Synthetic and natural organic photochromic systems for optic

Vol. 1 INTERNATIONAL JOURNAL OF PHOTOENERGY 1999

Synthetic and natural organic photochromicsystems for optic application

V. A. Barachevsky

Photochemistry Center of Russian Academy of Sciences 7a, Novatorov Street., Moscow, 117421, Russia

Abstract. Own results of the study and application of a number of organic synthetic photochromic sys-tems in the optical devices are presented. These results are compared with the data obtained for naturalphotochromic bacteriorhodopsines. It was shown that synthetic photochromic systems are characterized bymore wide functional possibilities as compared with the natural systems based on bacteriorhodopsine forapplication in the optical devices. Advances in application of synthetic and natural photochromic systemsfor light modulation and recording optical information as well as nonlinear transformation of laser irradia-tion are discussed.

1. INTRODUCTION

The phenomenon of a reversible color change forsubstances and systems under irradiation named pho-tochromism brought to the attention of scientists andengineers during the last 40 years because of real per-spectives for application in the optical devices of a dif-ferent type [1]:

Ahν1 //

Bhν2,kToo (1)

On the one hand, modern investigations in the fieldof photochromism are directed to the synthesis ofphotochromic compounds with usable polyfunctionalproperties [2]. This is caused by the wide perspec-tives of the modern synthetic chemistry. On the otherhand, the special hopes are counting on applicationof photochromic biological pigments from living sys-tems. These systems are different from synthetic com-pounds by high tolerance for irreversible phototrans-formations, that is, the high recurrence [3]. Among thebest-understood photosensitive proteins of a particularinterest is bacteriorhodopsine (BR) because of uniquephotochromic properties [3].

This paper presents the review of the results of theinvestigations carried out under direction of the authorin the field of the study of properties and application ofphotochromic systems of both types in optical devices.

2. PROPERTIES OF SYNTHTIC AND NATURALPHOTOCHROMIC SYSTEMS

Properties of photochromic compounds and sys-tems determine areas of its application. At presenttime,among possible areas of application the more im-portant ones are following:

-light modulators varying its light transmission au-tomatically as a function of the intensity of activatingirradiation;

-transformers of laser radiation;-devices for optical memory and optical processing.

E-Mail: [email protected]

Each of these optical devices imposes heavy demandson properties of photochromic compounds and sys-tems.

2.1. Photochromic properties.

Spectral characteristics. According to spectral charac-teristics all photochromic compounds can be separatedinto two groups:

-compounds exhibiting reversible transformation be-tween initial colorless and photoinduced color forms(positive photochromism) or between initial and pho-toinduced colorless ones (negative photochromism);

-compounds changing its color (spectra in the visibleregion) under irradiation.

Among the compounds from the first group areall photochromic spirocompounds from classes ofspiropyrans (1, Z=C), spirooxazines (2, Z=N) andchromenes (3) (see scheme 1 and 2).

R1

CO

Z

R2

hν1

hν2, kT R1

+ CO−

ZR2

Z= C,N

Scheme 1

At a sacrifice in varying the molecule structure thedifferent photoinduced colors can be obtained [4]. Thisprovides light modulation not only in narrow spectralranges but within road visible limits as well. The re-lated photoinduced spectral changes are observed forphotochromic systems based on spirooxazines too.

R1

R2

O

R3 hν1

hν2, kT

R1

R2

O

R3

Scheme 2

In contrast to spirocompounds photochromic sys-

2 V. A. Barachevsky Vol. 1

tems containing BR (4) and thioindigo dyes (5) exhibitcolor changing only. The last compounds are character-ized by spectra of trans- and cis-forms depending on amolecular structure (see scheme 3).

O

O

X

X hν

hν′ X

O O

X

X=O,S,Se

Scheme 3

The fundamental difference between spectral charac-teristics of initial and photoinduced forms for spiro-compounds, on the one hand, and BR and thioindigodyes, on the other hand, is due to the mechanism ofphotochromism. In the case of spirocompounds thephotodissociation of the -C−O- bond into pyran or ox-azine fragments and following cis-trans thermal iso-merization are realized. Since both fragments of spiro-compounds are engaged into the conjugation the spec-trum of the photoinduced form is arranged in the visi-ble region.

The sharp color change is observed for photochromicderivatives of phenoxyquinones (6) (see scheme 4).

Ar

O

O O

hν1

hν2

O

O

ArO

Scheme 4

This color change arises from the photoinducedtransformations between para- and ana-quinoid formsof these compounds.

In the case of BR and thioindigo dyes photoinucedtrans-cis isomerization is realized only. This photo-transformations can not lead to the sharp changeof spectral characteristics for the photochromic com-pounds of this type.

Kinetic characteristics. The rate of the photoinducedtransformations of photochromic compounds, as arule, depends on a character and a life-time of electron-excited states which participate in photochromic trans-formations. In principle, if photochromic transforma-tions are realized into exited singlet states, the time

of switching on can be achieved to 10−11–10−12 s. Theexception is photochromic transformation of BR. Mech-anism of photochromism for this compound includesthe intermediate molecular structures with the life-timewhich determines the transfer between initial and sta-ble photoinduced states [3].

The time necessary for the achievement of the re-quired photoinduced optical density is determined byquantum yields of the direct and inverse photochem-ical reactions, the extinction coefficients of the initialand photoinduced forms, and the rate of thermal relax-ation.

It was found that the above-mentioned photochromiccompounds into polymeric matrixes are characterizedby acceptable kinetic parameters.

The particular interest is kinetics of thermal relax-ation of the photoinduced form to the initial one.The life-time of the photoinduced form depending onthe structure of photochromic compounds as well asthe nature of the polymeric matrix can be changedfrom 10−11 s to several years. Among investigated pho-tochromic compounds the polymeric systems basedon phenoxynaphthacenequinone are characterized bythe greatest life-time of the photoinduced form (manyyears) [6].

Recurrence of photochromic transformations. Themain characteristic determining application of pho-tochromic systems is efficiency of the irreversible pho-tochemical and thermal reactions [7]. In the case of ni-trosubstituted spiropyrans which possess acceptablespectral and kinetic characteristics, the mechanismof phototransformation includes the reactive T-stateof the photoinduced form. The participation of thisstate leads to the high rate of photodegradation ofthese compounds. Unlike spiropyrans, spirooxazinesand chromenes do not require the introduction of nitro-substituents for the achievement of the acceptable life-time of the photoinduced form at usual temperatures.In this case, the T-state does not participate in pho-tochromic transformations. As a result, the photosta-bility of these compounds increases at least two ordersof magnitude as compared with spiropyrans.

Photochromic systems based on derivatives ofpheoxynaphtacenequinone exhibit the greater stabilityto photodegradation [6].

Essentially BR and thioindigo dyes do not prone tophotodegradation practically [7].

R1

O

O

O

R2

hν1

kT

R2

OO

O

R1kT

hν , kTR1

O

O

O

R2

Scheme 5

Vol. 1 Synthetic and natural organic photochromic . . . 3

Similar to other characteristics, the stability of in-vestigated photochromic compounds to irreversible re-actions is determined by mechanism of photochromictransformations. For spirocompounds the significantcontribution to increasing recurrence of photochromictransformations is exception of the T-state from pho-toprocesses. It seems likely that high recurrence forthe systems based on phenoxynaphthacenequinones isdue to formation of the intermediate σ -complex duringphototransformation (see scheme 5).

Essentially unlimited recurrence of photochromictransformation of BR and thioindigo dyes into poly-meric matrixes is linked with trans-cis photoisomer-ization processes. In this case photophysical processesare realized only instead of photochemical reactions forother photochromic compounds.

2.2. Polyfunctional properties. Besides above-mentioned photochromic properties, there are anotherones which are developed during the photochromictransformation. These new properties provide widen-ing application of organic photochromic compoundsand systems.

Photoinduced change of the refractive index. Thechange of the refractive index (∆n) under irradiationfor photochromic systems is due to the appearance andthe disappearance of the absorption band belonging tothe photoinduced form [8]. The maximum value of thephotoinduced change of the refractive index is linkedwith the difference of photoinduced optical density atthe maximum of the absorption band (∆Dmax) and thedrop steepness for this absorption band (dD/dλmax) bythe simple relation:

∆nmax = 1/d·∆Dmax ·dD/dλmax,

where d is the thickness of the photochromic film inmicrons.

Experimental maximum values of the photoinducedchange of the refractive index for layers based onspiropyrans exceed ∆n = 10−2 for the film with thethickness of d= 10–15 microns.

The photochromic organic systems exhibit photoin-duced birefringence (PB) under polarized irradiationas a result of the photoinduced transformation fromthe initial isotropic to the photoinduced anisotropicone [9]. This effect is associated by us, as a rule, withthe photoorientation of polar photochromic moleculesunder polarized light. Besides the photochromic filmswith spiropyrans, the photoinduced dichroism and PBwere found for the films based on BR [3]. The PB valueincreases in dark as a result of the additional orien-tation of the matrix molecules by photooriented pho-tochromic molecules [10]. It reaches to ∆n= 10−3. ThePB phenomenon is observed for Langmuir-Blodgett (LB)films based on photochromic azodyes [11] too.

Photoinduced emission. A number of the pho-tochromic compounds possesses the capability for gen-eration of fluorescent products from the nonlumines-cent initial substance under irradiation [5]. Among

these compounds are nitrosubstituted spiropyrans aswell as substances exhibiting trans-cis photoisomeriza-tion, namely thioindigo dyes and derivatives of phenox-yarylquinones.

Nitrosubstituted spiropyrans provide photoinducedemission in the 600–700 nm range depending onthe molecular structure. Introduction of electron-donor substituents into heterocycles leads to thebathochromic shift of the emission bands for the pho-toinduced merocyanine form [12]. Application of thesecompounds can realize the transformation of UV lightto visible irradiation. In the case of spiropyrans withboth fluorescent forms, light transformation into themore narrow spectral range is observed.

Nonlinear optical properties. The fundamental for-malism for describing the nonlinear optical responseof the photochromic systems is the expansion of op-tical induced polarization density as a power series inthe electric field of light:

P = χ(1)E+χ(2)E2+χ(3)E3+··· ,

where χ(1),χ(2),χ(3) are tensors of linear, second, third,and so on optical susceptibilities, E is the electric fieldof the light wave.

The first tensor χ(1) governs the normal absorptionand refraction properties under usual conditions of ir-radiation. The higher order tensors describe exhibitionof nonlinear optical effects. These effects require thesignificantly higher optical power as compared with lin-ear optical phenomena.

The nonlinear optical behavior of LB films preparedwith the use of photochromic spiran dye exhibiting neg-ative photochromism has been studied by us with agoal of obtaining second harmonic generation (SHG)[13] (see scheme 6).

CH3

N

+N

C16H33

H

H

O− OCH3

NO2

Scheme 6

This property of the photochromic films is governedby the second-order susceptibility. The analysis of theangular dependencies of the SHG intensity for this spi-ran dye shows that the optical susceptibility is charac-terized by the high value. It was found that the longmolecular axis to be parallel to the substrate surface.The domain size depends on subphase composition.Preparation of LB films from the usual aqueous sub-phase leads to the domain size which is below thewavelength of SHG (r < λ). The same films preparedfrom 0.05 M aqueous solution of KCl include domainsof more sizes (r> λ).

4 V. A. Barachevsky Vol. 1

Firstly nonlinear effects into photochromic systemswhich are due to the third-order susceptibility are ob-served by us for the solutions containing photochromicnitrosubstituted spiropyrans [14]. Two-photon charac-ter of photochromic transformation has been proved bythe quadratic relation between the photoinduced opti-cal density and the laser intensity. It is very interest-ing that this quadratic dependence is observed at thelow powers of photoexcitation. The further increasingof power leads to the transformation of the quadraticdependence to the linear one. This effect is associatedby us with decreasing the intensity of laser radiation be-cause of its absorption by the photoinduced form whichhas the absorption spectrum coinciding with the wave-length of laser radiation.

3. APPLICATION OF PHOTOCHROMIC MEDIA INOPTICAL DEVICES

The analysis of the above-mentioned results showsthat photochromic substances from classes of spiro-compounds, aryloxyquinones, thioindigo and azodyesas well as BR have polyfunctional properties providingtheir application in the optical devices of a differenttype.

3.1. Light modulators. The photochromic materialshave found a the wide practical use for the eye pro-tection from dangerous light irradiation [7]. The plas-tic photochromic lenses based on spirocompounds,namely spirooxazines and chromenes, have a chiefcommercial importance for the eye protection againstsunlight. This is due to the high photostability of thesecompounds as compared with spiropyrans as well asthe acceptable spectral characteristics. Recently, partic-ular emphasis is given to the synthesis of chromenesbecause of the less pronounced thermal dependenceof photochromic transformation of these compounds.The use of combination of several spirocompoundsprovides the possibility of making photochromic gog-gles with gray or brown photoinduced colors [15, 16].

Two-photon photochromism of spirocompoundswith the photoinduced absorption bands in the fieldof the wavelength of activating laser radiation [17, 18]opens perspectives of their application for the develop-ment of the devices providing the eye protection fromlaser radiation [17].

3.2. Frequency converters of irradiation. Pho-tochromic spirocompounds as well as thioindigo dyesexhibiting photoinduced emission as a result of the lin-ear interaction between photochromic media and lightirradiation are usable for the transformation of lightfrequency, especially into integrated optical schemes.As this takes place, the frequency range of this trans-formation is subject to wide variations (from UV toIR).

Nonlinear optical properties of photochromic mediabased on spiropyrans are adequate for the frequencytransformation by SHG of laser radiation.

3.3. Devices for optical memory and optical informa-tion processing. The properties of photochromic or-ganic systems are very attractive for using as recordingmedia in the optical devices working in real time.

The photoinduced change of the refractive index withconcurrent change of absorption provides recordingamplitude-phase holograms with the diffraction effi-ciency (DE) up to DE=15% [18]. The use of photobleach-ing for holographic recording permits to obtain deepholograms with the high angle selectivity [8]. Thus, itis possible to make holographic optical memory of thehigh information capacity.

The photochromic materials based on phenoxynaph-thacenequinones which are characterized by the verylong life-time of the photoinduced form (several years)can be used for making reversible bitwise optical mem-ory on disks. The same results can be achieved for me-dia based on BR mutants which are characterized bythe high stability of the photoinduced state too [3].

Application of photochromic recording media pos-sessing photoinduced fluorescence opens perspectivesfor the development of three-dimensional bitwise work-ing optical memory [19]. In this case, recording andread-out of optical information are realized with theuse of nonlinear two-photon processes for excitationof photochromic molecules in the medium volume.

PB gives the possibility of nondestructive read-out ofoptical information recorded into photochromic mediabecause it is carried out outside of the absorption bandof the photoinduced form. Rerecording is achieved byusing irradiation with orthogonal polarization. In thisconnections, the photoanisotropic photochromic mate-rials are especially acceptable for processing optical im-ages.

The organic photochromic media are usable for thedevelopment of the photochromic laser devices for pro-cessing of electrical signals, holographic filtration andinterferometry, outlining and other image transforma-tion. There are proposals for application of the pho-tochromic materials in the devices analyzing wide-bandsignals, in particular frequencies of refractive radar andhydroacoustic signals as well as in the linear opticalprocessors.

The possibility of making photochromic materialswith different photoinduced colors admits to developthe devices for the display of color information.

4. CONCLUSIONS

1. A number of photochromic systems based on syn-thesized substances from spirocompounds, ary-loxyquinones, thioindigo and azo dyes as well asnatural BR has been developed.

2. The study of these systems showed that polyfunc-tional properties provide the possibility of theirapplication in the optical devices of a differenttype.

3. It was demonstrated experimentally that devel-oped photochromic media are usable for the

Vol. 1 Synthetic and natural organic photochromic . . . 5

development of the light modulators with vari-able light transmission, frequency transform-ers of laser radiation, devices for two- andthree-dimensional optical holographic and bit-wise working optical memory as well as for pro-cessing optical information, signals and images.

ACKNOWLEDGEMENTS

This work was supported by the Russian Foundationfor Basic Researches (project no. 99-03-32021).

REFERENCES

[1] V. A. Barachevsky, Proc. SPIE 2968 (1997), 77.[2] V. A. Barachevsky, Opt. Mem. Neur. Networks, 6

(1997), 111.[3] N. Vsevolodov, Biomolecular Electronics, p. 275,

Birkhauser, Boston-Basel-Berlin, 1998.[4] V. A. Barachevsky, Proc. SOI, vol. 65, 1987, p. 114

(Russian).[5] V. A. Barachevsky, M. V. Alfimov, and V. B. Nazarov,

Proc. SPIE, vol. 3468, 1998, p. 293.[6] V. A. Barachevsky, Organic Photochromic and

Thermochromic Compounds (J. Crano andR. Guglielmetti, eds.), vol. 1, Plenum, New York,1998, p. 267.

[7] V. A. Barachevsky, Photochromic and its Applica-tion, Chemistry, Moscow, p. 277 (Russian).

[8] V. A. Barachevsky, Perspectives and Possibilitiesof Nonsilver Photography, Chemistry, Leningrad,1988, p. 112.

[9] V. A. Barachevsky, Proc. SPIE, vol. 1559, 1991,p. 184.

[10] V. M. Kozenkov, N. N. Vsevolodov, V. A.Barachevsky, et al., Light-Sensitive BiologicalComplexes and Optical Information Recording,AC USSR, Puschino, 1985, p. 156 (Russian).

[11] V. A. Barachevsky, Proc. SPIE, vol. 2208, 1995,p. 184.

[12] A. A. Ignatin, Yu. P. Strokach, and V. A.Barachevsky, Sci. Appl. Photo. 40 (1998), 243.

[13] G. K. Chudinova, I. A. Maslyanitsyn, V. A.Barachevsky, et al., Sci. Appl. Photo, 40 (1998), 231.

[14] V. F. Mandjikov, A. P. Darmanyan, V. A.Barachevsky, et al., Opt. Spectr. 32, 412 (Rus-sian).

[15] J. C. Crano, W. S. Kwak, C. N. Welch, and C. Ed.Mc Ardle, Blackie, Glasgow, 1992, p. 31.

[16] J. C. Crano, Fraives. Lab Talk (1996), 8.

[17] V. F. Mandjikov, V. A. Murin, and V. A. Barachevsky,Quant. Electronics (1973), 66 (Russian).

[18] Yu. P. Strokach, S. G. Kuzmin, V. F. Mandjikov,et al., Quant. Electronics 2 (1975), 2202 (Russian).

[19] V. A. Barachevsky, Proc. SPIE, vol. 3347, 1998, p. 2.

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