Tuning of photochemical and photophysical properties of [RuII(2,2'-bipyridine)2Lx] complexes using nonchromophoric ligand variations

ARTICLE

Tuning of photochemical and photophysical properties of [Ru^II(2,2'-bipyridine)₂L_x] complexes using nonchromophoric ligand variations

Simone D. Inglez^I; Francisco C. A. Lima^I; Mariana R. Camilo^II; Juliana F. S. Daniel^II; Evania D. A. Santos^I; Benedito S. Lima-NetoI, ^*; Rose M. Carlos^II

^IInstituto de Química de São Carlos, Universidade de São Paulo, CP 780, 13650-970 São Carlos-SP, Brazil

^IIDepartamento de Química, Universidade Federal de São Carlos, CP 676, 13565-905 São Carlos-SP, Brazil

ABSTRACT

MO calculations were carried out on a cis-[Ru(bpy)₂L_x](PF₆) ₂ complex, where bpy is 2,2'-bipyridine and L is 3-aminopyridine (complex 1 with x = 2), with the Gaussian 03 package using the DFT method. The electronic structure and molecular properties of 1 were characterized in vacuum and in acetonitrile solution, and the results were compared to those obtained from the complex with L = 5,6-bis(3-amidopyridine)-7-oxanorbornene (complex 2 with x = 1). The electronic spectra of 1 and 2 were investigated by TD-DFT. Experimental data from cyclic voltammetry, UV-visible spectroscopy, photochemical and photophysical experiments were compared to the theoretical data to discover the influence of L on the electronic transitions and to interpret the differences between the photochemical behaviors of these complexes.

Keywords: DFT, TDDFT, photophysical, photochemistry, ruthenium, 2,2'-bipyridine

RESUMO

Cálculos de orbitais moleculares para o complexo cis-[Ru(bpy)₂L_x](PF₆) ₂, onde bpy é 2,2'-bipiridina e L é 3-aminopiridina (complexo 1 com x = 2), foram realizados com o programa Gaussian 03 usando o método DFT. A estrutura eletrônica e as propriedades moleculares de 1 foram caracterizadas no vácuo e em solução com acetonitrila e comparadas com os resultados obtidos para o complexo com L = 5,6-bis(3-amidopiridina)-7-oxanorborneno (complexo 2 com x = 1). Os espectros eletrônicos dos complexos 1 e 2 foram investigados por TD-DFT. Os dados experimentais de voltametria cíclica, UV-vis, fotoquímica e fotofísica foram comparados com dados teóricos de maneira a estabelecer a influência de L nas transições eletrônicas e interpretar as diferenças entre os comportamentos fotoquímicos desses complexos.

Introduction

Previous studies conducted in our laboratories demonstrated that visible light photolysis of complex 1, cis-[Ru(bpy)₂L₂](PF₆ )₂, where bpy is 2,2'-bipyridine and L = 3-aminopyridine (3Amnpy), in CH₃CN solution, leads to ligand photosubstitution.¹ On the other hand, the similar complex 2, cis-[Ru(bpy)₂L](PF₆)₂, where L = 5,6-bis(3-amidopyridine)-7-oxanorbornene (3Amdpy₂oxaNBE), shows photophysical properties with emission in the visible region (λ_em = 600 nm; τ = 650 ns) and undergoes an electron transfer process using methylviologen (MV²⁺) as quenching agent.¹

The ligand 3Amdpy₂oxaNBE contains a cyclic olefin connected to two pyridine rings which are coordinated to the {Ru^II(bpy)₂} moiety in complex 2 (Scheme 1).¹ This bidentate monomer-ligand was obtained from a reaction between 3Amnpy and 5,6-bis-carboxylate-7-oxanorbornene monomer, so that, in the Ru^II complexes, the influence of the substitute groups in the meta-positions of the pyridine rings could be compared.¹

In view of the different photobehaviors of 1 and 2, the primary photoprocesses and the characterization of the electronic transitions of these complexes need to be understood. Thus, a theoretical investigation of 1 based on DFT calculations was carried out to gain further insight into the influence of the nonchromophoric ligand on the transition between the MLCT and MC states, which are closely related to the photochemical properties. In addition to what has already been calculated for complex 2,¹ a more detailed theoretical investigation was performed with 2 to improve the discussion. Investigations of the photoreactivity of 1 in various solvents and in different irradiation wavelengths were also performed.

The understanding of the consequences of these molecular perturbations is essential for the design of chemical systems with practical applications such as the conversion of radiant to chemical energy.^2-11

Experimental

Materials and procedures

Complexes 1 and 2 were prepared as previously described.¹ All solvents used were of HPLC grade. The optical spectra were recorded on a Varian spectrophotometer model Cary 500 NIR, using 1.00 cm path length quartz cells.

The NMR data were acquired using a Bruker DRX-400 spectrometer. The samples were prepared under argon and analyzed at room temperature using CD₃CN. The chemical shifts (δ) are given with reference to tetramethylsilane (TMS).

Monochromatic irradiations at 330, 440 and 500 nm were generated either using a 200 W xenon lamp in an Oriel model 68805 Universal Arc Lamp source selected with an appropriate interference filter (Oriel) or a RMR-600 model Rayonet Photochemical reactor using RMR-4200 lamps. The experiments were carried out at room temperature in 1.00 cm path length quartz cells with 4 polished sides capped with rubber septa. The magnetically stirred solutions (initial complex concentration of ca. 10^-5-10^-2 mol L^-1) were deoxygenated with argon. Potassium (tris-oxalate)ferrate(III) was used in actinometry.^12,13 The progress of the photoreactions was monitored either by UV-vis or ¹H NMR spectra.

The emission spectra at 25.0 ºC and 77 K were recorded on an Aminco-Bowman spectrofluorometer model J4-8960A with a high-pressure xenon lamp and an IP 28 type photomultiplier.

Computational methods

The calculations were made using the Gaussian 03 package.¹⁴ The starting molecular geometries were obtained at the HF/3-21G level of theory. The final molecular geometry optimizations were performed using the Kohn-Sham density functional theory (DFT).¹⁵ The Becke three-parameter hybrid exchange-correlation function (B3LYP)¹⁶ was used with the pseudo-potential basis set LanL2DZ.¹⁷ No symmetry condition was imposed. Vibrational frequencies were calculated from the analytic second derivatives to check the minimum on the potential energy surface. The natural bond orbital (NBO) calculation was made using the NBO 3.0 program, as implemented in the Gaussian 03 package.^18-20

The fragments {Ru^II(bpy)₂}, {3Amnpy} and {3Amdpy₂oxaNBE} were used to investigate the metal-ligand interaction energies and the composition of the orbitals. The molecular orbitals are expanded in the converged molecular or atomic orbital of these fragments. The Mulliken population of a fragment orbital in a molecular orbital was used to denote the percentage of the fragment orbital character of that molecular orbital. The differences between the one-electron energies of the appropriate virtual and occupied molecular orbitals were used as a first approximation for excitation energies.

Molecular orbital (MO) compositions and the overlap populations between molecular fragments were calculated using the AOMix program.^21,22 The atomic charges were calculated using NBO analyses.

Time-dependent density functional theory (TDDFT) was used to calculate the energies and intensities of the electronic transitions.^23,24 The electronic transitions were transformed into simulated spectra using the Swizard program²⁵ and Gaussian functions with half-widths of 25,000 cm^-1.

Results and Discussion

A general view of the structures for complexes 1 and 2 in CH₃CN is shown in Figure 1. Selected bond lengths and angles for the optimized geometry are given in Table 1. The X-ray data for [Ru(bpy)₃](PF₆)₂ are also listed.²⁶

Both complexes 1 and 2 have very similar geometrical arrangements. Each complex exhibits the Ru atom bounded to two bipyridyl ligands (bpy1 and bpy2) in cis configuration along with the L (3Amnpy or 3Amdpy₂oxaNBE) ligands. The bpy1 molecule (characterized by ^eqN1 and ^eqN7 pyridinic atoms), one pyridine N atom from bpy2 (^eqN19), one pyridine N atom of L (^eqN31) and the Ru atom lie roughly in the equatorial plane. The other N-pyridine atom (^axN25) of L is trans-positioned to a pyridine unit of the bpy2 (^axN13) located in the axial position. The complexes are characterized by C1 symmetry.

There is a satisfactory agreement between the theoretical data of complexes 1 and 2 concerning the bond length of each N-pyridinic atom and the central atoms (Table 1). The largest difference between complexes 1 and 2 concerns the bond lengths when one bpy ligand in [Ru(bpy)₃](PF₆)₂ is replaced by a different L (3Amnpy or 3Amdpy₂oxaNBE). The experimental bond lengths of the bpy N atoms for the tris-bpy complex are rather short in relation to complexes 1 and 2. The bond lengths Ru-N(L) for L = 3Amnpy (ca. 2.16 Å) and 3Amdpy₂oxaNBE (ca. 2.20 Å) are longer when compared with the Ru-N bond lengths observed for the bpy ligand, which is 2.05 Å. This could be caused by a diminished back bonding in complexes 1 and 2.

The polyhedral coordination of the complexes corresponds to an octahedral arrangement of the ligands in the coordination sphere of the metal. For complex 2, the polyhedron is slightly distorted, with the trans ^eqN1(bpy1)-Ru-^eqN31(3Amdpy₂oxaNBE) angle equal to 171º, for instance.

Considering that [Ru(bpy)₃](PF₆)₂ differs from 1 and 2 by replacing one bpy molecule for L, while the coordinated bpy molecules show similar N(bpy)-Ru-N(bpy) biting angles of 78 ºC as expected, the N(L)-Ru-N(L) angles in complexes 1 and 2 increase by 14-17 degrees. In addition, while the angle ^axN25(L)-Ru-^eqN31(L) for complex 2 (96.5°) is larger than that found for complex 1, 92.6°, the ^eqN1(bpy1)-Ru-^eqN31(L) angle decreases ca. 4.0º. The large bite ring could explain why the two py rings of the 3Amdpy₂oxaNBE are not orthogonal, contrary to the 3Amnpy molecules.

Additional information about complex binding was obtained by NBO charge analyses and charge decomposition analyses (CDA) using the AOMix program.^21,22

Table 2 shows the NBO charges for complexes 1 and 2. The calculated charge distributions show that Ru atoms carry similar positive charges, while the N-pyridinic atoms bound to Ru (N1, N7, N13, N19, N25, N31) and the N-substituted (N37, N38, N47) atoms in the L ligands are negatively charged. The highest negative charges are in the N-substituted atoms of each complex (N37, N38, N47) and the highest ones among them are located in complex 1 (N37 and N38). Investigation of the NBO charges in free bpy, 3Amnpy and 3Amdpy₂oxaNBE shows that no noticeable global charge transfer occurs between the pyridine units from either bpy or L (3Amnpy or 3Amdpy₂oxaNBE) and the Ru centre after L coordination, considering the fact that the NBO charge values in the N-pyridinic atoms are similar in each case. In counterpart, the negative charges in the N-substituted atoms differ 0.15-0.17 units between the 3Amnpy and 3Amdpy₂oxaNBE. This probably occurs because of the greater electronegativity of O in relation to N. As a consequence, the electron densities from the N37/N47 atoms migrate towards the amide-carbonyl moiety. The relative values of these transfers can indeed be deduced from the NBO orbital occupancies.

Table 3 shows the donation and back-donation charge transfer between different fragments for complexes 1 and 2. It can be observed that, while the back-donation from the {Ru^II(bpy)₂} moiety to L is insignificant, the donation from L to the {Ru^II(bpy)₂} moiety involves 0.56 electron. It is interesting to note the CDA between the {Ru^II(bpy)₂(3Amdpy₂ -)} and {(-oxaNBE)} fragments, suggesting that the oxaNBE moiety has a large contribution in the relative charge donation to the {Ru^II(bpy)₂} moiety.

Molecular orbital compositions

Further understanding of the nature of the coordination bonds and the electronic structure in the studied complexes can be provided by analysis of the valence molecular orbital composition. The energies and composition of the frontier molecular orbitals of complexes 1 and 2 are given in Table 4. The frontier molecular orbital representations are presented as Electronic Supplementary Information (Tables S1 and ^S2).

The theoretical data for complex 1 show that the HOMO orbital has a high percentage of Ru non-bonding d-orbital (86%), as occurs with 2.¹ In fact, the Ru orbitals are present in all HOMOs of 1 shown in the selected frontier orbitals, but it is not the case for 2. However, the type of L defines differences between the LUMOs from 1 and 2. The LUMO in 2 is exclusively located on the 3Amdpy₂oxaNBE ligand, whereas the bpy orbital predominates in the LUMO+1 and LUMO+2.¹ For complex 1, the π* orbitals of the bpy predominate in the LUMO, LUMO+1, LUMO+2 and LUMO+3 with a few Ru antibonding participation. Another substantial difference, considering the analyses of the frontier orbitals from 1 and 2, is the LUMO+12 situated 6 eV above the HOMO, which has a large Ru antibonding character (82%). The HOMO-1 in complex 1 comes basically from the Ru nonbonding orbitals, while the HOMOs -2, -3 and -4 have a sizeable contribution from the 3Amnpy.

The HOMO-LUMO gaps are 3.4 and 3.2 eV for 1 and 2 respectively, whereas the back-donation towards the L ligand is weak in both cases, as revealed by the charge donation analysis (Table 3). Considering a correlation between the HOMO-LUMO energy gap and the degree of charge delocalization,²⁷ it can be deduced that the Ru-bpy covalent interaction involves charge-donation from the L ligands to the {Ru^II(bpy)₂} fragment. Similar values were observed in the case of L = 3Amdpy₂oxaNBE (complex 2).¹

The presence of the 3Amdpy₂oxaNBE ligand in 2 decreases the oxidation potential of the metal complex to a less positive value compared to the 3Amnpy and bpy derivative complexes (E_1/2([Ru(bpy)₃]²⁺ ) = +1.29, E_1/2(1) = +1.11 and E_1/2(2) = +0.77 V vs. Ag/AgCl in CH₃CN). This occurs in agreement with the large Ru-L bond lengths in relation to Ru-bpy.¹ Since the HOMO-LUMO gap energies are unaffected by the different L, the net effect of L = 3Amdpy₂oxaNBE in 2 is to lower the energy of the ³MLCT states, as previously discussed.¹ Thus, when comparing complex 2 to the parent complex 1, this effect helps to decouple the ³MLCT from the higher energy ³MC states, resulting in an emissive ³MLCT, which does not occur in 1.

Theoretical and experimental electronic spectrum studies

The experimental and theoretical absorption spectra of complexes 1 and 2 in CH₃CN are characterized by two rather strong bands (Table 5; Figure 2). The relative positions and intensities are dependent on the nonchromophoric ligands. According to the experimental data (Table 5), the lowest energy absorption bands of complex 2 are red-shifted approximately 30 nm in relation to those of complex 1.

When the oxaNBE monomer-group is present in the substituted pyridine ligand, an extended conjugation is observed (Table 5), lowering the transition energy while increasing the oscillator strength. Similar tendencies are observed for the oxidation of Ru^II to Ru^III (Table 6). The higher metal oxidation potential of complex 1, compared to that of 2, reflects the stabilization of the +2 oxidation state by the 3Amnpy, the less σ donor ligand. Figure 3 points out that more electron-withdrawing ligands decrease the electron density at the metal center for the related series of complexes and lead to more positive potentials than the complexes with more electron-donating ligands.

The calculated spectrum of complex 1 presents four MLCT absorption transitions at 2.71 eV (HOMO-1→LUMO), 2.80 eV (HOMO→;LUMO+1), 3.61 eV (HOMO-1→LUMO+3) and 3.69 eV (HOMO→LUMO+5) (Table 5).

For complex 2, the lowest energy calculated absorption could be assigned as a HOMO-2→LUMO+1 electronic transition with an MLCT nature (Table 5). There are four electronic transitions at higher energies: HOMO-1→LUMO+4, HOMO-2→LUMO+4, HOMO→LUMO+6 and HOMO→LUMO+7. The energy differences among these transitions are not significant, ca. 0.1 eV, although the differences in the calculated intensities are substantial (f = 0.015-0.038).

In this context, for complex 2, it is noteworthy that the experimental absorption at 3.62 eV could be originated from the highest filled Ru d_π orbitals to the π* orbital of 3Amdpy₂oxaNBE. Furthermore, the calculations do not predict any MC transitions in the region studied for complex 2 (up to LUMO+20). These features correlate with the electrochemical behavior and structural features, such as the photophysical and photochemical results described in the next section.

Apart from the MLCT transitions, complex 1 has an MC transition at 3.78 eV (328 nm), on account of the HOMO→LUMO+12 transition. Because of the low intensity (f = 0.0060), this transition is obscured by the intense MLCT transition at 3.69 eV (f = 0.0683). The ground state calculation predicts 0.09 eV (726 cm^-1) as the energy gap between the first MLCT and the MC states.

Replacement of bpy by 3Amdpy₂oxaNBE does not have any significant influence on the position of the highest MLCT absorption at ca. 340 nm (3.64 eV) of the {Ru^II(bpy)₂} moiety (Table 6), although the ε value increases significantly on account of the contribution of the Ru→3Amdpy₂oxaNBE transition. In contrast, the 3Amnpy ligand shifts this band to 328 nm (3.78 eV), which matches the MC transition as attributed by the simulated spectrum. For a series of related [Ru^II(bpy)₂L_x)]^q+ complexes (Table 6), the experimental energy of this transition decreases in the order 3Amnpy (3.78 eV) > py (3.67 eV) > 3Amdpy₂oxaNBE (3.62 eV) > bpy (3.59 eV) > 4Amnpy (3.56 eV) > Cl^- (3.26 eV).

Table 7 presents the lowest energy bands for complexes 1 and 2 in different media. The absorption bands are solvent sensitive in agreement with the MLCT assignments.

Photochemistry studies

As reported in the earlier paper, complex 2 was photochemically unreactive.¹ On the other hand, the photolysis of complex 1 in CH₃CN caused photoreaction with a quantum yield of 0.26 at λ_exc = 440 nm, producing the cis-[Ru(bpy)₂(CH₃CN)₂] ²⁺ ion.¹ In order to better understand this photochemical process, the changes in the UV-vis spectra of complex 1 in a CH₃CN solution during photolysis with 440 nm light (I₀ = 1×10^-9 einstein s^-1) were analyzed (Figure 4a). A progressive depletion of the absorption band is noted at 464 nm, with concomitant blue shifts to 440 nm (Figure 4b) and then to 428 nm (Figure 4c). This implies that the formation of the cis-[Ru(bpy)₂(3Amnpy)(CH₃ CN)]²⁺ ion with λ_max = 440 nm occurs, followed by a second coordination of the π acceptor CH₃CN ligand, leaving a complex with a much smaller extinction coefficient when compared to complex 1. The λ_max of the mono-solvent complex coincides with the irradiation wavelength. Then, after 20 measurements, a depletion of the absorption of the first photoproduct species can be observed due to secondary photolysis (Figure 4b-c). The new band profile is similar to that shown by the cis-[Ru(bpy)₂(CH₃CN)₂] ²⁺ complex ion (Table 6). From these UV-vis spectrophotometer time scale measurements, these reactions occur quite smoothly. Isosbestic points are preserved even at a very high conversion. The quantum yield for the 3Amnpy photosubstitution at 440 nm was dependent on the irradiation time up to at least 3% decomposition of complex 1 due to the secondary photolysis.

The presences of the mono and bis-acetonitrile complexes during photolysis of 1 were confirmed by ¹H NMR measurements. A CD₃CN solution of cis-[Ru(bpy)₂(3Amnpy)₂ ]²⁺ was irradiated at room temperature with 420 nm light (I₀ = 1x10^-7 einstein s^-1). The ¹H NMR spectra were recorded with 30 min time intervals (Figure 5). Simple intensity arguments tell us that the peaks with chemical shifts between 6.9 and 9.5 ppm are from the bipyridine and Amnpy hydrogens.²⁹ During irradiation, decreasing and increasing resonance peaks were observed, indicating photorelease of 3Amnpy for 2 h. In particular, the signal at 9.0 ppm decreases and a new signal at 9.4 ppm appears. This indicates the replacement of 3Amnpy by acetonitrile in the metal coordination sphere. The shift of the bipyridine hydrogen signal at 9.0 ppm to a higher frequency indicates the change of the σ-donor nature of the trans ligand (3Amnpy versus acetonitrile). It is interesting to note that the shift of the 3Amnpy NH₂ signal from 4.42 to 4.52 ppm also indicates that the 3Amnpy is released. After four irradiation cycles, a well-resolved ¹H NMR spectrum is observed and assigned to the bis-acetonitrile complex. It differs from the spectrum of the sample irradiated for 30 min, attributed to the cis-[Ru(bpy)₂(3Amnpy)(CH₃ CN)]²⁺ complex ion. The final ¹H NMR spectrum profile was identical to the ¹H NMR spectrum of the cis-[Ru(bpy)₂(CH₃CN)₂](PF ₆)₂ species synthesized thermally, confirming the attribution.

Considering the observed stepwise photosubstitution of 1 in CH₃CN, photolyses in different solvents were carried out as a function of the irradiation wavelength (Figure 6). While thermal reactions were not observed up to 10 h at 25.0 °C, in all the irradiated solutions changes in the UV-vis spectra with blue shifted final spectra were observed. Table 7 shows the initial quantum yield data calculated for wavelengths where the ε value ratios between the starting complex 1 and the products were the largest.

Photolysis at 330 nm provides large quantum yields (Table 7). It is relevant that this wavelength is close to the calculated λ_max of the lowest-energy spin-allowed MC absorption band (Table 5).

The quantum yield data in Table 7 involve two simultaneous variables: λ_max(MLCT) and λ_irr. The variation of λ_irr should not influence the quantum yields if the deactivation from upper to lower energy excited states were 100% efficient. However, it is clear from Table 7 that photosubstitution quantum yields are quite dependent on λ_irr for all the studied cases. Furthermore, the interconversions to common states, presumably the lowest in energies, do not occur with 100% efficiency.

The significant decrease in the Φ values and the large dependence on λ_irr suggest a change in the nature of the lowest excited state from ³MC to ³MLCT. These results, combined with the absorption experiments and DFT calculations, confirm that the lowest energy reactive excited state is a ³MC for 1.

The tendency in the photochemical properties of complexes 1 and 2 parallels the changes in the photophysical properties found in the emission spectra.¹

Emission spectra

The emission spectra of 1, 2 and [Ru(bpy)₃](PF₆)₂ complexes in 4:1 EtOH/MeOH glasses at room and liquid nitrogen (77 K) temperatures are characterized by broad unstructured bands. These consisted of well-defined vibrational progressions with the energy differences between the two peaks ranging from 1,182 to 1,275 cm^-1 (8.46 to 7.84 nm), close to the ³MLCT transition found for [Ru(bpy)₃](PF₆)₂ at 1,262 cm^-1 (7.92 nm) (Figure 7).

In sharp contrast with the photochemical results, substitution of bpy in [Ru(bpy)₃](PF₆)₂ by 3Amdpy₂oxaNBE shifts the MLCT emission maximum to 565 nm, while the 3Amnpy ligand shifts this band to 590 nm. The excited state energy (0-0 energy) can be calculated according to the equation: E^0-0 = ν_max + 1.29 δν_1/2.³⁰ Although proposed for a Gaussian band shape, this relationship provides a qualitative basis for comparing the spectra of closely analogous complexes. For 2, [Ru(bpy)₃](PF₆)₂ and 1, the ν_max and δν_1/2 values, defined as the full width at half maximum, are 2.20, 2.16, 2.10 and 0.16, 0.11, 0.11 eV, respectively. Thus E^0-0 values are 2.42, 2.29 and 2.24 eV according to the assumption of a Gaussian band shape for 2, [Ru(bpy)₃](PF₆)₂ and 1 respectively. From these results, the ³MLCT spectroscopic energy gap between complexes 1 and 2 is 0.18 eV. The difference seems to be reasonable on account of the change in the electron densities about the metal centers after the attachment of the oxaNBE group to the substituted pyridine ligand.

Concerning the excitation of complex 2 at 450 nm at room temperature, the position of the emission maximum does not change when compared to the corresponding [Ru(bpy)₃](PF₆)₂ emission (577 nm). Complex 1 shows an emission at 592 nm with intensity much lower than that of complex 2 (Figure 7).

The observation of a weaker MLCT luminescence at the longer wavelength and the high sensitivity of Φ for λ_irr suggest that complex 1 has moved from the reactive to the unreactive category at room temperature, although still showing some intermediate nature (Table 7).

In the same way, going from 3Amnpy to 3Amdpy₂oxaNBE modifies the excited state order, where a reactive MC state is below or comparable in energy to the lowest MLCT state. These results are consistent with the observation of the extended π-conjugation over the 3Amdpy₂oxaNBE ligand. It increases the opportunity for electron delocalization at the triplet level. This delocalization should lower the nuclear reorganization energy that accompanies nonradiative decay of the ³MLCT, stabilizing the triplet states.

Scheme 2 shows a simple relative energy diagram illustrating the difference between the photochemical and photophysical properties of complexes 1 and 2. For complex 1, the energy gap between the MLCT state and the MC state is only 0.09 eV. Thus, the MLCT state lies close to the MC state and it is reached by direct light excitation. Consequently, the intersystem crossing will rapidly decay to the lower ³MLCT states, which is weakly emissive and will crossover preferentially to the ³MC state. The presence of the 3Amdpy₂oxaNBE ligand in 2 displaces the ³MC states towards energy levels which are much higher than the ³MLCT state. Therefore, when 2 is irradiated in the MLCT band, the system rapidly decays to the lower ³MLCT state, increasing its population and the emission without photoreaction.

CONCLUSIONS

The theoretical and experimental results demonstrated that the orbital nature of the lowest excited states changed when the pyridine ligands were connected to the oxaNBE group. The disturbances in the orbital nature introduced by this type of group can be used to tune the photochemical and photophysical properties of [Ru(bpy)₂(X-substituted-py)_x]²⁺ complexes: whereas complex 1 is photochemically reactive, complex 2 shows luminescence and is photochemically unreactive. The oxaNBE-substituted group showed unique properties when compared to other X-substituted-pyridines. This change is also related to the high degree of electron delocalization in the {Ru^II(3Amdpy₂oxaNBE)} unit on account of the more extended π conjugated system of the 3Amdpy₂oxaNBE ligand. Therefore, attaching the oxanorbornene unit to the 3Amnpy ligand seemed to be an effective way to prolong the MLCT (Ru→bpy) triplet lifetime.

As suggested by a referee, it cannot be ruled out that the lack of photoreaction from complex 2 is related to the chelating property of the ligand. Photochemical labilization of one of the two pyridines would not completely detach the ligand from the metal, offering a chance for a back coordinating reaction, which is obviously impossible in the case of complex 1.

Supplementary Information

Electronic Supplementary Information (contour plots of selected frontier molecular orbitals obtained from DFT calculations for complexes 1 and 2, Tables S1 and ^S2) is available free of charge at http://jbcs.sbq.org.br, as a PDF file.

Acknowledgments

We thank the financial support from FAPESP (Proc. 06/57577-4), CAPES and CNPq.

Received: April 13, 2009

Web Release Date: October 30, 2009

FAPESP helped in meeting the publication costs of this article.

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Publication Dates

• Publication in this collection
  18 Feb 2010
• Date of issue
  2010

History

• Accepted
  30 Oct 2009
• Received
  13 Apr 2009