The temperature effects on the spectral properties and photophysics of a new d(8)-d(8) dinuclear Pt-II chromophore, [Pt(ppy)(mu-Ph(2)pz)](2) (ppy is 2-phenylpyridine and Ph(2)pz is 3,5-diphenylpyrazolate), have been investigated. The thermochromic shifts are tentatively ascribed to intramolecular sigma interactions between the two pseudocofacial d(Z2) orbitals. Substantial emission profile changes occur in the solid state, solution, and doped polymer films

Castellano, Felix N.

Rachford, Aaron A.

English

Bowling Green State University: ScholarWorks@BGSU

Bowling Green State University ScholarWorks@BGSU Chemistry Faculty Publications Chemistry 12-2009 Thermochromic Absorption And Photoluminescence In [pt(ppy)(mu-ph(2)pz)](2) Aaron A. Rachford Felix N. Castellano Bowling Green State University, castell@bgsu.edu Follow this and additional works at: https://scholarworks.bgsu.edu/chem_pub  Part of the Chemistry Commons Repository Citation Rachford, Aaron A. and Castellano, Felix N., "Thermochromic Absorption And Photoluminescence In [pt(ppy)(mu-ph(2)pz)](2)" (2009). Chemistry Faculty Publications. 67. https://scholarworks.bgsu.edu/chem_pub/67 This Article is brought to you for free and open access by the Chemistry at ScholarWorks@BGSU. It has been accepted for inclusion in Chemistry Faculty Publications by an authorized administrator of ScholarWorks@BGSU. pubs.acs.org/ICPublished on Web 10/23/2009r 2009 American Chemical SocietyInorg. Chem. 2009, 48, 10865–10867 10865DOI: 10.1021/ic901156zThermochromic Absorption and Photoluminescence in [Pt(ppy)(μ-Ph2pz)]2Aaron A. Rachford and Felix N. Castellano*Department of Chemistry and Center for Photochemical Sciences, Bowling Green State University,Bowling Green, Ohio 43403Received June 16, 2009The temperature effects on the spectral properties and photo-physics of a new d8-d8 dinuclear PtII chromophore, [Pt(ppy)(μ-Ph2pz)]2 (ppy is 2-phenylpyridine and Ph2pz is 3,5-diphenylpyrazolate), have been investigated. The thermochromicshifts are tentatively ascribed to intramolecular σ interactionsbetween the two pseudocofacial dz2 orbitals. Substantial emissionprofile changes occur in the solid state, solution, and dopedpolymer films.Dinuclear transition-metal complexes have captured theimagination of researchers in a variety of applications in-cluding photocatalysis, solution and solid-state lumine-scence, and photoinduced energy and electron transfer.1-3In addition to these desirable characteristics, PtII chromo-phores possess the ability to tune their photophysical char-acteristics with a judicial choice of ancillary and charge-transfer (CT) ligands.4 Recently, it was demonstrated thatsystematic manipulations in the structure of bridging ligandscontrolled intramolecular Pt-Pt distances in select dinuclearstructures, ultimately tuning the photophysics in a predictivemanner.5-7 We note that temperature-dependent lumines-cence and photophysics have recently been observedfor pyrazolato-bridged trinuclear complexes of the coinagemetals.8In this contribution, we investigate the temperature effectson the spectral properties and photophysics in [Pt(ppy)(μ-Ph2pz)]2 (2; ppy is 2-phenylpyridine and Ph2pz is 3,5-diphenylpyrazolate). The thermochromic responses are ten-tatively ascribed to intramolecular σ interactions occurringbetween the two pseudocofacial dz2 orbitals. We also demon-strate significant emission profile changes in the solid state,solution, and chromophore-doped polymer films of 2. Pre-sented below are the structures of three dinuclear PtII com-plexes (1-3) investigated for their thermochromic response(see the Supporting Information for full synthetic details).These molecules feature two square-planar PtII centers with2-phenylpyridine as the CT ligand bridged by symmetricallysubstituted 3,5-R2-pyrazolates, where R = H (1), Ph (2), ort-Bu (3). These substitutions were selected to coarsely controlthe Pt-Pt distance.5-7 Compound 1 is expected to have thelargest Pt-Pt separation and exhibit mononuclear-like pho-tophysics. In contrast, 3 should provide the shortest Pt-Ptdistance and, thus, dinuclear-based photophysics. Accord-ingly, model chromophores 1 and 3 exhibit well-definedabsorption/emission properties, enabling comparativetemperature-dependent studies in concert with the title com-pound.The molecules were isolated in good yield as microcrystal-line solids or powders ranging in color from bright yellow (1)*To whom correspondence should be addressed. E-mail: castell@bgsu.edu.(1) (a) Roundhill, D. M.; Gray, H. B.; Che, C.-M. Acc. Chem. Res. 1989,22, 55–61. (b) Esswein, A. J.; Nocera, D. G. Chem. Rev. 2007, 107, 4022–4047.(c) Givaja, G.; Volpe, M.; Edwards, M. A.; Blake, A. J.; Wilson, C.; Schr€oder, M.;Love, J. B. Angew. Chem., Int. Ed. 2007, 46, 584–586. (d) Yamauchi, K.;Masaoka, S.; Sakai, K. J. Am. Chem. Soc. 2009, 131, ASAP.(2) (a) Lai, S.-W.; Chan, M. C. W.; Cheung, K.-K.; Peng, S.-M.; Che,C.-M. Organometallics 1999, 18, 3991–3997. (b) Ma, B.; Djurovich, P. I.;Yousufuddin, M.; Bau, R.; Thompson, M. E. J. Phys. Chem. C 2008, 112, 8022–8031.(3) (a) Alsindi, W. Z.; Easun, T. L.; Sun, X.-Z.; Ronayne, K. L.; Towrie,M.; Herrera, J.-M.; George, M. W.; Ward, M. D. Inorg. Chem. 2007, 46,3696–3704. (b) Jager, M.; Kumar, R. J.; Gorls, H.; Bergquist, J.; Johansson, O.Inorg. Chem. 2009, 48, 3228–3238.(4) (a) Brooks, J.; Babayan, Y.; Lamansky, S.; Djurovich, P. I.; Tsyba, I.;Bau, R.; Thompson,M. E. Inorg. Chem. 2002, 41, 3055–3066. (b) Develay, S.;Blackburn, O.; Thompson, A. L.; Williams, J. A. G. Inorg. Chem. 2008, 47,11129–11142. (c) Castellano, F. N.; Pomestchenko, I. E.; Shikhova, E.; Hua, F.;Muro, M. L.; Rajapakse, N. Coord. Chem. Rev. 2006, 250, 1819–1828. (d) Hua,F.; Kinayyigit, S.; Cable, J. R.; Castellano, F. N. Inorg. Chem. 2006, 45, 4304–4306. (e) Goeb, S.; Rachford, A. A.; Castellano, F. N. Chem. Commun. 2008,814–816. (f) Rachford, A. A.; Goeb, S.; Castellano, F. N. Inorg. Chem. 2008, 47,4348–4355. (g) Rachford, A. A.; Hua, F.; Adams, C. J.; Castellano, F. N. DaltonTrans. 2009, 3950–3954.(5) Ma, B.; Li, L.; Djurovich, P. I.; Yousufuddin, M.; Bau, R.;Thompson, M. E. J. Am. Chem. Soc. 2005, 127, 28–29.(6) Ma, B.; Djurovich, P. I.; Garon, S.; Alleyne, B.; Thompson, M. E.Adv. Funct. Mater. 2006, 16, 2438–2446.(7) Saito, K.; Nakao, Y.; Sakaki, S. Inorg. Chem. 2008, 47, 4329–4337.(8) (a) Omary, M. A.; Rawashdeh-Omary, M. A.; Gonser, M. W. A.;Elbjeirami, O.; Grimes, T.; Cundari, T. R.; Diyabalanage, H. V. K.;Gamage, C. S. P.; Dias, H. V. R. Inorg. Chem. 2005, 44, 8200–8210. (b)Omary, M. A.; Elbjeirami, O.; Gamage, C. S. P.; Sherman, K. M.; Dias, H. V. R.Inorg. Chem. 2009, 48, 1784–1786.10866 Inorganic Chemistry, Vol. 48, No. 23, 2009 Rachford and Castellanoto orange (2) to red (3) at room temperature (RT). However,the solids with the most steric bulk (2 and 3) exhibit drasticinstantaneous reversible color changes when cooled to 77 K.Provided in Figure 1 are representative digital color photo-graphs of powdered samples of 2 and 3. At RT, the solidsappear orange or red; upon exposure to liquid nitrogen, theabsorption immediately blue-shifts, producing yellow colora-tion. This is in direct opposition to earlier examples of solid-state thermochromism arising from intermolecular PtII-PtIIinteractions in the “red form” of both Pt(bpy)Cl2 andPt(bpy)(CN)2, where cooling to 10 K produced a systematiccontraction along the intermolecular z axis, red-shifting theabsorbance and emission spectra.9,10 Todelineate between anintra- versus intermolecularmechanism, the absorption spec-tra of dilute toluene solutions of 2 and 3 (∼7 10-5 M) werecollected between 273 and373K (Figure S1 in the SupportingInformation), where a red shift is clearly observed when thesamples are heated. This provides evidence that the thermo-chromism we observe in the solids and solutions most likelyarises from intramolecular interactions in both 2 and 3. Webelieve that the net σ overlap of the two dz2 orbitals decreasesas the temperature is lowered. However, more quantitativestudies on the molecular structure (X-ray) of the solidsremain a point of interest for future investigations.The absorption and emission spectra of all three chromo-phores in 2-methyltetrahydrofuran (MTHF) are provided inFigure 2. In agreementwith previous results, the lowest-energyCT absorptionmaximum systematically shifts to lower energyfrom 366 nm (1) to 406 nm (2) to 497 nm (3) as the steric bulkon the bridging pyrazolate increases. This is attributed toincreasing intramolecular Pt-Pt interactions (decreasingPt-Pt distances), culminating with the most red-absorbing(3). Consequently, the Franck-Condon state is assigned asmononuclear CT for 1, while 2 and 3 are assigned as dinuclearmetal-metal-to-ligand CT (MMLCT). The RT emission alsosystematically changes in shape and position across the series.The “mononuclear” model, 1, produces high-energy struc-tured photoluminescence with a maximum at 488 nm (Φem=0.07; τ=750 ns), consistent with a 3LC/MLCT excited state.5While two geometrical isomers of 1 are observable by 1HNMR (see the Supporting Information), both cis and transisomers exhibit identical photophysical behavior, consistentwith a mononuclear-like excited state. Here, cis and transdefine the relative positions of the nitrogen atoms in the twoppy ligands. The emission from 2 is broad and unstructuredcentered at∼625 nm (Φem=0.15; τ=2.0μs),most consistentwith MMLCT (3π*f σ*) parentage. TheMMLCT emissionfrom 3 is also broad and featureless and is themost red-shifted,λmax = 650 nm (Φem = 0.03; τ = 950 ns). The RT photo-luminescence assignments in 1 and 3 appear to be preserved at77K.However,2 generates amore complex emissionprofile at77 K (Figure 2). Hence, we further investigated the emissiontemperature dependence of 2 in MTHF.These data are presented in Figure 3. When the solution isheated through the glass transition temperature (Tg∼ 91K)11(panel A), the emission profile simply red-shifts and loses thehigh-energy vibrational structure. However, upon heatingfrom 180 to 300 K (panel B), the emission maximum at∼550nm concomitantly decreases with the appearance of a newemission maximum at ∼625 nm. Finally, only one photo-luminescence band near 625 nm is observed at 300 K, inagreement with the RT data in Figure 2. From the tempera-ture-dependent data, themolecule apparently displaysmono-nuclear (3LC at 480 nm and 3MLCT at 525 nm) emission inthe temperature range of 77-140 K.12,13 The 3MLCT emis-sion at 80K (525 nm) shifts to∼550 nm above 100 K. Above160 K, the ground state appears to traverse an activatedbarrier crossing, revealing dinuclear (MMLCT) emission.In essence, the ground-state geometry undergoes a reorgani-zation that somehow enforces more significant Pt-Pt σoverlap with potentially shorter Pt-Pt distances at higherFigure 1. Digital images illustrating the reversible thermochromic colorchanges of 2 (left) and 3 (right) as solid powders.Figure 2. Steady-state absorption and emission (inset) spectra of diluteMTHF solutions containing dinuclear PtII complexes. Emission spectrawere acquiredonargon-degassed solutions at concentrations of∼10-5M.Figure 3. Photoluminescence temperature dependence of 2 in MTHF(10-5 M; λex = 410 nm). Panel A depicts changes in the emission profilearound the glass transition of the solvent (Tg = 91 K). Panel B depictsemission profile changes at temperatures above the glass transitiontemperature.(9) Connick, W. B.; Henling, L. M.; Marsh, R. E.; Gray, H. B. Inorg.Chem. 1996, 35, 6262–6265.(10) Kato, M.; Kosuge, C.; Morii, K.; Ahn, J. S.; Kitagawa, H.; Mitani,T.; Matsushita, M.; Kato, T.; Yano, S.; Kimura, M. Inorg. Chem. 1999, 38,1638–1641.(11) Locknar, S. A.; Chowdhury, A.; Peteanu, L. A. J. Phys. Chem. B2000, 104, 5816–5824.(12) Colombo, M. G.; Brunold, T. C.; Riedener, T.; G€udel, H. U.;F€ortsch, M.; B€urgi, H.-B. Inorg. Chem. 1994, 33, 545–550.(13) Bailey, J. A.; Miskowski, V. M.; Gray, H. B. Inorg. Chem. 1993, 32,369–370.Communication Inorganic Chemistry, Vol. 48, No. 23, 2009 10867temperature. Variable-temperature excitation scans revealtemperature-dependent spectral changes that must be attrib-uted to modification in the ground-state structure (Figure S2in the Supporting Information). Approximating 160K as thecritical temperature provides a gross estimation of the activa-tion energy, 0.014 eV (113 cm-1, 0.32 kcal mol-1). Notably,dilute MTHF solutions of 3 simply display a smooth transi-tion in the emissionmaximumanda broadening in the profileas the temperature increases from 80 to 300 K (Figure S3 inthe Supporting Information). This behavior is not limited todilute solutions because we also observe similar behavior inmicrocrystalline filmsof 2depositedonglass slides (Figure S4in the Supporting Information). However, our model com-pounds 1 and 3 only exhibit a narrowing of the respective3LC/MLCT and 3MMLCT emission in solid thin films(Figure S5 in the Supporting Information). It thereforeappears that the ground-state structural reorganization isexclusive to 2.We have also investigated the emission temperature de-pendence of 2 in several host polymer films. Presented inFigure 4 is the emission temperature dependence of 2 in threedifferent polymer matrices of varying rigidity. While poly-(methyl methacrylate) (PMMA) is a rigid glass at RT (Tg =378 K),14 the rubbery Tecoflex EG-80A and Carbothaneprovide a less rigid environment (Tg < RT) and present theopportunity to probe both temperature and rigidity effectssimultaneously. In PMMA (panel A), the emission profile at77 K resembles that observed in MTHF at 77 K. The mostsignificant changes in the emission profile arise simply from aloss in the vibronic structure upon heating to RT. Weconclude that only mononuclear photoluminescence isobserved for a PMMA film of 2 throughout the temperaturerange of 77-300K.Only upon heating of the PMMA film totemperatures exceeding the glass transition does the emissionshift to much longer wavelengths similar to those seen inMTHF at 295 K (Figure S6 in the Supporting Information).Hence, the rigid nature of the polymer prevents the chromo-phore from undergoing important structural reorganiza-tional changes at higher temperatures. The incorporationof 2 into softer polymer matrices such as Tecoflex EG-80Aand Carbothane (panels B and C, respectively) yields dis-tinctively different spectroscopic behavior than that of rigidPMMA. During heating from 80 to 240 K in TecoflexEG-80A, the emission intensity decreases with a red shift inthe emission maximum to 550 nm. Upon further heating,photoluminescence decreases in intensity while a shoulder at∼625nmemerges. Carbothane provides similar transitions inthe emission profile (panel C). However, the shoulder at∼625 nm is now much more pronounced at 300 K. Controlexperiments of 1 in Tecoflex EG-80A do not exhibit similarspectral shifts (Figure S7 in the Supporting Information),further suggesting that geometric reorganization is necessaryto produce MMLCT photoluminescence in the softer poly-mer matrices.In conclusion, we have reported the temperature effects onthe absorption/emission properties of three dinuclear PtIIchromophores. The ability to design a molecule whosespectroscopic properties can be carefully controlled simplythrough variation in the temperature has been achieved. Thetitle compound demonstrates a thermochromic shift in thesolid state and solution, likely to arise through intramolecularσ interactions between the two pseudocofacial d8 PtII centers.It is rather interesting to note that this intramolecular ther-mochromism produces color changes in opposition to thecorresponding intermolecular processes occurring betweenPtII complexes in the solid state. Compound 2 was shown totraverse an activated barrier at temperatures above 160 K,demonstrated by dramatic changes in the emission profile.This phenomenon is observable in a variety of media and isconsistent with intimate intramolecular σ interactions occur-ring between the two metal centers in the ground state.Acknowledgment. This work was supported by fundingfrom the Air Force Office of Scientific Research (GrantFA 9550-05-1-0276) and the National Science Foun-dation (Grants CBET-0731153 and CHE-0719050).Dr. Tanya N. Singh-Rachford and Dr. XianghuaiWang are acknowledged for their assistance. We thank areviewer for valuable experimental suggestions.Supporting Information Available: Synthetic and experimen-tal details as well as additional spectroscopic measurements.Thismaterial is available free of charge via the Internet at http://pubs.acs.org.Figure 4. Photoluminescence temperature dependence of 2 in PMMA(A), TecoflexEG-80A (B), andCarbothane (C). The spectra displayed areat 80-300 K in 20 K increments; λex = 410 nm.(14) Handbook of Chemistry and Physics, 77th ed.; CRC Press: New York,1997.

Thermochromic Absorption And Photoluminescence In [pt(ppy)(mu-ph(2)pz)](2)

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Thermochromic Absorption And Photoluminescence In [pt(ppy)(mu-ph(2)pz)](2)

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