IR spectra of DPVBi pristine fi lm (bottom curve) and fi lm irradiated for one hour with I UV 1⁄4 1 mW cm À 2 (top curve). Spectra are offset 1 a.u. for clarity. 

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Context 1

... laser desorption and ionization time-of- fl ight Voyager Biospectrometry DE Pro Workstation (Perseptive Biosystems, USA) on pristine and irradiated fi lms deposited directly onto metallic sample holder (well plate) of the spectrometer. As DPVBi absorbs at the wavelength of the laser, matrix was not used. The pressure inside the workstation was kept at few 10 À 5 Pa so the laser, emitting at 337 nm, could not produce photo-oxidation of DPVBi fi lms. The polarization was positive and laser intensity was 2000 with 330 shots. Another method of MS, using Waters Acquity Tandem Quadrupole Detector with atmospheric solids analysis probe (ASAP) set to detect positive ions, was performed to compare results with LDI MS. All data were obtained and processed using the MassLynx 4.1 software. ASAP technique enables ionization of non-polar compounds and is particularly convenient for our study as materials need not to be dissolved. Sample preparation is simple: material is scratched from the fi lm with a clean capillary, which is immediately after- wards inserted in the chamber of the spectrometer. The polarization was positive, source temperature was at 150 ° C, capillary temperature at 400 ° C, corona and cone voltages were 3 keV and 20 V, respectively. Results form MLADI-TOF MS are in agreement with results from ASAP MS. We have carried out DFT modeling of molecular DPVBi and its possible photo-oxidation products with 0, 1 and 2 oxygen atoms attached, at B3LYP up to 6-311 þ þ G** level of theory using NWCHEM [28]. For each of them, we searched for the lowest-energy con- formers (CFs, including the varying of the O-binding sites), con- sidering in total about 300 geometries. The IR spectra were calculated for the lowest-energy CFs found and the obtained results compared with the experiment. For DPVBi molecule exactly four CFs were found with relative ground-state energies within only 1 kJ/mol. The absorption properties of the four CFs are barely distinguishable, since the frontier orbitals lie on the central two rings near attached carbons and are essentially not affected by the orientation of the outer rings by which the CF geometries differ. Photoluminescence intensity of DPVBi fi lms exposed to UV light in air falls exponentially with time. Upon prolonged exposure, their UV – vis absorption spectra change, indicating a chemical alteration of fi lm composition. Furthermore, when the partial pressure of oxygen is increased, these changes become faster. If, on the contrary, the fi lms are exposed to UV light in vacuum , neither their PL nor absorbance would change with the time of exposure (on the timescale and under UV illumination conditions of experiments presented here). Thus, the simultaneous presence of UV light and oxygen is needed for the degradation of DPVBi fi lms. In order to investigate photo-chemical changes of DPVBi fi lm composition upon its simultaneous exposure to UV light and air, a comparison of mass spectra of pristine and exposed fi lms was performed (Fig. 1). Mass spectra of DPVBi pristine thin fi lms show dominant peak at M DPVBi 1⁄4 510.6 g/mol, which corresponds to the mass of DPVBi molecule. Besides this, other smaller masses can be observed, which are attributed to fragments of DPVBi induced by MS technique. Irradiated samples show the presence of numerous new species – impurities (indicated by arrows), which can be divided into two groups. One group, with M 4 M DPVBi (solid arrows), is presumed to consist of oxygenated DPVBi molecules. These masses seem to follow a pattern: M x,y,z 1⁄4 M DPVBi þ x Δ M 1 þ y Δ M 2 þ z Δ M 3 , where M x,y,z is the mass of new species and x , y and z may take integer values 0, 1, 2 or 3 (inset of Fig. 1). Masses Δ M 1 1⁄4 14 g/mol, Δ M 2 1⁄4 15 g/mol and Δ M 3 1⁄4 16 g/mol correspond to the molecule gaining one oxygen atom and losing two, one or zero hydrogen atoms, respectively. The loss of two H atoms implies that O atom formed a bridge between two C atoms. If one H atom is lost, then it is expected that O forms a double bond with C and, fi nally, if no H atom is lost, it is plausible that – OH group is attached to C atom. The other group, with M o M DPVBi (dotted arrows), is supposed to consist of molecular fragments of oxygenated DPVBi (fragments of impurities with M 4 M DPVBi , induced by MS technique) and/or of photo-oxidation products. Results of MS suggest that the photo-oxidation of DPVBi happens when fi lms are exposed to air and UV light. The presence of dimers of DPVBi was not detected in irradiated samples. Relative intensity of peaks in mass spectra does not necessarily re fl ect relative number of different molecules present in a fi lm, as different species do not necessarily have the same ionization cross section. Thus, it is not possible to deduce which type of impurity is predominant. Further evidence for photo-oxidation is provided by infrared spectroscopy. The measured IR spectra of pristine and irradiated DPVBi fi lm are shown in Fig. 2. In the case of the irradiated sample new broad features can be observed in the regions around 1250, 1700 and 3300 cm . The new features have large width as a consequence of a superposition of a large number of characteristic vibrational frequencies with slightly different eigenvalues and by inhomogeneous broadening in the amorphous medium of DPVBi fi lm. IR spectra are in agreement with results obtained from MS: we assume that the feature around 1700 cm À 1 belongs to C 1⁄4 O bond ( Δ M 2 ), while the one around 1250 cm À 1 can be ascribed to C – O bond ( Δ M 1 , if O bound to two C atoms or Δ M 3 ). Broad feature around 3300 cm À 1 could be explained by the presence of – OH groups ( Δ M 3 ), possibly participating in hydrogen bonding [29]. While MS shows the presence of new masses which correspond to formation of hydroxyl groups, IR spectra do not show pronounced – OH peak. To give support to conclusions drawn in the previous para- graph, theoretical DFT molecular vibrational spectra were calculated for some possible photo-induced impurities. Comparison of the pristine DPVBi IR spectrum with the DFT result, given in Fig. 3a, shows that theoretical molecular spectra can well describe the experimental ones. The lower band properties (600 – 1600 cm À 1 ) are in a good agreement with the experiment. The upper band ( $ 3000 cm À 1 ) shows a red-shift relative to the measured spectra, but it is dif fi cult to discern whether the difference is a systematic error of DFT or a genuine difference between the molecular and thin fi lm DPVBi spectra. In Fig. 3b experimental IR spectrum of illuminated fi lm is compared with DFT molecular spectra of oxidized DPVBi, calculated at B3LYP/6-311 þ þ G** level, for the lowest energy CFs found. There are three important conclusions to be drawn from the comparison: (a) numerous lines found by DFT fall in the region of 1250 cm À 1 feature, many of them belonging to C – O bond, while only few of them belong to pure DPVBi molecule; (b) the feature about 1700 cm À 1 consists of two nearby broad peaks and DFT results suggest that these cannot belong to a molecular pristine DPVBi since there are no vibrations in its spectra between 1600 cm À 1 and 2900 cm À 1 (see Fig. 3a). In contradistinction, molecule with two O attached to a phenyl ring has an in-plane stretching of the C 1⁄4 O vibration at 1690 cm À 1 (denoted M 1 0,2,0 in Fig. 3b). The lowest energy M 0,2,0 found has an in-plane oscillation of the C 1⁄4 O bond at 1730 cm À 1 , in a very good agreement with the frequency of the second peak of the feature (both calculated at B3LYP/6-311 þ þ G** level); and, (c) DFT results for IR spectra of the molecular DPVBi with one or two hydroxyl groups ( M 0,0,1 and M 0,0,2 ) give strong peaks around 3590 cm À 1 corresponding to an – OH stretching mode. Since there are no strong peaks above 3100 cm À 1 in the measurement, we conclude that the noncovalent interactions between new species and surrounding molecules strongly damp and broaden vibrations of – OH group. From the MS and IR spectra one sees that new species are photo-oxidized DPVBi molecules or its fragments with one, two and even three oxygen atoms added, which can be singly or doubly bound to C atoms of DPVBi. Fig. 4a shows absorption spectra as a function of wavelength λ after different times of irradiation in air of 190 nm thick DPVBi fi lm deposited onto fused silica substrate and irradiated with I UV 1⁄4 0.4 mW cm À 2 . Pristine fi lm (no irradiation) shows two absorption bands at 242 and 355 nm, corresponding to two lowest excited electronic states. The energy gap is (3.0 7 0.1) eV, esti- mated from the long-wavelength absorption edge [30], in a good agreement with our TD-B3LYP/6-311 þ þ G** result for the fi rst excitation of DPVBi in the lowest vibrational state being at 3.14 eV. Irradiation with UV light induces gradual disappearance of both bands and emergence of new peak around 255 nm, re fl ecting a gradual chemical change in fi lm composition (degradation of DPVBi) and a formation of photo-oxidized products of DPVBi. The percent of change in the value of absorbance A at λ 1⁄4 355 nm is assumed to be roughly the same as the percent of impurities present in a fi lm. To track this change, A taken at 355 nm, normalized to its initial value A 0 at time t 1⁄4 0, is plotted in the inset of Fig. 4a as a function of irradiation time t . The decrease in normalized absorbance A / A 0 is close to linear at the beginning and then gradually slows down. Nonlinear part of the curve can be a consequence of change in degradation process dynamics due to a signi fi cant loss of DPVBi material (around 60%). From the linear part the rate R of change of normalized absorbance A / A 0 is de fi ned by A / A 0 1⁄4 1 À Rt /100 (the inset of Fig. 4a and for the fi lm from Fig. 4 it is around 0.22%/min. Effects which degradation in air has on the photoluminescence (PL) are shown in Fig. 4b as a series of spectra of a 190 nm thick fi lm ...

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... evidence for photo-oxidation is provided by infrared spectroscopy. The measured IR spectra of pristine and irradiated DPVBi film are shown in Fig. 2. In the case of the irradiated sample new broad features can be observed in the regions around 1250, 1700 and 3300 cm À 1 . The new features have large width as a consequence of a superposition of a large number of characteristic vibrational frequencies with slightly different eigenvalues and by inhomogeneous broadening in the ...

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... possible photo-oxidation products (their structure is given in the lower panel) denoted by M 0,0,1 (circles), M 0,0,2 (upward triangles), M 0,2,0 (downward triangles) and M1 0,2,0 (diamonds). Oxygen atoms are indicated as red. Arrows indicate the regions of broad features around 1250 and 1700 cm À 1 , which appear only in exposed films (see Fig. 2). Intensities (y-axis) in both (a) and (b) are in arbitrary units, while x-axis shows the wavenumber in cm À 1 . (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) Fig. 4. (a) Absorbance A vs. λ as a function of irradiation time t ¼0, 1, 2, 3, 4, 4.7, 5.7, ...