Temperature dependence of dual emission in ruthenium(II) complexes containing 3,3′-bi-1,2,4-triazine derivatives

Article abstract of DOI:10.1016/j.inoche.2010.05.019

Three new ruthenium(II) polypyridyl complexes with highly π-deficient ligands, [Ru(bpy)₂(btm)]²⁺ (1), [Ru(bpy) ₂(btb)]²⁺ (2) and [Ru(bpy)₂(btp)]²⁺ (3) (bpy = 2,2′-bipyridine, btm = 3,3′-bis(5,6-dimethyl-1,2,4- triazine, btb = 3,3′-bis(5,6-diphenyl-1,2,4-triazine, btp = 3,3′-bis(phenanthro[9,10-e][1,2,4]triazine) were synthesized and characterized. Electrochemical data together with molecular calculations show that the first redox process in these complexes is not bpy based. Complexes 1, 2 and 3 display luminescence in ethanol/methanol (4/1, V/V) at 80 K, and all three complexes exhibit a temperature switch from single (150 K) to dual (80 K) emission behavior.

Full text of DOI:10.1016/j.inoche.2010.05.019

Inorganic Chemistry Communications 13 (2010) 1018–1020
Contents lists available at ScienceDirect
Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Temperature dependence of dual emission in ruthenium(II) complexes containing
3,3′-bi-1,2,4-triazine derivatives
⁎
⁎
Yu Chen, Xu Zhou, Xu-Hui Wei, Bo-Le Yu, Hui Chao , Liang-Nian Ji
MOE Laboratory of Bioinorganic and Synthetic Chemistry, State Key Laboratory of Optoelectronic Materials and Technologies, School of Chemistry and Chemical Engineering,
Sun Yat-Sen University, Guangzhou 510275, PR China
a r t i c l e i n f o
a b s t r a c t
Three new ruthenium(II) polypyridyl complexes with highly π-deficient ligands, [Ru(bpy)2(btm)]2+ (1), [Ru
(bpy)2(btb)]2+ (2) and [Ru(bpy)2(btp)]2+ (3) (bpy=2,2′-bipyridine, btm=3,3′-bis(5,6-dimethyl-1,2,4-
triazine, btb=3,3′-bis(5,6-diphenyl-1,2,4-triazine, btp=3,3′-bis(phenanthro[9,10-e][1,2,4]triazine) were
synthesized and characterized. Electrochemical data together with molecular calculations show that the first
redox process in these complexes is not bpy based. Complexes 1, 2 and 3 display luminescence in ethanol/
methanol (4/1, V/V) at 80 K, and all three complexes exhibit a temperature switch from single (150 K) to
dual (80 K) emission behavior.
Article history:
Received 11 May 2010
Accepted 26 May 2010
Available online 4 June 2010
Keywords:
Ruthenium complexes
3,3′-Bi-1,2,4-triazine
Dual emission
© 2010 Elsevier B.V. All rights reserved.
Tremendous interest has been attracted to ruthenium(II) poly-
pyridyl complexes because of their potentials in molecular electronic
devices [1], as DNA structural probes or new therapeutic agents [2],
and as photosensitizers in the conversion of solar energy to chemical
or electrical energy [3]. Over the past decade quite a large amount of
data has been accumulated on the changes in the electrochemical and
photophysical properties of Ru(II) complexes. Although nearly all Ru
(II) heteroleptic complexes abide by Kasha's rule [4] and exhibit a
single emissive excited-state, a few Ru(II) complexes have been
recently found to possess two simultaneously emissive excited states
[5–10].
To observe dual emission from an individual molecule, the
molecule must exhibit two excited electronic states. However, it is
difficult to achieve because the excited states usually involve
occupation of orbitals extending over the entire molecule and space
at very modest energy separations. One possible approach for
obtaining dual emission in Ru(II) complexes is the coexistence of
3MLCT (metal-to-ligand charge transfer) with 3IL (intraligand) or
3ILCT (intraligand charge transfer) states [6,7]. Another viable way is
to separate the two excited electronic states by the energy barrier and
prevents interconversion of populations. Vos et al. reported the dual
emission of complex [Ru(bpy)2(pztr)]+ (Hpztr=3-(pyrazin-2-yl)-
1,2,4-triazole] in fluid solution [ethanol–methanol (4:1)] over the
temperature range 120–260 K [5]. Recently, dual emission was
observed from a family of heteroleptic Ru(II) complexes containing
substituted 1,10-phenanthroline ligands with extended conjugation
at room temperature in fluid solution [8]. Herein we present three
new ruthenium(II) diimine complexes with highly π-deficient
ligands. The dual emission behaviors of Ru(II) complexes are
investigated in solution over a wide temperature range.
The synthesis of Ru(II) complexes [Ru(bpy)2(btm)]2+ (1), [Ru(bpy)2
(btb)]2+ (2), and [Ru(bpy)2(btp)]2+(3) (bpy=2,2′-bipyridine,
btm=3,3′-bis(5,6-dimethyl-1,2,4-triazine, btb=3,3′-bis(5,6-diphe-
nyl-1,2,4-triazine, btp=3,3′-bis(phenanthro[9,10-e][1,2,4] triazine)
were achieved as shown in Scheme 1. The ligands were synthesized
on the basis of the method for the 1,2,4-triazine ring preparation
established by Case [11]. The Ru(II) complexes were obtained in
satisfactory yields (42–68%) by direct reaction of ligands with
appropriate mole ratios of the precursor complex cis-Ru(bpy)2Cl2 in
ethylene glycol. The product was purified by column chromatography
and characterized by NMR, ES–MS and elemental analyses. (Caution!
Perchlorate salts of metal complexes are potentially explosive and
should be handled in small quantity with care).
The electrochemical behaviors of the complexes have been studied
in CH3CN by cyclic voltammetry (Fig. S1). The oxidation of the Ru(II)
complexes shifts to more positive potential (1.43–1.53 V) in compar-
ison with that of [Ru(bpy)3]2+ (1.28 V) [12]. Extended-Hückel
calculation results show that the lowest unoccupied molecular orbital
(LUMO) energy of btm (−10.33 eV), btb (−10.25 eV) and btp
(−10.53 eV) are lower than that of bpy (−9.76 eV) (Fig. S2).
⁎
Therefore, the better π acceptor character of btm, btb and btp
stabilizes the ruthenium-based HOMO, rendering the oxidation of the
metal more difficult. For complex 1, the first reduction, which is
usually controlled by the ligand having the most stable lowest
unoccupied molecular orbital (LUMO), is assigned to a reduction
centered on the ligand btm. However, the situation in complexes 2
and 3 is different from that of complex 1. The LUMO and LUMO+1 on
btb (or btp) are similar except that the LUMO+1 is 0.05 eV higher in
⁎
Corresponding authors. Fax: +86 20 84035497.
E-mail addresses: ceschh@mail.sysu.edu.cn (H. Chao), cesjln@mail.sysu.edu.cn
(L.-N. Ji).
1387-7003/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2010.05.019

Y. Chen et al. / Inorganic Chemistry Communications 13 (2010) 1018–1020
1019
Scheme 1. Synthetic routes to the ligands and Ru(II) complexes.
energy, and the LUMO+1 orbitals of btb and btp are also significantly
lower in energy than the LUMO orbital of bpy. So the first and second
reductions of 2 (or 3) are assigned to the LUMO and LUMO+1 of btb
(or btp), and the third reduction involves the bpy ligand.
The absorption spectrum of Ru(II) complexes is characterized by
intense π–π* ligand transitions in the UV and metal-to-ligand charge
transfer (MLCT) transition in the visible region (Fig. S3). The bands
between 230 and 380 nm, especially the absorption bands near
348 nm are attributed to intraligand (IL) π–π* transitions, by
comparison with the spectra of [Ru(bpy)3]2+ [12]. The lowest energy
bands at 400–500 nm are assigned to metal–ligand charge transfer
(MLCT) transition. Two distinct MLCT bands might be expected for the
three complexes. However, as a result of the broad nature of the bands
and their relatively small wavelength separation, a broad MLCT band
with shoulder peak is observed in the spectra of these complexes.
The emission signals of three complexes are too weak to measure
at room temperature. Their emission spectra are recorded in ethanol–
methanol (4:1) rigid matrix at 80 K (Fig. 1). Emission decays of all
complexes are biexponential. The emission band maxima and excited-
state lifetime (τ) obtained are collected in Table 1. For complex 1, the
spectrum obtained at 80 K is resolved into two marked distinct bands;
one, as expected, at 596 nm, and an additional feature at 663 nm.
Normal curve fitting procedures yield lifetime of 4570 and 3670 ns
with ratios of 60 and 40%, respectively. Excitation spectra obtained at
both emission maxima are sufficiently similar to suggest that both
emissions are base on MLCT emissions. The excitation spectrum of the
596 nm emission shows an absorption maximum at 458 nm, while for
the 663 nm band a maximum is observed at 485 nm. On increasing
the temperature to 150 K, the contribution from the lower energy
component decreases, and the spectrum obtained is a single emission
being observed at 608 nm (Fig. 2). Similar observations are noticed for
complexes 2 and 3 (Figs. S4 and S5).
Table 1
Luminescence data for the ruthenium(II) complexesa.
Complex
150 K
max, nm
80 K
λ
λ
max1, nm
τ, μs
λ
max2, nm
τ, μs
[Ru(bpy)3]2+
1
2
3
600
608
605
602
580
596
578
579
4.37
4.57
4.27
4.37
663
688
667
3.67
4.81
5.42
Fig. 1. Emission spectra of complexes 1 (dash line), 2 (dot line), 3 (dash dot line) and
a
[Ru(bpy)3]2+ (solid line) at 80 K in 4:1 EtOH/MeOH.
Matrix, EtOH/MeOH=4:1.

1020
Y. Chen et al. / Inorganic Chemistry Communications 13 (2010) 1018–1020
Fig. 2. (a) Temperature-dependent emission spectra of complex 1 in ethanol/methanol (4:1, v/v). Spectra were recorded from 80 to 300 K at intervals of 10 K. The arrow shows the
intensity changes on decreasing the temperature. (b) Temperature-dependent luminescent decay rates of complex 1 in ethanol/methanol (4:1, v/v).
The simplest explanation for our data is that there are two MLCT
excited states localized on different ligands of certain Ru(II)
complexes, with the short-wavelength component being essentially
bipyridine-based, while the long-wavelength component is localized
predominantly on the more conjugated 3,3′-bi-1,2,4-triazine deriva-
tives ligand. For the three complexes at room temperature, after
excitation to the 1MLCT level, efficient intersystem crossing to the
lowest, strongly coupled bi-1,2,4-triazine based triplet state is
observed, and the main decay pathways are deactivation of the bi-
1,2,4-triazine based triplet state via the triplet metal-centered (1MC)
state by tunnelling and on non-radiative processes. Therefore, the
emission signals obtained for the three compounds at room
temperature are very weak, analogously to the other ruthenium–
triazine complexes. Over the temperature range 150–220 K, a single
emission with maxima around 600 nm was observed, indicated that
the excited-state is the bpy based more than the bi-triazine based.
Below 150 K, the solvent turned into rigid glassy matrix, and barrier
crossing would be slowed on such a condition, and as a consequence
strongly coupled bpy based and bi-1,2,4-triazine based triplet states
are observed.
Acknowledgment
This work was supported by the 973 Program of China
(2007CB815306), the NSFC (20771105, 20871122), the NCET (NCET-
06-0718), the Key Project (108103) of the Ministry of Education,
GDNSFC (9351027501000003) and Sun Yat-Sen University.
Appendix A. Supplementary material
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.inoche.2010.05.019.
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In summary, we have shown that three Ru(II) complexes with
highly π-deficient ligands can exhibit a temperature switch from
single (150 K) to dual (80 K) emission behavior. The present results
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