Tunable solid-state fluorescence of pyrene-1-carboxamides

Pyrene reacts with potassium thiocyanate and organic isothiocyanates in the presence of trifluoromethanesulfonic acid to afford primary and secondary pyrene-1-carbothioamides in high yields. These compounds were efficiently oxidatively desulfurized with Oxone® to the corresponding carboxamides. The amides display solid-state fluorescence with quantum efficiencies up to 62%, originating from monomers, aggregates (such as preformed dimers), and/or excimers, depending on the substituent at the nitrogen atom. Single crystal X-ray diffraction characterization of one highly emissive compound supports this assumption.

The reported synthetic route to pyrene-1-carboxamides relies on the reaction of pyrene-1-carboxylic acid chloride with amines.The amides can then be transformed into thioamides via a reaction with Lawesson reagent. 17,23,24 this paper we report an efficient method of synthesis of primary and secondary pyrene-1-carbothioamides directly from pyrene via a Friedel-Cras-type reaction with potassium thiocyanate and organic isothiocyanates, respectively.The thioamides were then desulfurized into the corresponding amides using Oxone®.Furthermore, we discovered that most of the synthesized amides are uorescent not only in solution, as was reported earlier, but also in the solid state.Solid-state emission is a phenomenon of great practical importance, [25][26][27] but is relatively rare in the case of planar aromatic compounds such as pyrene and its derivatives.We found that the solid-state emissive properties of synthesized pyrene-1-carboxamides strongly depend on the substituent on the nitrogen atom and for bulky substituents uorescence quantum yields as high as 60% were obtained.

Results and discussion
9][30][31] We recently reported that a reaction of pyrene with ethoxycarbonylisothiocyanate afforded Nethoxycarbonyl-pyrene-1-carbothioamide in 95% yield. 32rimary thioamides were obtained in a reaction of electron-rich arenes (1,2-diethoxybenzene, ferrocene) with potassium thiocyanate in methanesulfonic acid. 28,33,34erein we report that pyrene reacts with potassium thiocyanate in the presence of 4 equiv. of TfOH to afford the primary thioamide 1a in 63% yield (Scheme 1).The reaction was performed in dichloromethane at room temperature.Under the same conditions, reaction of pyrene with alkyl-and aryl isothiocyanates afforded secondary thioamides 1b-j in 78-96% yield.Thioamides 1a-j were conveniently isolated, aer quenching the reaction mixtures with water and extraction, via column chromatography on silica gel.
Aer having elaborated an efficient direct method for thioamidation of pyrene, we explored the possibility of extending this method to the synthesis of pyrenyl amides using isocyanates instead of isothiocyanates.We chose the reaction of pyrene with phenyl isocyanate as a model.Unfortunately, we found that this reaction provided the expected N-phenylpyrene-1carboxamide only in low ($30%) yield.Because of this unsatisfactory result, we changed our approach in order to nd another synthetic route to pyrenyl amides.
Since a variety of simple and efficient methods of desulfurization of thioamides to amides has been reported in the literature, 35 we decided to check whether this transformation could be applied to the synthesis of pyrenyl amides.
In order to complete the series for photophysical studies, we also synthesized pyrene amides 2k-m bearing bulky substituents at the nitrogen atom (Scheme 2).Since the isothiocyanates required for syntheses of these compounds via Friedel-Cras reaction were not available we prepared them from pyrene-1carboxylic acid and the corresponding amines in the presence of N,N 0 -dicyclohexylcarbodiimide (DCC).The isolated yields of 2k-m were in the range of 85-90%.
All synthesized compounds were fully characterized by spectroscopic methods and elemental analyses.

Solid-state uorescence of pyrene-1-carboxamides 2a-m
Solution uorescence of various pyrene-1-carboxamides was recently thoroughly studied by Konishi et al. [14][15][16] During the course of our synthetic work, we discovered that these compounds are also solid-state emitters.6][27] It critically depends on interactions of molecular uorophores (p-p interactions, formation of H-or J-aggregates, excimers, etc.) in powders, crystals or solid lms. 38e measured solid-state excitation spectra, emission spectra and uorescence quantum yields of amides 2a-d, g, h, k-m (compound 2j proved non uorescent).Corresponding spectra are displayed in Fig. 1 and spectroscopic data are gathered in Table 1.
It is visible in Table 1 that substituents at the nitrogen atom strongly inuence solid-state emissive properties of the synthesized amides.On one side, the primary amide 2a shows a weak structured violet uorescence (F F ¼ 6%), that is similar in shape to its uorescence spectrum when dissolved in chloroform (Fig. 2A).However, compared to the solution spectrum, the solid-state emission spectrum of 2a is red-shied by 25 nm, and a long tail can be observed at wavelengths over 450 nm.This observation suggests that the interaction of uorophores inuencing solid-state uorescence is in this case rather limited.On the other side, the solid-state excitation/emission spectra of the secondary amides have a wide variety of shapes, with corresponding quantum efficiencies varying from 0.2% to 62%, as a function of the substituents.When analyzing in detail the solid-state emission spectra, we can distinguish several cases: (i) one compound (2g) shows a clear monomer contribution, peaking at 414 nm, associated with a structured shape and a very low uorescence quantum yield (F F ¼ 0.2%); (ii) several compounds (2b-c, 2k-m) display a large red-shied single band, with quantum yields in the 7-62% range; and (iii) two compounds (2d and 2h) show a well-apparent shoulder at the violet edge of the spectrum (406-416 nm) and a main emission band in the range of 450-600 nm, with intermediate quantum yields (12% and 20%, respectively).The spectral signatures highlighted in the two latter cases are clear indications that the solid-state emission originates from aggregates such as preformed dimers, or excimers.Moreover, the excitation spectra depicted in Fig. 1 all show a main maximum band around 363-369 nm, but also eventually a shoulder (or even a well-dened separated band) at longer wavelengths, in the range of 400-433 nm (amides 2b, 2d, 2h, 2l, see Table 1).This observation supports the presence of aggregates in the ground state.In spite of this variety of uorescence properties, a general tendency can be drawn i.e. that: the increase of the size of the alkyl substituent at the nitrogen atom brings about an increase in the uorescence quantum yield.Indeed, the amides 2c and 2j bearing a bulky N-tert-butyl and 1-adamantyl groups are the most efficient solid-state emitters of the series (F F ¼ 59% and 62%, respectively).On the other hand, N-phenyl substitution (compound 2g) leads practically to disappearance of the solidstate emissive properties.The N-benzyl amide 2h shows moderate uorescence quantum yield (20%) and the attachment of the second and third phenyl ring to the sp3 carbon (amides 2l and 2m, respectively) brings about decrease of the uorescence quantum yield.
Practically, the colors of solid-state uorescence emitted by the investigated series of amides are shown in the CIE diagram (Fig. 3).As a consequence of the different uorescence spectral shapes of these pyrene derivatives, the solid-state emission expands over a wide range of colors, from violet-blue to green and even yellow, depending on the substituents.
It is worth noting that the most efficient compounds (2c and 2j) emit in the blue region.Interestingly, the former compound has co-ordinates close to those required for deep-blue emission (y < 0.15; x + y < 0.30, cf.ESI †). 39The substituents at the nitrogen   atom shi the uorescence into the blue-green (compound 2b) or even the green-yellow region (compound 2l).However, this bathochromic shi is accompanied by a decrease in emission efficiency.
In order to deepen our investigation of the photophysical properties of pyrene carboxamide derivatives in the solid-state, we recorded uorescence decays for the whole series 2a-d, g-h, k-m at variable emission wavelengths.The time-correlated single photon counting (TCSPC) method was employed for short decays (decay time constants < 35 ns), whereas the laser ash uorometry was used for longer decays (including decay time constants > 35 ns).The Fig. 4 displays the solid-state uorescence decay curves of the compounds 2a-d, g-h, k-m recorded using these experimental methods, at their respective maximum emission wavelengths.In all cases, the uorescence decays reveal a multiexponential behavior, which reects the complexity of these systems in the solid-state.They were satisfactorily tted with a sum of two, three or four exponentials, and the corresponding parameters are compiled in the Table 2.As was mentioned in the previous paragraph, the derivatives 2a and 2g mostly show monomer emission, and constitute a rst group of compounds.This translates into relatively short uorescence decays, with time constants in the range of a few nanoseconds for 2a, and even in the range of several hundreds of picoseconds for 2g.Thus, the different time constants may represent different populations of monomers in the solid.The time-resolved uorescence investigations reveal a very specic situation for compounds 2b, 2d and 2h (the second group of compounds).Indeed, they show a strong emission wavelength dependence of their uorescence decays, as is highlighted in Fig. 5, with a multiexponential decay at short emission wavelength (recorded at the blue edge of the emission spectrum), and a short rise time followed by multiexponential decay at long emission wavelength (recorded at the red edge of the emission spectrum).A global analysis was carried out on these data sets (see Table 2), which shows that the short rise time measured at long emission wavelength (0.3-1.6 ns) has the same order of magnitude as the shorter decay time recorded at short emission wavelength (0.5-0.7 ns).This nding may be consistent with the dynamics of fast excimer formation in the solid-state, and is rather compatible with the spectral features described in the previous paragraph.In this frame the longest decay time constant, determined for 2b, 2d and 2h to be 23.7 ns, 24.8 ns, and 34.1 ns, respectively, corresponds specically to excimer emission, since its relative fraction of intensity increases at longer emission wavelength (Table 2).Then the last group of four compounds, namely 2c and 2j-l, exhibits intermediate behavior between purely monomeric emission (such as 2a and 2g) and well-dened excimer formation (such as 2b, 2d and 2h).On the one hand, the 2c and 2k-m derivatives do not show any rise time at long wavelength emission, which excludes the excimer formation mechanism in these cases.On the other hand, their uorescence decays are rather long and their emission spectra are broad, red-shied, without any vibrational structure, which is inconsistent with simple monomer emission.We could then assume that, for 2c and 2k-m, solid-state  Table 2 Fitting parameters of solid-state fluorescence decays (s i , A i , f i ) for compounds 2a-d, g-h, k-m, recorded by the time-correlated single photon counting system (TCSPC) or by the laser flash photolysis spectrometer a Selected emission wavelength by means of a monochromator with a 50 nm bandwidth.b Intensity fractions were calculated using the following equation: , obtained for a global analysis of the full set of uorescence decay curves, calculated for TCSPC experiments.d Recorded by the TCSPC system, l exc ¼ 370 nm. e Recorded by the laser ash photolysis spectrometer, l exc ¼ 355 nm.uorescence arises from strongly emissive aggregates or dimers preformed in the ground state.Several populations of emitters may exist in the solid state, as is suggested by the multiexponential decays recorded for these compounds.It is worth noting that the situation of compounds 2c and 2k is quite unique, with extremely long decay components (s 1 $ 10 ns and s 2 $ 45 ns for 2c; s 1 $ 12 ns and s 2 $ 70 ns for 2k) associated with unprecedented high quantum yields (59% and 62%, respectively).Therefore, for these two derivatives we expect that the formation of aggregates in the crystalline structure and the specic p-p interaction between pyrenyl groups are extremely favorable, thus leading to such effective solid-state uorescence.
[42] X-ray diffraction study of 2c We intended to compare molecular packings in the crystals of the synthesized amides but we were able to obtain crystals suitable for X-ray diffraction analysis only for 2c.The compound crystallized in the space group P2 1 /c of the monoclinic system aer a long period of standing (2-3 weeks) in a concentrated solution in DMSO in an open tube (presumably moisture slowly diffused into the solution and decreased the solubility of 2c).The molecular structure of 2c is shown in Fig. 6.
The pyrenyl moiety is slightly bent, with the angle between the planes of rings C1-C2-C3-C4-C15-C14 and C7-C8-C9-C10-C11-C16 $4 .The amide group is twisted by 60.6(2) degrees from the average plane of the pyrenyl moiety.Together with the C1-C17 bond length greater than 1.5 Å this suggests that there is only a very weak electronic conjugation between these units.
The crystallographic structure reveals that the molecules of 2c form p-stacked dimers with short C/C distances (Fig. 7), thus suggesting strong electronic interactions between the two pyrenyl moieties, which is in very good agreement with the photophysical investigations that were described and discussed in the previous section.This strong p-p interaction in the ground state seems to be at the origin of the very high solid-state uorescence quantum yield of the 2c derivative, which is associated with its very slow uorescence decay.
The intermolecular N1 -H1/O1 hydrogen bonds formed between the molecules packed along the crystallographic c direction bring about the formation of innite chains of Hbonded molecules, with the N/O vector almost parallel to c direction and adjacent pyrenyl moieties in the crystal structure located at an angle of 63.0(3) degrees (Fig. 8).
Crystal packing viewed along b direction conrms the formation of columns of p-stacked dimers of 2c.Along the crystallographic a direction the crystal structure is stabilized only by van-der Waals interactions of the tert-butyl moieties (Fig. 9).

Conclusion
We have developed a simple and efficient synthetic route to primary and secondary pyrene-1-carbothioamides and carboxamides.The amides emit uorescence in the solid state with quantum efficiencies up to $60%.The primary amide in the solid state mainly displays structured monomer emission, whereas several secondary amides show a dynamic excimer formation leading to red-shied emission.The strong solidstate uorescence of the most efficient uorophores, N-tertbutyl-and N-(adamant-1-yl)pyrene-1-carboxamide can be attributed to preformed dimers or aggregates in the ground state, associated with very long decay times.

Fig. 4
Fig. 4 Fluorescence decays of 2a, b, d, g, h, l, m in the solid state, recorded by the TCSPC system at l exc ¼ 370 nm (A), or laser flash fluorometry at l exc ¼ 355 nm (B).

Fig. 5
Fig.5Fluorescence decays of (A) 2b, (B) 2d, (C) 2h in the solid state, recorded by the TCSPC system at two different emission wavelengths: blue edge of the emission spectrum (blue dots) and red edge of the emission spectrum (red dots) (l exc ¼ 370 nm).

Fig. 7
Fig. 7 Dimers of p-stacked molecules of 2c, related by the crystallographic center of symmetry.

Fig. 8
Fig.8View of the N-H/O hydrogen bond in the context of crystal packing of 2c along a* direction.The atoms directly involved in the H-bond network are represented with thermal ellipsoids at 90% probability level, to enhance the pattern.The remaining atoms are represented as wires in gray or in green in order to differentiate between the adjacent p/p interacting chains of H-bonded atoms.
H and 13 C NMR spectra were recorded in CDCl 3 (if not indicated) or in DMSO-d 6 on a Bruker ARX 600 MHz (600 MHz for 1 H and 151 MHz for 13 C).Chemical shis were referenced relative to solvent signals.Spectra were recorded at room temperature (291 K), chemical shis are in ppm and coupling constants in Hz.IR spectra were run on an FT-IR Nexus spectrometer.Column chromatography was carried out on silica gel 60 (0.040-0.063 mm, 230-400 mesh, Fluka).Elemental analyses were performed at the Laboratory of Microanalysis in the Centre of Molecular and Macromolecular Studies in Łódź, Poland.

Table 1 Solid
-state absorption and emission data of pyrene-1-carboxamides 2a-d, g, h, k-mAmideExcitation a l max /nm Emission a l max /nm F F /%2a (R ¼ H)a Shoulders are noted (sh).