Broad-Band NIR Transient Absorption ... - ACS Publications

Communication pubs.acs.org/Organometallics

Broad-Band NIR Transient Absorption Spectroscopy of an “AllCarbon”-Bridged Bimetallic Radical Cation Complex Josef B. G. Gluyas,† Alexandre N. Sobolev,‡ Evan G. Moore,*,§ and Paul J. Low*,† †

School of Chemistry and Biochemistry and ‡Centre for Microscopy Characterization and Analysis, University of Western Australia, 35 Stirling Highway, Crawley 6009, Australia § School of Chemistry and Molecular Biosciences, University of Queensland, Brisbane 4072, Australia

Downloaded by UNIV OF PRINCE EDWARD ISLAND on August 31, 2015 | http://pubs.acs.org Publication Date (Web): August 5, 2015 | doi: 10.1021/acs.organomet.5b00534

S Supporting Information *

ABSTRACT: Broad-band near-infrared (NIR) transient absorption (TA) spectroscopy has been used for the first time to probe an all-carbon-bridged organometallic radical cation complex. The compound [{Ru(PPh3)2Cp}2(μ-CCCC)]+ ([1]+) was investigated in dichloromethane and acetonitrile solutions, using laser excitation at 700, 800, 900, and 1000 nm; these wavelengths span the NIR absorption band envelope. The resulting TA spectra were found to be independent of excitation wavelength and consist of an excited state absorption feature with a peak at ca. 1150 nm and the corresponding bleach signal of the ground-state NIR absorption band, which both decay to 0 over the 50 ps time window investigated. Data were analyzed globally and fit collectively for each of the four different excitation wavelengths, with the resulting best fit to a biexponential decay function indicating two processes with slightly different time scales of ca. 1.5 and ca. 9.0 ps involved in the relaxation to the ground state.

U

lived excited states that arise from photoinduced charge transfer provides valuable information concerning solvent dynamics and electronic relaxation processes, from which back electron transfer rates can be extracted. The compound [{Ru(PPh3)2Cp}2(μ-CCCC)] (1) is an archetypal example of an “all-carbon”-bridged bimetallic organometallic complex. Many previous studies of 1 and related systems have described the “wirelike” properties of the polyyndiyl ligand and the delocalized electronic structure of radical cations such as [1]+.30 It has recently been shown that the delocalized radical cation [1]+ exhibits multiple transitions in the NIR region, which have been attributed to π−π* (or charge resonance) and MLCT processes, the latter gaining significant intensity in conformers in which the two Cp rings are more or less orthogonal.12,31,32 Although the ground-state potential energy surface that encompasses these different rotamers is shallow, leading to population of a range of conformers in solution at room temperature, the excited-state barriers are as yet undetermined. Here, broad-band pump− probe TA NIR spectroscopy on the femtosecond time scale has been used to further investigate the excited states of [1]+ generated by photoexcitation at a number of wavelengths that span the NIR absorption envelope. Treatment of the neutral complex {Ru(PPh3)2Cp}2(μ-C CCC) (1) with 1 equiv of ferrocinum hexafluorophosphate in dichloromethane followed by precipitation with diethyl ether afforded [1]PF6 as a forest green powder in 57% yield.32 Crystallization by layer diffusion of diethyl ether into a dichloromethane solution allowed the isolation of crystals suitable for X-ray diffraction. The complex crystallized with an

nderstanding the optical properties and associated electronic structures of compounds of the form [E− bridge−E]•(±) (where E is an electrophore) has been the subject of a great deal of research over the past 50 years, underpinning much of our knowledge concerning the fundamental aspects of electron-transfer reactions and dynamics.1−5 While the term is not strictly accurate in all cases, openshell [E−bridge−E]•(±) radicals are conventionally referred to as mixed-valence (MV) compounds. Typically information concerning the localization or delocalization of the unpaired electron in MV complexes is inferred from the band shape of the critical intervalence charge transfer (IVCT), or charge resonance, band using the relationships initially developed by Hush.6,7 However, caution must be exercised when analyses are based purely on the electronic band shape, since the presence of multiple IVCT transitions, localized interconfigurational transitions, MLCT/LMCT transitions, vibronic coupling effects, and rotational dynamics leading to conformers with distinct electronic characters can result in multiple overlapping transitions.8−13 Thus, the interpretation of NIR spectra is nontrivial in many cases. Very often, further support from spectroscopic evidence, increasingly augmented by computational investigations,14 is required to fully elucidate the electronic character of MV complexes. Pump−probe transient absorption (TA) spectroscopy15 has been used to explore the underlying electronic structure of MV systems with comparatively simple absorption spectra.16−29 The majority of such time-resolved investigations have focused on weakly coupled, valence-localized MV complexes featuring bridging ligands based on nitrogen-containing aromatics16−20 or cyanide;21−28 a more recent investigation focusing on a related neutral organic mixed valence molecule has also been noted.29 Analysis of the time-dependent spectra from the short© 2015 American Chemical Society

Received: January 19, 2015 Published: August 5, 2015 3923

DOI: 10.1021/acs.organomet.5b00534 Organometallics 2015, 34, 3923−3926

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Organometallics

transitions, corresponding to a lower energy π−π* type transition40,41 and higher energy MLCT processes, the relative intensities of which are sensitive to the precise details of the molecular geometry.12,13 In an attempt to probe these processes individually using TA NIR spectroscopy, a range of “pump” energies were chosen (Figure 2) with the intention of biasing the excited state populations toward one or the other of the species responsible for the two differing transitions visible in the NIR band envelope. Hence, transient absorption spectra in the NIR region for [1]PF6 were collected in degassed dichloromethane and degassed acetonitrile solutions using excitation wavelengths of 700, 800, 900, and 1000 nm (Figure 3 and Figures S1 and S4 in


essentially cis orientation of the cyclopentadiene rings (Figure 1), notable as the parent complex 1 has been observed in both

Figure 1. Plot of the ion pair from the structure of [1]PF6·3CH2Cl2. Solvent molecules and hydrogen atoms have been excluded for clarity. Selected bond lengths (Å) and angles (deg): Ru1−C1 1.926(5), C1− C2 1.252(8), C2−C3 1.351(8), C3−C4 1.249(9), C4−Ru2 1.948(6); Ru1−C1− C2 167.1(5), C1−C2−C3 174.9(6), C2−C3−C4 178.6(7), C3−C4−Ru2 169.7(6).

cis33,34 and trans34,35 forms in the solid state. There is a small (0.02 Å) degree of asymmetry in the most precisely determined Ru(1)−C(1) and Ru(2)−C(4) bond lengths of [1]PF6, which may hint at a degree of valence localization in the solid state. Pleasingly, the bond lengths along the six-atom RuCCCCRu chain in [1]+ appear to display a more limited long/short alternation in comparison to that found in cis-134 (Ru−C(1) 2.01(3), C(1)−C(2) 1.24(4), C(2)−C(2′) 1.31(4) Å) and various solvates,33 which fits well to the expectations of previous IR spectroscopic32 and computational13,32 investigations. As discussed elsewhere,12 the NIR absorption band envelope of [1]+ (Figure 2) deviates from the single asymmetric band predicted by the two-state model for strongly coupled MV or delocalized complexes.3 In the case of [1]+ as reported elsewhere12 and in the case of related bimetallic radical cations13,36−39 it has recently been recognized that the NIR absorption envelope consists of at least two overlapping

Figure 3. Observed TA signals in the NIR region from 900 to 1350 nm at various time delays from t = 0.5 (red) to t = 50 ps (blue) for [1]PF6 in degassed dichloromethane using λex 900 nm.

the Supporting Information). The shapes of the resulting TA signals were found to be independent of the excitation wavelength and consist of both an excited-state absorption feature with a peak at ca. 1150 nm and the corresponding bleach signal of the ground-state NIR absorption band, which both decay to 0 over the 50 ps time window investigated. The corresponding kinetic plots of the TA dynamics integrated over 10 nm increments from 900 to 1350 nm for λex 900 nm in dichloromethane are shown in Figure 4 (see also Figures S2 and S5 in the Supporting Information). These decay data were analyzed globally at all wavelengths and fit collectively for each of the four different excitation wavelengths in each solvent to a biexponential decay function, as shown in eq 1 It = A1 exp−1/ τ1(t ) + A 2 exp−1/ τ2(t ) + y0

(1)

where It is the intensity of the transient absorption data at time t, τ1 and τ2 are the lifetimes, A1 and A2 are pre-exponential scaling factors, and y0 is a horizontal offset (∼0) accounting for any noise in the spectral data. The resulting best fit of the data yielded decay constants of τ1 = 1.46 ± 0.07 ps and τ2 = 9.44 ± 0.47 ps for [1]+ in degassed dichloromethane, with the corresponding fits and residuals shown in Figure 4 and Figure S2 in the Supporting Information. In degassed acetonitrile solution, very similar decay constants of τ1 = 1.44 ± 0.07 and τ2 = 8.76 ± 0.39 ps were obtained using the same fitting procedure (Figures S5 and S6 in the Supporting Information). Since the biexponential decay kinetics observed for the transient absorption features are independent of the excitation

Figure 2. NIR absorption spectrum of [1]PF6 recorded in degassed dichloromethane. The black arrows indicate laser excitation wavelengths used for broad-band NIR TA experiments. 3924


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Supporting Information) reveals a progressive blue shift of the TA data at longer time delays and spectral narrowing, which are classical signatures of thermal relaxation processes.46,47 In summary, these data are the first time-resolved spectroscopic measurements to be reported for an all-carbonbridged radical cation complex. Notably, in comparison to simpler cyano-bridged [(NH3)5MII−CN−MIII(CN)5]− compounds, the longer lifetime of the τ1 decay component we obtain for [1]+ is most likely due to the longer four-atom bridge between metal atoms, decreasing the rate of back electron transfer. For previously reported class II mixed valence complexes such as [(NH3)5MII−CN−MIII(CN)5]− (M = Fe, Ru, Os), the terminal ν(CN) mode has been identified as one of the key acceptor vibrations using ps-TRIR spectroscopy.45 In this context, the high-frequency ν(CCCC) mode may play a similar role as an energy acceptor, being able to dissipate a significant quanta of energy from the excited state.48 To this end, we are currently pursuing further investigations of [1]+ using time resolved infrared (TRIR) and time-resolved resonance Raman (TR3) spectroscopy in order to better understand these aspects of the excited-state decay, which may reveal further fascinating aspects of the “wirelike” properties of this bridging ligand. Similarly, we are also investigating related systems with longer all-carbon bridges and analogous C CCN ligands.

Figure 4. Kinetic plots and corresponding global fit of observed transient absorption signals from t = 0.5 to t = 50 ps in 10 nm wavelength increments from 900 nm (red) to 1350 nm (blue) for [1]PF6 in degassed dichloromethane using λex 900 nm. Note that the log scale is on the x axis.

wavelength (and intramolecular rotation is typically on the picosecond time scale),42,43 it is unlikely that they correspond to decay of the individual π−π* and MLCT excited states associated with the different rotational conformers. Instead, it appears that efficient internal conversion converts the higher energy MLCT to the lower energy π−π* state within 100 fs of excitation. As such, there remain two excited-state relaxation mechanisms operating in this complex. The shorter lifetime component can be readily attributed to the ground state recovery process (reverse electron transfer), with a lifetime of ca. 1.45 ps which matches the recovery of the ground state “bleach” signal. The second process with a slightly longer ca. 9 ps lifetime is manifested by a much weaker excited-state absorption feature centered at 1080 nm (see Figures S3 and S6 in the Supporting Information). Similar behavior has been previously reported21 by Barbara and Hupp, who undertook variable-wavelength TA studies on a [(NH3)5FeII−CN−RuIII(CN)5]− mixed-valence complex. In that case, a long-lived component was also observed which was assigned to a small fraction of RuIIFeIII excited-state complexes which decay via an unassigned higher energy RuIIIFeII excited state. A similar situation may apply here, in which case the longer-lived lifetime may correspond to a higher energy excited state formed on the sub-picosecond time scale. Alternately, the spectral features we observe for [1]+ at longer delays may be due to vibrational cooling of an initially formed hot ground state upon back electron transfer. This type of behavior has been reported in more recent studies on the relaxation processes occurring in [(NH 3 ) 5 M II −CN− MIII(CN)5]− complexes (M = Ru, Os),22,44,45 with back electron transfer taking place on the order of τ < 0.5 ps to a vibrationally hot ground state, which further decays over the τ = 2−6 ps (M = Ru) or τ = 1.8−19 ps (M = Os) time scale. In the present case, for [1]+, the weak solvent dependence of the τ2 lifetime suggests that solvent interactions play an important role in the relaxation kinetics for this process, and the observed trend agrees with that expected on the basis of the thermal diffusivities of acetonitrile in comparison to dichloromethane (10.7 × 10−8 vs 8.81 × 10−8 m2 s−1, respectively).46 Similarly, inspection of the normalized TA data (see Figure S7 in the

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.5b00534. Full synthetic, spectroscopic, and crystallographic experimental details and plots of the transient absorption signals, kinetic data, and kinetic deconvolutions for [1]PF6 at λex 700, 800, 900, and 1000 nm in degassed dichloromethane and acetonitrile solution (PDF) Crystallographic data for [1]PF6·3CH2Cl2 (CCDC1043279) (CIF)

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AUTHOR INFORMATION

Corresponding Authors

*E.G.M.: tel, +61 7 3365 3862; e-mail, [email protected]. *P.J.L.: tel, +61 8 6488 3045; e-mail, [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the ARC (DP140100855). P.J.L. and E.G.M. hold ARC future fellowships (FT12010073 and FT100100795). Ultrafast measurements were undertaken at the Photochemistry and Ultrafast Laser Spectroscopy (PULSE) facility, School of Chemistry and Molecular Biosciences, established with financial support by the University of Queensland (MEI-2013000106).

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REFERENCES

(1) Creutz, C. Prog. Inorg. Chem. 1983, 30, 1−73. (2) Richardson, D. E.; Taube, H. Coord. Chem. Rev. 1984, 60, 107− 129. (3) Brunschwig, B. S.; Creutz, C.; Sutin, N. Chem. Soc. Rev. 2002, 31, 168−184.

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Organometallics (4) Heckmann, A.; Lambert, C. Angew. Chem., Int. Ed. 2012, 51, 326−392. (5) Hankache, J.; Wenger, O. S. Chem. Rev. 2011, 111, 5138−5178. (6) Hush, N. S. Prog. Inorg. Chem. 1967, 8, 391−444. (7) Hush, N. S. Electrochim. Acta 1968, 13, 1005−1023. (8) Demadis, K. D.; Hartshorn, C. M.; Meyer, T. J. Chem. Rev. 2001, 101, 2655−2686. (9) Concepcion, J. J.; Dattelbaum, D. M.; Meyer, T. J.; Rocha, R. C. Philos. Trans. R. Soc., A 2008, 366, 163−175. (10) D’Alessandro, D. M.; Keene, F. R. Dalton Trans. 2004, 3950− 3954. (11) Fox, M. A.; Le Guennic, B.; Roberts, R. L.; Brue, D. A.; Yufit, D. S.; Howard, J. A. K.; Manca, G.; Halet, J.-F.; Hartl, F.; Low, P. J. J. Am. Chem. Soc. 2011, 133, 18433−18446. (12) Parthey, M.; Gluyas, J. B. G.; Schauer, P. A.; Yufit, D. S.; Howard, J. A. K.; Kaupp, M.; Low, P. J. Chem. Eur. J. 2013, 19, 9780− 9784. (13) Parthey, M.; Gluyas, J. B. G.; Fox, M. A.; Low, P. J.; Kaupp, M. Chem. Eur. J. 2014, 20, 6895−6908. (14) Parthey, M.; Kaupp, M. Chem. Soc. Rev. 2014, 43, 5067−5088. (15) Stolow, A.; Bragg, A. E.; Neumark, D. M. Chem. Rev. 2004, 104, 1719−1758. (16) Matsui, K.; Nazeeruddin, M. K.; Humphrybaker, R.; Gratzel, M.; Kalyanasundaram, K. J. Phys. Chem. 1992, 96, 10587−10590. (17) Song, X. Q.; Lei, Y. B.; Vanwallendal, S.; Perkovic, M. W.; Jackman, D. C.; Endicott, J. F.; Rillema, D. P. J. Phys. Chem. 1993, 97, 3225−3236. (18) Strouse, G. F.; Schoonover, J. R.; Duesing, R.; Boyde, S.; Jones, W. E.; Meyer, T. J. Inorg. Chem. 1995, 34, 473−487. (19) Abbott, L. C.; Arnold, C. J.; Ye, T.-Q.; Gordon, K. C.; Perutz, R. N.; Hester, R. E.; Moore, J. N. J. Phys. Chem. A 1998, 102, 1252−1260. (20) Dattelbaum, D. M.; Hartshorn, C. M.; Meyer, T. J. J. Am. Chem. Soc. 2002, 124, 4938−4939. (21) Walker, G. C.; Barbara, P. F.; Doorn, S. K.; Dong, Y. H.; Hupp, J. T. J. Phys. Chem. 1991, 95, 5712−5715. (22) Kliner, D.; Tominaga, K.; Walker, G. C.; Barbara, P. F. J. Am. Chem. Soc. 1992, 114, 8323−8325. (23) Tominaga, K.; Kliner, D. A. V.; Johnson, A. E.; Levinger, N. E.; Barbara, P. F. J. Chem. Phys. 1993, 98, 1228. (24) Reid, P. J.; Silva, C.; Barbara, P. F.; Karki, L.; Hupp, J. T. J. Phys. Chem. 1995, 99, 2609−2616. (25) Son, D. H.; Kambhampati, P.; Kee, T. W.; Barbara, P. F. J. Phys. Chem. A 2002, 106, 4591−4597. (26) Kambhampati, P.; Son, D. H.; Kee, T. W.; Barbara, P. F. J. Phys. Chem. A 2000, 104, 10637−10644. (27) Macpherson, B. P.; Bernhardt, P. V.; Hauser, A.; Pagès, S.; Vauthey, E. Inorg. Chem. 2005, 44, 5530−5536. (28) Chen, Y.-J.; Odongo, O. S.; McNamara, P. G.; Szacilowski, K. T.; Endicott, J. F. Inorg. Chem. 2008, 47, 10921−10934. (29) Maksimenka, R.; Margraf, M.; Köhler, J.; Heckmann, A.; Lambert, C.; Fischer, I. Chem. Phys. 2008, 347, 436−445. (30) Bruce, M. I.; Low, P. J. Adv. Organomet. Chem. 2004, 50, 179− 444. (31) Bruce, M. I.; Denisovich, L. I.; Low, P. J.; Peregudova, S. M.; Ustynyuk, N. A. Mendeleev Commun. 1996, 6, 200−201. (32) Bruce, M. I.; Low, P. J.; Costuas, K.; Halet, J.-F.; Best, S. P.; Heath, G. A. J. Am. Chem. Soc. 2000, 122, 1949−1962. (33) Bruce, M. I.; Hall, B. C.; Kelly, B. D.; Low, P. J.; Skelton, B. W.; White, A. H. J. Chem. Soc., Dalton Trans. 1999, 3719−3728. (34) Bruce, M. I.; Hinterding, P.; Tiekink, E. R.; Skelton, B. W.; White, A. H. J. Organomet. Chem. 1993, 450, 209−218. (35) Bruce, M. I.; Ellis, B. G.; Gaudio, M.; Lapinte, C.; Melino, G.; Paul, F.; Skelton, B. W.; Smith, M. E.; Toupet, L.; White, A. H. Dalton Trans. 2004, 1601. (36) Fitzgerald, E. C.; Ladjarafi, A.; Brown, N. J.; Collison, D.; Costuas, K.; Edge, R.; Halet, J.-F.; Justaud, F.; Low, P. J.; Meghezzi, H.; Roisnel, T.; Whiteley, M. W.; Lapinte, C. Organometallics 2011, 30, 4180−4195.

(37) Lehrich, S. W.; Hildebrandt, A.; Rüffer, T.; Korb, M.; Low, P. J.; Lang, H. Organometallics 2014, 33, 4836−4845. (38) Pfaff, U.; Hildebrandt, A.; Korb, M.; Schaarschmidt, D.; Rosenkranz, M.; Popov, A.; Lang, H. Organometallics 2015, 34, 2826− 2840. (39) Kong, D.-D.; Xue, L.-S.; Jang, R.; Liu, B.; Meng, X.-G.; Jin, S.; Ou, Y.-P.; Hao, X.; Liu, S.-H. Chem. - Eur. J. 2015, 21, 9895−9904. (40) Maurer, J.; Winter, R. F.; Sarkar, B.; Záliš, S. J. Solid State Electrochem. 2005, 9, 738−749. (41) Lear, B. J.; Chisholm, M. H. Inorg. Chem. 2009, 48, 10954− 10971. (42) Beeby, A.; Findlay, K. S.; Low, P. J.; Marder, T. B.; Matousek, P.; Parker, A. W.; Rutter, S. R.; Towrie, M. Chem. Commun. 2003, 2406−2407. (43) Greaves, S. J.; Flynn, E. L.; Futcher, E. L.; Wrede, E.; Lydon, D. P.; Low, P. J.; Rutter, S. R.; Beeby, A. J. Phys. Chem. A 2006, 110, 2114−2121. (44) Doorn, S. K.; Dyer, R. B.; Stoutland, P. O.; Woodruff, W. H. J. Am. Chem. Soc. 1993, 115, 6398−6405. (45) Doorn, S. K.; Stoutland, P. O.; Dyer, R. B.; Woodruff, W. H. J. Am. Chem. Soc. 1992, 114, 3133−3134. (46) Tan, X.; Gustafson, T. L.; Lefumeux, C.; Burdzinski, G.; Buntinx, G.; Poizat, O. J. Phys. Chem. A 2002, 106, 3593−3598. (47) Ye, X.; Demidov, A.; Rosca, F.; Wang, W.; Kumar, A.; Ionascu, D.; Zhu, L.; Barrick, D.; Wharton, D.; Champion, P. M. J. Phys. Chem. A 2003, 107, 8156−8165. (48) Kober, E. M.; Caspar, J. V.; Lumpkin, R. S.; Meyer, T. J. J. Phys. Chem. 1986, 90, 3722−3734.

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Broad-Band NIR Transient Absorption ... - ACS Publications

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