Processing and Morphology of the Aggregates

1.2.3 Optical Properties 39

1.2.4 Processing and Morphology of the Aggregates 40

The overview in a with its magnification in b shows a typical P2 aggregate distribution, where some larger aggregates can be seen to be highly transparent for electrons. The tiny aggregate magnified in e reaches already a width of 40 nm, while its internal structure is apparently highly ordered, as can be assumed from the high aspect ratio of 7.5 and the straight facets, which are almost like in a conventional crystal.

The micrograph in d shows a magnification of a dark spot, of which only two were visible in the overview a.

These are agglomerates of very small aggregates, which did not have such regular shapes and such aspect ratios like the above described aggregate. They might have a more disordered internal structure, but are not amorphous as can by seen on their distinct facets.

1.2.4.2 Precipitation from DCM – n-heptane mixtures

Similar P2 J-aggregates can be obtained also by precipitation from a concentrated P2 dispersion within anhydrous DCM when added into a larger amount of anhydrous n-heptane (n-C7), in which a substrate was placed for the coverage with P2 aggregates. By this technique the aggregates grow also on the substrate surface which is directed downwards when the liquid phase has enough access to this surface, e.g. when its not sticking tightly to the bottom of the vessel. The results of J-aggregates processed by this precipitation technique is shown in Fig. 27.

Fig. 26: SEM micrographs of P2 aggregates on silicon deposited by drop-casting from DCM

P2 has been dissolved within anhydrous DCM (CH2Cl2) to a concentration of 2.1 mM by sonicating it for 5 min and keeping the dispersion sealed for a few hours in the dark. A drop of this dispersion has been dried onto a smooth silicon wafer under ambient conditions. In-lens, EHT: 10 kV, WD: 4 mm, aperture:

30 µm

e b

c d

a

1.2.4 Processing and Morphology of the Aggregates 41

The P2 aggregates and their distribution differed not much from the morphology obtained by the drop-casting technique directly from anhydrous DCM (Fig. 26). The precipitation technique in n-heptane led to a macroscopically more homogeneous distribution of the aggregates over the whole substrate without significant macroscopic concentration gradients nor drying rings, in contrast to the drop-casting technique.

This technique is also less sensitive to impurities within the solvent, because the n-heptane is not allowed to dry completely onto the substrate, thus more impurities and monomers can stay within the remaining solvent.

By the drop-casting technique also amorphous residues can be deposited on the substrate if the solvent was not pure enough. The microscopic morphology and size-distribution of the aggregates is similar for both deposition techniques. There might be a slight tendency that the rod-shaped aggregates, which were deposited by the precipitation technique, agglomerated slightly more, as can be seen in Fig. 27b.

1.2.4.3 Testing other Solvents: Dichlorobenzene and Diethyl Ether

As reported by Takahasi et al. [44] 1,2-dichlorobenzene, also called ortho-DCB or o-DCB, does also not coordinate the central zinc atom of zinc porphyrins, similar to DCM, in contrast to e.g. alcohols. As o-DCB has a much higher boiling point of 180°C, compared to that of DCM, being 40°C, the self-assembly at elevated temperatures may differ from that at room temperature from DCM. We investigated the resulting aggregates deposited by evaporating some drops of P2 dispersed within commercial o-DCB at 110°C (Fig.

28).

Fig. 27: SEM micrographs of P2 aggregates on silicon deposited by precipitation in n-C

7

20 µl of the same 2.1 mM dispersion of P2 within anhydrous DCM as in Fig. 26 were injected into 1 ml of anhydrous n-heptane (n-C7) containing an additional amount of 30 µl of anhydrous DCM. The aggregates precipitated completely within seconds and macroscopic homogeneous onto all walls of the vessel and the inside placed silicon wafer. In-lens, EHT: 10 kV, WD: 4 mm, aperture: 30 µm

b a

1.2.4 Processing and Morphology of the Aggregates 42

The SEM micrographs revealed quite similar rod-shaped structures as known from DCM dispersions. The absorption spectrum of P2 dispersed within o-DCB, shown in Fig. 29, was also very similar to that in DCM, revealing a major fraction of J-aggregates. To this dispersion also a small amount of THF was added in order to see if it is possible to evaporate the THF, which has a boiling point of 66°C, from the mixture with o-DCB, what might be indicated by a recovery of the self-assembly of P2.

Fig. 28: SEM micrographs of P2 J-aggregates from a o-DCB dispersion

A 0.1 mM dispersion of P2 in 1,2-dichlorobenzene (o-DCB) was drop-cast onto an FTO substrate. The solvent was evaporated in a drying oven at ca. 110°C. In-lens, EHT: 5 kV, WD: 5 mm, aperture: 20 µm

d

a b

c

1.2.4 Processing and Morphology of the Aggregates 43

After the addition of anhydrous DCM to the P2-o-DCB dispersion the absorption spectrum did not change significantly and still indicated mainly the presence of J-aggregates by the red-shifted Soret band (shoulder) and the red-shifted Q bands like that at 632 nm. After addition of THF, the J-aggregates disassembled almost completely within 40 min, what could be seen by the vanishing of the red-shifted Soret band and Q bands while the monomeric bands rose. The Soret band maximum slightly shifted from 432 to 435 nm due to the common solvent effect of the polar THF. The solution was has been annealed until it large bubbles started to rise at about 120°C. After a few seconds the boiling stopped and the temperature was maintained for 5 more minutes. The following absorption measurement of this solution showed almost no qualitative change compared to the former one containing THF in the o-DCB solution. The Soret band shifted partly back to 434 nm and the Q bands also by about 1-3 nm, showing that the THF was only partly removed again. The deposits from this annealed P2-THF-o-DCB solution were amorphous.

Also the deposition from the solvent diethyl ether was tested, which has a very similar molecular structure as THF, but is known to have very different properties, due to its acyclic conformation shielding the lone pairs of the oxygen [47]. The used commercial diethyl ether contained 0.2 % water and therefore has been distilled once to reduce the water content. The absorption spectrum of the P2 within diethyl ether was very similar to that within anhydrous DCM and o-DCB, revealing also predominantly J-aggregates. The aggregates have been drop-cast from this dispersion onto a mesoporous TiO2 layer, which contained additional macropores.

The J-aggregate morphology and size-distribution was also very similar to that obtained from anhydrous DCM dispersions, as can be seen in Fig. 30.

Fig. 29: Absorption spectra of P2 in o-DCB and with a trace of DCM and THF

a) Into a 0.1 mM dispersion of P2 within commercial dichlorobenzene (o-DCB) 50 µl anhydrous dichloromethane (DCM) were added (≈ 11 Vol-%) and afterwards 50 µl anhydrous THF. Whereas the DCM did not change the absorption spectrum significantly, the addition of THF led to a disassembly of the P2 J-aggregates within about 40 min. b) The solution was then heated up to 120°C until the first bubbles arose and was kept at this temperature for about 5 min.

300 400 500 600 700 800

0.0 0.2 0.4 0.6 0.8

1.0 435

400 600 800

0 1 2 3

Absorbance

Wavelength / nm 600632 560590 552 432

P1 in DCB + DCM + THF after 40 min.

300 400 500 600 700 800

0.0 0.1 0.2

0.3 P2 in 1,2-DCB

+DCM +THF 5 min. at 120 °C

Absorption

Wavelength / nm

a) b)

diethyl ether THF

P2

1.2.4 Processing and Morphology of the Aggregates 44

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