Photochromic properties of six 5O-n-alkyl , 6 ′-CN substituted spironaphthoxazines

The photochromic properties of a series of six 5-O-n-alkyl, 6′-CN substituted spironaphthoxazines has been investigated in toluene and methanol solution. The 6′-CN and 5-O-n-alkyl substitutions lead to a bathochromic shift of the UV spectra of the closed forms. The λmax of the visible spectra of the open forms decrease with solvent polarity. Most of the photochromic parameters have been quantitatively determined. Among them, photocoloration and photobleaching quantum yields and molar absorption coefficients of the open forms have been derived from a numerical fitting of the photokinetic curves recorded under continuous monochromatic irradiation. Results show that the photo-steady-state properties are relatively independent of the 5-O-n-alkyl chain length, but are more sensitive to the size of the indolinic nitrogen substituent.


INTRODUCTION
It is well known that photochromic molecules undergo reversible rearrangements between two main states characterised by strongly differing spectral parameters [1].For the purpose of practical applications, such as variable transmission glasses [2] and future optochemical memories [3], it is possible to identify among the numerous organic photochromic substances those exhibiting the required properties [4].Once a photochromic series has been elected, its basic features can be modulated by playing with the nature and the position of a large variety of substituents [5].
Spirooxazines represent one of the most important photochromic series [6,7].Their photochromism can be understood by the presence of a photo-thermal equilibrium between the "closed" spiro isomer and the "open" planar merocyanine (Scheme 1).
Spirooxazines have a greater photostability than the more widely studied structurally similar spiropyrans.It has been shown recently that their photochromic and solvatochromic properties are very sensitive to the donor/acceptor character and the position of their substituents [8].So, a spirooxazine with an acceptor substituent on the naphthoxazine side will be called: pull-type, while a spirooxazine with a donor substituent on the indoline side will be called push-type.In the same way, a donor on the indoline and an acceptor on the naphthoxazine gives rise to a push-pull compound [9].Despite the great number of studies devoted to spirooxazines, information concerning structure-photochromic properties relationships is only available for a restricted number of compounds [10,11].
The purpose of this paper is to investigate quantitatively the photochromic properties of six 5-O-n-alkyl, 6 -CN substituted spironaphthoxazines (push-pull).For the sake of comparison, one O-n-alkyl substituted compound (push-type), one 6 -CN substituted compound (pull-type) and the non-substituted spironaphthoxazine [12] (neutral) were also investigated in the same conditions.Table 1 gives the details of the substitutions for the whole set of molecules under investigation.

Spectral measurements. Absorption spectra
were recorded on thermostated Varian Cary 50, Varian Cary 100 and Hewlett-Packard 8452A spectrophotometers.Solvents were of the highest available spectroscopic grade (Acros Organic).

Photokinetic monitoring. Photokinetic data
(Abs vs. λ vs. t) matrices were recorded on an Ocean Optics fibre optic diode array spectrophotometer.The photochemical irradiation was derived at 90 • from a 200 W high-pressure, IR filtered, mercury lamp equipped with single-line (313, 365, or 546 nm) interference filters.The monochromatic light intensity was determined directly in the stirred reactor using either an acidic aqueous solution of potassium ferrioxalate Table 1.List of differently substituted spironaphthoxazines and their polar character.
or a fresh toluene solution of Aberchrom 540 (I 0 546 = (3.5 ± 0.1) The relative photon flux at the output of the irradiation fibre was checked by a home-made semi-conductor photosensor before and after each experiment.Experiments must be designed in order to reveal the effect of the direct and reverse quantum yields, i.e., using at least two different irradiation wavelengths λ and λ .These wavelengths must be selected so that ε B /ε A ≠ ε B /ε A as in Fischer's classical method [17].Moreover, to take into account the possible variations of the photokinetic factor, the evolution of the absorbance at the irradiation wavelength (Abs ) must be monitored continuously throughout the irradiation process.The photochemical reactor was a 1 cm × 1 cm optical path quartz cell containing 2 cm 3 solution, it was closed with a Teflon bung.The whole set-up was enclosed inside a thermostatic block (T = 283-323 K).

Data treatment.
The photokinetic system under consideration included three global processes: from which three equations can be established according to the photokinetic laws [18,19].
(1) where Φ AB = quantum yield of the A → B photochemical reaction, ε A = molar absorption coefficient of the photosensitive substrate A at the irradiation wavelength, l = optical path, I 0 = incident photon flux and F = photokinetic factor.
Their combination is the basis for the data treatment.An home-made software [20] was used for the numerical integration of the differential equation and parameter optimisation.Photochromic parameters were extracted during a numerical curve fitting procedure.To start the fitting procedure, all the unknown parameters are put at arbitrary values, then they are optimised by iteration until a minimum is reached between the experimental photokinetic curves and the model.Two quantum yields: Φ AB and Φ BA and three molar absorption coefficients of the open form: ε B (first irradiation wavelength), ε B (second irradiation wavelength) and ε B (λ max ) are to be determined.All the other parameters, such as the closed form spectrum (ε A vs. λ) and the thermal bleaching rate constant (k BA ) are known from independent measurements.The reliability of the solution is ensured by showing that whatever the initial starting values the same optimised parameters are obtained.

Analysis of the absorption spectra of the closed forms.
The spectra were recorded in toluene and methanol.Figure 1 shows that 6 -CN substitution leads to a bathochromic shift of the less energetic band (compounds 1 and 2 vs. 8 and 9).Among each group,    Complete results, gathered in Table 2, confirm the 6 -CN bathochromic shift (λ max = 368-373 nm (1 to 7) vs. 319-347 nm (8 and 9).

closed form gives rise to an open form (Scheme 1).
There is a strong variation of the absorption spectrum in the visible region (Figure 2).From such records, the λ max of the open forms can be measured directly while specific photokinetic curves can be extracted.Leaving the solution in the dark allows thermal bleaching to be monitored.λ max and k BA are solvent sensitive.

Solvatochromic effect on the open form spectra.
Figure 3 shows that for push-pull compounds like 2, 5, and 6, there is a very significant linear correlation between the wavenumber ν max (B) and the Brooker's blue parameter (χ B ) [21].Such behaviour, where the λ max decreases with the solvent polarity is known as negative solvatochromism [22].On the other hand, neutral compound 9 was confirmed to exhibit positive solvatochromism by the existence of an excellent linear correlation of its ν max (B) with the Brooker's red parameter (χ R ) (not shown).

Kinetic study of the thermal bleaching.
Thermal bleaching occurs when a previously irradiated solution is kept in the dark.Careful data treatment of the thermal bleaching kinetics show that in all cases more than 98% of the decay can be interpreted accurately by a monoexponential curve (Figure 4).This result excludes the occurrence of aggregation process of the open forms in our experimental conditions [23].Activation energies of the thermal decays were determined from Arrhenius plots.Figure 5 shows the plot obtained in toluene and methanol during the investigation of compound 7.
The main results are gathered in Table 3.For all the molecules, the thermal bleaching rate constant k BA is solvent sensitive.On the other hand, all the pushpull compounds (2 to 7) give rise to similar activation energies without any significant solvent effect.For pulltype compound 1, E a decreases with solvent polarity, while it increases with push-type and neutral molecule 8 and 9.These reverse effects are compensated on pushpull molecules.

Determination of the quantum yields of photoreactions and absorption coefficients of the open form.
Because the photoconversion rate (i.e., the per- centage of open form) is not known, neither the molar absorption coefficient (ε B ) nor the photocoloration and photobleaching quantum yields (Φ AB and Φ BA ) can be reached directly.A quantitative kinetic analysis of the photokinetic curves recorded under continuous monochromatic irradiation is needed.This analysis involves a curve fitting procedure based on differential kinetic equation established from the macroscopic mechanism of the reaction.Several photokinetic curves were recorded using three different irradiation wavelengths successively, two in the UV region: 313 and 365 nm and one in the visible region: 546 nm.In each experiment, three wavelengths were monitored simultaneously: one corresponds to the irradiation wavelength; the two others are close to the λ max of the open form.For each experiments, nine photokinetic curves need to be fitted simultaneously (Figure 6).
After the fitting procedure has succeeded, photochromic parameters such as quantum yields and molar absorption coefficients of open form are delivered.They are gathered in Table 4.
The results show that all the push-pull molecules (2 to 7) behave very similarly with a photocoloration quantum yield ranging from 0.11 to 0.135 and a molar absorption coefficient from 60800 to 67700.On the other side, push-type 8 and neutral 9 compounds are more difficult to investigate due to their faster thermal decay [24,25].Higher photocoloration quantum yield, but lower molar absorption coefficient is observed for the pull-type compound 1.

Structure-properties relationship: effect of the CN, O-alkyl and N-alkyl substitutions.
From the careful examination of Table 1 some structural similarities can be seen.Compounds 2 to 7 are push-pull type molecules, while 1 is pull-type, 8 push-type and 9 neutral.These structural features are reflected by their  kinetic and spectral parameters.This structure effect can be illustrated by plotting the activation energies in methanol vs. the activation energies in toluene E a (met) vs. E a (tol) (Figure 7).
5-O-n-Alkyl, 6 -CN substituted push-pull compounds 2, 3, 4, 5, 6, and 7 behave similarly while pull-type, push-type and neutral compounds 1, 8, and 9 do not.Looking more carefully at the [2][3][4][5][6][7] cluster shows that the effect of the O-n-alkyl chain length from O-methyl to O-hexadecyl is negligible (sub-cluster [2, 3, 4, 5]).On the contrary, the effect of the substituents on the nitrogen atom of the indoline moiety seems to be more prominent: on the sub-cluster [2,6,7] it can be seen that the activation energy in methanol increases with the size of the nitrogen substituent.A possible explanation is related to the charge distribution within the open forms.Compounds 2 to 7 are expected to exhibit larger charge delocalisation as they bear a donor sub- stituent on the indoline moiety and an acceptor on oxazine.Such push-pull like molecules are known to give rise to zwitterionic merocyanine forms.These zwitterionic merocyanines are of interest because they are expected to self-aggregate in appropriate solvents [26].On the other hand, a donor substituent on the indoline moiety (push-type compound 8) or acceptor substituent on the oxazine side (pull-type compound 1) exhibit a lesser charge delocalisation.They are consequently qualified of "intermediate" in the diagram in Figure 7.In the case of unsubstituted spironaphthoxazine 9 (neutral), the corresponding merocyanine is mainly under a quinoïd form.
Such splitting into separate sub-sets would also appear by plotting several other photochromic properties [27], for instance ε B vs. Φ AB , k BA (tol) vs. k BA (met) or λ max (B) (tol) vs. k BA (tol).

CONCLUSION
5-O-n-alkyl, 6 -CN substituted spironaphthoxazines are characterised by very strong photochromic behaviour due to some favourable parameters.The bulky alkyl nitrogen substituent increases this feature.Small bleaching parameters (k BA and Φ BA ) and rather high coloration parameters (ε B and Φ AB ) are responsible of this effect.From the whole set of photochromic parameters an absolute classification of the set of 6 -CNsubstituted spirooxazines (1 to 7) was obtained.Considering the photo-steady-state colorability in toluene solution, the following sequence was found: 7 ≈ 6 > 2 ≈ 3 ≈ 4 ≈ 5 > 1.It has been shown that a less than 10 −4 molar solution of push-pull compounds (2 to 7) in methanol solution can easily reach an intense blue colour (Abs ≈ 2 in a 1 cm cell) under 365 nm irradiation (filtered high pressure 200 w mercury arc).Another very interesting property is that their open form has a zwitterionic structure giving a chance for future observation of self-aggregation.

5 -
OCH 3 substitution (electron donating) leads to the appearance of a longer wavelength shoulder (compounds 2 and 8 vs. 1 and 9).

Figure 3 .
photochromic solution, the photoisomerisation of the

Figure 4 .
Figure 4. Variation of the absorbance during the thermal bleaching of compound 7 in toluene solution.The solid line corresponds to fitting (insert: the linearity of the same kinetics with an ordinate log scale shows its mono-exponential character).

Figure 5 .
Figure 5. Arrhenius plot for the thermal bleaching of compound 7 in methanol and toluene.

Figure 7 .
Figure 7. E a (met) vs. E a (tol) diagram showing the structure properties relationship of the various compounds under consideration.

Table 2 .
Spectral characteristics of the closed forms in toluene (tol) and methanol (met) solutions.