Photoinduced electron transfer reaction between the excited state ruthenium (II) polypyridyl complexes and quinones has been investigated in cetyltrimethylammonium bromide using luminescent quenching techniques. The complexes have the absorption and emission maximum in the range 452 - 468 nm and 594 - 617 nm respectively. The static nature of quenching is confirmed from the ground state absorption studies. The association constants for the complexes with quinones are calculated from the Benesi-Hildebrand plots using absorption spectral data. The value of quenching rate constant ( kq) is highly sensitive to the nature of the ligand and the quencher, the medium, structure and size of the quenchers. Compared to the aqueous medium, the electron transfer rate is altered in CTAB medium. The oxidative nature of the quenching is confirmed by the formation of Ru 3+ ion and quinone anion radical.
Luminescence quenching is a very important technique to get adequate information about the structure, properties and reactions of luminescent molecules. Quenching is an important process by which the luminescence intensity of the luminophore is reduced by various processes like molecular rearrangements, excited state reactions, ground-state complex formation and collisional quenching [
Quinones are important class of organic compounds used in various electrochemical processes during biological energy storage and storage. They have a good redox behavior and act as an efficient electron acceptor. Many biological processes like respiration operate only due to the active quinone species [
A well known surfactant is cetyltrimethylammonium bromide. A series of surfactant ruthenium (II) bipridyl derivatives have been reported previously [
The three [Ru(NN)3]2+ complexes [NN = bipyridine (bpy), 4,4'-dimethyl-2,2'-bipyridine (dmbpy), 4,4'-di-t-butyl, 2,2'-bipyridine (dtbpy)] were synthesized from known procedures [
SYSTRONICS 2203 double beam spectrophotometer was used to measure the absorption spectral characteristics. Emission studies were carried out using JASCO-FP-8600 spectrofluorometer. The sample concentrations were maintained at 4 × 10−5 M. Quenchers had the concentrations between 4 × 10−6 and 2.8 × 10−7 M. Luminescence measurements and absorption measurements were carried out at room temperature. The sample solutions of same concentration was used for both absorbance and emission measurements. The solutions used for emission studies and excited state lifetime measurements were deaerated for about 30 mts using dry N2 gas purging. Excited state lifetime was measured using laser flash photolysis technique. An applied photophysics SP-Quanta Ray GCR-2(10) Nd YAG laser was used as an excitation source.
The quenching rate constant (kq) was calculated using Stern-Volmer plot equation,
I 0 / I = 1 + k q τ 0 [ Q ]
where I0 and I are the emission intensities in the absence and presence of quencher respectively and τ 0 is the emission lifetime of the Ru(II) complex in the absence of the quencher.
The structure of the ligands and the quenchers used in the present study are shown in
The photophysical properties like absorption maxima, emission maxima and emission lifetime of the [Ru(NN)3]2+ complexes are depicted in
Complex | Absorption maximum (nm) | Emission maximum, (nm) | Life time τ (ns) | |||
---|---|---|---|---|---|---|
aq | CTAB | aq | CTAB | aq | CTAB | |
[Ru(bpy)3]2+ | 448 | 452 | 596 | 594 | 650 | 387 |
[Ru(dmbpy)3]2+ | 458 | 459 | 605 | 606 | 360 | 250 |
[Ru(dtbpy)3]2+ | 457 | 468 | 625 | 617 | 510 | 290 |
aqueous medium. The emission maxima range from 452 nm to 461 nm. All the three Ru(II) complexes show a strong ligand centred (LC) π-π* transition in the region 260 nm to 280 nm region and a low energy absorption in the 452 to 461 nm region assigned to the dπ-π* MLCT transition. In [Ru(bpy)3]2+ complex, the MLCT absorption maximum is at 448 nm in the aqueous medium. There is a bathochromic shift in the absorption maximum to the tune to 10 nm due to the introduction of dmbpy, dtbpy ligands in the aqueous medium. The change in medium from homogeneous to microheterogeneous medium also alters the photophysical properties.
These data show that micelles lower the energy level of 3MLCT states, thereby they stabilize them relative to the d-d state. The absorption spectrum is recorded in water-acetonitrile medium (9:1 v/v).
bring the ruthenium (II) complexes close to the cationic micelle. The results show the importance of hydrophobic effect over the electrostatic forces.
The photoinduced ET reactions of [Ru(NN)3]2+ complexes with quinones in presence of cationic surfactant CTAB has been studied using luminescence technique. The absorption spectral studies of [Ru(bpy)3]2+, [Ru(dmbpy)3]2+, [Ru(dtbpy)3]2+ complexes with the incremental addition of para quinones are performed in order to check the formation of ground state complex in aqueous and CTAB medium. There is a shift in the MLCT absorption maximum of these complexes with the addition of quinone derivatives indicating that there is ground state complex formation and this concluded the static nature of quenching in the present experimental condition.
The absorption spectral studies of [Ru(bpy)3]2+ complexes with incremental concentration of 1,4-benzoquinone shows a slight shift in the MLCT absorption maximum, confirming the formation of ground state complex (
Quencher | [Ru(bpy)3]2+ | [Ru(dmbpy)3]2+ | ||
---|---|---|---|---|
Ka (M−1) | krad (s−1) | Ka (M−1) | krad (s−1) | |
1,4-benzoquinone | 2.1 × 10 4 | 4.09 × 10 5 | 4.71 × 10 4 | 4.62 × 10 5 |
2-methyl-1, 4-benzoquinone | 1.17 × 10 5 | 7.10 × 10 5 | 1.93 × 10 4 | 1.15 × 10 6 |
complexes and quinones in CTAB media is probably Vander Waals or hydrophobic in nature. The first order rate constant, which is got by the ratio of association constant and quenching rate constant shows that their values lie in the range of 104 to 105 M−1.
The Stern-Volmer plot for the oxidative quenching of *[Ru(dmbpy)3]2+ by 1,4-benzoquinone in CTAB medium is shown in
The oxidative nature of the quenching of ruthenium (II) polypyridyl complexes
Quencher | [Ru(bpy)3]2+ | |
---|---|---|
M−1・s−1 | ||
Aqueous | CTAB | |
2-chloro 1,4-benzoquinone | 6.01 × 109 | 3.849 × 1010 |
1,4-benzoquinone | 2.461 × 109 | 1.166 × 1010 |
2-methyl-1,4-benzoquinone | 4.071 × 109 | 3.369 × 1010 |
2,6-dimethoxy-1,4-benzoquinone | 1.452 × 109 | 3.318 × 1010 |
[Ru(dmbpy)3]2+ | [Ru(dtbpy)3]2+ | ||
---|---|---|---|
M−1・s−1 | |||
Aqueous | CTAB | Aqueous | CTAB |
5.55 × 109 | 3.965 × 1010 | 3.11 × 109 | 1.306 × 1010 |
1.452 × 1010 | 4.761 × 1010 | 1.38 × 1010 | 5.959 × 1010 |
1.563 × 1010 | 2.237 × 1010 | 2.001 × 109 | 2.76 × 1010 |
1.35 × 109 | 7.865 × 1010 | 4.484 × 109 | 3.02 × 1010 |
is confirmed by the transient absorption spectrum. Argon bubbled CTAB (0.04 M) solution of Ru(II) complex were excited at 355 nm under laser flash photolysis. The transient absorption spectrum of *[Ru(dmbpy)3]2+ in the presence of 0.001 M 2,6-ditertiary butyl-1,4-benzoquinone in CTABat 1 μs, after 355 nm laser flash photolysis is shown in
The present study clearly establishes the effect of cationic micelle on the quenching of [Ru(bpy)3]2+, [Ru(dmbpy)3]2+ and [Ru(dtbpy)3]2+ with para-quinones. Absorption spectral data show that the quenching is static in nature. The value of quenching rate constant is sensitive to the medium, nature and structure of the ligand and the quenchers used, the electron transfer distance between the luminophore and quencher. The detection of Ru3+ species and formation of quinone anion radical confirms the oxidation nature and electron transfer nature of quenching. Thus the change in medium from homogeneous to microheterogeneous strongly influences the kq due to the presence of hydrophobic interaction.
The authors declare no conflicts of interest regarding the publication of this paper.
Celin, T.S. and Raj, G.A.G. (2019) Micellar Effect on Photoinduced Electron Transfer Reactions of Ruthenium(II) Polypyridyl Complexes with Quinones: Effect of CTAB. Open Journal of Inorganic Chemistry, 9, 1-10. https://doi.org/10.4236/ojic.2019.91001