Chemical Physics Letters 386 (2004) 336–341

www.elsevier.com/locate/cplett

Ultrafast singlet energy transfer competes with intersystem crossingin a multi-center transition metal polypyridine complex

Johan Andersson a,*, Fausto Puntoriero b, Scolastica Serroni b, Arkady Yartsev a,Torbj€orn Pascher a, Tom�a�s Pol�ıvka a, Sebastiano Campagna b, Villy Sundstr€om a

a Department of Chemical Physics, Chemical Centre, Lund University, P.O. Box 124, S-221 00 Lund, Swedenb Dipartimento di Chimica Inorganica, Chimica Analitica e Chimica Fisica, University of Messina, Via Sperone 31, I-98166 Messina, Italy

Received 21 November 2003; in final form 13 January 2004

Published online:

Abstract

Transition metal polypyridine complexes are finding widespread applications within many areas of chemistry. For their light-

induced processes, the generally accepted picture is that all function emanates from triplet states because the singlet states initially

prepared by light absorption are depopulated via intersystem crossing on the 100-fs time scale, before they are significantly involved

in chemical reactions. Here we show that this is not always true. With ultrafast spectroscopy applied to a (ruthenium)3–osmium

complex we show that transition metal polypyridine complexes can be designed where energy transfer between excited singlet states

located on different metal centers efficiently competes with intersystem crossing, thus decreasing population of the lower-lying triplet

states and concomitant energy loss.

� 2004 Elsevier B.V. All rights reserved.

1. Introduction

Transition metal polypyridine complexes, in particu-

lar made of octahedral d6 metals like Ru(II) and Os(II),

have played and continue to play key roles in the de-

velopment of several fields of chemical science, including

photochemistry, photoinduced electron and energy

transfer, electrochemistry, chemi- and electro-lumines-

cence, solar energy conversion, and in more recent

times, supramolecular chemistry, in particular within itssubtopics of light-activated molecular machines and

devices [1–10]. The generally accepted model of the ex-

cited state properties of transition metal polypyridine

complexes (common to most transition metal com-

plexes) assumes very fast light-induced population of the

lowest-lying excited state, without regard of its multi-

plicity, since the heavy-atom effect induces strong spin-

orbit coupling and makes intersystem crossing (ISC)

* Corresponding author. Fax: +46-46-22-24119.

E-mail address: johan.andersson@chemphys.lu.se (J. Andersson).

0009-2614/$ - see front matter � 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.cplett.2004.01.081

extremely fast. For example, in the case of Ru(II) and

Os(II) polypyridine complexes, the lowest-lying metal-to-ligand charge-transfer triplet state (3MLCT) is pro-

duced on the sub-picosecond timescale and with unitary

efficiency from the 1MLCT level initially prepared by

direct light irradiation [3,11–13]. Exceptions to this rule

can however be found; very recently a Ru(II) compound

was reported, in which the intersystem crossing rate is

on the 100-ps timescale and with an efficiency lower than

unity [14]. Perdeuteration of the polypyridine ligandswas assumed to be responsible for such a slow rate of

ISC in that particular species.

Most photochemical studies carried out in the last

decades (bimolecular quenching processes, electro-

chemiluminescence, etc.) assume the picture outlined

above, with the notable recent exception of photoin-

duced electron injection in semiconductors in particular

cases [15,16]. This implies that in the design of (supra-molecular) multicomponent systems based on transition

metal polypyridine complexes, driving forces for fun-

damental processes like inter-component photoinduced

electron and energy transfer are calculated by con-

sidering the energy of the triplet states of the metal

Fig. 2. Scheme of energy transfer and relaxation processes within the

OsRu3 complex. Excitation of the singlet (blue) state (process 2) leads

to ultrafast singlet–singlet energy transfer (SS) towards the Os core

that competes with ISC crossing denoted by double arrows. Excitation

of the triplet (red) state (process 1) initiates slower triplet–triplet (TT)

energy transfer. All processes are probed by means of excited state

absorption from the Os triplet (process 3). Wavy arrows denote vi-

brational relaxation and cooling in triplet states of both metal centers.

See text for details.

J. Andersson et al. / Chemical Physics Letters 386 (2004) 336–341 337

components [3,5]. This aspect is particularly important

for artificial metal-based light-harvesting antennas

designed to be part of more complex systems for artifi-

cial photosynthesis [17–20].

Here we provide a new model for energy transferwithin multi-centered transition metal complexes. The

new model is a result of an investigation of the excited-

state relaxation and decay processes occurring in a

tetranuclear mixed-metal complex {Os((l-2,3-dpp)-Ru(bpy)2)3}

8þ (2,3-dpp¼ 2,3-bis(20-pyridyl)pyrazine;bpy¼ 2,20-bipyridine), hereafter denoted as OsRu3, de-

picted in Fig. 1. This complex behaves as an artificial

antenna system, since all the light energy absorbed bythe peripheral Ru-based chromophores is quantitatively

transferred to the central Os-based core [21,22]. To

allow a deeper insight into the complicated excited state

dynamics of OsRu3, the transient absorption method

employing �30-fs pulses is used to follow the time

evolution of the excited states [23]. The results demon-

strate that the excited-state dynamics depend on exci-

tation wavelength and, a comparison with the excitedstate dynamics of monomeric Os(2,3-dpp)2þ3 (mOs) and

Ru(2,3-dpp)2þ3 (mRu) complexes, enabled us to con-

struct a new model of excited state processes occurring

in polypyridine transition metal complexes that is de-

picted in Fig. 2. This new model has two key features:

(1) Ru-to-Os singlet–singlet energy transfer (660 fs)

(process SS in the figure), which effectively competes

with intra-unit relaxation processes (ISC and vibrationalrelaxation). (2) An order of magnitude slower (600 fs),

and hence slower than the intra-unit relaxation pro-

cesses, energy transfer from the triplet states localized on

the Ru-centers to the triplet state localized on the Os-

core (process TT in Fig. 2). The fact that the singlet–

singlet transfer favorably competes with the intra-unit

relaxation processes opens possibilities of constructing

Fig. 1. Representations of the OsRu3 complex, charges omitted. Different co

light-harvesting devices with total quantum efficiency

close to unity, utilizing the maximum photon energy andminimizing triplet-mediated photochemical side reac-

tions, as this route of energy transfer decreases popu-

lation of the triplet.

lors of the ligands represent bpy ligand (black) and dpp ligand (green).

Tra

ns.

Abs.

(a.u

.)

0.0

1.0

Ab

s (a

.u.)

26 24 22 20 18 16 14 12 10

Energy (x103 cm-1)

(a)

(b)

400 500 600 700 800 1000 Wavelength (nm)

0.0

2 1

Fig. 3. (a) Absorption spectra of OsRu3 (purple), Ru 4 (red), mOs

338 J. Andersson et al. / Chemical Physics Letters 386 (2004) 336–341

2. Experimental

In order to probe the excited state dynamics reflecting

energy transfer and other relaxation processes, time re-

solved fluorescence or transient absorption from excitedstates involved in the transfer processes can be used. In

this work we have chosen the transient absorption

method since it is known to be able to probe the dy-

namics of both singlet and triplet states of transition

metal complexes, as well as their internal dynamics

[13,23,24,29]. Time-resolved differential absorption

measurements were conducted in the conventional

manner using a setup based on a 1-kHz amplifiedTi:sapphire laser system, Clark MXR CPA 2001, de-

livering �30-fs pulses [23]. Excitation and probe pulses

were generated by noncollinear optical parametric am-

plifiers (NOPA plus). For all measurements, the inten-

sities of the probe, reference, and pump beams were

measured by Si photodiodes. With proper software se-

lection of the pulses, absorption changes as low as 10�5

could be recorded accurately. The polarizations of pumpand probe beams were kept at the magic angle and the

excitation pulse intensity of �1014 photon/cm2 used was

far from the saturation limit considering the optical

cross-section of the molecules. All the experiments were

performed in acetonitrile solution at room temperature.

Measured kinetics were analyzed with either single-

kinetics multi-exponential fitting or a global analysis

of larger data sets, details are given in the text below.

(green) and mRu (orange) by full symbols, while open symbols visu-

alize the spectral region corresponding to a direct absorption of the

triplet states. Grey vertical lines denote the excitation wavelengths used

for excitation of triplet (1) and singlet (2) states of the complexes. The

spectra are not in the same scale. (b) Transient absorption spectrum of

the final triplet state of OsRu3, measured at 1.6 ps after excitation,

when the excited state population is in the Os-3MLCT state.

3. Results and discussion

3.1. Selection of excitation and probing wavelengths

We will first briefly discuss the spectroscopic basis for

the excitation and probing processes of the studiedmolecules. The absorption spectra of the studied com-

plexes shown in Fig. 3a are the basis for the choice of

excitation wavelengths. The excited state manifold of

transition metal complexes hold optically active singlet

and triplet states. This leads to complex spectral prop-

erties with many bands corresponding to the various

transitions. In the absorption spectra of the studied

complexes the main features are color coded (Fig. 3a)and correlated to the transitions used to prepare a

molecule in a particular excited state (see the energy

level scheme of Fig. 2). With 675-nm excitation, we

prepare the complexes in the lowest 3MLCT states. For

the OsRu3 complex, the overlapping absorption bands

of the Ru- and Os-units makes selective excitation of the

two different metal centers impossible, but we signifi-

cantly excite the Ru-part (via the Ru-dpp 3MLCTtransition). Due to the differences between Os and Ru

metals, the singlet–triplet absorption transition is much

more efficient in Os(II) mononuclear complexes than in

Ru(II) ones. However, looking at the absorption spec-

trum of the all-ruthenium tetranuclear compound{Ru((l-2,3-dpp)Ru(bpy)2)3}

8þ, Ru4 (Fig. 3a), it is clear

that there is also a significant contribution from Ru-

based singlet–triplet MLCT transitions to the absorp-

tion of the OsRu3 complex at 675 nm. Actually, at

675 nm the extinction coefficient of OsRu3 is 16,100

M�1 cm�1, and the extinction coefficient of Ru4 is 5400

M�1 cm�1, showing that the Ru-centered MLCT tran-

sitions probably have a somewhat smaller contributionto the absorption at 675 nm, than the Os-centered one.

This is verified by the OsRu3 kinetics obtained for this

excitation wavelength (Fig. 4a), exhibiting a 59% in-

stantaneous rise of the signal. With 500-nm excitation,

we preferentially prepare OsRu3 in the 1MLCT state

localized on the Ru-dpp part of the molecule, by virtue

of the 3/1 Ru/Os ratio. The same wavelengths, 500 and

675 nm, were used to excite the singlet and triplet states,respectively, of the monomeric complexes.

0 5 10 15 20 25

0.0 0.2 0.4 0.6 0.8 1.0

0 5 10 15 20 25

(a)

Timedelay (ps)

∆Abs

. (a.

u.)

∆Abs

. (a.

u.)

(b)

Fig. 4. (a) Kinetic traces of the OsRu3 complex recorded after exci-

tation of the singlet state (blue) and of the triplet state (red). Solid lines

represent multi-exponential global fits of the kinetics yielding the fol-

lowing rise components: instantaneous, s1 ¼ 80 fs, s2 ¼ 600 fs and

s3 ¼ 12 ps. For 675-nm excitation (red) the following amplitudes

were obtained, instantaneous¼)59%, A1 ¼ �22%, A2 ¼ �10% and

A3 ¼ �9%, and for 500-nm excitation (blue), instantaneous¼)36%,

A1 ¼ �49%, A2 ¼ �4%, A3 ¼ �9%. (b) Kinetic traces recorded after

excitation of the singlet state of mRu (orange) and of the triplet state of

mOs (green). Fitting (solid lines) yields the following time constants

and amplitudes. mRu excited at 500 nm (orange): instantaneous rise

()81%), s1 ¼ 95 fs ()19%), s2 ¼ 250 ps (3%); mOs excited at 675 nm

(green): instantaneous rise ()80%), s1 ¼ 40 fs (60%) and s2 ¼ 13 ps

()20%). To obtain satisfying fits, a long (1 ns) component was used in

all kinetics to account for the decay of the long-lived triplet state

localized at the Os core. Precision of lifetimes and amplitudes is

�10–20%.

J. Andersson et al. / Chemical Physics Letters 386 (2004) 336–341 339

The transient absorption spectra of the studied mol-

ecules (Fig. 3b) are typical of this type of molecules [25],

and constitute the source of information for the selec-

tion of probe wavelengths. All spectra exhibit ground

state bleach in the spectral region of the ground state

absorption and extended excited state absorption fromthe singlet and triplet states, to the red of the ground

state bleach. We found that wavelengths around 1000

nm are sensitive to the appearance of the Os-dpp triplet

excited states and have therefore used this wavelength to

monitor the energy flow from the Ru-units to the central

Os-part of the molecule. However, the excited state

spectra of these complexes are broad, lacking distinct

features, and have contributions from both singlet and

triplet excited states of the Ru- and Os-centers. It is

therefore crucial to use the monomeric complexes to get

information on the intra-unit processes. The same probe

wavelength, 1000 nm, was used to measure the internal

dynamics of the monomeric complexes.

3.2. Excited state dynamics of the monomeric building

blocks, mOs and mRu

The excited state dynamics of the OsRu3 tetranuclear

complex is expected to contain contributions from pro-

cesses within the building blocks, as well as processes

reflecting the energy transfer between the different units.Therefore, in order to establish a basis for interpreting

the results for the OsRu3 complex, we start by charac-

terizing and discussing the dynamics of the monomeric

mRu and mOs building blocks. The choice of the mo-

nomeric species as models for the individual subunits of

OsRu3 requires comment. Some properties (including

orbital energies) of the bridging ligand 2,3-dpp change

when a second metal center is coordinated [5,22]. Inparticular, the relevant MLCT bands shift to the red on

passing from mono- to tetranuclear complexes. How-

ever, the rates of the various fast decay processes oc-

curring within the subunit(s) are not expected to change

significantly (this is confirmed by our results), with the

exception of the lifetime of the lowest-lying 3MLCT

state, which anyway does not play any role in the present

discussion. We decided to use mononuclear species asmodels for the intra-unit decay rates also because the use

of other polynuclear systems (which could be considered

closest to the studied OsRu3 system as far as the spec-

troscopic properties are concerned) unavoidably would

have introduced other inter-unit processes, which would

make it difficult to resolve the intra-unit processes.

A great deal of knowledge exists for the excited state

relaxation processes of various monomeric transitionmetal complexes of the type studied here [13,23,25–27].

Following excitation to a 1MLCT state, ultrafast inter-

system crossing to the lower-lying 3MLCT state occurs.

The triplet state is long lived and has a lifetime on the

tens to hundreds of nanosecond timescale, sometimes

even on the microsecond timescale, depending on the

molecule [3]. Excess optical energy deposited in the

molecule results in vibrational relaxation and energydissipation (cooling) on the sub-picosecond to few

picoseconds time scale [3,26,28]. Optical excitation of a

symmetric complex has been observed to result in

preparation of an excited state delocalized over all the

ligands and the localization to one of the ligands, driven

by solvent interactions occurs on the sub-100 femto-

second time scale [29]. All these processes could be

identified in the mRu and mOs complexes. Hence,3MLCT excitation (675 nm) of mOs results in instan-

taneous formation of the excited state absorption at

1000 nm, followed by rapid (40 fs) decay, a slower

340 J. Andersson et al. / Chemical Physics Letters 386 (2004) 336–341

(13 ps) rise and a very slow (>1 ns) decay (Fig. 4b and

Table 1), representing excited state localization, vibra-

tional cooling and triplet state decay, respectively.

Triplet state lifetimes of transition metal complexes are

known to be much too long [3] to be accurately deter-mined in these short-time scale measurements. For the

mRu complex excited to the 1MLCT state (500-nm ex-

citation), we observe an instantaneous rise of excited

state absorption due to the 1MLCT state, followed by a

further rise (95 fs) due to the formation of the 3MLCT

state (intersystem crossing and localization) and by a

much slower decay representing cooling and decay of

the triplet excited state (Fig. 4b and Table 1). Since in-tersystem crossing and excited state localization occurs

on similar time scales (see discussion for mOs), the two

processes proceed in concert for mRu excited to the

singlet state, and when mRu appears in the 3MLCT

state the excitation is already localized. This information

on the monomeric complexes is crucial for the identi-

fication below of the energy transfer pathways in

OsRu3.

3.3. Energy transfer in OsRu3

Due to the efficient transfer of energy from the

peripheral Ru-units to the central Os-core in the tetra-

nuclear OsRu3 complex [21,22], following optical exci-

tation of the Ru chromophores we expect to observe

unique dynamics (not present in the monomeric com-plexes) associated with the transfer of energy from

the Ru-units to the Os-core. Thus, the excited state

absorption kinetics of OsRu3 following excitation at

675 nm (Fig. 4a) carries a �500-fs rise time component

(as obtained from a single-kinetics multi-exponential fit)

not present in any of the monomeric complexes, show-

ing that the Os-3MLCT state is formed with this time

constant. The OsRu3 kinetics with 675-nm excitationalso contain instantaneous, sub-100-fs, and �10-ps rise

time components, attributable to direct excitation of the

Ru- and Os-chromophores, localization and cooling

processes, as discussed for the monomeric complexes.

The very slow (>1 ns) decay of the kinetics corresponds

to the triplet state lifetime of the Os-center – the final

emitter. Exciting to the Ru-1MLCT state of OsRu3

(500-nm excitation) makes the rise of the excited stateabsorption faster – a single-kinetics fit shows that the

Table 1

Multi-exponential fitting results

Molecule kexc (nm) ainst% s1 (ps) a1% s2 (ps) a2%

OsRu3 500 )36 0.080 )49 0.6 )4675 )59 0.080 )22 0.6 )10

mOs 675 )80 0.040 60 – –

mRu 500 )81 0.095 )17 – –

ainst – amplitude of instantaneous rise component; ainf – amplitude of long

�10–20%.

reason is a decreased amplitude of the �500 fs rise time

and increase of the amplitude of the sub-100-fs rise time

(Fig. 4a). The �10-ps time constant remains the same

with approximately the same amplitude for both exci-

tation wavelengths. The similarity of all time constantsof the OsRu3 kinetics excited at 500 and 675 nm fulfills

the requirements for a global fit including the total data

set. The energy level scheme of Fig. 2 also suggests that

the observed lifetimes should be very similar for both

excitation wavelengths, with only a variation in ampli-

tudes. We therefore performed a global fit to the two

kinetic traces in Fig. 4a, to obtain more reliable lifetimes

than those generated by the single-kinetics fits, and nowobtained the time constants 80 and 600 fs, 12 ps and a

very long >1 ns (see Table 1). The decrease in amplitude

of the 600-fs time constant in going from 675 to 500-nm

excitation (see Table 1), we interpret as opening up of an

additional energy transfer pathway when the Ru-1MLCT state of OsRu3 is excited, the singlet–singlet

energy transfer (process SS in Fig. 2) from the Ru-cen-

ters to the Os-center. From the amplitudes of the 600-fsTT process observed with 675 and 500 nm excitation, we

can estimate the SS energy transfer time to 60 fs, if we

assume that the intersystem crossing time in the Ru-unit

of OsRu3 is the same as that of mRu (95 fs, see Table 1).

We believe that this is a valid assumption, since ISC in

transition metal complexes has been shown to generally

occur on the �100-fs time scale [29]. It should also be

considered that since the 500 nm excitation populatesmore efficiently the Ru centers than the Os core com-

pared to the 675 nm excitation (see above and compare

the amplitudes of instantaneous rise with 500 and

675 nm excitation in Fig. 4a), if the additional SS energy

transfer would not be active the amplitude of the 600 fs

TT energy transfer should increase on passing to the 500

nm excitation. Based on these considerations, the 60 fs

estimate for the SS energy transfer process is probablyan upper limit. Note that the observed rise time (80 fs) of

the excited state absorption after Ru-1MLCT state ex-

citation of OsRu3 (Fig. 4a) is slower than the singlet–

singlet transfer time estimated from the amplitude ratio

of the 600-fs component measured with 675 and 500-nm

excitation, because it measures the arrival of the exci-

tation in the Os-3MLCT state and thus includes ISC in

the Os-unit. Our results show that there are two path-ways of energy flow from the outer Ru-moieties to the

s3 (ps) a3% s4 (ps) a4% ainf :% Comment

12 )9 210 13 87 Global fit

12 )9 – – 100 Global fit

13 )20 – – 40 Single-trace fit

– – 250 3 97 Single-trace fit

lived, >1 ns decay component. Precision of lifetimes and amplitudes is

J. Andersson et al. / Chemical Physics Letters 386 (2004) 336–341 341

Os-core in the tetranuclear OsRu3 complex – a very fast

singlet channel (sSS 6 60 fs) and an order of magnitude

slower triplet-triplet channel sTT ¼ 600 fs. This explains

previous failures to resolve energy transfer processes

in similar coupled complexes [30] – all excited statedynamics on a time scale longer than �500 fs reflect

processes in the acceptor-unit. Finally, it should be

mentioned that the results presented here are the first for

this kind of multi-centered transition metal complexes,

with a time resolution capable of monitoring the ultra-

fast inter-unit processes. More work in the future will

therefore naturally add more features and more details

to the picture presented here.We have shown that an OsRu3 tetranuclear transi-

tion metal complex excited to the singlet states of the

peripheral Ru-units exhibits ultrafast 6 60-fs energy

transfer to the central Os-core in competition with ISC

to the lower-lying triplet state. Energy transfer involving

the triplet states is at least an order of magnitude slower.

Together with previous observations of ultrafast elec-

tron transfer processes from transition metal complexesto a semiconductor [15,16], the present results show that

such molecules can be designed to perform extremely

fast (among the fastest chemical processes observed) and

efficient energy-harvesting and converting functions.

This opens the possibility to construct very large systems

with maintained efficiency and maximum utilization of

the photon energy. The very short residence time of

excitation energy on a metal center will in additionminimize destructive photochemical processes in an

extended energy converting system.

Acknowledgements

This work was supported by funding from the

Swedish Energy Agency (STEM), DESS (Delegationenf€or Energif€ors€orjning i Sydsverige), the Swedish Re-

search Council, the K&A Wallenberg Foundation, the

Crafoord Foundation, the EU contract no HPRI-

CT-1999-00041 and MIUR (Ministero dell�Istruzione,dell�Universit�ae della Ricerca).

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