
the photoexcited MLCT states, as these CT states could be
populated by internal conversion from the original photoexcited
MLCT states. Furthermore, a direct photoexcitation to a ligand
localized CT band can produce such charge separated species
instantly, without undergoing charge diffusion in excited states.
The populated ligand localized CT excited states represents in-
tramolecular charge separated states where electrons and counter
positive charges (holes) reside on electron-acceptor and electron-
donor ligands, respectively. In recent times, such a controlled
intramolecular charge separation in metal–polypyridyl complexes
aRadiation and Photo Chemistry Division, Bhabha Atomic Research Center,
Mumbai, India. E-mail: hnghosh@barc.gov.in
bCentral Salt and Marine Chemicals Research Institute (CSIR), Bhavnagar,
364002, Gujarat, India. E-mail: amitava@csmcri.org
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c1dt10266d
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is desired for photovoltaic applications such as solar energy
conversion,6,7 photocatalytic activity, electrochemiluminescence
etc.8 In particular, the metal–polypyridyl complexes comprising
ligand localized CT states were found to be useful in the field of
dye sensitized solar cells (DSSC).7,9 So, the design and synthesis
of ruthenium complexes exhibiting these properties are important
in a way similar to optimization of photovoltaic devices based on
such complexes.
the electron withdrawing nature of the catechol moiety of the L1
ligand. Therefore, it will be interesting to study the photophysical
property of complex 2 and complex 3 to identify the ligand
localized CT states. In this present study, the ligand localized CT
excited states (ILCT and LLCT states) are explored by carrying out
comparative optical absorption and emission studies of complexes
1, 2 and 3 under different ambient conditions such as aerated vs.
de-aerated, protonated vs. de-protonated (at different pH) and
ambient vs. low temperature. We have also carried out the time
resolved emission measurements (using the time correlated single
Recent experimental as well as theoretical studies have described
the role of ILCT and LLCT states in achieving a rapid intramolec-
ular charge separation in metal–polypyridyl complexes which
comprises of electron-donor and electron-acceptor ligands.10–15 In
ruthenium–polypyridyl complexes, the singlet → triplet excited
state conversion (intersystem crossing process (ISC)) is accom-
plished in an ultrafast time scale (<100 fs).17,18 The inherent lower
energetics13 of 3ILCT or 3LLCT states with respect to the 3MLCT
excited state, makes these ligand localized CT states active for
photoluminescence in the near-IR region. The photoluminescence
of such complexes were experimentally observed in the 650–
850 nm region.19 The internal conversion from 3MLCT to 3ILCT
3
photon counting-TCSPC technique) to explore the MLCT →
3ILCT or 3LLCT internal conversion process. Our studies indicate
strong LLCT behaviour of complex 3 in addition to weak ILCT
behavior of complex 2.
2. Experimental section
a) Materials
RuCl3·xH2O, 2,2¢-bipyridyl, 3,4-dimethoxy benzaldehyde, ac-
etaldehyde and 4-(N,N-dimethylamino) benzaldehyde were pro-
cured from Sigma-Aldrich and were used as received. Pyridine
and ethanol, used as solvent, were dried and distilled prior
to use. Nanopure water (Barnsted System, USA) is used for
making aqueous solutions. All other reagents were of AR grade
and procured from S.D. Fine Chemicals (India). HPLC grade
acetonitrile (E. Mark, Bombay, India) is used for all spectrophoto-
metric studies. Solvents were degassed thoroughly with dinitrogen
gas (IOLAR grade purity) before use in the preparation of
all standard solutions. 4-(N,N¢-dimethylamino-phenyl)-propanal
(A), pyridacyl pyridinium iodide salt (B), 4-(2,2¢-bipyridinyl-4-
yl)vinyl-benzene-1,2-diol (L1), complexes 1 and 1·Me2 (SI) were
synthesized following a previously reported procedure.23a,b
3
or LLCT states is reported as a dominant fast phase emission
3
quenching of the MLCT state in the sub ns time domain.20,21
Herein, the chemical nature of ancillary ligands defines the ILCT
or LLCT character of final excited electronic states in RuII–
polypyridyl complexes. For instance, Chou and co-workers16 have
3
3
demonstrated a shift from ILCT to LLCT character in ligand
localized CT states by the subsequent incorporation of electron
withdrawing moieties in axial N-heterocyclic ligands. It suggests
that ILCT or LLCT transitions are originating from specific
substitution of N-donor ligands. So, in an effort to explore the
ILCT and LLCT excited state, we have designed and synthesized a
new series of ruthenium–polypyridyl complexes 1, 2 and 3 (Scheme
1) by varying the chemical nature of the ancillary ligands. The
ligand centered charge separation in complex 2 and 3 (after pho-
toexcitation) is expected from the facile electron donor character
of N,N-dimethylaniline (ph-NMe2) moiety directly attached to the
bpy fragment of L2 ligand. Earlier, Zhang and co-workers22 have
reported 3ILCT states in a dimethylamino-a-styryl substituted
RuII–diimine complex which is structurally related to complex 2 in
this study. Moreover, in the case of complex 3, the ligand centered
charge separated excited states which are produced by the electron
donor L2 ligand can be stabilized by the low lying p* (LUMO)
level of the p-conjugated bipyridyl (bpy–CH CH–) moiety and
b) Analytical methods
Microanalyses (C, H, N) were performed using a Perkin–Elmer
4100 elemental analyzer. FTIR spectra were recorded either as
KBr pellets or as acetonitrile solutions in a cell fitted with a KBr
window, using a Perkin–Elmer Spectra GX 2000 spectrometer. 1H
NMR spectra were recorded on a Bruker 200 MHz FT NMR
(model: Avance-DPX 200) using CD3CN and CD3OD as the
solvents and tetramethylsilane (TMS) as an internal standard;
ESI MS measurements were carried out on a Waters QTof-Micro
Scheme 1 Molecular structure of complexes 1, 2 and 3.
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instrument. Electronic spectra were recorded with a Shimadzu
UV-3101 PC spectrophotometer; while steady state luminescence
spectra were recorded using a Perkin–Elmer LS 50B luminescence
spectrofluorimeter fitted with a red-sensitive photomultiplier.
Electrochemical experiments were performed on a CH-660A
electrochemical instrument with a conventional three-electrode
cell assembly. A saturated Ag/AgCl reference and a platinum
working electrode were used for all measurements. Ferrocene was
added at the end of each measurement of the cyclic voltametric
experiment as an internal standard.
using alumina as the stationary phase and acetonitrile as the
eluent, afforded the desired complex as a dark orange solid. Yield:
1
30%; MS (ESI-MS) m/z: 834 (M+ - PF6); H NMR (200 MHz,
CD3CN, ppm): d 8.66 (2H, d, J = 7 Hz, 2HL2 5,6); 8.51 (5H, d,
J = 8.2, Hbpy 3,3¢, HL3¢); 8.09–8.01 (6H, m, Hbpy 6,6¢, HL2,6,6¢); 7.79–7.72
(7H, m, Hbpy 4,4¢, Hphenyl, HL2); 7.42–7.36 (5H, m, Hbpy 5,5¢,HL2,5¢); 6.85
(2H, d, J = 9 Hz, HPhenyl); 3.04 (6H, s, HL 3 ). IR (KBr pellet,
CH
2
cm-1): 847 (–PF6). Calculated for RuC38H33N7P2F12: C 46.63, H
3.40, N 10.02; Found: C 46.5; H 3.38; N 9.98. E1/2 (in CH3CN):
1.25 (Ru2+/Ru3+); -1.21 (L1/L1 ) vs. Ag/AgCl.
-
Complex 3. To 30 mL DMF, 75 mg (0.282 mmol) of
RuCl3·3H2O and 45 mg (0.282 mmol) of 2,2¢-bipy was added.
After stirring in the dark for 2 h at 90 ◦C and maintaining
an Ar atmosphere, 85 mg (0.282 mmol) of 4-[2-(4¢-methyl-2,2¢-
bipyridinyl-4-yl)vinyl] benzene-1,2-diol (L1) was added at 120 ◦C
and stirred for 4 h. 77.5 mg (0.282 mmol) of 4-(2,2¢-bipyridin-4-
yl)-N,N-dimethylaniline (L2) was added to this reaction mixture
and was heated to 145 ◦C for 4 h more. After evaporation of DMF
under reduced pressure, aqueous solution of excess KPF6 was
added and sonicated for 5 min. An orange solid was precipitated
and collected by filtration after repeated washing with cold water
andether. Chromatographic purificationofthe desired compound,
using alumina as the stationary phase and acetonitrile as the
eluent, afforded the desired complex as a dark orange solid. Yield:
30%; Calculated for RuC47H41N7O2P2F12: C 50.6, H 3.67, N 8.7;
Found: C 51.0, H 3.7, N 8.51; MS (ESI-MS) m/z: 837 (M+ - 2PF6);
1H NMR (200 MHz, CD3CN, ppm): d 9.68 (H, s, H3L3); 8.65 (2H,
d, J = 7 Hz, 2Hbpy 6,6¢); 8.49 (3H, d, J = 8.2, HL2 3¢,6¢, HL2 3¢); 8.05 (4H,
c) Synthesis
(i) 4-(N,N-dimethylamino-phenyl)-propanal (A). 4-N,N-
dimethylamino benzaldehyde (16.6 g) and acetaldehyde (5.6 mL)
were dissolved in 40 mL of ethanol (95%). The mixture was cooled
to 0 ◦C and to this 36 mL of 28% NaOH solution was added slowly
then the solution was stirred vigorously and the temperature of
the reaction mixture was maintained under 10 ◦C (15 min). After
stirring further for 30 min at room temperature, 200 mL of water
was added and the desired compound was extracted three times
with 50 mL of ether. Then the ether layers were collected and
washed twice with 50 mL of brine solution. Then ether solution
was dried over anhydrous MgSO4. Evaporation of ether followed
by distillation at reduced pressure gave the product as a highly
viscous yellow oil. This compound was further used for the next
step without any purification.
(ii) Pyridacyl pyridinium iodide salt (B) was synthesized follow-
ing the literature procedure.24
t, J = 7.4 Hz, HL2 4¢5, Hbpy 4,4¢); 7.81–7.66 (9H, m, Hbpy 5,5¢, Hethenyl
,
(iii) 4-(N,N-dimethylamino-phenyl)-(2,2¢-bipyridine). Pyrida-
cyl pyridinium iodide salt (B) (5.2 g, 0.015 mol) was dissolved
in 50 mL glacial acetic acid in a 250 mL round bottom flask. To
this solution ammonium acetate (12.0 g, 0.13 mol) was added
and stirred at 100 ◦C. Further, 3.0 g (0.015 mol) of 4-(N,N-
dimethylamino-phenyl)-propanal (A) was added to this in three
intervals of 1.5 h at 100 ◦C. This overall reaction mixture was
stirred at 100 ◦C for 16 h. Then the reaction mixture was allowed
to cool to room temperature and acetic acid was removed under
vacuum. To this about 50 mL of water was added. The pH of this
solution was adjusted to ~8.0 and then the desired compound was
extracted with chloroform. The chloroform layer was dried over
anhydrous MgSO4 and evaporated to get L2 in its crude form.
This was purified by gravity chromatography using silica as the
stationary phase and methanol–chloroform (5 : 95; v/v) as the
eluent. Yield: 0.80 g (18%). ES-MS; (M+): 275 (100%); 1H NMR
(200 MHz CD3OD; ppm): d 8.71–8.63 (m, 1H, Hpyridyl), 8.42 (d,
J = 8.0 Hz, 1H, Hpyridyl), 7.72 (m, 4H, Hpyridyl), 7.27 (d, J = 8 Hz, 1H,
Hpyridyl), 7.33–7.26 (m, 1H, 1Hpyridyl), 6.91 (d, J = 8 Hz, 1H, Hphenyl),
6.85 (d, J = 8 Hz, 2H, Hphenyl), 3.06 (s, 6Hmethyl). IR (KBr pellet,
cm-1): 1604 (C C, C N).
H
L1 5¢, HL26), Hphenyl 5,6, HL2 3,5); 7.55 (2H, s, HL1 3), 7.29–7.23 (4H, m,
H
bpy 3,3¢), HL2 5¢,6¢); 6.85 (2H, d, J = 9 Hz, HL2 2,6); 6.76 (3H, d, J = 9
Hz, Hethenyl, HL1 5, HL2 6); 3.04 (6H, s, HL2 CH3); 3.03 (3H, s, HL1 CH3);
IR (KBr pellet, cm-1): 3435 (–OH), 847 (–PF6). E1/2 (in CH3CN):
1.24 (Ru2+/Ru3+); -1.22 (L1/L1 ) vs. Ag/AgCl.
-
d) Picosecond time-resolved fluorimeter. Time-resolved fluo-
rescence measurements were carried out using a diode laser based
spectrofluorometer from IBH (UK). The instrument works on the
principle of time-correlated single-photon counting (TCSPC).25 In
the present work, a 406 nm (FWHM ~ 66 ps, 1 MHz) diode laser
was used as the excitation light sources, and a TBX4 detection
module (IBH) coupled with a special Hamamatsu PMT was used
for fluorescence detection.
3. Results and discussion
(a) Synthesis
Pyridacyl pyridinium iodide salt (B) and 4-(N,N¢-dimethylamino-
phenyl)-propanal (A) were allowed to react for the synthesis of L2
following the basic synthetic methodology reported earlier.23d The
synthesis of ligand L1 is reported earlier.23a,b Complex 2 was synthe-
sized following the typical procedure by reacting Ru(bpy)2Cl2 with
L2 in ethanol and was isolated as the hexafluorophosphate salt.
Complex 3 was synthesized following a one pot synthesis, where
RuCl3·3H2O was initially reacted with one equivalent of 2,2¢-
bipyridine at 90 ◦C in DMF solution. To this resulting mixture, L1
was added and allowed to react with Ru(bpy)Cl3 produced in situ,
and the reaction temperature was raised to 120 ◦C. To this resulting
Complex 2. To 50 mL ethanol, 80 mg (0.154 mmol)
of Ru(bpy)2Cl2·2H2O and 42 mg (0.154 mmol) of 4-(N,N-
dimethylamino)-2,2¢-bipyridine was added. This reaction mixture
was heated to reflux for 8 h. To this an aqueous solution of excess
KPF6 was added and stirred for few minutes. Then ethanol was
removed under reduced pressure and an orange solid suspended in
water was obtained. This orange solid was then filtered and washed
with cold water and ether. Column chromatographic purification,
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Scheme 2 i: CH3CHO, EtOH, 28% NaOH, 0◦C; ii: Py, I2, reflux, 90 min, N2 atmp. iii: CH3COOH, CH3COO-NH4+.
reaction mixture that contains Ru(bpy)L1, L2 was added and the
temperature was raised to 145 ◦C. At the end of the reaction, DMF
was removed under reduced pressure and the crude product was
further purified by column chromatography to achieve the desired
purity. Analytical and spectroscopic data obtained for complexes
2 and 3, agreed well with the proposed structure for the respective
complexes.
b) Cyclic voltammetry
Electrochemical studies revealed that the RuII/III redox potential
(with respect to Ag/AgCl) for complexes 1, 2 and 3 is +1.32 V,
+1.25 V and +1.24 V, respectively. This difference in the RuII/III
redox potential is reflected in the observed redshift of the
MLCT band for complex 3 as compared to the heteroleptic
complex 1.
(c) Spectroscopic properties: UV-vis absorption spectra
Fig. 1 Optical absorbance spectra of (a) complex 1, (b) complex 2 and
(c) complex 3 in neutral (bold lines) and (a¢) complex 1, (b¢) complex
2 and (c¢) complex 3 in acidic (dash-dot lines) conditions in acetonitrile
(concentrations of all complex were kept the same).
The optical absorption spectra of complexes 1, 2 and 3 were
measured at the same concentration in acetonitrile and presented
in Fig. 1. Electronic spectra recorded for L1 (Fig. S3a, ESI†)
revealed a prominent p–p* transition at 336 nm, along with an
n–p* transition at an even shorter wavelength. Presumably this
absorption band moves to about 351 nm in complex 1. However,
our earlier studies suggest that the absorption band in this region
(348–351 nm) is predominantly due to intra-ligand (bpy or L1)
or inter-ligand (L1[p]–bpy[p*])-based transitions.23c The transition at
351 nm has some contribution from the dpRu(II) → p*L1-based
MLCT transition; while the transition band with a maxima
at 457 nm is predominantly a dpRu(II) → p*L1/bpy-based MLCT
transition. Such a dpRu(II) → p*L1/bpy-based MLCT transition band
at ~457 nm has been observed earlier by many researchers for
various Ru(II)–polypyridyl complexes.23f No appreciable change
in the electronic spectra for complex 1 is observed on addition
of HNO3 (pH ~ 1) as this acidity is not sufficient for proto-
nation of the hydroxyl functionality of the pendant catechol
moiety.
charge transfer transitions. The MLCT electronic transitions
for complex 2 are expected to be somewhat similar to that of
complex 1. Thus, the broader absorption band of complex 2 (Fig.
1b) in the longer wavelength region (l > 350 nm) represents
overlapped MLCT and ILCT electronic transitions. Earlier, Zhang
and coworkers22 reported the presence of an ILCT electronic
transition in related Ru(II)–polypyridyl complexes which comprise
–NMe2 groups as electron donating units on diimine ligands. In
this present study, the ILCT states are evidenced in complex 2 by
a decrease in optical density after protonation (Fig. 1b¢). In the
presence of HNO3, an ILCT transition is expected to be eliminated
after protonation of the –NNMe2 -centre (–NH+
formation) and
NMe
2
is perhaps reflected by the decrease in the absorbance in the 350–
410 nm region. On the contrary, the protonation of the –NNMe
-
2
centre (–NH+ 2 formation) is also expected to lower the L2-based
NMe
The absorption spectra recorded for L2 (Fig. S3b, ESI†) reveals
that the tail of the absorption band with a maxima at 336 nm
extends beyond 500 nm. An absorption band at 336 nm was
assigned to the p–p*-based transition, while the weaker absorption
having a tail beyond 500 nm can be assigned to intramolecular
LUMO energy level and thus with the narrower HOMO–LUMO
gap, the dpRu(II) → p*L2/bpy-based MLCT transition is expected
to shift to the longer wavelength. Thus, the observed absorption
spectra for the protonated complex 2 in the 410–600 nm region
is the result of two opposing influences (Fig. 1b¢). The spectral
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change in the 350–400 nm region and the slight decrease in the
overall optical density (350–550 nm region) after protonation of
complex 2 signifies the presence of an ILCT electronic transition
in the neutral complex 2.
However, the broad absorption spectrum of complex 3 (Fig.
1c) is a little more affected by the L1 and L2 ligand’s electronic
properties. The presence of the –NMe2 functionality in L2 and the
extended conjugated system in the catechol-based bpy derivative
(L1) are expected to favour the interligand charge transfer tran-
sitions. Earlier, Chi, Chou and their co-workers8 have reported
LLCT transitions in osmium–polypyridyl complexes containing
both electron donor and acceptor ligands. The combination of
MLCT and LLCT absorption bands was responsible for the
broad absorption spectrum in the longer wavelength region. In
the case of complex 3, the HOMO–LUMO energy gap for this
pL2(HOMO) → p*L1-based transition is expected to be narrower
than expected for a pL2(HOMO) → p*bpy-based transition in
complex 2 and accounts for the longer wavelength absorbance
with the tail extending to ~575 nm (Fig. 1c). This interligand
pL2(HOMO) → p*L1-based transition is affected significantly in
acidic solution (pH ~ 1) due to the protonation of the –NMe2
functionality in L2 ligand; while this is expected to shift the
dpRu(II) → p*L2/bpy -based MLCT transition to longer wavelength
(Fig. 1c¢). The protonation of the –NMe2 group suppresses the
electron donor character of the L2 ligand which inhibits the
interligand electron transfer processes, a condition essential for
the LLCT excited state.26 Thus, the significant decrease in the
absorption band on protonation of complex 3 indicates a strong
contribution of LLCT electronic transition in the neutral complex
3. However, unambiguous assignment of the ligand localized CT
bands in the steady state absorption spectra is difficult due to
the presence of various overlapping spectral bands. So, emission
spectroscopy is used to develop a better insight of ligand localized
CT states.
Fig. 2 Emission spectra of (a) complex 1, (b) complex 2 and (c) complex
3 in neutral (bold lines) conditions and (a¢) complex 1, (b¢) complex 2
and (c¢) complex 3 in acidic (dash-dot lines) conditions in acetonitrile (all
emission spectra are normalized with respect to the same optical density).
energetic 3ILCT excited states. Earlier, the low emissive behaviour
of the ILCT state has been suggested by Charlot and co-workers28
by considering the absence of heavy atom spin density on the
3ILCT state which decreases the radiative transition from 3ILCT
states in comparison to 3MLCT states. The low emission quantum
yield of the structurally related Ru(II)–polypyridyl complex was
reported by Zhang and co-workers and attributed to ILCT states.22
Thus, the presence of 3ILCT excited states could be the reason of
observing low emission yield for complex 2. On addition of HNO3
(excess H+ ions), the emission yield of complex 2 is further reduced
by a factor of ~2.9 (Fig. 2b¢). The drastic reduction in the emission
quantum yield is possibly due to increase in non-radiative decay
on protonation of the NMe2 group of L2 ligand. This also indicates
a weak ILCT character in complex 2.
Fig. 2c shows the emission spectra of complex 3 in acetonitrile.
The emission maximum of complex 3 is observed to be redshifted
at 640 nm as compared to emission maxima of complex 1 and 2.
The emission quantum yield of complex 3 (f3 = 0.0006) is observed
to be extremely low as compared to that of complex 1 (f1 = 0.023)
and complex 2 (f2 = 0.016) and this cannot be explained alone by
the non-radiative decay channel caused by the H-bonding network
d) Steady state emission spectroscopy of complexes 1, 2 and 3
The photoluminescence spectra of ruthenium(II)–polypyridyl
complexes are largely attributed to the 3MLCT states in the
literature which are populated via an ultrafast ISC process from
directly photoexcited 1MLCT states.17,18 Fig. 2a shows the emission
spectra of complex 1 in aerated acetonitrile. The emission peak at
621 nm is assigned to the 3MLCT → S0 transition. The emission
quantum yield of the complex 1 is observed to be less (f1 = 0.023)
3
of the catechol moiety as observed in complex 1 or ILCT state
3
as observed in complex 2. This can be attributed to the LLCT
with respect to that of Ru(bpy)3(PF6)2 (f = 0.062 in acetonitrile
at room temperature with lmax
states which are suggested by large redshifted absorption spectra
of complex 3 (Fig. 1c). The extremely low emission quantum yield
of complex 3 suggests a very low lying LLCT state (energy gap
= 611 nm).1,27 It is known in
emission
the literature that the catechol moiety forms a hydrogen bonded
adduct with the surrounding polar solvent molecules, which
favours a non-radiative loss of photoexcitation energy.23c This is
further supported by the fact that the emission yield increases
two fold in complex 1Me2 (RuII(bpy)2(bpy–CH CH–ph(OMe)2))
where the –OH group is replaced with an –OMe group in the Me2L1
ligand. Thus, the lower emission quantum yield of complex 1 is
attributed to non-radiative decay through a H-bonding network of
the catechol moiety. Fig. 2b shows the emission spectra of complex
2 in acetonitrile with a maxima at 623 nm. The emission quantum
yield 0.016 (f2) of complex 2 is observed to be lower as compared to
that of complex 1 (f1 = 0.023). The lower emission quantum yield
of complex 2 can be due to the crossing of 3MLCT states to lower
3
law29) as compared to the MLCT manifold. In this regard, Chi,
Chou and coworkers16 have suggested shallow potential energy
surfaces of low lying 3LLCT excited states (T1) which under
extreme conditions of surface crossing between T1 and S0 states
(Scheme 4) can yield an extremely low emission yield on account of
the thermally activated vibrational states near the crossing point.30
Also, McCusker and co-workers14 have reported the excited state
(ES) vibronic coupling to ground states (S0) and observed a
low emission yield of a RuII complex on account of intra-ligand
electron delocalization. So, the extremely low emission quantum
yield of complex 3 in comparison to that of complex 1 and 2 can
be assigned to strong LLCT character of complex 3. Interestingly,
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ILCT or LLCT excited states. The emission measurements are
carried out at 77 K in an ethanol–methanol mixture (4 : 1 v/v).
Fig. 3 shows the uncorrected emission spectra of complex 1, 2
and 3 at 77 K. The emission spectral feature of complex 1, 2
and 3 in the 570–670 nm region are observed to be same as that
of Ru(bpy)3 complex reported by Balzani and co-workers,38 The
spectral features in this region (570–670 nm) can be assigned to
3
vibronic progression in the MLCT states. Unlike the emission
spectra observed at room temperature, the emission intensity of
complex 3 is observed to be comparable to that of complex 1 and
2 at 77 K. Interestingly, complexes 2 and 3 exhibit a new emission
band in the 670–760 nm region. The new redshifted emission band
in 670–750 nm region is attributed to the ILCT excited states of
complex 2 and LLCT excited states of complex 3. The emission
decay kinetics are measured at 77 K and presented in the ESI (Fig.
S10†). The lifetime of the ILCT and LLCT states are observed to
be ~9 ms and ~22 ms, respectively.
Scheme 3 Straight arrows (1) and (5) represent direct S0 → MLCT and
S0 → LLCT transitions; curved arrows (2), (3) and (4) indicate radiationless
transitions; bold arrows indicates dominant photo processes.
Fig. 3 Uncorrected emission spectra of (a) complex 1, (b) complex 2 and
(c) complex 3 at 77 K in an ethanol–methanol mixture (v/v 4 : 1).
Scheme 4 A simplified pictorial representation of different photophysical
processes in complex 1 (blue), 2 (green) and 3 (red) (Common ground state
(S0), 1MLCT and 3MLCT states are shown for the sake of convenience
only).
f) Time resolved emission (TCSPC) studies of complexes 1, 2 and 3
The photophysical properties of ruthenium–polypyridyl com-
plexes are largely dominated by MLCT transitions. On pho-
1
on addition of HNO3 (excess H+ ion), the emission yield increases
by a factor of four (Fig. 2c¢). In acidic conditions (acetonitrile–
HNO3), the protonation of the NMe2 moiety of the L2 ligand
suppresses the electron donor character of the L2 ligand which
in turn suppresses the strong LLCT character of complex 3.
Therefore, we observed an increased emission yield of protonated
complex 3. This indicates strong LLCT character in complex 3.
Furthermore, the emission yield of protonated complex 2 and
complex 3 are observed to be of same order. This suggests that the
observed emission of complex 2 and complex 3 in acidic conditions
is purely due to the 3MLCT state.
toexcitation in the visible region, a directly populated MLCT
excited state undergoes an ultrafast ISC process and 3MLCT
excited states are formed with almost unit quantum yield.17,18
3
The lifetime of MLCT excited states are reported in the sub-
microsecond domain.29b Fig. 4a shows the emission decay kinetics
of complex 1 at 621 nm (lmaxemission) monitoring wavelength in
aerated acetonitrile using 406 nm laser pulse (FWHM ~ 66 ps)
photoexcitation. The decay kinetics are best fitted bi-exponentially
with time constants of 3.6 ns (7%) and 150 ns (93%) (Table 1).
Similar, bi-exponential decay profiles have been reported in our
earlier studies on related Ru(II) and Os(II)–polypyridyl complexes
viz the OsII(bpy)2(L1) complex.32 The longer component (~150
3
ns) is attributed to a p*(bpy) → dRu(II)-based MLCT transition as
e) Steady state emission studies at low temperature (77 K)
it matches well with the emission decay kinetics of a RuII(bpy)3
complex in aerated solution (Fig. S9a, ESI†). The non-radiative
behaviour of p*L1 → dpRu(II) transitions, observed previously in
steady state emission measurements, is attributed to the shorter
decay component (3.6 ns). The bi-exponential decay process in
complex 1 is reconfirmed in de-aerated conditions (Fig. S8b, ESI†)
where emission kinetics are best fitted with 6 ns (6%) and >315 ns
(94%) time constants. Fig. 4b shows the emission decay kinetics of
The ligand localized CT states (ILCT or LLCT) emission is too
low to observe in emission spectroscopy at room temperature. The
ligand localized excited states are lower in energy as compared to
3MLCT states. As a result, the vibrational coupling to the ground
state is relatively high for ligand localized CT states. So, the low
emissive behaviour of ligand localized CT states can be due to
thermally activated non-radiative decay processes associated with
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3
domain39) or due to the radiationless decay nature of the ILCT
Table 1 Time resolved emission kinetics data of complex 1, 2 and 3 in
different conditions
states. Fig. 4c shows the emission decay profile of complex 3 in
aerated acetonitrile at 640 nm monitoring wavelength. The decay
profile is best fitted with 200 ps (97%), 3.5 ns (2%) and 74 ns (1%)
time constants (Table 1). Similar decay kinetics are observed in
de-aerated conditions (Fig. S8c, ESI†) and best fitted with 200 ps
(95%), 3.2 ns (3.2%) and >500 ns (1.8%) time constants (Table 1).
Such a short component (200 ps) and its large amplitude (~97%)
are not observed in complex 1, complex 2 and the 1 : 1 : 1 physical
mixture of RuII(bpy)3, L1 and L2 molecular entities (Fig. S9d,
ESI†). In the case of complex 3, the collective effect of electron
donor L2 ligand and electron acceptor L1 ligand introduces
energetically lower lying 3LLCT excited states (supported by low
Complex
Complex
Complex
lex = 406 nm
1 (lem = 621 nm) 2 (lem = 623 nm) 3 (lem = 640 nm)
Neutral
Acetonitrile
Air (O2)
3.6 ns (7%)
150 ns (93%)
700 ps (47%)
80 ns (53%)
200 ps (97%)
3.5 ns (2%)
74 ns (1%)
Neutral
Acetonitrile
Inert (N2)
6 ns (6%)
315 ns (94%)
700 ps (45%)
>300 ns (55%)
200 ps (96%)
3.2 ns (3.2%)
>500 ns (1.8%)
Acidic (HNO3) 4.2 ns (14%)
380 ps (85%)
6 ns (7.4%)
140 ns (7.6%)
340 ps (87%)
3.7 ns (8%)
140 ns (5%)
Acetonitrile
Air (O2)
150 ns (86%)
3
temperature emission measurements) to MLCT manifolds. The
lower energetics 3LLCT can significantly quench the 3MLCT state
of complex 3. Earlier, the crossing from 3MLCT to lower energetic
3LLCT excited state are reported to be as fast as <30 ps.33 So,
the shortest component (200 ps) of complex 3 in acetonitrile
3
solvent can reasonably be attributed to 3MLCT → LLCT internal
conversion. The second and third time components (3.5 ns and
74 ns in aerated conditions) are too low (total 3%) to assign
properly. Earlier, Yeh et al.36 have shown a 3ILCT → 3LLCT
conversion time as 4.9 ns. So, a 3.5 ns time constant can either
3
3
be associated with ILCT → LLCT conversion or related with
3MLCT states of the L1 ligand (p*(bp–CH CH–catechol) → dRu(II) process).
The longer component (74 ns in aerated condition or >500 ns
in de-aerated conditions) can be assigned to 3MLCT excited
states.
Fig. 4 Emission decay kinetics of (a) complex 1, (b) complex 2 and (c)
complex 3 at respective emission peak wavelengths in aerated conditions.
(lex = 406 nm; laser source ~66 ps FWHM).
3
3
The physical interpretation of the MLCT → LLCT internal
conversion process in complex 3 can be given by the intramolecular
electron transfer process as depicted in Scheme 3. In the photoex-
cited 3MLCT state of complex 3, the presence of the strong oxidant
Ru(III) ion helps in removing an electron from the bp–ph–NMe2
moiety of the L2 ligand and produces interligand (between L1
and L2 ligand) charge separated excited states (LLCT states). This
behavior is supported by earlier studies1,34 in reductive quenching
of 3MLCT states of *[(bpy)∑-Ru(III)(bpy)2]2+ complexes in presence
complex 2 in aerated acetonitrile monitored at 623 nm (lmaxemission).
The emission decay kinetics is bi-exponentially fitted with time
constants of 700 ps (47%) and 80 ns (53%)(Table 1). Similar bi-
exponential decay kinetics (700 ps (45%) and >300 ns (55%),
Table 1) is observed in de-aerated conditions (Fig. S8, ESI†) which
3
rule out MLCT state quenching by dissolved O2 in acetonitrile
3
as reported in the literature.15 Furthermore, this emission decay
profile is not observed in a 1 : 1 physical mixture of Ru(bpy)3
and L2 molecular entity (Fig. S9c, ESI†). This indicates that the
shortest time component (700 ps) is associated with the inherent
photophysical process of complex 2. In complex 2, the presence
of a strong electron donating N,N¢-dimethylaminophenyl moiety
on the bpy ligand (L2) introduces energetically lower lying 3ILCT
excited states (supported by low temperature emission spectra)
of triethylamine or N,N¢-dimethylaniline. So, 3MLCT → LLCT
internal conversion can be interpreted as an intramolecular
electron transfer reaction5,35 Furthermore, the photoexcitation
through strong LLCT absorption bands of complex 3 can directly
populate the interligand charge separated excited states (LLCT
state) as depicted in Scheme 3.
g) Time resolved emission studies (TCSPC) of complexes 1, 2 and 3
at low pH
3
to MLCT states manifold. This can cause significant emission
quenching of 3MLCT states via internal conversion to 3ILCT
3
excites states (Scheme 4).31 Earlier, the 3MLCT → ILCT internal
Steady state emission measurements of complex 1, 2 and 3 in the
presence of excess H+ ions revealed suppression of ligand localized
CT states due to protonation of the N,N¢-dimethylaminophenyl
moiety of L2 ligand in complex 2 and 3. So, the time resolved
emission studies (TCSPC) are carried out in acidic conditions
(adding HNO3 in acetonitrile) to understand the 3MLCT excited
state conversion to ligand localized CT states (ILCT in complex
2 and LLCT in complex 3). Fig. 5a shows the decay profile of
complex 1 in the presence of excess of H+ ions which is best fitted
with time constants of 4.2 ns (14%) and 150 ns (86%) (Table 1). The
decay kinetics of complex 1 in acidic conditions is observed to be
similar to that in neutral conditions (Fig. 4a). The slight increase of
conversion is reported to be as fast as ~100 ps by Charlot et al.28
So, in this present study, the 700 ps component can be assigned
3
to 3MLCT → ILCT internal conversion process. The longer time
component (80 ns in aerated conditions or >300 ns in de-aerated
3
conditions) is associated with remaining MLCT excited states.
The longer lifetime component of de-aerated sample (>300 ns)
may not be accurate due to instrumental limitations of the 500 ns
time scale imposed by the 1 MHz repetition rate of the 406 nm
laser excitation source. Further, the lifetime of the populated
3ILCT states may not be observed in TCSPC measurements either
due to their longer lifetime (reported in the > microsecond time
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spectrum of complex 3 is redshifted and broadened in comparison
to that of complex 1 and 2 on account of the superimposed
LLCT and MLCT states in the 350–700 nm region. The extremely
low emission quantum yield of complex 3 is attributed to the
3
quenching of 3MLCT states by the lower energetic LLCT states
which are low-emissive at room temperature. The ligand localized
CT state (ILCT and LLCT) emission of complex 2 and 3 are
revealed by the appearance of a new luminescence band in the
670–760 nm region at 77 K. On photoexcitation of complex 3, the
LLCT excited state is populated directly via the LLCT absorption
band (350–700 nm) or indirectly through internal conversion
from photoexcited 3MLCT (400–600 nm). The internal conversion
3
process is studied by quenching of MLCT emission using time
Fig. 5 Emission decay kinetics of protonated (a) complex 1, (b) complex
2 and (c) complex 3 at respective emission peak wavelengths in aerated
conditions (lex = 406 nm; laser source ~ 66 ps FWHM).
resolved emission studies. The internal conversion to ILCT and
LLCT excited states are observed to be as fast as ~700 ps and ~200
ps time scale in complex 2 and 3, respectively. This study helps
to understand the effect of ligand localized excited state on the
photophysical property of heteroleptic ruthenium(II) polypyridyl
complexes.
first component (14%) can be attributed to a decrease in its 3MLCT
energy level (L1 ligand) which populates more from 3MLCT states
involving bpy ligand as per the energy gap law.37 Fig. 5b shows
the emission decay profile of protonated complex 2 and best fitted
with multi-exponential time constants of 380 ps (85%), 6 ns (7.4%)
and 140 ns (7.6%) (Table 1). A similar decay profile is observed for
complex 3 (Fig. 5c) which is multi-exponentially fitted with time
constants of 340 ps (87%), 3.7 ns (8%) and 140 ns (5%) (Table
1). It is interesting to observe a similar emission decay profile for
complex 2 and complex 3 (Fig. 5) in acidic conditions in contrary
to their widely different emission decay profile of non-protonated
complex 2, complex 3 (Fig. 4) and 1 : 1 : 1 physical mixture of
RuII(bpy)3, L1 and L2 molecular species (Fig. S9d, ESI†). The
ligand localized CT excited states of complex 2 and complex 3 are
different in nature (ILCT vs. LLCT) and also different in energetics
(LLCT lower energetic than ILCT excited state). As a result the
internal conversion process in two complexes occurs with different
timescales (700 ps vs. 200 ps in Fig. 4b and 4c) and with different
strength (45% vs. 96% in Fig. 4b and 4c). In acidic conditions,
the ligand localized CT states are suppressed and two complexes
exhibit similar decay kinetics in TCSPC measurements. The lower
emission yield of complex 2 in acidic conditions as compared to
that in neutral conditions (Fig. 2b and 2b¢), suggests the initiation
of a new non-radiative decay process due to the protonated NMe2
moiety of the L2 ligand. Therefore, the 360 20 ps time constant
is assigned to the decay of the 3MLCT state due to a non-radiative
process in the protonated complex 2 and 3.
Acknowledgements
We thank Dr T. Mukherjee and Dr S. K. Sarkar of BARC and Dr P.
K. Ghosh of CSMCRI for their constant encouragement. HNG
thanks BRNS while AD thanks DST and BRNS for financial
assistance. We also like to thank Prof. Anindya Dutta of IIT,
Mumbai and Dr V. Sudarsan of Chemistry Division, BARC for
their kind help in recording time-resolved photoluminescence
kinetics.
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