The effects of amino acid functionalisation on the optoelectronic properties and self-assembly of perylene bisimides

Here we report on ten water-soluble perylene bisimides that are functionalised with the amino acids L-alanine, L-aspartic acid, L-glutamic acid, L-phenylalanine, L-histidine, L-leucine, L-methionine, L-valine, L-tryptophan, and L-tyrosine. Reduction potentials, absoprtion and emission spectra, molar absorptivity, quantum yield, and rheology are obtained and the data interpreted for each species in aqueous solution or hydrogels in order to provide a comprehensive understanding of the subtle effects of amino acid functionalisation on the optoelectronic and supramolecular properties.


Introduction
Perylene bisimides (PBIs, also referred to as perylene diimides or PDIs) are a class of compounds possessing several features that make them very useful in a wide variety of organic electronic devices.For example, PBIs possess high molar absorptivity in the visible spectrum and high thermal and photo stability owing to a low-lying triplet excited state [1] which gives them high fluorescence quantum yield (QY) [2,3].This allows them to be used as light absorbers in organic solar cells (OSCs) [4][5][6] and as emitters in organic light emitting diodes (OLEDs) [7,8].PBIs also possess high electron affinity (EA) and can be readily reduced which, combined with their propensity to self-assemble [9][10][11], makes them effective electron acceptors or electron transport mediators in a variety of devices [12][13][14].In addition, these reduced states are often differently coloured from the neutral state making PBIs suitable for electrochromic devices [11,15,16].Finally, PBIs can be readily functionalised at the imide positions or at the bay and ortho positions of the perylene core in order to fine-tune the optoelectronic, solubility, and self-assembly properties to better suit their desired application [17][18][19][20][21].
Although PBIs were first discovered in 1913 [22], interest in their potential has increased dramatically in the last few decades as research in organic electronic devices has intensified.With this increased interest comes the importance for developing PBI materials that are readily processable from green, environmentally friendly solvents [23,24].We have developed amino acid-functionalised PBIs which are water soluble in neutral to high pH and function as low molecular weight gelators [25][26][27][28].By studying their absorption and rheological properties, we have found that the choice of amino acid has a profound impact on PBI self-assembly and optoelectronic properties [29].As a result, amino acid choice greatly affects PBI performance in electronic devices.We have observed this effect when we fabricated electrochromic [15], OSC [12], and photocatalytic [30] devices with our amino acid-functionalised PBIs.Here we have explored a library of amino acid-functionalised PBIs (figure 1) and performed a comprehensive characterisation of their optical, electronic, and self-assembling properties, comparing both new and previously reported species, in order to more fully understand the subtle effects of the choice of amino acid.Some of the materials are new while the others have been previously reported.By collating and studying the data for these PBIs together in one study, we hope to discern new trends or patterns that will lead to a deeper understanding of the nature of these materials.Our aim is not only to increase our knowledge of the structure-property relationship in amino acid-functionalised PBIs and find a link between aggregation types, chemical structure, and performance, but also to demonstrate to the wider community the ease with which a common organic material can be made water soluble without compromising its desirable properties.
We focus here on the effect of aggregation by these PBIs in water, providing a direct comparison between PBIs under the same conditions.In water, these PBIs aggregate, with the aggregated species formed being dependent on the exact solution pH, temperature, process history, and salt concentration.Process dependence in supramolecular systems is a common observation.As such, comparing between isolated reports is difficult in general and a comprehensive study is required.Here, to understand the differences between species, we have measured the cyclic voltammogram (CV) in organic solution (to probe the monomeric state) and compared this to the aggregated species in water.We also compare the optical properties of the different species across a range of pH values.These data allow us to probe the effect of amino acid substitution on the optoelectronic properties of the molecule.
The materials were all analysed both at a concentration of 5 mg ml −1 in solution at various pHs and in the gel state.To carry out the analyses, stock solutions of each PBI were prepared by dissolving in deionised water with 1 molar equivalent of NaOH (0.1 M) and leaving to stir overnight.The solutions were then adjusted to the desired pH with 1 M NaOH or 1 M HCl.To prepare the gels, a pre-weighed amount of glucono-δ-lactone (GdL) was added to the solutions and then gently shaken until the GdL was visibly dissolved.The samples were left overnight at room temperature to gel.
PBIs are well known for their high EA and ease of reduction to the dianion form which can be measured via cyclic voltammetry, appearing as two well-resolved reduction waves.As these PBIs are water soluble at high pH, we measured their CVs in water, pH adjusted to 9 and 6, with 40% v/v 0.1 M NaCl as an electrolyte.Each scan was run five times at 50, 100, and 200 mV s −1 scan rates.Since we are looking to directly compare across our series and maintain constant solution conditions, we provide data here, some data nominally repeats the literature but this direct comparison eliminates variables that arise from differences in pH, concentration, electrolyte, film thickness etc in the previously reported studies.We highlight that a key aspect is that the data collected depends on the aggregation state of the molecule and hence we report here on the specific aggregation states of these PBIs and, as such, there are subtle differences in some data from previous work which can be ascribed to these variables.
The CV results are shown in the SI, figures S53-S92.A representative example of an aqueous CV for PBI-V is shown in figure 2. In this study, there was not found to be any meaningful difference when the measurements were taken at different pH values.The two well-defined reduction waves that are typical of the CVs of PBIs are not seen for these aqueous measurements.Instead, only one reduction event appears to be present for each PBI each with an onset of approximately −0.4 V to −0.5 V. To further explore the reduction Scheme 1.The general reaction method for synthesizing amino acid-functionalised PBIs.R represents the different amino acids side chains as indicated in figure 1. behaviour of these PBIs in aqueous media, differential pulse voltammograms (DPVs) were measured, also shown in the SI, figures S53-S92.Many of these show very broad reduction peaks with some bearing a semblance of two distinct reduction peaks.This implies that the PBIs are indeed undergoing their typical two-electron reduction in aqueous solution but solvent effects are inhibiting resolution in the CVs.
In order to obtain more diagnostic data and study any differences between the electronic properties of the PBIs, CVs were measured in N,N'-dimethylformamide (DMF) with 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF 6 ) electrolyte.PBI-H was found to be poorly soluble in DMF and most other organic solvents except for dimethyl sulfoxide (DMSO) containing 2% v/v trifluoroacetic acid (TFA).The CVs are provided in the SI, figures S53-S92, and a representative example is shown in figure 2. When measured in DMF, two well-resolved reduction waves are shown for almost all PBIs.In organic solvents, reduction onsets occur at approximately −0.8 V and E 1/2 potentials for first and second reductions are around −0.9 V and −1.1 V, respectively.Similar results were also found via DPV measurements (SI, figures S53-S92).From the onset of reduction in organic solvents, the EA of each PBI was calculated [32].Previous reports provided the activation energy (EA) and ionisation potential (IP) based on calculations from measurements taken in aqueous media [10] while in this study we have chosen to provide those values based on measurements in organic media.These were all found to be around 4.0 eV, which is typical for PBIs.Oxidation potentials of the PBIs could not be measured electrochemically.Instead, the band gap for each PBI was inferred from the onset of absorption in each species' UV-vis absorption spectrum (see below).The ionisation energies (IEs) were then calculated by adding the band gaps to the EAs.These were found to be around 6.1 eV for PBI- A, -F, -L, -M, -V, -W, and -Y. PBI-E, -D and -H, which are expected to be more water soluble, had slightly larger IEs of 6.2 eV.Complete electrochemical data are provided in table 1 and a graphical representation of the PBIs' electronic properties is shown in figure 3.These data show that the electronic properties of these water-soluble PBIs are not greatly affected by their amino acid substituents.This is to be expected given that the electronic properties of PBIs tend to be dictated by the core perylene unit [6,18] and imide functionalisation has little if any effect.
The electronic characterization results indicate that these species are well suited for application as electron acceptors or electron transport mediators, and for several of these species we have previously demonstrated exactly that.Our previous studies have demonstrated that PBI-A can be used as an electron acceptor and electrochrome in electrochromic devices [15] and PBI-A along with PBI-F and PBI-V have been shown to work as electron transport mediators for photocatalyzed hydrogen evolution [30].Furthermore, each PBI except for PBIs D, E, and M, has been employed in the electron transport interlayer of OSCs [12].Given that our newest PBI's D, E, and M have nearly identical electronic properties in both organic and aqueous media to those previously reported, that bodes well for their potential application in similar catalytic or electronic devices.
During this study, we observed that optical properties are correlated with amino acid functionalization and so we performed a comprehensive optical characterization at pH 9. The absorption spectra of each PBI in water were measured to determine molar absorptivity.Dilution series were prepared from each stock solution mentioned above.Molar absorptivity spectra are shown in figure 4. Dilution series spectra and  absorption maxima calibration curves are provided in the SI, figures S2-S41.Each PBI absorbs strongly between 600 nm and 420 nm with the characteristic PBI maxima at approximately 540 nm and 500 nm for the 0-0 and 0-1 vibronic bands, respectively, of the S 0 -S 1 transition.The exact values, along with other optical data, are provided in table 2. In contrast to the electronic properties, amino acid functionalisation has a significant impact on optical properties, particularly the 0-0/0-1 vibronic band intensity ratio.This ratio is influenced by the aggregation of PBI molecules [33][34][35] which is here controlled by the amino acid functionalisation.A high 0-0/0-1 ratio is indicative of high PBI solubility and less aggregation while a lower ratio indicates greater aggregation [36].As the newest PBIs, the 0-0/0-1 ratios for PBIs D, E, and M are of particular interest.Indeed, PBI-D and PBI-E possess the highest 0-0/0-1 ratio (1.4 each) followed by PBI-F, -H, -L, -M, -V, -W, and -Y (all ranging from 0.9 to 1.1) and PBI-A with the lowest (0.7).
With their acidic functional groups, PBI-D and -E are expected to be the most water soluble at high pH which also makes them less likely to associate and aggregate in solution.This results in their high 0-0/0-1 ratio and is also likely responsible for their molar absorptivity spectra integrating to 0.40 M −1 while all others have molar absorptivity spectra that integrate to around 0.33 M −1 .PBI-M is in line with the other PBIs' 0-0/0-1 ratio being closer to unity which indicates that it and the other species are aggregating more in solution.PBI-A has the smallest functional group which likely results in its own unique aggregation and its much lower 0-0/0-1 ratio.An interesting observation is that the UV-vis absorption spectrum and so the degree of aggregation for PBI-D found here differs from that reported previously at pH 8 [31].This may be due to the PBS buffer used in that work or some other experimental aspect leading to differences in the aggregated state and shows the importance of directly comparing data under the same conditions as we do here.
The effects of pH on the absorption profiles were also explored by adjusting the pH of the solutions.The spectra are shown in the SI, figures S2-S41.At basic pH both carboxylic acid groups in the amino acids will be deprotonated.In general, most PBIs are solubilised above pH 7 and there is little if any difference observed in the absorption profiles as pH is increased.Around pH 5-6, the absorption intensity begins to decrease as the PBIs become less soluble.For PBI-D, -E, -W, and -Y an inversion in the 0-0/0-1 intensity ratio is observed.This is indicative of the PBIs changing their aggregation in solution as one of the carboxylate groups becomes protonated around this pH and may be due to their amino acid moieties possessing additional H-bonding capabilities.Finally, at pH 4 the absorption intensity is drastically reduced and most PBIs are observed to precipitate from the solution, implying all carboxylate groups are protonated and they are no longer soluble in aqueous media.The pHs at which we observe these changes are linked to the apparent pK a s of each of the PBIs (table 3) [9], which as stated is related to the protonation of the two carboxylic acid groups present on the molecule.The PBIs that have more than two ionisable groups (D and E) therefore show more than two pK a values, as they have more groups that can be protonated/deprotonated as the pH is changed, which changes the solubility of the aggregates formed in solution.The one exception to this trend is PBI-H, which is observed to undergo the change in aggregation as the pH is lowered from 9 to 7. No further change is observed below pH 7 and PBI-H remains soluble at low pH.This is due to the imidazole functional group from the histidine moiety, which is readily protonated in acidic conditions allowing PBI-H to remain soluble in acidic solution.
Fluorescence and QY data were also obtained for each PBI.Unlike electronic potential and optical absorption, fluorescence and QY have not been previously reported for any of these PBIs.Many PBIs possess quite high QYs, close to unity, which makes them useful as fluorophores in a variety of sensing and imaging applications [37][38][39].However, in aqueous media PBI fluorescence tends to be suppressed due to aggregation [39][40][41].Fluorescence spectra are shown in figure 4 and QY calibration curves are provided in the SI, figures S42-S51.Emission and QY data are provided in table 2. From the absorption data it is clear that amino acid functionalisation has a profound impact on PBI aggregation in solution and so it follows that there would also be an effect on emission and QY.As shown in figure 4 and table 2, each PBI has a small Stokes shift, none higher than 0.1 eV.This is typical for PBIs, where the rigidity of the molecule and low lying triplet energy level reduces opportunities for non-radiative energy loss [1,2].The Stoke shift for PBI-W is particularly low implying a greater rigidity in the molecule than the other PBIs.The QYs for each PBI were measured using perylene orange [42] as a photoluminescent standard.Each QY was measured in triplicate in pH 9 water.Once again, significant differences between the PBIs were found in their QYs.The QYs of PBI-D and -E are the highest (81% and 79%, respectively) followed by -V and -L (66% and 65%, respectively), -A and -H (both 47%), -F (5%), -M (4%), and finally -W and -Y, which both have QYs that are barely detectable (reported as <1%) and could reasonably be described as non-emissive.
As mentioned above, water-soluble PBIs tend to have quenched emission due to aggregation so given the fact that most of these amino acid-functionalised PBIs produce high QYs, in particular PBI-D and -E, is very encouraging.By charting the QYs from highest to lowest (figure 5), a trend is apparent.The highest QYs belong to PBIs with acidic functional groups (-D and -E), those with aromatic functional groups (-F, -W, -Y) have the lowest QYs, and those with aliphatic groups (-A, -L, -V) are in the middle.The acidic PBI-D and -E are expected to be the most water-soluble of the set and this follows from previous reports that show PBIs with large water-solubilising functional groups at the imide positions do possess high QYs in aqueous solution [39,43].As discussed above, it is likely that the more hydrophilic functional groups of PBI-D and -E decrease their tendency to aggregate in solution, thereby reducing their ability to dissipate excitation energy through non-radiative pathways.This follows from the absorption data which showed PBI-D and -E having the largest 0-0/0-1 intensity ratio indicating the least aggregation compared to the other PBIs.Aliphatic PBI-A, -L, and -V are aggregating in solution to a greater extent, as indicated by their 0-0/0-1 ratio, which accounts for their lower QYs.The aromatic PBI-F, -W, and -Y have the most suppressed QYs which indicates that, in addition to aggregating in solution, they possess alternative non-radiative energy loss pathways, likely  provided by their bulky aromatic substituents.Exceptions to this trend include PBI-H, which is in line with the aliphatic PBIs; and PBI-M, which is like the aromatic PBIs.The S heteroatom of PBI-M is likely suppressing QY in PBI-M by providing non-radiative energy loss pathways while the aromatic imidazole of PBI-H must be unable to quench emission in the same manner as the groups in PBI-F, -W, and -Y, leading to a similar QY as PBI-A.Several of these water-soluble PBIs possessing high QYs is very promising for their application in bioimaging and labelling where their amino acid functional groups would be very advantageous [38,39].Fluorescence applications of our water soluble PBIs is not area of research our group has pursued before but given these results that is something we will have to reconsider, especially concerning the remarkable results for PBIs D and E.
We have previously shown that amino acid-functionalised PBIs can be reduced to the radical anion and dianion upon excitation at 365 nm [10,25,29].This results in a change in absorption profile and in the observed colour of the PBI solution or thin film from red to purple.For this study we have compared photochromic and electrochromic data for our previously reported PBIs and to our newest PBIs.Each PBI was dissolved at 5 mg ml −1 in water pH-adjusted to 9 and excited at 365 nm for 60 s in a 0.1 mm pathlength cuvette.The absorption profiles before and after excitation are shown in the SI, figures S2-S41, and a representative example is provided in figure 6.For each material, including the new PBIs D, E, and M, excitation results in a decrease in absorption intensity between 600 nm and 420 nm and the appearance of new bands at 725 nm, 810 nm, and 975 nm, which are indicative of the radical anion [44].Previous studies have shown that this radical anion is stable enough to persist for many hours and is correlated with an increase in the photo-conductivity of thin films formed of these PBIs [10,25], an important factor for their application in conductive devices as we have previously demonstrated [10,12,15].
The lowest intensity radical peaks are seen for those with aromatic groups (F, H, W, and Y), as has been previously reported [10], as well as for the new material PBI-M.Once again, it seems the S heteroatom of methionine endows PBI-M with characteristics similar to those of the aromatic amino acid PBIs.PBIs D and E, while clearly demonstrating radical formation, do not appear to stand out compared to the previously reported aliphatic PBIs.Clearly, further investigation into the radical anion formation of these PBIs is necessary and this can be achieved through spectroelectrochemisty.Reduction of the PBIs can not only be induced via UV excitation, but also via electrochemical means.By applying a negative potential over several minutes to PBI in an electrolyte solution, reduction to radical anion and, in some cases, dianion can be achieved.
Solutions of each PBI were prepared at 0.5 mg ml −1 in 0.04 M NaCl solution.The solutions were placed in a spectroelectrochemical cell with Pt mesh working electrode and subject to a negative potential of −0.6 V for 15 min.Absorption measurements were recorded every 60 s.The spectroelectrochemistry results are shown in the SI, figures S93-S102, and a representative example is shown in figure 6.Each PBI was successfully reduced to the radical anion as indicated by the appearance of bands at 725 nm, 810 nm, and 975 nm after about 60 s of reduction.The same colour change to purple was also observed around the Pt To compare the amount of radical anion and dianion formed for each PBI, the normalised absorption intensities at 725 nm and 610 nm after reduction for 15 min are plotted and shown in figure 7. We have seen that all PBIs have essentially identical reduction properties (figure 3) so any variance in the concentration of reduced species produced will be due to other properties that affect radical and dianion stability such as solvation or aggregation.The least intense radical peaks are shown by PBIs M and W, with A, D, E, and Y showing higher intensities and F, H, L, and V the highest.This may seem contradictory to our previous statement that PBI-F and PBI-H have been shown to have lower radical peak intensity.Here, absorption measurements are being taken with a constantly applied negative potential while the photochromic results shown above and previously reported results involve absorption measurements taken after an excitation with UV light.In other words, electrochemical absorption results are indicative of the ease of reducing the species and forming the radical while photo-excitation absorption results have more to do with the stability of the radical itself.Clearly, radical stability is negatively affected by aromatic side groups, a finding that we have previously observed [10].All PBIs except W show an increase in their 0-0/0-1 ratios upon reduction, indicating that the reduction is inducing a change in their aggregation.As discussed above, this increase in 0-0/0-1 ratios implies the PBIs are aggregating less in solution and becoming more water-soluble, which is to be expected for charged species in water [45].The sole exception is PBI-W, which displays only a small amount of radical anion formation and little change in its 0-0/0-1 ratio.Only PBI-D and -M indicate dianion formation with a new peak appearing at 610 nm.PBI-M in particular is readily reduced to the dianion form, which accounts for its relatively low intensity at 725 nm after 15 min.It has been reported that reduction of the radical anion to form the dianion can be inhibited through aggregation [46][47][48].Thus, for PBI-D and -M, the decrease in aggregation and increase in solubility caused by reduction to the radical anion is enough to enable further reduction to the dianion.The other PBIs, including PBI-E, remain sufficiently aggregated at these concentrations after reduction to the radical anion such that further reduction is inhibited under these conditions.The fact that PBI-E does not seem to share PBI-D's ability to form dianion is a most interesting result that warrants further investigation.Nonetheless, the apparent ease with which these PBIs, excluding PBI-W, can be reduced in aqueous solution further reinforces their great potential as acceptors or electron transport mediators.PBI-M in particular being readily reduced to the dianion is of particular interest for this sort of application, as PBI dianions are known to be more stable than the radical anions and possess high energy excited states upon photoexcitation, enabling access to high energy electron transfer reactions [30,45].
We have previously investigated the rheological properties of hydrogels formed from some of the PBIs reported here, in particular PBI-A [9,28,49].These properties include stiffness (G ′ -the storage modulus and G ′′ -the loss modulus) and strength (percentage strain at which the gel is breaking) of the gel.These values can determine what applications the gel can be used for.As we have done for our previously reported PBIs, we sought to form gels with our new PBIs and compare their rheological properties to the others.Again, any data reported herein has been collected anew for this study unless otherwise specified.Rheological data can provide further insights into the relationship between molecular structure and optoelectronic properties by revealing the types of aggregates that these materials form in solution.PBI gels were formed by preparing solutions and decreasing the pH.To ensure a slow and uniform pH change, we used the hydrolysis of GdL as described above [25].After addition of GdL, the samples were allowed to stand overnight.At this point, the final pH of these samples was between 4.8 and 3.2, depending on the gelator used.
The formation of hydrogels is highly dependent on how the materials can aggregate which in turn is dependent on molecular structure and solubility.Given how PBI-D and PBI-E were shown to be more water soluble than the others, we curious to see if these new PBIs would gel at all and, if so, how their gel properties would differ.All of the PBIs apart from PBI-W were able to form self-supporting gels (SI, figure S103).PBI-W precipitated from solution upon lowering the pH.This further highlights the difference in PBI-W from the other systems and indicates that tryptophan is not conducive to self-assembly in this sort of environment.The rheological properties of the successful gels were tested using strain and frequency sweeps.As typical for low molecular weight gels, all gels broke at low strain (<20%).Consistent with our other work on PBI gelators [25][26][27][28][29], the gels showed a slight increase in the storage (G ′ ) and loss (G ′′ ) moduli at high frequency.PBI-M showed the largest G ′ at 4500 Pa and PBI-D the lowest at 100 Pa with the other gelators being in the region between these two values.For PBI-D, the lowest tanδ and non-linearity in its frequency sweeps (SI, figure S105(b)) likely indicate that it does not form a true gel despite displaying gel-like properties in the inversion tests.Once again, this is most likely due to its high-water solubility, and therefore is a highly viscous liquid rather than a gel network.The fact that PBI-D does appear to form a true gel further highlights that, although they displayed very similar optical properties and high-water solubility, the slight difference in their structures can result in significantly different behaviour, as was observed in their electrochromism discussed above.On the other hand, PBI-M showing the largest G ′ and G ′′ values is very encouraging for its future application in hydrogel research where a stiffer gel is required.Aside from these two examples, there does not seem to be any other obvious trends from the type of amino acid used and the stiffness, yield point or tanδ.The final bulk rheological properties are of course dependent on rate of gelation, temperature, concentration of gelator, final pH, etc and not just on the molecule used.It is therefore not surprising that no direct link between properties and molecular structure could be found here.A full list of rheological properties is found in table 3, with the strain and frequency sweeps shown in the SI, figures S104 and S105.
To assess the structures that formed the gels, small angle neutron scattering (SANS) was used.SANS showed that the gels fit to three different models (full data and fits are in the SI, but a summary of the data in context is provided below, figure S106 and tables S2-S4).PBI-A, PBI-L, and PBI-V all fit best to a combination of a flexible elliptical cylinder and a power law; PBI-F, PBI-H, PBI-W, and PBI-Y all fit best to a combination of a flexible cylinder and a power law; and PBI-D, PBI-E and PBI-M fit to just a flexible elliptical cylinder.The differences in the structures (i.e.flexible cylinder versus flexible elliptical cylinder) reflect the differences in the materials' molecular structure and how they aggregate together.A cylinder versus an elliptical cylinder refers to the cross section of the aggregated fibre, with a larger axis-ratio meaning that the structure is more elliptical.The power law refers the correlation length of the structures present, with the fractal scaling referring to different structures.A smaller Kuhn length would suggest that the fibres are more flexible.The aliphatic PBIs seem to aggregate in a similar way to each other and form fibres with an elliptical profile, while the aromatic PBIs form fibres that are more circular.This is possibly due to the smaller aliphatic groups allowing the molecules to stack closely together in their fibres while the aromatic groups cause the molecules to spread out more while forming fibres resulting in less elliptical and more circular cylinders with smaller Kuhn lengths.However, without detailed data that shows how they are packed we are unable to prove this is the case.In terms of the flexibility, fibres formed of PBIs D, E, and M seem to be more similar to the aliphatic species, albeit with some slight differences.Fibres of PBI-D and E being more tape like in structure (higher axis ratio) with longer Kuhn lengths correlate well with QY data above which showed very high QYs for the hydrophilic PBIs and very low for the hydrophobic aromatic PBIs.Their particular aggregation resulting in more flexible fibres likely correlate with the aromatic PBIs quenching emission which is not the case for the hydrophilic the elliptical, tape like structures of the hydrophilic PBIs D and E and aliphatic PBIs A, L, and V. PBI-M, despite forming tape like structures, has quenched emission like the aromatic species but this, as mentioned above, is likely more do to with the effects of the heteroatom.
In all cases, the models suggest that the gels are the result of the entanglement of long, one-dimensional objects as expected.Interestingly, despite PBI-W not forming a gel, but rather a precipitate, the SANS implies that similar structures to the other systems are present.Presumably the degree of entanglement for PBI-W is different, explaining why a gel is not formed.The structures formed in gelation do not appear to have any link with the rheological properties of the gels.Nonetheless, the apparent correlation between fibrous structure and QY highlights our observations in the past of how important the aggregation type is to performance of these materials.

Conclusions
We set out to study how subtle changes in molecular structure can influence aggregation and hence the electronic and optical properties of a range of amino acid substituted PBIs.Due to the process dependence of the aggregation of these PBIs in water, this study provides the first direct comparative data for this library under the same set of conditions.PBI functionalisation has been extensively studied and it is known that imide functionalisation in particular affects solubility and aggregation properties rather than the inherent electronic properties.However, aggregation also can also lead to indirect influences on the optoelectronic properties, thereby providing an efficient and atom-economical method of tailoring optoelectronic or rheological properties to suit various device needs and hydrogel applications.
Our data show that the electronic properties of these water-soluble PBIs are not greatly affected by their amino acid substituents.This is to be expected given that the electronic properties of PBIs tend to be dictated by the core perylene unit and imide functionalisation has little if any effect.However, in contrast, there is a strong correlation between amino acid group and solubility/solution aggregation.For example, PBI-D and -E possessed the most hydrophilic amino acids making them the most water-soluble as shown by their absorption spectra.PBIs with more hydrophobic functional groups (A, F, H, L, M, V, W, Y) aggregate to a greater degree.The degree of aggregation directly affects the QYs of the solutions, with the less aggregated PBI-D and -E having the highest QYs and the PBIs substituted with the more hydrophobic amino acids having the lowest.Indeed, PBI-W and PBI-Y are essentially non-emissive.
Reducing the PBIs electrochemically led to differences in the concentration of the radical anion that is formed.Since these PBIs have essentially identical reduction properties when measured in an organic solvent in the monomeric state, any variance in the concentration of reduced species produced can be ascribed to the aggregation in water, as such, this is again affected by the hydrophobicity of the amino acid side groups.The radical stability is negatively affected by aromatic side groups.The lower degree of aggregation for the more hydrophilic PBI-D and -M allows further reduction to the dianion.The other PBIs, including PBI-E, remain sufficiently aggregated at these concentrations after reduction to the radical anion that further reduction is inhibited under these conditions.The fact that PBI-E does not seem to share PBI-D's ability to form dianion is really interesting.We note that the stability of the radical anion due to aggregation can be a real benefit in terms of lack of sensitivity to oxygen.There is a strong link between solubility, the aggregates formed, and their ability to form and stabilise the radical anion and dianions.
Hence, our comprehensive examination of these water-soluble amino acid functionalised PBIs has clearly demonstrated that electronic properties of the PBI core is not affected by the choice of amino acid substituent in the monomeric state in an organic solvent.However, the optical properties, ease of reduction to radical anion and dianion, as well as the rheological properties of gels that can be formed can be readily influenced by simple and subtle changes in structure.This demonstrates the ease with which common organic functional materials can be made not just water-soluble, but also carefully tuned to suit the needs of their application, whether it be an OSC, OLED, electron-transport mediator, or hydrogel.All of these solutions are water-based and so present environmental benefits and the easy synthesis means a range of different properties can be tuned in by the hydrophobicity of the amino acid.For example, PBI-D and PBI-E have high QYs along with high water solubility making them well suited for applications in bioimaging.The aliphatic PBIs A, L, and V as well as H also showed substantial QY values which warrants further investigation into their emissive device potential.PBI-M's very encouraging rheological properties along with its ease of reduction to the dianion warrant investigation in electrochromic devices.

Figure 1 .
Figure 1.Amino acid-functionalised PBIs, PBI-R, presented in this report, where R represents the different amino acid side chains, named using one letter codes.

Figure 3 .
Figure 3. Graphical representation of electronic energy gaps for water-soluble PBIs.

Figure 4 .
Figure 4. Molar absorptivity (solid lines) and emission (dotted lines) spectra for PBIs (a) A (b) D (c) E (d) F (e) H (f) L (g) M (h) V (i) W and (j) Y measured in water pH-adjusted to 9. Emission spectra were obtained with an excitation wavelength of 500 nm.

Figure 5 .
Figure 5. Graphical representation of the PBI quantum yields from lowest to highest.

Figure 7 .
Figure 7. Normalised absorption at 725 nm (solid bars) and 610 nm (striped bars) after applying a potential of −0.6 V for 15 min.

Table 3 .
Rheological properties of GdL gels.G ′ and G ′′ are taken from the frequency sweeps at 10 rad s −1 .Yield point quote from where G ′ deviates from linearity.Data are averaged from three repeat samples.pH values are taken from six repeat samples.'NG' means no gel was formed.
* Indicates that it is likely that this material is not a true gel due to the non-linearity in the frequency sweeps, despite being invertible and showing a yield and flow point in the strain sweep.