UV-Vis

The produced oligomers were soft flexible solids. Their colours ranged from dark purple to slightly pink depending on their BODIPY concentration. In Figure 32 the optical appearance of a Diol-ED-2003-Oligomer 3.1 film can be seen.

Figure 32: Photo of the Diol-ED-2003-Oligomer 3.1

49 The oligomers were irradiated with λ= 365 nm and λ= 510 nm light in order to study the degradation behaviour. The experiments have been carried out in two different solvents as a degradation of the oligomers was observed in DMF without any irradiation. Therefore, the measurements have also been performed in MeOH to rule out any polymer instability.

4.9.1.1 Diol-ED-2003-Oligomer 1 in DMF

To investigate the degradation of the oligomer at λ= 365 nm the sample has been dissolved in DMF and irradiated for 5 min before each measurement. The measurements were carried out without any interruption or overnight storage. Only every second measurement is plotted to simplify the spectrum and ease the interpretation. As it can be seen in Figure 33 the absorption decreases linearly with each irradiation step. Additionally, the broad signal from λ= 600 - 780 nm shows a significant reduction in absorption. Since for the absorption measurements of the monomeric BODIPYs no absorption could be determined at these wavelengths, it was assumed that these signals are not originating from any BODIPY compounds.

The exact structures underlying this signal have not been further characterized.

After 60 min the absorption maximum was reduced to 66 % of the starting value.

50 Figure 33: Absorbance spectra of Diol-ED-2003-Oligomer-1 in DMF after different irradiation times with λ=365 nm light Full spectrum (top), zoomed in spectrum (bottom).

An additional sample was dissolved in DMF as well and irradiated with λ= 510 nm light in 5 min steps. As it can be seen in Figure 34 the degradation process is much slower compared to an irradiation with λ= 365 nm light. In comparison, the intensity at the absorption maximum was only reduced to 89 % of the starting value after 60 min. Additionally, a big jump can be seen between 20 min and 30 min. In between these time points the sample was stored overnight in the dark before the next measurements were performed.

This indicates that the oligomer may not be stable in DMF in the dark, although unexpected as the urethane and urea functionalities present in the oligomer should be stable under these conditions.

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0.0 0.2 0.4 0.6 0.8

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0.3

51 Figure 34: Absorbance spectra of Diol-ED-2003-Oligomer-1 in DMF after different irradiation times with λ= 510 nm light Full spectrum (top), zoomed in spectrum (bottom).

In order to evaluate the stability in other solvents, a reference measurement was performed in MeOH. To this end the absorption of the oligomer was measured and the sample was subsequently stored overnight in the dark.

400 500 600 700

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0.4

52 As it can be seen in Figure 35 the decrease in absorbance is very miniscule and can be neglected. In MeOH the oligomer shows a long-term stability in the dark, unlike in DMF. This significant influence of the solvent on the degradation behaviour of the oligomers was therefore taken into consideration for further experiments.

Figure 35: Determination of the stability of Diol-ED-2003-Oligomer-1 in methanol over 18 h in the dark.

510 520 530 540 550 560

0.75 0.80 0.85

Absor ba nce / a .u.

Wavelength / nm

0 h 18 h

53 4.9.1.2 Diol-ED-2003-Oligomer 1 in MeOH

With the stability in MeOH established the degradation of the oligomers upon irradiation was investigated in the new solvent as well. To this end the sample was dissolved in MeOH and irradiated with λ= 365 nm light for 60 min before each measurement. The measurements were carried out without any interruption or overnight storage. As it can be seen in Figure 36 the absorption decreases linearly with each irradiation step. In contrast to the irradiation in DMF however, the process is much slower. Even after 8 h the degradation is less advanced than in DMF after just 60 min, roughly decreased by a factor of 20. After 8 h the absorbance at the maximum was reduced to 87 % of the starting value.

Figure 36: Absorbance spectra of Diol-ED-2003-Oligomer-1 in methanol after different irradiation times with λ= 365 nm light. Full spectrum (top), zoomed in spectrum (bottom).

400 500 600 700

54 Additionally, Figure 37 shows the degradation of Diol-ED-2003-Oligomer-1 in MeOH after irradiation with λ= 510 nm light. The absorbance peak shows a steady and linear decrease, nevertheless again considerably slower than for λ= 365 nm as in DMF. Only a small decrease in absorption can be observed with a reduced absorbance at the maximum to 93 % of the starting value.

Figure 37: Absorbance spectra of Diol-ED-2003-Oligomer-1 in methanol after different irradiation times with λ= 510 nm light. Full spectrum (top), zoomed in spectrum (bottom).

400 500 600 700

55 4.9.2 GPC-measurements

On account of the instability of the polymers in DMF as described in chapter 4.9.1.1, the irradiation prior to the GPC measurements in DMF, has to be performed in MeOH. The sample was therefore dissolved in MeOH and irradiated for the specific time period. The solvent was evaporated subsequently, and the sample again dissolved in DMF with 10 mM LiBr. To keep the degradation of the oligomers in DMF to a minimum, the GPC measurements were carried out right after dissolution. Diol-ED-2003-Oligomer 2.2 and Diol-ED-2003 Oligomer 3.1 were further discussed as they delivered the best results in terms of their gel permeation chromatograms prior to irradiation.

4.9.2.1 Diol-ED-2003-Oligomer 2.2

In Figure 38 the GPC measurements of Diol-ED-2003-Oligomer 2.2 after different irradiation times with light at λ= 365 nm can be seen. The decrease in intensity of Peak 1 is clearly visible, suggesting a degradation of the oligomer. Furthermore, the peak seems to shift slightly towards higher retention times. The peak of the supposed degradation product, Peak 2 is rising with longer irradiation times as the chains get split up into smaller structures. Other peaks at 18- 22 min retention time show no visible change and most probably correspond to uncharacterised side products.

56 Figure 38: GPC chromatogram of Diol-ED-2003-Oligomer 2.2 after 0 h, 18 h and 72 h irradiation at λ = 365 nm.

A molecular weight of Mn= 41000- 45000 g mol^-1 against polystyrene standards was determined, as summarised in Table 1. Slight deviations can be due to measurement inconsistencies like varying backpressure of the system or small errors during the sample preparation. The dispersity Đ however is unexpectedly high, in regard to the step-growth polymerisation a dispersity Đ of 2 would have been expected. The values of peak 2 have to be interpreted with caution as the negative system peak lies close to it. The determination of the molecular weight in this area is very inaccurate which can lead to a wrong dispersity Đ as it can be seen in Table 1 with the values below.

12 14 16 18 20 22 24

57 Table 1: Summary of molecular weight data of Diol-ED-2003-Oligomer 2.2 irradiated with light at λ= 365 nm for certain time intervals. Molecular weights were determined via GPC in DMF with 10 mM LiBr against polystyrene standards.

0 h 18 h 72 h

Mn Peak 1 / kDa 41.051 43.529 44.916

Mw Peak 1 / kDa 351.235 170.945 167.501

Dispersity Đ 8.556 3.927 3.792

Mn Peak 2 / kDa 0.011 0.177 0.037

Mw Peak 2 / kDa 0.025 0.017 0.029

Dispersity Đ 2.322 0.095 0.778

4.9.2.2 Diol-ED-2003-Oligomer 3.1

The GPC measurements of Diol-ED-2003-Oligomer 3.1 after different irradiation times with light at λ= 365 nm can be seen in Figure 39.

The data looks similar to the GPC data of Diol-ED-2003-Oligomer 2.2 as the same degradation process can be observed. However, Peak 2 shows no increase in intensity after 18 h irradiation with λ= 365 nm light in comparison to the non-irradiated oligomer. Even more, the intensity appears to be lower than in the non-irradiated polymer. Nevertheless, since a clear and significant rise in intensity is visible after 72 h, this was interpreted as a measuring error probably due to the vicinity to the system peak.

58 Figure 39: GPC chromatogram of Diol-ED-2003-Oligomer 3.1 after 0 h, 18 h and 72 h of irradiation at λ= 365 nm.

As summarised in Table 2 a molecular weight of Mn= 57000- 64000 g mol^-1 against polystyrene standards was determined.

The dispersity Đ, again unexpectedly high is comparable to Diol-ED-2003-Oligomer 2.2. That said, the synthesis of Diol-ED-2003-Oligomer 3.1 resulted in slightly longer polymeric chains, possibly due to stoichiometric imbalances stemming from weighing errors.

12 14 16 18 20 22 24

0 2 4 6 8 10

Int en sity / a .u.

retention time / min

72 h 18 h 0 h 1

2

59 Table 2: Summary of molecular weight data of Diol-ED-2003-Oligomer 3.1 irradiated with light at λ= 365 nm for certain time intervals. Molecular weights were determined via GPC in DMF with 10 mM LiBr against polystyrene standards.

0 h 18 h 72 h

The GPC chromatograms of the remaining synthesised oligomers are not shown since the best results with the lowest dispersity Đ were achieved with Diol-ED-2003-Oligomer 2.2 and Oligomer 3.1, with Diol-ED-2003-Oligomer 3.1 showing slightly longer oligomers. Because of the low percentage of BODIPY in relation to the whole oligomer mass, no influences on the overall polymer properties were expected. However, due to the small batch sizes overall and the therefore connected difficulties in maintaining a stochiometric balance, differences in properties like molecular weight or dispersity Đ were present after all. In the low milligram area it is difficult to weigh in chemicals correctly, especially regarding the balance error at such small amounts. An upscale of this reaction may lead to a smaller weighing error and, which enables a better stoichiometric balance that is needed for higher molecular weights.

The influence of the addition order of the reactants during the synthesis on the polymer length is predicted to be low as the overall mol of monomers is maintained constant. However, 2 different synthesis routes were chosen because the monomeric addition order can have an influence on the distribution of compound 4 (Diol-BODIPY) in the backbone of the polymer. From the GPC data this could not be further verified.

60

4.10 Gel synthesis and degradation

In order to determine the degradability of the BODIPY- hydrogels, gels with different BODIPY contents have been synthesised based on isocyanate-chemistry as described above. They were all irradiated both with light at λ= 510 nm as well as with λ= 365 nm and investigated after certain time intervals. As a control, a BODIPY-free gel was produced and tested under the same conditions.

Furthermore, a kinetic analysis of the degradation process of a selected gel, gel 3, was performed and the presence of the BODIPY-structure further verified by the absorption and emission spectra of the gel.

4.10.1 Degradation studies under 510 nm irradiation

Although all gels were extensively washed with acetone and water, Gel 1 still showed a slight colouring of the solvent. This was maybe due to the large BODIPY content used in the synthesis of this hydrogel. Small amounts of unreacted compound 4 (Diol-BODIPY) are probably still leaching out of the polymer matrix.

However, since already only a miniscule amount of BODIPY is enough for a strong response in fluorescence and free BODIPY should have no significant influence on the gel degradation, no further washings were performed.

The irradiation process was carried out in acetone at approximately 5 °C over the course of 5 days. Not only did the gels decrease in size but they also changed in colour and consistency. A comparison of the gels before and after irradiation, photographed under UV-light can be seen in Figure 40.

61 Figure 40: Comparison of BODIPY-hydrogels with different fluorophore concentration before and

after irradiation at λ= 510 nm for 5 days, photographed under UV-light. Left Gel 1 (1:2), middle Gel 2 (1:28), right Gel 3 (1:140).

Gel 2 and Gel 3 lost most of the glow under UV- light, though not visible in Figure 40 a faint glow still remained. The Hydrogels visibly shrunk in size and partly lost their structural integrity, which later manifested in disintegration of the gels upon heavy shaking. This indicates breakage of some crosslinks, however, not enough to allow a full dissolution of the gel.

Gel 1 shows like Gel 2 and Gel 3 a slight decrease in size, even though less than for the other two. In contrast to the other two, an interesting change in colour can be seen upon placing it under UV-light. The colour changed from yellow to orange.

This leads to the assumption that the BODIPY- framework was not fully destroyed during the photocleavage process. The colour shift from yellow to orange rather indicates a further advanced leaching-out-process of the BODIPY out of the hydrogel into the solvent, as this effect was also observed during the handling with various BODIPY solutions of different concentration.

before

after 1 cm

1 cm

62 Most probably the observed higher rate of degradation of Gel 2 and Gel 3 compared to Gel 1 is due to aggregation caused quenching. The high concentration of BODIPY leads to a close vicinity of the photocages resulting in a quenching effect. The lower concentration in Gel 2 and Gel 3 allowed an increased distance of the BODIPY molecules which prevented an interaction between them.

Additionally, a higher BODIPY concentration can lead to a strong absorption of the irradiated light, in turn preventing a deeper penetration of the radiation into the hydrogel since the light is already absorbed at the edges of the hydrogel. The photocleavage effect can therefor only take place in the outer layers of the hydrogel. In general, in order to achieve an efficient degradation process, it is not only required to use the right concentration but also ensure a uniform distribution within the polymer with no local accumulations. Finally, the differences before and after the irradiation were also visible under ambient light, depicted in Figure 41.

The colour of Gel 2 and Gel 3 clearly faded, the solution of gel 1 however, intensified a bit in colour. This may be due to some unreacted and not incorporated BODIPY still leaching out of the hydrogel over the duration of 5 days.

Figure 41: Comparison of BODIPY-hydrogels with different fluorophore concentration before and after λ= 510 nm irradiation after 5 days. Left Gel 1 (1:2), middle Gel 2 (1:28), right Gel 3 (1:140).

before

after 1 cm

1 cm

63 4.10.2 Degradation studies under 365 nm irradiation

The irradiation process with light at λ= 365 nm was carried out under the same conditions as for λ= 510 nm. However, already after 72 h a significant change could be observed.

In Figure 42 and Figure 43 a comparison of the gels before and after irradiation can be seen under ambient and UV-light, respectively. Gel 2 and Gel 3 completely dissolved and a colourless liquid remained and Gel 1 significantly shrunk in size, the slight colouring of the solvent completely faded.

Figure 42: Comparison of BODIPY-hydrogels with different fluorophore concentration before and after irradiation with light at λ= 365 nm after 72 h. Left Gel 1 (1:2), middle Gel 2 (1:28), right Gel 3

(1:140).

Analogous to the irradiation at 510 nm Gel 1 degraded at the slowest rate. As described above this may be caused by a self-quenching effect, limiting the practicable amount of incorporated BODIPY.

before

after 1 cm

1 cm

64 Based on the information received from the experiments, the optimum concentration for an effective degradation should therefore be in the area between Diol:Jeffamine = 1:28 and Diol:Jeffamine = 1:140.

Figure 43: Comparison of BODIPY-hydrogels with different fluorophore concentration before and after irradiation with λ= 365 nm light after 72 h. Left Gel 1 (1:2), middle Gel 2 (1:28), right Gel 3

(1:140).

In regard to the degradation rate of Gel 2 and Gel 3 the setup was checked after 24 h to ensure a working experiment. From a subjective point of view the degradation of the Gel 2 was slightly faster than that of Gel 3.

Finally, these experiments showed that only very small BODIPY concentrations have to be implemented into a polymeric structure in order to achieve photosensitive degradation behaviour. Too high concentrations can have an advert effect and limit the degradation rate. This indicates the possibility to synthesise photolabile polymers with only marginal effect on polarity or other properties as only small amounts of fluorophore are introduced in the polymeric structure.

before

after 1 cm

1 cm

65 4.10.3 Degradation studies of a control gel

As a control, a hydrogel has been prepared without any addition of BODIPY compounds. The simple, crosslinked polyurea gel has been synthesised analogous to the BODIPY hydrogels and was irradiated with λ= 365 nm light for 5 days. In Figure 44 a comparison of the hydrogel before and after irradiation with λ= 365 nm can be seen. No degradation could be observed, demonstrating a stable polymeric backbone during UV- irradiation.

Figure 44: Comparison of a hydrogel synthesised analogous to BODIPY-hydrogels but without fluorophore addition before and after irradiation with light at λ= 365 nm for 5 days.

The reason why the gels degraded much faster under λ= 365 nm irradiation than under λ= 510 nm despite a higher absorbance of the fluorophore at λ= 510 nm may be due to different power outputs of the used lamps. The actual brightness of the lamps could not be determined, however, the power output of the UV-lamps is roughly tenfold higher than that of the used green light LEDs with 64 W compared to the 6 W, respectively. Additionally, the light spectrum of the UV-lamp is much broader and less well defined as the light from the LED. Overall, giving an explanation for the apparently higher degradation rate at λ= 365 nm compared to λ= 510 nm.

before

after 1 cm

1 cm

66 4.10.4 Kinetic studies of the hydrogel degradation

Since the different power outputs of the used lamps has been a major parameter affecting the degradation rate in the previous experiments, the process was further examined. To this end the photo degradation of the hydrogels was observed via fluorescence emission spectroscopy on a Jobin Yvon Fluorolog 3 spectrometer allowing irradiation at the different wavelengths with roughly the same intensity as well as recording the absorbance and emission spectra after regular time intervals.

Gel 3 was used for the experiment. It was stored in deionized water and used in its swelled state, the sample was placed between 2 glass slides without any additional solvent.

In Figure 45 the fluorescence emission spectra recorded after regular time intervals can be seen. The gel was continuously excited with light at λ= 365 nm.

67 Figure 45: Fluorescence emission spectra of gel 3 recorded after regular time intervals and continuous irradiation at λ= 365 nm. Top (full spectrum), bottom (zoomed).

A continuous degradation process can be observed with a reduced emission intensity of 91 % at the maximum after 180 min.

Analogous, the gel was irradiated at 510 nm as well. In Figure 46 the fluorescence emission recorded after regular time intervals and continuous irradiation at λ= 510 nm is depicted.

In contrast to the irradiation experiments in the solvent it can be seen that the degradation process is considerably faster at λ= 510 nm compared to the irradiation at λ= 365 nm. After 180 min the fluorescence intensity at the emission maximum was reduced to 76 %. Furthermore, the kinetics at the different wavelengths are found to be in correlation with their absorbance intensities.

500 525 550 575 600 625 650 675

0

520 530 540 550 560 570 580 590

500000

68 Irradiation near the absorption maximum leads to the fastest degradation rate, at λ= 510 nm the hydrogels degrade roughly the factor 2.5 faster than at λ= 365 nm.

This is most likely due to the higher absorbance at 510 nm which is also by a factor of 2.5 higher compared to at 365 nm.

Figure 46: Fluorescence emission spectra of Gel 3 recorded after regular time intervals and continuous irradiation at λ= 510 nm light, Top (full spectrum), bottom (zoomed).

Overall, in combination with the results of the degradation experiments in the solvent in chapter 4.9.1, as well as of the control gel in chapter 4.10.3 it can be proven that the degradation process observed in Figure 41 as well as Figure 42 is in fact due to photocleavage of the BODIPY-compound. Additionally, it can also be seen that the absorption and emission of the incorporated BODIPY does not significantly change compared to its free unbound state. The maximum absorbance and emission for the unbound compound 4 (Diol-BODIPY) were

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69 measured at λ= 538 nm and λ= 557 nm respectively and at λ= 526 nm and λ= 547 nm for the incorporated compound.

The small difference was expected due to the incorporation in the macromolecular system and possible changes in the chemical environment.

70

5. Outlook and conclusion

In conclusion, the work presented here reports the synthesis of a novel bifunctional green-light photo-labile BODIPY monomer, compound 4 (Diol-BODIPY), and its

In conclusion, the work presented here reports the synthesis of a novel bifunctional green-light photo-labile BODIPY monomer, compound 4 (Diol-BODIPY), and its

Im Dokument Bifunctional BODIPYs for green-light photolabile macromolecular applications (Seite 58-0)