Molecular Encapsulation of Naphthalene Diimide (NDI) Based π‐Conjugated Polymers: A Tool for Understanding Photoluminescence

Abstract Conjugated polymers are an important class of chromophores for optoelectronic devices. Understanding and controlling their excited state properties, in particular, radiative and non‐radiative recombination processes are among the greatest challenges that must be overcome. We report the synthesis and characterization of a molecularly encapsulated naphthalene diimide‐based polymer, one of the most successfully used motifs, and explore its structural and optical properties. The molecular encapsulation enables a detailed understanding of the effect of interpolymer interactions. We reveal that the non‐encapsulated analogue P(NDI‐2OD‐T) undergoes aggregation enhanced emission; an effect that is suppressed upon encapsulation due to an increasing π‐interchain stacking distance. This suggests that decreasing π‐stacking distances may be an attractive method to enhance the radiative properties of conjugated polymers in contrast to the current paradigm where it is viewed as a source of optical quenching.

Introduction p-Conjugated polymers are an important class of semiconductors with unique and tunable optoelectronic properties,w hich hold great promise for the development of solution-processible and low-cost sustainable energy applications,s uch as organic photovoltaics (OPVs), organic lightemitting diodes (OLEDs), organic field-effect transistors (OFETs), sensors and energy storage devices. [1,2] In particular,n aphthalene diimide (NDI) based p-conjugated polymers are among the most interesting electrontransporting semiconductors given their remarkable track record as high-performance non-fullerene acceptors (NFAs) in OPVs [2][3][4][5] and high mobility n-channel semiconductors in OFETs. [2,6] NDIse xcellent device performance and many favorable properties (e.g., appropriate energy levels,f acile synthesis,l ow synthetic cost, good electron mobility,h igh solubility as well as thermal and (photo)chemical stability) have therefore made this electron-depleted core extremely popular in plastic electronics. [2,[7][8][9] However,since NDI-based polymers such as N2200 [P(NDI-2OD-2T)] or PCE8 [P(NDI-2OD-T)] are based on quite large aromatic units,t hey have ag reat propensity to aggregate in both solution and as thin films. [5] This can directly affect the conjugated polymer properties (both optical and electronic) as well as their miscibility with other polymers and hence greatly influences their device performance.G aining greater synthetic control over the intermolecular interactions of these materials is therefore very important but remains challenging.
Recently,i th as become apparent that one of the most important obstacles that must be overcome in conjugated polymer design is the suppression of non-radiative decay in narrow band gap conjugated polymers. [10] Numerous recent studies have revealed that the high non-radiative voltage loss encountered in organic photovoltaics (OPVs), which holds back their efficiency, is primarily ac onsequence of the low solid state photoluminescence quantum yields (PLQYs) of the blend materials. [11,12] Hence,i ncreasing the PLQY of narrow band gap conjugated polymers is essential for next generation OPV,aswell as being appealing for light emitting diodes and biological imaging.I ti sc ommonly assumed that the close packing of conjugated polymers results in the creation of additional non-radiative decay pathways.T herefore,t he most promising strategy to increase the solid-state PLQY of conjugated polymers is to molecularly isolate the chains either through covalent or non-covalent encapsulation. [13][14][15][16][17][18][19][20][21] Polymer dilution in ahost matrix can also be used to increase PLQY but this cannot suppress intramolecular interactions,oreven intermolecular in some cases. [22] Herein, we report the synthesis and characterization of an ovel encapsulated conjugated NDI polymer and report the resulting effect of molecular encapsulation on its optical properties. Forour study we chose to encapsulate aP(NDI-T) derivative as it is one of the most successful n-type conjugated polymers used in organic photovoltaics. [4] Thus,o ur findings are applicable and of relevance to virtually all polymer-polymer solar cell devices.W ep resent anovel synthetic methodology which represents as ignificant advance in the functionalization of NDI-based monomers and allows for amuch wider use of solubilizing/bulky functional groups on the N-atom of the naphthalene diimide.T his strategy enables the tuning of intermolecular interactions in NDI-based polymers in an unprecedented fashion, and thereby allows invaluable fundamental insight into their solid-state optical properties.O ur key finding is that upon increasing conjugated polymer interstrand separation, the photoluminescence is quenched due to the suppression of an AIEE (aggregation induced enhanced emission) type phenomena which is present in the parent polymer.
Here, Br 2 -NDA was first converted into Th 2 -NDA through at raditional Stille cross-coupling reaction and subsequently functionalized with am ore versatile aniline, such as DMTAn (Figure 1). Our methodology therefore allows for amuch wider use of solubilizing/functional groups at the imide nitrogen positions,w hich would not be possible via the traditional route (indicated in the middle). Th 2 -NDA was obtained in 93 %u sing commercial Br 2 -NDA or 39 % from the "home-made" mixture of brominated products.The presence of other, unwanted brominated products in the "home-made" Br2-NDAs ample therefore affords al ower yield upon isolation/purification. Next, Th 2 -NDA was reacted with DMTAn,l eading to the formation of Th 2 -NDI-OMe in 34 %y ield. Subsequent treatment of Th 2 -NDI-OMe with excess sodium ethane thiolate successfully generated the crude demethylated product (Th 2 -NDI-OH), which was encapsulated with 1,12-dibromododecane to give the encapsulated NDI monomer (E-NDI-T)inatwo-step yield of 34 %.
Thestructure of the encapsulated NDI monomer; E-NDI-T,w as determined by X-ray crystallography (single crystals grown via the layering technique using chloroform and methanol), which confirmed that the straps indeed shield the NDI core ( Figure 2). Analysis of the packing shows minimum distances of % 12 between the centroids of the NDI cores (see Supporting Information), with the closest intermolecular separations being between the alkyl chains,of the order of 4 .A nalysis of the packing shows minimum distances of % 12 between the centroids of the NDI cores (see Supporting Information), with the closest intermolecular separations being between the alkyl chains,o ft he order of 4 .Incontrast, non-encapsulated NDIs often show close p-p stacking distances in the region of % 3.3 , [34] demonstrating the ability of the macrocycle to spatially separate the chromophores effectively.
Them olecule is centrosymmetric in the crystal structure, with dihedral angles of % 408 8 between the NDI and thiophene units.T his is consistent with other reports on NDI-T based polymers,w hich indicate al arge degree of non-coplanarity between the thiophene and the naphthalene core. [35][36][37] Thec onjugated polymers ( Figure 3) were prepared through adirect arylation polymerization (DArP) procedure, using N,N'-bis(2-octyldodecyl)-2,6-dibromo-1,4,5,8-naphthalene diimide (Br 2 -NDI-2OD)a st he co-monomer.D ArP was chosen as the preferred method for this study since it is an emerging,green approach that enables the direct coupling of aryl halides (Ar-X) and (hetero)aryls (Ar-H), while avoiding toxic side products. [39][40][41][42] Furthermore,r ecent studies have demonstrated that polymers prepared via DArP can achieve very similar molecular masses (M n and M w )a nd device performances compared to traditional methods. [39,42] As observed, the polymer series consists of the reference polymer P(NDI-2OD-T),a ni n-between, bulky dimethoxyphenyl polymer P(NDI-DMP-T),and lastly the encapsulated P(E-NDI-T) polymer.All polymers were obtained in similar number average molecular weights (M n )o fa pproximately 15 kDa (Table 1) and display good solubility in various solvents.
Thes olution and thin film absorption and emission spectra of the three NDI polymers are shown in Figure 4 and summarized in Table 3. As observed, the spectral profiles of the solution absorption spectra are relatively featureless and nearly identical for all polymers,h aving l max values of 545 nm, 549 nm, and 546 nm for P(NDI-2OD-T), P(NDI-DMP-T) and P(E-NDI-T),r espectively.T he thin film absorption spectra, on the other hand, show am uch clearer difference between the different polymers.F or P(NDI-2OD-T), l max is significantly red-shifted (by 51 nm) to 596 nm on going from solution to solid state.I nc ontrast, P(NDI-DMP-T) and P(E-NDI-T) display only minor redshifts of 3a nd 1nm, respectively.Inother words,with increased shielding of the polymer backbone,the formation of lower energy species is increasingly suppressed, with aconcomitant retention of the absorption profile when going from solution to thin film. This trend is consistent with previous studies on encapsulated conjugated materials. [13,14,43] Similarly,t he photoluminescence (PL) of the three polymers in solution is almost identical, with a l max at % 636 nm. This suggests that, in solution, the emission originates from isolated chains.I nt hin film, on the other hand, the spectra are again progressively red-shifted according to the "exposure" of the polymer chains to one another. For P(NDI-2OD-T),t he l max red-shifts from 636 nm to 725 nm when going from solution to solid state.F or P(NDI-DMP-T) instead, the PL maximum is only red-shifted by 31 nm (from l max = 639 nm to 670 nm) in film compared to solution, thus indicating some remnant interstrand interaction. Thed iscrepancy with the absorption results,w hich hint instead at am ore significant suppression of interpolymer interactions,c onfirms the higher sensitivity of luminescence vs.a bsorption spectroscopy.I mportantly however, the P(E-NDI-T) PL spectrum remains essentially identical when going from solution to the solid state,with the l max only red-shifted by 5nm(from 635 nm to 640 nm). This observation confirms effective suppression of solid-state interactions and thus lower energy states in P(E-NDI-T).

Film Microstructure
Grazing incidence wide-angle X-ray scattering (GI-WAXS) was performed on all three polymers to elucidate the origins of the observations from the optical spectroscopy. Figure 5s hows the two-dimensional scattering data and the corresponding radial integrals,w ith Table 2c ontaining the extracted d-spacings of the novel polymeric materials.
TheGIWAXS data for the parent polymer P(NDI-2OD-T) exhibits three prominent scattering features,aDebye-Sherrer-ring at 0.26 À1 ,apartial ring scattering ring at 0.62 À1 ,and aDebye-Sherrer-ring at 1.4 À1 .These features are consistent with previous reports of closely related polymers, [5,[44][45][46] and as-such, the observed scattering features may be ascribed to typical 100 lamellar packing with adistance of 25 (Q = 0.26 À1 )a nd 010 interchain stacking with   as eparation of 4.5 (Q = 1.39 À1 ). Thes econd prominent scattering feature at 0.62 À1 is not consistent with ah igher order n00 reflection, however,t he lengthscale of this feature 10.1 correlates well with the chain backbone repeat distance,c onsistent with the findings of Rivnay et al. [44] The isotropic 100 and 010 reflections indicate that the film comprises randomly orientated crystalline grains with no preferential ordering either in-or out-of-plane. On addition of the dimethoxyphenyl group in P(NDI-DMP-T),t he lamella scattering feature becomes less prominent and is further reduced for the fully encapsulated polymer P(E-NDI-T),i ndicating as ignificant reduction in the long-range order of these systems.Whilst the 010 feature is broad, it can clearly be seen that by going from the parent polymer P(NDI-2OD-T) to the non-encapsulated P(NDI-DMP-T) and finally P(E-NDI-T),the stacking distance along the p-conjugated plane increases and becomes more diffuse, as expected from the structural changes.T herefore,w ith the combined photophysical and crystallographic characterization we can report with confidence that in the solid state,we are systematically increasing the distance and thus reducing the interactions between adjacent polymer chains.

Photoluminescence
To further investigate the effects of the interchromophore interactions,t he PLQYs of the three different polymers in both solution and solid state were also measured. Theresults are summarized in Table 3. As observed, the solution PLQYs of all polymers are relatively low,with 3.6 %, 4.1 %and 3.2 % for P(NDI-2OD-T), P(NDI-DMP-T) and P(E-NDI-T),r espectively.H owever,s urprisingly,t he thin film PLQYs for both P(NDI-2OD-T) and P(NDI-DMP-T) are enhanced considerably,r eaching 9.2 %a nd 6.1 %, respectively,w hile P(E-NDI-T) only exhibits asmall increase,going from 3.2 to 3.7 %. Thei ncrease in PLQY for both P(NDI-2OD-T) and P(NDI-DMP-T) can therefore be associated with aggregation-induced enhanced emission (AIEE), whereas P(E-NDI-T) mostly suppresses intermolecular interactions and hence retains its PLQY at higher concentrations (i.e., in thin films).
ThePLlifetimes and rate constants for radiative (k r )and non-radiative (k nr )decay provide further insight (Table 3). In line with the PLQYs,f or all polymers the weighted-average PL lifetimes (t AVG )a re very similar in solution, yielding comparable values of k r and k nr .C ombined with the nearindistinguishable profiles of their solution PL spectra, it can be concluded that the PL properties of the isolated chains of P(NDI-2OD-T), P(NDI-DMP-T) and P(E-NDI-T) are very similar.
Fora ll polymers,t he values of the average lifetime t AVG values are lower in solution than in thin films.F or P(NDI-2OD-T),b oth solutions and thin films follow as imilar biexponential decay,inwhich the fast-decaying component (t 1 ) dominates exciton deactivation. As seen, the PL lifetime increases from t 1 = 0.31 ns (or t AVG = 0.43 ns) in solution to 0.89 ns (or t AV G = 1.16 ns) in film. This is manifested in anear three-fold decrease in k nr in films of P(NDI-2OD-T) (k nr = 7.86 10 8 s À1 )c ompared to the solution value (k nr = 22.47 10 8 s À1 ). In contrast, k r is effectively unchanged on going to the solid state.T he absence of any additional fast-decaying PL components in thin film, and the near identical values of k r for P(NDI-2OD-T) in solution and thin film suggest that Jaggregation is likely not responsible for its enhanced PL in the solid state. [47,48] For P(NDI-DMP-T),asimilar trend is observed. However,t he difference between solution and thin films is much smaller due to partial suppression of interchromophore interactions.T he average lifetimes for P(NDI-DMP-T) are 0.47 ns in solution and 0.65 ns in thin film, again closely matching the difference in PLQYs (4.1 %insolution vs.6.1 % in thin film). This is similarly due to ad ecrease in k nr in thin film compared to solution, albeit as maller one than for P(NDI-2OD-T) (14.45 10 8 s À1 in thin film vs.2 0.40 10 8 s À1 in solution). There is again no significant change in k r .I n contrast, when going from solution to thin films the PL characteristics of P(E-NDI-T) remain relatively unaffected due to the suppression of intermolecular interactions by molecular encapsulation. This translates to a t AV G of 0.44 ns in solution and 0.45 ns in thin film, which is comparable to the small difference in PLQYs (3.2 %i nsolution versus 3.7 %in thin film). Hence, k r and k nr are effectively unchanged. Table 3: Summary of the photophysical properties of P(NDI-2OD-T), P(NDI-DMP-T) and P(E-NDI-T).  To summarize,asthe interactions between polymer chains are increased in the order P(E-NDI-T) ! P(NDI-DMP-T) ! P(NDI-2OD-T) an enhancement of solid-state PL is observed due to an improved suppression of non-radiative decay pathways.T he combined photophysical and crystallographic characterisation clearly shows that the substantial decrease in non-radiative decay for P(NDI-2OD-T) in thin film is ac onsequence of stronger intermolecular interactions between polymer chains.

Discussion
It is commonly assumed and observed that conjugated polymers undergo substantial photoluminescence quenching on going from solution to the solid state.O ur combined morphological and photophysical results show the surprising result that i) the extremely well-known polymer P(NDI-2OD-T) undergoes strong aggregation induced enhanced emission and ii)t his phenomenon originates from the interchain packing.W e( and others) have previously shown that molecular encapsulation allows ap olymer to retain, to some extent, solution properties when in the solid state. [13,14] In this work, we find the same concept applies to P(E-NDI-T) as both the absorption and emission shapes,yields and lifetimes are almost identical. However,incontrast to almost all other reported conjugated polymers,t he emissivity of the nonencapsulated derivative increases in the solid state (as opposed to decreasing), resulting in arelatively high emission PLQY for this wavelength region. Thep henomenon of aggregation induced enhanced emission is well known and has been reported in numerous conjugated polymer systems, including those containing naphthalene diimide units. [49] Crucially however, in virtually every example of conjugated polymers AIEE-specific groups (such as vinylenes or TPEs), that are well known to imbue AIEE-like properties,a re introduced for this reason. [50][51][52][53][54][55] There are almost no examples of a" conventional" conjugated polymer undergoing aggregation induced enhanced emission. Moreover,i ti sr emarkable that it has not been previously observed in P(NDI-2OD-T),which have been extensively studied. In fact, we can find only one other example of AIEE in ac onventional (i.e., without the inclusion of AIEE-specific groups) conjugated polymer,where emissive efficiencywas enhanced through the formation of close contacts between adjacent polymer chains. [56] To determine the origin of the observed AIEE in this system it is useful to consider all the possible mechanisms through which it could be occurring. NDI-based small molecules have previously been shown to undergo enhanced emission in the solid state predominantly through J-aggregation. [57][58][59][60][61] However,t he p-stacking distance as measured by GIWAXS for P(NDI-2OD-T) is % 4.5 ,w hich is considerably longer than those reported for small molecule NDIs displaying J-aggregates (< 4 ). We also do not observe as harpening of the absorption spectrum or an enhancement in k r in the solid state,w hich are common spectroscopic signatures of J-aggregation. Furthermore,t he red-shifted emission of NDI-T based polymers has been demonstrated to originate from the crystallization induced planarization of the conjugated backbone rather than J-aggregates. [62] We observe as tepwise reduction in AIEE when going from P(NDI-2OD-T) to P(NDI-DMP-T) and P(E-NDI-T) films, which correlates extremely well with the increased 010 distances interlayer separation. Thus,t he crystallization of the p-conjugated faces of the polymer results not only in ap lanarization, but also in the suppression of non-radiative decay pathways,and thereby asubstantially increased photoluminescence efficiency.W eb elieve that this is the first observation of this effect in an on-vinylene containing conjugated polymer and suggest that it could provide anovel design strategy for obtaining good emissive efficiencyi n narrow band gap conjugated polymers-a major hurdle that must be overcome to increase the efficiency of OPV devices. [10] Finally,w en ote that P(NDI-2OD-T) and related polymers are the most successful conjugated polymers used in all-polymer solar cells [63] and this previously unreported AIEE may provide ac lue as to why they have performed so well.

Conclusion
This article reports the synthesis of molecularly encapsulated, naphthalene diimide based conjugated polymers through the development of new synthetic pathways.T he encapsulated polymer allows us to determine the exact effect of interpolymer interactions on the optical properties of the conjugated polymer.S urprisingly,i nt he solid state,t he encapsulated polymer has ah igher energy,b ut lower photoluminescence quantum yield compared to the naked reference polymer,which is in contrast to the current conventional wisdom and aviolation of the energy gap "law". [64,65] Instead, this work reveals ap reviously unrecognized aggregation induced enhanced emission phenomena in one of the most well studied conjugated polymers P(NDI-2OD-T).Structural characterization confirms that this AIEE is due to the crystallization of the p-conjugated faces of the polymer chains.T herefore,w eb elieve that understanding and having control over the crystallization and solid-state behavior of conjugated polymers may be key to enhance their emissive properties.O ur results conclusively demonstrate that solid state photoluminescence in conjugated polymers can actually be enhanced through closer packing (which is often beneficial for charge transport) suggesting that high PLQY and high charge carrier mobility may not be as mutually exclusive as is commonly believed.