4,4 -BPDC

Photophysical Processes in Dimethyl 4,4 -Biphenyldicarboxy-late (4,4I-BPDC). The ultraviolet absorption spectrum of dimethyl 4,4 -biphenyldicarboxyl ate was examined in both HFIP and 95% ethanol. In each case two distinct absorption maxima were recorded, an intense absorption near 200 nm and a slightly less intense absorption near 280 nm. The corrected fluorescence excitation and emission spectra of 4,4 -BPDC in HFIP at 298°K shows a single broad excitation band centered at 280 nm with a corresponding broad structureless emission band centered at 340 nm. At 77°K, the uncorrected phosphorescence spectra shows a single broad structureless excitation band centered at 298 nm, and a structured emission band having maxima at 472 and 505 nm with a lifetime, t, equal to 1.2 seconds. 

In 95% ethanol, 4,4 -BPDC shows a strong fluorescence and relatively weaker phosphorescence (relative intensity 10-2 times the fluorescence). This indicates, though perhaps unexpected, the lowest energy singlet state is a (ir,ir ) state... 

Energy Transfer Studies with Dimethyl Terephthalate (DMT) and 4,4 -BPDC. Several attempts were made to determine if energy transfer could occur from an excited DMT molecule to a 4,4 -BPDC molecule in a rigid ethanol glass at 77°K. These studies were accomplished by adding various amounts (20 - 50 mole percent) 4,4 -BPDC to a known concentration (5.0 x 10"4 M) of DMT. The change in emission intensity at 418 nm, which is exclusively emission from DMT, was then measured with excitation at 298 nm. 

The results of this study show a definite quenching of the 418 nm phosphorescence emission of DMT. One would expect that the quenching effect, in a rigid glass, would fit the Perrin model (73). A plot in In 4>0/4> versus concentration of 4,4 -BPDC yielded a straight line, the slope of which was identified with NV. The radius, R, of the active volume of quenching sphere was calculated by the following equation ... 

In addition, it can be shown for the concentration range of the 4,4 -BPDC used, assuming each molecule occupies a spherical volume, the average radius of this volume is about 108 a. This calculation predicts, on the average, the probability of an excited DMT molecule having a 4,4 -BPDC molecule within the required 15 A for energy transfer to occur by the exchange mechanism, which would be spin allowed, is small. 

If neither mode of energy transfer is acceptable, a different explanation of the apparent quenching of the DMT phosphorescence must be put forth. It must be recalled that both DMT and 4,4 -BPDC absorb 298 nm light, which introduces the argument that competitive absorption causes the apparent quenching effect. 

Since 4,4 -BPDC has no phosphorescence emission at 418 nm, light absorbed by the 4,4 -BPDC molecules is essentially lost to the detector monitoring the 418 nm emission. This can easily be seen by monitoring the change in excitation spectra of the mixed solutions as a function of 4,4 -BPDC concentration. At zero concentration of 4,4 -BPDC and 5.0 x 10-4 M DMT concentration, since the rotation of the chopper limits the amount of light reaching the sample, most of the available light is absorbed by the DMT and the excitation maxima at 250 and 298 nm reflect the... 

The conclusion from the monomer solvent studies is that, in nearly equal molar solutions, DMT and 4,4 -BPDC compete for absorption of the 298 nm radiation. However, the results also show that, even in equal concentrations, the DMT emission, when excited by 298 nm light, is several times as intense as the 4,4 -BPDC emission at 472 nm. It must be emphasized that these studies do not preclude the existence of energy transfer from excited DMT to 4,4 -BPDC. From the volume calculation used above, it can be shown that a concentration of v 0.1 M 4,4 -BPDC is needed to assume an occupied volume with radius of 15 8, the required distance for the exchange mechanism. 

Photophysical Processes in Pol,y(ethy1eneterephthalate-co-4,4 -biphenyldicarboxyl ate) (PET-co-4,4 -BPDC). The absorption and luminescence properties of PET are summarized above. At room temperature the absorption spectrum of PET-co-4,4 -BPDC copolymers, with concentrations of 4,4 -BPDC ranging from 0.5 -5.0 mole percent, showed UV absorption spectra similar to that of PET in HFIP. The corrected fluorescence spectra of the copolymers in HFIP exhibited excitation maxima at 255 and 290 nm. The emission spectrum displayed emission from the terephthalate portion of the polymer, when excited by 255 nm radiation, and emission from the 4,4 -biphenyldicarboxylate portion of the polymer when excited with 290 nm radiation. 

Figure 6). The phosphorescence spectra of the copolymer yarns showed excitation in the 305 - 310 nm range with corresponding emission maxima at 480 and about 515 nm, corresponding lifetimes equal 1.2 seconds. In the copolymer yarns containing 0.5 - 2.0 mole percent 4,4 -BPDC a small shoulder was observed at 452 corresponding to the PET homopolymer phosphorescence. 

As the concentration of 4,4 -BPDC increases, an increase in the intensity of the band at 289.5 nm was observed. This is the result in the increased intensity of the A - lL- transition of the 4,4 -BPDC in this region. In dilute HFIP solutions the copolymers show a fluorescent emission in the 326 - 338 nm range when excited with 255 nm radiation. This emission corresponds to emission from the terephthalate units of the copolymer. 

In the yarns, the fluorescence of the 4,4 -biphenyldicarboxy-late unit is distinct and predominate both at 298 and 77°K. Examination of the phosphorescence spectra of the PET and PET-co-4,41-BPDC yarns revealed three emission maxima. In the PET homopolymer excitation with 310 nm radiation produced an emission at 452 nm from the terephthalate chromophore. In the copolymers excitation with either 305 or 310 nm radiation produced emission spectra with distinct maxima at 480 and 515 nm (t 1.2 sec), and a shoulder near 452 nm (t = 1.2 sec). The maxima in the phosphorescence spectra were assigned as emission from the 4,4 -biphenyldicarboxylate units of the copolymer. The observed emissions are bathochromatically shifted from the emission of 4,4 -BPDC in a glassed solvent. This is supported by the observed emissions from solid 4,4 -BPDC at 520 and 560 nm (t =. 3 sec) when excited with 340 or 356 nm radiation. 

Phototendering of PET and PET-co-4,4 -BPDC Filament Yarns. Both "P T homopolymer and PET-co-4,4 -BPDC copolymers were irradiated from 20 to 80 hours in the photolysis chamber. In order to account for the lamp aging, the phototendering rate curves were plotted as percent loss tenacity versus total quanta/cm2 of exposure, rather than irradiation time. The phototendering rate curves for the homopolymer PET and PET-co-4,4 -BPDC copolymers show that all the samples became weaker and showed a decrease in percent elongation to break as total quanta/cm of exposure was increased (Figure 21). 

The 4,4 -BPDC copolymers exhibited effects similar to that reported for 2,6-ND. Even though the absolute rates of phototendering differed, the change in the rate from homopolymer to 4.0 mole percent copolymer were similar. The 2,6-ND copolymer showed a 65 percent decrease in rate compared to 59 percent for the 4,4 -BPDC copolymer. 

Fluorescence Analysis of Irradiated PET and PET-co-4,4 -BPDC Yarns. The presence of a material, which emits a blue-green fluorescence, on photooxidized PET has been reported previously (2, 21). This fluorescent material, which emits at 460 nm when excited by 342 nm energy, has been proved to be monohydroxy-tere-phthalate. 

On the other hand, the exposed copolymer yarn containing 4.0 mole percent 4,4 -BPDC still exhibits the normal terephthalate fluorescence (388 nm emission) as the major band in the emission spectrum when excited with 342 nm energy. 

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