J. Phys. Chem. B 1997, 101, 1775-1781
1775
Solid-State Fluorescence of Aromatic Dicarboxamides. Dependence upon Crystal Packing Frederick D. Lewis* and Jye-Shane Yang Department of Chemistry, Northwestern UniVersity, EVanston, Illinois 60208 ReceiVed: October 8, 1996; In Final Form: December 19, 1996X
The fluorescence of six secondary arenedicarboxamides has been investigated in single crystals, powders, melts, and ethanol solution. Single crystals of four rodlike arenedicarboxamides which pack in one-dimensional tapes display dual emission attributed to excited-state monomers and dimers. These tapes have a constant 5-Å separation between the long axes of neighboring arenes but differ in the arene-arene dihedral angle. The frequency shift between monomer and dimer fluorescence and the intensity and vibronic structure of the dimer emission are dependent upon the ground-state geometry. Single crystals of two naphthalenedicarboxamides which pack in two-dimensional sheets and have no close contacts between neighboring arenes within the same sheet display only monomer emission. Melted and resolidified samples display broad structureless emission attributed to fluorescence from a mixture of excited monomers and dimers with different ground-state geometries.
Introduction
CHART 1
Secondary dicarboxamides are known to pack in the solid state either as one-dimensional hydrogen-bonded tapes or as two-dimensional hydrogen-bonded sheets.1,2 In the cases of N,N ′-dimethylarenedicarboxamides (Chart 1), the rodlike arenedicarboxamides 1-4 form one-dimensional tapes in which the up-down translational arrangement of amide-amide hydrogen bonding allows infinite chains of neighboring arenes to adopt either offset face-to-face (Figure 1a) or edge-to-face geometries (Figure 1b and 1c).2 In contrast, two isomeric naphthalenedicarboxamides, 5 and 6, form two-dimensional sheets (Figure 1d and 1f, respectively) which pack in either single layers (Figure 1e) or bilayers (Figure 1g) in which there are no close arene-arene contacts within a sheet.2b The geometric relationship between neighboring arenes in both the tapes and sheets is significantly different from those of the parent arenes, all of which adopt the common edge-to-face or herringbone relationship.3 These differences in crystal packing might be expected to influence the excited-state behavior of the arenedicarboxamides in the crystalline state. Fluorescence spectroscopy provides a powerful technique for the study of excited-state chromophore-chromophore interactions in the gas phase,4 solution,5 and the solid state.6 Arenes which pack in the herringbone geometry display structured fluorescence attributed to the monomer or free exciton.7 In contrast, arenes which pack either in infinite stacks (e.g., 9,10dichloroanthracene) or in pairs (e.g., pyrene) display broad redshifted fluorescence attributed to a species variously described as an excited dimer, excimer, or deeply trapped exciton.7,8 Previous studies of the luminescence of polymorphs of aromatic hydrocarbons which pack in infinite stacks have indicated that the shift in frequency for dimer vs monomer fluorescence is sensitive to the plane-to-plane separation and relative orientation (head-to-head or head-to-tail) of adjacent arenes.9 Theoretical investigations of excimers have also indicated that excimer frequency shifts should be dependent upon the lateral displacement and dihedral angle between neighboring arenes, as well as the plane-to-plane separation.10,11 We report here the solidstate fluorescence behavior of 1-6. Arenes hydrogen-bonded as tapes display both monomer and dimer fluorescence in single X
Abstract published in AdVance ACS Abstracts, February 15, 1997.
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crystals and powders, whereas arenes hydrogen-bonded as sheets display only monomer fluorescence. These properties are related to the ground-state equilibrium geometry of the crystal. Broad emission attributed to dimers is observed for the melts formed from both tape and sheet-forming arenes. Results The synthesis and crystal structures of the N,N ′-dimethylarenedicarboxamides 1-6 have been reported.2 The dicarboxamides are fluorescent both in solution and in the solid state. The fluorescence excitation spectra of dilute solutions are similar in appearance to the absorption spectra. The fluorescence emission spectra of 2-5 in ethanol solution display vibrational structure. The positions of their 0,0 bands are reported in Table 1. Diamides 1 and 6 display unstructured emission, and their solution 0,0 band positions are estimated from the crossing point of the excitation and emission spectra. The fluorescence emission and excitation spectra of single crystals were obtained from crystals mounted on a thin glass rod (see Experimental Section). The orientation of the crystal relative to the exciting light beam was changed to locate the face of the crystal which provided optimum fluorescence intensity upon front face excitation. It was found that the optimum spectrum was obtained when the electric vector of the incident light is approximately parallel to the long axis of © 1997 American Chemical Society
1776 J. Phys. Chem. B, Vol. 101, No. 10, 1997
Lewis and Yang
Figure 1. Schematic representations of arene-arene (rectangles and bold lines) geometries in the hydrogen-bonded tapes of (a) xylenedicarboxamide 1, (b) stilbenedicarboxamide 2 and biphenyldicarboxamide 3, and (c) diphenylacetylenedicarboxamide 4 and in the hydrogen-bonded sheets of (d) 2,6-naphthalenedicarboxamide 5 within a sheet and (e) between the neighboring sheets and of (f) 1,4-naphthalenedicarboxamide 6 within a sheet and (g) between the neighboring sheets.
the chromophores. For example, in the case of the stilbenedicarboxamide 2, the optimum intensity was obtained by exciting the (100) face of the crystal, which corresponds to the plane
perpendicular to the long axis (hydrogen-bonding axis) of the one-dimensional tape. Excitation at other faces of the crystal resulted in very weak and/or broadened, structureless fluores-
Solid-State Fluorescence of Dicarboxamides
J. Phys. Chem. B, Vol. 101, No. 10, 1997 1777
TABLE 1: Absorption and Fluorescence Spectral Data λmax,abs, nm, in EtOH λ0,0,flu, nm, in EtOH λ0,0,flu,c nm, single crystal λmax, ex,e nm, single crystal ∆υm-ex,f cm-1, single crystal
1
2
3
4
5
6
264 272a 279 353 5500
328 358b 361 (362)d 442 3600
280 320b 318 (316)d 360 2300
302, 320 330b 359 (362)d 447 3900
286, 296, 326, 340 351b 357 (355)d
292 336a 338
a From the crossing point of the excitation and emission specta. b From the emission 0,0 band. c From the excitation 0,0 band. d From the emission 0,0 band. e From the maxima of excimer emission. f From the maxima of monomer and excimer emission.
Figure 2. Fluorescence emission and excitation spectra of xylenedicarboxamide 1 (A) in ethanol solution (----) (λex ) 260 nm and λem ) 300 nm) and in a single crystal (s) (λex ) 260 nm and λem ) (a) 300 and (b) 390 nm) (insert: λex ) 280, 290, 300, and 310 nm) and (B) (c) in the powder (λex ) 240 nm and λem ) 400 nm) and (d) in the melt (λex ) 320 nm and λem ) 450 nm).
cence. Fluorescence spectra were also obtained for powdered samples sandwiched between glass plates and for melted samples obtained by rapid heating of powdered samples supported on a glass plate followed by air cooling. All fluorescence emission and excitation spectra of arenedicarboxamides 1-6 in ethanol solution and the powdered and melted forms (but not in the single crystals) are fluorescence intensity normalized. The fluorescence spectra of the xylenedicarboxamide 1 in ethanol solution and in the single crystal are shown in Figure 2A. Excitation of the crystal at 260 nm, near the maximum of the solution absorption band, results in the observation of two emission bands. The higher energy band is similar in wavelength to the emission band observed in ethanol solution. The excitation spectrum of the lower energy band has a maximum at 312 nm, well to the red of the onset of the solution absorption band. The lower energy band displays a vibrational structure which becomes less well-resolved as the excitation wavelength increases (Figure 2A insert). The emission spectra of 1 in the powder or solidified melt (Figure 2B) are broad and structureless, with maxima at longer wavelengths than that of the longwavelength band of the single crystal. The fluorescence spectra of the stilbenedicarboxamide 2 in ethanol solution and in the single crystal are shown in Figure 3A. Excitation at 320 nm, near the maximum of the solution absorption band, results in an emission spectrum similar to that observed from 2 in ethanol solution. However, excitation at 395 nm, beyond the red edge of the solution absorption spectrum, results in weakly structured emission with a maximum at 442 nm. The fluorescence spectra of 2 obtained from a
Figure 3. Fluorescence emission and excitation spectra of stilbenedicarboxamide 2 (A) in ethanol solution (----) (λex ) 320 nm and λem ) 420 nm) and in a single crystal (s) (λex ) (a) 320 and (b) 395 nm and λem ) (c) 402 and (d) 440 nm) and (B) (e) in the powder (λex ) 330 nm and λem ) 430 nm) and (f) in the melt (λex ) 320 nm and λem ) 430 nm).
powder and solidified melt are shown in Figure 3B. Excitation of the powder at 320 nm results in a broad emission spectrum. The two high-energy maxima are at approximately the same position as the maxima in the spectrum of the single crystal; however, the 362-nm band observed in the single crystal is not observed in the powder, presumably due to reabsorption. The spectrum of the melt is broader than that of the powder. The solution and single-crystal spectra of 3 are shown in Figure 4. As is the case for 2, excitation of the single crystal of 3 near the maximum of the solution absorption band (280 nm) results in weakly structured emission similar to that observed from 3 in solution. Excitation at 325 nm, beyond the red edge of the solution absorption band, results in broad emission with a maximum at 360 nm. Qualitatively similar results are obtained for the single crystal of 4 (Figure 5). However, the structured shorter wavelength emission of 4 is strongly red-shifted with respect to the solution spectrum. The fluorescence spectra of 3 and 4 were also obtained from powders and solidified melts (spectra not shown). These spectra differ from those of the single crystals in much the same way as observed for 2. The fluorescence emission and excitation spectra of the 2,6naphthalenedicarboxamide 5 in ethanol solution and in a single crystal are shown in Figure 6A. Comparison of the emission spectra shows that the 0,0 band is much weaker in the crystal than in solution and that the vibronic bands are red-shifted by ∼5 nm in the crystal. The onset of the excitation spectrum is red-shifted in the crystal, and thus, reabsorption may account
1778 J. Phys. Chem. B, Vol. 101, No. 10, 1997
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Figure 4. Fluorescence emission and excitation spectra of biphenyldicarboxamide 3 in ethanol solution (----) (λex ) 280 nm and λem ) 340 nm) and in a single crystal (s) (λex ) (a) 280 and (b) 325 nm and λem ) (c) 350 and (d) 400 nm).
Figure 6. Fluorescence emission and excitation spectra of 2,6naphthalenedicarboxamide 5 (A) in ethanol solution (----) (λex ) 326 nm and λem ) 367 nm) and in a single crystal (s) (λex ) 320 nm and λem ) 390 nm) and (B) (a) in the powder (λex ) 315 nm and λem ) 390 nm) and (b) in the melt (λex ) 320 nm and λem ) 390 nm).
Figure 5. Fluorescence emission and excitation spectra of diphenylacetylenedicarboxamide 4 in ethanol solution (----) (λex ) 304 nm and λem ) 350 nm) and in a single crystal (s) (λex ) (a) 290 and (b) 394 nm and λem ) (c) 400 and (d) 450 nm).
for the weak 0,0 band in the crystal. The emission spectra of both the solution and single crystal are independent of the excitation wavelength. Excitation of the single crystal beyond the red edge of the solution spectrum does not result in the appearance of a new fluorescence band. Fluorescence emission and excitation spectra of 5 obtained from a powder and from a solidified melt are shown in Figure 6B. The spectra of the powder are similar to that of the single crystal except that the 0,0 emission band cannot be detected and the excitation spectrum is broader in the powder. The emission spectrum of the melt displays a broad tail at longer wavelengths. The fluorescence emission and excitation spectra of the 1,4naphthalenedicarboxamide 6 in ethanol solution and in a single crystal are shown in Figure 7. The maxima of the structureless emission bands are similar in solution and the crystal. The band shapes are similar except for an increase in intensity of the longwavelength tail for the crystal. A change in the excitation wavelength from 320 to 340 nm results in a small red shift (ca. 700 cm-1) in the emission maximum. The fluorescence spectra of 6 in a powder or melt are similar to that in the single crystal. The position of the 0,0 bands for the higher energy fluorescence excitation and emission of the single-crystal dicarboxamides 1-6 are summarized in Table 1. Also reported in Table 1 are the maxima of the lower energy bands observed in the single crystals of 1-4 and the frequency difference of fluorescence maxima between these bands and the higher energy bands (υm - υex). Fluorescence decay times for dilute solutions, single crystals, and powders were determined by single photon counting (see Experimental Section). Decay times (τ), preexponentials (A),
Figure 7. Fluorescence emission and excitation spectra of 1,4naphthalenedicarboxamide 6 in ethanol solution (----) (λex ) 290 nm and λem ) 385 nm) and in a single crystal (s) (λex ) (a) 320 and (b) 340 nm and λem ) (c) 393 nm).
and the contribution of each decay component to the total fluorescence for 1, 2, and 5 are reported in Table 2. The fluorescence decay of 2-6 in ethanol solution can be fit by a single exponential. Dual-exponential decay of 1 in solution is attributed to the presence of two or more ground-state conformational isomers.12 The fluorescence decays of the single crystals or powders cannot be fit by a single exponential. Satisfactory fits required the use of either two or three exponentials. Discussion Structure and Fluorescence of Hydrogen-Bonded Tapes. The crystal structures of the dicarboxamides 1-6 have been reported,2 and their hydrogen-bonded modes are schematically presented in Figure 1. The neighboring phenyl-phenyl geometries for dicarboxamides 1-4 in the same hydrogen-bonded tape can be depicted as shown in Figure 8, in which the arene long axis is shown as a filled circle in the middle of a line representing the width of a benzene ring. The 5-Å radius of the large circle in Figure 8 corresponds to the optimum amide-
Solid-State Fluorescence of Dicarboxamides
J. Phys. Chem. B, Vol. 101, No. 10, 1997 1779
TABLE 2: Fluorescence Decay Times of Arenedicarboxamides 1, 2, and 5 compd
medium
λex
λem
τ1, ns
A1
F1, %
τ2, ns
A2
F2, %
1
EtOH crystal crystal EtOH crystal crystal powder powder EtOH crystal powder
260 260 310 300 320 365 330 365 320 320 330
300 300 370 390 390 440 390 440 370 390 390
17.7 7.14 1.21 0.56 1.47 1.28 0.82 1.56 12.0 4.00 1.78
0.16 0.16 0.67 1.00 0.90 0.99 0.96 0.93 1.00 0.85 0.63
11.2 5.0 10.2 100 31.4 85.5 60.7 71.4 100 38.2 49.2
26.6 25.9 9.85
0.84 0.84 0.18
88.8 95.0 22.3
35.8
0.15
67.5
13.2 5.30 4.31 4.23
0.05 0.006 0.03 0.06
15.7 2.1 10.0 12.5
44.6 61.3 38.0 32.6
0.05 0.003 0.01 0.01
52.9 12.4 29.4 16.1
36.7 3.00
0.15 0.36
61.8 47.4
0.01
3.4
2
5
Figure 8. Schematic representations of the neighboring phenyl-phenyl geometries of arenedicarboxamides 1-4 in the same hydrogen-bonded tapes, in which the arene long axis is shown as a filled circle in the middle of a bold line representing the width of a benzene ring. The large circle illustrates the optimum 5-Å amide-amide hydrogenbonding distance.
amide hydrogen-bonding distance. The benzene rings in 1 adopt a ladder structure in which the benzenes are parallel but nonoverlapping. The stilbenes and biphenyl in 2 and 3 are nonplanar and have dihedral angles of 28.1° and 35.5°, respectively, both for benzene rings within the same molecule and for adjacent molecules within the same tape. The diphenylacetylenes in 4 are planar, and adjacent molecules adopt an edge-to-face geometry with a dihedral angle of 67.9°. The closest arene-arene contacts are essentially the same for all four crystals. As a consequence of the fixed 5-Å distance between long axes, an increase in the dihedral angle is accompanied by an increase in the plane-to-plane separation, z, but a decrease in the lateral displacement, x of the adjacent molecules. It should be noted that the different geometrical relationships for 1-4 shown in Figure 8 are interconvertable by rotating the arenes about their long axis. Counterclockwise rotation of the arene located at the center of the circle by 28°, 36°, and 68°, respectively, converts the phenyl-phenyl geometries in 2, 3, and 4 to the parallel offset geometry of 1. The barrier to such rotations in the crystal would depend on the rigidity of the packing of arenes and the hydrogen-bonding scaffolds. The emission spectra of 1-4 in the single crystal display both a short wavelength band, similar to that observed in solution, and a long-wavelength band, which is not observed in solution (Figures 2-5). We assign these bands to the singlet excited monomers and dimers, respectively. The relatively short decay times for both bands (Table 2) are consistent with their assignment as fluorescence rather than phosphorescence. The
τ3, ns
7.74
A3
F3%
χ2 1.08 1.00 1.06 1.20 1.22 1.27 1.16 1.03 1.08 0.93 1.12
dependence of the fluorescence spectrum on the orientation of single crystals indicates that neither emission can be attributed to impurities. The shorter wavelength emission attributed to monomer fluorescence from single crystals of 1, 2, and 3 is similar in appearance to the solution spectra except that the highest energy shoulder in the solution spectrum is much weaker in the crystal (Figures 2-4). Reabsorption of fluorescence by the strongly absorbing single crystal is most likely responsible for this observation and for the appearance of the maximum in the single-crystal excitation spectrum near the onset of the solution excitation spectrum. In the case of 4, large red shifts in the fluorescence emission and excitation spectra are observed in the single crystal vs solution. Similar shifts are observed for the parent diphenylacetylene13 and may be a consequence of the herringbone packing of the planar chromophore. The observation of monomer fluorescence requires the use of excitation wavelengths which are strongly absorbed by the crystal. Thus, monomer emission could arise from both surface and bulk molecules. Since each molecule in the crystals of 1 and 2 has the same geometry, the multiple-exponential decay observed for 1 and 2 (Table 2) is attributed to emission from different sites within the crystal. Multiple-exponential monomer decay has also been observed for linear arrays of naphthyl-labeled [n]ladderanes in solution and attributed to electronic interactions among imperfectly stacked naphthalenes.14 It is interesting to note that the decay times of 1 are somewhat shorter in the crystal vs solution but that the decay times of 2 are substantially longer in the crystal. The short singlet lifetimes of stilbenes in solution are a consequence of twisting about the double bond,15 a decay pathway which is unavailable in the crystal. The longer wavelength fluorescence assigned to dimers is observed upon long-wavelength excitation, which should penetrate the crystal. Thus, dimer fluorescence is characteristic of the bulk of the crystal. Only in the case of 1 is dimer as well as monomer fluorescence observed upon short-wavelength excitation. This may reflect the much lower absorption intensity of benzene vs biphenyl, stilbene, or diphenylacetylene, which would permit greater penetration of light into the bulk of the crystal. The rise time for dimer fluorescence of 1 and 2 is shorter than the exciting lamp profile (
