Protein‐like Enwrapped Perylene Bisimide Chromophore as a Bright Microcrystalline Emitter Material

Abstract Strongly emissive solid‐state materials are mandatory components for many emerging optoelectronic technologies, but fluorescence is often quenched in the solid state owing to strong intermolecular interactions. The design of new organic pigments, which retain their optical properties despite their high tendency to crystallize, could overcome such limitations. Herein, we show a new material with monomer‐like absorption and emission profiles as well as fluorescence quantum yields over 90 % in its crystalline solid state. The material was synthesized by attaching two bulky tris(4‐tert‐butylphenyl)phenoxy substituents at the perylene bisimide bay positions. These substituents direct a packing arrangement with full enwrapping of the chromophore and unidirectional chromophore alignment within the crystal lattice to afford optical properties that resemble those of their natural pigment counterparts, in which chromophores are rigidly embedded in protein environments.

Dedicated to Professor Jürgen Kçhler on the occasion of his 60th birthday Abstract: Strongly emissive solid-state materials are mandatory components for many emerging optoelectronic technologies,but fluorescence is often quenched in the solid state owing to strong intermolecular interactions.T he design of new organic pigments,w hichr etain their optical properties despite their high tendency to crystallize, could overcome such limitations.H erein, we showanew material with monomerlike absorption and emission profiles as well as fluorescence quantum yields over 90 %i ni ts crystalline solid state.T he material was synthesized by attaching two bulky tris(4-tertbutylphenyl)phenoxy substituents at the perylene bisimide bay positions.These substituents direct apackingarrangement with full enwrapping of the chromophore and unidirectional chromophore alignment within the crystal lattice to afford optical properties that resemble those of their natural pigment counterparts,inwhich chromophores are rigidly embedded in protein environments.
Solid-state fluorescent organic materials [1][2][3] are of considerable interest as they enable abroad variety of applications in (opto-)electronics,f or example,o rganic light emitting diodes (OLEDs), [4] waveguiding, [5,6] solid-state lasers [7][8][9] luminescent sensors, [10] or as fluorescent labels for (bio)medical research [11] as well as security printing technologies. [12] Unfortunately,while pigments are the preferred colorants for these applications compared to dyes owing to their superior thermal, photochemical, and chemical robustness,o nly very few pigments exhibit ad ecent fluorescence. [13] In contrast to the bright luminescence with quantum yields (F F )o fu pt o unity for many organic chromophores in diluted solutions, intermolecular interactions in the solid state open up amulti-tude of non-radiative relaxation pathways that quench the fluorescence emission. [14][15][16] Owing to their high molar absorptivity and excellent chemical and thermal stability as well as lightfastness, perylene bisimides (PBIs) are an exceptional class of organic colorants,b oth as soluble dyes with fluorescence quantum yields up to unity and as pigments. [17] However,a ll commercial PBI pigments with their red, maroon, and black shades are non-fluorescent, [18] and it remains ac hallenge to overcome the prevailing fluorescence quenching pathways and to design PBI-based solid-state emitter materials that maintain the intense fluorescence that is commonly observed for PBI dyes in diluted solutions.S ystematic studies on PBI dye aggregates carried out in our laboratory as well as others during the last decade have identified excimer formation, [19,20] symmetry-breaking charge separation, [21] and several triplet state population processes [22] as the main fluorescencequenching pathways.B ecause all of these processes are favored by close p-p-stacking of PBI dyes, [23] much effort has been dedicated to the steric shielding of the chromophore core at imide,h eadland, or bay positions [24][25][26][27][28] as well as by isolating individual chromophores in ah ost matrix. [29] Other recent approaches include the organization of perylene dyes in orthogonally crossed arrangements or at magic angle slipping in which the excitonic coupling vanishes. [30] Interestingly,w hile by these measures solid-state fluorescence quantum yields of up to 59 %c ould indeed be achieved, the absorption and fluorescence spectra of the solid-state materials in all examples still reveal substantial electronic interactions between the PBI dyes whose large p-scaffolds appear to attract each other in sometimes rather unexpected ways with the concomitant deterioration of the fluorescence properties.
In this contribution, we report as terically fully enwrapped. [31][32][33][34][35] PBI derivative whose solid-state absorption and emission properties fully match those in solution. Accordingly,m onomer-like vibronic progressions in the absorption and emission spectra and, most importantly,afluorescence quantum yield close to unity under ambient conditions were obtained. This exciting result is rationalized by single crystal X-ray analysis,w hich unveils ac omplete enwrapping of the PBI fluorophore in the solid state ( Figure 1). Thus,aspecific environment of the dye is created, quite similar to the embedding of chromophores in protein matrices of green fluorescent proteins [36] or natural light-harvesting pigment [37] in which non-fluorescent relaxation pathways are slowed down by chromophore isolation and rigidification, and high chromophore concentrations are realized without compromising the dyes desirable photo-functional properties.
Owing to the crystallographic inversion center located in the center of gravity of the molecule (space group P " 1), the twist angle between both naphthalene mono-imide subunits is 08 8. Thephenoxy substituents are bent towards opposing faces of the PBI p-scaffold with dihedral angles of 31.1(3)8 8 (](C2-C1-O-C aryl )a nd ](C8-C7-O-C aryl )), thereby efficiently shielding the chromophores p-scaffold ( Figure 1b). Owing to the bulky 4-tert-butylphenyl moieties,t he individual chromophores are well-separated in the solid state with an interplanar distance of 8.1 and at ransversal and longitudinal displacement of 3.7 and 7.9 ,r espectively (Supporting Information, Figure S2). Thepacking arrangement of PBI 1 is characterized by strictly parallel oriented molecules with several CÀH···O and CÀC···p interactions and at otal void volume of 34 3 .M ost importantly,i na ddition to the jacketing provided by each two tert-butylphenyl units attached at the ortho-position of the 1,7-bay-substituents (dark blue colored, Figures 1b-d), full chromophore enclosure is realized by tert-butylphenyl units of the neighboring chromophores (grey and light blue colored, Figures 1c,d). This desired feature became possible by the molecular design with three tert-butylphenyl units attached to the 2,4,6positions of the chromophore-bound phenoxy units.T he perfect chromophore embedment is corroborated by the absence of co-crystallized solvent molecules and an appreciably high density for an organic crystal of 1.156 gcm À3 .A ll these structural features can likewise be found in the isolated bulk material according to powder XRD analysis (Supporting Information, Figure S3 d) and do account for the bright solidstate luminescence of PBI 1 (see below).
Theoptical properties of PBI 1 were investigated by UV/ Visa bsorption and fluorescence spectroscopy,b oth in dichloromethane solutions and in the solid state ( Figure 2 and Supporting Information, Figures S3 a-c). Thes olid-state absorption profile of PBI 1 in reflection mode was determined on an ensemble of microcrystals in aBaSO 4 trituration, applying the Kubelka-Munk theory ( Figure 2). [38] Both absorption spectra reveal sharp S 0 !S 1 transitions located at 559 nm (solution) and 557 nm (solid), respectively,each with awell-resolved vibronic progression at intervals of 1370 cm À1 and am olar extinction coefficient (e max )o f7 .2 10 4 m À1 cm À1  with labeling of the carbon atoms of the chromophore core (red) and b) its solid-state molecular structure determined by single crystal structure analysis (ellipsoids set to 50 %p robability). [50] Packing arrangement of PBI 1 in the solid state as viewed along the c) long and d) short molecularc hromophore axis (ellipsoids of the PBI p-scaffold set to 50 %probability and colored in orange and its 2,4,6-tris(4-tert-butylphenyl)phenoxy substituents colored in dark blue;2 ,4,6-tris(4-tert-butylphenyl)phenoxy substituents of adjacent chromophores are illustrated as space filling model and are alternatinglyc olored in gray and light blue).
for the lowest energy 0,0-vibronic transition ( Table 1). The monomer transition is bathochromically shifted by 1200 cm À1 (35 nm, solution) compared to the bay-unsubstituted N,N'dicyclohexyl-3,4:9,10-perylenetetracarboxylic acid bisimide owing to the electron-donating character of the phenoxy substituents. [28] Thehigher energetic S 0 !S 2 transitions can be observed at 410 nm (solution) and 408 nm (solid), respectively,w hich is absent in the absorption spectrum of the parent chromophore.Thus,owing to the absence of significant inter-chromophore interactions in the solid state,the absorption spectral features are essentially identical to those in solution with the exception of aslightly reduced ratio for the intensities of the apparent 0-0 and 0-1 vibronic transitions (A 0-0 /A 0-1 )o f1 .61 (solid) compared to 1.79 (solution), which may suggest av ery weak exciton coupling. [39] Foranaccurate comparison of the fluorescence properties of PBI 1 in solution and in the solid state,the intrinsic steadystate emission spectra have to be acquired without any contamination by reabsorption, which is rather challenging for solid-state samples of chromophores with high tinctorial strengths (e max )a nd small Stokes shifts (Dṽ Stokes ). Whereas highly diluted solutions of PBI 1 (OD < 0.05; c 0 < 10 À6 m)are routinely investigated using standard fluorescence spectrometers,t he spectroscopic investigations on micrometer-sized single crystals of PBI 1 were performed with an optical polarization microscope equipped with af iber optic-coupled CCD spectrometer.Thus,single crystals of different sizes and thicknesses were grown from dichloromethane/hexane 3:7 solutions and subsequently investigated (Supporting Information, Figure S4) to obtain nearly reabsorption-free solidstate emission spectra ( Figure 2). These data are most accurate and allow for aq uantitative analysis of the intrinsic solid-state fluorescence properties of PBI 1.
In contrast to previously reported 1,7-diphenoxy-substituted PBI derivatives, [27,28] PBI 1 displays as olid-state fluorescence spectrum that is an almost perfect mirror image of its absorption profile with distinct vibronic progressions at 565, 608, and 663 nm (each separated by 1360 cm À1 ). It is remarkable that even the emission spectrum of PBI 1 in dichloromethane solution is more broadened, less structured, and more red-shifted by 490 cm À1 (Figure 2a nd Supporting Information, Figure S3 a), which hitherto could only be observed for natural pigments (i.e., protein-encapsulated chromophores) [30,31] but not for synthetic pigments.Likewise, the Stokes shift (Dṽ Stokes = 260 cm À1 )and the full width at half maximum (FWHM em = 570 cm À1 )o ft he shortest wavelength emission of microcrystalline PBI 1 is approximately half of that of its monomer emission in solution (Table 1). These observations can be rationalized by ar igidification of the chromophore p-scaffold by its enclosing environment in the solid matrix, thereby preventing structural relaxations of the excited molecules (as well as their environment) as it is given in solution. In accordance with this interpretation, an increased fluorescence lifetime of 7.70 ns was observed for the crystalline sample compared to the value of 5.20 ns in dichloromethane solution ( Supporting Information, Figure S3 b). Most importantly,t he bright fluorescence of PBI 1 in solution (F F = 100 %) is also retained in the solid state with af luorescence quantum yield F F of at least 90 %a fter correcting the apparent solid-state emission quantum yield (F F * = 82 %) for unavoidable reabsorption losses (Figure 2c and Supporting Information, Figure S3 c). [40] To gain further insights into the optical properties of this unique highly luminescent solid-state material, oriented microplatelets of PBI 1 were grown on quartz substrates [41] from dichloromethane/hexane 4:6s olutions and their anisotropy was investigated by optical polarization (fluorescence) microscopy ( Figure 3a nd Supporting Information, Figures S5-6). Thecrystallinity of these samples was demonstrated by selected area electron diffraction (SAED) and out-ofplane XRD experiments performed on microcrystals grown on acarbon-coated copper grid, confirming an identical solidstate packing arrangement as determined by single crystal structure analysis (see above;Supporting Information, Figures S7 a-c). Independent on the substrate,t he rhomboid crystals exhibit edge lengths of several tens of micrometers and angles of 1078 8 and 738 8 (Figure 3b), respectively,w ith thicknesses of several hundred nanometers according to AFM cross-section analysis (Supporting Information, Figure S7 d- [a] FWHM was derived as twice the distance between the maximum to the closest edge at half-maximum of the unsymmetrically shaped absorption or emission band to prevent falsification by overlapping transitions.
[b] Ensemble property of crystalline material.
[c] Single microcrystal investigatedw ith an optical polarization microscope equipped with af iber-coupled CCD spectrometer. f). Accordingly,o wing to the high tinctorial strength, the emission spectra of those microplatelets are partially contaminated by reabsorption (see above) and no absorption spectra could be recorded in transmission mode.S AED and XRD experiments unambiguously confirmed that the investigated microcrystals are oriented with their (010) plane parallel to the surface (Figure 3b and Supporting Information, Figures S7 a-c). Thef ast-growing directions of the crystals,w hich give rise to their rhomboidal shape,a re presumably the (100) and (001) lattice planes as evidenced by the simulated Bravais-Friedel-Donnay-Harker (BFDH) morphology (Figure 3b). Thelong molecular axes of the PBI chromophores and thus their S 0 !S 1 transition dipole moments are unidirectionally aligned along the short rhomboid diagonal that intersects the long one at an angle of 868 8 (Figure 3b). Accordingly,r homboid crystals of PBI 1 appear deep red in optical polarization microscopy when the transmitting light is polarized along the short rhomboid diagonal but become fully transparent when the polarization is rotated by 908 8 (Figure 3b and Supporting Information, Figure S6). This behavior becomes even more obvious in polarizationdependent fluorescence microscopy experiments starting with polarizer, analyzer,a nd short rhomboid diagonal all aligned parallel (Figures 3a,c). By rotating the short diagonal out of this arrangement the bright orange fluorescence gradually decreases and is completely absent at an angle of 908 8 (Figures 3a,c). Apparently,t his process is characterized by ahuge optical anisotropy as the emission intensity at 606 nm is reduced by 99 %ataperpendicular orientation (Figure 3a). Theh igh quality of the microcrystals and the perfect unidirectional alignment of the chromophores is thereby reflected in an outstanding dichroic ratio, D 606nm ,of82and an order parameter, S 606nm ,o f0.96. [42][43][44] Thespectroscopic data accordingly fully meet our expectations based on the packing features.Different from conventional pigments or other p-conjugated thin-film and solidstate materials in which electronic couplings from closely packed chromophore units lead to alterations of the spectral features with the commonly observed fluorescence quenching, the outstanding optical properties of PBI 1 are fully conserved and even improved in the solid state.Such features were hitherto exclusive for natural pigments in which evolutionarily optimized protein environments afford chromophore isolation and chromophore rigidification. Fort his reason, natural light-harvesting pigments can have quite high dye densities to afford the desired high absorption cross sections for the capture of sunlight without undesired "concentration"-quenching of the photo-excited state.F or PBI 1based thin films or crystals,the chromophore concentration is even higher than for any natural pigment because the chromophore part, that is,t he perylene bisimide dye, constitutes 27.4 %o ft he total molecular mass of PBI 1 (c = 0.76 mol L À1 ; [29] Supporting Information, Figure S9 a). For most protein-based colorants this value is considerably smaller (Supporting Information, Figures S8 and S9), for example,0 .8 %f or the green fluorescent protein, [45] 0.54-0.85 %f or rhodopsin, [46,47] 9.7 %f or the Fenna-Matthews-Olson photosynthetic light-harvesting protein complex. [48,49] In summary,w er eported au nique perylene bisimide solid-state emitter, which readily crystallizes from dichloromethane/hexane solutions to form rhomboid microcrystals with absolute solid-state fluorescence quantum yields greater than 90 %. Thes terically demanding threefold tert-butylphenyl-functionalized 1,7-phenoxy substituents efficiently shield the planar chromophore core within the crystal lattice, providing complete wrapping and preventing electronic coupling between adjacent molecules,w hile maintaining an exceptional high chromophore concentration of more than 27 %i nt he solid state.T his well-defined intrinsic dilution of the chromophore core,l ike it is exclusively illustrated by many natural light-harvesting pigments,a ccounts for its remarkable solid-state luminescence and gives rise to au nique emission spectrum with aw ell-resolved vibronic progression. Thep erfect unidirectional alignment of the chromophores within the single crystalline material enables ah uge optical anisotropy with adichroic ratio of 82, making PBI 1 ah ighly attractive solid-state material for photonic applications.