Quantum yields of photocoloration and molar absorption coefficients of ferrocenyl substituted benzo and dibenzochromenes . Comparison with their phenyl-homologues

The photochromic properties of three ferrocenyl-[2H]-benzopyrans were investigated under monochromatic irradiation and compared with those of their phenyl homologues. The UV/visible spectra of the closed and open forms are reported together with the quantum yields of photocoloration. It was shown that the ferrocenylsubstitution induces the formation of a new band in the 500–700 nm range in the open forms, however, it does not affect significantly the UV spectra of the closed forms. Ferrocenylsubstitution was also shown to increase the thermal bleaching rate constants and to decrease the photocoloration quantum yields. For most compounds, photochromic behaviour was not sensitive to the irradiation wavelength. However, for the methyl ferrocenyl chromene, the open form spectrum was slightly dependent of the irradiation wavelength. The influence of the ferrocenyl group and other structural features on the photochromic properties are discussed.


INTRODUCTION
[2H]-benzopyrans (elsewhere called 2H-chromenes) are photochromic compounds.They undergo a photocleavage of the C − O bond leading to a photomerocyanine form often referred as the open form (OF) (Scheme 1) [1].Due to electronic delocalization, the absorption range of the open form is shifted towards longer wavelengths, i.e. from the UV to the visible region.The thermally reversible closure of the photomerocyanine gives the starting [2H]-benzopyran which is often referred as the closed form (CF).
Their photochromic behaviour is characterized by the UV spectra of the closed and open forms (ε CF λ and ε OF λ ), by the thermal bleaching kinetic rate constants (k ) and by the value of the quantum yield of the reversible photocoloration (φ col ).These photochromic parameters are greatly influenced by the nature of the R 1 and R 2 groups placed on the Csp3 in the 2-position [2] and by the size of the aromatic moiety (benzo[h] or dibenzo[f , h] chromene).Moreover, the replacement of a phenyl group by a ferrocenyl one at the 2-position [3] leads to open forms with two λ max resulting in a strong absorption range extension in the visible region [4].

UV spectra of the closed forms.
Spectral analysis of the closed form [λ (nm), ε(L • mol −1 • cm −1 )] has been realized in toluene and the results are reported in Table 1 (see Figure 1 for the detailed UV spectra of 2a and 2b).
Table 1 shows that for each compound, there are several well-defined bands or shoulders that appear in the same narrow spectral range (1 st λ max : 315-317, 2 nd λ max : 326-333, 3 rd λ max : 340-345, 4 th λ max : 354-356, 5 th λ max : 374-375 nm).Spectra of the ferrocenyl substituted compounds 1a, 2a, 3a are not significantly different from those of their phenyl homologues 1b, 2b and 3b.On the other hand, phenanthrenocompounds 3a and 3b exhibit a characteristic strong band at 374-375 nm which appear to be specific for the phenanthreno-annellation.These results confirm that the pyranic moiety is responsible for the lower energy absorption bands of the closed form.Figure 1 shows the similarity between UV spectra of the closed forms of ferrocenyl-and phenyl-homologues.

Effect of the irradiation wavelength on UVvisible spectra of the open forms of compound 1a in methanol.
The photochromism of these compounds has been investigated in four solvents: acetonitrile (a), ethanol (e) or methanol (m) and toluene (t) using independently two monochromatic irradiation wavelengths (313 and 366 nm).Usually, they lead to the same distribution of open forms with the same UV/visible spectra.However, in the case of 1a in methanol, an unexpected effect has been observed.Both irradiation wavelengths lead to the formation of a broad spectrum exhibiting three bands at 387, 470 and 609 nm.But, when the irradiation is performed at 366 nm, the absorbances at 470 and 609 nm are smaller and the 387 nm band remains stable (i.e., there is no bleaching in the dark for this specific band) (Figure 2).Such behaviour was already observed under polychromatic irradiation.A possible explanation would be the presence of a wavelength and irradiation time dependent proportion of the various expected photoisomers CCC, CTT, CTC, TTC and TTT (Figure 3).They are able to undergo, either thermally or photochemically induced interconversions.If the thermal interconversion is slow, the photochemical interconversion prevails.Then, depending of their relative molar absorption coefficients at the UV irradiation wavelength, different distributions are likely to be established.The more energetic 313 nm irradiation could induce the partial photo-destruction of the long-lived photoisomer monitored at 387 nm letting it to accumulate under 366 nm irradiation.

Kinetic study of the thermal decay.
Under continuous monochromatic irradiation, a rather large proportion (from 42 to 95%) of the open forms was reached at steady state.As it has been shown with compound 1a in methanol, depending on the nature of the irradiated compound, the irradiation wavelength, the photon flux, the solvent and the duration of the irradiation, several isomers are likely to be present.After the irradiation has been shut down, the system undergoes a thermal bleaching, however this process is not 100% reversible, a certain amount of the coloured species (hereby specified as long-lived species) remains relatively stable within the time-scale of observation and do not participate to the mono-exponential decay that has been recorded for all the compounds.Results are gathered in Table 2. Within the time-scale of our kinetic technique (i.e., 0.1 s to 10000 s), a large amplitude thermal decay has been recorded for all compounds under investigation.These kinetics were characterised by a first-order rate constant k .Lower k values are for compounds 2a and 2b in toluene, while higher k is in the more polar solvent acetonitrile (1b).Our measurements show also that the replacement of the phenyl group by a ferrocene increases the thermal bleaching: k (2a) > k (2b) and k (3a) > k (3b).The same effect is observed when a methyl group replaces a phenyl one: k (1b) > k (2b) and when a phenanthrene nucleus replaces a naphthalene one: k (3b) > k (2b) and k (3a There is a slight irradiation wavelength effect on the percentage of stable long-lived species formed after irradiation.The origin of this effect is likely to be the same than for compound 1a in methanol solution (i.e., a variable distribution of several possible photoisomers).However, after standing for a long time in the dark (several hours or days), a slow thermal bleaching occurs [7].This spontaneous long-time decay was never complete, ferrocenyl compounds 1a, 2a and 3a appeared always to contain, in toluene solution and at room temperature 1, 4.8, and 10% of the open forms, respectively.This long-term stable proportion was less important for the phenyl-homologues, respectively 0.4, 0.8 and 1% for 1b, 2b, and 3b.The presence of such small quantities of stable open form is likely to be related to a thermal equilibrium between the closed and open stereoisomers (i.e., a slight thermochromic effect).

Kinetic modelling of the Absorbance vs time curves under continuous monochromatic irradiation.
From kinetic modelling of Absorbance vs time photokinetic curves [8], quantitative UV-visible spectra of the reversible open forms (lifetime < 1000 s) and quantum yields of photocoloration can be extracted.The molar absorption coefficients of the reversible open forms (OF) that have been obtained after kinetic modelling are gathered on the Table 3. Figure 4 presents the calculated spectra of compounds 1b, 2a, 2b and 3a as they have been obtained from the kinetic modelling.For ferrocenyl-containing compounds 2a and 3a, the main feature is the presence  of a specific band within the 580-610 nm range [5].For compounds 2a and 2b, another characteristics is the presence on the lower wavelength part of the spectra of a small "pattern" or less similar to the UV spectra of the corresponding closed form, but redshifted by ≈ 33 kJ • mol −1 in 2a and ≈ 41 kJ • mol −1 in 2b.It is likely that this pattern would be related to the 12 electrons system of the vinyl-naphthalene moiety encountered either in the closed or open forms.Moreover, as expected, replacement of a methyl by a phenyl increases the conjugation and hence the molar absorption coefficient (ε OF (2b) > ε OF (1b)); λ max is also red-shifted (λ max (2b) > λ max (1b)).This effect is no longer true for ferrocenyl-compounds 2a and 3a, where phenanthrenyl-(to naphthyl-) substitution induces a blue-shift (λ max (2a) > λ max (3a)) and decreases the molar absorption (ε OF (2a) > ε OF (3a)).

Determination of the quantum yields of the reversible photocoloration process
It was assumed that these quantum yields were not irradiation wavelength sensitive for 313 and 366 nm (Table 4).This hypothesis was justified because the values that have been extracted independently from the two irradiation wavelength were not significantly different within our experimental accuracy.Values given have been obtained by averaging data from 313 and 366 nm irradiation.Quantum yield for compound 3b is not given because it cannot be extracted with a sufficient precision due to lack of convergence in the iterative calculation.Ferrocenyl-compounds 2a and 3a exhibit low quantum yields.For compound 2a and 2b we have obtained (φ(2b) ≈ 4 × φ(2a)).A possible explanation for this weak photoreactivity would be a deactivation of the excited state (either thermally, by fluorescence or by intersystem crossing to a low reactive triplet intermediate) due to the presence of the ferrocene group.Quantum yields are also solvent sensitive, being higher in the less polar solvent.This effect could be interpreted by the presence of a non-reactive short lived state (triplet or transient photoisomer) giving rise to deactivation in the more polar solvent as already described for spiropyrans systems [9].φ col also increases slightly when a phenanthrene nucleus replaces a naph- thalene one φ col (3a) > φ col (2a).As the exact molar absorption coefficient of the photoisomers responsible for the non-reversible photocoloration (Figure 2) was not known, the corresponding quantum yields were not determined.
The quantum yield of photocoloration φ col and the thermal fading rate constant k can be correlated to the experimental photosteady state absorbance (Abs PSS ).Highly colourable compound 2b (Abs PSS (470) ≈ 1.9) exhibits a high φ col with a low k .On the other side, weakly colourable compound 3a (Abs PSS (582) ≈ 0.45) has a low photocoloration quantum yield and a high thermal fading rate constant.In between, lie medium colourable compounds 1b and 2a (Abs PSS (λ max ) ≈ 0.9-1.1).The origin of such behaviour can be understood from the photosteady state equation, which shows that Abs PSS is an increasing function of the ratio φ col /k .The coloration of the solution is proportional to the open form concentration ([OF]) [10].Then at the photosteady state (PSS), eq.(1) (derived from eq. (3) = 0) holds.It shows that [OF] PSS is proportional to the ratio φ col /k :

CONCLUSION
Kinetic modelling of Absorbance vs time curves recorded under monochromatic irradiation of a series of ferrocenyl-benzopyrans and their phenyl-homologues has provided interesting results about their quantitative spectral, kinetic and photochromic properties.It has been shown that the UV spectra of the closed forms was not sensitive to the presence of the ferrocenyl-group at the 2-position.Kinetic measurements have also shown that the rate constant of thermal fading was higher with ferrocenyl compounds than with their phenyl-homologues.However, an unexpected stable open form has been observed with the methyl ferrocenyl-compound 1a in methanol solution.A wavelength and solvent dependent long-lived photoisomer distribution has been assumed.
The molar absorption coefficients of the reversible open forms and the corresponding quantum yields of photocoloration have been determined.The presence of ferrocenyl-group in the 2-position of the benzopyrans decreases the quantum yield but it leads to a new visible absorption band in the 550-700 nm range.The photocoloration behaviour can be quantitatively correlated to their photochromic parameters.

Photochromic solutions.
The photochromic solutions were obtained by dissolving the corresponding chromenes in spectroscopic grade anhydrous solvents.All the concentrations were in the 3 × 10 −4 mol • L −1 range.

Monochromatic irradiations.
The monochromatic irradiation was derived from a filtered 200 W high pressure mercury lamp using a liquid optical fibre able to deliver around ≈ 130 W • m −2 in the 300-400 nm mercury UV lines with a 10 nm bandwidth.Potassium ferrioxalate was used to determine the photon flux with a precision better than 5%.at 313 nm: The photon flux was checked periodically using a home-made semiconductor light sensor.The photochemical reactor was a 1 cm × 1 cm thermostated and stirred quartz cell placed in the compartment of a HP8451A diode array spectrophotometer.Several wavelengths including the irradiation one were monitored simultaneously.The data as Abs vs time photokinetic curves were stored and processed using a home-made software.

Thermal relaxation. Thermal decay occurred
after the irradiation photon flux was shut down.Absorbance vs time were recorded during this process.The thermal bleaching constants k at 25 • C were calculated using monoexponential eq. ( 2) [11][12][13] Abs In this equation Abs t is the absorbance measured at a time t, Abs o is the absorbance measured at time 0 and Abs PSS is the absorbance at the photo-steady state.Short irradiation times (less than 300 s) were selected in order to minimise photodegradation and long-lived species formation.

Photocoloration quantum yields and molar absorption coefficients of the transient OF.
In or- der to extract the photochromic parameters (φ col and ε OF ), experimental photokinetic curves (Abs vs t) were fitted using a kinetic modelling procedure.The model was derived from the simplest photochromic mechanism: CF → OF (hν); OF → CF ( ) which gives rise to the differential and phenomenological equations: ε CF is taken at the irradiation wavelength, F is the photokinetic factor [8,11], F = (1 − 10 −Abs )/Abs and [C] 0 is the total concentration of the photochromic compound.
The calculated Abs values were obtained by numerical integration using "guessed" values of the unknown parameters, then they were fitted on the experimental ones using a minimisation algorithm able to adjust the initial "guessed" values of the parameters at their optimum.At the end of the fitting procedure, the estimated values of the parameters were delivered.The uniqueness of the solution was tested by showing that whatever the initial values of the guessed parameters calculation arrived to the same minimum at convergence.

Scheme 2 .
Scheme 1. Ring opening and ring closure of benzo-chromenes under irradiation.

Figure 1 .Figure 2 .
Figure1.Spectra of typical closed forms in toluene (compounds 2a (ferrocenyl-) and phenyl-homologue 2b).Note the presence on the two homologue compounds of the similar intensity and wavelength range for the four spectral bands.

Figure 3 .
Figure 3. Main stereoisomers of the open form.

Figure 4 .
Figure 4. UV-visible spectra of the reversible open forms of compounds 1b, 2a, 2b and 3a in toluene solution.

Table 1 .
UV spectra of compounds

Table 2 .
% (after irradiation) of long-lived isomers of the open form and thermal bleaching rate constant k for compounds 1-3 (both monitored at λ obs ) ( T = 298K).

Table 3 .
Molar absorption coefficients of the transient open forms (OF).

Table 4 .
Photocoloration quantum yields of various compounds in various solvents.