Spectroscopic properties of two highly substituted PAH heteroanalogs: octakis(pyrazol-1-yl)naphthalene and octakis (3,5-dimethylpyrazol-1-yl)naphthalene

Two heteroanalogs of octaphenylnaphthalene, octakis(pyrazol-1-yl)naphthalene and octakis(3,5-dimethylpyrazol-1-yl)naphthalene have been prepared and fully characterized by NMR ( 1 H and 13 C). Although they are very insoluble in ordinary solvents, such as ethanol and ethyl acetate, absorption and emission spectra were recorded. These experiments show that the new compounds behave photophysically like other highly substituted naphthalenes


Synthesis ISSN 1551-7004
Page 613  ARKAT USA, Inc Octakis(pyrazol-1-yl)naphthalene (OPzN).0.5 g (7.3 mmol) of pyrazole in 5 mL of anhydrous THF were placed in a three-necked round bottomed flask provided with a reflux condenser, argon atmosphere and magnetic stirring.Then, 0.29 g (7.3 mmol) of NaH (60% oil dispersion) were added and the reaction mixture was heated to 65 o C for 1 h.After cooling down at room temperature, 0.25 g (0.92 mmol) of octafluoronaphthalene were added and the mixture was heated under reflux for 2 h.Compound OPzN precipitated from the reaction mixture, was filtered off, washed with water and purified by column chromatography using CH2Cl2/ethanol
In the case of the octakis(3,5-dimethylpyrazol-1-yl)naphthalene (OMe 2 PzN) the assignment of 1 H and 13 C signals was carried out through ( 1 H-1 H) COSY and ( 1 H-13 C) HMQC experiments.The quaternary carbon atoms, C-3', C-5', C-3" and C-5" were assigned using an ( 1 H-13 C) HMBC experiment.A NOESY experiment shows only one clear cross peak between the methyl groups at 2.00 and 2.07 ppm.Since these peaks belong to the same substituent, we assign them to the substituents at positions 1, 4, 5, 8 because the 3'-methyl at 2.00 (up) is close to the 5'-methyl at 2.07 ppm (down) in the peri positions.The assignment of the nitrogen atoms to pyrazoles at positions 1 and 2 of the naphthalene ring was based on ( 1 H-15 N) HMBC experiments; for OPzN the experiment was performed at 320 K.The assignment of N-1 and N-2 for each pyrazole was made by analogy with the chemical shifts of 1-phenylpyrazole [8].The results are reported in Table 1.
The fact that both OPzN and OMe2PzN show only a family of signals in 1 H, 13 C and 15 N NMR is in contrast of what we have found in hexakis(pyrazol-1-yl) and hexakis(3,5-dimethylpyrazol-1yl) benzenes [5][6][7] were several conformers are generally seen.Although the aim of this paper is not related to the study of the conformational aspects of per(pyrazol-1-yl)naphthalenes, the simplicity of the 1 H and 13 C NMR spectra is indicative, in the case of OPzN, of a freedom of rotation of the eight residues and in the case of OMe2PzN of a conformation with the highest symmetry like the udududud one.
udududud ud Scheme 3 In the case of 1,8-bis(1,2,3-triazol-1-yl)naphthalene it has been demonstrated by X-ray crystallography that the substituents adopt an ud conformation [9,10].Absorption spectra for OPzN and OMe2PzN in ethanol and ethyl acetate are shown in Figures 2  and 3, respectively.OPzN exhibits an absorption band centered at 280 nm which corresponds to naphthalene (this compound displays a very structured absorption band centered at this wavelength [11]), with a red shifted shoulder around 325 nm probably as a consequence of the substitution in the aromatic ring.In the OMe2PzN spectra the main absorption is centered around 294 nm with an absorption tail which extends above 400 nm.Fluorescence spectra of both OPzN and OMe2PzN in the same solvents are also shown in Figures 2 and 3. When OPzN in ethanol is excited at 325 nm it presents a broad structured emission with maxima centered at 360 nm and 385 nm while in ethyl acetate it displays a red shifted broad and structureless fluorescence band centered around 410 nm.Again the OMe2PzN exhibits only a red shifted structureless emission band around 509 nm in both solvents.

Absorption and Emission Spectra
Examination of the spectra of differently substituted derivatives of naphthalene indicates that when substitution increases, both red shift and broadening of the vibrational fine structure of the absorption and emission bands occurred, probably as a consequence of the loss of planarity in the naphthalene chromophore as well as the increase in rotational and conformational structures of these substituted derivatives [3,11].It can be concluded that substitution on naphthalene shifts

Conclusions
Absorption spectra of OPzN and OMe2PzN (in EtOH and EtOAc) present two bands each, around 280 and 325 nm and 294 and 350 nm, respectively.Emission spectra in EtOAc for both compounds exhibit a broad, structureless blue fluorescence centered at around 410 nm for OPzN and 509 nm for OMe2PzN, naphthalene itself having a very structured emission beginning at 320 nm.Pyrazolyl and 3,5-dimethylpyrazolyl substitution on naphthalene shifts the fluorescence of these derivatives OPzN and OMe2PzN to longer wavelengths, therefore fulfilling the preliminary conditions to study their electrogenerated chemiluminescence properties.To go further on, more soluble compounds have to be prepared, for instance, using pyrazoles with long alkyl chains at position 4.

Figure 2 .Figure 3 .
Figure 2. Absorption and emission spectra of OPzN in (a) ethanol and (b) in ethyl acetate.

Table 1 .
Continued a At 320 K For the assignment of the 13 C NMR signals to the quaternary naphthalene ring atoms C-1 and C-2 (
 ARKAT USA, Inc Incthe absorption and fluorescence bands of these derivatives to longer wavelengths, indicating that substitution has a greater effect than the nature of the substituent itself in these highly substituted naphthalene derivatives.Melting points were determined in a microscope hot stage apparatus and on a SEIKO DSC 220C instrument.Column chromatography was performed on silica gel Merck 60 (70-230 mesh) using the appropriate eluent.1Hand 13 C-NMR spectra in solution were recorded on a Bruker DRX 400 at 400.13 and 100.62 MHz, respectively.The assignments are based on NMR experiments with pulse field gradients: ( 1 H-1 H) COSY, ( 1 H-13 C) HMQC, ( 1 H-13 C) HMBC and ( 1 H-15 N) HMBC.Chemical shifts (δ) are given from TMS for 1 H and 13 C NMR and from external (capillary) nitromethane for 15 N NMR.Infrared spectra were recorded on a Philips PU 9714 spectrometer and mass spectra were performed on a VG AutoSpec spectrometer using fast-atom bombardment (FAB+) and matrix meta-nitrobenzyl alcohol (NBA).