The effects of ligand substitution and deuteriation on the spectroscopic ...

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www.rsc.org/pps | Photochemical & Photobiological Sciences

The effects of ligand substitution and deuteriation on the spectroscopic and photophysical properties of [Ru(LL)(CN)4 ]2− complexes†‡ Margit Kovács,a Kate L. Ronayne,b Wesley R. Browne,§c William Henry,c Johannes G. Vos,c John J. McGarveyb and Attila Horváth*a Received 16th August 2006, Accepted 20th October 2006 First published as an Advance Article on the web 6th November 2006 DOI: 10.1039/b611825a The spectroscopic characterisation of a series of [Ru(LL)(CN)4 ]2− complexes, where LL = 1,10phenanthroline (phen) and its methyl- and phenyl-substituted derivatives and several deuteriated isotopologues are reported. The optical and vibrational properties of these complexes are compared with that of the series of 2,2 -bipyridine (bipy) derivatives and analogous [Ru(LL)3 ]2+ complexes. It has been demonstrated that substitution at the 4,4 positions of bipy and 4,7-positions of phen by electron donating (CH3 ) and withdrawing (C6 H5 , COO− ) groups induces a pronounced blue and red shift, respectively, in the lowest energy 1 MLCT absorption band of [Ru(LL)(CN)4 ]2− . The energy of the emission originating from the 3 MLCT excited state is found to be dependant on the nature of the vibrational modes of the aromatic rings and the electron donating and/or withdrawing properties of the substituents. Single-mode Franck–Condon analysis indicates that methyl substitution leads to a significant increase in the Huang–Rhys factor (SM ), while phenyl substitution results in a decrease in SM for both series (bipy and phen) of complexes. The rate of non-radiative (knr ) and radiative decay (kph ) to the ground state and the parameters of thermally activated deactivation pathways (A4th , DE 4th and Add , DE dd ) were estimated from the temperature dependence of luminescence quantum yields and lifetimes. It has been demonstrated that the non-radiative decay rate and the temperature dependent decay processes are more efficient for bipy complexes than for phen derivatives due to the rigidity of the latter ligand.

Introduction The bottom-up approach to the design of molecular-level devices has opened up new opportunities not only for basic research but also for the emergent areas of nanoscience and nanotechnology.1 The fundamental constituents of light driven molecular assemblies are the photosensitisers, which, by definition, have high molar absorptivity in the visible spectrum and long lived excited states capable of electron or energy transfer. First reported by Bignozzi et al.2 [Ru(bipy)(CN)4 ]2− and related [Ru(LL)(CN)4 ]2− complexes (where LL is a bidentate polypyridyl ligand, such as 2,2 -bipyridine and 1,10-phenanthroline), can be regarded, structurally, as the simplest inorganic light sensitizer units available for application in the supramolecular photonic a Department of General and Inorganic Chemistry, University of Pannonia, PO Box 158, Veszprém, H-08201, Hungary. E-mail: attila@vegic.vein.hu; Fax: ++36 88 6245 48; Tel: ++36 88 624341 b School of Chemistry and Chemical Engineering, Queen’s University Belfast, Stranmillis Road, Northern Ireland, UK BT9 5AG. E-mail: j.mcgarvey@ qub.ac.uk c National Centre for Senor Research, School of Chemical Sciences, Dublin City University, Dublin 9, Ireland. E-mail: johannes.vos@dcu.ie † This paper was published as part of the special issue to commemorate the 70th birthday of Vincenzo Balzani. ‡ Electronic supplementary information (ESI) available: Analytical data for ligands and complexes, resonance-Raman spectra, transient absorption spectra and photophysical data. See DOI: 10.1039/b611825a § Present address: Stratingh Institute, University of Groningen, Nijenborgh 4, 9747AG, Groningen, The Netherlands. E-mail: w.r.browne@ rug.nl

444 | Photochem. Photobiol. Sci., 2007, 6, 444–453

devices.3 These polypyridyl tetracyanoruthenates have received attention for various applications such as light harvesting units for photosynthetic systems,4 quenchers,5 photosensitisers for TiO2 solar cells6 and as humidity sensors.7 Detailed studies of the solvatochromic,8 temperature and pH9 dependence of the luminescence properties of [Ru(bipy)(CN)4 ]2− and other mixed ligand cyano-polypyridyl ruthenium(II) complexes8 have shown that judicious choice of the polypyridyl ligand provides for the selection of desired photophysical characteristics. In addition, the presence of CN− ligands allows for efficient interaction with their immediate environment and as bridges in polynuclear complexes.10 At a more fundamental level, [Ru(LL)(CN)4 ]2− is perhaps the simplest model system for the very extensive range of ruthenium polypyridyl complexes available (e.g. [Ru(bipy)3 ]2+ ). In bis- and tris-homoleptic complexes an interesting and, in some respects, controversial question regarding their excited state properties is the localisation/delocalisation of electron density on a single ligand or over several ligands in the lowest triplet metal to ligand charge transfer state (3 MLCT). One of the consequences of the apparently simple structure of [Ru(LL)(CN)4 ]2− complexes is that electron delocalisation in the MLCT states is by default localised on only one polypyridyl ligand. The nature of the diimine ligand in the complexes [Ru(LL)3 ]2+ ,11 and [Ru(LL)2 X2 ],12 (where X is a halide or pseudohalide ion, such as Cl− and CN− or SCN− ) has been confirmed to play a crucial role in determining the efficiency of both radiative and nonradiative deactivation of the lowest emissive 3 MLCT excited state.

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Although, similar effects are assumed for the [Ru(LL)(CN)4 ]2− series of complexes, systematic studies have been performed on only a few of these complexes2,3,13 and of their supramolecular derivatives.4,5,14 Hence, the preparation and examination of a representative series of substituted bipy and phen based complexes is expected to provide a more thorough understanding of this type of complex, enabling a detailed analysis of the electronic structure of their 3 MLCT excited states and their radiative and non-radiative decay pathways. In addition, the introduction of methyl groups at different positions (phen) and the use of isotopic labeling, both partial (ph2 phen) and perdeuteriation, enables an extra dimension of the excited state properties to be explored. In this article, the spectroscopic characterisation of several new [Ru(LL)(CN)4 ]2− complexes (where LL = 2,9-dimethyl-1,10phenanthroline, 4,7-dimethyl-1,10-phenanthroline, 5,6-dimethyl1,10-phenanthroline and 4,7-diphenyl-1,10-phenanthroline) are reported and compared with that of previously reported complexes (LL = 2,2 -bipyridine, 1,10-phenanthroline, 4,4 -dimethyl2,2 -bipyridine, 4,4 -dicarboxy-2,2 -bipyridine, 4,4 -diphenyl-2,2 bipyridine), Scheme 1. Several deuteriated isotopologues have been prepared to facilitate characterisation by electronic, vibrational and NMR spectroscopy. The excited state electronic structure of this class of complex is explored to deepen our

Scheme 1

understanding of the properties of the lowest-lying 3 MLCT excited states of the [Ru(LL)(CN)4 ]2− complexes and on their deactivation processes.

Experimental Materials All solvents employed were of HPLC grade. For emission measurements UVASOL grade solvents were employed. The reagents employed in synthetic procedures were of reagent grade or better. All deuteriated ligands were prepared according to the method of Browne et al.15 K2 [Ru(LL)(CN)4 ] (LL = 2,2 -bipyridine (1), 4,4 dimethyl-2,2 -bipyridine (2), [H8 ]-1,10-phenanthroline (5)) were synthesized according to procedures reported previously.16 General procedure to prepare complexes with ligand 3a–b, 4, 6, 7, 8, 9a–e K4 [Ru(CN)6 ] was first converted to tetrabutylammonium salt (TBA) by cation exchange chromatography over Sephadex SPC25. The tetrabutylammonium salt of [Ru(CN)6 ]4− and 1 eq. of the diimine ligand were heated in a water–N-methylpyrrolidone

Ligands described in the text (names e.g., 5a indicate the [Ru(phen)(CN)4 ]2− complex formed from phen ligand).


Photochem. Photobiol. Sci., 2007, 6, 444–453 | 445

mixture (1 : 1) at 90 ◦ C for 24 h, the solution was maintained at pH 3.5 to 4.5 by addition of HClO4 . At the end of reaction the mixture was neutralised by TBA+ OH− and evaporated to dryness. The residue was washed with a small amount of cold water to dissolve the complex formed and filtered to remove unreacted ligand and TBAClO4 . The filtrate was reduced in vacuo and the components were separated by chromatography on Sephadex G-15 column, eluted with water. The product (orange fraction) was eluted after two unidentified red–brown fractions. After evaporation of the solvent the solid was redissolved in small amount of acetone and precipitated by addition of diethyl ether. The product, TBA2 [Ru(LL)CN4 ], is very hygroscopic and where sufficient amounts were obtained, the TBA+ salt of the complex was metathesised by addition a concentrated solution of KClO4 to an acetone solution of the complex. For analytical data see electronic supporting information (ESI‡). Apparatus and physical methods

Counting apparatus (TCSPC). The reduced v2 and residual plots were used to judge the quality of the fits; s ± 2.5%.

Results and discussion Electronic spectroscopy The absorption spectra of the bipy based complex (1a) and its substituted derivatives (2a and 3a) and those of phen (5a) and other phen based complexes (6, 7, 8, 9a) are shown in Fig. 1 and are in agreement with spectra reported previously. For all complexes, ligand deuteriation has no significant effect on the electronic absorption spectra. The spectra are typical of Ru(II) diimine complexes, with metal-to-ligand charge transfer (1 MLCT) bands in the visible region, and intense ligand-centered (1 LC) transitions in the UV range. For both the bipy and phen series, a red shift of the 1 MLCT bands with increasing electron withdrawing strength of the substituent (i.e. CH3 < H < C6 H5 , in the 4,4 - and 4,7-positions, respectively) is observed.

1

H NMR spectra were recorded on a Varian Unity300 (300 MHz) NMR spectrometer. Peak positions are relative to residual solvent peaks. Infrared spectra were recorded on a BioRad FTS60A spectrophotometer. UV/Vis absorption spectra were recorded on a Specord S-100 diode array UV/Vis spectrophotometer. Ground state resonance-Raman spectra of the complexes were recorded at 457.9 nm using an Argon ion laser (Spectra Physics model 2050) as the excitation source. The laser power at the sample was typically 30–40 mW. The Raman backscatter was focused onto the entrance slit of a single stage spectrograph (JY Horiba HR640), which was coupled to a CCD detector (Andor Technology DV420OE). Transient resonance-Raman spectra were recorded using the single-colour pump and probe method in which the leading edge of the pulse excites the molecules and the trailing edge probes the resultant Raman scattering. The excitation source was a pulsed laser (Spectra Physics Q-switched ND:YAG, GCR-3) at 354.67 nm with a typical pulse energy of ca. 3 mJ at the sample. The Raman backscatter was focused onto the entrance slit of a double-stage spectrograph (Spex 1870) which was coupled to an ICCD (Andor Technology DH501). A flow-cell of in-house design was used to minimise thermal and/or photodegradation of the samples over the 15 min accumulation times. For Raman techniques, a 50 : 50 (v/v) mixture of acetonitrile and toluene was used as the calibration solution.17 Emission spectra (accuracy ±2 nm) were recorded at 298 and 77 K using a Perkin-Elmer LS50B luminescence spectrophotometer, equipped with a red-sensitive Hamamatsu R928 PMT detector, interfaced with an Elonex PC466 employing Perkin-Elmer Fl WinLab custom built software. Emission and excitation slit widths were 5 nm at 77 K and 10 nm at 298 K. Emission spectra are corrected for photomultiplier response. 10 or 2 mm path length quartz cells were used for recording spectra. Emission measurements at 77 K were carried out using the Perkin Elmer low temperature luminescence accessory (L2250136). Emission spectra measured at 77 K ethanol–methanol (4 : 1) were subjected to single mode Franck–Condon analysis as described previously.14a,17 Quantum yields and room-temperature luminescence lifetime measurements were carried out as described earlier.14a,17 Luminescence lifetime measurements at 77 K were carried out using an Edinburgh Analytical Instruments (EAI) Time-Correlated Single-Photon 446 | Photochem. Photobiol. Sci., 2007, 6, 444–453

Fig. 1 Room-temperature absorption spectra of complexes at 298 K in H2 O: (i) LL = 1a (dashed), 2a (solid line), 3a (dash dot), 4 (dotted); (ii) LL = 5a (dotted), 6 (short dash-long dash), 7 (solid), 8 (dashed), 9a (dash dot dot dot).

For the bipy based systems, the most pronounced red shifts are induced by substitution with carboxylic acid groups (4). The kmax of the lowest energy absorption bands are found at higher energies for the series of phen complexes than the equivalent bipy complexes. Similar observations were reported earlier for tris-diimine complexes of ruthenium(II).11 It was demonstrated


that the methyl and phenyl substitutions at 4,7 positions for [Ru(phen)3 ]2+ resulted in blue and red shifts, respectively, with less pronounced changes than observed for the tetracyano complexes. By contrast, for the series of bipy based tris-diimine complexes, i.e., [Ru(bipy)2 (LL)]2+ , [Ru(bipy)(LL)2 ]2+ and [Ru(LL)3 ]3 2+ (where LL is a 4,4 substituted bipy) both methyl and phenyl substitutions induce a red shift, with a more pronounced effect upon phenyl substitution. Surprisingly, for the phen complexes replacement of methyl groups with phenyl groups results in a smaller shift in the 1 MLCT absorption bands than for the bipy complexes. For the Me2 phen based complexes 6–8, the effect of substitution pattern on the electronic properties is quite dramatic. Substitution in the 5,6-positions results in only minor perturbation of the 1 MLCT absorption band, and a small red-shift in the 1 LC band at ∼280 nm. In contrast, substitution in the 4,7- or the 2,9-positions results in a blue-shift in the 1 MLCT absorption band. The effect of substitution in the 5,6-position is relatively minor, reflecting the double bond character of the C5-C6 bond. By contrast, substitution in the 4,7- and particularly in the 2,9-positions results in considerable changes to the electronic properties as demonstrated by the absorption spectra (Fig. 1 and Table 1) and by the 1 H NMR data (Table S1, ESI†). A reasonable qualitative explanation of the substituent effects observed for the tetracyano and tris diimine complexes can be reached by considering the r-donor and p-acceptor properties of both the unchanged (spectator) ligands, such as the cyanide ligands of the mono-diimine complexes and bipy or phen ligands for the tris-diimine complexes, and the diimine ligands substituted by either electron donating or electron withdrawing groups. Namely, phenyl substitution either on bipy or on phen results in a decrease in energy of the p* orbital of the substituted ligand, and the stabilization of the metal centered t2g orbitals. The extent of this latter stabilization is influenced by the nature of the spectator ligands also. This influence is minor for the strong r-donor and p-acceptor ligands possessing filled p-orbitals, such as cyanides Table 1 Electronic absorption and emission data Complex

LL

kabs /nm

e/M−1 cm−1

kem /nm

[Ru(LL)(CN)4 ]2−

bipy 4,4 -dmb 4,4 -dpb 4,4 -dcb phen 2,9 dmphen 4,7-dmphen 5,6-dmphen ph2 phen

400 395 417 425 385 363 379 380 396

3400 3580 4550 6620 4720 4520 6780 5610 10520

624 621 651 666 610 609 605 613 650

b

[Ru(bipy)(LL)2 ]2+

4,4 -dmb 4,4 -dpba 4,4 -dmb 4,4 -dpba

456 465 457 457

11000 19700 — —

630 635 630 630

b

[Ru(LL)3 ]2+

bipy dmb dpb

452a 459a 474a

14600 14900 28000

628 631 632

b

[Ru(LL)3 ]2+

phen 4,7-dmphen ph2 -phen

447a 445a 460a

18100 25300 29500

604 613 510

a

Measured in methanol–ethanol solvent mixture. b From ref. 11d.

Scheme 2

(see Scheme 2). In addition, for the phenyl substituted tris diimine complexes, it is the substituted ligand which has the lowest energy p* orbital. Hence a considerable red shift is observed for the MLCT band in the absorption spectra of both tetracyano and tris diimine complexes. The methyl substitution induces a destabilization of the p* orbital of the substituted ligand which accompanies an increase in energy of metal centered t2g orbitals. Again, this latter change is influenced by the donor–acceptor properties of the spectator ligands. Considering the ligand field strength of bipy, phen and CN− , the smallest destabilization is expected for cyanides, while a significant increase in energy of t2g states is expected for bipy. The energy of the spectator ligand’s p* orbital (cyanides and bipy or phen) should be less sensitive to the methyl substitution than the p* orbital of the substituted ligand. Hence the p* orbital of the spectator bipy has significantly lower energy than the p* orbital of the methyl substituted ligand in [Ru(bipy)2 (LL)]2+ , [Ru(bipy)(LL)2 ]2+ complexes. These effects result in small red shifts for bipy based tris diimine complexes, a very small blue shift for phen based tris diimine complexes and significant blue shift for the tetracyano complexes.11d Overall, the shifts in 1 MLCT bands can be rationalised in terms of the r- and p-donor properties of the polypyridyl ligand (LL) and the extent of the mixing of dp(Ru) orbitals with (a) the p(LL) orbitals and (b) the anti-bonding orbitals of the LL, p*(LL) and the cyanide ligands, p*(CN− ), predominantly of those which are in-plane of LL. The dominance of the p*(LL) is expected hence it will be sensitive to the nature of the substituents replaced. Resonance-Raman spectroscopy Resonance-Raman spectra (rR) were recorded in aqueous solution at 457.9 nm for complexes 1a/b, 2a/b, 3a/b, 5a/b and 9a–e (Fig. 2, S1 and S2, ESI‡). The bands observed in all cases are attributed to neutral ligand modes. Deuteriation results in modification of the spectra in terms of peak position and the overall intensity of the spectra are decreased (usually with the exception of the symmetric ligand based breathing mode at ≈1490 cm−1 for 1a, 2a and 3a and at ≈1421 cm−1 for 1b, 2b and 3b). It is also interesting to note that many of the peak positions are conserved between the nondeuteriated bipyridyl complexes, e.g. all contain peaks at ca. 1030, 1280, 1314, 1490 and 1606 ± 10 cm−1 . A similar phenomenon is observed for the deuteriated complexes where the major spectral features of the D8 -bipy (1b) complex at 1008, 1421 and 1539 are observed (±10 cm−1 ) in the spectra of the D12 -dmb and D16 -dpb complexes. The resonance-Raman spectra of the phenanthroline complexes, 5a and 5b, and the diphenylphenanthroline isotopologues,



Fig. 2 Ground state resonance-Raman spectra (in H2 O, kex = 457.9 nm) for (a) complex 1a; (b) 1b; (c) 2a; (d) 2b; (e) 3a; (f) 3b.

9a–e, were recorded in aqueous solution at kex = 457.9 nm. As for 1–3, all bands arise from neutral ligand modes. The spectra of 5a and 5b show an overall increase in signal to noise ratio compared with those of the bipyridyl based complexes. The diphenylphenanthroline based compounds have quite rich spectra with marked changes occurring on deuteriation of the ligands (Fig. S2. ESI‡). Substitution at the 2,9 position (9b) leads to a number of spectral changes. 9c shows the effect of deuteriation of the ligand at the 3,8 and phenyl positions as a large decrease in the S/N ratio coupled with an increase in the number of spectral features observed, leading to a more complex spectrum. The rR spectra for 9d and 9e are similar in both band position and relative intensity, indicating the C5/C6 atoms are, largely, isolated from the remainder of the ligand in terms of vibrational modes.

Fig. 3 Emission spectra in water at room temperature of (i) 1a (dashed), 2a (solid line), 3a (dash dot), 4 (dotted); (ii) 5a (dotted), 6 (dash dot dot dot), 7 (solid), 8 (dashed), 9a (dash dot).

Generally, the shape of the emission spectra indicates the dominance of the 0–0 transition, which is attributed to the extensive delocalisation of the electron promoted to a p*(LL). This is true for the bipy (1a), dmb (2a), phen (5a), and dmphen (6–8) complexes, however, at room temperature, the 0–1 transition is more intense for 3a and for 9a than the 0–0 transition. The emission from the deuteriated complexes is more intense than those of the respective non-deuteriated complexes, suggesting a higher luminescence quantum yield and slower non-radiative decays (vide infra).

Emission spectroscopy in fluid media The steady state luminescence spectra recorded in aqueous solution at room temperature exhibit an intense emission from the 3 MLCT excited state (Fig. 3), which is typical of ruthenium(II) polypyridine luminophores. The energy of the emission is sensitive to the nature of polypyridine skeleton as well as to the electron withdrawing properties of the substituents in the same manner as the 1 MLCT absorption bands, with the derivatives of the rigid phen ligands emitting at higher energy than their corresponding bipy complexes (Table 1) in liquid aqueous media. The influence of substitution of bipy and phen at the 4,4 and 4,7 positions, respectively, is more pronounced for phen complexes than the bipy complexes especially at low temperature (Fig. 4 and Tables 1 and S2, ESI‡). The differences between E 00 of methyl and phenyl substituted derivatives of the bipy and phen complexes are 885 and 1375 cm−1 , respectively. 448 | Photochem. Photobiol. Sci., 2007, 6, 444–453

Emission spectroscopy in rigid media The fine structure of the emission spectra recorded at low temperature in glassy matrix is shown in Fig. 4. Methyl substitution results in a small blue shift, while phenyl substitution results in a pronounced red shift. These qualitative observations are reproduced by the data derived using a single-mode, Franck– Condon analysis of emission spectral profiles. The parameters estimated by this method are given in Table S2, ESI.‡ The data indicate that the Huang–Rhys factor (SM ) is sensitive to the structure of the diimine ligand also and to the substitution of two hydrogen atoms of the polypyridine ligand with either methyl or phenyl groups. The rigid phenanthroline ligand, 5a, has a lower value of SM compared with 1a. The effect of methyl and phenyl substitution is opposite; the methyl groups increase the SM parameter, while a decrease is observed with phenyl groups. The


Excited state resonance Raman

Fig. 4 Emission spectra in ethanol–methanol glass (4 : 1) at 77 K: (i), 1a (dashed), 2a (solid line), 3a (dash dot); 4 (dotted); (ii) 5a (dotted), 6 (dash dot dot dot), 7 (solid), 8 (dashed), 9a (dash dot).

full or partial deuteriation of the polypyridine ligand, leads to a very small blue shift in the emission spectra detected at 77 K, which has been confirmed by the E 0–0 data obtained by a singlemode Franck–Condon analysis of emission spectra. Transient absorption spectroscopy The transient absorption spectra of 1a–5a are similar to those of reported for their respective tris-homoleptic ([Ru(LL)3 )2+ ]) complexes.18 Strong absorption features at ∼360 nm and at ∼480 nm are observed for bipy complex (Fig. S3, ESI‡) and for other bipy derivatives assigned to an IL transition of the coordinated anion LL*− formed under MLCT excitation due to the promotion of an electron from t2g metal orbital to p* orbital of the diimine ligand, and an LMCT transition respectively. A relatively small band peaking at 720 nm is assigned as absorption of solvated electron. It is reasonable to suppose that this species is generated by two photonic process. Namely photon absorption by the long lived triplet excited complex *3 [RuIII (bipy− )(CN)4 ]2− leads to electron ejection from a higher energy excited state. The spectrum of the phen complex (Fig S3, ESI‡), which has higher signal/noise ratio than that of bipy complex due to the weak absorption of [Ru(phen)(CN)4 ]2− at the excitation wavelength, shows a blue shift of IL transition (kmax ≈ 330 nm) compared to IL band in the spectrum of bipy complex and a bleaching at 400 nm. The absorption features at longer wavelengths suggests the appearance of photo-products.

Transient resonance-Raman (TR2 ) spectra for 1a/1b, 2a/2b and 3a/3b were recorded in aqueous solution (Fig. 5, S4 and S5, ESI‡). As for the resonance-Raman spectra of the complexes (vide supra), deuteriation results in significant modification of the spectra. On comparing the spectra of 1a, 2a and 3a, there are two very striking characteristics. The first is the presence of several features, which are almost coincident with those observed in the ground state resonance-Raman spectra and secondly the shifts in peak position of the excited state Raman spectra, on substitution at the 4,4 position, are very different to changes observed in the ground state spectra. Only the characteristic features of the ligand anion radical at 1284, 1212 and 1016 cm−1 in the spectrum of 1a can be seen in those of 2a and 3a at 1288, 1211 and 1002 cm−1 and 1288, 1204 and 1017 cm−1 , respectively. At higher wavenumbers it is difficult to assign which peaks in the spectra of the disubstituted bipyridyls correspond to those in the spectrum of the unsubstituted bipyridyl species and this is particularly pronounced for 3a. On deuteriation of the polypyridyl ligand, this effect becomes even more marked; a tentative assignment of the shifts associated with the 4,4 substitution shows that in 1b features at 977, 1338 and 1508 cm−1 are shifted to 978, 1375 and 1538 cm−1 for 2b and 973, 1433 and 1571 cm−1 for 3b. With the possible exception of 3b, none of the excited state bands observed are found in the resonance-Raman ground state spectra. It proved difficult to obtain excited state resonance-Raman spectra of the complexes containing the perprotio and perdeuterio phenanthroline ligands, which may be attributable to the weak Raman cross-section of the ligand. However, transient absorption spectra indicate that 354.67 nm is not resonant with an excited state absorption of 5a/5b (vide supra) indicating that the weak signal is due to weak resonance with the excited state absorption. Indeed, almost all of the bands in 5a/5b are observed in the ground state spectra suggesting the excited state is only weakly populated in both cases. For 9a–9e there is a significant increase in the signal to noise ratio compared with the unsubstituted phenanthroline complexes, and no features can be resolved below ∼1200 cm−1 .

Fig. 5 TR2 spectra of 1a (lower trace) and 1b (upper trace) in H2 O, kexc = 354.67 nm.



Deuteriation of the phenanthroline 2,9 position leads to some changes in the TR2 spectrum and, as for the ground state resonance Raman, further ligand deuteriation to D12 diphenylphenanthroline (9c), leads to more pronounced differences. The same trend as for the ground state spectra is continued on going from D12 -phenanthroline to D14 and D16 . Comparing the spectra of 9d and 9e, we can see that there are marked similarities between both. Comparing 9a and 9b, the intensity of the strong band at 1401 cm−1 appears to be reduced and shifted to lower wavenumbers. The general shifting of peaks to lower wavenumbers on deuteriation can be seen from the 1440 cm−1 band in 9a which is found at 1437 cm−1 (9b), 1434 cm−1 (9c), 1431 cm−1 (9d) and 1428 cm−1 (9e). Luminescence quantum yields and lifetimes Luminescence quantum yields and lifetimes determined at ambient temperature using Ar saturated aqueous solutions and at 77 K in ethanol/methanol (4/1) glass are given in Table S3, ESI.‡The 3 MLCT states of the phenanthroline derivatives are found to be more emissive and longer lived than the bipyridine complexes. It is important to note that the lifetime of 9a is comparable with that of [Ru(phen)3 ]2+ . The luminescence quantum yields measured at ambient and at low temperature (∼77 K) of the phenanthroline derivatives are considerably higher than those of the bipyridine complexes, and the highest values were measured for 6 (φ lum = 0.044 at 25 ◦ C) and for 7 (φ lum = 0.043 at 25 ◦ C). The luminescence lifetimes of bipy derivatives are between 3 and 4 ls while the emission lifetime of the phen complexes are ∼10 ls or longer at 77 K (Table S3, ESI‡). The 77 K lifetimes are increased by an order of magnitude compared to those measured at room temperature. This may be rationalised by the presence of at least one temperature dependent nonradiative deactivation channel operating from the triplet excited state to the ground state. The temperature dependence of the emission lifetimes has been examined between 275 and 340 K in aqueous solution (Fig. 6). These values provide a solid data base for estimating the rate coefficient of non-radiative decay directly to the ground state and parameters of temperature dependent photophysical processes such as the pre-exponential factor and apparent activation energy. Deuteriation of diimine ligands results in a moderate (5–20%) increase in the lifetime of the triplet luminescent excited species both at room temperature and at low temperature (77 K), depending on the skeletal vibrational modes of the diimine ligand, on the nature of substituents and on the extent and position of the ligand deuteriation. Rate of photophysical processes The 3 MLCT excited state of Ru(II) polypyridyl complexes possessing three closely lying triplet emitting level11d generally decay via four different channels at room temperature: (a) by phosphorescence, (b) directly to the ground state via a nonradiative process, (c) through the 4th MLCT state of moderate triplet character11e,f and (d) through the metal centered 3 d–d state (i.e., there are two possible temperature dependent decay channels). However the lifetime data between 275 and 340 K for all [Ru(LL)(CN)]2− complexes investigated shows a relatively straightforward dependence on temperature. Hence a reasonable 450 | Photochem. Photobiol. Sci., 2007, 6, 444–453

Fig. 6 Temperature dependence of the luminescent lifetime in water: (i) 5a (䊊), 6 (), 7 (), 8 (), 9a (䉫); (ii) 5a (䊊), 5b (䊉), 9a (䉫), 9b (), 9e ().

fitting can be obtained by using eqn (1): 1 + exp −DE RT s= kd + A exp −DE RT

(1)

where kd = kph + knr , and A is the pre-exponential factor of the apparent potential barrier (DE). The rate of phosphorescence has been determined by using the luminescence quantum yield, and the observed decay lifetime (kph = φ lum /s). The three other parameters (knr , A and DE) have been estimated by the fitting procedure described elsewhere (see Table S4, ESI‡).3d The data obtained for the complexes of a given diimine ligand and its deuteriated derivative indicate a small difference between the activation parameters. In addition, it was pointed out that DE values estimated for the lifetime vs. temperature data series measured for a complex in water and in D2 O resulted in apparently the same activation energy.3d,19 Hence, it is reasonable to reduce the number of fitting parameters to one DE, three kd and three A parameters for three series of lifetime-temperature data including that measured for the complex of a given ligand LL and its deuteriated derivative in water and the complex dissolved in D2 O. In this way the number of fitted parameters is reduced by keeping the objective functions at approximately the same level.


The data obtained by this procedure are summarized in Table S5, ESI.‡ As expected, the data derived by this procedure are very similar to that of collected in Table S4, ESI.‡ It should be noted that the knr and A parameters given in Table S5, ESI,‡ indicate their dependence on the nature of the diimine ligand, the deuteriation of the diimine ligand and the effect of the replacement of water with D2 O, more clearly than that those estimated by three parameter fitting using single s–T data set. So one can conclude that the phenyl substitution either on bipy or phen results in a decrease in both knr and A, while the methyl substitution on bipy leads to an increase of knr and A. These substituents induce an opposite change in the apparent activation energy of the bipy complex, while the phenyl substitution of the phen complex does not change the activation barrier. However, it has been pointed out that it is better to regard the parameters A and DE as weighted averages of the frequency factors and potential barriers for deactivation through channels (c) and (d). It has also been demonstrated that using the lifetime data of 1a measured at various temperatures in CH3 OH, CH3 OD and CD3 OD19 the fitting by eqn (2) resulted in reasonable values for kd , A4th , DE 4th and DE dd by keeping Add as constant (1.5 × 1014 s−1 ). −DEdd 4th 1 + exp −DE + exp kB T kB T s= (2) −DE4th dd kd + A4th exp kB T + Add exp −DE kB T This procedure produced essentially the same DE 4th values (788– 791 cm−1 ) and very similar data for DE dd (4060–4260 cm−1 ) in all three solvents. Although the kph values derived by using eqn (2) were 5–15% higher and the knr data were smaller by 10–15% than that obtained using eqn (1), their dependence on solvent deuteriation indicated virtually similar trends as the data estimated by use of a simple single exponential expression (1). It was also apparent from this fitting that the DE 4th values do not depend on solvent deuteriation (788 < DE 4th < 791 cm−1 for MeOH and its deuteriated derivatives). Considering that the potential surface of various states in a given complex are generally not affected significantly by ligand deuteriation a procedure, very similar to the one used for CH3 OH, CH3 OD and CH3 OD, has been applied for s–T data measured for the series 9a–e. The same DE 4th and DE dd data were derived for the three series of data measured for 9a, 9b and 9e resulting in the characteristic photophysical parameters shown in (Table S6, ESI‡). The data provided in Tables S4–6, ESI,‡ are useful for discussion of the photophysics of [Ru(LL)(CN)4 ]2− complexes. Rate of phosphorescence The rates of radiative decay are very similar for all complexes and range between 2.2 × 104 and 7.7 × 104 s−1 . Apart from 6, the phen derivatives have slower radiative decay rates than the bipy complexes. Although there is a very small difference in kph values obtained for bipy, dmb and dpb complexes. Furthermore the highest radiative decay rate is obtained for dpb (3a) within the series of bipy derivatives, while it is the ph2 phen complex (9a), which has the slowest emission decay rate. The phosphorescence rate seems to be rather sensitive to the position of dimethyl substitution for phenanthroline derivatives. The complete or

partial deuteriation of diimine ligands results in no significant change in the rate of phosphorescence. Rate of non-radiative direct decay The non-radiative decays are more rapid for bipy than for phen based complexes. In both cases, phenyl substitution of the polypyridyl ligand results in a decrease in the rate of this decay. Methyl substitution leads to an insignificant change in the rate of non-radiative decay for bipy complex but a considerable effect is observed for phen complexes. In addition the rate of decay of dmphen complexes is dependent on the position of methyl groups. Substitution at the 5,6 positions leads to a slight increase in the rate of non-radiative decay with respect to the phen complex whereas knr is decreased for the 2,9-substituted isomer and this decrease is more pronounced for the 4,7-dmphen. For the bipy based complexes, the rate of non-radiative decay is found to decrease slightly on deuteriation of the ligand. In the case of the ph2 phen isotopologues, partial and complete deuteriation of the polypyridyl ligand results in a surprisingly small change in knr . Rate of temperature dependent processes It is reasonable to expect that the rate, and hence the efficiency of the temperature dependent deactivation processes would be influenced strongly by the electronic structure of the polypyridyl ligands. However using the three parameters (knr , A and DE) assumption it is rather difficult to deduce a relationship between the structure of the ligand and the rate or efficiency of the temperature dependent deactivation from the triplet excited state of [Ru(LL)(CN)4 ]2− complexes. On the other hand by deuteriation of LL and using deuteriated solvents the number of parameters can be reduced and more reliable knr , A and DE data can be obtained (compare the data given in Tables S4 and S5, ESI,‡ respectively). So it has been assumed that using three sets of lifetime vs. temperature data a reasonable estimation to obtain knr , A4th , DE 4th , Add and DE dd can be also performed by eqn (2). Table S6, ESI,‡ shows the data obtained by this procedure for 1a investigated in three various solvents, for phen measured in H2 O and D2 O and D8 -phen measured in water, and for three isotopologues of the ph2 phen complex. The data suggest that although both apparent energy gaps (DE 4th , DE dd ) are sensitive to the electronic structure of the LL ligand, it is DE dd which is strongly affected by the rigidity of the ligand and the degree of the delocalization of the electron promoted from the t2g orbital to the *p orbital of the diimine ligand on excitation. As the extent of delocalization of the electron promoted to the diimine ligand increases (bipy < phen < dpphen) the potential barrier for the 3 MLCT → 3 dd transition decreases due to changes in the displacement of the potential energy surfaces of these two excited states along the nuclear coordinates.

Conclusions The synthesis and the characterisation of two series of complexes of type [Ru(LL)(CN)4 ]2− are presented in this contribution. The properties of a series of 4,4 -substituted 2,2 -bipyridine as well as a series of 4,7-substituted 1,10-phenanthroline complexes have been compared. Substitution of the diimine ligand with groups of varying electron withdrawing properties is an effective tool for



tuning the energy level of MLCT excited states. The data clearly demonstrate that the luminescence spectra as well as the luminescence quantum yield and lifetime of the luminescent species can be tuned by modifying the skeleton and the substituents of LL. It has been concluded that because of the mixing of p*(CN) and p*(LL) the relative effects of these modifications on LL are more enhanced than the effects observed for [Ru(LL)2 (CN)2 ] and [Ru(LL)3 ]2+ complexes. In addition these complexes show strong solute–solvent interaction that provide an another very efficient tool for tuning the energy level of 3 MLCT. Hence the new [Ru(LL)(CN)4 ]2− complexes can be regarded as important basic building blocks of various molecular devices in the future.

Acknowledgements The authors thank Hungarian Scientific Research Fund (OTKA K63494), Enterprise Ireland, the EU Framework 5 IHP Training Network Susana Contract HPRN-CT-2002-00185 and the EPSRC (UK) GR/M45696 for financial support. K. R. also acknowledges ANDOR Technology Ltd. for financial support.

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