J. Phys. Chem. 1996, 100, 19315-19320
19315
Photochromism and Thermochromism Driven by Intramolecular Proton Transfer in Dinitrobenzylpyridine Compounds A. Corval,† K. Kuldova´ ,† Y. Eichen,‡,§ Z. Pikramenou,‡,| J. M. Lehn,‡ and H. P. Trommsdorff*,† Laboratoire de Spectrome´ trie Physique, UniVersite´ J. Fourier Grenoble 1, CNRS (UMR 5588) BP 87, 38402 St Martin d’He` res Cedex, France, and Laboratoire de Chimie Supramole´ culaire, Institut Le Bel, UniVersite´ Louis Pasteur, 4 rue Blaise Pascal, 67000 Strasbourg, France ReceiVed: August 13, 1996; In Final Form: September 26, 1996X
The photoinduced proton transfer reaction taking place in 2-(2′,4′-dinitrobenzyl)pyridine (R-DNBP) and in some of its derivatives is characterized by IR, visible, and NMR spectroscopy. The enamine “NH” structure of the blue phototautomer is confirmed by the analysis of the IR spectra of R-DNBP and its deuterated analogue. 2D NOESY 1H NMR data indicate that this tautomer is predominantly in the cis configuration. In the 2-(2′,4′-dinitrobenzyl)phenanthroline derivative, the stabilization of the phototautomer is sufficient to make it thermally accessible. A quantitative analysis of the resulting thermochromism indicates that in toluene solutions the ground state energy of the “NH” form is lowered to 2.9 kcal mol-1 above the thermodynamically stable “CH” tautomer compared to more than 8 kcal mol-1 in the parent R-DNBP compound.
I. Introduction 2-(2′,4′-Dinitrobenzyl)pyridine (R-DNBP, 1), known since
1925, undergoes a photochromic process.1 This process results from an intramolecular proton transfer (PT) after irradiation in the first electronic transition in the near-UV spectral region. The photogenerated metastable tautomer absorbs in the visible (λmax ≈ 550 nm) and is relatively long-lived. For instance, at 298 K the lifetime is 4.7 s in ethanol2 and about 4.6 h in the crystal.3,4 These compounds have been extensively studied in †
Universite´ J. Fourier Grenoble 1. Universite´ Louis Pasteur. Present address: Department of Chemistry, Technion-Israel Institute of Technology, Technion City, Haifa 32000, Israel. | Present address: Department of Chemistry, University of Edinburgh, King’s Buildings, West Mains Rd, Edinburgh EH9 3JJ, UK. X Abstract published in AdVance ACS Abstracts, November 1, 1996.
solution and in the solid state in order to characterize the reaction mechanism and the structure of the tautomer. The proposed pathways leading to the phototautomers and the structure of the different tautomers are presented in Scheme 1. Recent interest in such photochromic compounds is motivated by potential applications as optically bistable systems for optical data storage. Applications of this kind require the optimization of the efficiency of the photoreaction and the control of the lifetime of the metastable form. In view of a “molecular engineering” approach to this optimization, the reaction mechanism must be well understood. Regarding R-DNBP, different reaction pathways have been proposed in previous work, and even the structure of the blue phototautomer (PT state) is not unambiguously established. In this paper new spectroscopic data are presented that establish the structure of the colored tautomer and that lead to a quantitative evaluation of the conversion obtained. An analysis of the di-deuterated derivative 2 (deuterated in the benzylic position) and the N-methyl analogue 4 of the “NH” form of 1 confirms the assignment of the phototautomer as the enamine “NH” form 3. 2D NOESY 1H NMR data of 5, the N-benzyl analogue of 4 serving as a model of the “NH” tautomer, are used to determine its dominant configuration. We also report the thermochromism of 2-(2′,4′-dinitrobenzyl)1,10-phenanthroline 6. This compound had been especially designed and synthesized with the objective of stabilizing the PT state, and the lifetime of the “NH” tautomer in room temperature toluene solutions was indeed found to be increased by a factor of 5 × 103 compared to R-DNBP. This stabilization, which was attributed to hydrogen bonding between the transferred proton and the additional adjacent pyridine nitrogen,2 is sufficient to make the PT state thermally accessible so that this compound is also thermochromic. Quantitative studies of this thermochromism enable the direct measurement of the energy difference between the imine and enamine tautomers as well as of the extinction coefficient of the unstable “NH” tautomer.
‡ §
S0022-3654(96)02433-1 CCC: $12.00
II. Experimental Section Material. R-DNBP 1 (Lancaster) was used after careful purification using column chromatography and recrystallization © 1996 American Chemical Society
19316 J. Phys. Chem., Vol. 100, No. 50, 1996 SCHEME 1
SCHEME 2
from ethanol. The preparation of 2-(2′,4′-dinitrobenzyl)-1,10phenanthroline 62, N-methylenamine 4, and N-benzylenamine 5 was reported elsewhere.5 Deuteration of R-DNBP 1. Two drops of dry triethylamine and a 20-fold molar excess of ethanol-d1 were added to a 10 mL solution of 1 g of 1 in dry THF. The solution was stirred under an argon atmosphere at 50 °C for 1 week. Solvents were then removed under reduced pressure, and the product was recrystallized from ethanol-d1. NMR characterization showed 92-95% deuteration exclusively at the benzylic position. For the studies of photochromism, methanol and ethanol were dried by standard methods. For the study of the thermochromism, solvents (toluene, dichloromethane, and acetonitrile, Normapur, for analysis) were used without further purification. Toluene-d8 (Aldrich) was stored over dried molecular sieves. Infrared Spectra. IR spectra were recorded with a Bruker IFS 25 Fourier transform spectrometer. The concentration of the samples was about 2% in NaCl or KBr pellets. An appropriately filtered (250-380 nm band-pass) 150 W xenon lamp was used for the irradiation of the pellets (typical irradiation time: 4 min). UV-Visible Spectra. The UV-visible absorption spectra were recorded with a Perkin-Elmer λ9 spectrophotometer. A home-built cryostat, operating in the temperature range 100340 K, was used for the study of the thermochromism. NMR Spectra. 1H NMR spectra were recorded on a 400 MHz Bruker instrument. Temperature dependent measurements were performed on a 200 MHz Bruker instrument. III. Structure of Phototautomer III.1. Review of Previous Structural Assignments. Many o-nitro aromatic compounds are known to exhibit photochromism resulting from an intramolecular proton transfer.6,7 A necessary requirement for the observation of this property is the presence of a nitro group ortho to a benzylic hydrogen.6,8-10 Evidence for the photoinduced intramolecular abstraction of a benzyl hydrogen by the o-nitro group has been obtained by
Corval et al. various spectroscopic techniques such as NMR and IR.11 On the basis of transient absorption measurements, it has been proposed that the PT occurs in the excited state, producing a colored aci-nitro structure that exists in equilibrium with its deprotonated anionic form6,7,10,12 (Scheme 2). This equilibrium was suggested to be solvent dependent, shifting toward the acinitro form in nonpolar solvents and toward the deprotonated form in polar solvents. When one of the benzylic hydrogen atoms is replaced by a pyridyl group, a subsequent PT from the o-nitro group to the heterocyclic nitrogen may be envisioned, since in solution the protonated nitro group is more acidic than the protonated pyridine group. It was also demonstrated that the reprotonation of the benzylic carbon, being too slow with respect to the protonation of the pyridine ring, does not occur on this time scale. In R-DNBP 1, which is photochromic in both the solid state and in solutions, the photogenerated metastable blue form was therefore assigned to the aci-nitro form13,14 or to the enamine “NH” form.1,8 Previous absorption measurements in low-temperature solutions8,14 as well as IR spectra of irradiated R-DNBP in KBr pellets15 did not lead to an unambiguous conclusion regarding the nature of the colored form. An aci-nitro structure (or an anionic one, depending on solvent and on pH) of the colored form was proposed for the colored form of R-DNBP in solution. This assignment was based on the analogy with the photochromism of 2,4-dinitrotoluene16 and on molecular orbital calculation using PPP semiempirical SCF ASMO CI,17 MOLCAO, and PPP methods.18,19 Klemm et al.,20,21 on the other hand, assigned the long-lived species absorbing at 550 nm, observed even in nonpolar solvents after nanosecond flash photolysis, to the “NH” form because an analogous transient is not detected in the photochromism of tetranitrophenylmethanes under similar conditions. Further support of this assignment comes from the similarity of the visible absorption of the blue form of R-DNBP with the spectrum of the N-methylenamine 4.22 More recently, two bands at 1303 and 1635 cm-1 observed in time-resolved resonance Raman spectroscopy were considered to be characteristic of the blue form. The 1635 cm-1 band was tentatively assigned to the CdC stretching of a quinoid or azamerocyanin form.23 A comparison between the resonance Raman spectra of irradiated R- and γ-DNBP (4-(2′,4′-dinitrobenzyl)pyridine) in methanol and the corresponding Nmethylenamines, as well as with molecules having, respectively, the structure of the quinoid form and of the aci-nitro form,24 led also to the conclusion that the metastable blue form corresponds to the “NH” form, thus supporting earlier reports. To conclude this short review, we note that even though the most recent results favor the “NH” structure of the PT form, further experimental evidence is desirable in order to firmly establish this assignment. The electronic absorption spectra of irradiated R-DNBP in ethanol at 173 K and of the reference compound 4 are shown in Figure 1. Irradiation of R-DNBP was performed at low temperature because of the relatively short lifetime (e5 s) of the colored form at room temperature. The similarity of the absorption spectra of these two species strongly favors the assignment of the phototautomer to the “NH” form. III.2. Reexamination of the IR Spectra of the PT Form. The ambiguity of previous structural assignments of the PT form of R-DNBP15 by IR spectroscopy was mainly due to the very low conversion to the PT form (1-4%) that could be obtained in these experiments. The IR spectra of R-DNBP presented here are obtained for samples where the conversion is higher
Photochromism and Thermochromism
J. Phys. Chem., Vol. 100, No. 50, 1996 19317
Figure 2. Comparison of the IR spectrum of 4 (top) with the spectra recorded for R-DNBP 1 after and before irradiation in the near-UV (bottom). Figure 1. Absorption spectra of compounds 1, 4, and 6 in alcohol solutions, at room temperature (4 and 6) and 173 K (1). The continuous and dotted lines correspond, respectively, to spectra taken before and after irradiation in the near-UV.
by at least 1 order of magnitude (30-40%). Additional support for the present assignments comes from a comparison with the IR spectra of compound 4, having an electronic structure similar to that of the “NH” form of R-DNBP, and with the spectra of the deuterated analogue of R-DNBP 2. Infrared spectra of irradiated and nonirradiated samples of R-DNBP were recorded in NaCl and KBr pellets. The spectra are very similar, and only those in NaCl pellets are shown (Figures 2 and 3). The samples, irradiated either as pellets or, prior to compression, in powder form in order to optimize the conversion, exhibit the characteristic deep blue color. The principal lines that can be attributed to the blue form are observed at the following frequencies (values in cm-1): (a) 3388; (b) 1558; (c) 1290; (d) 1165; (e) 1121. The major change in the IR spectra, already previously reported,15 is the appearance of a strong absorption band at 1290 cm-1 in the irradiated sample. The expected decrease upon irradiation in the -CH2- absorption band at ∼2860 cm-1 is not detected and may be below our detection limit. Such a disappearance of the C-H stretching bands around 3000 cm-1 has been observed in a chemically produced blue PT form of γ-DNBP25 but has never been reported for the photoinduced or thermally produced blue form in mixtures presumably because of the low concentration of the metastable species. Most notable is the appearance of the new line (a) at 3388 cm-1, i.e., in the O-H or N-H stretching region, which increases in intensity with irradiation time. This band was previously assigned to either the “NH” form or a “OH” structure.15 Since the characteristic frequencies of O-H and N-H groups are known to be (within 100 cm-1) 3680 and 3350 cm-1, respectively,26 the assignment of line (a) to a “NH” structure of the PT form is favored. The broad absorption bands characteristic of the nitro groups at ∼1350 and ∼1520 cm-1 are unchanged in going to the blue form. The chemical nature of the nitro groups is therefore not significantly altered in the photoproduct. In addition to the appearance of the new bands cited above, the band at 1567 cm-1, attributed to the sCdNs group15 and therefore not present in the IR spectrum of compound 4, decreases in intensity upon irradiation (Figure 4). This is the first observation of such a decrease, a decrease that is expected for a proton transfer to the pyridine nitrogen. Another interesting feature in the IR spectrum of compound 4 (Figure 2) is the presence of an intense band at 1290 cm-1 at the same position as line c in the PT form, which is absent in
Figure 3. IR spectra of R-DNBP 1 in NaCl pellets before and after irradiation in the near-UV.
Figure 4. IR spectra of R-DNBP 1 in the region of the sCdNs modes of pyridine before and after irradiation in the near-UV.
the nonirradiated R-DNBP sample. Lines d and e lie in the frequency range of the C-H bending vibrations and are also visible in 4. Another band, common to the IR spectra of the irradiated sample and of compound 4, is observed at 1635 cm-1 (marked with a star in Figure 2) and has previously been considered to be characteristic of the quaternary ammonium Nδ+ structure of azamerocyanines22 (see, however, discussion below). Irradiation of the deuterated analogue of R-DNBP 2 leads to the appearance of three new bands at 2513, 1540, and 1283 cm-1. These lines correspond to the a, b, and c lines observed in the blue form of protonated R-DNBP. The isotope shift of line a from 3388 cm-1 in R-DNBP to 2513 cm-1 in d2-R-DNBP has a magnitude expected for the N-H (N-D) vibration. The ensemble of these observations supports the assignment of the blue form of R-DNBP to the “NH” configuration of 3. Irradiation of the phenanthroline derivative 6 does not lead to noticeable changes in the IR spectra, neither in KBr and NaCl pellets nor in Nujol suspension. The blue color is visible in KBr pellets, but the concentration of the blue form is below our detection limit. This compound is believed to crystallize
19318 J. Phys. Chem., Vol. 100, No. 50, 1996 in two or more polymorphs of which only the less abundant is photochromic. Polymorphism and the coexistence of photochromic and nonphotochromic phases have been observed for a number of other DNBP derivatives.27 The assignment of the blue PT form of 6 to the “NH” form is nevertheless plausible because of the great similarity of the visible absorption spectra of the blue forms of R-DNBP and of 6 in solution (Figure 1). III.3. Electronic Structure of the “NH” Form. The present assignment by IR of the metastable phototautomer of R-DNBP to the “NH” structure confirms the assignment proposed by Takahashi based on resonance Raman spectroscopy.24 Two mesomeric configurations can be envisioned for the “NH” tautomer: a quinoid structure such as the one shown for compounds 3-5; a zwitterionic structure with a positive charge at the pyridine nitrogen and a negative charge at the benzylic carbon atom or at one of the nitro groups. It has been argued by Klemm and Klemm that a purely quinoid structure (sHCdC
