Synthesis, Structural Characterization, and Luminescence Switching of Diarylethene-Conjugated Ru(II)-Terpyridine Complexes by trans-cis Photoisomerization: Experimental and DFT/TD-DFT Investigation

Article abstract of DOI:10.1021/acs.inorgchem.7b03096

We synthesized and thoroughly characterized a new family of diarylethene-conjugated mononuclear Ru(II)-terpyridine complexes and investigated in detail their photophysical, electrochemical, and spectroelectrochemical behaviors. Interestingly, the compounds show moderately strong room-temperature luminescence predominantly from their ³MLCT state with luminescence lifetime varying between 8.43 and 22.82 ns. Because of the presence of diarylethene unit, all the monometallic complexes underwent trans-to-cis photoisomerization upon interaction with UV light with substantial changes in their absorption and luminescence spectra. Reverting back from the cis to the trans form is also made possible upon treatment with visible light or by heat. Trans-to-cis isomerization leads to almost complete quenching of luminescence, while backward cis-to-trans isomerization gives rise to restoration of the original luminescence for all the complexes. Thus, on-off and off-on emission switching was made possible upon successive interaction of the complexes with UV and visible light. Computational investigation involving density functional theory (DFT) and time-dependent DFT methods was done for proper assignment of the experimental absorption and emission spectral bands in the complexes. Finally, experimentally observed trend on the absorption and emission spectral behaviors of the complexes upon photoisomerization was also compared with the calculated results.

Full text of DOI:10.1021/acs.inorgchem.7b03096

Article
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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Synthesis, Structural Characterization, and Luminescence Switching
of Diarylethene-Conjugated Ru(II)-Terpyridine Complexes by trans−
cis Photoisomerization: Experimental and DFT/TD-DFT Investigation
Poulami Pal, Shruti Mukherjee, Dinesh Maity, and Sujoy Baitalik*
Department of Chemistry, Inorganic Chemistry Section, Jadavpur University, Kolkata 700032, India
S
* Supporting Information
ABSTRACT: We synthesized and thoroughly characterized a
new family of diarylethene-conjugated mononuclear Ru(II)-
terpyridine complexes and investigated in detail their photo-
physical, electrochemical, and spectroelectrochemical behav-
iors. Interestingly, the compounds show moderately strong
room-temperature luminescence predominantly from their
3MLCT state with luminescence lifetime varying between 8.43
and 22.82 ns. Because of the presence of diarylethene unit, all
the monometallic complexes underwent trans-to-cis photo-
isomerization upon interaction with UV light with substantial
changes in their absorption and luminescence spectra.
Reverting back from the cis to the trans form is also made
possible upon treatment with visible light or by heat. Trans-to-cis isomerization leads to almost complete quenching of
luminescence, while backward cis-to-trans isomerization gives rise to restoration of the original luminescence for all the
complexes. Thus, “on−off” and “off-on” emission switching was made possible upon successive interaction of the complexes with
UV and visible light. Computational investigation involving density functional theory (DFT) and time-dependent DFT methods
was done for proper assignment of the experimental absorption and emission spectral bands in the complexes. Finally,
experimentally observed trend on the absorption and emission spectral behaviors of the complexes upon photoisomerization was
also compared with the calculated results.
due to their outstanding photophysical and redox proper-
ties.23−26 Both bipyridine- and terpyridine-type ligands have
widely been utilized to synthesize a large variety of Ru(II)
complexes. But the octahedral complexes based on bis-
terpyridine-type ligands are superior over tris-bipyridine type
with regard to the construction of achiral multinuclear
architectures.27−33 However, this class of complexes often
suffers from their low intrinsic quantum yields because of
closeness of energy between the radiative metal-to-ligand
charge transfer (MLCT) states and nonradiative metal-centered
(MC) states arising out of their distorted octahedral geo-
metries.34 Nonetheless, it had already been shown that their
room-temperature emission properties can be improved by
adopting different strategies. In general, improved luminescence
properties can be accomplished by synthetic routes, that is, by
insertion of electron-donating or -accepting substituent35−39 as
well as polyaromatic chromophores40−43 to control the
electronic coupling of π−π* excited state and simultaneously
present MLCT states. The use of cyclometalated ligands can
INTRODUCTION
■
Smart molecular systems capable of reversibly changing their
physicochemical properties upon treatment with different
external stimuli are receiving great attention in recent years
with regard to their probable practical applications such as for
the design of molecular switches and memory devices.1−6
Among the various external stimuli, light is the most suitable
and convenient, because it is a clean energy source and required
wavelength of light can easily be used for exciting a particular
molecular fragment within very short time scale. To this end,
photoisomerization studies of some organic molecules such as
azobenzenes, diarylethenes, and spiropyrans have been
extensively investigated.7−13 But reports on similar photo-
isomerization studies with metal complexes are relatively few in
the literature compared to pure organic compounds.14−17
Coordination complexes based on transition metals have gained
increasing attention in recent years as materials in photovoltaic,
optoelectronic, photonic devices as well as in sensors and in
different photocatalytic processes.18−22 The versatility of the
complexes is related to their widely tunable optoelectronic
properties, which can be incorporated upon variation of the
metal as well as the ligand. Among the various coordination
complexes, those derived from Ru(II) metal and in
combination with polypyridine ligands contributed significantly
44−47
3
give rise to destabilization of the nonemitting MC state.
Alternatively, incorporation of different heterocyclic rings can
Received: December 8, 2017
© XXXX American Chemical Society
A
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PhCH2PPh3Br) were synthesized following reported literature
procedures.67,79 [(tpy-PhCH3)RuCl3] was prepared by refluxing
RuCl3·3H2O and tpy-PhCH3 (1:1 molar ratio) in EtOH. The
terpyridine ligands incorporating diarylethene were synthesized by
modification of the literature procedures adopted by Schmehl80 and
Magennis,81 and the synthetic details along with the relevant
characterization data were given in Supporting Information (Figures
S1−S8).
give rise to less distorted planar ground-state configura-
tion.48−52 The key factor for the improvement of their
photophysical properties with respect to their probable practical
application is an extension of the electron delocalization and
the minimization of nonradiative relaxation channels.53−58
With regard to our ongoing research interest for designing
luminescent terpyridine-based Ru(II) complexes,59−68 we
report herein a new family of heteroleptic Ru(II) complexes
of composition [(tpy-PhCH3)Ru(tpy-pvp-X)](BF4)2 (Chart 1)
Synthesis of the Metal Complexes. A general procedure as
described below was adopted for the synthesis of the complexes.
[(tpy-PhCH3)Ru(tpy-pvp-H)](BF4)2·H2O (1). [Ru(tpy-PhCH3)Cl3]
(0.13 g, 0.25 mmol) and AgBF4 (0.17 g, 0.87 mmol) were successively
added to 40 mL of acetone and stirred under refluxing condition for
∼3 h. The white solid of AgCl was quickly filtered out, and the filtrate
was treated with ∼40 mL of EtOH, and the remaining Me2CO was
rotary evaporated. Tpy-pvp-H (0.11 g, 0.26 mmol) was then added in
its finely powdered form to the solution and refluxed for 6 h with
continuous stirring under argon protection. The red compound that
deposited upon cooling was collected by filtration and thoroughly
washed with chloroform and dried. Acetonitrile solution of the
resulting compound was subjected to column chromatography (silica
gel) and eluted with MeCN−PhCH3 (10:1, v/v) mixture. The desired
compound was collected upon evaporation to a small volume.
Recrystallization of the complex from MeCN−MeOH (1:1, v/v)
mixture was done for further purification (0.16 g, yield: 61%). Anal.
Calcd for C51H40N6B2F8ORu: C, 59.78; H, 3.92; N, 8.17. Found: C,
Chart 1
1
59.67; H, 3.99; N, 8.05%. H NMR (300 MHz, deuterated dimethyl
sulfoxide (DMSO-d6)): δ 9.45 (s, 2H, 2H3′), 9.41 (s, 2H, 2H3′), 9.07
(t, 4H, J = 7.0 Hz, H6), 8.47 (d, 2H, J = 7.9 Hz, 2H7), 8.33 (d, 2H, J =
7.7 Hz, 2H7), 8.04−7.97 (m, 6H, 4H4 + 2H8), 7.70 (d, J = 7.7 Hz,
2H, 2H8), 7.57−7.52 (m, 8H, 4H3 + 2H11 + 2H12), 7.46 (nr, 1H,
H13′), 7.42 (d, 1H, J = 7.2 Hz, H9), 7.34 (d, 1H, J = 7.2 Hz, H10),
7.26 (t, 4H, J = 6.2 Hz, H5), 2.49 (s, 3H, CH3). Electrospray
ionization mass spectrometry (ESI-MS; positive, MeCN) m/z =
417.81 (100%) [(tpy-PhCH3)Ru(tpy-pvp-H)]2+ and m/z = 922.52
(5%) [(tpy-PhCH3)Ru(tpy-pvp-H)(BF4)]+.
by incorporating a photoisomerizable diarylethene group within
the ligand frame (tpy-pvp-X). Apart from incorporating π-
delocalization in the ligand frame, diarylethenes can also be
considered as model trans/cis photoisomerization system-
s.69−73Some reports are available on photoisomerization studies
of azo-bridged complexes based on terpyridine li-
gands,4,14−17,74−78 but to our knowledge, no such studies
have hitherto been performed on luminescent Ru(II)
complexes containing coupled terpyridine-diarylethene motifs.
It is expected that conjunction of linear bis-tridentate Ru(II)
complex and diarylethene group could lead to the construction
of potential molecular-switching systems. This paper deals with
synthesis, structural characterization, photophysical, electro-
chemical, and detailed photoisomerization studies of a new
series of heteroleptic Ru(II) complexes of composition [(tpy-
PhCH3)Ru(tpy-pvp-X)](BF4)2. For fine-tuning of the optical
properties as well as the rate of photoisomerization process, the
electronic nature of tpy-pvp-X was varied with X = H, Me, Cl,
NO2, and Ph group. Finally, computation works were also
performed on both trans and cis forms of the complexes by
employing density functional theory (DFT) and time-depend-
ent (TD) DFT methods for getting some insight about the
electronic structure of the complexes and also for appropriate
interpretation of experimentally observed absorption and
emission bands.
[(tpy-PhCH3)Ru(tpy-pvp-Me)](BF4)2·H2O (2). Yield: 0.16 g (63%).
Anal. Calcd for C52H42N6B2F8ORu: C, 59.96; H, 4.06; N, 8.18. Found:
1
C, 59.79; H,4.18; N, 8.28%. H NMR (300 MHz, DMSO-d6): δ 9.48
(s, 2H, 2H3′), 9.43 (s, 2H, 2H3′), 9.09 (t, 4H, J = 6.7 Hz, H6), 8.47
(d, 2H, J = 7.4 Hz, 2H7), 8.35 (d, 2H, J = 7.1 Hz, 2H7), 8.05 (t, 4H, J
= 7.1 Hz, H4), 7.97 (d, 2H, J = 7.5 Hz, 2H8), 7.61−7.53 (m, 10H,
4H3 + 2H8 + 2H11 + 2H12), 7.48 (nr, 1H, H9), 7.35 (d, 1H, J = 9.0
Hz, H10), 7.29−7.24 (m, 4H, H5), 2.34 (s, 3H, CH3), 2.49 (s, 3H,
CH3). ESI-MS (positive, MeCN) m/z = 425.10 (100%) [(tpy-
PhCH3)Ru(tpy-pvp-CH3)]2+ and m/z = 937.24 (12%) [(tpy-PhCH3)-
Ru(tpy-pvp-Me)(BF4)]+.
[(tpy-PhCH3)Ru(tpy-pvp-Cl)](BF4)2·2H2O (3). Yield: 0.17 g (65%).
Anal. Calcd for C51H41N6B2F8ClO2Ru: C, 56.71; H, 3.83; N, 7.78.
Found: C, 56.60; H, 3.99; N, 7.62%. 1H NMR (300 MHz, DMSO-d6):
δ 9.43 (s, 2H, 2H3′), 9.38 (s, 2H, 2H3′), 9.04 (t, 4H, J = 6.6 Hz, H6),
8.45 (d, 2H, J = 8.0 Hz, 2H7), 8.31 (d, 2H, J = 8.1 Hz, 2H7), 8.04−
7.96 (m, 6H, 4H4 + 2H8), 7.72 (d, 2H, J = 8.5 Hz, 2H8), 7.57−7.46
(m, 10H, 4H3 + 1H9 + 1H10 + 2H11 +2H12), 7.26 (t, 4H, J = 6.0,
H5), 2.53 (s, 3H, CH3). ESI-MS (positive, MeCN) m/z = 435.01
(100%) [(tpy-PhCH3)Ru(tpy-pvp-Cl)]2+.
[(tpy-PhCH3)Ru(tpy-pvp-NO2)](BF4)2·H2O (4). Yield: 0.16 g (60%).
Anal. Calcd for C51H39N7B2F8O3Ru: C, 57.11; H, 3.66; N, 9.14.
Found: C, 57.01; H, 3.78; N, 9.01%. 1H NMR (300 MHz, DMSO-d6):
δ 9.50 (s, 2H, 2H3′), 9.43 (s, 2H, 2H3′), 9.09 (t, 4H, J = 6.2 Hz, H6),
8.52 (d, 2H, J = 7.5 Hz, 2H7), 8.35 (d, 2H, J = 7.4 Hz, 2H7), 8.30 (d,
2H, J = 7.6 Hz, 2H8), 8.07−8.05 (m, 6H, 4H4 + 2H8), 7.96 (d, 2H, J
= 7.5 Hz, H12), 7.71 (nr, 2H, 1H9 + 1H10), 7.58−7.53(m, 6H, 4H3 +
2H11), 7.29−7.27 (nr, 4H, H5), 2.49 (s, 3H, CH3). ESI-MS (positive,
MeCN) m/z = 440.59 (100%) [(tpy-PhCH3)Ru(tpy-pvp-NO2)]2+
and 968.21(10%) [(tpy-PhCH3)Ru(tpy-pvp-NO2)(BF4)]+.
[(tpy-PhCH3)Ru(tpy-pvp-Ph)](BF4)2·3H2O (5). Yield: 0.17 g (62%).
Anal. Calcd for C57H48N6B2F8O3Ru: C, 60.06; H, 4.24; N, 7.37.
Found: C, 59.82; H, 4.67; N, 7.05%. 1H NMR (300 MHz, DMSO-d6):
δ 9.47 (s, 2H, 2H3′), 9.41 (s, 2H, 2H3′), 9.07 (t, 4H, J = 7.5 Hz, H6),
EXPERIMENTAL SECTION
■
Materials. RuCl3·xH2O, AgBF4, and different para-substituted
benzaldehydes were purchased from Sigma. Other analytical reagent-
grade chemicals as well as solvents were procured from local vendors.
The precursors that were used for the synthesis of the ligands such as
4′-(p-methylphenyl)-2,2′:6′,2″-terpyridine (tpy-PhCH3), 4′-(p-bromo-
methyl phenyl)-2,2′:6′,2″-terpyridine (tpy-PhCH2Br), and 4′-
(2,2′:6′,2″-terpyridyl-4)-benzyltriphenyl phosphonium bromide (tpy-
B
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Figure 1. Perspective view of 22+ with 50% probability ellipsoid plots.
8.48 (d, 2H, J = 8.2 Hz, 2H7), 8.33 (d, 2H, J = 8.1 Hz, 2H7), 8.04−
7.99 (m, 6H, 4H4 + 2H8), 7.82−7.72 (m, 6H, 1H12 + 2H13 + 2H14
+ 1H15), 7.62−7.52 (m, 9H, 4H3 + 2H8+ 2H11 + 1H12), 7.47 (d,
1H, J = 7.5 Hz, H9), 7.40 (d, 1H, J = 7.3 Hz, H10), 7.26 (t, 4H, J = 6.3
Hz, H5), 2.49 (s, 3H, CH3). ESI-MS (positive, MeCN) m/z = 455.85
(100%) [(tpy-PhCH3)Ru(tpy-pvp-Ph)]2+.
Instruments and Physical Methods. Descriptions of different
instruments and detailed procedures for various physical measure-
ments were given in the Supporting Information. Computational
details were also described in the Supporting Information.
resonances) and within the chemical shift range of 7.34−7.71
ppm are clearly due to the ethylenic double bond.
Mass Spectra. ESI mass spectra of complexes were
acquired in MeCN and presented in Figures S13−S16
(Supporting Information). Figure S15 (Supporting Informa-
tion) shows that the isotopic distribution pattern of the peak at
m/z = 435.01 (3) and 455.85 (5), separated by 0.5 Da, match
nicely to the calculated pattern for [(tpy-PhCH3)Ru(tpy-pvp-
Cl)]2+ and [(tpy-PhCH3)Ru(tpy-pvp-Ph)]2+ species, respec-
tively. Thus, the most abundant peak in all the complexes
indicates dipositive cation, [(tpy-PhCH3)Ru(tpy-pvp-X)]2+.
The weak peak at m/z = 922.52 for 1 (Figure S13, Supporting
Information), m/z = 937.24 for 2 (Figure S14, Supporting
Information), and m/z = 968.21 for 4 (Figure S16, Supporting
Information) indicates the presence of the monopositive ion
[(tpy-PhCH3)Ru(tpy-pvp-X)(BF4)]+.
Description of the Crystal Structure of [(tpy-PhCH3)Ru(tpy-
pvp-Me)](ClO4)2 (2). Perspective view of 22+ together with its
atom numbering is provided in Figure 1, and representative
bond distances and bond angles are summarized in Table S2
(Supporting Information). Crystal structure indicates distorted
octahedral geometry around Ru(II) center with two terpyridine
ligands coordinated in meridional fashion. 22+ crystallizes in
monoclinic system with C2/c space group. Crystal structure
also indicates the trans orientation of two substituted aryl
groups around the ethylenic double bond. The bite angles are
within the range of 78.80°(2)−102.10°(2), whereas the trans
angle are within the range of 158.0°(2)−178.80°(2). Ru(II)−N
distances in 22+ vary between 1.940(5) and 2.064(6) Å. The
bond distance between Ru(II) and N atom of central pyridine
moiety of tpy unit is found to be substantially shorter compared
with the two terminal pyridine rings due to substantial overlap
between t2g orbital of Ru(II) and π* orbitals of the said pyridine
moiety. The relatively large Ru−N distances associated with the
terminal pyridine units are due to the strain developed on the
terpyridine moiety of the ligand upon transformation from their
transoid to the cisoid conformation in the resulting complex.
DFT and TD-DFT Calculations. Geometry optimizations
of both trans and cis forms of the complexes were performed in
MeCN by Gaussian 09 program82 employing DFT method
with Becke’s three-parameter hybrid functional and Lee−
Yang−Parr’s gradient-corrected correlation functional (B3LYP)
level of theory.83,84 Either 6-31G(d) or 6-31G* basis set was
employed for the C, H, and O. By contrast, SDD basis set was
employed for heavier Ru atom. The optimized structures of the
complexes are shown in Figure S17 (Supporting Information),
and metrical parameters are summarized in Tables S2−S6
(Supporting Information). Geometrical parameters indicate
RESULTS AND DISCUSSION
■
Synthesis and Characterization. The terpyridine ligands
were obtained in appreciable yields by reacting 4′-(2,2′:6′,2″-
terpyridyl-4)-benzyltriphenylphosphonium bromide (tpy-
PhCH2PPh3Br) with different 4-substituted benzaldehydes in
dichloromethane under argon atmosphere at ∼0−5 °C. All the
ligands were purified by column chromatography and
recrystallization techniques. For the synthesis of the metal
complexes, [(tpy-PhCH3)Ru(Me2CO)3]3+, obtained upon
treating [(tpy-PhCH3)RuCl3] and AgBF4 in appropriate
molar ratio, was used, as it functions as a superior precursor
compared to [(tpy-PhCH3)RuCl3] with regard to reaction time.
Column chromatography and recrystallization techniques were
again employed for purification of the complexes. Character-
ization of the ligands and their metal complexes were
performed by using NMR and mass spectroscopic techniques
along with their elemental (C, H, and N) analyses. All the
characterization data have been provided in the Experimental
Section. Single-crystal X-ray structure of complex 2 was also
determined in the solid state (Table S1, Supporting
Information).
1
NMR Spectra. H and {1H−1H} homonuclear correlated
spectroscopy (COSY) NMR spectra of 1−5 were acquired in
DMSO-d6 and presented in Figure S9 and Figures S10−S12
(Supporting Information), respectively. In spite of complexity
in the spectra, all the peaks were tentatively assigned by their
COSY spectra as well as by comparing the data with related
complexes.59−68 The singlet that appears at ∼2.5 ppm (not
shown in Figure S9, Supporting Information) and counts for
three protons in each case, except 2 can be assigned as the
protons of −CH3 group of tpy-PhCH3 moiety. For 2, two
singlets are observed at 2.34 and 2.49 ppm due to two types of
−CH3 group within the complex. Two closely situated singlets
that appeared at the most downfield region are assignable to the
H3′ proton associated with two tpy motifs situated at the
opposite site of the Ru(II) center. Interestingly, H9 and H10
protons that appeared as a pair of doublets (although not
clearly visible in all cases due to overlapping with other proton
C
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substantial distortion from idealized octahedral geometry in
each case. The calculated Ru−N bond distances vary between
2.006 and 2.113 Å, and for the methyl derivative whose single-
crystal X-ray structure is available, the correlation between
calculated and experimental metrical parameters was found to
be good (Table S2, Supporting Information).
terpyridine moiety of tpy-pvp-X. Very intense bands in the
UV region are primarily due to π−π* transitions within tpy-
PhCH3 as well as tpy-pvp-X. Calculated lowest-energy band in
cis form was blue-shifted, and the extent of shift depends on the
electronic behavior of the substituent X.
Natural transition orbital (NTO) analysis plots as well as
electron density difference map (EDDM) are also useful for
appropriate assignment of the bands. NTO plot, which is
primarily based on calculated transition density matrices,
indicates that the lowest-energy band is primarily composed
of both MLCT and ILCT transitions (Figures S29−S33,
Supporting Information). EDDM plots (Figure S34, Support-
ing Information) again support the conclusion drawn from
NTO plots, and mixed MLCT and ILCT nature for the said
band gets verified.
Unrestricted Kohn−Sham (UKS) computations directly on
the triplet state of both the trans and cis form of the complexes
were performed to get some insight about their luminescence
behaviors. Very little variation of the geometrical parameters is
observed among the ground and excited states (Tables S2−S6,
Supporting Information). The calculated luminescence max-
imum of the complexes was found to vary between 612 and 635
nm (Table S17, Supporting Information).
Upon photoisomerization from trans to cis form, coordina-
tion environment around the Ru(II) center remains almost
unaltered (Tables S2−S6, Supporting Information). The most
affected part upon isomerization is the diarylethylene unit in
the complex as expected. The calculated results indicate that
the magnitude of dihedral angle between the aryl rings across
the double bond in all complexes is ∼179.9°, while the
corresponding angle in the cis form is ∼7.7°. It would of
interest to compare the relative strain in the corresponding
ligands. To this end, we optimized the geometry of both cis and
trans forms of free ligands. Interestingly, the dihedral angle
between the aryl groups across the double bond in the free
ligands is almost identical to that of the metal complexes.
Moreover, the difference of energy among the two forms of the
metal complexes [varying between 24.42 kJ/mol (3) and 25.00
kJ/mol (4)] is almost the same as those of the free ligands
[lying between 23.49 kJ/mol (tpy-pvp-Cl) and 24.55 kJ/mol
(tpy-pvp-NO2)]. Thus, it appears that the amount of strain
incorporated across the diaryethylene unit upon photo-
isomerization is almost same for both metal complexes as
well as the ligands.
Experimental Absorption Spectral Behaviors. Absorp-
tion spectra of 1−5 acquired in MeCN are displayed in Figure
2, and related data are presented in Table 1. The spectral
Frontier molecular orbital (FMO) sketch (Figures S18−S22,
Supporting Information) as well as their compositions show
that, with few exceptions, highest occupied molecular orbitals
(HOMOs) are primarily located on Ru(II) and to some extent
on the vinylphenyl and tpy-PhCH3 moiety (Tables S7−S11,
Supporting Information), while the lowest unoccupied
molecular orbitals (LUMOs) are based on the terpyridines.
For nitro derivative, lower-energy LUMOs are mainly localized
on nitrobenzene part of tpy-pvp-NO2. On going from trans to
their cis forms, only small differences are observed in the
compositions of MOs. The distribution of electronic charge
density in the complex backbone can be seen by plotting
electrostatic surface potential (ESP) over the total electron
density (Figure S23, Supporting Information). The electron-
rich portion is indicated by green color, while electron-deficient
area is indicated by blue color in the ESP plots. Trans-to-cis
isomerization does not alter the electronic charge density
around the complex backbone to a great extent as expected.
To obtain calculated UV−vis spectra, TD-DFT computa-
tions were also performed in MeCN. Calculated data along the
band assignment were provided in Tables S12−S16 (Support-
ing Information). The FMOs that participate in the lowest-
energy band were displayed in Figures S24−S28 (Supporting
Information). The calculated band that obtained between 477
nm (for 3 and 4) and 491 nm (for 5) in their trans forms is a
combination of metal-to-ligand (Ru(II) → terpyridine) charge
transfer (MLCT) and intraligand (phenylvinyl → terpyridine)
charge transfer (ILCT) transitions (Figures S24−S28, Support-
ing Information). In case of nitro derivative, the lowest-energy
band consists of combined charge transfer from Ru(II) and
phenylvinyl part to the nitrobenzyl unit of tpy-pvp-NO2. Thus,
taking into consideration the participation of the relevant
FMOs, the calculated lowest-energy band can be assigned as
admixture of both MLCT and ILCT. The next higher band
obtained between 353 nm (for 1) and 372 nm (for 5) is
primarily due to ILCT transition from phenylvinyl to
Figure 2. UV−Vis absorption spectra of 1−5 in acetonitrile.
signature of the complexes is more or less similar containing
several very intense peaks in the UV and one moderately
intense band in the visible region. Outcomes of TD-DFT
calculations and literature data of related complexes suggest the
band that appears at ∼500 nm can be assigned as admixture of
1
1[RuII(d)6] → [RuII(d)5tpy-pvp-X(π*)1] MLCT as well as
phenylvinyl → terpyridine ILCT transitions. The next higher-
energy band that appears between 354 and 364 nm, depending
upon the electronic nature of the substituent (X) on tpy-pvp-X,
is primarily ILCT in nature, although some π−π* character is
also involved therein. The intense bands that appeared in the
UV region primarily consist of π−π* transitions of the
coordinated tpy-PhCH3 and tpy-pvp-X ligands. The correlation
between calculated and experimental data was found to be
reasonably good. The MLCT band is red-shifted in the
complexes compared to the parent [Ru(tpy)2]2+ (474 nm)25
probably because of extended delocalization induced by
diarylethenes moiety at the 4′-position of the terpyridine unit
of tpy-pvp-X. In addition, the ILCT band is also red-shifted
with enhanced electron acceptor capability of X.
Experimental Luminescence Spectral Behaviors.
Steady-state emission spectra of 1−5 were acquired in MeCN
D
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at room temperature (RT) as well as in EtOH−MeOH (4:1, v/
v) glass at 77 K. The spectra are shown in Figure 3a and Figure
S35 (Supporting Information), while relevant data are provided
in Table 1. One moderately intense luminescent band in the
range between 661 (1) and 670 nm (4) at RT and between 653
(1) and 662 nm (5) at 77 K was observed in each case upon
excitation at their MLCT maxima. Luminescence lifetime
acquired at RT were found to vary between 8.43 ns (2) and
22.82 ns (4) (Figure 3b), while at 77 K the lifetimes increased
significantly and vary between 44.96 μs (2) and 62.04 μs (4)
(Figure S36, Supporting Information).
On the basis of the luminescence data of the related Ru(II)-
terpyridine complexes, it can be concluded that the origin of
emission in the present complexes is predominantly 3MLCT in
nature.25−28 Blue shift of the emission maximum and
substantial increase in intensity and lifetime in frozen glass is
3
also indicative of typical MLCT emitters. The energy (E00) of
the 3MLCT state was also evaluated from their emission
maximum at 77 K (varying between 1.87 and 1.90 eV). Thus,
the most important characteristic of the present complexes is
that they are luminescent at RT with moderately long lifetimes
compared to nonluminescent [Ru(tpy)2]2+ (τ = 0.25 ns) parent
complex.34 In addition, improved emission characteristics of the
complexes were acquired without too much lowering of their
excited-state energy. It may be mentioned that reasonably good
correlation between the calculated (based on UKS) and
experimentally observed emission maximum was observed for
the trans form of complexes (Table S17, Supporting
Information). The luminescence band is also red-shifted in
1−5 compared to the parent [Ru(tpy)2]2+. The red shift,
although to a very small extent, was found to depend upon the
electron-accepting nature of X in tpy-pvp-X.
For better assessment of deactivation dynamics, emission
spectra and lifetimes of the complexes were acquired by varying
temperature (from 265 to 323 K), and the experimental
findings are shown in Figure 4 and Figure S37 (Supporting
Information). Irrespective of their electronic nature, lumines-
cence intensity, quantum yield, and luminescence lifetime
gradually decrease upon increasing temperature for all
complexes. Nonlinear regression analysis of temperature versus
lifetime data by using eq 1 led to an idea about the involvement
of different deactivation channels in the excited-state decay
process.23,85
(τ(T))−1 = (k1 + k2exp[−ΔE/RT])/(1 + exp[−ΔE/RT]
(1)
where k1 corresponds to the temperature-independent rate
constant, which is the summation of radiative (kr) and
3
nonradiative (knr) rate constants from the MLCT state to
the ground state at low temperature (77 K). K2 corresponds to
temperature-dependent rate constant, which takes into account
the rate of population of the 3MC state from 3MLCT state, and
ΔE corresponds to the activation energy required for this
process.
Nonlinear fit of the experimental data to eq 1 gives rise to the
values of k1, k2, and ΔE {Figure 4c,d and Figure S37c,d,
Supporting Information}. The estimated ΔE values were found
to vary between 1863 36 and 3391 49 cm−1, depending
upon the nature of X. The values of k1 (varying between 1.61 ×
104 and 2.22 × 104 s−1) that were obtained from 77 K emission
data were utilized for fitting eq 1. Calculated k2 value ranges
between 8.56 × 1010 and 1.32 × 1013 s−1. Substantial increase in
the value of ΔE is noticed in all complexes compared to the
E
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Figure 3. (a) Luminescence spectra (λex = 495 nm) and (b) luminescence decays (λex = 450 nm) of 1−5 in MeCN at RT.
Figure 4. Effect of temperature on excited-state decay profiles for 2 (a) and 4 (b) in acetonitrile. Temperature-dependent lifetime data along with the
values of different parameters and the corresponding nonlinear fit are shown for 2 (c) and 4 (d).
reference, [Ru(tpy)2]2+ (ΔE = 1500 cm−1),23 and the increase
is probably due to excited-state delocalization induced by
extensively π-delocalized diarylethene unit incorporated onto
the tpy moiety. Moreover, the difference in energy between the
short-lived component is probably due to MLCT level, while
the second long-lived component probably appears from the
equilibrated triplet state of substituted diarylethenes moiety and
3MLCT, although no direct proof of the prediction could be
given. The 77 K emission profiles for all complexes show clear
vibrational features with the vibrational spacings (varying
between 1442 and 1470 cm−1), which indicate quite substantial
contribution from the triplet IL emission in the luminescence at
77 K (Figure S35, Supporting Information). In essence, a new
family of luminescent bis-terpyridine complexes of Ru(II) with
moderately long lifetime were designed without substantial
3
3
3
emitting MLCT and nonemitting MC states is a delicate
function of the electronic nature of substituent X in the
complexes. In addition, the extent of energy gap is also reflected
in the measured lifetime of the complexes. We demonstrated
earlier that incorporation of appropriate donor and acceptor
units or π-conjugated moiety onto the 4′-position of the
terpyridine unit could adjust the relative energies of closely
situated excited states, and by judicious choice of ligands, we
were able to successfully tune the luminescence characteristics
of the complexes in a predictable maner.59−68 In the present
3
decrease of their MLCT energy.
Redox Properties. The electrochemical behaviors of 1−5
were investigated in MeCN through cyclic voltammetry (CV)
and square wave voltammetry (SWV; Table S18 and Figure
S38, Supporting Information). All the complexes exhibit one
reversible oxidation and several quasi-reversible and/or
irreversible reductions. The oxidation of the complexes with
their E1/2 values varying between +1.26 and +1.28 V
correspond to a RuII/RuIII process, whereas the waves in the
negative potential domain correspond to the reductions of
coordinated tpy moieties of both kinds. The reduction
3
systems, it is expected that the energy of the MC state is
almost constant, while the energy of the emitting 3MLCT state
is lowered substantially by incorporating diarylethenes onto the
tpy unit and thereby slowing the 3MLCT→3MC surface-
crossing efficiency. The complexes in the present study exhibit
biexponential decay at RT with first-component lifetimes of
0.68−2.15 ns followed by a second long-lived component
(lifetime ranges between 8.43 and 22.82 ns in MeCN). The first
F
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Figure 5. Spin density plot for one-electron oxidized (a) and one-electron reduced (b) form of 2 in acetonitrile. (c) Absorption spectral changes
upon electrochemical oxidation of 2 at 1.3 V in acetonitrile at room temperature.
Figure 6. UV−Vis absorption (a, c) and luminescence (b, d) spectral changes of (a, b) [(tpy-PhCH3)Ru(tpy-pvp-Me)]2+ (2) and (c, d) [(tpy-
PhCH3)Ru(tpy-pvp-NO2)]2+ (4), in acetonitrile−DCM (1:50, v/v) upon irradiation with UV light. Excitation wavelength for recording the emission
is 495 nm.
behaviors of tpy-pvp-NO2 and the corresponding Ru(II)
complex (4) show interesting features. In the free ligand,
three successive reversible couples are observed, which are
found to be shifted cathodically in the complex. These ligand-
centered electron-transfer processes are characteristic of the
multiple electrons accepting ability of the nitro group.
moieties in the remaining complexes are also in line with
calculated results. Although we assigned the oxidation process
in all the complexes, it is primarily based on Ru(II) center, but
calculated results hint the involvement of both Ru as well as
arylethylene unit in the oxidation process for all the complexes
except 4.
The spin densities of one-electron oxidized and one-electron
reduced forms of 1−5 are presented (Figure 5a,b and Figure
S39, Supporting Information). Except 4, the spin density in the
oxidized form of the complexes is located on both Ru center
and entire arylethylene unit, while in the reduced form the spin
density is mainly localized on the terpyridine moieties along
with small localization on arylethylene unit. In case of 4, the
spin density is localized predominantly on Ru center in the
oxidized form, while on the entire nitro-arylethylene unit in the
reduced form. Thus, DFT calculation supports our assignments
for oxidation process involving Ru(II) center and for reduction
involving nitro-aryl moiety of tpy-pvp-NO2 in 4. Moreover, the
assignment of the reduction processes involving the terpyridine
Spectroelectrochemical investigations were performed for the
complexes in MeCN at RT in the spectral domain of 300−950
nm. Spectral changes that take place upon oxidation of
complexes at 1.3 V are presented in Figure 5c and Figures
S40 and S41 (Supporting Information). Upon electrochemical
oxidation, the broad MLCT band at ∼500 nm gradually loses
its intensity with concomitant evolution of a broad band at
∼710 nm. Spectral changes are accompanied by a well-defined
isosbestic point. The broad band that evolved at considerably
higher wavelength (in the range of 700−715 nm) is due to
ligand-to-metal (tpy-pvp-X→RuIII) charge transfer (LMCT)
transition.
G
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Photoisomerization Studies. As the complexes possess
one diarylethene moiety in their structural unit, they will be
expected to undergo trans → cis isomerization in presence of
light. Although isomerization studies were attempted with both
visible as well as UV light, the rate is very slow with visible light.
In addition, as the rate of trans → cis photoisomerization is
extremely slow in polar solvents like MeCN or DMSO, we
performed the trans → cis isomerization studies in mixed
MeCN−DCM (1:50 v/v) mixture (DCM = dichloromethane).
Upon irradiation with UV light, all the complexes exhibit
substantial absorption spectral changes due to trans → cis
isomerization (Figure 6 and Figures S42 and S43, Supporting
Information). The intensity of both MLCT as well as ILCT
band gradually decreases in all cases except nitro-compound
(4), although the extent of decrease is more marked for the
ILCT band as expected. At the same time, the absorbance of
the π−π* transition bands increased to some extent. For 4, the
absorbance of the MLCT band remains constant, while the
absorbance of ILCT band decreases gradually upon photolysis
(Figure 6c). The appearance of isosbestic points during spectral
changes clearly indicates that only two species are exchanging
during the photoisomerization process. For the completion of
the isomerization process, almost 90 min of photolysis is
required.
observed blue shift of the emission band is reproduced by UKS
calculations.
We are not very sure about the actual reason for the
quenching of emission upon trans→cis isomerization. The
present complexes exhibit biexponential decay profiles implying
the existence of two excited states (3MLCT and 3ILCT) under
equilibrium. As trans→cis isomerization induces significant
strain and leads to decrease the extent of π-conjugation,
although to a small extent, in the aromatic backbone on and
around the double bond, the excited-state equilibrium between
3
3MLCT and ILCT probably gets hampered, which in turn
3
facilitates thermally activated crossing to the MC state. Thus,
quenching of luminescence upon trans→cis photoisomerization
3
is probably due to population of the nonradiative MC state
caused by disturbance of excited-state equilibrium between
3
3MLCT and ILCT.
Absorption titration data were utilized to calculate the rate
constant of the trans-to-cis photoisomerization process in the
complexes. The photoisomerization process at the early stage
follows first-order reaction kinetics as the photo- and thermal
cis-to-trans isomerization rate is very slow in this stage (Figures
S48−S50, Supporting Information). Estimated values of the
rate constants range between 3.45 × 10−4 and 7.46 × 10−4 s−1
at a light intensity of 0.11 W (Figures S48−S50, Supporting
Information). The quantum yields of trans-to-cis photo-
isomerization process were also measured (Table 2). It appears
that the values of the quantum yields of the photoisomerization
process are low in general and vary between 1.0 × 10−4 and 1.6
× 10−4.
The cis → trans backward process in the complexes proceeds
very slowly in the dark, but the rate is accelerated upon
increasing the temperature or upon treatment with visible light.
MLCT band gets slightly blue-shifted upon trans to cis
photoisomerization, which is also reflected in the calculated
results (Figure 7 and Figures S44−S47, Supporting Informa-
Table 2. Quantum Yield and Rate Constants for the
Photoisomerization of Complexes 1−5
compounds
quantum yield (Φ × 104)
rate constant (kiso × 104, s−1
)
1
2
3
4
5
1.3
1.2
1.6
1.0
1.2
3.69
3.45
7.46
3.83
3.54
1H NMR titrations of 1 and 2 were also performed in
CDCl3−DMSO-d6 (4:1, v/v) upon irradiation with UV light at
different time intervals to get some support in favor of trans→
cis isomerization process (Figures S51 and S52, Supporting
Information). Unfortunately, the main spectral signature of the
complexes remains almost unaltered with only small upfield
shift of few protons. For 1, H9 and H10 protons associated
with ethylenic double bond as well as H11 and H12 protons
adjacent to the double bond that probably appeared within the
broad multiplet (6.96−6.87 ppm for 1) gets small upfield
shifted (6.92−6.82 ppm). In addition, H8 proton in the phenyl
ring on tpy side, which is again adjacent to double bond, also
gets upfield shifted (7.46 to 7.41 ppm). Spectral profiles of 2
also follow a more or less similar trend to that of 1. The upfield
shift of the said protons, although to a very small extent, may be
due to decrease of conjugation on going from trans to cis form
by taking into consideration the twisted structure around the
ethylenic double bond.
Figure 7. Overlay of the calculated (dotted lines) and experimental
(solid lines) absorption spectra of both trans (red) and cis (blue)
forms of [(tpy-PhCH3)Ru(tpy-pvp-H)]2+ (1) in acetonitrile.
tion). Slower rate of isomerization in more polar solvents like
MeCN and DMSO is due to large degree of solvation, which in
turn leads to increase of the effective rotor volume of the
complex cations.
In the luminescence spectra, substantial quenching of
emission along with small blue shift occurred for all complexes
upon irradiation with UV light (Figure 6b,d and Figures S42b,d
& S43b, Supporting Information). Although the steady-state
emission intensity decreases substantially, the lifetime of the
complexes decreases only to a small extent on going from trans
to cis form. It is of interest to note that backward isomerization
from cis → trans form again restores the original emission of
the complexes. Thus, “on−off” and “off-on” emission switching
was made possible for all the complexes upon successive
interaction with UV and visible light. The experimentally
CONCLUSIONS
■
A new family of heteroleptic bis-terpyridine complexes of
Ru(II) by incorporating diarylethene moiety at the 4′-position
of the terpyridine unit have been designed and characterized in
H
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this work. Two important facts regarding the luminescence
properties of the complexes are observed. First, higher
conjugation of the terpyridyl ligand leads to a stabilization of
the 3MLCT states resulting in improved luminescence
characteristics of the complexes at room temperature. The
results presented in this study indicate that the complexes show
moderately strong room-temperature emission having lifetimes
2 orders of magnitude higher than [Ru(tpy)2]2+ complex, and
the observed lifetimes vary between 8.43 and 22.82 ns,
depending upon the electronic nature of the substituent (X)
on the terminal phenyl group of the ligand, tpy-pvp-X. In
addition, the improved luminescence characteristics of the
complexes were acquired without too much lowering of their
3MLCT energy. Second, introduction of isomerizable diary-
lethene unit induces trans-to-cis photoisomerization with
substantial changes in their absorption and emission spectra
upon interaction with UV light. Cis-to-trans backward isomer-
ization was also feasible upon treatment with either visible light
or heat. One of the most important aspects of the present work
is that trans-to-cis isomerization leads to almost complete
quenching of luminescence, while backward cis-to-trans
isomerization gives rise to the restoration of the original
luminescence for all the complexes. Accordingly, present
diarylethene-conjugated Ru(II) terpyridine complexes could
be promising building blocks in the field of information storage
and photoswitching molecular devices, because trans−cis and
cis−trans conformational change is linked with their “on−off”
and “off−on” emission switching.
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.7b03096.
■
S
DFT and TD-DFT calculated structural parameters
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Accession Codes
CCDC 1588932 contains the supplementary crystallographic
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AUTHOR INFORMATION
Corresponding Author
*E-mail: sbaitalik@hotmail.com, sujoyb@chemistry.jdvu.ac.in.
Phone: 91-033-2414-6666.
■
ORCID
Sujoy Baitalik: 0000-0001-9720-6889
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
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We gratefully acknowledge the financial support received from
SERB-DST [Grant No. EMR/2015/001163], New Delhi,
India. TCSPC facility under the DST-PURSE program of
Dept. of Chemistry (Jadavpur Univ.) is also gratefully
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