Optical Behavior of Conjugated Pt-Containing Polymetallaynes

ARTICLE pubs.acs.org/JPCB

Optical Behavior of Conjugated Pt-Containing Polymetallaynes Exposed to Gamma-Ray Radiation Doses Augusto Batagin-Neto,† Erika S. Bronze-Uhle,† David M. Fernandes,† Ilaria Fratoddi,*,‡ Iole Venditti,‡ Franco Decker,‡ Enrico Bodo,‡ Maria Vittoria Russo,‡ and Carlos F. O. Graeff † † ‡

Department of Physics, FC-UNESP, Av. Eng. Luiz Edmundo Carrijo Coube 14-01, 17033-360 Bauru, Brazil Department of Chemistry, University of Rome “Sapienza”, P. le A. Moro 5, 00185 Rome, Italy

bS Supporting Information ABSTRACT: The effect of 60Co γ irradiation on the absorption and emission spectra of the organometallic polymer [
Pt(PBu3)2—CtC—C6H4—C6H4—CtC
]n (Pt-DEBP) in chloroform and toluene solutions for dosimetry applications has been studied. The system Pt-DEBP/chloroform can be used for dosimetric applications in two different ways: (i) monitoring of absorption spectra changes for higher doses (higher than 1 Gy), and (ii) monitoring of emission spectra changes for low doses (below 1 Gy). The response of the polymer solution to γ ray doses has been interpreted with the aid of theoretical studies based on time dependent density functional theory (TD-DFT) calculations on the absorption bands of a model complex and of the possible fragments coming from the degradation of the polymer backbone. It has been proposed that the observed changes are promoted by the attack of radicals, from the radiolysis of the solvent, on the polymer triple bonds.

1. INTRODUCTION Ionizing radiations have been widely used in medicine and food industry.1,2 However, due to their potential harm to life, for example, as an inducer of cancer, there is a clear demand for methods that can quantify the dose used in a given situation.3
6 In this regard, conjugated organic polymers have been proposed as promising materials to produce dosimeters, mainly due to their opto-electronic properties, easy processing. and relative low cost.7
11 Recently, poly [2-methoxy-5-((20 -ethylhexyl)oxy)p-phenylenevinylene] (MEH-PPV) in chloroform solution has been proposed as a low dose dosimeter.7 The dose has been determined by measuring blue shifts in the UV
vis spectra absorption maxima due to γ irradiation. In this work we extend the investigation to 1,10 -bis(ethynyl)-4,40 -biphenyl bridging Pt(II) centers (Pt-DEBP), i.e., [
Pt(PBu3)2—CtC—C6H4— C6H4—CtC
]n which belongs to the class of polymetallaynes. Polymetallaynes emerged in recent years as materials belonging to one of the most stimulating research areas.12,13 These soluble and stable polymers feature the well-known advantages of organic polymers, together with the functionality provided by the presence of metal centers, linked through a π-conjugated moiety. Polymetallaynes find broad applicability in electronic and electro-optical devices, including chemical sensors,14
16 organic light emitting devices (OLEDs),17 and photovoltaics.18,19 Despite the wide number of reported properties and applications,20 studies on the behavior of these materials under γ-ray irradiation are unknown; the degradation of polymetallaynes is not understood r 2011 American Chemical Society

in detail and, to our knowledge, there are no reports on their use as a dosimeter. In this work we report on the optical characterization of a polymetallayne containing ten Pt(II) metal centers, namely poly[1,10 -bis(ethynyl)-4,40 -biphenyl(bis(tributylphosphine))Pt(II)] (Pt-DEBP) compared with the binuclear model molecule trans,trans-[ClPt(PBu3)2—CtC—C6H4—C6H4—CtC—Pt(PBu3)2Cl] (Pt2-DEBP). Its behavior under a γ ray dose has been investigated in chloroform and toluene solutions by means of absorption and emission studies and interpreted with the aid of density functional theory (DFT) calculations. The study in toluene solution seeks to elucidate if the effects are also observed in the absence on heavy atoms in the solvent, which is suggested to be a key factor in the process.7 The results aim to understand how generally this class of materials can be used as dosimeters and to propose these polymers for expected biomedical applications.

2. EXPERIMENTAL SECTION 2.1. Materials. All reactions have been performed under an inert argon atmosphere. Solvents have been dried on Na2SO4 before use. All chemicals, unless otherwise stated, have been obtained from commercial sources and used as received. The compound 1,10 bis(ethynyl)-4,40 -biphenyl, H—CtC—C6H4
C6H4—CtC—H Received: January 26, 2011 Revised: April 28, 2011 Published: June 09, 2011 8047

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The Journal of Physical Chemistry B (DEBP) has been prepared according to literature methods,21 as well as the complex trans-[PtCl2(PBu3)2].22 The organometallic polymer [
Pt(PBu3)2—CtC—C6H4—C6H4—CtC
]n (PtDEBP) was synthesized from equimolar amounts of dichloride square planar complex trans-[Pt(PBu3)2Cl2] in the presence of monomer DEBP in NHEt2 as solvent and base.23 The model molecule, i.e., binuclear complex trans,trans-[ClPt(PBu3)2—CtC —C6H4—C6H4—CtC—Pt(PBu3)2Cl] (Pt2-DEBP), has been prepared following the literature procedure.24 The purification of the crude products was performed by means of chromatographic separation with 70
230 mesh alumina (Merck), by using n-hexane/ dichloromethane mixtures as the eluting phase. 2.2. Characterization of Materials. Pt-DEBP. IR (film, cm
1): 2094 (νCtC), 1602 (νCdC), 401, 393. UV (CHCl3): 373.2 nm. 1H NMR (CDCl3, δ ppm): 0.93 (t, 18H,
CH3, J = 9.00 Hz), 1.45 (q, 12H, P—CH2—CH2—CH2
), 1.62 (m, 12H, P—CH2— CH2
), 2.15 (m, 12H, P—CH2
), 7.29 (d, 4H, Ar—H), 7.44 (d, 4H, Ar—H). 31P NMR (δ ppm, CDCl3 (J195Pt
31P, Hz)): 3.72 (2358), 7.52 (2371) with intensity ratio 4/1, corresponding to a ten metal units oligomer. Elemental analysis (%), found (calculated for the Pt10P20Cl2C240H612): C = 58.61, (60.06); H = 7.84 (7.81). Pt2-DEBP. FTIR (film, cm
1): 2118 (νCtC), 1603 (νCdC). UV (CHCl3): 342.0 nm. 1H NMR (CDCl3, δ ppm): 0.93 (t, 36H,
CH3), 1.45 (m, 24H,
CH2—CH2—CH3), 1.56 (m, 24H,
CH2—CH2—CH3), 2.02 (m, 24H, P—CH2), 7.44
7.28 (dd, 8H, Ar—H). 31P NMR (δ ppm, CDCl3 (J195Pt—31P, Hz)): 7.52 (2372). Elemental analysis (%), found (calculated for Pt2P4Cl2C64H116): C = 52.80 (52.83), H = 8.48 (8.43). 2.3. Methods. Fourier transform infrared (FTIR) spectra have been recorded as Nujol mulls or as films deposited from CHCl3 solutions by using ZSM-5 cells, on a Bruker Vertex 70 Fourier transform spectrometer. 1H and 31P nuclear magnetic resonance (NMR) spectra have been recorded in CDCl3 on a Bruker AC 300P spectrometer at 300 and 121 MHz, respectively; the chemical shifts (ppm) have been referenced to TMS for 1H NMR by assigning the residual 1H impurity signal in the solvent at 7.24 ppm (CDCl3). 31P NMR chemical shifts are relative to H3PO4 (85%). Preliminary UV
vis spectra have been recorded in CHCl3 solutions at room temperature on a Varian Cary 100 instrument and photoluminescence spectra have been performed on a Perkin-Elmer LS 50 fluorescence spectrometer. All measurements have been carried out at room temperature using quantitative solutions of the polymers in CHCl3. 2.4. Dosimetry Study. Initially, stock solutions of 23.3 mL with concentration 0.3 mg/mL of Pt-DEBP in chloroform or toluene solvents have been prepared. From these, four samples with different concentrations have been prepared and dispensed in 4 mL glass vials (Wheaton 13-425) for each solvent. The concentrations used ranged from 0.0113 to 0.0500 mg/mL. All solutions have been prepared at room temperature under low illumination conditions to avoid undesirable photoreactions and stored in a refrigerator (T = 10 °C). For each concentration, five samples have been prepared, one has been kept as the reference sample, while the other four have been irradiated in pairs to test reproducibility. The samples have been irradiated at room temperature in a cobalt therapy unit (CGR - Model Alcyon II) with appropriate acrylic build-ups (thickness of 0.5 cm). The samples have been analyzed by UV
vis and fluorescence spectroscopy. Measurements of UV
vis spectroscopy have been performed on a Shimadzu (UV mini 1240). The fluorescence measurements have been performed

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Figure 1. Optical absorption spectra of Pt-DEBP in chloroform before and after irradiation with γ rays at different radiation doses (C = 0.0250 mg/mL).

using a Varian fluorimeter (Cary Eclipse). The samples have been excited with a wavelength of 390 nm. IR spectra were recorded with a Vertex 70 Fourier transform infrared spectrometer (Bruker) using a micro ATR (Attenuated Total Reflectance) diamond accessory. 2.5. Theoretical calculations. The calculations have been performed with Gaussian03 and Gaussian09.25 For the energy calculations and geometry searches, the DFT method has been used with various functionals (B3LYP, O3LYP, and BMK),26
28 the 6-311þG(d)29 basis on the first atoms and the 6-311þG(df) on chlorine. The Pt atom has been treated with the Lanl effective core potential (ECP) and a double-ζ (DZ) or triple-ζ (TZ) basis.30 Excited state calculations have been performed by means of the TD-DFT method.31 Calculations have been developed on the bimetallic complex with hydrogen substituted phosphines (PH3) because of the structural complexity of Bu3 chains in PBu3 ligands.

3. DOSIMETRY STUDY Figure 1 shows the typical optical absorption spectra before and after γ ray irradiation for Pt-DEBP in chloroform. The principal absorption peaks are due to π f π* absorption and are localized on the conjugated system. Similar to what has been observed in MEH-PPV,7 the main peak position of the absorption spectra shifts to lower wavelengths (blue shift), followed by the reduction in the amplitude of the absorption band after irradiation. Two different samples have been independently prepared and irradiated under the same conditions. As can be seen, the results are highly reproducible; notice that there are two measurements for each irradiation. Figure 2 shows the shift in the main peak position as a function of dose for samples with different concentrations in chloroform solutions. Each point plotted is the average of at least two samples. As can be seen, there is a linear shift of the peak position with dose, independent of concentration. Notice that for the highest doses and smaller concentrations, signs of saturation in the effect can be observed. Figure 3 shows the typical fluorescence spectra of Pt-DEBP in chloroform before and after irradiation. The excitation bands in the emission spectra are due to π f π* absorption (localized on 8048

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Figure 2. Main peak position as a function of dose for Pt-DEBP/CHCl3 samples with different concentrations. The lines were obtained by fitting as described in the text.

the conjugated system). The emission from the singlet state are observed between 400 and 450 nm and triplet emission (phosphorescence) bands are observed above 500 nm. Contrary to what is observed in MEH-PPV, even for low doses (1 Gy), noticeable changes are observed in the spectrum. New bands are clearly detected at longer wavelengths as well as a resulting displacement of the spectrum maxima. Similar results are obtained for the other concentrations. We can note that the absorption and emission spectra exhibit a distinct sensitivity to the used doses. The absorption spectrum shows significant changes only after exposure to relatively high doses (above 15 Gy), while the emission spectrum appears to be primarily sensitive to low doses (lower than 15 Gy). No significant shifts are observed in the absorption and emission spectra in toluene solutions, even for high doses (see Supporting Information). Figure 1 shows that the Pt-DEBP/chloroform system is a possible dosimeter for doses above 1 Gy. In fact, it is possible, using linear fits, to write eq 1 that fits all of the data presented in Figure 2: λmax ¼ 4:81474ðCDÞ
0:559035ðDÞ þ λ0max

ð1Þ

where λmax is the main peak position after irradiation, C is the solution concentration, D is the used dose, and λ0max = 373.46 nm is the main peak position of the nonirradiated samples (details in the Supporting Information). The absorption spectra shift dependence with the concentration and solvent type indicates that the effect has a nature similar to that observed in MEH-PPV solutions with concentrations smaller than 0.0500 mg/mL32 (a complex dependence with the concentration is observed for higher concentrations).7 In this system it has been proposed that the observed changes are promoted by the attack of radicals, from the radiolysis of the solvent, on the polymer backbone.7,32 These attacks would reduce the conjugation length of the polymer and consequently induce blue shifts in the absorption main peak. Our results for the Pt-DEBP/toluene system reinforces the hypothesis of radical attack. Because it is known that aromatic compounds have lower chemical changes under ionizing irradiation

Figure 3. Emission spectra of the samples of Pt-DEBP sample in CHCl3 before and after irradiation at different doses (C = 0.0113 mg/mL).

(compared to aliphatic compounds),33 less radical formation is expected and a quenching in the effect should be observed. In ref 7, it is also suggested that the solvent radiolysis is increased due to higher interaction cross sections of chlorine atoms with ionizing radiation. In our case, Pt-DEBP solutions, the Pt atom presents the highest cross section of the system and a direct interaction of the polymer chains with the radiation was expected. However, the absence of effect in Pt-DEBP/toluene system indicates that the solvent plays a crucial hole in the process and direct interactions have a secondary importance. It can be understood considering that, given the low concentrations of the studied solutions, the possibility of radiation interaction with solvent molecules is greater than with the polymer chain. Aiming to better understand the spectrum alteration, optical absorption (Figure 4A) and photoluminescence (Figure 4B) measurements have been carried out by comparing the optical behavior of different polymer substructures: (i) polymetallayne with ten Pt(II) metal centers (Pt-DEBP), (ii) dinuclear model molecule Pt2-DEBP, and (iii) DEBP precursor (without metal centers). It can be observed from the absorption features that the delocalization extending through the conjugated chain increases from monomer to binuclear complex and to polymeric compound, because the absorption peaks are due to π f π* transitions and are localized on the conjugated system. However, the difference in the peak position between Pt-DEBP and Pt2DEBP is relatively small, and this indicates that the effective conjugation length is not increased significantly with the number of units. The photoluminescence properties of these polymeric systems have been extensively analyzed before: Pt containing organic ethylene polymers and monomers have been studied and discussed by Kohler et al.;34 Pt(II) alkynyl polymers incorporating diethynylcarbazole units have been analyzed by Liu et al.;35 Pt-acetylide oligomers and Pt-containing poly(aryleneethylene)s have been also analyzed recently.36,37 In conjugated organic polymers and monomers, phosphorescence from the first triplet state T1 is very weak, unless the system contains a heavy atom such as Pt, which greatly enhances the amount of spin
orbit coupling (see Figure 4B). In the latter case the triplet radiative decay becomes comparable in time with the intrinsic nonradiative decay rate34 and the triplet emission from 8049

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Figure 4. Optical characterization of DEBP, Pt2-DEBP, and Pt-DEBP: (A) absorption and (B) emission spectra in CHCl3 solution.

Figure 5. Chemical structure of binuclear complex [Cl
Pt(PH3)2
DEBP
Pt(PH3)2Cl].

the T1 state becomes directly detectable with conventional steady state spectroscopy. The phosphorescence bands may be not readily detectable unless a very cold sample is analyzed.36,37 This triplet state turns out to be insensitive to the length of the oligomer, thereby confirming the notion that the triplet exciton is strongly confined at least more than the singlet exciton.34 To assess the details of the electronic transition involved in the emission spectrum, electronic structure calculations were carried out. For spectra simulation, our model system is the binuclear complex Cl
Pt(PH3)2
DEBP
Pt(PH3)2Cl, which is reported in Figure 5. A gas phase optimization of the DEBP structure produced a geometry in which the two phenyl groups are slightly twisted by an angle of about 40°.38 Although the crystallographic evidence for the dinuclear complex Pt2-DEBP is in favor of a coplanar structure in the solid state,27 a distortion from planarity is also expected in solution. This is evidenced in Figure 4A; the position of the main peak of the binuclear complex is at 342 nm and the absorption peak maximum of the decamer at 373 nm. As the polymer does not present a planar structure, the π system does not extend for many repeating units, so there is not a significant increase in the length of conjugation with the number of units; in fact, an effective conjugation length of 6 repeat units is reported in the literature for similar polymetallaynes.39 However, in the following calculations, we have adopted a molecular geometry in which we have constrained the two rings to be on the same plane while the rest of the degrees of freedom have been fully optimized. We have performed the excited state calculations for both the coplanar and the twisted conformations because the main absorption line (a πfπ*) transition is obviously dependent upon the relative geometry of the two π systems on the phenyl groups. The best agreement is obtained with the B3LYP functional and by constraining the structure to a planar geometry. The results obtained with a DZ quality basis or a TZ one on the Pt atom are almost coincident. Our simulated spectra are reported in Figure 6. In the same figure we have also reported the spectra obtained with the O3LYP functional that, however, produces a slightly worse agreement.

Figure 6. Experimental absorption profile for the binuclear [Cl
Pt(PH3)2
DEBP
Pt(PH3)2Cl] compared to the positions of the calculated transitions.

The main absorption line is due to a π f π* transition, which brings the ground state to the fifth excited singlet state through the promotion of a HOMO electron (orbital 93) to the π* LUMOþ2 orbital (orbital 96). Examples of the involved orbitals (Kohn
Sham orbitals) are reported in Figure 7. Similar shapes and behavior have also been found for Pt containing poly(aryleneethynylene)s.40 The weaker band at 290 nm can be described by a more complex transition that involves the electron transfer from the π orbitals HOMO
1, HOMO
2, or HOMO
3 to the Pt
P empty antibonding orbitals. Note that the absorption spectrum main peak of nonirradiated Pt-DEBP decamer, centered at about 373 nm, can be attributed to transitions between the frontier orbitals π
π*, localized mainly on the aromatic rings regions (Figure 7). Then, considering the solvent radiolysis, the question is how the radical attack could change the absorption spectra. In this regard, two different pathways can be considered: (i) triple bonds saturation by radicals incorporation and (ii) the polymer chain degradation induced by radiation or radical attack (considering that the direct radiation interaction has a secondary importance in the effect). FTIR analysis and additional B3LYP geometric optimizations followed by TD-DFT calculations on various possible polymer 8050

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Figure 7. Orbitals involved in the main transitions in the absorption spectra.

Figure 8. Simulated experimental absorption profiles for the binuclear complex compared to those of possible compounds resulting from fragmentation.

fragmentation channels have been performed to check whether it has been possible to identify the degradation products. The fragment compounds we have studied are reported in Figure 8, together with their absorption spectra, and the absorption spectrum of the nondegraded compound. We have arbitrarily convoluted the theoretical absorption energies with a Gaussian function of width 0.2 eV. As can be seen, only compounds B and D absorb in the 300
350 nm range, the region where the absorption bands shift during γ ray degradation. Initially, this result suggests that the main degradation pathway is tC—Pt bond cleavage and the breaking of the polymer into units along the bond interconnecting the phenyl groups. However, the FTIR analysis does not confirm the chain scission on the biphenyl

group, because no change is observed in the regions 822, 1485, and 1602 cm
1 after irradiation41,42 (see Supporting Information). Additionally, this conjugation length reduction can be associated with a blue-shift in the fluorescence spectra (see Figure 4B), which does not agree with the observed shift in Figure 3. Considering the possibility of radical incorporation in the polymer backbone, given the stability promoted by the resonance in the aromatic rings, it is reasonable to consider that the radical attack occurs preferentially at the triple bond and the Pt site. To better assess this point, FTIR spectra have been collected and a decrease of the intensities of the CtC stretching mode at about 2100 cm
1 has been observed (see Supporting Information). This result is also in agreement with UV
vis data, because the π
π* transitions (Figure 7) are not affected by the cleavage of the Pt—Ct bond. In fact, the radical addition does not promote a significant shift of the spectrum, as observed for low irradiation doses. On the other hand, for higher doses, an increased radical formation is expected and thus attacks on the aromatic ring are possible, promoting the observed shifts. No evidence of incorporation of radicals on sites close to Pt were observed. Regarding the emission spectra of the nonirradiated polymer, we can observe two bands located at 420 and 555 nm (see Figure 4). These emission bands can be due to fluorescence of a singlet excited state or to phosphorescence by the lowest lying triplet state. The strong feature at 420 nm can be ascribed clearly to fluorescence from the singlet state because of its close proximity to the absorption band (an energy that is too high for a triplet state). Moreover, the first optically active singlet found by TD-DFT calculations is located at 384 nm, which is not very far from the experimental fluorescence band origin of 400 nm. The second, less prominent feature at 2.2 eV (λ = 550 nm), might be due to a phosphorescence band. We have two ways in which it is possible to calculate the first triplet energy location: the first one is simply by using the unrestricted Kohn
Sham (UKS) theory with the same functional/basis used for the singlet and take the energy difference with it. The latter one is to perform a TD-DFT calculation for the triplet states. However, it has to be kept in mind that a TD-DFT calculation with B3LYP functional on a triplet state can have a deviation as 8051

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Figure 9. Stable geometry and spin density of the lowest triplet state.

large as 0.5 eV42 and therefore we have repeated the same computation also with the BMK functional, which has proven to be more accurate in predicting singlet
triplet splittings. Furthermore, we have to choose a geometry for the triplet emission. In general, if the radiative decay time is long enough, one should use the triplet optimized geometry. In the present case, however, the radiative relaxation time is comparable to the nonradiative one and therefore we have used also the ground state singlet geometry. In the singlet geometry the triplet calculated with the UKS/ B3LYP theory lies respectively 2.73 eV above the ground state (λ = 454 nm) while those calculated with TD-DFT are found to lie at 2.42 eV (λ = 512 nm) using the B3LYP functional and 2.64 (λ = 469 nm) using the BMK one. The geometry of the triplet has been optimized at the UKS/ B3LYP level of theory. The main difference between the geometry of the ground singlet state and that of the triplet state is the position of the Pt
P bonds, which are perpendicular to the π system in the singlet and parallel to them in the triplet. The geometry of the triplet is reported in Figure 9 along with the unpaired spin density: the triplet state has its parallel spin electrons localized in the π/π * system made by orbitals 93 (HOMO) and 94 (HOMOþ1). We have found that this relaxed triplet state lies 1.88 eV above the ground state (λ = 660 nm). TD-DFT theory at the triplet geometry locates the first triplet state at 1.63 eV (760 nm) with B3LYP and 1.76 eV (701 nm) using BMK. Given the typical deviations of singlet
triplet gap determinations using DFT methods, we can surmise that the weak band is probably due to a triplet
singlet fast radiative decay taking place at a geometry that turns out to be similar to that of the singlet ground state. This conclusion is strengthened by an analogous result36 where the S0
T1 transition energies computed using the ground state geometries are in better agreement with the experimental data than those computed using triplet geometries that underestimated the band position by 0.4
0.5 eV. Considering the radical incorporation scheme outlined above, for low radiation doses, halogen addition on the triple bonds are expected. These new halogen functions in the polymer chain may act as efficient emission centers, promoting more flexibility to the polymer backbone for relaxation of excited structures. In fact, new bands at higher wavelength in the fluorescence are observed even at low doses (1 Gy). On the other hand, profound alterations in the fluorescence spectra shape are observed for high doses, suggesting strong modification in the initial emission centers. Given the complexity of the involved phenomena, further investigations are being conducted to better understand these effects and will be subject of a later publication. In comparison to MEH-PPV, Pt-DEBP in chloroform solution has a lower sensitivity when the dose assessment is through the UV
vis absorption spectra. MEH-PPV solutions with 0.075 mg/mL in CHCl3 show a shift of about 40 nm in the absorption

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spectrum after exposure to 30 Gy.7 In Pt-DEBP the best result is found for a concentration of 0.0113 mg/mL, where the shift is of only 17 nm. This low sensitivity may be associated with lower conjugation length of the polymer, which makes it insensitive to small changes in the main chain. The advantage of using PtDEBP with respect to MEH-PPV and other polymers8
11 is related to the fluorescence response (Figure 3), which suggests high sensitivity at low doses for this kind of polymers. More work is underway to understand this effect.

4. CONCLUSIONS Solutions of the organometallic polymer Pt-DEBP with different concentrations have been used to study the effect of 60Co γ irradiation on the absorption and emission spectra for dosimetric applications. The results indicate that Pt-DEBP in CHCl3 can be used as a γ ray dosimeter for doses higher than 1 Gy using the absorption spectra changes. Shifts in the position of the main peak of the absorption spectrum of the solutions had an approximately linear relationship with the used dose, and some saturation effects have been observed at high doses. This linearity with dose depends on the concentration in a predictable way. Changes in the fluorescence spectrum suggest that the system can also be used for doses below 1 Gy. So far, the response of PtDEBP to increasing γ ray exposure has been interpreted as due to radical attack on the polymer backbone, likewise for the case of already investigated polymers. FTIR analysis suggests that the effects are related to radical attack on the polymer triple bonds. The halogen incorporation promotes the formation of new emissive centers in the polymer while small changes are promoted in the absorption spectrum. We have fully characterized by TD-DFT calculation the absorption spectrum of the binuclear complex Pt2-DEBP by using a very similar model system. The main absorption is due to a π f π* transition that brings the system into its fifth singlet excited state. The following radiative decay process has two components: a fluorescence due to singlet states and a phosphorescence one due to the direct emission from the triplet state triggered by the Pt atom. The triplet emission time scale is comparable with the geometric relaxation time if not faster and we have found that the calculations better agree with experiments when the emission takes place at a geometry of the initial singlet ground state. ’ ASSOCIATED CONTENT Supporting Information. Extensive figures (absorption, emission, FTIR spectra) and analytical and spectral characterization data. This material is available free of charge via the Internet at http://pubs.acs.org.

bS

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Fax þþ3906490324.

’ ACKNOWLEDGMENT We acknowledge for the financial support of this research MAE-MIUR Progetti di Ricerca Scientifica e Tecnologica Bilaterale 2008-2010, CNPq (Brazil), FAPESP (Brazil), CAPES (Brazil), and CNEN (Brazil) and Ateneo Federato AST 2008 (26F09MA27). E.B. acknowledges computational support of the CASPUR and CINECA Supercomputing Centers. 8052

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