**4. Results and discussion**

For optical experiments focused on studying excitation energy dependence of the energy transfer efficiency, we used highly diluted (optical density of 0.009 at 671 nm, concentration less than 10 μM) aqueous solution of PCP complexes. Such a low concentration is very important as on the one hand it strongly reduces the inner filter effect, but this also yields a thin layer of PCP complexes on a graphene surface. As a result, we minimize the fraction of

Finally, we fabricated structures for the evaluation of the effect of polymer layer (in our case PVA) on both interaction with graphene and photostability. To this end, samples were fabricated with the concentration of PVA varying between 0.2 and 0.002%. The obtained solutions were either drop-casted or spin-coated on single-layer graphene substrates. In the case of the latter approach, the concentration of PCP had to be adjusted to be slightly higher, as spin coating strongly reduces the number of PCP complexes within the focal volume of the

The optical properties of hybrid nanostructures comprising light-harvesting complexes and graphene-based materials were studied using absorption and fluorescence spectroscopy in the visible spectral region. Absorption spectra were obtained using Cary 50 spectrophotometer, while fluorescence in solution was measured using Fluorolog 3 spectrofluorometer. A Xenon lamp with a double grating monochromator was used for excitation and the signal was detected with a thermoelectrically cooled photomultipliertube characterized by a dark current

Fluorescence intensity maps were measured with an inverted fluorescence wide-field Nikon Eclipse Ti-U microscope equipped with an Andor iXon Du-888 EMCCD detector. For each sample, a series of 50 images were acquired in orderto allow forreliable statistics. Every image was collected for a different sample area, which allows for minimization of any photobleach‐ ing of the PCP fluorescence. Immersion objective with a magnification of 100× (Plan Apo, Nikon) and a numerical aperture of 1.4 was used, which provides a spatial resolution of about 300 nm. As a light source, we used LED illuminators (405, 480 and 530 nm) equipped with appropriate bandpass filters. Excitation power was equal to 50 μW. Fluorescence of PCP was extracted by combining a dichroic mirror (Chroma T650lxpr) and a bandpass filter (Thorlabs FB 670-10). Fluorescence intensity maps and kinetics were collected with the electron multiplying gain of 300× and acquisition times of 0.25 or 0.5 s, depending on the experimen‐ tal conditions. White-light transmission images were recorded with the same microscope, with

Spectrally and time-resolved fluorescence measurements were performed using a home-built confocal fluorescence microscope described in detail in [53]. The sample was placed on a piezoelectric translation stage. We used pulsed laser excitation at 405, 485 and 640 nm (repetition rate of 20 MHz, average power of 30 μW, power density of ~1MW/cm−2). Impor‐ tantly, PCP can be efficiently excited at 405 (Soret band), at 485 (Per) and at 640 nm (excited states of chlorophylls). The laser beam was focused on the sample by LMPlan 50× objective (Olympus) with a numerical aperture of 0.5. Fluorescence was first filtered by a longpass

a halogen lamp V2-A LL (12 V, 100 W) as a light source.

PCP that is not coupled to graphene, thus takes no part in the energy transfer.

focused laser.

of less than 100 cps.

**3.4. Experimental techniques**

162 Recent Advances in Graphene Research

In this section, we describe three experiments, where interactions between the PCP com‐ plexes and both rGO and epitaxial graphene were investigated. Since the PCP complexes are soluble in water, we mixed them with rGO and investigated for the energy transfer between the light-harvesting complexes and rGO. In the case of epitaxial graphene, we focus on two aspects: the dependence of the energy transfer efficiency from PCP complexes to graphene on the excitation wavelength and the influence of sample preparation on the strength of the interactions in such hybrid nanostructures.

#### **4.1. PCP with reduced graphene oxide**

Interactions between various emitters and rGO were studied so far only in solution, where ensemble averaging can smear out subtle effects associated with the interaction between emitters and rGO. Our idea was to prepare mixtures with controlled concentration of both PCP and rGO, deposit solutions on glass coverslips and image fluorescence with high spatial resolution and high sensitivity [26]. In all experiments, the concentration of PCP was main‐ tained constant.

**Figure 8.** Fluorescence maps of (a) PCP-only reference sample and PCP/rGO mixture samples with varied rGO concen‐ tration. The excitation wavelength was 530 nm.

In **Figure 8**, we show a sequence of fluorescence maps collected for PCP complexes mixed with varied amount of rGO. The concentration of rGO was varied by orders of magnitude, so that the influence can be seen in a pronounced way. The excitation wavelength was 530 nm, but the results are qualitatively identical for the other two excitation wavelengths used in the

experiment. Incorporation of rGO induces substantial changes in fluorescence images of PCP: while forthe PCP-only sample the distribution of intensity is pretty much uniform, mixing the photosynthetic complexes with rGO leads to a pattern that features high-intensity spots on an otherwise uniform background. Importantly, the intensity of these isolated bright spots is approximately 10-fold enhanced as compared to the intensity measured for the reference sample (PCP-only). At the same time, the fluorescence intensity away from the bright spots in the PCP/rGO hybrid system is quenched compared to the reference. Furthermore, the number of the bright spots increases with rGO concentration. We can exclude any contribution to this emission from GO that might be present in our sample due to non-complete reduction, as the fluorescence of GO occurs in a spectral range between 350 and 550 nm, and the emission of the PCP complexes is strongly shifted to the red. This proves that for ensemble of PCP complexes, both in the reference and in the hybrid structures (PCP/rGO) studied in this work, there is no other contribution to the measured signal.

**Figure 9.** Histograms for PCP and PCP mixed with rGO with indicated concentrations obtained for average fluores‐ cence intensities calculated from 50 fluorescence intensity maps for each rGO concentration. The excitation wavelength of 480 nm was used.

Statistical information about observed effects is obtained by collecting a series of more than 50 images for each sample configuration and for every excitation wavelength. In this way, we strongly reduce any possible influence of particular sample preparation or a way the experi‐ ment was carried out. Next, the fluorescence images are analysed by plotting a histogram of all the intensities measured [26]. This procedure can be applied for any single map, but also to all the maps measured for a given excitation wavelength and rGO concentration. The result of this procedure carried out of the excitation wavelength of 480 nm is displayed in **Figure 9**. The distribution of fluorescence intensity measured for the reference sample, containing only PCP complexes, features Gaussian shape, indicative of statistical distribution of concentra‐ tion variation across the substrate. By contrast, the results extracted for PCP complexes mixed with rGO are more complicated. Although the majority of the intensity distribution can be approximated with Gaussian shape, similarly as in the case of the reference sample, the maximum of this distribution shifts towards lower values with increasing rGO concentra‐ tion. In addition, we find considerable contribution of isolated spots spread out randomly across the images, as shown in **Figure 8**, and this contribution is larger with increasing concentration of rGO in the sample. Not only the shift to lower emission intensities with incorporating rGO to the mixture as well as emergence of high-intensity spots in fluores‐ cence images is systematic in nature, but both effects exhibit monotonic dependence on the rGO concentration in the initial solution. Indeed, for the highest rGO concentration (0.25 mg/ ml), we find the largest number of bright fluorescence spots, and furthermore they exhibit the highest intensity. The results of fluorescence imaging of PCP/rGO mixtures strongly suggest that the incorporation of rGO yields both quenching of emission and formation of strongly enhanced spatially localized emitting sites. The results of fluorescence imaging show thus that the interaction between rGO and photosynthetic complexes of PCP is more complex than as discussed in a simple image of fluorescence quenching.

experiment. Incorporation of rGO induces substantial changes in fluorescence images of PCP: while forthe PCP-only sample the distribution of intensity is pretty much uniform, mixing the photosynthetic complexes with rGO leads to a pattern that features high-intensity spots on an otherwise uniform background. Importantly, the intensity of these isolated bright spots is approximately 10-fold enhanced as compared to the intensity measured for the reference sample (PCP-only). At the same time, the fluorescence intensity away from the bright spots in the PCP/rGO hybrid system is quenched compared to the reference. Furthermore, the number of the bright spots increases with rGO concentration. We can exclude any contribution to this emission from GO that might be present in our sample due to non-complete reduction, as the fluorescence of GO occurs in a spectral range between 350 and 550 nm, and the emission of the PCP complexes is strongly shifted to the red. This proves that for ensemble of PCP complexes, both in the reference and in the hybrid structures (PCP/rGO) studied in this work,

**Figure 9.** Histograms for PCP and PCP mixed with rGO with indicated concentrations obtained for average fluores‐ cence intensities calculated from 50 fluorescence intensity maps for each rGO concentration. The excitation wavelength

Statistical information about observed effects is obtained by collecting a series of more than 50 images for each sample configuration and for every excitation wavelength. In this way, we strongly reduce any possible influence of particular sample preparation or a way the experi‐ ment was carried out. Next, the fluorescence images are analysed by plotting a histogram of all the intensities measured [26]. This procedure can be applied for any single map, but also to all the maps measured for a given excitation wavelength and rGO concentration. The result of this procedure carried out of the excitation wavelength of 480 nm is displayed in **Figure 9**. The distribution of fluorescence intensity measured for the reference sample, containing only PCP complexes, features Gaussian shape, indicative of statistical distribution of concentra‐ tion variation across the substrate. By contrast, the results extracted for PCP complexes mixed with rGO are more complicated. Although the majority of the intensity distribution can be approximated with Gaussian shape, similarly as in the case of the reference sample, the maximum of this distribution shifts towards lower values with increasing rGO concentra‐ tion. In addition, we find considerable contribution of isolated spots spread out randomly

there is no other contribution to the measured signal.

164 Recent Advances in Graphene Research

of 480 nm was used.

**Figure 10.** (Upper row) White-light transmission image of a graphene aggregate and intensity map of PCP fluores‐ cence measured for 480-nm excitation for exactly the same location. (Lower row) Intensity profiles extracted along the cross ections marked with red lines in both images.

In order to get some insight into the possible origin of this bimodal behaviour, we also imaged a large rGO aggregate, as shown in **Figure 10**. The correspondence between transmission image and fluorescence image indicates that the same object is probed in both experiments. Even without any detailed analysis, the comparison between the two images suggests that enhanced emission occurs for PCP complexes at the edges of the flake and perhaps on its thin sections. By contrast, when the thickness of the aggregate is large, the fluorescence of PCP is efficiently quenched.

The relation between transmission and fluorescence images can be quantified using for instance Pearson correlation coefficients calculated for the two transmission-emission pair cross sections marked with red lines in **Figure 10**. For one line, we obtain a negative coeffi‐

cient of −0.55, meaning that the high intensity of fluorescence correlates with dips in trans‐ mission images. A contrasting effect can be seen for the other line, where the coefficient of approximately 0.5 was obtained. Therefore, in this case, low transmission combines with quenched fluorescence. Both correlation and anti-correlation between profiles obtained for transmission and fluorescence images can be readily seen in the panels in **Figure 10**, where corresponding cross sections are plotted. These results may suggest that when a region outside the flake is considered, the correlation between emission and transmission is low. For a thin rGO flake, we find negative correlation, which could imply that PCP complexes placed at the edges of rGO experience fluorescence enhancement. Finally, for the thickest part of the flake, where both transmission and fluorescence exhibit strong decrease, the quenching is the dominant effect.

The results of fluorescence imaging of PCP/rGO hybrid nanostructures show bimodal character of the interactions. On the one hand, the fluorescence of the majority of PCP complexes is quenched; however, there are numerous localized spots characterized with considerably higher intensity. This effect depends on the rGO concentration, in particular the number of these bright spots increases with rGO concentration. While the quenching of fluorescence was observed for many graphene-based hybrid nanostructures, the enhance‐ ment is far less frequent. The exact mechanism of fluorescence enhancement is not clear, but the results show unambiguously the complexity of interactions in graphene-based photosyn‐ thetic hybrid nanostructures. Future work, which includes spectrally and temporally re‐ solved studies, as well as experiments on large rGO flakes are in orderto answer some of these questions.

#### **4.2. Energy transfer from PCP to graphene: excitation wavelength dependence**

The broad absorption of PCP complexes allows forinvestigating the dependence of the energy transfer efficiency on the excitation wavelength [54]. The rationale behind this experiment is that as graphene is a conductor, and contains high concentration of free carriers, it should be possible to affect the behaviour of these electrons locally using focused laser excitation. At the same time, with PCP as a donor, it is still possible to excite emission and probe the energy transfer dynamics.

In this experiment, we used epitaxial monolayer graphene transferred on 285-nm SiO2 substrate. The optical properties of such a hybrid nanostructure were probed by time-resolved fluorescence microscopy with three excitation wavelengths of 405, 485 and 640 nm. All these wavelengths excite the emission of the PCP complexes, either via direct excitation of chloro‐ phyll molecules or via intra-complex energy transfer from Per molecules.

In **Figure 11**, we compare fluorescence spectra measured for PCP complexes deposited on graphene with the reference. The excitation wavelength was 405 nm. This result shows substantial decrease of fluorescence intensity upon deposition of PCP on graphene, which can be tentatively attributed to the energy transfer. The decrease of fluorescence intensity is the strongest for the 405-nm excitation and the weakest for the 640-nm excitation, which again suggests that there is indeed a dependence of the energy transfer efficiency on the excitation wavelength. However, comparison of bare intensities of emission could be misleading as the values might depend on many factors that can be sometimes difficult to control experimen‐ tally. However, we note that the shape of the PCP emission spectrum on graphene remains unaffected, and is identical to previously published [37], which indicates that depositing PCP on graphene leaves no measureable effect on the energy transfer pathways within the PCP complexes.

cient of −0.55, meaning that the high intensity of fluorescence correlates with dips in trans‐ mission images. A contrasting effect can be seen for the other line, where the coefficient of approximately 0.5 was obtained. Therefore, in this case, low transmission combines with quenched fluorescence. Both correlation and anti-correlation between profiles obtained for transmission and fluorescence images can be readily seen in the panels in **Figure 10**, where corresponding cross sections are plotted. These results may suggest that when a region outside the flake is considered, the correlation between emission and transmission is low. For a thin rGO flake, we find negative correlation, which could imply that PCP complexes placed at the edges of rGO experience fluorescence enhancement. Finally, for the thickest part of the flake, where both transmission and fluorescence exhibit strong decrease, the quenching is the

The results of fluorescence imaging of PCP/rGO hybrid nanostructures show bimodal character of the interactions. On the one hand, the fluorescence of the majority of PCP complexes is quenched; however, there are numerous localized spots characterized with considerably higher intensity. This effect depends on the rGO concentration, in particular the number of these bright spots increases with rGO concentration. While the quenching of fluorescence was observed for many graphene-based hybrid nanostructures, the enhance‐ ment is far less frequent. The exact mechanism of fluorescence enhancement is not clear, but the results show unambiguously the complexity of interactions in graphene-based photosyn‐ thetic hybrid nanostructures. Future work, which includes spectrally and temporally re‐ solved studies, as well as experiments on large rGO flakes are in orderto answer some of these

**4.2. Energy transfer from PCP to graphene: excitation wavelength dependence**

phyll molecules or via intra-complex energy transfer from Per molecules.

The broad absorption of PCP complexes allows forinvestigating the dependence of the energy transfer efficiency on the excitation wavelength [54]. The rationale behind this experiment is that as graphene is a conductor, and contains high concentration of free carriers, it should be possible to affect the behaviour of these electrons locally using focused laser excitation. At the same time, with PCP as a donor, it is still possible to excite emission and probe the energy

In this experiment, we used epitaxial monolayer graphene transferred on 285-nm SiO2 substrate. The optical properties of such a hybrid nanostructure were probed by time-resolved fluorescence microscopy with three excitation wavelengths of 405, 485 and 640 nm. All these wavelengths excite the emission of the PCP complexes, either via direct excitation of chloro‐

In **Figure 11**, we compare fluorescence spectra measured for PCP complexes deposited on graphene with the reference. The excitation wavelength was 405 nm. This result shows substantial decrease of fluorescence intensity upon deposition of PCP on graphene, which can be tentatively attributed to the energy transfer. The decrease of fluorescence intensity is the strongest for the 405-nm excitation and the weakest for the 640-nm excitation, which again suggests that there is indeed a dependence of the energy transfer efficiency on the excitation wavelength. However, comparison of bare intensities of emission could be misleading as the

dominant effect.

166 Recent Advances in Graphene Research

questions.

transfer dynamics.

**Figure 11.** (Left) Fluorescence spectra of the PCP complexes deposited on graphene (blue) and on a glass substrate (black). The excitation wavelength was 405 nm. (Right) Comparison between average fluorescence transients measured for three excitation wavelengths, as indicated.

The initial assignment of the observed reduction of the emission intensity of PCP complexes deposited on graphene to the energy transfer from the chlorophylls in the photosynthetic complex to graphene is confirmed by time-resolved fluorescence spectroscopy. In **Figure 11**, we display average fluorescence transients measured for the excitation wavelengths of 405, 485 and 640 nm. The results are compared with the decay obtained for PCP complexes deposited on glass, and it has been checked that the obtained transient very weakly changes with the excitation wavelength in this case. As described in detail in [54], we find some degree of variation of fluorescence decays measured for a given excitation wavelength. It is expect‐ ed, as in this experiment, that we do not control the separation between graphene and PCP complexes, and in turn introduce inhomogeneity of the interaction between the two struc‐ tures across the substrate. Furthermore, it has been shown that for graphene deposited on silica, the local structure of graphene is also quite inhomogeneous with islands of high and low mobility of carriers [55]. We might therefore assume that such non-uniformity contrib‐ utes to some degree to the observed spreading of fluorescence transients, although the scale of these inhomogeneities is less than 100 nm, as compared to the resolution of our micro‐ scope of about 1 μm.

Nevertheless, for both excitation wavelengths (and for 485-nm excitation as well), we observe significant shortening of fluorescence lifetime for PCP complexes deposited on graphene as compared with the reference. In addition, the 405-nm excitation yields very fast decays, while exciting with 640 nm results in considerably longer decays, regardless of the inhomogeneity of the data. It is also striking that most of them exhibit almost monoexponential behaviour, which could suggest that majority of PCP complexes within the laser spot couples to gra‐ phene with a comparable strength. In no case, however, for PCP deposited on graphene we observe long (~4-ns) decay component attributable to PCP complexes isolated from graphene.

The key conclusion from these experiments is that the energy transfer to graphene depends on the excitation wavelength. Indeed, shortening of the fluorescence decay, accompanied with a decrease of the overall fluorescence intensity, observed for all used excitation wavelengths (405, 485 and 640 nm) strongly suggests that the energy absorbed by PCP complexes is efficiently dissipated into the graphene layer. Furthermore, much shorter fluorescence decay times measured for 405-nm excitation prove that the energy transfer for this excitation wavelength is more efficient compared to 640-nm excitation. The average decay times are equal to 0.5 and 1.4 ns, respectively, what translates to the energy transfer efficiencies of 87 and 65%. This is the first experimental observation of such an effect, which distinguishes graphene as a totally unique acceptor of energy in such hybrid assemblies. The qualitative picture display‐ ing this fact is shown in **Figure 12**.

**Figure 12.** Schematics showing the effect of the excitation wavelength on the efficiency of the energy transfer between PCP complexes and graphene.

For molecular systems, where the energy transfer takes place between two dipole moments, the decay of a donor is independent of the excitation wavelength. This is a reminiscence of the fact that light has no effect on the surrounding of the molecules participating in the energy transfer. Apparently, for PCP complexes on graphene the situation is different. Clear influ‐ ence of the excitation wavelength on the energy transfer indicates that in addition to populat‐ ing PCP-excited states, laser changes also the local properties of graphene. A scenario that can explain this effect relies on the fact that focused laser excited electrons in graphene in a similar way as in metallic nanoparticle, forcing them to oscillate in a confined space defined by a monolayer of graphene on the one hand, and the size of the laser spot on the other. As a consequence, electronic excitations in chlorophylls in PCP can see graphene as a metallic nanoparticle with specific character that can influence energy dissipation.

Based on these results, we conclude that energy quenching in graphene is driven not only by dipole-dipole interaction but also by a mechanism associated with light-induced oscillations of electrons in graphene. Indeed, exciting electrons in graphene has an effect of its dissipa‐ tive efficiency, which opens avenues in interfacing electronic and plasmonic character of graphene in hybrid nanostructures and controls the electronic dynamics of such systems with light.

#### **4.3. Energy transfer from PCP to graphene: influence of sample structure**

phene with a comparable strength. In no case, however, for PCP deposited on graphene we observe long (~4-ns) decay component attributable to PCP complexes isolated from graphene.

The key conclusion from these experiments is that the energy transfer to graphene depends on the excitation wavelength. Indeed, shortening of the fluorescence decay, accompanied with a decrease of the overall fluorescence intensity, observed for all used excitation wavelengths (405, 485 and 640 nm) strongly suggests that the energy absorbed by PCP complexes is efficiently dissipated into the graphene layer. Furthermore, much shorter fluorescence decay times measured for 405-nm excitation prove that the energy transfer for this excitation wavelength is more efficient compared to 640-nm excitation. The average decay times are equal to 0.5 and 1.4 ns, respectively, what translates to the energy transfer efficiencies of 87 and 65%. This is the first experimental observation of such an effect, which distinguishes graphene as a totally unique acceptor of energy in such hybrid assemblies. The qualitative picture display‐

**Figure 12.** Schematics showing the effect of the excitation wavelength on the efficiency of the energy transfer between

For molecular systems, where the energy transfer takes place between two dipole moments, the decay of a donor is independent of the excitation wavelength. This is a reminiscence of the fact that light has no effect on the surrounding of the molecules participating in the energy transfer. Apparently, for PCP complexes on graphene the situation is different. Clear influ‐ ence of the excitation wavelength on the energy transfer indicates that in addition to populat‐ ing PCP-excited states, laser changes also the local properties of graphene. A scenario that can explain this effect relies on the fact that focused laser excited electrons in graphene in a similar way as in metallic nanoparticle, forcing them to oscillate in a confined space defined by a monolayer of graphene on the one hand, and the size of the laser spot on the other. As a consequence, electronic excitations in chlorophylls in PCP can see graphene as a metallic

Based on these results, we conclude that energy quenching in graphene is driven not only by dipole-dipole interaction but also by a mechanism associated with light-induced oscillations of electrons in graphene. Indeed, exciting electrons in graphene has an effect of its dissipa‐ tive efficiency, which opens avenues in interfacing electronic and plasmonic character of

nanoparticle with specific character that can influence energy dissipation.

ing this fact is shown in **Figure 12**.

168 Recent Advances in Graphene Research

PCP complexes and graphene.

An important aspect, frequently overlooked, in discussing the optical properties of hybrid nanostructures, where the interactions are distance-dependent, is the design and fabrication of layered structures. In the case of the experiment where we studied the influence of excitation energy on the efficiency ofthe energy transferfrom PCP complexes to graphene, we used dropcasting to deposit the PCP solution on a graphene monolayer. Since it was aqueous solution, once water evaporated, PCP complexes are assumed to fall down on the graphene surface. This method, while allowing for making structures with rather well-defined geometry, results in the pigment-protein complexes being fully exposed to ambient conditions. This in turn speeds up degradation of the pigments, and the complexes as well. One of the most com‐ mon ways to increase their protection against oxygen is to embed them in polymer matrix [56].

**Figure 13.** (Left) Fluorescence intensity decays of PCP deposited on graphene, diluted beforehand in water or in PVA aqueous solutions with varied concentrations. (Right) Corresponding kinetic curves of integrated emission intensity spectra measured in 10 min with acquisition time of 1 s. The excitation wavelength was 485 nm.

In **Figure 13**, we compare fluorescence decay curves measured for PCP complexes in water and in PVA matrix with varied concentration of the polymer, for the samples drop-casted on a single-layer graphene. The lowest concentration of PVA gives almost identical result as pure aqueous solution, in other words, the fluorescence decay is substantially shorter than the reference. This, in addition with almost monoexponential character of the decay, indicates that almost all of the PCP complexes interact with graphene, which means that the energy is transferred from chlorophylls to graphene. As the PVA concentration increases, the charac‐ ter of the decay changes dramatically. It is no longer monoexponential, and it features a long decay tail, reminiscent of the decay characteristic for PCP complexes that are uncoupled to graphene. This result can be understood in terms of a thicker-layer formation for a solution with higher content of PVA polymer. Such a scenario would then lead to comparatively smaller fraction of the PCP complexes that couple with graphene, most of them would be too far away from the graphene layer to experience its presence at all. This interpretation is strongly

supported by the increase of total fluorescence intensity observed with increasing concentra‐ tion of PVA in solution. In fact, this increase is over an order of magnitude, although the amount of the solution deposited on a substrate is in all cases identical.

There is also another important aspect that must be considered when fabricating graphenebased hybrid nanostructures. An alternative approach to prepare layers of fluorophores on graphene substrates would be through spin coating of a solution of PCP complexes in PVA matrix. The obtained layers are much more uniform than those made by simple drop-casting, and it shows no systematic dependence on the PVA concentration. Rough estimations, based on the distribution of emission intensities measured as emission spectra or decay curves, suggest that the uniformity of the layers prepared with spin coating is about a factor of two better in terms of a standard deviation, as compared to the drop-casted samples. At the same time, the concentration of PCP complexes within the focal volume of the laser would dimin‐ ish considerably as compared with the drop-casting approach; thus, this parameter must be carefully adjusted.
