Luminescence Properties of 1,8-Naphthalimide Derivatives in Solution

Article pubs.acs.org/JPCC

Luminescence Properties of 1,8-Naphthalimide Derivatives in Solution, in Their Crystals, and in Co-crystals: Toward RoomTemperature Phosphorescence from Organic Materials Barbara Ventura,*,† Alessio Bertocco,‡ Dario Braga,‡ Luca Catalano,‡ Simone d’Agostino,‡ Fabrizia Grepioni,*,‡ and Paola Taddei*,§ †

Istituto ISOF-CNR, Via P. Gobetti 101, 40129 Bologna, Italy Dipartimento di Chimica G. Ciamician, Università di Bologna, Via F. Selmi 2, 40126 Bologna, Italy § Dipartimento di Scienze Biomediche e Neuromotorie, Università di Bologna, Via Belmeloro 8/2, 40126 Bologna, Italy ‡

S Supporting Information *

ABSTRACT: Crystalline 1,8-naphthalimide derivatives bearing a bromine atom at the 4-position and a 2-, 3-, or 4methylpyridine at the imidic N-position have been synthesized, and their co-crystals with the coformer 1,4-diiodotetrafluorobenzene have been obtained via mechanochemistry. The structure of crystals and co-crystals has been characterized by means of X-ray diffraction and Raman and IR analysis. The luminescence properties of the derivatives have been explored both in solution and in their solid crystals and co-crystals. All of the compounds exhibit weak fluorescence in air-equilibrated solutions at room temperature and both fluorescence and phosphorescence at low temperature. In air-free solvent, all of the derivatives show phosphorescence at room temperature, at variance with the unsubstituted 1,8-naphthalimide model. Solid crystals display a red-shifted fluorescence with an increased emission quantum yield as compared to solution, whereas co-crystals show different behaviors. For all of the solid compounds, phosphorescence could be observed at room temperature by means of a gated detection. The dependence of the luminescence features of the solid materials on the intermolecular interactions that occur in the lattice is discussed.

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halogen bonds25−27 between suitable coformers and electronrich substituents of the organic components or their π systems. The external heavy atom effect caused by halogen bonding promotes intersystem crossing and enables efficient roomtemperature phosphorescence in these solid materials. Other factors that contribute to the effectiveness of the phenomenon are the dilution of the chromophore in the solid given by the coformers, which reduces self-quenching, and the prevention of vibrational losses due to the rigid structure of the lattice. Quantum yields in the solid state up to 50% have been reported, in particular for solids where spin−orbit coupling is already effective in the starting material, such as in aromatic carbonyls.21 We report here on newly designed 1,8-naphthalimide derivatives, conceived as suitable candidates for room-temperature phosphorescence. 1,8-Naphthalimides represent a class of molecules intensely studied for their rich optical properties, which dramatically depend on the nature of the substituents, in particular at the 4-position.28−30 Electron-donating or electronwithdrawing groups at the 4-position determine the existence of

INTRODUCTION Phosphorescence at room temperature is an optical phenomenon that generally belongs to inorganic and organometallic compounds and is rarely observed for purely organic materials. Phosphorescent metal-containing materials are commonly used in optoelectronics and in biological and medical fields as chemical sensors and luminescent probes.1−8 The search for purely organic phosphorescent materials is an appealing challenge in these fields, due to the high performances of organic materials in terms of electron transport, their reduced toxicity as metal-free substances, and their low cost. Examples of organic dyes that show phosphorescence in diluted oxygenfree solutions have been reported9−11 and point out the essential role of substituents, such as carbonyls and halogen atoms, which enhance spin−orbit coupling. Encapsulation of the dyes, often with heavy-atom coguests, in constrained systems that block diffusion-controlled quenching phenomena such as nanocontainers (cyclodextrins, cavitands, zeolites)12−17 or polymer matrixes,18−20 is an explored strategy to pursue room-temperature organic phosphorescence. Recently, a turning point in this direction has come from crystal engineering, with the design of solid structures based on the cocrystallization of organic dyes and halogenated coformers.21−24 The key point is the formation, in the solid structure, of © 2014 American Chemical Society

Received: May 19, 2014 Revised: July 15, 2014 Published: August 5, 2014 18646

dx.doi.org/10.1021/jp5049309 | J. Phys. Chem. C 2014, 118, 18646−18658

The Journal of Physical Chemistry C

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a strongly fluorescent lowest lying charge-transfer (CT) excited state and marked reducing or oxidizing capability, and these derivatives found lots of attraction as sensors,31−33 components for organic light-emitting diodes,34,35 donor or acceptor partners in supramolecular arrays for light energy collection and conversion.36−40 On the contrary, 1,8-naphthalimides without substituents at the 4-position, or derivatized with weakly withdrawing/donating groups, show very poor fluorescence, due to the presence of an upper lying triplet excited state nearly isoenergetic with the lowest singlet excited state, which renders intersystem crossing the main deactivation process.41−43 This class of compounds finds use as redox active cleavage agents.42,44 In this Article, three novel 1,8-naphthalimide derivatives substituted at the 4-position with a bromine atom and at the Nposition with a 2-, or 3-, or 4-methylpyridine have been designed (see Scheme 1); they differ in the location of the

crystals 22·I2F4 and 42·I2F4. The effect of substituents and molecular arrangement in the solid state on the luminescence properties of the compounds has been analyzed and discussed.

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EXPERIMENTAL SECTION Synthesis of Compounds 1−4. All solvents and chemicals were purchased from Sigma-Aldrich and used without further purification; doubly distilled water was used. Compounds 2, 3, and 4 were all synthesized by using a modification of a previously reported procedure.73 1: To a suspension of 1,8-naphthalic anhydride (400 mg, 2.0 mmol) in ethanol (25 mL) was added a slight excess of 33% aqueous ammonia (ca. 0.5 mL). The suspension was heated until a clear orange solution was obtained, which was refluxed overnight. The solution was then allowed to slowly cool to room temperature, and colorless needle-like crystals grew from the solution. The product was recovered by filtration and washed with cold ethanol (5 × 2 mL) and bidistilled water (5 × 2 mL). Yield: 66%. Crystallographic data for 1 were extracted from the Cambridge Structural Database (reference code NAPOIM01) and used to calculate the corresponding X-ray powder pattern; a comparison of the calculated pattern with that measured on the ground crystals is shown in Supporting Information Figure S12. 2: To a solution of 4-bromo-1,8-naphthalic anhydride (430 mg, 1.54 mmol) in tetrahydrofuran (ca. 100 mL) was added 1 equiv of 2-picolylamine (0.16 mL, 1.52 mmol) at room temperature. The solution was left under stirring for 24 h; as no precipitate was obtained, the solvent was removed by rotary evaporation. The remaining solid was collected and washed with a saturated aqueous solution of K2CO3 (5 × 2 mL), and with bidistilled water (5 × 2 mL). Recrystallization from ethanol/dichloromethane (2:1) yielded a white powder. Yield: 40%. ESI−MS (in methanol): m/z = 367.0 (100%), 368.0 (20%), 368.9 (100%), 370.0 (20%) [M + H+]. 3: To a solution of 4-bromo-1,8-naphthalic anhydride (426 mg, 1.54 mmol) in tetrahydrofuran (ca. 100 mL) was added 1 equiv of 3-picolylamine (0.16 mL, 1.52 mmol) at room temperature. The solution was left under stirring for 72 h; a pale-yellow precipitate was obtained, which was collected by filtration and washed with THF (5 × 2 mL), with a saturated aqueous solution of K2CO3 (5 × 2 mL), and with bidistilled water (5 × 2 mL). Recrystallization from hot dimethylformamide yielded pale-yellow crystals. Yield: 63%. ESI−MS (in methanol): m/z = 367.0 (100%), 368.0 (20%), 368.9 (100%), 370.0 (20%) [M + H+]. 4: To a solution of 4-bromo-1,8-naphthalic anhydride (430 mg, 1.54 mmol) in tetrahydrofuran (ca. 100 mL) was added 1 equiv of 4-picolylamine (0.16 mL, 1.52 mmol) at room temperature. The solution was stirred for 6 h; a white precipitate was obtained, which was collected by filtration and washed with THF (5 × 2 mL), with a saturated aqueous solution of K2CO3 (5 × 2 mL), and with bidistilled water (5 × 2 mL). Recrystallization from hot dimethylformamide yielded a white powder. Yield: 60%. ESI−MS (in methanol): m/z = 367.0 (100%), 368.0 (20%), 368.9 (100%), 370.0 (20%) [M + H+]. Several crystallization experiments in pure and mixed solvents were attempted on crystalline powders of 2, 3, and 4, but only in the case of 3 could single crystals suitable for Xray structural determination be obtained (see below).

Scheme 1. Molecules Described in This Studya

a The 1,8-naphthalimide (1) derivatives are all substituted at the 4position with a bromine atom, and at the imidic N-position with 2-, 3-, or 4-methylpyridine (2, 3, and 4, respectively). The C6F4I2 molecule (I2F4) has been used as a coformer.

pyridine nitrogen with respect to the naphthalimide ring. The role of the bromine substituent is to promote intersystem crossing through an internal heavy atom effect, avoiding the generation of fluorescent CT states due to the weak withdrawing ability of the group. The methylpyridine substituent has been introduced as an anchoring group able to form a halogen bond with the pluri-halogenated coformer I2F4 (Scheme 1), with the aim of producing n2·I2F4 cocrystals. The different position of the pyridine nitrogen should play a role in controlling the displacement of the naphthalimide units in the final solid material, and the methylene spacer has a function in distancing the aromatic units and avoiding stacking effects. Only crystalline powder of 1,8-naphthalimide derivatives could be obtained, with the exception of 3, for which full structural characterization could be obtained via single-crystal X-ray diffraction; reaction of the functionalized 1,8-naphthalimides with the coformer I2F4 resulted in the formation of the 22·I2F4 and 42·I2F4 co-crystals. Raman and IR characterization of the co-crystals has been performed with the aim of investigating the nature and the strength of the halogen bond. Actually, vibrational techniques proved to be powerful tools to detect the formation of halogenbonded adducts. The photophysical properties of molecules 2−4 have been first analyzed in solution, at room temperature in aerated and air-free solvent, and at low temperature in glassy matrix. Reference 1,8-naphthalimide (1) has been studied in the same conditions for comparison purposes. The study has been then addressed to the solid materials, that is, crystalline 1−4 and co18647



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Figure 1. Ball-and-sticks representation of molecule 3, as observed in its crystals (a); packing view down the crystallographic a-axis (b) and spacefilling representation of a columnar stacking (c). A π-stacking interaction (d) is also at work between the (aminomethyl)pyridines (distance between the ring planes of ca. 3.5 Å). HCH omitted for clarity.

Synthesis of Co-crystals. Cocrystallization of solid 2−4 with the halogenated coformer 1,4-diiodotetrafluorobenzene (I2F4) was explored both from solution and directly in the solid-state. All of the co-crystallization experiments from solution were fruitless, while good results were obtained by mechanochemistry. In the solid-state reactions, crystalline 2, 3, or 4 were ground together in a 2:1 molar ratio with the coformer I2F4; the reaction was conducted in a ball-milling apparatus Retsch MM 20 for 99 min at 20 Hz. Co-crystals formation, in the case of 2 and 4, was detected by comparison of experimental powder patterns for the solid-state reaction products and the reagents (Supporting Information Figure S13), and by Raman spectroscopy (see below). Crystal Structure Determination. Single-crystal data for compound 3 were collected at room temperature on an Oxford X’Calibur S CCD diffractometer equipped with a graphite monochromator (Mo Kα radiation, λ = 0.71073 Å). Data collection and refinement details are listed in Supporting Information Table S1. All non-hydrogen atoms were refined anisotropically; HCH atoms for all compounds were added in calculated positions and refined riding on their respective carbon atoms. SHELX9774 was used for structure solution and refinement on F2. The program PLATON75 was used to calculate hydrogen-bonding interactions. CYLview76 and Mercury77 were used for molecular graphics. Powder Diffraction Measurements. X-ray powder diffractograms in the 2θ range 5−40° (step size, 0.02°; time/ step, 20 s; 0.04 rad soller; 40 mA × 40 kV) were collected on a Panalytical X’Pert PRO automated diffractometer equipped with an X’Celerator detector and in Bragg−Brentano geometry, using Cu Kα radiation without a monochromator. The program Mercury77 was used for simulation of X-ray powder patterns on the basis of single-crystal data, when available. Chemical and structural identity between bulk material and single crystal for 3

was verified by comparing experimental and simulated powder diffraction patterns (see Supporting Information Figure S14). Raman and IR Spectroscopy. Raman spectra were recorded on a Bruker MultiRam FT-Raman spectrometer equipped with a cooled Ge-diode detector. The excitation source was a Nd3+-YAG laser (1064 nm) in the backscattering (180°) configuration. The focused laser beam diameter was about 100 μm, and the spectral resolution was 4 cm−1. The reported spectra were recorded with a laser power at the sample of about 60 mW. No sample degradation upon laser irradiation under these conditions was observed (actually, the reported spectra are coincident with those recorded at laser powers as low as 1 mW). IR spectra were recorded on a Nicolet 5700 FTIR spectrometer, equipped with a Smart Orbit diamond ATR accessory and a DTGS detector; the spectral resolution was 4 cm−1. Optical Spectroscopy and Photophysics. The solvents used were spectroscopic grade from C. Erba. Solid-state determination made use of powder samples placed inside two quartz slides. Absorption spectra of solutions were recorded with a PerkinElmer Lambda 950 UV−vis−NIR spectrophotometer. Reflectance spectra of solid samples were acquired with a Perkin−Elmer Lambda 9 UV/vis/NIR spectrophotometer equipped with a 60 mm integrating sphere and converted in absorption spectra using the Kubelka−Munk function.78,79 Emission spectra were collected in right-angle mode for solutions and in front-face mode for solids with an Edinburgh FLS920 fluorimeter equipped with Peltier-cooled Hamamatsu R928 PMT (200−850 nm), and corrected for the wavelengthdependent phototube response. Fluorescence quantum yields of samples in solution were evaluated from the area of the corrected luminescence spectra with reference to 2-(1naphthyl)-5-phenyloxazole (α-NPO) in cyclohexane (ϕfl = 0.94),80 using the method of Demas and Crosby.81 Deaerated 18648



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by comparison of experimental X-ray powder patterns for the ball-milling products with those of the solid reagents. No cocrystal formation could be observed upon reaction of 3 with I2F4. Absorption and Luminescence Properties in Solution. The spectroscopic and photophysical properties of compounds 2, 3, and 4 and reference 1 have been explored in four different solvents (toluene, TOL; tetrahydrofuran, THF; dichloromethane, DCM; and acetonitrile, ACN) at room temperature and in toluene glassy matrix at 77 K. Normalized absorption and emission spectra of 1 and the three derivatives 2−4 at room temperature are shown in Figure 2 and Supporting Information Figure S1; relevant absorption and fluorescence data are summarized in Table 1.

samples were obtained by bubbling the solutions with Ar for ca. 20 min in homemade 10 mm optical path cells and then sealing. The concentration of the solutions was adjusted to have A < 0.1 at the excitation wavelength. Absolute photoluminescence quantum yields of solid samples were measured on the same fluorimeter equipped with a 6 in. Labsphere integrating sphere, according to the method reported by Ishida et al.82 Each measurement was repeated from three to four times. The limit of detection of the system is 2%. Fluorescence decays were obtained with an apparatus based on a Nd:YAG laser (Continuum PY62-10) with a 35 ps pulse duration, 1.0 mJ/pulse, 355 nm, and a streak camera (Hamamatsu C1587 equipped with M1952). Right-angle and front-face modes were employed for solutions and solid samples, respectively. The luminescence signals from 100 to 600 laser shots were averaged, and the time profile was measured in a wavelength range of ca. 50 nm around the emission maximum. The overall time resolution of the system after the deconvolution procedure is 10 ps.83 Phosphorescence spectra were acquired by means of an Edinburgh FLS920 fluorimeter, using a time-gated spectral scanning mode and a μF920H Xenon flashlamp (pulse width 0.9.41−43 The series 2−4 shows even lower fluorescence quantum yields, from 5 to 10 times lower than those of 1 in the different solvents, and accordingly shorter lifetimes (Table 1). This effect can be attributed to the presence of the heavy bromine substituent that further promotes the already effective intersystem crossing process. In toluene frozen matrix at 77 K, all of the compounds show both fluorescence and phosphorescence emission. In Figure 3 are reported the fluorescence spectra of 1 and 2−4 recorded with a fluorimeter and the phosphorescence spectra isolated by means of a gated detection. It is worthwhile to note that even without using a gated setup, the phosphorescence of the 2−4 series is clearly visible at 77 K, emerging from the tail of fluorescence, whereas for 1 it is weakly distinguishable (Supporting Information Figure S2). From Figure 3 it is clear that while fluorescence has similar features for all of the compounds, peaking at ca. 400 nm, the phosphorescence spectra of the 2−4 series are bathocromically shifted by ca. 20 18650



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extension),47 500 cm−1 (lateral ring expansion + (C−I + C−F) symmetric stretching),47 and 441 cm−1 (in-plane ring deformation),47 which shifted to 1376, 498, and 438 cm−1, respectively. An opposite behavior (i.e., a blue-shift) was observed for the Raman bands (Figure 5) at 1609 cm−1 (aryl quadrant stretching),47 403 cm−1 (out-of-plane ring twisting),47 333 cm−1 (out-of-plane ring rocking),47 and 158 cm−1 (symmetric C−I stretching + ring elongation)47 and the IR bands (Supporting Information Figure S8) at 1456 cm−1 (aryl semicircle stretching),47 938 cm−1 (C−F side-to-side stretching),47 757−740 cm−1 (C−I antisymmetric stretching),47 and 561 cm−1 (out-of-plane ring deformation),47 which shifted to 1612, 409, 334, 159, 1462, 941, 760−748, and 563 cm−1, respectively. Analogous trends were observed by Shen et al. for co-crystals constructed by I2F4 and polyaromatic hydrocarbons based on C−I···π halogen bonding.48 Also, some vibrational bands assignable to the constituent 2 underwent some changes in both IR and Raman spectra upon 22·I2F4 co-crystal formation. In the C−H stretching region, the Raman modes (Figure 5) due to the aromatic components at 3144 cm−1 (2-substituted pyridine moiety)49 and 3074 cm−1 (naphthalimide moiety)50 shifted to lower wavenumber values (3142 and 3067 cm−1, respectively), while the aliphatic C−H stretching mode weakened and up-shifted from 2970 to 2972 cm−1 (Figure 5). The latter trend was observed in previously analyzed (S)/(R)2-(1,8-naphthalimido)-2-quinuclidin-3-yl (NMiABCO)−I2 adducts51 and in diazabicyclooctane-diiodoperfluorocarbons complexes.52 The blue-shift and decrease in intensity of the aliphatic C−H stretching mode has been related to a more acidic character of the H atoms adjacent to the N atom accepting the halogen bond as a consequence of the n → σ* donation.51,52 In our case, the blue-shift of the aliphatic CH stretching mode (localized on the CH2 group adjacent to the pyridine N atom) was relatively small if compared to the abovementioned studies; therefore, a certain n → σ* donation may be hypothesized in the 22·I2F4 co-crystal, although it appeared not so extensive (and definitely less extensive than in 42·I2F4, see below), because no significant downshift of the Raman C−I symmetric stretching at 158 cm−1 was observed, although this band appeared strengthened. Actually, as required by the n → σ* character of the halogen bond, the C−I bond lengthens on co-crystal formation, and consequently the weaker C−I force constant should result in a decreased stretching wavenumber value.52,53 Upon 22·I2F4 formation, the CO antisymmetric stretching mode50,54 shifted to lower wavenumber values (i.e., from 1663 to 1656 cm−1), while the aromatic ring stretching mode49,50 at 1349 cm−1 shifted to 1351 cm−1 (Figure 5). Some changes were observed also in the 1200−1100 cm−1 range, where the aromatic CH and ring modes are reported to fall49,50 together with the C−F diagonal stretching of I2F4,47 falling in the free coformer at 1135 cm−1. Analogous shifts were observed also in the corresponding IRactive modes (Supporting Information Figure S8): CH stretching (3100−2900 cm−1 range), CO antisymmetric stretching (at about 1650 cm−1),50,54 aromatic C−H bending + ring stretching (at about 1370 cm−1),54 ring stretching (1145 cm−1),50 aromatic C−H bending (at about 1080 cm−1),50,54 and CO bending (at about 650 cm−1).50 Analogous IR blueshifts have been reported in the bands of pyrene upon cocrystal formation with I2F4.24

Figure 4. Emission spectra from optically matched air-free TOL solutions of 2 (black), 3 (red), and 4 (green) at room temperature. Excitation at 340 nm, A340 = 0.080.

is shown in the picture of Supporting Information Figure S5. Measured phosphorescence lifetimes are of the order of 300 μs (Table 2 and Supporting Information Figure S6). An estimation of the phosphorescence emission quantum yields of 2−4 has been performed and led to values reported in Table 2. The whole luminescence quantum yield of TOL airfree solutions of the compounds has been measured and subtracted by the fluorescence contribution, measured in airequilibrated solutions (Table 1). For all of the examined compounds, the fluorescence quantum yield is, in fact, unaffected by the presence of molecular oxygen in solution, due to the short fluorescence lifetime of the order of 100 ps (a confirmation is shown in Supporting Information Figure S7). The estimated values are of the same order of magnitude of fluorescence quantum yields, that is, (5−6) × 10−3. Overall it can be inferred that the bromine substituent is governing the photophysical properties of the compounds in solution, and the different substituents on the N-position appear not to produce any effect, if not a slight decrease of both fluorescence and phosphorescence quantum yields in 2, probably due to the presence of the methylene bridge that electronically isolates the pyridine group from the naphthalimide ring. Vibrational Characterization of Crystals and Cocrystals. Figures 5, 6, and Supporting Information Figures S8, S9 show the Raman and IR spectra of the ball-milling 22· I2F4 and 42·I2F4 products; the effective co-crystal formation was deduced by comparing these spectra with those of the solid constituents, which are reported to this purpose. On co-crystallization of 2 and 4 with I2F4, several changes were observed in both Raman and IR spectra; they appeared more pronounced in the spectra of 42·I2F4, although well evident also in those of 22·I2F4. These findings suggest that the compounds 2 and 4 interact with I2F4 in a different manner, indicating that the interaction mode is affected by the position of the pyridine nitrogen in the naphthalimide derivative. In both 22·I2F4 and 42·I2F4 co-crystals, the main vibrational bands due to I2F4 shifted to different extents and modified their relative intensity upon co-crystal formation; moreover, also bands assignable to the other constituent (2 or 4, respectively) appeared to change. With regards to 22·I2F4, going from free I2F4 to the cocrystal, some modes of the coformer shifted to lower wavenumber values. This is the case of the Raman bands (Figure 5) at 1383 cm−1 (symmetric ring contraction and F 18651



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Figure 5. 3250−2900 cm−1 (top) and 1750−100 cm−1 (bottom) Raman spectra of 2 (black), I2F4 (blue), and 22·I2F4 (red). The bands of I2F4 are indicated with an asterisk.

diiodoperfluoroalkanes and I2F4.52,55−59 This tendency may be easily recognized also in the 42·I2F4 co-crystal, whose vibrational spectra showed a definitely stronger N···I interaction than in 22·I2F4; actually, steric hindrance, which is evidently higher in 22·I2F4, may reduce or prevent the effective halogen atom approach to the donor site, influencing the binding pattern and the lattice organization in the co-crystals.60 The most significant spectral features revealing the stronger nature of the N···I interaction in 42·I2F4 are the shifts of the C−I and aromatic C−H stretching modes. Unlike 22·I2F4, going from free I2F4 to 42·I2F4, the Raman antisymmetric and the IR symmetric C−I modes shifted to lower wavenumbers, that is, from 158 and 757 cm−1 to 150 and 750 cm−1, respectively (Figure 6 and Supporting Information Figure S9). These red-shifts reflect the lengthening (i.e., the weakening) of

All of the observed changes may be assigned to the intermolecular interactions between 2 and I2F4. Vibrational data suggest that upon co-crystal formation, constituent 2 underwent packing rearrangements also involving modifications in the CO···H−C hydrogen-bonding patterns, as previously reported for NMiABCO−I2 adducts;51 π−π stacking interactions as well as changes in C−H···I, C−I···I−C, and C−F···H interactions, reported for co-crystals between I2F4 and polyaromatic hydrocarbons24,48 as well as pyridine-based systems,55 could not be excluded. Actually, I2F4 has been reported to self-assemble with electron-rich aromatic systems (i.e., picolyl derivatives) through π−π stacking interactions.55 On the other hand, the general ability of the pyridine moiety to work as an effective electron donor and halogen-bond acceptor is well-known in co-crystals with both α-ω18652



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Figure 6. 3250−2900 cm−1 (top) and 1750−100 cm−1 (bottom) Raman spectra of 4 (black), I2F4 (blue), and 42·I2F4 (red). The bands of I2F4 are indicated with an asterisk.

the C−I bond upon the n → σ* donation from the pyridyl N atom to iodine and have been reported as diagnostic for N···I halogen-bond interactions.52,57 As a result of this donation, going from 4 to 42·I2F4, the aromatic C−H mode shifted to higher wavenumbers and decreased in intensity (i.e., from 3059 to 3068 cm−1 in Raman, Figure 6, and from 3053 to 3057 cm−1 in IR, Supporting Information Figure S9), due to the increased acidic character of the hydrogen atoms. Shifts of nearly the same extents have been reported in pyridine−I2F4 co-crystals58 and upon N···I halogen-bonding interaction between the pyridyl nitrogen of 1,3-bis-pyridylmethylcalix[4]arene and I2F4.55 For the latter co-crystal, Messina et al. have reported a N···I distance of 2.90 Å; therefore, it is not unexpected that

the band shifts appeared of minor extents than that observed for stronger I2 complexes.59 The intermolecular interactions in 42·I2F4 also affect other Raman and IR vibrations of both I2F4 and 4 constituents, as easily observable in Figure 6 and Supporting Information Figure S9. With the exception of the already discussed C−I modes, the bands above assigned to the I2F4 coformer underwent shifts analogous to those observed in 22·I2F4, while the changes of the bands due to the other constituent (i.e., 4) appeared more pronounced. They definitely involved the vibrational modes due to both the naphthalimide module (see, for example, the blue-shift of the CO symmetric and antisymmetric stretching modes at about 1700 and 1660 cm−1, Figures 6 and Supporting Information Figure S9)50,54 and the 18653



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1 shows absorption and fluorescence spectra red-shifted ca. 30 nm with respect to the molecule in solution, with a structured emission nearly mirroring the absorption bands. The crystals of 2−4, while showing absorption bands below 400 nm similar to the solution case, display emission red-shifted from 30 to 50 nm as compared to solution, with broader spectra, in particular for 2 and 3 (Figure 7a). Absolute fluorescence quantum yields measured for the solids are significantly higher, from 5 to 10 times, than mean values observed in solution (Tables 3 and 1). Co-crystals 22·I2F4 and 42·I2F4 behave differently from their respective crystals and show pronounced differences one from the other (Figure 7b and Table 3). 22· I2F4 has a broad and red-shifted emission (further red-shifted with respect to solid 2) with a very low emission quantum yield (below 0.02, instrumental resolution). On the contrary, 42·I2F4 shows a sharp fluorescence spectrum peaking at 422 nm, similarly to solid 4, but with an emission quantum yield enhanced more than twice. We safely assume that the observed increases in yield are real, considering that determination of emission spectra in solid compounds can be affected by reabsorption phenomena, especially in case of small Stokes shift, and this effect could reduce the measured quantum yield values.82,61 In organic solids, fluorescence spectral shifts and changes in the quantum yield with respect to solution depend on the arrangement/packing of the molecules in the solid lattice.62−66 It has been shown that two competing factors are mainly influencing the optical properties of the material: (i) the presence of strong π−π interactions between face-to-face closely spaced molecules, that generally leads to red-shifted fluorescence and a decrease of emission efficiency;66−68 and (ii) the immobilization of the molecules in the lattice, which causes the disappearance of nonradiative deactivation pathways and an increase of emission quantum yield.68,69 The same effects as (i) could originate also from CT interactions and hydrogen bonding between molecules.70,71 In our case, the crystal structure of compound 3 has pointed out that the molecular units are placed in a columnar π−π stacked arrangement with interplanar distances of the order of 3.5 Å. The red-shifted emission in passing from solution to the solid state can thus be ascribed to this molecular arrangement, whereas the increase in quantum yield is less straightforward: it can be inferred that rigidity effects prevail over coupling effects. In the absence of crystal structures for 2, 4, and 1, we can only assume similar molecular interactions for these crystals in view of the observed optical features. In the co-crystals, the presence of the coformers is clearly influencing the intermolecular arrangement. On the basis of the luminescence results, in fact, we can gather that strong π−π interactions between the naphthalimide units

pyridine component (see, for example, the changes in the Raman ranges near 1200, 1070, and 1000 cm−1, Figure 6).49 All of these findings showed that the formation of the 42· I2F4 co-crystal was governed by the N···I halogen-bonding interaction, but also by changes in CO···H−C hydrogenbonding patterns and rearrangements of the aromatic component. Absorption and Luminescence Properties of Crystals and Co-crystals. Steady-state and time-resolved spectroscopic properties of powder samples of crystalline 2, 3, 4, and reference 1 and of the co-crystals 22·I2F4 and 42·I2F4 have been investigated in the solid state. Measurements have been performed on uncrushed powder samples. Absorption and emission spectra of the crystals and the cocrystals are shown in Figure 7a and b, respectively. The main spectral and photophysical data are reported in Table 3.

Figure 7. Absorption (full lines) and fluorescence (dashed lines) spectra of (a) 1 (gray), 2 (black), 3 (red), and 4 (green) and (b) 22· I2F4 (blue) and 42·I2F4 (purple) in the solid state. Excitation at 340 nm.

Table 3. Absorption, Fluorescence and Phosphorescence Parameters at Room Temperature in the Solid State 1 2 3 4 22·I2F4 42·I2F4

a λmax abs /nm

λmax fl /nm

360 348 347 342 379 357

404 440 448 424 468 422

ϕflb

τfl/psc

λmax phos/nm

± ± ± ±

300 (30%); 1140 (70%) 75 (40%); 450 (60%) 230 (50%); 563 (50%) 183 (45%); 470 (55%) 290 (80%); 645 (20%) 78 (45%); 240 (55%)

− 648 686 616 638, 716, 788 (sh) 590, 630, 682

0.097 0.037 0.021 0.039 −e 0.092

0.007 0.001 0.002 0.002

± 0.004

τphos/msd − 0.75 0.30 0.68 0.24 0.22

(49%); (64%); (85%); (54%); (64%);

3.9 (51%) 1.5 (51%) 3.2 (15%) 1.0 (46%) 0.82 (36%)

a Absorption maxima from reflectance spectra converted using Kubelka−Munk function. bAbsolute emission quantum yields. cFluorescence lifetimes, measured with a streak camera apparatus (see Experimental Section). dWeak signals, mean values from three to five measurements. eBelow instrumental resolution.

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occur in 22·I2F4, leading to a very poor fluorescence (although other effects could account for the weak emission, as discussed below), whereas 42·I2F4 is characterized by more “isolated” molecules. A confirmation of the different arrangement of the units in the two co-crystals comes from vibrational analysis that evidenced how 42·I2F4 is governed by N···I halogen-bonding interactions between the naphthalimide units and the coformers and CO···H−C hydrogen-bonding patterns between adjacent chromophores, at a difference with 22·I2F4 where C− I···π halogen bonding seems to prevail, besides π−π stacking interactions as well as changes in CO···H−C, C−H···I, C− I···I−C, and C−F···H interactions. Fluorescence decays of all of the examined solids are in the subnanosecond regime and have been measured by means of a picosecond laser and a streak camera device in a front-face setup (see Experimental Section). The decays are characterized by a multiexponential behavior that can be reasonably fitted with a biexponential function; the results are collected in Table 3. Overall the weighted average values of the lifetimes in the solids are longer than those observed in solution, paralleling the increase in emission quantum yield. The multiexponential behavior of luminescence decays in solids of organic materials, often observed,69,72 can be ascribed to the presence and inhomogeneous distribution of exciton traps, such as impurities or structural defects of the crystal. The measured lifetimes of solids 2−4 (Table 3) appear to be composed of a short component of the order of 100−200 ps, and a longer one of the order of 400−500 ps. In co-crystal 22·I2F4, the short lifetime is the main component of the decay, accounting for the low fluorescence yield of the crystal, and we cannot exclude that in this case, besides the molecular arrangement, a larger number of structural defects is responsible for the observed features. The lifetime of 42·I2F4 appears quite short if combined with the value of the emission quantum yield; the results suggest that together with a decrease of knr the molecules in the crystal also experience an increase of kr. We did not have evidence of excimer emission in any of the studied solids. Despite the improved fluorescence efficiency of the molecules in the crystals with respect to solution, we investigated the phosphorescence properties of the solids, assuming that the intersystem crossing in the single units remains efficient, in particular for the co-crystals where the proximity of halogenated coformers should operate an external heavy atom effect. Weak but discernible emission spectra have been obtained in gated detection at RT for all of the compounds except parent 1. The crystals of 2−4 show broad emission bands in the region 600−800 nm with lifetimes of the order of hundreds of microseconds to a few milliseconds (decays fitted with biexponential functions), as shown in Figure 8a and Table 3. The observed spectra can be ascribed to the phosphorescence of the crystals, and the increasing red-shift in the order 3 > 2 > 4, in agreement with the fluorescence trend (Figure 7a), indicates that the interactions between molecules in the crystals influence in the same way singlet and triplet energy levels. Co-crystals show a more structured phosphorescence with a shorter lifetime (

Luminescence Properties of 1,8-Naphthalimide Derivatives in Solution

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