The ultrafast decay rates of photoexcited View the MathML source ions have been measured in the condensed phase. The mechanism for decay is internal conversion, and the decay rate is a strong function of the charge on the ion. A bottleneck in the ground state recovery has also been detected, and its interpretation is discussed

Colin A. Bates

David L. Andrews

Dunn

Ebbesen

Ehrler

Garduno-Mejia

Gunnarsson

Hands

Heath

Ian D. Hands

Janette L. Dunn

Kato

Khaled

Kovalenko

Lawson

Michael J. Hope

Michael P. Higlett

Reed

Sassara

Stepanov

Stephen R. Meech

Trulove

Wang

English

Crossref

Observation of ultrafast internal conversion in fullerene anions in solution

Hope, Michael J.

Higlett, Michael P.

Andrews, David L.

Meech, Steven R

Hands, Ian D.

Dunn, Janette L.

Bates, Colin A.

University of East Anglia digital repository

This article appeared in a journal published by Elsevier. The attachedcopy is furnished to the author for internal non-commercial researchand education use, including for instruction at the authors institutionand sharing with colleagues.Other uses, including reproduction and distribution, or selling orlicensing copies, or posting to personal, institutional or third partywebsites are prohibited.In most cases authors are permitted to post their version of thearticle (e.g. in Word or Tex form) to their personal website orinstitutional repository. Authors requiring further informationregarding Elsevier’s archiving and manuscript policies areencouraged to visit:http://www.elsevier.com/copyrightAuthor's personal copyObservation of ultrafast internal conversion in fullerene anions in solutionMichael J. Hope a, Michael P. Higlett a, David L. Andrews a, Stephen R. Meech a,*, Ian D. Hands b,Janette L. Dunn b, Colin A. Bates ba School of Chemical Sciences, University of East Anglia, Norwich NR4 7TJ, UKb School of Physics and Astronomy, University of Nottingham, Nottingham NG7 2RD, UKa r t i c l e i n f oArticle history:Received 19 March 2009In ﬁnal form 23 April 2009Available online 3 May 2009a b s t r a c tThe ultrafast decay rates of photoexcited C60 and C260 ions have been measured in the condensed phase.The mechanism for decay is internal conversion, and the decay rate is a strong function of the charge onthe ion. A bottleneck in the ground state recovery has also been detected, and its interpretation isdiscussed. 2009 Elsevier B.V. All rights reserved.1. IntroductionThe spectroscopy, structure and chemistry of the negatively-charged fullerene ions (Cn60 ; where 1 6 n 6 6, generically referredto here as fullerides) continue to attract the attention of a largebody of scientists. This is not only because of the critical role theseions play in facilitating superconductivity in certain fullerene-alka-li metal salts [1], but also on account of their role in electron trans-fer reactions (itself a result of the high electron afﬁnity of C60) andas intermediates in fullerene photochemistry [2]. In C60, whilst ananosecond timescale characterizes intersystem crossing fromthe lowest singlet state [3], relaxation amongst the higher-lyingsinglet states is revealed by ultrafast studies [4,5]. In contrast,there have been relatively few studies of the excited state dynam-ics of the fullerides. Fullerides can readily be generated by syn-thetic and electrochemical means, and in deoxygenated solutionsthey are relatively stable, allowing electronic spectra to be mea-sured for solutions of anions with n = 1, 2 or 3 [6–8]. While absorp-tion spectra have been recorded, there is only a single report of anexcited state lifetime, by Ehrler et al. [9] who measured the decayof the excited monoanion in the gas phase using time-resolvedtwo-photon two-colour photoelectron spectroscopy. In this Letter,we report measurement of the excited state decay lifetimes of themono- and dianion of C60 in the condensed phase. To our knowl-edge, this is the ﬁrst lifetime determination of the dianion in anymedium. The ultrafast ground state recovery times that are ob-served allow us to draw inferences concerning both the mecha-nism of the radiationless decay and the strength of vibroniccoupling in solution, including a dependence on the charge of theion.2. ExperimentalFor the majority of the measurements described here the anionswere generated electrochemically. Molecular C60 (sublimed,99.9 + %, Aldrich) was dissolved in a 5.4:1 toluene:acetonitrile sol-vent containing 0.1 M tetrabutylammonium hexaﬂuorophosphateas a supporting electrolyte. The solution was placed in a spectro-electrochemical cell (2 mm optical pathlength) and deoxygenatedby passing solvent-saturated nitrogen gas through it. It was thenkept under a nitrogen atmosphere throughout the experiment. Acyclic voltammogram was recorded and reduction potentials forthe mono- and dianion were determined. The results, 0.5 V and1.0 V, respectively, are in good agreement with the literature val-ues [8,10]. Electronic absorption spectra were recorded using anear-IR-sensitive multichannel detector, setting the potential at0.8 V and 1.3 V for the mono- and dianion, respectively, asshown in Fig. 1. The spectra were measured as a function of timeafter initiation of the reduction, while monitoring the amount ofcharge transferred. Once the spectra had reached a steady value,the extinction coefﬁcient at the wavelength of maximum absor-bance was determined from the Beer–Lambert law under theassumption of complete reduction to the desired anion. Duringthe reduction of C60 to C260 , an isosbestic point was observed at990 nm and the 1075 nm peak disappeared, consistent with thepresence of only two absorbing species and complete electrochem-ical interconversion. For C60 at 1075 nm, an extinction coefﬁcientof 16 000 M1 cm1 was obtained, while for C260 at 950 nm, a valueof 16 800 M1 cm1 was measured. These results, which are lowerlimits, are within the ranges previously reported in the literature[8,11,12]. From these extinction coefﬁcients, one can predict nano-second radiative lifetimes for both these ions.The structure in the monoanion spectrum shown in Fig. 1a hasbeen discussed in detail in the context of the dynamic Jahn–Tellereffect in an earlier paper [13]. The spectrum comprises (at least) 4distinct bands, assigned to the T1g T1u transition in which both0009-2614/$ - see front matter  2009 Elsevier B.V. All rights reserved.doi:10.1016/j.cplett.2009.04.066* Corresponding author. Fax: +44 1603592003.E-mail address: s.meech@uea.ac.uk (S.R. Meech).Chemical Physics Letters 474 (2009) 112–114Contents lists available at ScienceDirectChemical Physics Lettersjournal homepage: www.elsevier .com/locate /cplet tAuthor's personal copyground and excited states are subject to vibronic coupling via aT1  h Jahn–Teller effect. The observed spectrum was shown tobe consistent with states possessing D3d distorted geometries.The C260 spectrum, Fig. 1b, reveals a simpler structure, with onlytwo distinct bands being observed, both of appreciable intensity.This is consistent with a simple static Jahn–Teller coupling modelproposed originally by Lawson et al. [14,15] in which the T1u andT1g states split into E1u and A2u and E1g and A2g, respectively, toyield the observed pair of transitions, E1g E1u and A2g E1u. Amore realistic dynamical treatment complementary to that at-tempted in Ref. [13] would be likely to yield a much more compli-cated structure for the bands. However, the lack of detail in thefeatures observed precludes any attempt to produce a more realis-tic model.To monitor the dynamics of the ground state recovery, we use asingle-colour pump–probe experiment. The laser source was ahome-built, mode-locked and cavity-dumped titanium:sapphirelaser described in detail elsewhere [16]. For these measurements,intracavity mirrors were exchanged to optimize gain in the 900–1000 nm region. Optimum performance was at 925 nm, producingexcitation into the lowest energy excited state of the dianion, andalso exciting the monoanion with only a small excess energy ofapproximately 1500 cm1 (Fig. 1). At 925 nm, the laser output con-sisted of ca. 40 fs pulses of about 20 nJ energy per pulse at a repe-tition rate of 300 kHz. Pulses were split into pump and probe in theratio 10:1, routed through a conventional pump–probe geometryand recombined in the spectroelectrochemical cell (spot radiusca. 100 lm). The intensity of the transmitted probe beam wasmonitored as a function of pump–probe delay.3. Results and discussionThe transient behaviour of the monoanion solution in tolu-ene:acetonitrile, for pump and probe wavelengths both of925 nm, is shown in Fig. 2a. The transmitted probe intensity in-creases within the 50 fs time resolution of the experiment, consis-tent with bleaching of the ground state. A minor, pulse-limiteddecrease in intensity at around time-zero is succeeded by a domi-nant decrease occurring on a picosecond timescale, which eventu-ally settles to a long-lived offset that relaxes on a much longertimescale. (This slow relaxation, with a lifetime exceeding 100 ps,could not be accurately determined, although from the observedrecovery of the baseline it was established that the sample hadcompletely relaxed before the arrival of the next pump pulse,3 ls later.) The minor pulse-limited transient at around t = 0 is as-signed to ultrafast coherent processes in the solution (includingthe electronic response of the solvent). The data after 100 fs wereﬁtted to a single exponential relaxation plus an essentially con-stant offset, I(t) = exp (t/s) + a. The exponential recovery time, s,was determined to be 3.5 ± 0.5 ps with the very slowly relaxing off-set a = 0.2.The pulse-limited increase in transmission followed by an expo-nential decrease is consistent with internal conversion (IC) fromthe excited state directly repopulating the ground state. Thesemeasurements on the ground state population conﬁrm the assign-ment of radiationless decay through IC by Ehrler et al. [9], based onthe observed ultrafast excited state decay time. The 3.5 ps groundstate recovery in solution is on the same picosecond timescale asthe 2.2 ps excited state decay time measured in the gas phase. Thisresult veriﬁes that the radiationless decay in C60 is dominated byintramolecular relaxation, and that it is at most only weakly inﬂu-enced by the solvent environment. Intramolecular IC may be pro-moted by a structural change between the ground and excitedelectronic states. This result thus highlights the potential signiﬁ-cance of the Jahn–Teller distortion, since the latter provides amechanism for generating the required difference in the excitedand ground state geometries. The ground state recovery time wasalso measured in chemically-generated C60 in THF solution andfound to be 4.0 ± 0.5 ps. This is again consistent with a negligiblerole for the solvent in determining the rate of IC. The implicationof these results is that the key relaxation processes are intrinsicto the fulleride itself. In addition, the similarity of the excited statedecay rate in the gas phase (where the C60 was excited with3600 cm1 excess energy), and those reported here in solution,suggests only a small effect of the excess energy on the lifetime.Another feature also emerges in the ground state recovery cy-cle; the measurement in Fig. 2 reveals the development of a signif-icant population in a comparatively long-lived intermediate state(the long-lived offset accounting for about 20% of the originalbleach). Since this bottleneck state is populated on a picosecondtimescale, it can be assumed to occur in a spin-allowed process,and so competition from intersystem crossing can be ruled out.There are a number of possible assignments for a bottleneck statein the ground state recovery. IC repopulates the electronic groundstate with about 104 cm1 of excess energy in the accepting modesof the radiationless transition. This excess energy will initially be00.51-2 0 2 4 6 8 10 12 14Time/ps-2 0 2 4 6 8 10 12 14Time/psNormalised Intensity00.51Normalised IntensityFig. 2. Ultrafast ground state recovery of two fullerene anions: (a) the dynamicsobserved for C60; (b) C260 . The circles are experimental data and the solid lines ﬁts tomono- or bi-exponential recovery kinetics (plus a constant offset on the picosecondtimescale). For (a), the time-zero response has not been included in the analysis.700 800 900 1000 1100Wavelength/nm00.20.40.60.81AbsorbanceFig. 1. Absorption spectra of electrochemically generated fullerene anions intoluene:acetonitrile solution: (a) C60 (solid line), and (b) C260 (dashed line).M.J. Hope et al. / Chemical Physics Letters 474 (2009) 112–114 113Author's personal copyredistributed to low-frequency intramolecular modes of C60 beforeultimately being transferred to solvent translational and rotationalmotions. The vibrationally-hot state attained after intramolecularvibrational relaxation might have a different electronic spectrumto the equilibrated C60 at 293 K, leading to the observation of along-lived change in absorbance. However, the observed offset per-sists for at least 100 ps, suggesting that the transfer of vibrationalenergy from the low-frequency modes of C60 to the solvent wouldhave to be rather slow compared to the timescales observed withother large organic molecules (such as azulene [17]). This suggestsan unusually weak coupling between the low-frequency modes ofthe solute and the solvent modes.An alternative assignment for the bottleneck state is the popu-lation, through internal conversion, of low-lying vibronic states inthe Jahn–Teller ground state manifold. In particular, a D3d distortedC60 anion has ground vibronic states of T1u, T2u and Gu symmetry[13,18]. However, the product T1u  hg associated with vibroniccoupling between a set of electronic T1u states and a ﬁvefolddegenerate set of vibrations of hg symmetry decomposes in icosa-hedral symmetry as T1u  T2u  Gu  Hu. Thus, there is a set of Hulevels above the ground vibronic states possessing an energy thatis too high to be thermally accessible by an ion in its ground state.These vibronic states could be populated during the vibrationalrelaxation on the ground state surface following IC. Again, theobservations require a lifetime for these states in excess of100 ps. However, it is worth noting in this context that the groundvibronic states {T1u, T2u, Gu} conceptually arise from combinationof the ten ground state D3d wells that occur in the limit of strongvibronic coupling. On the other hand, the Hu states arise from nineground state and an excited D3d well containing one quantum ofvibration. Therefore, relaxation from the Hu states to the otherstates formally occurs with the loss of a quantum of vibrational en-ergy. If the coupling to the solvent is weak, then this process maynot be efﬁcient and the Hu state relatively long-lived.A ﬁnal possibility is a photo-induced chemical reaction of theexcited C60 ion, leading to a change in absorbance that is irrevers-ible on the picosecond timescale, and which recovers only throughdiffusion of the product out of the excited volume to be replaced byfresh, ground-state molecules. This mechanism would be consis-tent with the very slow recovery time. However, it is unlikely thata bimolecular process could compete with the ultrafast IC. In addi-tion, no long-term photochemistry was detected in the steady-state absorption spectra recorded before and after irradiation.One possible mechanism for an intramolecular reaction is electronphotodetachment in a multi-photon process. This is energeticallyfeasible via a two-photon process (the electron afﬁnity of C60 is2.68 eV [19], and the photon energy is 1.34 eV) but since the peakpower is only about 1.5 GW cm2, this mechanism requires a highphotodetachment cross-section at the threshold energy. However,we did not observe a permanent change in the absorption intensityover time, as would be expected for a chemical process with a 20%yield, suggesting that any such process is reversible. Further theindependence of the relaxation rate on solvent polarity is an unex-pected feature if charge transfer to solvent state is signiﬁcant.In Fig. 2b, the ground state recovery for electrochemically gen-erated C260 is shown, again for a pump and probe wavelength of925 nm. The ground state recovery for the dianion is clearly muchfaster than for C60, the former comprising a dominant recoverytime of 400 fs followed by a minor (10%) slower recovery of3.0 ± 1.5 ps and, as was seen in the monoanion, a 20% long-lived(>100 ps) offset. The long component can again be assigned toeither a vibronically hot ground state formed during vibrationalrelaxation, or to the multi-photon photodetachment mechanismdiscussed for the monoanion.The most remarkable feature of the photophysics of C260 is thevery fast IC from the excited state, compared with that for C60. Asalready discussed by Ehrler et al. [9], the fast IC in the C60 is consis-tent with a very strong coupling between the ﬁrst excited andground electronic state. This is in marked contrast to the case forthe C60 itself, where IC is so slow that the spin-forbidden intersys-tem crossing dominates the radiationless decay [4]. The present re-sult shows that this coupling is stronger still for the dianion. This,in turn, suggests a greater Franck–Condon overlap between the ex-cited and ground state vibronic wavefunctions, which may be aconsequence of the different Jahn–Teller distortions operating inthe two ions. Once again, a distortion of the nuclear frameworkmay prove to be a key component of the mechanism for excitedstate decay.In conclusion, the excited state decays of two fullerene anionshave been measured in the condensed phase for the ﬁrst time.The decay mechanism has been identiﬁed as ultrafast internal con-version. A comparison with the excited state lifetime of the mono-anion in the gas phase [9] indicates that intramolecular relaxationdominates, and that the amount of excess energy in the initially ex-cited state exerts only a weak inﬂuence on the lifetime. The datashow that the rate of internal conversion, and by inference thevibrational coupling of ground and excited state, is sensitive tothe charge on the anion. Finally, for both fullerides, a bottleneckhas been detected in the ground state recovery cycle. Amongst sev-eral alternative explanations – which could not be distinguished bythe present measurements – all share one striking feature; theystrongly indicate that the anion photophysics is relatively insensi-tive to any coupling between the fulleride and solvent.AcknowledgementsWe are grateful to EPSRC for ﬁnancial support for this work(Grant Nos. GR/S43894/01 and GR/S43900/01).References[1] O. Gunnarsson, Alkali-Doped Fullerides, Narrow-Band Solids with UnusualProperties, World Scientiﬁc, Singapore, 2004.[2] C.A. Reed, R.D. Bolskar, Chem. Rev. 100 (2000) 1075.[3] T.W. Ebbesen, K. Tanigaki, S. Kuroshima, Chem. Phys. Lett. 181 (1991) 501.[4] A. Sassara, G. Zerza, M. Chergui, V. Ciulin, J.D. Ganiere, B. Deveaud, J. Chem.Phys. 111 (1999) 689.[5] A.G. Stepanov, M.T. Portella-Oberli, A. Sassara, M. Chergui, Chem. Phys. Lett.358 (2002) 516.[6] G.A. Heath, J.E. McGrady, R.L. Martin, J. Chem. Soc. Chem. Commun. (1992)1272.[7] T. Kato et al., Chem. Phys. Lett. 186 (1991) 35.[8] M.M. Khaled, R.T. Carlin, P.C. Trulove, G.R. Eaton, S.S. Eaton, J. Am. Chem. Soc.116 (1994) 3465.[9] O.T. Ehrler, J.P. Yang, C. Hättig, A. Unterreiner, H. Hippler, M. Kappes, J. Chem.Phys. 125 (2006) 074312.[10] Q. Xie, E. Perez-Cordero, L. Echegoyen, J. Am. Chem. Soc. 114 (1992) 3978.[11] T. Kato, Laser Chem. 14 (1993) 155.[12] P.C. Trulove, R.T. Carlin, G.R. Eaton, S.S. Eaton, J. Am. Chem. Soc. 117 (1995)6265.[13] I.D. Hands, J.L. Dunn, C.A. Bates, M.J. Hope, S.R. Meech, D.L. Andrews, Phys. Rev.B 77 (2008) 115445.[14] D.R. Lawson, D.L. Feldheim, C.A. Foss, P.K. Dorhout, C.M. Elliott, C.R. Martin, B.Parkinson, J. Phys. Chem. 96 (1992) 7175.[15] D.R. Lawson, D.L. Feldheim, C.A. Foss, P.K. Dorhout, C.M. Elliott, C.R. Martin, B.Parkinson, J. Electrochem. Soc. 139 (1992) L68.[16] J. Garduno-Mejia, S.R. Meech, Opt. Commun. 259 (2006) 840.[17] S.A. Kovalenko, R. Schanz, H. Hennig, N.P. Ernsting, J. Chem. Phys. 115 (2001)3256.[18] J.L. Dunn, C.A. Bates, Phys. Rev. B 52 (1995) 5996.[19] X.B. 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Observation of ultrafast internal conversion in fullerene anions in solution

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