Photoinduced electron and energy transfer and pH-induced modulation of the photophysical properties in homo- and heterobimetallic complexes of ruthenium(II) and rhodium(III) based on a heteroditopic phenanthroline- terpyridine bridge

DOI: 10.1021/ic400449c

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Inorganic Chemistry p. 7933 - 7946 (2013)

Update date:2022-07-30

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Authors:

Maity, Dinesh

Bhaumik, Chanchal

Karmakar, Srikanta

Baitalik, Sujoy

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Article abstract of DOI:10.1021/ic400449c

Homo- and heterobimetallic complexes of compositions [(bpy) ₂Ru^II(phen-Hbzim-tpy)Ru^II(tpy/tpy-PhCH ₃/H₂pbbzim)]⁴⁺ and [(bpy)₂Ru ^II(phen-Hbzim-tpy)Rh^III(tpy-PhCH₃/H ₂pbbzim)]⁵⁺, where phen-Hbzim-tpy = 2-[4-(2,6-dipyridin-2- ylpyridin-4-yl)phenyl]-1H-imidazole[4,5-f][1,10]phenanthroline, bpy = 2,2′-bipyridine, tpy = 2,2′:6′,2″-terpyridine, tpy-PhCH₃ = 4′-(4-methylphenyl)-2,2′:6′,2″- terpyridine, and H₂pbbzim = 2,6-bis(benzimidazol-2-yl)pyridine, have been synthesized and characterized by elemental analyses, electrospray ionization mass spectrometry, and ¹H NMR spectroscopy. The absorption spectra, redox behavior, and luminescence properties of these bimetallic complexes have been thoroughly investigated and compared with those of monometallic [(bpy)₂Ru^II(phen-Hbzim-tpy)]²⁺ and [(tpy-PhCH₃)Rh^III(tpy-Hbzim-phen)]³⁺ model compounds. The electrochemistry of the complexes shows a reversible Ru ^II/III oxidation in the anodic region and an irreversible Rh ^III/I reduction and several ligand-based reductions in the cathodic region. Steady-state and time-resolved luminescence data at room temperature show that an efficient intramolecular electronic energy transfer from the metal-to-ligand charge-transfer (MLCT) excited state of the [(bpy) ₂Ru^II(phen-Hbzim-tpy)] chromophore to the MLCT state of the tpy-containing chromophore [(phen-Hbzim-tpy)Ru^II(tpy/tpy- PhCH₃/H₂pbbzim)] occurs in all three unsymmetrical homobimetallic complexes. On the other hand, for both heterometallic dyads, an efficient intramolecular photoinduced electron transfer from the excited ruthenium moiety to the rhodium-based unit takes place. The rate constants for the energy- and electron-transfer processes have been determined by time-resolved emission spectroscopy. The influence of the pH on the absorption, steady-state, and time-resolved emission properties of complexes has been thoroughly investigated. The absorption titration data were used to determine the ground-state pK values, whereas the luminescence data were utilized for determination of the excited-state acid dissociation constants. In effect, deprotonation of the azole NH moieties of the complexes leads to a substantial lowering of the MLCT absorption and emission band energies.

Full text of DOI:10.1021/ic400449c

Article

pubs.acs.org/IC

Photoinduced Electron and Energy Transfer and pH-Induced

Modulation of the Photophysical Properties in Homo- and

Heterobimetallic Complexes of Ruthenium(II) and Rhodium(III) Based

on a Heteroditopic Phenanthroline−Terpyridine Bridge

Dinesh Maity, Chanchal Bhaumik, Srikanta Karmakar, and Sujoy Baitalik*

Inorganic Chemistry Section, Department of Chemistry, Jadavpur University, Kolkata 700032, India

* Supporting Information

ABSTRACT: Homo- and heterobimetallic complexes of

compositions [(bpy)₂Ru^II(phen-Hbzim-tpy)Ru^II(tpy/tpy-

PhCH₃/H₂pbbzim)]⁴⁺and [(bpy)₂Ru^II(phen-Hbzim-tpy)-

Rh^III(tpy-PhCH₃/H₂pbbzim)]⁵⁺, where phen-Hbzim-tpy = 2-

[4-(2,6-dipyridin-2-ylpyridin-4-yl)phenyl]-1H-imidazole[4,5-

f ][1,10]phenanthroline, bpy = 2,2′-bipyridine, tpy = 2,2′:6′,2″-

terpyridine, tpy-PhCH₃= 4′-(4-methylphenyl)-2,2′:6′,2″-

terpyridine, and H₂pbbzim = 2,6-bis(benzimidazol-2-yl)-

pyridine, have been synthesized and characterized by elemental

analyses, electrospray ionization mass spectrometry, and H

NMR spectroscopy. The absorption spectra, redox behavior,

and luminescence properties of these bimetallic complexes

have been thoroughly investigated and compared with those of

monometallic [(bpy)₂Ru^II(phen-Hbzim-tpy)]²⁺and [(tpy-PhCH₃)Rh^III(tpy-Hbzim-phen)]³⁺model compounds. The electro-

chemistry of the complexes shows a reversible Ru^II/IIIoxidation in the anodic region and an irreversible Rh^III/Ireduction and

several ligand-based reductions in the cathodic region. Steady-state and time-resolved luminescence data at room temperature

show that an eﬃcient intramolecular electronic energy transfer from the metal-to-ligand charge-transfer (MLCT) excited state of

the [(bpy)₂Ru^II(phen-Hbzim-tpy)] chromophore to the MLCT state of the tpy-containing chromophore [(phen-Hbzim-

tpy)Ru^II(tpy/tpy-PhCH₃/H₂pbbzim)] occurs in all three unsymmetrical homobimetallic complexes. On the other hand, for both

heterometallic dyads, an eﬃcient intramolecular photoinduced electron transfer from the excited ruthenium moiety to the

rhodium-based unit takes place. The rate constants for the energy- and electron-transfer processes have been determined by

time-resolved emission spectroscopy. The inﬂuence of the pH on the absorption, steady-state, and time-resolved emission

properties of complexes has been thoroughly investigated. The absorption titration data were used to determine the ground-state

pK values, whereas the luminescence data were utilized for determination of the excited-state acid dissociation constants. In

eﬀect, deprotonation of the azole NH moieties of the complexes leads to a substantial lowering of the MLCT absorption and

emission band energies.

which has found maximum use, contains bidentate chelating

sites like 2,2′-bipyridine (bpy) or 1,10-phenanthroline (phen),

either directly coupled or linked by a spacer.¹The second type

of ligand, on the other hand, contains tridentate chelating sites

such as 2,2′:6′,2″-terpyridine (tpy).^1,4A large number of

mono-, bi-, and multimetallic complexes are now known,

notably built around bpy, phen, or tpy ligands, and have been

used for various applications.¹⁻⁷The choice of the bridging

ligands based on bpy-type chelating sites is appropriate from

electronic and photophysical viewpoints. In terms of structure,

however, such a choice is not ideal because substitution of a

single position in a bpy ligand will lead to the formation of

diastereomeric products. The tridentate tpy-type ligand is more

INTRODUCTION

■

Intramolecular electron- and energy-transfer processes in di-

and oligonuclear d⁶transition-metal polypyridine complexes

whose mononuclear entities are linked by conjugated bridging

ligands have received considerable attention because coopera-

tive interactions between metal centers in these complexes give

rise to properties that are useful for constructing photo-

molecular devices.¹⁻³The directional ﬂow of electron and/or

energy gets facilitated when there is asymmetry in the

multicomponent system. Such asymmetry can be introduced

by either using ligands that diﬀer in donor−acceptor properties

or using heterometallic sites. The features associated with

bridging ligands such as length, rigidity, topology, conjugation,

charge, and numbers of dissociable hydrogen atoms play

important roles in this context.¹⁻³Two types of polypyridine

bridging ligands are generally used for this study. The ﬁrst type,

Received: February 20, 2013

Published: June 25, 2013

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Chart 1

appealing from the viewpoint of constructing linear, rodlike

polynuclear complexes.⁴However, usually such complexes are

practically nonluminescent at room temperature and their

excited-state lifetimes (τ = 0.25 ns for [Ru(tpy)₂]²⁺)^1aare also

very short and therefore are the major deterrents for them to

act as photosensitizers. Consequently, much eﬀort has been

devoted to designing and synthesizing tridentate polypyridine

ligands that can produce ruthenium(II) complexes with

enhanced emission quantum yields and excited-state lifetimes.⁴

Compared to the tris-bidentate and bis-terdentate complexes,

rigidly linked homo- and heterometallic assemblies comprising

a combination of bis-terdentate and tris-bidentate complexes

are extremely rare, despite the very interesting properties that

could be expected from the unique attributes of each type of

complex.⁸⁻¹¹

Recently, we reported the synthesis, structural character-

ization, and photophysical and anion-sensing properties of

several terpyridylimidazole ligands and their ruthenium(II) and

osmium(II) complexes.¹²In our search for the appropriate bpy-

tpy- or phen-tpy-type bridging ligand, we have found the 2-[4-

(2,6-dipyridin-2-ylpyridin-4-yl)phenyl]-1H-imidazole[4,5-f ]-

[1,10]phenanthroline (phen-Hbzim-tpy) system, which has yet

to be exploited in the area for the construction of rigidly linked

homo- and heterometallic complexes comprising a combination

of bis-terdentate and tris-bidentate moieties (Chart 1). During

the course of our investigation, Hamelin and co-workers

synthesized a Ru-bpy-based dyad containing the same ligand

phen-Hbzim-tpy and showed that the complex upon photo-

excitation is able to catalyze selective sulﬁde oxygenation

involving an oxygen atom transfer from water to the

substrate.¹³In this contribution, we will demonstrate the

formation of unprecedented bimetallic systems comprising one

metal center (Ru^II) coordinated by three bidentate ligands (bpy

or phen) and a second metal center (Ru^IIor Rh^III) coordinated

by two tridentate (tpy/tpy-PhCH₃/H₂pbbzim) ligands. Selec-

tive incorporation of a Rh^IIIunit in this way is expected to be

diﬃcult owing to the very robust conditions required for

coordination. However, such units are highly attractive as

energy donors because of their high excited-state energies.

Moreover, all of the complexes have dissociable imidazole NH

proton(s). We will be interested in studying pH-induced

switching of the photophysical properties of these complexes

because the design and synthesis of complexes that reversibly

interchange between diﬀerent forms prompted by external

stimuli are currently of great interest in both fundamental and

applied research.¹⁴⁻¹⁶The pH (protonation/deprotonation)

can be used as an external stimulus for rapid switching of the

molecular properties toward the development of promising

functional molecules.^17,18Indeed, depending upon the extent of

perturbation of these properties, proton-driven molecular

switches can be developed.¹⁴⁻¹⁸The present study is

concerned with the synthesis, structural characterization,

redox activities, and pH-induced modulation of the photo-

physical properties of three new homodimetallic ruthenium-

(II)−ruthenium(II) and two heterodimetallic ruthenium(II)−

rhodium(III) complexes based on the use of heteroditopic

phen-Hbzim-tpy as a bridging ligand. As will be seen, signiﬁcant

changes in the absorbances and steady-state and time-resolved

luminescence properties of such complexes can be brought

about by changing the pH of the solution.

EXPERIMENTAL SECTION

■

Materials. Reagent-grade chemicals obtained from commercial

sources were used as received. Solvents were puriﬁed and dried

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according to standard methods. 1,10-Phenanthroline (phen),

2,2′:6′,2″-terpyridine (tpy), 2,6-pyridinedicarboxylic acid, and 1,2-

phenylenediamine were purchased from Sigma-Aldrich. 1,10-Phenan-

throline-5,6-dione,¹⁹4′-(p-methylphenyl)-2,2′:6′,2″-terpyridine (tpy-

PhCH₃),^12e,204′-(p-formylphenyl)-2,2′:6′,2″-terpyridine (tpy-

PhCHO),²¹2,6-bis(benzimidazol-2-yl)pyridine (H₂pbbzim),²²the

bridging 2-[4-(2,6-dipyridin-2-ylpyridine-4-yl)phenyl]-1H-imidazole-

[4,5-f ][1,10]phenanthroline (phen-Hbzim-tpy) ligand,¹³and cis-[Ru-

(bpy)₂Cl₂]·2H₂O²³were synthesized according to literature proce-

dures. AgClO₄was prepared from silver carbonate and perchloric acid

and recrystallized from benzene. [(tpy-PhCH₃)RuCl₃] and

[(H₂pbbzim)RuCl₃] were prepared by the reaction of RuCl₃·3H₂O

with tpy-PhCH₃and H₂pbbzim in a 1:1 molar ratio in reﬂuxing

ethanol. Again, [(tpy-PhCH₃)RhCl₃] and [(H₂pbbzim)RhCl₃] were

prepared by the reaction of RhCl₃·3H₂O with tpy-PhCH₃and

H₂pbbzim in a 1:1 molar ratio in reﬂuxing ethanol. The monometallic

ruthenium(II) complex of composition [(bpy)₂Ru(phen-Hbzim-

tpy)](ClO₄)₂was prepared by a slight modiﬁcation of a previously

published literature procedure.¹³

was redissolved in a minimum volume of acetonitrile and then

subjected to silica gel column chromatography (eluent: acetonitrile).

The eluent was rotary evaporated to a small volume (∼5 mL), and

anion-exchange reaction with NaClO₄gave rise to the desired

compound. The compound was ﬁnally recrystallized from an

acetonitrile−methanol (1:1, v/v) mixture in the presence of a few

drops of aqueous 10⁻⁴M perchloric acid (145 mg, yield 60%). Anal.

Calcd for C₆₉H₅₂N₁₄Cl₄O₁₈Ru₂: C, 48.49; H, 3.07; N, 11.47. Found: C,

48.47; H, 3.09; N, 11.49. H NMR (300 MHz, DMSO-d₆, δ/ppm):

14.63 (s, 1H, NH imidazole), 9.57 (s, 2H, H3′), 9.19−9.11 (m, 6H,

2H3′ + 4H6), 8.91−8.85 (m, 6H, 4H3″ + 2H9), 8.75−8.67 (m, 4H,

2H7 + 2H8), 8.56 (t, 1H, J = 8.0 Hz, H4″), 8.24 (t, 2H, J = 7.8 Hz,

H4), 8.16−7.98 (m, 8H, 2H4 + 4H4′ + 2H6′), 7.87 (d, 2H, J = 5.3 Hz,

H6′), 7.63−7.55 (m, 4H, 2H5′ + 2H11), 7.48 (d, 2H, J = 5.3 Hz,

H10), 7.38 (t, 2H, J = 6.5 Hz, H5′), 7.33−7.22 (m, 6H, 4H3 + 2H5),

7.15 (t, 2H, J = 6.3, H5). ESI-MS (positive, CH₃CN) m/z 318.89

(100%) [(bpy)₂Ru(phen-Hbzim-tpy)Ru(tpy)]⁴⁺, 424.89 (22%)

[(bpy)₂Ru(phen-bzim-tpy)Ru(tpy)]³⁺. UV−vis [CH₃CN; λ_max/nm

(ε/M⁻¹cm⁻¹)]: 486 (35660), 460 (sh, 32250), 365 (br, 35750),

332 (sh, 49660), 308 (sh, 71410), 286 (100750).

[(bpy)₂Ru(phen-Hbzim-tpy)Ru(tpy-PhCH₃)](ClO₄)₄·2H₂O (3). This

compound was prepared in the same way as 2 using [(tpy-

PhCH₃)RuCl₃] as the starting material. Yield: 65%. Anal. Calcd for

C₇₆H₅₈N₁₄Cl₄O₁₈Ru₂: C, 50.73; H, 3.25; N, 10.90. Found: C, 50.71;

H, 3.28; N, 10.92. ¹H NMR (500 MHz, DMSO-d₆, δ/ppm): 14.72 (s,

1H, NH imidazole), 9.54 (s, 2H, H3′), 9.43 (s, 2H, H3′), 9.19 (d, 2H,

J = 7.5 Hz, H6), 9.09 (m, 4H, 2H6 + 2H9), 8.86 (d, 2H, J = 7.5 Hz,

H3″), 8.83 (d, 2H, J = 8.0 Hz, H3″), 8.70 (nr, 4H, 2H7 + 2H8), 8.34

(nr, 2H, H8), 8.22 (t, 2H, J = 7.5 Hz, H4), 8.12−8.05 (m, 8H, 4H4′ +

2H6′ + 2H4), 7.98 (br, 2H, H6′), 7.87 (d, 2H, J = 5.0 Hz, H11), 7.64

(nr, 2H, H10), 7.57−7.55 (m, 6H, 4H3 + 2H7), 7.36 (t, 4H, J = 6.5

Hz, H5′), 7.29−7.27 (m, 4H, H5), 2.50 (s, 3H, CH₃). ESI-MS

(positive, CH₃CN): m/z 341.26 (100%) [(bpy)₂Ru(phen-Hbzim-

tpy)Ru(tpy-PhCH₃)]⁴⁺, 454.70 (18%) [(bpy)₂Ru(phen-bzim-tpy)Ru-

(tpy-PhCH₃)]³⁺. UV−vis [CH₃CN; λ_max/nm (ε/M⁻¹cm⁻¹)]: 492

(45160), 458 (br, 34580), 364 (br, 33830), 331 (br, 66160), 310 (sh,

99080), 286 (141160).

Preparation of the Ligand phen-Hbzim-tpy. tpy-PhCHO (337

mg, 1.00 mmol), 1,10-phenanthroline-5,6-dione (230 mg, 1.10 mmol),

and ammonium acetate (1.6 g, 20 mmol) were stirred in acetic acid

(30 mL), and the mixture was reﬂuxed for 2 h with continuous stirring.

Upon cooling to room temperature, a pale-yellow crystalline

compound deposited. The resulting compound was collected by

ﬁltration, washed several times with water, and then air-dried. The

compound was ﬁnally recrystallized from a chloroform−methanol

(1:1, v/v) mixture, and the desired compound was obtained as a light-

yellow crystalline solid (340 mg, 0.64 mmol, yield 65%). H NMR

(300 MHz, DMSO-d₆, δ/ppm): 13.93 (s, 1H, NH imidazole), 9.04 (s,

2H, H3′), 8.95 (d, 2H, J = 7.5 Hz, H6), 8.82−8.75 (m, 2H, H9), 8.68

(d, 2H, J = 7.8 Hz, H3), 8.49 (d, 2H, J = 6.6 Hz, H8), 8.22 (d, 2H, J =

7.8 Hz, H7), 8.04 (t, 2H, J = 7.6 Hz, H4), 7.84 (br, 4H, 2H10 +

2H11), 7.54 (t, 2H, J = 5.8 Hz, H5). ESI-MS: m/z 528.22 ([L + H]⁺).

Anal. Calcd for C₃₄H₂₁N₇: C, 77.40; H, 4.01; N, 18.58. Found: C,

77.38; H, 4.03; N, 18.55.

Synthesis of the Metal Complexes. The complexes were

prepared under oxygen and moisture-free dinitrogen using standard

Schlenk techniques.

[(bpy)₂Ru(phen-Hbzim-tpy)Ru(H₂pbbzim)](ClO₄)₄·2H₂O (4).

[(H₂pbbzim)RuCl₃] (73 mg, 0.14 mmol) was suspended in ethylene

glycol (30 mL) and heated at 100 °C with continuous stirring. To the

suspension was added 1 (180 mg, 0.15 mmol), and the reaction

mixture was again heated at 180 °C for 12 h. The resulting solution

was cooled, and the perchlorate salt of the complex was precipitated by

pouring the solution into an aqueous solution of NaClO₄·H₂O (1.0 g

in 10 mL of water). The precipitate obtained was ﬁltered, washed with

water, and dried under vacuum. The compound was then puriﬁed by

silica gel column chromatography using a mixture of CH₃CN and 10%

aqueous KNO₃(10:1, v/v) as the eluent. Subsequent anion-exchange

reaction with NaClO₄·H₂O gave rise to the desired compound. The

compound was ﬁnally recrystallized from an acetonitrile−methanol

[(bpy)₂Ru(phen-Hbzim-tpy)](ClO₄)₂·4H₂O (1). To a stirred suspen-

sion of cis-[Ru(bpy)₂Cl₂]·2H₂O (0.52 g, 1 mmol) in ethanol (50 mL)

was added solid AgClO₄(0.43 g, 2.1 mmol). After 0.5 h, the

precipitated AgCl was removed by quick ﬁltration, and to the ﬁltrate

containing [Ru(bpy)₂(EtOH)₂]²⁺was added solid phen-Hbzim-tpy

(0.64 g, 1.2 mmol). The resulting mixture was then reﬂuxed for 5 h,

during which time the color changed from blood red to reddish-yellow.

Upon cooling to room temperature, a red precipitate was obtained,

ﬁltered, and dried under vacuum. The compound was then puriﬁed by

recrystallization from an acetonitrile−methanol (1:1, v/v) mixture in

the presence of a few drops of aqueous 10⁻⁴M perchloric acid (1.03 g,

yield 85%). Anal. Calcd for C₅₄H₄₅N₁₁Cl₂O₁₂Ru: C, 53.52; H, 3.74; N,

12.71. Found: C, 53.50; H, 3.72; N, 12.69. ¹H NMR [300 MHz,

DMSO-d₆, δ/ ppm; see Scheme S1 (Supporting Information) for

proton numbering]: 14.59 (s, 1H, NH imidazole), 9.17 (d, 2H, J = 8.1

Hz, H3″), 8.90−8.79 (m, 8H, 2H3″ + 2H3′ + 2H6 + 2H9), 8.71 (d,

2H, J = 7.8 Hz, H3), 8.56 (d, 2H, J = 8.2 Hz, H8), 8.30 (d, 2H, J = 8.3

Hz, H7), 8.22 (t, 2H, J = 7.7 Hz, H4), 8.14−8.04 (m, 6H, 2H4′ +

2H10 + 2H11), 7.95 (br, 2H, H4′), 7.85 (d, 2H, J = 5.3 Hz, H6′),

7.62−7.55 (m, 6H, 2H5′ + 2H6′ + 2H5), 7.36 (t, 2H, J = 6.5 Hz,

H5′). UV−vis [CH₃CN; λ_max/nm (ε/M⁻¹cm⁻¹)]: 460 (19750), 426

(sh, 15330), 326 (br, 51330), 288 (112250).

[(bpy)₂Ru(phen-Hbzim-tpy)Ru(tpy)](ClO₄)₄·2H₂O (2). A mixture of

[Ru(tpy)Cl₃] (62 mg, 0.14 mmol), AgBF₄(92 mg, 0.47 mmol), and

30 mL of acetone were reﬂuxed with continuous stirring for 4 h. After

the solution cooled to room temperature, the precipitated AgCl was

removed by ﬁltration. A total of 40 mL of EtOH was then added to the

ﬁltrate. Acetone was removed by rotary evaporation. To the resulting

solution was added an ethanol solution of complex 1, and the solution

was reﬂuxed for 6 h. Upon cooling, the deep-red compound that

deposited was ﬁltered and then dried under vacuum. The compound

(1:1, v/v) mixture in the presence of a few drops of aqueous 10⁻⁴

perchloric acid (168 mg, yield 63%). Anal. Calcd for

C₇₃H₅₄N₁₆Cl₄O₁₈Ru₂: C, 49.06; H, 3.04; N, 12.54. Found: C, 49.04;

H, 3.06; N, 12.56. ¹H NMR (300 MHz, DMSO-d₆, δ/ppm): 15.05 (s,

2H, NH imidazole), 14.67 (s, 1H, NH imidazole), 9.67 (s, 2H, H3′),

9.20 (d, 2H, J = 7.6 Hz, H6), 9.04 (d, 2H, J = 8.1 Hz, H9), 8.91−8.85

(m, 6H, 2H7 + 2H8 + 2H11′), 8.79−8.71 (m, 4H, H3″), 8.63 (t, 1H, J

= 7.9 Hz, H10′), 8.24 (t, 2H, J = 7.8 Hz, H4), 8.16−8.11 (m, 4H, 2H4′

+ 2H11), 8.00−7.95 (m, 4H, 2H4′ + 2H10), 7.87 (d, 2H, J = 5.3 Hz,

H6′), 7.68−7.59 (m, 8H, 2H3 + 4H5′ + 2H6′), 7.52 (d, 2H, J = 5.4

Hz, H12), 7.38 (t, 2H, J = 6.5 Hz, H5), 7.27 (t, 2H, J = 6.4 Hz, H13),

7.00 (t, 2H, J = 7.4 Hz, H14), 6.07 (d, 2H, J = 8.1 Hz, H15). ESI-MS

(positive, CH₃CN): m/z 338.32 (100%) [(bpy)₂Ru(phen-Hbzim-

tpy)Ru(H₂pbbzim)]⁴⁺, 450.79 (78%) [(bpy)₂Ru(phen-Hbzim-tpy)-

Ru(Hpbbzim)]³⁺, 675.71 (10%) [(bpy)₂Ru(phen-Hbzim-tpy)Ru-

(pbbzim)]²⁺. UV−vis [CH₃CN; λ_max/nm (ε/M⁻¹cm⁻¹)]: 480

(33660), 425 (sh, 22580), 373 (br, 44000), 348 (sh, 79830), 334

(sh, 76750), 315 (sh, 78500), 286 (128160).

[(tpy-PhCH₃)Rh(tpy-Hbzim-phen)](BF₄)₃·3H₂O (5). A solution of

[(tpy-PhCH₃)Rh(acetone)₃]³⁺was prepared by stirring a mixture of

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Scheme 1. Synthesis of Homo- and Heterometallic Complexes

[(tpy-PhCH₃)RhCl₃] (75 mg, 0.14 mmol) and AgBF₄(92 mg, 0.47

mmol) in acetone (30 mL) for 4 h and removing the AgCl precipitate.

The ﬁltrate was added slowly to a methanol−chloroform solution of

phen-Hbzim-tpy (120 mg, 0.23 mmol), and the solution was reﬂuxed

for 3 h, during which time the color of the solution changed from light

yellow to orange. Upon cooling, the yellow-orange compound that

deposited was collected by ﬁltration. The compound was puriﬁed by

silica gel column chromatography using acetonitrile−water (10:1, v/v)

as the eluent. The eluents were reduced to a small volume (∼5 mL)

when a microcrystalline compound deposited. Further puriﬁcation was

carried out by recrystallization of the compound from a methanol−

water (5:1, v/v) mixture in the presence of a few drops of aqueous

10⁻⁴M perchloric acid (98 mg, yield 55%). Anal. Calcd for

C₅₆H₄₄N₁₀B₃F₁₂O₃Rh: C, 53.03; H, 3.01; N, 11.04. Found: C, 53.01;

H, 3.03; N, 11.02. ¹H NMR (300 MHz, DMSO-d₆, δ/ppm): 14.62 (s,

1H, NH imidazole), 9.25 (s, 2H, H3′), 9.21−9.18 (m, 6H, 2H3′ +

4H6), 9.09−9.05 (m, 2H, H9), 8.58−8.50 (m, 8H, 4H7 + 4H8),

8.21−8.17 (m, 6H, 4H4 + 2H11), 8.08−7.98 (m, 6H, 4H3 + 2H10),

7.52 (d, 4H, J = 8.1 Hz, H5), 2.48 (s, 3H, CH₃). ESI-MS (positive,

CH₃CN): m/z 499.39 (100%) [(tpy-PhCH₃)Rh(phen-bzim-tpy)-

(Na)]³⁺. UV−vis [CH₃CN; λ_max/nm (ε/M⁻¹cm⁻¹)]: 355 (20750),

315 (55420), 287 (59000).

deposited upon standing overnight was ﬁltered. The compound was

then puriﬁed by silica gel column chromatography using acetonitrile as

the eluent. The eluents were reduced to a small volume (∼5 mL), and

to it was then added an aqueous solution of NaClO₄·H₂O when a red

crystalline compound deposited. Further puriﬁcation was carried out

by recrystallization of the compound from a mixture of MeCN and

MeOH (1:5) in the presence of a few drops of aqueous 10⁻⁴

perchloric acid (186 mg, yield 69%). Anal. Calcd for

C₇₆H₆₂N₁₄Cl₅O₂₄RuRh: C, 47.13; H, 3.23; N, 10.12. Found: C,

47.10; H, 3.24; N, 10.14. H NMR (300 MHz, DMSO-d₆, δ/ppm):

14.64 (s, 1H, NH imidazole), 9.71 (s, 2H, H3′), 9.62 (s, 2H, H3′),

9.27−9.25 (m, 4H, H6), 9.16 (br, 2H, H9), 8.87 (t, 4H, J = 9.5 Hz,

H3″), 8.73 (nr, 4H, 2H7 + 2H8), 8.48−8.41 (m, 4H, 2H7 + 2H8),

8.23 (t, 4H, J = 7.7 Hz, H4), 8.13 (t, 4H, J = 7.2 Hz, H4′), 8.00 (t, 4H,

J = 5.8 Hz, H6′), 7.86 (d, 4H, J = 5.2 Hz, 2H10 + 2H11), 7.67−7.59

(m, 8H, 4H3 + 4H5′), 7.37 (t, 4H, J = 6.4 Hz, H5), 2.52 (s, 3H, CH₃).

ESI-MS (positive, CH₃CN): m/z 273.21 (100%) [(bpy)₂Ru(phen-

Hbzim-tpy)Rh(tpy-PhCH₃)]⁵⁺, 341.50 (57%) [(bpy)₂Ru(phen-bzim-

tpy)Rh(tpy-PhCH₃)]⁴⁺. UV−vis [CH₃CN; λ_max/nm (ε/M⁻¹cm⁻¹)]:

460 (32500), 428 (sh, 30330), 370 (br, 56580), 288 (158330).

[(bpy)₂Ru(phen-Hbzim-tpy)Rh(H₂pbbzim)](ClO₄)₅·2H₂O (7). Start-

ing from [(H₂pbbzim)RhCl₃], the preparation of 7 was the same as

that described for 4, except the temperature and reﬂuxing time were

200 °C and 24 h, respectively. Yield: 55%. Anal. Calcd for

C₇₃H₅₄N₁₆Cl₅O₂₂RuRh: C, 46.43; H, 2.88; N, 11.87. Found: C,

[(bpy)₂Ru(phen-Hbzim-tpy)Rh(tpy-PhCH₃)](ClO₄)₅·4H₂O (6). To

an ethanol solution (20 mL) of [(tpy-PhCH₃)Rh(EtOH)₃]³⁺,

generated from 75 mg (0.15 mmol) of [(tpy-PhCH₃)RhCl₃], was

added a second ethanol solution (30 mL) of 1 (180 mg, 0.15 mmol).

The solution was reﬂuxed 10 h, after which it was allowed to evaporate

slowly at room temperature. The microcrystalline product that

46.40; H, 2.90; N, 11.85. H NMR (500 MHz, DMSO-d₆, δ/ppm):

15.07 (s, 1H, NH imidazole), 14.69 (s, 2H, NH imidazole), 9.83 (s,

2H, H3′), 9.18 (t, 4H, J = 8.7 Hz, H3″), 8.89 (d, 4H, J = 8.0 Hz, 2H6

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+ 2H11′), 8.86 (d, 2H, J = 8.5 Hz, H9), 8.77 (d, 2H, J = 8.0 Hz, H8),

8.73 (d, 2H, J = 6.5 Hz, H7), 8.36 (t, 1H, J = 7.7 Hz, H10′), 8.24 (t,

2H, J = 7.7 Hz, 2H4), 8.22−8.15 (m, 6H, 4H4′ + 2H11), 8.00 (t, 2H, J

= 6.2 Hz, 2H10), 7.88 (s, nr, 4H, H6′), 7.69 (d, 2H, J = 7.5 Hz, 2H3),

7.65 (d, 2H, J = 5.5 Hz, 2H12), 7.61 (t, 2H, J = 6.5 Hz, H5′), 7.58 (t,

2H, J = 6.5 Hz, H5′), 7.38 (t, 2H, J = 6.5 Hz, H5), 7.18 (s, nr, 2H,

H13), 7.01 (s, nr, 2H, H14), 6.10 (d, 2H, J = 7.5 Hz, H15). ESI-MS

(positive, CH₃CN): m/z 338.63 (18%) [(bpy)₂Ru(phen-Hbzim-

tpy)Rh(Hpbbzim)]⁴⁺, 451.16 (100%) [(bpy)₂Ru(phen-Hbzim-tpy)

Rh (pbbzim)]³⁺, 676.24 (68%)[(bpy)₂Ru(phen-bzim-tpy)Rh-

(pbbzim)]²⁺. UV−vis [CH₃CN; λ_max/nm (ε/M⁻¹cm⁻¹)]: 460

(25830), 386 (br, 48160), 332 (sh, 54410), 287 (141660).

Physical Measurements. Elemental (C, H, and N) analyses were

performed on a Perkin-Elmer 2400II analyzer. Electrospray ionization

mass spectra (ESI-MS) were obtained on a Micromass Qtof YA 263

mass spectrometer. ¹H and {¹H−¹H} COSY NMR spectra of the

complexes were obtained either on a Bruker Avance DPX 300 MHz or

a 500 MHz spectrometer using a DMSO-d₆solution.

Electronic spectra were recorded on a Shimadzu UV-1800

spectrophotometer. To determine the pK_avalues of the complexes,

spectrophotometric titrations were carried out with a series of

MeCN−aqueous buﬀer (3:2, v/v) solutions containing the same

amount of complex (10⁻⁵M) and the pH was adjusted in the range 2−

12. A Robinson−Britton buﬀer was used in the study.²⁴The pH

measurements were made with a Beckman Research model pH meter.

The pH meter responded reproducibly to variation of the hydrogen

ion concentration, and as such, the pH meter readings were referred to

as the pH. The individual pK values were evaluated from the two

segments of the spectrophotometric titration data and using the

equation

Experimental uncertainties were as follows: absorption maxima, 2

nm; molar absorption coeﬃcients, 10%; emission maxima, 5 nm;

excited-state lifetimes, 10%; luminescence quantum yields, 20%; redox

potentials, 10 mV.

RESULTS AND DISCUSSION

■

Synthesis and Characterization of the Complexes. The

general synthetic procedures for the homo- and heterobime-

tallic ruthenium(II) and rhodium(III) complexes are given in

Scheme 1. The key monometallic ruthenium(II) compound

[(bpy)₂Ru^II(phen-Hbzim-tpy)]²⁺has been prepared by a

modiﬁcation of the previously described procedure.¹³We

have reported earlier that in terms of the reaction rate and yield

the solvated cation [(bpy)₂Ru(EtOH)₂]²⁺, generated by

treating stoichiometric amounts of cis-[(bpy)₂RuCl₂] and

AgClO₄in ethanol, acts as a better precursor relative to cis-

[(bpy)₂RuCl₂] itself.^12,18Thus, the monometallic complex 1

was readily obtained by reacting [(bpy)₂Ru (EtOH)₂]²⁺with

the bridging ligand phen-Hbzim-tpy in the ratio 1:1 in reﬂuxing

ethanol. As an alternative approach, the same compound was

prepared previously by reacting phen-Hbzim-tpy directly with

[(bpy)₂RuCl₂].¹³However, the procedure took a considerably

longer time. Moreover, in our procedure, as the resulting

compound got precipitated upon cooling to room temperature,

the puriﬁcation process became simpler. This monometallic

precursor complex containing a free terpyridine site is allowed

to react with either [(tpy/tpy-PhCH₃)RuL₃] or [(tpy-PhCH₃)-

RhL₃] (L = ethanol) in reﬂuxing ethanol for 6 h for the

synthesis of the unsymmetrical homobimetallic ruthenium(II)−

ruthenium(II) complexes and 10 h for the heterobimetallic

ruthenium(II)−rhodium(III) compounds, respectively. [(tpy/

tpy-PhCH₃)RuL₃] or [(tpy-PhCH₃)RhL₃] (L = acetone or

ethanol) was obtained by ﬁrst replacing the chloride ligands of

the complexes [(tpy/tpy-PhCH₃)RuCl₃] or [(tpy-PhCH₃)-

RhCl₃] by solvent molecules using AgBF₄in reﬂuxing acetone

and then rotary evaporating with ethanol. On the other hand,

dehalogenation of the ruthenium(III) and rhodium(III)

precursors [(H₂pbbzim)RuCl₃] and [(H₂pbbzim)RhCl₃] with

AgBF₄in acetone was far less eﬃcient. Thus, both complexes

[(bpy)₂Ru^II(phen-Hbzim-tpy)Ru^II(H₂pbbzim)]⁴⁺and

[(bpy)₂Ru^II(phen-Hbzim-tpy)Rh^III(H₂pbbzim)]⁵⁺have been

straightforwardly prepared by reacting [(H₂pbbzim)RuCl₃] or

[(H₂pbbzim)RhCl₃] with 1 in ethylene glycol in the temper-

ature range 180−200 °C, followed by anion metathesis with

NaClO₄. All compounds were then puriﬁed by silica gel column

chromatography using the appropriate eluent. Finally, the

compounds were recrystallized from an acetonitrile−methanol

(1:1, v/v) mixture in the presence of a few drops of aqueous

10⁻⁴M perchloric acid to keep the imidazole NH proton(s)

intact. The compounds were characterized by elemental (C, H,

and N) analyses, ESI-MS, UV−vis, and ¹H NMR spectroscopic

measurements, and the results are given in the Experimental

Section. ESI-MS spectra of the complexes and their simulated

isotopic patterns are shown in Figures S1−S6 (Supporting

Information).

A − A₀

A_f− A₀

pH = pK − log

(1)

Steady-state emission spectra were recorded on a Perkin-Elmer

LS55 ﬂuorescence spectrophotometer. The room temperature spectra

were obtained in acetonitrile solutions, while the spectra at 77 K were

recorded in 4:1 (v/v) ethanol−methanol glass. Photoluminescence

titrations were carried out with the same sets of solutions as those

made with spectrophotometry. The quantum yields were determined

by a relative method using [Ru(bpy)₃]²⁺as the standard.

η ²

A_stdI_r

A_rI_std

Φ = Φ

std

(2)

std

Time-correlated single-photon-counting (TCSPC) measurements

were carried out for the luminescence decay of the complexes. For

TCSPC measurement, photoexcitation was made at 440 nm using a

picosecond diode laser (IBH Nanoled-07) in an IBH Fluorocube

apparatus. The lifetimes of the complexes were also recorded as a

function of the pH of the solution. The ﬂuorescence decay data were

collected on a Hamamatsu MCP photomultiplier (R3809) and were

analyzed by using IBH DAS6 software.

The electrochemical measurements were carried out with a BAS

epsilon electrochemistry system. A three-electrode assembly compris-

ing a platinum or glassy carbon working electrode, a platinum auxiliary

electrode, and a Ag/AgCl reference electrode was used. Cyclic

voltammetric (CV) and square-wave voltammetric (SWV) measure-

ments were carried out at 25 °C in an acetonitrile solution of the

complexes (approximately 1 mM), and the concentration of the

supporting electrolyte tetraethylammonium perchlorate was main-

tained at 0.1 M. The electrochemical measurements were carried out

with oxygen-free solutions made by purging with puriﬁed nitrogen.

The potentials measured were compensated for by the iR drop in the

cell. Under the experimental condition used, the reversible oxidation of

the ferrocene/ferrocenium ion was observed at 0.36 V. For variable-

pH electrochemical measurements, a MeCN−aqueous (3:2, v/v)

solution of the complexes was used. The E_1/2value of the ferrocene/

ferrocenium couple in the above solvent mixture at approximately pH

7 is 0.26 V.

¹H NMR Spectra. The H NMR spectra of the complexes

have been recorded in DMSO-d₆at room temperature, and the

assignments made for the observed chemical shifts, according

to the numbering (Figures S7 and S8, Supporting Information),

are listed in the Experimental Section. The spectral assignments

of the complexes have been made with the help of their

{¹H−¹H} COSY NMR spectra (Figures S9 and S10,

Supporting Information) and relative areas of the peaks and

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Table 1. Spectroscopic and Photophysical Data of 1−7

luminescence

298 K

77 K

λ_max

Φ/

k_r/10⁵

k_nr/10⁷

λ_max

compound

absorption λ_max/nm (ε/M⁻¹cm⁻¹

)

τ/ns

10⁻³

s⁻¹

k_en/k_et/s⁻¹

460 (19750), 426 (sh, 15330), 326 (br, 51330), 288 (112250)

607

657

151

297

19.59

0.46

589

650

0.31

0.26

486 (35660), 460 (sh, 32250), 365 (br, 35750), 332 (sh, 49660),

308 (sh, 71410), 286 (100750)

2.37,

15.68

1.54

6.42,

0.98

42.12,

6.36

5.71 × 10⁷

4.77 × 10⁷

6.44 × 10⁶

492 (45160), 458 (br, 34580), 364 (br, 33830), 331 (br, 66160)

310 (sh, 99080), 286 (141160)

660

685

455

608

606

2.83,

18.42

4.02

14.20,

2.18

35.19,

5.40

645

678

442

590

588

0.30

0.33

0.15

0.32

0.25

480 (br, 33660), 425 (sh, 22580), 373 (br, 44000), 348 (sh,

79830), 334 (sh, 76750), 315 (sh, 78500), 286 (128160)

10.71,

76.75

10.57

9.81,

1.37

9.23,

1.28

355 (20750), 315 (55420), 287 (59000)

1.15,

3.52

78.08 679, 222

80.11,

26.19

460 (32500), 428 (sh, 30330), 370 (br, 56580), 288 (158330)

460 (25830), 386 (br, 48160), 332 (sh, 54410), 287 (141660)

11.98,

148

105

88.15,

7.12

7.46,

0.66

1.56 × 10⁵

7.75 × 10⁶

9.56,

69.67

24.40

25.52,

3.50

10.20,

1.40

In CH₃CN. MeOH−EtOH (1:4, v/v) glass. k_encorresponds to the rate constant for energy transfer in 2−4, while k_etcorresponds to the rate

constant for electron transfer in 6 and 7.

taking into consideration the usual ranges of J values for bpy,

tpy in the bimetallic complexes shifts to a signiﬁcantly upﬁeld

region compared to the monometallic ruthenium(II) complex 1

because this proton lies above the shielding region of a pyridine

ring of the other tpy capping ligand.

Absorption Spectral Studies. The absorption spectral

characteristics of the homobimetallic ruthenium(II)−

ruthenium(II) and heterometallic ruthenium(II)−rhodium(III)

complexes were studied in acetonitrile solutions at room

temperature, and spectral data are catalogued in Table 1. For

purposes of comparison, the UV−vis absorption spectral

behaviors of phen-Hbzim-tpy and monometallic model

compounds 1 and 5 were studied under the same experimental

conditions.

tpy, tpy-PhCH₃, and H₂pbbzim.¹²The H NMR spectra for

complexes 1−4 are shown in Figure S7 (Supporting

Information), while those for complexes 5−7 are presented

in Figure S8 (Supporting Information). The H NMR spectra

of the complexes show the occurrence of a fairly large number

of resonances, some of which are overlapped with each other.

As may be seen in Figures S7 and S8 (Supporting

Information), all of the resonances in the complexes, barring

three, occur in the range of 7.00−9.25 ppm. Of the three

disparate signals, the one that appears as a singlet at 2.50 ppm

in 3 and at 2.52 ppm in 6 (not shown in Figures S7 and S8,

Supporting Information), accounting for three protons, is

clearly due to −CH₃protons of the coordinated tpy-PhCH₃

moiety. The second-highest ﬁeld resonance, which appears as a

doublet at 6.07 ppm for 4 and at 6.10 ppm for 7, is attributable

to H15 of the H₂pbbzim moiety, because this proton

experiences maximum shielding due to the anisotropic ring-

current eﬀect of the adjacent pyridine rings. The third distinct

signal, which is most downﬁeld-shifted, is observed either as a

singlet or as a broad feature in the region 14.67−15.07 ppm due

to the imidazole NH proton(s) of the coordinated phen-

Hbzim-tpy and H₂pbbzim ligands. The imidazole NH protons

are profoundly downﬁeld-shifted in all of the complexes due to

hydrogen bonding with DMSO-d₆. In the case of 4 and 7, the

occurrence of two distinct NH signals is observed in the region

between 14.67 and 15.07 ppm due to the presence of two

diﬀerent kinds of imidazole NH protons with diﬀerent chemical

environments. By a comparison of the chemical shifts of the

NH protons of 1−3, the peaks at 14.67 ppm in 4 and at 14.69

ppm in 7 can be attributed to NH of phen-Hbzim-tpy, while

the peaks observed at 15.05 ppm in 4 and at 15.07 ppm in 7 are

due to NH of H₂pbbzim.

Figure 1 shows the UV−vis absorption spectra of equimolar

solutions of the parent monometallic (1) and homobimetallic

Figure 1. UV−vis absorption spectra of equimolar solutions of

complexes 1−4 in acetonitrile at room temperature. The spectrum of

phen-Hbzim-tpy in dimethylformamide−acetonitrile (1:9, v/v) is also

included.

(2−4) ruthenium(II) complexes in acetonitrile. Complex 1

displays an electronic absorption spectrum containing bpy and

phen-Hbzim-tpy π → π* transitions at 288 and 326 nm and

overlapping Ru(dπ) → bpy and Ru(dπ) → phen-Hbzim-tpy

charge-transfer transitions (MLCT) between 426 and 460 nm.

Previously, it was reported that bpy and phen-Hbzim-tpy π →

π* transitions occurred at 285 and 330 nm, while Ru(dπ) →

bpy charge-transfer transitions (MLCT) occurred at 433 and

459 nm in 0.1 M sodium phosphate buﬀer (pH 6.8).¹³It is

well-known that ruthenium(II) polypyridine complexes of this

type typically display ligand-based π → π* transitions from

It is of interest to note that the H3′ protons, which appear as

a singlet at 8.89−8.79 ppm in 1, are considerably shifted to a

downﬁeld region in both the homo- and heterobimetallic

complexes when the free terpyridine site in 1 is coordinated

with a second metal center (Ru^IIand Rh^III). Moreover, the

presence of two closely situated singlets at 9.54 and 9.43 ppm

in 3 and at 9.71 and 9.62 ppm in 6 clearly indicates the two

diﬀerent chemical environments of the H3′ protons. It is also to

be noted that the H3 proton of the tpy moiety of phen-Hbzim-

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each polypyridine ligand in the UV region of the spectrum and

MLCT transitions for each acceptor ligand in the visible

region.^1−4,12In this broad-band system, the overlapping

contributions from two types of ligands cannot be distin-

guished, although the expectation is that the MLCT transition

involving the phen-Hbzim-tpy ligand should be at a slightly

lower energy than those involving bpy ligands.

Luminescence Spectral Studies. The photoluminescence

spectral behaviors of the complexes were studied in acetonitrile

solutions at room temperature and at 77 K in ethanol−

methanol (4:1, v/v) glass. Table 1 summarizes the emission

maxima, quantum yields, and lifetimes of homo- and

heterobimetallic complexes. The emission properties of the

model complexes 1 and 5 are also reported in Table 1. Figure 3

In the homobimetallic complexes, there is another intense

MLCT band in the lower-energy region in addition to the

MLCT bands for the monometallic complex 1, and this can be

assigned as Ru(dπ) → tpy/tpy-PhCH₃/phen-Hbzim-tpy

charge-transition transition(s). The MLCT band in the range

of 480−492 nm is typical of bis(terpyridine)ruthenium(II)

complexes.^4,12Upon coordination of a second Ru^IIcenter to

the parent monometallic complex 1, substantial stabilization of

the π* orbital of phen-Hbzim-tpy occurs, and this stabilization

is reﬂected in the red shift of the phen-Hbzim-tpy-based π →

π* transitions that now occur around 348 and 365 nm in the

bimetallic complexes compared to 284 and 338 nm in free

phen-Hbzim-tpy and at 288 and 326 nm in 1.

The UV−vis absorption spectra of the heterobimetallic

ruthenium(II)−rhodium(III) complexes (6 and 7) in an

acetonitrile solution at room temperature are shown in Figure

2. For the purpose of comparison, the spectra of 1 and 5 are

Figure 3. Photoluminescence spectra of 1−4 at room temperature in

acetonitrile (a) and at 77 K in methanol−ethanol (1:4, v/v) glass (b).

compares the photoluminescence spectra of equimolar

solutions of homobimetallic ruthenium(II) complexes 2−4,

along with the monometallic precursor complex 1 at both room

temperature and 77 K. Complex 1 displays a luminescence

spectrum with its maximum at 607 nm at room temperature

and at 589 nm at 77 K. Complex 1 was reported to be emissive

at room temperature with an emission maximum at 604 nm and

a lifetime of 812 ns in 0.1 M sodium phosphate buﬀer (pH

6.8),¹³typical of the ruthenium(II) polypyridine family.¹⁻⁴The

unsymmetrical bimetallic ruthenium(II) complexes (2−4), on

the other hand, upon excitation at their MLCT absorption

maxima, display a broad emission maximum in the range of

657−685 nm at room temperature and between 645 and 678

nm at 77 K. Luminescence in these complexes also occurs from

an MLCT state, which is, by comparison with a large number of

closely related complexes, predominantly triplet in charac-

ter.^1−4,12Upon coordination of a second Ru^IIcenter to the

remote tpy site of the bridging phen-Hbzim-tpy ligand, the

emission maximum in all of the bimetallic complexes is red-

shifted with concomitant quenching of the emission intensity

relative to the monometallic complex. The less intense and

lower-energy emission of the unsymmetrical bimetallic

complexes is explained by the presence of a Ru(tpy)₂-type

Figure 2. UV−vis absorption spectra of equimolar solutions of

complexes of 1 and 5−7 in acetonitrile and of the ligand phen-Hbzim-

tpy in dimethylformamide−acetonitrile (1:9, v/v) at room temper-

ature.

also presented in Figure 2. Complex 5 does not absorb in the

visible, whereas in the UV region, three principle bands with

maxima at 355, 315, and 287 nm can be identiﬁed. The

heterometallic complexes (6 and 7), on the other hand, show

intense bands in both the visible and UV regions. For 6 and 7,

the lowest-energy MLCT band practically coincides with that of

the ruthenium(II) model compound 1. Thus, the visible region

is characterized by the MLCT transitions of the ruthenium(II)

component. The UV region is dominated by ligand-centered

transitions of both the ruthenium(II) and rhodium(III)

components. Comparison of the spectra of 6 and 7 with that

of the rhodium(III) model complex (5) allows us to assign the

shoulders at 370 nm for 6 and 386 nm for 7, corresponding to

the distinct features of the rhodium(III) polypyridyl (tpy/phen-

Hbzim-tpy) component.^25,26Thus, the absorption spectra of

the heterobimetallic dyads are an approximate superposition of

those of the mononuclear models. The substantial additivity of

the spectroscopic properties of the molecular components in

the ruthenium(II)−rhodium(III) binuclear complexes (6 and

7), investigated in this study points toward a relatively weak

degree of metal−metal electronic coupling.²⁷⁻²⁹

chromophore. The small energy diﬀerence between MLCT

and ³MC states in ruthenium(II) tridentate polypyridine

complexes compared to the bidentate ligands is due to an ill-

ﬁtted octahedral arrangement, which, in turn, is responsible for

the poor room temperature luminescence properties of

Ru(tpy)₂-type complexes.^4,12Upon going from a ﬂuid solution

to frozen glass, the emission maxima get blue-shifted with

signiﬁcant increases of the intensities and quantum yields,

which are the characteristics of typical MLCT emitters.^1−4,12,18

At 77 K, each spectrum displays a well-deﬁned vibronic

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progression in the lower-energy region with spacing of ∼1401

cm⁻¹for 1, ∼1418 cm⁻¹for 2, ∼1403 cm⁻¹for 3, ∼1438 cm⁻¹

for 4, ∼1102 cm⁻¹for 6, and ∼1234 cm⁻¹for 7, which are

similar to those reported for [M(bpy)₃]²⁺, [M(tpy)₂]²⁺(M =

Ru^IIand Os^II), and the other polypyridine complexes of

ruthenium(II) and osmium(II) and can be attributed to

aromatic stretching vibrations of the ligands.^19,21For a better

understanding of the complete kinetic schemes of the excited-

state deactivation of these complexes at lower temperatures,

detailed temperature-dependent lifetime measurements includ-

ing those at 77 K will be required. Unfortunately, because of the

lack of such a facility, we are unable to address this issue

presently.

Table 2. Electrochemical Data for 1−7 in Acetonitrile

oxidation

compound

E_1/2(ox)/V

reduction E_1/2(red)/V

1.31

1.33

−1.34, −1.53, −1.78, −1.90

−1.19, −1.34, −1.43, −1.55, −1.89,

−2.17

1.32

−1.16, −1.34, −1.55, −1.88, −2.15

−1.13, −1.36, −1.50, −1.90, −1.98

−0.75, −1.20, −1.55, −1.99

−0.70, −1.34, −1.56, −1.88, −2.13

−0.61, −1.18, −1.35, −1.55, −1.85

1.36, 1.12

1.33

1.37

All potentials are referenced against the Ag/AgCl electrode with E_1/2

= 0.36 V for a Fc/Fc⁺couple. Reversible electron-transfer process

Photoluminescence spectra of ruthenium(II)−rhodium(III)

complexes (6 and 7) as well as 1, obtained upon visible MLCT

excitation on their absorbance-matched solutions at room

temperature as well as at 77 K, are presented in Figure 4. It is

with a platinum working electrode. E_1/2values obtained from SWV

using a glassy carbon electrode.

negative potential window (0 to −2.0 V) (Figures S11 and S12,

Supporting Information). In ruthenium(II) polypyridyl com-

plexes, the highest occupied molecular orbital (HOMO) is

normally localized on the metal center and oxidative processes

are therefore metal-based, whereas the lowest unoccupied

molecular orbital (LUMO) is usually ligand-based and the

reduction processes are therefore ligand-centered, in agreement

with literature data and the reversibility of most of the

processes.¹⁻⁴Thus, the oxidation at 1.31 V for 1 can been

assigned as a Ru^II/Ru^IIIprocess. Previously, an irreversible

oxidation behavior was reported in 0.1 M sodium phosphate

buﬀer (pH 6.8) for 1.¹³Reductively, it displays four couples

because of the successive reductions of both phen-Hbzim-tpy

and bpy ligands.

The homobimetallic complexes except 4 also exhibit one

reversible oxidation at slightly more positive potential

compared with 1 and ﬁve successive quasi-reversible or

irreversible reductions in the negative potential window (0 to

−2.3 V) (Figures S11 and S12, Supporting Information). Thus,

the reversible oxidation at 1.33 V for 2 and at 1.32 V for 3

against the Ag/AgCl reference electrode has been assigned as a

simultaneous two-electron Ru^II/Ru^IIIoxidation process. This is

quite obvious because the distance between the two Ru^II

centers is quite large. Complex 4, on the other hand, displays

two successive reversible one-electron oxidations at 1.36 and

1.12 V, with the ruthenium bound to the tpy site of the phen-

Hbzim-tpy ligand oxidizing at lower potential. It is of interest to

note that when the tpy site of the phen-Hbzim-tpy ligand in 1 is

coordinated to another Ru^IIcenter to form bimetallic

complexes, stabilization of the phen-Hbzim-tpy π* orbital is

observed. This leads to a shift to the more positive potential of

the phen-Hbzim-tpy-centered reductions.

The square-wave voltammogram of the rhodium(III) model

compound 5 shows four successive reduction waves in the

cathodic region (Figure S13, Supporting Information). The ﬁrst

reduction process occurring at −0.75 V can be assigned as the

reduction of the Rh^IIIcenter by comparing the reduction

potentials of other rhodium(III) polypyridine complexes.²⁵⁻²⁷

The poorly reversible character of this process is as expected on

the basis of the general redox behavior of rhodium(III)

polypyridine complexes.^29,30The remaining reduction couples

are due to the successive reductions of both phen-Hbzim-tpy

and bpy ligands as expected.

Figure 4. Photoluminescence spectra of 1, 6, and 7 at room

temperature in acetonitrile (a) and at 77 K in methanol−ethanol (1:4,

v/v) glass (b).

observed that an emission maximum at 608 nm for 6 and at 606

nm for 7 at room temperature and at 590 nm for 6 and at 588

nm for 7 at 77 K is observed for both heterometallic dyads

close to that of the ruthenium(II) model compound 1. Thus,

both ruthenium(II)−rhodium(III) complexes were found to

exhibit ruthenium-based emission. The excitation spectra of the

ruthenium(II)-based emission for the heterometallic dyads

matched very closely to their absorption spectra. Again a

comparison of the luminescence intensity and quantum yield of

1 with those of 6 and 7 indicates that signiﬁcant quenching of

the ruthenium(II)-based emission occurs in both heterometallic

dyads.

Electrochemical Properties. The electrochemical behav-

iors of homo- and heterometallic complexes were studied by

CV and SWV in an acetonitrile solution at room temperature.

For the purpose of comparison, the electrochemical behaviors

of the monometallic model compounds (1 and 5) were also

studied under the same experimental conditions. The relevant

electrochemical data are summarized in Table 2.

The heterobimetallic ruthenium(II)−rhodium(III) com-

plexes 6 and 7 exhibit one reversible oxidation and ﬁve

successive quasi-reversible reductions (Figures S13 and S14,

Supporting Information). In the anodic region, oxidation of the

Complex 1 is found to undergo one reversible oxidation at

1.31 V and in the positive potential window (0 to +1.6 V) and

four successive quasi-reversible or irreversible reductions in the

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Ru^IIcenter occurs at slightly more positive (1.33 V for 6 and

1.37 V for 7) potential compared with 1. The ﬁrst reduction

occurring at −0.70 V for 6 and at −0.61 V for 7 is quasi-

reversible, and the reversibility of both couples increases as the

scan rate is increased. By a comparison of the reduction

potential (−0.75 V) of the model compound 5 and taking into

account the irreversibility and the higher peak current for this

peak relative to the ruthenium oxidation, it can be concluded

that the ﬁrst reduction corresponds to the Rh^III/Rh^Icouple in

both 6 and 7. The Rh^III/Rh^Icouple is expected to be

irreversible because of generation of the d⁸rhodium(I) species,

which would prefer to be square planar and is thus expected to

undergo subsequent ligand loss.²⁹Both complexes also display

ﬁve successive reductions up to −2.3 V and can be assigned as

the reductions of phen-Hbzim-tpy as well as bpy/tpy-PhCH₃/

H₂pbbzim ligands. Thus, the electrochemical data indicate that

the mixed-metal systems (6 and 7) possess a ruthenium-based

HOMO and a rhodium-based LUMO. This would suggest that,

although the lowest-lying spectroscopic transition seen in the

electronic absorption spectroscopy of the heterometallic

complexes was a Ru → polypyridine charge-transfer transition,

excitation of these molecules to this state should be followed by

intramolecular electron transfer to the lower-energy rhodium

acceptor orbital. Because both 6 and 7 have a [(bpy)Ru^II(phen-

Hbzim-tpy)] chromophore, which has been shown to be

emissive at room temperature, it should be possible to use

emission spectroscopy and emission lifetime measurements to

probe the intramolecular electron-transfer quenching of the Ru

→ polypyridine charge-transfer excited state by the rhodium-

based electron acceptor.²⁷⁻²⁹

Intercomponent Energy Transfer in Unsymmetrical

Homobimetallic Ruthenium(II) Complexes. As was already

mentioned, the emission intensity, quantum yield, and lifetime

of the Ru^II-centered emission in all three bimetallic ruthenium-

(II) complexes (2−4) are considerably less compared to those

of the monometallic parent compound 1. This indicates that

the second Ru^IIcenter coordinated to the tpy site of phen-

Hbzim-tpy probably induces a quenching eﬀect on the

[(bpy)₂Ru^II(phen-Hbzim-tpy)]-centered component. In princi-

ple, two predominant paths exist for the quenching of

[(bpy)₂Ru(phen-Hbzim-tpy)]²⁺-based luminescence in the

unsymmetrical bimetallic complex: reductive electron-transfer

quenching of the [(bpy)₂Ru(phen-Hbzim-tpy)]²⁺-based excited

state by the Ru^IIcenter of the [(tpy/tpy-PhCH₃/H₂pbbzim)-

Ru(phen-Hbzim-tpy)]²⁺component of the molecule and

electronic energy transfer from the [(bpy)₂Ru(phen-Hbzim-

tpy)]²⁺chromophore to the tpy-containing chromophore. We

have estimated the extent of luminescence quenching of the

[(bpy)₂Ru(phen-Hbzim-tpy)]²⁺chromophore by measuring

the luminescence intensities of equimolar solutions of three

bimetallic ruthenium(II) complexes as well as the monometallic

parent compound 1, keeping identical experimental conditions

in all cases. The result obtained shows that about 99% of the

luminescence intensity of the [(bpy)₂Ru(phen-Hbzim-tpy)]²⁺

chromophore is quenched by the tpy-containing chromophore

[(tpy/tpy-PhCH₃/H₂pbbzim)Ru(phen-Hbzim-tpy)]²⁺in the

unsymmetrical bimetallic complexes 2−4. The excited-state

energy of the [(bpy)₂Ru(phen-Hbzim-tpy)]-based component

is 2.10 eV, while that of the [(tpy/tpy-PhCH₃/H₂pbbzim)Ru-

(phen-Hbzim-tpy)]-based component lies in the range of

1.83−1.92 eV, estimated from the emission maxima at 77 K in

the complexes (Figure 3b). Therefore, the free-energy change

for energy transfer from an excited-state [(bpy)₂Ru(phen-

Hbzim-tpy)]-based component to a ground-state [(tpy/tpy-

PhCH₃/H₂pbbzim)Ru(phen-Hbzim-tpy)]-based component

lies in the range of −0.18 to −0.27 eV. This thermodynamically

favored process accounts for the observed quenching.

The quenching, in principle, can take place by an electron-

transfer process also. However, to be feasible, the process has to

be thermodynamically favorable. The photoinduced oxidative

quenching in the unsymmetrical bimetallic complexes 2−4

*[(bpy)₂Ru^II(phen‐Hbzim‐tpy)Ru^II(tpy)]⁴⁺

→ [(bpy)₂Ru^III(phen‐Hbzim‐tpy)Ru^I(tpy)]⁴⁺

ΔG° = E°([(bpy)₂Ru^II(phen‐Hbzim‐tpy)Ru^II(tpy)]⁵⁺

/[(bpy)₂Ru^II(phen‐Hbzim‐tpy)Ru^II(tpy)]⁴⁺

)

− E°([(bpy)₂Ru^II(phen‐Hbzim‐tpy)Ru^II(tpy)]⁴⁺

/([(bpy)₂Ru^II(phen‐Hbzim‐tpy^•−)Ru^II(tpy)]³⁺

(3)

− E°°

is highly unfavorable because the free energy change according

to eq 3 is too high, lying in the range between +0.38 and +0.42

eV. Thus, evaluation of the redox potentials (Table 2) and

excited-state energies of the unsymmetrical bimetallic com-

plexes suggests that the electron-transfer process is energetically

uphill, while the excitation energy transfer is exergonic.

The luminescence decay proﬁles of the three unsymmetrical

bimetallic and monometallic precursor complexes in acetoni-

trile at room temperature are shown in Figure S15 (Supporting

Information). The room temperature luminescence decays of

the donor−acceptor complexes are double exponentials on

both the red and blue edges of the emission compared with the

monoexponential decay of the parent monometallic complex 1.

All three bimetallic complexes (2−4) also possess shorter

excited-state lifetimes than the parent monometallic complex 1.

The rate constant k_enfor energy transfer can be calculated using

eq 4

k_en= 1/τ − 1/τ⁰

(4)

where τ⁰is the emission lifetime of the monometallic precursor

complex 1 (151 ns) and τ is the luminescence lifetime of the

unsymmetrical bimetallic complexes (15.7−76.8 ns). Thus, the

values of k_enat room temperature lie in the range of 6.4 × 10⁶−

5.7 × 10⁷s⁻¹. The reason for such fast energy transfer is likely

due to overlap between the donor emission and acceptor

absorption. The energy-transfer process is also very eﬃcient in

low-temperature glasses. The structured luminescence charac-

teristic of the [(bpy)₂Ru(phen-Hbzim-tpy)]²⁺chromophore is

absent in the spectrum of the bimetallic complexes 2−4. In the

literature, intramolecular energy transfer of several types of

unsymmetrical bimetallic ruthenium(II) complexes derived

from heteroditopic bpy-tpy-type ligands has been reported.³¹

Energy-transfer processes occur in two ways, viz., Dexter-type³²

through-bond electron exchange or Forster-type³³through-

space Coulombic interaction. However, in most cases, it is not

easy to ascertain which particular mechanism plays the

dominant role. Electron exchange becomes important where

the spacer is either highly conjugated or relatively short, while

the Coulombic mechanism becomes important when either the

spacer is saturated or the transition dipoles are well-separated.³⁴

Photoinduced Electron Transfer in the Heterobime-

tallic Ruthenium(II)−Rhodium(III) Complexes. A compar-

ison of the emission intensity and quantum yield of the model

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monometallic ruthenium(II) complex 1 with those of the

heterometallic ruthenium(II)−rhodium(III) complexes 6 and 7

indicates that signiﬁcant quenching of the ruthenium(II)-based

emission occurs in both heterometallic dyads. The energy of

the MLCT triplet state is 2.26 eV for 6 and 2.30 eV for 7,

estimated from the energies at the intersection point of the

absorption and emission band of the dyads at room

temperature. The energy of the intercomponent charge-transfer

state can be obtained from electrochemical data of the dyads as

the diﬀerence in the potentials for oxidation of Ru^IIand for

reduction of Rh^III.²⁷⁻²⁹Thus, values of 2.03 eV for 6 and 1.98

eV for 7 are obtained from the redox data for the energy of the

charge-transfer state. No correction for electrostatic work terms

is required in this case because of the charge shift character

(*Ru^II−Rh^III→ Ru^III−Rh^II) of the process involved. Thus, the

driving force for electron-transfer quenching from the MLCT

state (*Ru^II−Rh^III) to the electron-transfer state (Ru^III−Rh^II) is

−0.23 eV for 6 and −0.32 eV for 7. The emission lifetimes of

the dyads were also measured at room temperature, and the

observed decays for the complexes are presented in Figure S15,

Supporting Information. For both dyads studied, the emission

decay was biexponential, involving a short-lived minor

component and a longer-lived major component. The

excited-state lifetimes of 6 and 7 are 148 and 69.67 ns,

respectively, compared to 151 ns for 1. The rate constant of

photoinduced electron transfer (k_et) can be calculated using eq

Figure 5. Changes in the absorption (a and b) and photoluminescence

spectra (c and d) of 3 with variation of the pH in acetonitrile−water

(3:2, v/v). The insets show the change of absorbance (a and b) and

luminescence (c and d) with the pH.

deprotonation process occurred between pH 7.50 and 11.00,

accompanying the following spectral features: the intensities of

the MLCT band at 493 nm and the valley at 398 nm increased

along with a small red shift, while the intensities of the band at

352 nm decreased. During the second deprotonation processes,

the successive absorption curves passed through two new

isosbestic points appearing at 372 and 330 nm. The ﬁrst

spectral change observed between pH 2.5 and 7.5 is probably

due to dissociation of the protons on the protonated imidazole

ring, while the second step can be assigned to deprotonation of

the proton on the neutral imidazole ring, as shown in Scheme

It is of interest to see the spectral changes for 4 because it has

a more complicated structure with respect to having diﬀerent

types of NH protons with diﬀerent chemical environments

compared to the above two complexes. The spectral changes

that occur for 4 as a function of the pH are shown in Figure 6.

At ﬁrst, upon an increase of the pH from 2.5 to 6.0, the band at

480 nm decreased slightly in intensity and two isosbestic points

at 430 and 504 nm appeared; the spectral changes observed

here are due to dissociation of one proton from the protonated

imidazole ring. As the pH is increased from 6.0 to 6.6, the

absorption maximum at 494 nm is progressively red-shifted to

511 nm, with isosbestic points arising at 510, 421, and 357 nm;

upon further increases of the pH between 6.6 and 7.6, the

bands are additionally red-shifted through a new set of

isosbestic points at 520 and 440 nm until the maximum at

514 nm appeared at pH 7.6. Thus, these second and third

deprotonation processes indicate that the two NH protons

associated with the H₂pbbzim moiety are successively

deprotonated in the pH range 6.0−7.6. The last deprotonation

step occurred between pH 7.60 and 10.50 probably because of

deprotonation of the neutral imidazole NH proton associated

with the phen-Hbzim-tpy moiety.

k_et= 1/τ([(bpy)₂Ru(phen‐Hbzim‐tpy)Rh(tpy‐PhCH₃)]⁵⁺

− 1/τ([(bpy)₂Ru(phen‐Hbzim‐tpy)]²⁺

(5)

This allows us to calculate k_etfor 6 as 1.56 × 10⁵s⁻¹and that

for 7 as 7.75 × 10⁶s⁻¹.

The emission properties of heterometallic dyads were also

studied in a rigid matrix at 77 K (4:1 EtOH−MeOH). Both

complexes exhibit a structured MLCT emission with a

maximum centered around 590 nm, and no quenching of the

typical MLCT ruthenium-based emission takes place, clearly

indicating that the electron-transfer process does not occur

under these experimental conditions. This behavior is in line

with what is expected to occur for processes that are slightly

exergonic in ﬂuid solution.^27,28

pH-Induced Modulation of the Photophysical Proper-

ties of the Complexes. Upon coordination of the metal

center(s) to the bridging phen-Hbzim-tpy ligand, imidazole

NH proton(s) of phen-Hbzim-tpy as well as the terminal

H₂pbbzim ligand in the complexes became appreciably acidic.

Inasmuch as the imidazole NH group becomes appreciably

acidic, we have been interested in studying the inﬂuence of the

pH on the absorption, steady-state, and time-resolved emission

behaviors of the complexes.

Spectrophotometric Studies of Protonic Equilibria.

Spectrophotometric titrations of the complexes have been

studied quantitatively in acetonitrile−water (3:2, v/v) solutions

over the pH range 2.5−12. The spectral changes that occur for

2 and 3 with variation of the pH are shown in Figures 5 and

S16 (Supporting Information).

It is clear that both complexes underwent two successive

deprotonation processes over the pH range 2.5−12.0. Upon an

increase of the pH of 3 from 2.5 to 7.5, the band at 495 and 310

nm and the valley at 400 nm decreased, while the shoulder at

353 nm increased slightly in intensity with the appearance of

two isosbestic points at 373 and 328 nm. The second

Upon close inspection of the changes in the spectral proﬁles

with increases of the pH, the occurrence of four successive

deprotonation steps becomes evident, as shown in Scheme 3

over the pH range 2.5−12.0.

The changes that occur for 6 over the pH range 2.5−12.0 are

shown in Figure S17 (Supporting Information). Similar to 2

and 3, this complex also undergoes two successive deprotona-

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Scheme 2

tion processes over the pH range 2.5−12.0, as shown in

Scheme 2.

Finally, the absorption spectral changes that occur for 7 with

variation of the pH are shown in Figure 7. In contrast to 4, the

complex underwent three successive deprotonation processes

over the pH range 2.5−12.0. As expected, the ﬁrst step is due to

deprotonation of the protonated imidazole NH of the phen-

Hbzim-tpy moiety. It is to be noted that dissociation of this

proton occurred at a lower pH compared with that of 4 because

of the increased positive charge on the complex. In contrast to

4, dissociation of two NH protons of the H₂pbbzim moiety

occurs in a single step. The last deprotonation step is again

from the neutral imidazole moiety associated with the phen-

Hbzim-tpy ligand.

The individual pK values of complexes 2−4, 6, and 7 have

been evaluated using eq 1 from the diﬀerent segments of the

spectrophotometric titration data and are presented in Table 3.

Clearly, the pK values of the heterometallic complexes 6 and 7

are less compared with their homometallic analogues 2−4

because of the relatively more electron-withdrawing nature of

rhodium(III) than that of ruthenium(II). It is of interest to

note that, with deprotonation of the coordinated azole ligands,

red shifts of the MLCT bands occur to some extent for all

complexes. This is an expected trend because, with an increase

of the electron density at the metal center due to the azole

ligand deprotonation, it becomes easier to transfer an electron

to the acceptor π * orbitals of the polypyridine ligands.

Figure 6. Changes in the absorption (a−d) and photoluminescence

(e−h) spectra of 4 with variation of the pH in acetonitrile−water (3:2,

v/v). The insets show the change of absorbance (a−d) and

luminescence (e−h) with the pH.

Scheme 3

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the contrary, the same complex acted as “on−oﬀ” emission

switch with an emission quenching factor of ∼1.4 over the pH

range of 7.5−12.0.

Parts e−h in Figure 6 show the eﬀects of variation of the pH

on the luminescence spectral proﬁles of 4. Similar to the

absorption spectra, photoluminescence spectral proﬁles also

show that the complex underwent four successive deprotona-

tion processes over the pH range 2.5−12.0. When the pH is

increased from 2.5 to 6.0, an increase in the emission intensity

at 693 nm occurs (Figure 6e). On the other hand, upon an

increase in the pH from 6.0 to 12.0, the emission intensities of

the complex were found to decrease gradually in three

successive steps (Figure 6f−h), with the emission maximum

red-shifted from 693 to 818 nm. Complete quenching of the

luminescence intensity of the complex occurs at pH ∼11.00.

The eﬀect of an increase of the pH on the luminescence

spectrum of 6 is shown in Figure S17c,d (Supporting

Information), and the inset shows enhancement and quenching

of the luminescence intensity at 612 nm as function of the pH

of the solution. Again, similar to the bimetallic ruthenium(II)

complexes, it is seen that, in the low pH range of 2.5−6.0,

enhancement of the luminescence intensity of the band at 612

nm occurs. On the other hand, upon an increase of the pH

from 6.0 to 12.0, the emission intensities were found to

decrease gradually, with the emission maximum red-shifted

from 612 to 624 nm. Thus, the emission versus pH proﬁle in

this case again consists of two reverse proﬁles corresponding to

two successive excited-state deprotonation processes of the

coordinated phen-Hbzim-tpy moiety in the complexes over the

pH region from 2.5 to 12.0.

The emission spectral changes of 7 as a function of the pH

are shown in Figure 7d−f. Upon an increase of the pH from 2.5

to 5.5, the intensity of the emission maximum at 610 nm

increased by about 131%. By contrast, when the pH is increased

from 5.5 to 7.8, a signiﬁcant decrease (48%) of the emission

intensities occurs around 610 nm. Upon a further increase of

the pH from 7.8 to 9.5, again a sharp decrease in the emission

intensity was observed and the emission maximum was slightly

red-shifted from 610 to 625 nm. Clearly, the emission spectral

changes are associated with three excited-state deprotonation

processes, and each of the processes dealt with the same

proton(s) as UV−vis spectral titrations. The changes of the

relative intensities versus pH are shown in the insets of Figure

7d−f.

Figure 7. Changes in the absorption (a−c) and photoluminescence

(d−f) spectra of 7 with variation of the pH in acetonitrile−water (3:2,

v/v). The insets show the change of absorbance (a−c) and

luminescence (d−f) with the pH.

Spectroﬂuorometric Studies of Protonic Equilibria. It

has been observed that deprotonation of the imidazole NH

protons of the complexes shifts the MLCT bands to higher

wavelengths, and it has been of interest to us to ﬁnd out the

extent of changes that occur in the excited states of the

complexes upon deprotonation. Photoluminescence titrations

of the homo- and heterobimetallic complexes with variation of

the pH were carried out in the same way as was already

described for spectrophotometric measurements.

The eﬀect of the pH on the luminescence spectra of 2 and 3

is already shown in Figures 5 and S16 (Supporting

Information). When the pH is increased from 2.5 to 7.5,

signiﬁcant increases in the emission intensities occur (Figures

5c and S16c, Supporting Information) in both complexes. On

the other hand, upon increasing the pH from 7.5 to 12.0, the

emission intensities were found to decrease gradually with a

small blue shift of the emission maximum. Thus, the emission

versus pH proﬁle consists of two reverse proﬁles corresponding

to two successive excited-state deprotonation processes of the

coordinated phen-Hbzim-tpy moiety in the complexes over the

pH region from 2.5 to 12.0. It is of interest to note that the

complex acted as an “oﬀ−on” emission switch with an emission

enhancement factor of ∼3.0 over the pH region of 2.5−7.5. On

The eﬀect of the pH on the luminescence lifetimes of the

complexes is shown in Figures 8 and S18−S21 (Supporting

Information). All bimetallic complexes exhibited biexponential

decays compared with the monoexponential decay for the

parent monometallic ruthenium(II) complex 1. The lifetime

versus pH proﬁles of all complexes consist of two reverse

proﬁles over the pH range 2.5−12.0. When the pH is increased

from 2.5 to 7.5, an increase in the luminescence lifetimes occurs

Table 3. Ground- and Excited-State pK Values of 2−4, 6, and 7 in CH₃CN−H₂O (3:2, v/v)

ground state

excited state

compound

pK₁

pK₂

pK₃

pK₄

pK₁*

pK₂*

pK₃*

pK₄*

5.66

5.55

5.04

4.38

4.32

9.33

9.18

6.14

7.97

7.02

7.21

8.81

9.09

5.36

4.76

4.85

4.63

3.76

9.58

9.20

6.39

8.15

7.21

7.55

8.83

9.12

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MLCT state is localized on phen-Hbzim-tpy (the excited state

has increased negative charge and is therefore less acidic).³⁶⁻³⁹

CONCLUSION

■

In conclusion, we have successfully designed a series of redox-

active and photoactive homobimetallic ruthenium(II)−

ruthenium(II) and heterobimetallic ruthenium(II)−rhodium-

(III) complexes by using a heteroditopic phenanthroline−

terpyridine bridge. The absorption spectra, redox behavior, and

luminescence properties of these bimetallic complexes have

been thoroughly investigated and compared with those of

monometallic model compounds. Steady-state and time-

resolved luminescence data at room temperature show that

an eﬃcient intramolecular electronic energy transfer from the

³MLCT excited state of the [(bpy)₂Ru^II(phen-Hbzim-tpy)]-

Figure 8. Change of the luminescence lifetimes of 7 with variation of

the pH in acetonitrile−water (3:2, v/v). The insets show the decay

proﬁles of 7 as a function of the pH.

in all complexes. On the other hand, upon an increase in the

pH beyond 7.5, the luminescence lifetimes were found to

decrease gradually. It may be mentioned that the intensity of

the steady-state emission maxima of the complexes increases

initially and then decreases gradually as the pH of the solution

is gradually increased. It may be mentioned that the

luminescence lifetimes of the complexes measured in a 3:2

(v/v) acetonitrile−water buﬀer solution are less compared to

the lifetimes of the complexes in a pure acetonitrile medium.

The spectrophotometric data were used previously to

determine the ground-state pK values of the complexes, and

the luminescence data can be utilized to determine the excited-

state acid dissociation constants (pK*). Because the pH ranges

coincide with those observed for ground-state deprotonation of

the coordinated phen-Hbzim-tpy as well as H₂pbbzim moieties

of the complexes in the UV−vis titration, the above

luminescence spectral changes are thus assigned to the

excited-state deprotonation of the same proton, as in the

ground state. Excited-state pK values could be obtained by

using eq 6 from the luminescence intensity and lifetime data

following the methods developed by Ireland and Wyatt.³⁵

based chromophore to the MLCT state of the tpy-containing

chromophore [(phen-Hbzim-tpy)Ru^II(tpy/tpy-PhCH₃/

H₂pbbzim)] occurs in all three unsymmetrical homobimetallic

complexes. On the other hand, for both heterometallic dyads,

an eﬃcient intramolecular photoinduced electron transfer from

the excited-state Ru^IImoiety to the Rh^III-based unit takes place.

The rate constants for the energy- and electron-transfer

processes have been determined by time-resolved emission

spectroscopy. Another point of interest is that the emitting

properties of the compounds are strongly dependent on the

protonation state of the imidazole rings. In this context, the

inﬂuence of the pH on the absorption, steady-state, and time-

resolved emission properties of the complexes has been

thoroughly investigated. This opens the possibility of the

application of such compounds as proton-driven molecular

switches toward the development of molecular-based electronic

and photonic devices.

ASSOCIATED CONTENT

■

* Supporting Information

ESI-MS (positive), H and H−¹H COSY NMR spectra, cyclic

and square-wave voltammograms, changes in the absorption

and photoluminescence spectra with variation of the pH, and

changes in the excited-state lifetimes with variation of the pH of

the complexes. This material is available free of charge via the

Internet at http://pubs.acs.org.

pK_a* = pH + log τ_acid/log τ_base

(6)

where the pH is the inﬂection point of the curve of the

emission intensity as a function of the pH. τ_acidand τ_base

correspond to the lifetimes of the protonated and deprotonated

states, respectively. The lifetime values are experimentally

obtained at pH levels well above and well below the midpoint,

where τ is relatively invariant with the pH. Excited-state pK*

values, calculated in this way, are summarized in Table 3. The

results indicate that the pK₁* values of the complexes are lower

than their respective pK₁values, while the pK₂* values are

higher compared to the pK₂values. A comparison of ground-

and excited-state acid dissociation constants is useful because

the relative change in the pK value provides qualitative

information on the localization of charge in the MLCT

states.³⁶⁻³⁹It is observed that the excited-state pK₁values of

the complexes are less than the ground-state pK₁values,

indicating that the excited-state electron in the complexes was

mostly delocalized over the bpy/tpy moiety rather than the

imidazole part of phen-Hbzim-tpy. The increased acidity in the

AUTHOR INFORMATION

Corresponding Author

*E-mail: sbaitalik@hotmail.com.

■

Notes

The authors declare no competing ﬁnancial interest.

ACKNOWLEDGMENTS

■

Financial assistance received from the Department of Science

and Technology, New Delhi (Grant SR/S1/IC-33/2010), is

gratefully acknowledged. Thanks are due to the DST for

providing the Time-Resolved Nanosecond Spectroﬂuorimeter

in PURSE programme at the Department of Chemistry of

Jadavpur University. D.M. and S.K. thank the CSIR for their

fellowship.

excited state of the complexes is a consequence of Ru^II

→

ligand charge transfer principally to ligands (bpy/tpy) not

involved in the proton transfer; there is a net drain of the

electron density from phen-Hbzim-tpy in the excited state via

electron donation to the hole of π symmetry on the ruthenium

atom. The second excited-state pK₂* value is higher than the

ground-state value for the same process, implying that the

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