Probing excited state charge transfer dynamics in a heteroleptic ruthenium complex.

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Cite this: Phys. Chem. Chem. Phys., 2014, 16, 219

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Probing excited state charge transfer dynamics in a heteroleptic ruthenium complex Rajib Ghosh and Dipak K. Palit*

Published on 15 October 2013. Downloaded on 09/10/2014 14:40:46.

Dynamics of metal to ligand charge transfer in the excited states of ruthenium polypyridyl complexes, which have shown promise as materials for artificial solar energy harvesting, has been of immense interest recently. Mixed ligand complexes are especially important for broader absorption in the visible region. Dynamics of ultrafast vibrational energy relaxation and inter-ligand charge transfer processes in the excited states of a heteroleptic ruthenium complex, [Ru(bpy)2(pap)](ClO4)2 (where bpy is 2,2 0 -bipyridine and pap is 2-(phenylazo)pyridine) have been investigated using femtosecond to nanosecond time-resolved transient absorption spectroscopic techniques. A good agreement between the TA spectrum of the lowest excited 3MLCT state of [Ru(bpy)2(pap)](ClO4)2 complex and the anion radical spectrum of the pap ligand, which has been generated using the pulse radiolysis technique, Received 12th September 2013, Accepted 15th October 2013 DOI: 10.1039/c3cp53886a

confirmed the charge localization at the pap ligand. While the lifetime of the inter-ligand charge transfer from the bpy to the pap ligand in the 3MLCT state is about 2.5 ps, vibrational cooling of the pap-localized 3

MLCT state occurs over a much longer time scale with a lifetime of about 35 ps. Ultrafast charge

localization dynamics observed here may have important consequences in artificial solar energy harvesting www.rsc.org/pccp

systems, which employ heteroleptic ruthenium complexes.

1. Introduction Charge transfer is a pivotal step in the conversion of solar energy to photochemical and photoelectrical energy in dye-sensitized solar cells, photoinduced water oxidation catalysis and artificial photosynthesis. Photoinduced charge-transfer processes in ruthenium(II) polypyridyl complexes have long been exploited for solar-energy harvesting due to strong metal to ligand charge transfer (MLCT) absorption in the visible region and relatively large stability in oxidized and reduced forms.1–6 Moreover, spectroscopic and photophysical properties can easily be tuned by introducing appropriate ligands. A great deal of effort has been spent to synthesize various ruthenium complexes containing different ligands in order to improve light absorption characteristics in the solar spectral region as well as to improve the charge transfer characteristics. Mixed ligand (heteroleptic) complexes found special importance for application in photovoltaic systems due to the broadened MLCT absorption envelope with increased absorption cross section and thus a more efficient use of the solar spectrum for photon energy conversion. In any of such molecular systems, effective use of the photoexcitation energy is possible only if efficient charge separation is achieved in the excited state. Thus, detailed understanding of metal to ligand charge transfer (MLCT) processes Radiation & Photochemistry Division, Bhabha Atomic Research Centre, Mumbai-400085, India. E-mail: [email protected]; Fax: +91-22-25505151; Tel: +91-22-25595091

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and excited-state relaxation dynamics of ruthenium polypyridyl complexes are of great importance, for both fundamental reasons and potential applications in solar energy conversion. A great deal of research effort has been devoted towards molecular level understanding of the real time excited state dynamics of the ruthenium polypyridyl complexes.7–15 Extensive studies on a prototype [Ru(bpy)3]2+ complex using time resolved spectroscopic techniques revealed ultrafast ISC (o50 fs) process followed by vibrational relaxation to populate the relaxed 3MLCT state in a few picosecond time domain.7–11 Another fundamental property is the localized nature of the electronic charge in the relaxed MLCT excited state. In the homoleptic D3 symmetric complexes like [Ru(bpy)3]2+, the excited state is expected to be symmetric and the charge transfer induced excited electron should be evenly shared between the three ligands. However, contrary to this expectation, it has been well established that the relaxed excited state is best described as one formally reduced bpy and two neutral ones (i.e. [Ru(III)(bpy)(bpy)2]2+) and thus charge is localized on one of three ligands and the charge localization occurs on a femtosecond timescale.12–21 Symmetry breaking via vibrational trapping or solvent fluctuation has been proposed as the possible mechanism for charge localization.22 Though ultrafast dynamics of the prototype [Ru(bpy)3]2+ have been extensively studied to discern ISC, vibrational relaxation and charge localization processes, real time dynamics studies of mixed ligand (heteroleptic) complexes have not been extensively undertaken and studies on such complexes has gained impetus

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in recent years.23–28 As mentioned earlier, heteroleptic ruthenium complexes have gained a lot of interest in photovoltaic systems due to the broadened MLCT absorption envelope covering the solar spectrum due to multiple MLCT bands corresponding to different ligands. Effective use of the photoexcitation energy depends on the temporal kinetics of charge distribution around different ligands. The ultrafast spectroscopy of low-symmetry complexes is therefore of special interest not only for addressing fundamental questions regarding excited state relaxation mechanisms but also for ultimate manipulation of excited state processes in molecular assemblies employing these types of complexes as photosensitizers. Time resolved vibrational studies on nanosecond timescales on many mixed ligand complexes strongly suggest that the photoexcited electron in the relaxed MLCT state is localized on the lowest energy ligand, at least on long timescales. Thus excitation to higher energy MLCT excited states will undergo inter-ligand electron transfer (ILET) to generate the charge localized lowest energy MLCT state. However, the real time dynamics of the charge localization processes with subpicosecond time resolution are still scarce and only a few studies are reported in literature.18,26–28 To explore the real time dynamics of charge transfer and vibrational relaxation of heteroleptic complexes, we have studied the excited state dynamics of [Ru(bpy)2(pap)]2+, (where bpy = 2,2 0 -bipyridine and pap = 2-(phenylazo)pyridine, Scheme 1) using subpicosecond transient absorption spectroscopy. This complex allowed us to monitor the charge transfer from the bpy to pap ligand because of the stronger p-acidity of the pap ligand in comparison to that of the bpy ligand and thus the lowest MLCT state (either in the singlet or in the triplet manifold) will be localized on the pap ligand, whereas, bpy-localized MLCT states will be the next higher energy state. Optical excitation to the bpy-localized 1MLCT state will ultimately lead to the formation of the lowest energy 3MLCT state during the course of the excited state relaxation process which has been monitored by femtosecond transient absorption spectroscopy. Nanosecond laser flash photolysis and pulse radiolysis experiments confirmed the formation of charge localized state. The present work describes the ultrafast charge localization and vibrational relaxation dynamics in this heteroleptic complex in a few organic solvents as well as in aqueous solution.

2. Experimental section The perchlorate salt of the complex, [Ru(bpy)2(pap)](ClO4)2 was synthesized following a reported procedure29 and purified by column chromatography. Organic solvents used for spectroscopic measurements were of HPLC grade (Spectrochem, India) and were used as received. MilliQ grade water was used for the experiment in aqueous medium. Steady-state absorption spectra were recorded on a Thermo-Electron model Biomate 5 spectrophotometer. Transient absorption experiments with about 5 ns time resolution were performed using a Laser Kinetic Spectrometer (Edinburgh Instruments, UK, model LP920). The sample in a 10 mm 10 mm quartz cuvette was excited using 532 nm laser pulses of 5 ns duration and 20 mJ per pulse energy generated using a frequency doubled Nd-YAG laser (Thales Laser SA, France, model: SAGA). Continuum light (250–1000 nm) from a pulsed 450 W xenon arc lamp was used as the optical probe. Time-resolved transient absorption spectra in the 300–950 nm region were recorded using an ICCD camera (Andor, UK, Model: iStar-320T) and the temporal absorption profiles at selected wavelengths were recorded using a photomultiplier tube (Hamamatsu R920) connected to a 200 MHz digital oscilloscope. Electron pulse radiolysis experiments were performed using a set-up which has already been discussed in detail elsewhere.30 TA profiles and time-resolved TA spectra were recorded with subpicosecond time resolution (B120 fs) using a pump–probe spectrometer, which has been described in detail elsewhere.31 Briefly, it uses a amplified laser system based on a Ti : sapphire oscillator (Thales Optronique SA, Elancourt, France). 400 nm laser pulses (pump) of 5–10 mJ per pulse and about 50 fs duration and its polarization set at the magic angle were used to excite the solution of ThT flowing through a 2 mm quartz cell and the excited states were probed using a white light continuum (470–1000 nm) generated by focusing of about 1 mJ per pulse energy on a 2 mm thick sapphire plate. TA profiles have been recorded using a spectrometer in a dual-beam configuration using a pair of interference filters of 10 nm bandwidth for selecting the monitoring wavelength and signal detection combining a photodiode detector and boxcar integration. Ground-state geometry was optimized by Density Functional Theory (DFT) calculations using B3LYP hybrid functional and 6-31G(d,p) basis set.32 Effective core potential (ECP) was used for ruthenium. Vibrational frequencies were calculated to verify the minimum energy geometry. Single point energy calculations were also performed using the same basis set and functional. All calculations were carried out using the GAMESS software package.33

3. Results and discussion 3.1.

Scheme 1

Structure of [Ru(bpy)2(pap)]2+.

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Steady state studies

Steady state absorption spectra of [Ru(bpy)2(pap)]2+ in acetonitrile and water are shown in Fig. 1. Close similarities in these spectra recorded in two solvents of widely different polarities suggest that the electronic structure and energies of the excited singlet MLCT


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Paper Table 1 TDDFT (B3LYP/6-31++G, ECP (Ru)) calculated electronic excitation energies of [Ru(bpy)2(pap)]2+

Electronic transition


S0 S0 S0 S0 S0 S0 S0 S0

Fig. 1 Steady state absorption spectra of [Ru(bpy)2(pap)]2+ in acetonitrile and water.

(or 1MLCT) state are not significantly solvent dependent. Each of these absorption spectra consists of two absorption bands – the lowest energy absorption band in the 440–580 nm region with a maximum at 490 nm and the other in the 330–440 nm region with a maximum at 370 nm. The important point to be noted here is that the lowest energy absorption band is significantly red shifted as compared to that of [Ru(bpy)3]2+, in which the lowest energy absorption maximum appears at ca. 450 nm. Occurrence of the lowest energy absorption band in the case of [Ru(bpy)2(pap)]2+ is assigned to the transition from the ground electronic (S0) state to the pap-localized 1MLCT state, in which charge transfer takes place from Ru(dp) to the lower energy p* state of the strongly p-acidic pap ligand, whereas the higher energy absorption band in [Ru(bpy)2(pap)]2+ arises due to the bpy-localized 1 MLCT state (Scheme 2).29 Further, this transition appears at higher energy as compared to that of the [Ru(bpy)3]2+ complex because the stronger p accepting character of the pap ligand stabilizes the Ru(dp) orbital leading to increase in the p*(bpy) ’ Ru(dp) transition energy. Systematic studies on the series of homoleptic and heteroleptic complexes have already established a general trend in the shift of the absorption bands to lower energy with increase in p accepting character of the ligands.26 The absorption feature in the UV region (below 320 nm) originates from the ligand centred transition as

-


Oscillator strength

Dominant orbital contribution

1.93 (640) 2.17 (570) 2.7 (460) 2.82 (438) 2.84 (435) 3.02 (412) 3.07 (403) 3.15 (393)

0.001 0.000 0.080 0.040 0.01 0.015 0.025 0.06

HOMO HOMO HOMO HOMO HOMO HOMO HOMO HOMO

- LUMO 1 - LUMO 2 - LUMO - LUMO + 1 - LUMO + 2 4 - LUMO 1 - LUMO + 1 1 - LUMO + 2

well as the higher energy MLCT transitions. However, the assignment of these higher energy transitions is not important for the present study. 3.2.

Theoretical calculations

Assignment of these absorption bands in the 320–600 nm region has been delineated by the TDDFT calculations. Transition energies, oscillator strengths and dominant orbital contributions corresponding to different electronic transitions are presented in Table 1 and the electronic charge distribution in the molecular orbitals involved in different kinds of transitions are shown in Fig. 2. It becomes evident from Fig. 2 that three occupied molecular orbitals (i.e. HOMO, HOMO 1 and HOMO 2) mainly consist of the Ru(dp) orbital, whereas the lowest unoccupied orbital (LUMO), which is located mainly at the pap ligand as well as the next two higher unoccupied orbitals (LUMO + 1 and LUMO + 2) are located at either of the two bpy ligands. It is also important to note that the three lowest electronic excitations are associated with the charge transfer from the metal to the pap ligand, among which the S0 - S3 transition (to appear at ca. 460 nm) has appreciable oscillator strength. Considering the uncertainties associated with the transition energies calculated using TDDFT, the lowest energy absorption band observed in the 440–550 nm region can be assigned to the 1MLCT state corresponding to charge transfer from Ru(dp) to pap(p*). On the other hand, the higher energy electronic transitions are mainly associated with the charge transfer from Ru(dp) to bpy(p*) and the transition appears below 430 nm. Significant differences in the energy levels between pap and bpy ligands create distinct MLCT energy manifolds corresponding to the excitation of different ligand localized MLCT states. Thus, it can be concluded that 400 nm excitation preferentially (though not exclusively) populates the bpy-localized 1MLCT excited state with significant vibrational energy. 3.3.

Scheme 2 Simplified energy level diagram to compare the MLCT transition of [Ru(bpy)2(pap)]2+ and [Ru(bpy)3]2+.

S1 S2 S3 S4 S5 S6 S7 S8

Transition energy/eV (nm)

Nanosecond laser flash photolysis

Fig. 3 presents the differential TA spectrum of the [Ru(bpy)2(pap)]2+ complex in acetonitrile recorded following photoexcitation using 532 nm laser pulses. The most prominent feature of this spectrum is the negative absorption band at 500 nm, which is flanked by two weaker excited state absorption (ESA) bands appearing in the 420–450 and 570–650 nm regions. The position of the negative absorption band coincides with the lowest energy band of the steady state absorption spectrum of the complex (Fig. 1).


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Fig. 2 Molecular orbitals associated with the different 1MLCT transitions calculated using TDDFT method.

Therefore, this band in the differential TA spectrum is assigned to bleaching of the ground state following photoexcitation. Appearance of two ESA bands at both sides of the bleaching band possibly suggests distortion of a broad ESA band extended from 400 to 650 nm because of its overlap with the strong bleaching band. Knowing the absorption coefficient for the steady state absorption at 500 nm and the number of molecules populating the excited state following photoexcitation, the true transient spectrum of the excited state of the complex was calculated and the same is also shown in Fig. 3. The later is characterized by two distinct ESA bands with the maxima at ca. 550 and 450 nm. This spectrum is tentatively assigned to the 3 MLCT state (vide infra). In the earlier section, we discussed that the lowest energy absorption band is assigned to the MLCT state arising due to Ru(dp) to pap(p*) charge transfer and we designate here it as

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the pap-localized 3MLCT state. Therefore, in the lowest excited triplet state, charge is localized on the pap ligand and the TA spectrum should reveal the characteristic features of the pap anion radical. To delineate this aspect, we recorded the transient absorption spectrum following irradiation of a solution of the pap ligand in acetonitrile using an electron pulse. Generation of an anion radical of organic molecules in acetonitrile using pulse radiolysis, in which anion radicals are produced due to direct capture of solvated electrons by the molecule, is well known.34 In Fig. 3, we have compared the anion radical spectrum of the pap ligand with the corrected TA spectrum of the complex. Perfect agreement of these two spectra in the 500–700 nm region clearly suggests that the lowest energy excited state of the complex is characterized by the pap-localized 3MLCT state. In the inset of Fig. 3, we have shown two temporal profiles recorded at 500 and 570 nm following photoexcitation using 532 nm pulses of 5 ns duration, along with the single exponential best fit functions. These temporal profiles reveal the kinetics of bleach recovery and excited state decay, respectively. These two processes are complimentary, i.e. complete decay of the excited state leads to quantitative recovery of the ground state. The lifetime of the 3MLCT state has been determined to be 4.7 ns. One important point to be noted here is that the lifetime of the 3MLCT state of the [Ru(bpy)2(pap)]2+ complex is much shorter than that of the prototype Ru(bpy)32+ complex (a few ms).3 This can partly be attributed to the lower energy of the 3 MLCT state of the [Ru(bpy)2(pap)]2+ complex as compared to that of the homoleptic Ru(bpy)32+ complex, and the shorter lifetime is explained by the energy gap law. Another factor, which also may be responsible for shortening the lifetime of the 3 MLCT state of the [Ru(bpy)2(pap)]2+ complex, is strong nuclear displacement in the lowest energy triplet excited state, which imparts stronger vibronic coupling with the ground state leading to faster excited state deactivation. This has indeed been observed in several ruthenium and other metal complexes.35 The effect of the lower energy gap between the 3MLCT and the ground electronic states as well as large nuclear displacement in the 3MLCT state are revealed in the nonemissive nature of the complex. 3.4.

Fig. 3 (a) Steady state absorption spectrum in acetonitrile (inverted); (b) differential TA spectrum recorded immediately after photoexcitation of [Ru(bpy)2(pap)]2+ complex using nanosecond laser pulses; (c) TA spectrum of the anion radical of pap ligand using pulse radiolysis; (d) true TA spectrum of the 3MLCT state of the [Ru(bpy)2(pap)]2+ complex. Inset: temporal profiles (circles) recorded at 500 nm and 570 nm along with the single exponential fit function (solid line) calculated by deconvolution with the laser pulse width (6 ns).


Excited state dynamics in sub-ps time domain

Transient absorption studies in the sub-ps time-domain has revealed the early time relaxation dynamics following photoexcitation of the [Ru(bpy)2(pap)]2+ complex. Fig. 4 presents the time-resolved differential TA spectra recorded following photoexcitation of [Ru(bpy)2(pap)]2+ in acetonitrile solution at room temperature using 400 nm laser pulses of 50 fs duration. 400 nm laser excitation dominantly excites the complex to the 1 MLCT state, in which the electronic charge is localized at the bpy ligand (we designate this state as the bpy-localized 1MLCT state). TA spectra recorded at 0.2 ps delay time are characterized by a negative absorption band with the maximum at 500 nm, which arises due to bleaching of the ground state following photoexcitation, as well as an ESA band appearing in the 530–720 nm region, with an apparent maximum at ca. 570 nm. In the sub-8 ps time domain, the ESA band rises in the 530–590 nm region


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ESA band in the 570–650 nm region is quite evident. This small evolution of the transient absorption spectra in the 7–100 ps time-domain possibly can be assigned to the vibrational cooling process occurring in the pap-localized 3MLCT state. A detailed analysis of the wavelength dependence of the temporal profiles is expected to unravel its occurrence (vide infra). Beyond 100 ps delay time, the TA spectra do not show any significant evolution up to about 1 ns delay time (not shown in Fig. 4) and, further, the TA spectra recorded at 100 ps delay time agrees well with that recorded in the nanosecond time-domain using nanosecond laser flash photolysis (Fig. 3). Temporal profiles recorded at a few selective wavelengths following photoexcitation of the complex are shown in Fig. 5 along with the best multi-exponential fit functions. Each of these profiles could be fitted with a three or four exponential function consisting of decaying (d) and/or rising (r) components, including a long-lived component with a lifetime longer than a few ns. Lifetimes of the components associated with the best fit function for a temporal profile are also given in the inset. We mentioned earlier that the bpy-localized 1MLCT state, which is populated by photoexcitation, undergoes ISC processes to populate the bpy-localized 3MLCT state in the time domain faster than the time resolution of our spectrometer. Therefore, all the components associated with the temporal profiles presented in Fig. 5 represent the processes occurring in the triplet manifold. Fig. 4 Transient absorption spectra of [Ru(bpy)2(pap)]2+ in acetonitrile upon 400 nm photoexcitation.

accompanied by a concomitant decay in the 570–700 nm region, leading to the appearance of a temporary isobestic point at ca. 590 nm. In this time domain, partial recovery of the bleach band is also observed possibly due to increase in absorbance in the 480–550 nm region. Appearance of the temporary isobestic point at 590 nm suggests the occurrence of a process of conversion between two distinct electronic states. It is well established that in ruthenium polypyridyl complexes, intersystem crossing (ISC) processes, i.e. conversion of the 1MLCT state to the 3MLCT state, are completed in the o100 fs time domain.7–10 Therefore, population of the bpy-localized 3MLCT state can be assumed to be completed within the time resolution of our spectrometer via the ISC process from the bpy-localized 1MLCT state, which is populated following photoexcitation. Hence, the time evolution of the TA spectra in sub-8 ps time domain is assigned to the conversion of the bpy-localized 3MLCT state to the pap-localized 3MLCT state. Population of the pap-localized 3MLCT state has been evident from the comparison of the spectroscopic characteristics of the TA spectra with the pap anion radical spectrum (see Section 3.3). With further increase in delay time up to 100 ps, the maximum of the ESA band shifts marginally towards the blue region, which is accompanied by a marginal increase in intensity of the bleach band. Because of significant overlap of the ESA and the bleach bands, it is truly not justified to make any assignment for this evolution of the time-resolved TA spectra recorded in this time domain. However, a marginal decrease in intensity of the


Fig. 5 Temporal dynamics of [Ru(bpy)2(pap)]2+ in acetonitrile at different wavelengths along with triexponential fit functions (solid lines). Lifetimes (in picoseconds) are given in the inset.


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The ultrafast component with the lifetime of 0.3 0.1 ps, which is associated with the temporal profiles recorded in the 500–570 nm region can be assigned to the vibrational relaxation in the bpy-localized 3MLCT state, which is populated by the ultrafast ISC process from the bpy-localized 1MLCT state. Occurrence of this process has already been reported in the relaxation of the excited states of many other ruthenium complexes.7–9 The component with the lifetime of 2.5 0.1 ps, which has been associated with all the temporal profiles recorded in the 500–600 nm region, is attributed to the charge transfer from the bpy ligand to the pap ligand, i.e. the process of conversion of the bpy-localized 3MLCT state of the pap-localized 3MLCT state. Thus, the present results show the ultrafast nature of the inter ligand charge transfer dynamics in the 3MLCT state of a heteroleptic ruthenium complex. Earlier work of Papanikolas and coworkers on osmium polypyridyl complexes also revealed ultrafast inter-ligand charge transfer dynamics.18 In addition, a recent paper also reported the lifetime of the inter-ligand charge transfer process in the excited state of a ruthenium terpyridyl complex as about 2.5 ps.28 Ultrafast localization of electronic charge in the excited state has also been observed for a DNA light switch complex, which however occurs on a relatively longer timescale.25 The lifetime of the third component is wavelength dependent and becomes longer as the probe wavelength is tuned from 650 nm (lifetime is 15 ps) to 540 nm (lifetime is 35 ps). The later value remains nearly similar as the probe wavelength is further tuned towards blue. Wavelength dependence of the lifetime of this component leads us to assign it to the vibrational cooling of the pap-localized 3MLCT state via dissipation of thermal energy to the solvent.31 We have also studied the temporal dynamics of the excited state of the complex in methanol and aqueous solution at a few selective wavelengths (Fig. 6). In these solvents too, the excited state relaxation dynamics could be described by the occurrence of three processes, namely, vibrational energy relaxation in the bpy-localized 3MLCT state, inter-ligand charge transfer from the bpy to the pap ligand and vibrational cooling in the pap-localized 3 MLCT state. Time constants associated with these processes are also seen to be very similar in all three solvents. Thus, the observed dynamics are more or less independent of the solvent characteristics, in spite of the fact that the characteristics of these solvents, such as polarity and solvation time, are significantly different. For example, the lifetimes of the inter-ligand charge transfer process have been found to be in the range 2.5–2.8 ps in acetonitrile, water and methanol, which have different solvation times (htisol is 0.5, 0.25 and 5 ps, respectively) and polarities (p* values are 0.66, 1.09, 0.58, respectively). Lack of solvent dependence of the lifetime of the inter-ligand charge transfer process possibly can be explained by similar polarities of the bpy localized 3MLCT state and pap-localized 3MLCT state. In acetonitrile and water, the time constant for the charge localization process is significantly longer than the solvation time. This suggests that intramolecular reorganization energy governs the kinetics of the charge transfer process and not the solvent reorganization process. It is known that charge accumulation at the pap ligand results in significant change in


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Fig. 6 Temporal dynamics of [Ru(bpy)2(pap)]2+ in methanol and water at few selective wavelengths along with triexponential fit functions (solid line). Lifetimes (in picoseconds) are given in the inset.

the length of the azo bond, which contributes to a large intramolecular reorganization energy.36 The ultrafast nature of the inter-ligand electron transfer process is possibly facilitated by the sufficiently strong coupling between two diabatic electronic states, namely the bpy-localized 3MLCT and pap-localized 3MLCT states. We could not monitor the dynamics in low polarity solvents because of poor signal to noise ratio of the data due to sparing solubility of the complex in these solvents. Detailed enumeration of the mechanistic aspects of the charge localization dynamics will require of tuning the energy levels of two MLCT states by introducing different donor and/or acceptor substituents on the bpy and pap ligands and this may be an important aspect of further studies. Another important aspect of the present study is the long lifetimes (B30–40 ps) of the vibrational cooling of the pap-localized 3MLCT state, which is populated following interligand charge transfer from the bpy-localized 3MLCT state. This deserves attention, since the earlier studies using transient absorption techniques report that the vibrationally relaxed 3 MLCT state of Ru(bpy)32+ is populated within a few ps following photoexcitation.7,8 A similar time scale has also been proposed from the results of fluorescence upconversion experiments.9 Despite such consensus in the earlier studies, the issue of vibrational relaxation in ruthenium polypyridyl complexes remains a subject of debate because recent time resolved infrared and Raman


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Fig. 7 Simplified schematic potential energy diagram showing excited state processes of [Ru(bpy)2(pap)]2+ in acetonitrile.

studies demonstrated that vibrational relaxation in the 3MLCT state does not become complete within 1 ps, rather takes 20 ps for complete relaxation.23 Ultrafast transient absorption measurements of Hammarstrom et al.24 also revealed the occurrence of a process with about 10 ps time constant, which is in disagreement with the earlier reports of McCusker and coworkers.27 These differences in the results obtained were attributed to the differences in the probe wavelengths employed. In the earlier study of McCusker et al., the TA kinetics of Ru(bpy)32+ were probed in the 400–500 nm region, where ground-state recovery was the dominant process.8 In contrast, the 360 nm probe falls within the absorption band of the bpy anion radical (bpy ) species and, thus, absorption of the thermally relaxed 3MLCT state, where most of the solvation and vibrational relaxation processes are expected to be observed. Based on the above arguments, we may expect prominent dynamical features for vibrational relaxation in the wavelength region (460–600 nm) of the pap anion radical absorption. Our present transient absorption study monitors the dynamics in the 480–600 nm region and the component with the wavelength dependent lifetime is obviously assigned to the vibrational relaxation process. Based on the above discussion, we describe the relaxation dynamics in the excited states of the [Ru(bpy)2(pap)]2+ complex using the simplified potential energy surface diagram presented in Fig. 7. Because of the limited time resolution (B120 fs) of our spectrometer, the intersystem crossing process from the bpylocalized 1MLCT to the bpy-localized 3MLCT state could not be monitored in the present work.

4. Conclusions In conclusion, we have probed the nanosecond and subpicosecond dynamics of the [Ru(bpy)2(pap)]2+ complex using transient absorption spectroscopic techniques. Nanosecond TA spectra show that the lowest energy 3MLCT state, in which the electronic charge is localized on the pap ligand, decays with a


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4.7 ns lifetime. Time-resolved absorption spectroscopy on the femtosecond time scale reveals that charge transfer from the bpy ligand to the pap ligand takes place on the picosecond timescale. Although the present results do not provide detailed mechanistic understanding of the electron transfer processes, it clearly reveals that the two diabatic 3MLCT states are strongly coupled to induce ultrafast charge transfer processes. The present experimental results also support the recent literature reports which claim that vibrational cooling in the 3MLCT manifold does not complete within a picosecond, but rather takes at least tens of picoseconds. The observations reported here may have important consequences for the design of supramolecular systems based on heteroleptic complexes for light harvesting systems, as the charge localized state will determine the charge separation kinetics in a covalently linked donor–acceptor system or interfacial electron injection efficiency in dye sensitized solar cells.

Acknowledgements Authors gratefully acknowledge Dr Dipanwita Das’ help in synthesizing the complex.

Notes and references 1 V. Balzani, A. Juris, M. Venturi, S. Campagna and S. Serroni, Designing Dendrimers Based on Transition-Metal Complexes. Light-Harvesting Properties and Predetermined Redox Patterns, Acc. Chem. Res., 1998, 31, 26. 2 V. Balzani, A. Juris, M. Venturi, S. Campagna and S. Serroni, Luminescent and Redox-Active Polynuclear Transition Metal Complexes, Chem. Rev., 1996, 96, 759. 3 K. Kalyanasundaram and M. Gratzel, Applications of functionalized transition metal complexes in photonic and optoelectronic devices, Coord. Chem. Rev., 1998, 177, 347. 4 O. S. Wenger, Long-range electron transfer in artificial system with d6 and d8 metal photosensitizers, Coord. Chem. Rev., 2009, 253, 1439–1457. 5 A. Hagfeldt, G. Boschloo, L. Sun, L. Kloo and H. Pettersson, Dye-Sensitized Solar Cells, Chem. Rev., 2010, 110, 6595. 6 B. O’Regan and M. Gratzel, A low-cost high-efficiency solar cell based on dye sensitized colloidal TiO2 films, Nature, 1991, 353, 737. 7 N. H. Damrauer, G. Cerullo, A. Yeh, T. Boussie, C. V. Shank and J. K. McCusker, Femtosecond Dynamics of Excited-State Evolution in [Ru(bpy)3]2+, Science, 1997, 275, 54. 8 J. K. McCusker, Femtosecond Absorption Spectroscopy of Transition Metal Charge-Transfer Complexes, Acc. Chem. Res., 2003, 36, 876. 9 A. C. Bhasikuttan, M. Suzuki, S. Nakashima and T. Okada, Ultrafast Fluorescence Detection in Tris(2,2 0 -bipyridine)ruthenium(II) Complex in Solution: Relaxation Dynamics Involving Higher Excited States, J. Am. Chem. Soc., 2002, 124, 8398.


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