Synthesis and characterization of naphthalimide-functionalized polynorbornenes

Abstract Highly fluorescent and photostable (2-alkyl)-1H-benzo[de]isoquinoline-1,3(2H)-diones with a polymerizable norbornene scaffold have been synthesized and polymerized using ring-opening metathesis polymerization. The monomers presented herein could be polymerized in a living fashion, using different comonomers and different monomer ratios. All obtained materials showed good film-forming properties and bright fluorescence caused by the incorporated push–pull chromophores. Additionally, one of the monomers containing a methylpiperazine functionality showed protonation-dependent photoinduced electron transfer which opens up interesting applications for logic gates and sensing. Graphical abstract

Other interesting potential applications of naphthalimide derivatives are related to organic electronics where they are especially applied in organic light emitting diodes (OLEDs) [2,3] or in organic photovoltaics (in non-fullerene acceptors and n-type polymers) [4].
Another opportunity to modify naphthalimide-type dyes is to introduce functionalities via the imide nitrogen. This is typically utilized to tune solubility and compatibility or to attach naphthalimides to other materials [18,20]. Alternatively, polymerizable groups can be attached to obtain functional polymers. One interesting approach is to molecularly imprint these naphthalimide-decorated materials again leading to optochemical probes [21,22].
All of the above mentioned modification techniques can be combined, leading to photoswitchable, pH-sensitive polymers which were used to detect lysosomes in cancer cells [23].
In many applications, covalent attachment to the polymer matrix controls the distribution of the dye within the material and prevents the dye from leaching out. In particular, for biological labeling or special sensing applications, the dye molecules should be placed at a distinct location of the polymer chain, thus making a rational polymerization and labeling process necessary.
Ring-opening metathesis polymerization (ROMP) [24] has been recognized as a powerful polymerization technique due to its high functional group tolerance. Thus, a number of dye-functionalized polymers have been successfully obtained [25,26], while the living nature of ROMP allowed the precise placement of dye molecules in dedicated segments of block copolymers and combining them with stimuli-responsive comonomers [27]. This renders ROMP the method of choice for the synthesis of naphthalimide-functionalized polymers which is the objective of this work. Furthermore, the photophysical properties of the obtained materials are characterized and discussed.

Results and discussion
The overall reaction scheme is depicted in Scheme 1. Starting from 4-bromo-1,8-naphthalic anhydride, imides are prepared which are typically used for tuning the solubility of the dye or connecting the naphthalimide to a material. We use the imide functionality to link the functional naphthalimide chromophores to a norbornene residue which can be polymerized using ring-opening metathesis polymerization (ROMP). In the resulting 4-substituted 1,8-naphthalimide (formally, 6-substituted (2alkyl)-1H-benzo[de]isoquinoline-1,3(2H)-dione) system, substituents are introduced in 6-position which can be varied and have a strong influence on the photophysical properties [28,29].
As shown in Figs. 1 and 2 as well as in Table 1, the substituent at position 6 in the N-alkyl-1H-benzo[de]isoquinoline-1,3(2H)-dione system has a strong influence on the spectroscopic characteristics. Replacing the bromine substituent in 6 with electron-donating substituents creates a donor-acceptor system with the naphthalimide moiety acting as an acceptor, leading to a bathochromic shift. Comparing the absorption maxima of 6-10 in DMSO to those in DCM, the absorption maximum of the methoxy substituted derivative 7 is red-shifted in DMSO by about 20 nm compared to the absorption maximum in DCM which is a consequence of the increased solvent polarity. Notably, this effect was not as pronounced for the aminosubstituted compounds 8-10. Furthermore, the UV-Vis absorption of naphthalimide-type chromophores is very sensitive to the conformation of the amino moiety which is reflected in 10 having a significantly red-shifted absorption maximum compared to 8 and 9 which expectedly show similar UV-Vis spectra [32].
Due to the heavy bromine substituent, the fluorescence of 6 is quenched. 7-10 show intense fluorescence and the Stokes shifts are between 70 and 100 nm for all compounds. The emission maximum of diethylaminosubstituted compound 10 is bathochromically shifted compared to 7-9, 7 being the emitter with the lowest emission wavelength maximum.
Monomers 6-10 were copolymerized with dimethyl bicyclo[2.2.1]hept-5-ene-2,3-dicarboxylate Overall, we observed an increasing polydispersity index (PDI) and shorter chain lengths with increasing dye load. This was especially the case for monomer 8 and 10; however, dye-functionalized polymeric materials could be successfully prepared using these monomers. Monomer 9 was successfully copolymerized with the two additional comonomers (12 and 13, see Scheme 3) as indicated by similar polydispersity indices for all three copolymers. DSC (differential scanning calorimetry) measurements revealed that the glass transition temperature (T g ) was expectedly mainly governed by the bulk comonomers. A small increase in T g was detected for monomers 6-7 with increasing dye load, whereas increased dye loadings for 8-10 led to a decreased T g .
UV-Vis absorption and photoluminescence spectra (k ex = 395 nm) of drop-casted films of polymers co(M 1 -
The absorption and photoluminescence of the dyefunctionalized monomers and the derived polymers are very similar to the free dye monomers 6-10 in solution. Thus, stacking of the naphthalimides which would lead to strong shifts in the photophysical properties is avoided.
In addition, monomer 9 and all related copolymers with this monomer are pH-sensitive due to the possibility to protonate the piperazine group. In Fig. 4, the UV-Vis spectra of the protonated and unprotonated form of the monomer are shown. The absorption maximum of 9 at 402 nm is blue shifted to 375 nm if the pyrazine is protonated, e.g., by the addition of trifluoroacetic acid. Upon addition of a base (e.g., triethylamine), the original spectrum is observed so that this reaction can be considered reversible. Thus, copolymers with 9 are possible candidates for polymer based pH-sensing materials, using similar read-out techniques as proposed by Trupp et al. using the same dye functionality in a hydrogel matrix [34]. The possibility to use copolymers of 9 as sensor materials will be exploited in the future.

Conclusion
In this contribution, different naphthalimide dyes have been successfully linked to norbornene monomers. All derivatives were subsequently copolymerized with different norbornene monomers leading to random copolymers. Narrow polydispersity indices and good control of the molecular weight suggest a living polymerization which can be used to design special macromolecular architectures. The optical properties of the polymers are practically identical to those of the dye monomers showing that p-p stacking of the naphthalimide dyes is avoided.    2H, 3 J HH = 6.4 Hz, -N-CH 2 -CH 2 -), 3.65-3.59 (m, 2H, OH-CH 2 -CH 2 -) ppm; 1 H NMR spectra were found to be identical with the ones described in Ref. [15].