The synthesis and fluorescence behavior of a series of bis(trisilylalkyl)anthracene molecules is described. The photodegradation of these molecules under UV light has been monitored and compared to a commercially available fluorescent optical brightener. There is a relationship between the structure and the rate of photo decay. The materials with more bulky substituents exhibit the greater stability towards UV. For bis(triphenylsilyl)anthracene the photostability appears to be comparable with a commercially available optical brightener, but the molecule may be susceptible to thermal decay.
The development of environmentally friendly routes towards improved crop production is a goal of global concern. One particularly attractive proposition studied by a number of groups including the team at Reading, is the development of spectral filters in polyethylene cladding used for commercially grown protected crops. For example, a reduction in plant height has been observed in a range of species under polyethylene films containing a far-red blocking dye, thereby, offering an alternative method of producing compact high quality plants without using chemical growth regulators [
In this paper we extend our studies into modifying the film’s absorption in the short wavelength end of the visible spectrum. It has been shown that UV blocking films may provide an environmentally friendly way of reducing pests and disease; for example, it has been found that whitefly incidence is considerably reduced for crops grown under such films [
A number of reports of the preparation of bis(alkylsilyl)anthracenes appear in the literature, particularly the bis(trimethylsilyl)anthracene [
Synthetic route to 9,10-disilylanthracene.
The absorption spectra of the 9,10-bis(trialkyl/triphenylsilyl)anthracenes showed a distinctive three-peak system as shown in Figure
Absorption characteristics for 9,10-bis(trialkyl/triphenylsilyl)anthracenes.
Functional alkyl/aryl group | Absorption |
---|---|
Methyla | 402 |
Ethyla | 406 |
Propyla | 407 |
Butyla | 408 |
Phenylb | 415 |
UV spectra of the bis(trialkyl/triphenylsilyl)anthracenes synthesized in this work.
The fluorescence spectra of the bis(triphenyl)anthracene is shown in Figure
UV/visible and fluorescence spectra for 9,10-bis(triphenylsilyl) anthracene.
9,10-Bis(trialkylsilyl/triphenylsilyl)anthracenes were investigated for their relative photostabilities by using the unfiltered radiation from a medium pressure mercury arc lamp (see Section
Summary of kinetic data for 9,10-bis(trialkyl/triphenylsilyl)anthracenes.
Functional alkyl/aryl group | Rate constant ( |
Half-life |
---|---|---|
Methyla |
|
744 |
Ethyla |
|
2059 |
Propyla |
|
2346 |
Butyla |
|
2461 |
Phenylb |
|
7702 |
In order to gain a better understanding of the degradation processes, an attempt was made to analyze the photoproducts, specifically for the ethyl-silyl0-substituted compound. Analysis of the photoproduct by mass spectrometry gave a parent ion of 438 amu (9,10-bis(triethylsilyl)anthracene has a molar mass of 406 amu). The additional mass is highly suggestive of endoperoxide formation at 9,10 positions [
The effect of oxygen on the photoprocess was further explored by the irradiation of 9,10-bis(triethylsilyl)anthracene under aerated and deaerated conditions. Oxygen was removed from the solution (hexane 1 mg/10 mL) by a freeze-pump-thaw/purging process (see Section
Kinetic measurements of 9,10-bis(triethylsilyl)anthracene in solution in the presence and absence of oxygen.
Rate constant ( |
Half-life |
|
---|---|---|
Absence of oxygen (in solution) |
|
1856.4 |
Presence of oxygen (in solution) |
|
1596 |
The photo degradation behavior of the anthracene chromophores was investigated with two photostabilizers, firstly 4-methyl-2,6-di-
Summary of kinetic data for 9,10-bis(trialkyl/triphenylsilyl)anthracenes with BHT.
Type of alkyl/aryl group | Rate constant (+BHT) ( |
Half-life |
---|---|---|
Methyl |
|
798 |
Ethyl |
|
2189 |
Propyl |
|
2388 |
Butyl |
|
2632 |
Phenyla |
|
103972 |
The significant increase in the photostability of the phenyl silyl anthracene is consistent with the report that bulky substituents, such as the triphenylsilyl groups, on the 9,10-positions of anthracenes produce a significant increase in fluorescence quantum yield [
A polymethyl methacrylate (PMMA) film was prepared by dissolving the polymer in dichloromethane (DCM), containing the 9,10-bis(triethylsilyl)anthracene, followed by casting on a glass plate and evaporating to dryness. A similar film was prepared under an argon atmosphere using a glove bag. The films were removed from glass plates once dried and placed under UV light, and their decay was measured at regular intervals; the results are shown in Table
Kinetic measurements of 9,10-bis(triethylsilyl)anthracene in films in the presence/absence of oxygen.
Rate constant ( |
Half-life ( |
|
---|---|---|
Absence of oxygen (PMMA film) |
|
83160 |
Presence of oxygen (PMMA film) |
|
59400 |
A polyacrylonitrile film was prepared by dissolving the polymer in DMF and incorporating 9,10-bis(triethylsilyl)anthracene, followed by evaporation. UV/visible and fluorescence spectra of the film were obtained before and after irradiation. UV irradiation of the film continued for 7 hours (Figure
Fluorescence spectra of polyacrylonitrile/9,10-bis(triethylsilyl)anthracene film (dashed line) before UV (solid line) after UV irradiation of 7 hours (25200 seconds).
Change of absorbance with time for 9,10-bis(triethylsilyl)anthracene in polyacrylonitrile film under UV light; first-order plots suggest a half-life of
A polyethylene (LDPE) film was prepared by dissolving the polymer in warm DCM and incorporating 9,10-bis(triethylsilyl)anthracene. The solution was cast onto a glass layer while the warm solvent evaporated. The UV/visible and fluorescence spectra of the film were obtained prior to irradiating with UV light and at various times after irradiation commenced. In this case, the irradiation of the film continued for 210 minutes till the UV/visible spectral readings became difficult to measure accurately, and at the end of 210-minute period both UV and fluorescence spectra were recorded and compared. As for the polyacrylonitrile film, the final fluorescence was much reduced and in line with expectations based on the decrease in absorbance. The results for the chromophore stability in the films are listed in Table
Stability of 9,10-bis(triethylsilyl)anthracene to UV light in polyacrylonitrile and in polyethylene.
Polymer film | Rate constant |
Half-life |
---|---|---|
Polyacrylonitrile |
|
19440 |
Polyethylene |
|
7050 |
On the basis of the photostabilities listed above, the stability of PMMA films containing either bis(triethylsilyl) or bis(triphenylsilyl)anthracenes was tested in sunlight over a period of 6 days in the absence and presence of BHT (carried out in August and in sunny days continuously). The results are summarized in Table
The photodegradation of 9,10-bis(triphenylsilyl)/ethylsilyl anthracene in sunlight in PMMA.
Day | Absorbance of phenylsilyl film | Absorbance of phenylsilyl film with BHT | Absorbance of ethylsilyl film | Absorbance of ethylsilyl film with BHT |
---|---|---|---|---|
0 | 0.93 | 1.67 | 2.50 | 2.36 |
3 | 0.23 | 0.69 | 0.26 | 0.87 |
6 | 0.16 | 0.4 | 0.21 | 0.71 |
In conclusion 9,10-bis(trialkylsilyl/triphenylsilyl)anthracene systems have been investigated by synthesizing the numbers of 9,10-disilylalkyl substituted anthracenes and 9,10-bis(triphenylsilyl)anthracene. The compounds have been characterised by various photostability tests followed by calculating their rate constants (
The stability of chromophores dissolved in polymer films was somewhat increased, with rates of decomposition varying with different polymers; here removal of oxygen significantly increased stability. In terms of potential applications, this indicates that films on the inside of glass houses should exhibit improved stability.
In terms of potential application, only the 9,10-bis(triphenylsilyl)anthracene showed comparable UV stability relative to some of the commercially available chromophores tested. Unfortunately this compound was not sufficiently thermally stable for the targeted application. That being said, the stabilization effects shown by the additives discussed above allowed substantial lifetime improvements in films prepared with the commercial chromophores allowing both in field testing (Figure
Horticultural tunnels undergoing field trials at the University of Reading; the light purple colored films contain stabilized optical brighteners.
All chemicals used during this research were purchased from Aldrich, BDH, Lancaster synthesis, and Acros chemicals and were used as supplied, unless stated otherwise. All solvents used during reactions, workups, and purification procedures were used as supplied, unless stated otherwise. Solvent purification and drying procedures were generally adopted from the books by Perrin and Armarego [
Deoxygenation was achieved by the use of Freeze-Pump-Thaw and Purging methods as described by Perrin and Armarego [
Photostability measurements for chromophores were carried out both in solution and polymer films. The chromophores were dissolved in solvent (hexane or DCM 10 mg in 10 mL) and placed in 1 cm × 1 cm UV cells, subsequently irradiated using the unfiltered radiation from a medium pressure mercury arc lamp (340 nm). The UV absorption of the solutions was measured at regular intervals until the absorption could no longer be accurately measured.
Appropriate polymer films were synthesized by dissolving the chromophores
A solution of Mp 111–113°C (Lit. 115–117°C) I.R.; 1H NMR (250 MHz) CDCl3 13C NMR (60 MHz) CDCl3 UV/vis Spectrum: (
Yield 2.65 g, 53% Mp 74–76°C I.R.;
1H NMR (250 MHz) CDCl3
13C NMR (60 MHz) CDCl3 UV/vis Spectrum: (
Yield 0.407 g, 8% Mp 51°C I.R.;
1H NMR (250 MHz) CDCl3
13C NMR (60 MHz) CDCl3 UV/vis Spectrum: (
Yield 0.568 g, 11% Mp 31°C I.R.;
1H NMR (250 MHz) CDCl3
13C NMR (60 MHz) CDCl3 UV/vis Spectrum: (
Under an argon atmosphere, a solution of Yield 0.08 g (<1%) Mp 260°C (decom.) (Lit. 108°C) [ I.R.; 1H NMR (250 MHz) CDCl3 13C NMR (60 MHz) CDCl3 UV/vis Spectrum: (
See Scheme 1H NMR (250 MHz) CDCl3