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Vol. 07 INTERNATIONAL JOURNAL OF PHOTOENERGY 2005

Solid state photochromism of spiropyrans

B. S. Lukyanov,1 A. V. Metelitsa,1 N. A. Voloshin,1 Yu. S. Alexeenko,1

M. B. Lukyanova,1 G. T. Vasilyuk,2 S. A. Maskevich,2 and E. L. Mukhanov1

1 Institute of Physical and Organic Chemistry, Rostov State University, Rostov-on-Don, 344090, Russia2 Y. Kupala State University of Grodno, Ozheshko 22, Grodno 230023, Belarus

Abstract. A series of spiropyrans possessing photochromic properties in solid state were investigated.UV irradiation (λmax = 365 nm) of the thin vacuum deposited spiropyran films results in conversion ofthe colorless form to the photocolored one. The reaction is photochemically and thermally reversible. Thephotochromic activity of the solid material and polystyrene matrix has been compared. Raman spectroscopyhas been used for studying the influence of metallic surface on the adsorbed spiropyran layers.

1. INTRODUCTION

An active research work is carried out all over the worldin the field of nonsilvered light sensitive materials, suchas organic photochromic compounds capable of col-oration under irradiation in various spectral regions [1].

Spiropyrans is one of the major classes of or-ganic photochromes [2, 3]. The photochromic behav-ior of spiropyrans is usually enhanced if a moleculepossesses strong electron acceptor substitutes suchas a nitro group in the 6′ and/or 8′ positions of its2H-chromene fragment.

Het

1′O

2′

3′ 4′

5′

6′

7′8′

However, the replacement of the nitro group byother π -acceptor results in effective coloration andgood adhesion. This could lead to deposition of thephotosensitive layer on the end-window of glass fiberoptic. It has been proposed that spiropyrans couldbe used for recording digital information [3]. Becauseof the technological importance of this class of com-pounds we will report here about the investigation ofsome new spiropyrans.

2. METHODS AND MATERIALS

All spiropyrans I–IV [2, 4, 5] have been purified withcolumn chromatography.

The structures of all synthesized compounds havebeen determined with elemental analysis, IR and 1HNMR spectroscopy.

1H NMR spectra have been detected using a NMRspectrometer “Varian Unity 300” (300 MHz, USA) inCDCl3. Chemical shifts are reported in δ (ppm) relativeto tethramethylsilane.

Electronic absorption spectra of the spiropyransolid films before and after irradiation have been de-tected by using “Specord UV VIS” spectrophotometerequipped with special rotating mirror. The vacuumthermal deposition process was carried out for obtain-ing the transparent spiropyran thin films on quartz orglass plates. The VUP-4 apparatus was used for vacuumdeposition (at ∼ 1.5 × 10−5 Torr, temperature of evap-oration ∼ 250–400 ◦C) for both spiropyrans and silverfilms. The control of the film homogeneity had beenrealized with a polarity interference microscope MPI-5(PZO Warsawa, Poland produced) by scanning two co-ordinate axis and observing interference signals. TheRaman spectra were recorded with a DFS-52M (LOMO)spectrometer and ILA-120 argon laser (Carl Zeiss) wasused as an excitation source. The power density oflaser radiation was about 10 mW/mm2. A 250 W high-pressure mercury discharge lamp (DRSh-250) was usedin irradiation. The absorption spectra were detectedwith the spectrophotometer “Specord M 40” (Germany).

3. RESULTS AND DISCUSSION

3.1. Photochromism of the thin films of spiropy-ran I–IV. It has been found that compound I exhibitsphotochromic properties not only in solutions but alsoin the solid state.

H3C CH3

N O CH3

HOC

CH3

I

Figure 1 shows the effect of the UV irradia-tion time (365 nm ∼ 27307 cm−1) on the vacuum de-posited thin films. Irradiation results in conversion of

18 B. S. Lukyanov et al. Vol. 07

400 500 600 700

λ, nm

1,2

1,0

0,8

0,6

0,4

0,2

0,0

D

32 28 24 20 16

ν , 103 cm−1

1

2

3

4

5

1

54

32

Figure 1. The effect of the time of UV-irradiation on the

absorption spectra of the thin film of spiropyran I (1) 0,

(2) 30, (3) 120, (4) 240, (5) 360 sec.

colorless (360 nm) form of I into photocolored onewith λmax615 nm (∼ 16260 cm−1), (Figure 1). Photocol-oration reaction was found thermally (+70 ◦C) and pho-tochemically reversible. The half-life of a colored formis about 5 minutes (Figure 2).

Thin solid spiropyran II and III films were studied.

N

OO

CH3

O

HOC CH3

II

CH3H3C

ON

CH2PhHOC

OCH3

III

Similar behavior has been noticed in compounds IIand III (Figures 3 and 4). UV irradiation (365 nm) of vac-uum deposited spiropyrans II and III films results inconversion of colourless forms to photocolored ones.The reaction is photochemically reversible. Heating ofphotocolored forms results in stabilising photocoloredform of III, whereas II shows reversible behavior.

It would be of interest to compare the pho-tochromic behavior of the thin solid films of the stud-ied spiropyrans I, II, and III with the 1,3,3-trimethyl-6′-

400 500 600 700

λ, nm

1,0

0,8

0,6

0,4

0,2

0,0

D

32 28 24 20 16

ν , 103 cm−1

543

2

1

1

2

34

5

Figure 2. The absorption spectra of the back dark reaction

of compound I after different time interval (1) photoin-

duced form; (2)–(5) spectra recorded after 5, 15, 30, and

60 mins after irradiation, respectively.

nitro-indolinospiro-pyran V, which is considered as oneof the best photochromic material.

H3C CH3

OO

HOC

OCH3

IV

H3C CH3

CH3

N O NO2

V

The photocoloration rate of thin solid films ofspiropyrans I–IV is faster than those of the thin spiropy-ran V films (Figure 5). But in polymeric polystyrenematrixes the photocoloration rate of spiropyran V ishigher than one of spiropyrans I–IV (Figure 6).

Spiropyran I contains the nitro group and has themaximum rate in polymeric matrix, but at transition tothin solid films spiropyran IV greatly exceed the com-pound I in rate.

Initial rate constants (asymptotic slope with t→0[6]) are given in the table. The table shows, that spiropy-rans IV in a solid phase exceeds 21 times compound V,and besides, enables reversible record of the optical in-formation in solid state.

Vol. 07 Solid state photochromism of spiropyrans 19

400 500 600 700

λ, nm

1,0

0,8

0,6

0,4

0,2

0,0

D

32 28 24 20 16

ν , 103 cm−1

17

23456

6

5

4

3

21

7

Figure 3. The effect of the time of UV-irradiation on the

absorption spectra of the thin film of spiropyran II (1) 0,

(2) 30, (3) 120, (4) 240, (5) 270, (6) 360 sec, (7) after

heating at 70 ◦C–“erasing”.

3.2. Raman studies of thermo- and phototransfor-mation. We have studied the structure and also thethermo- and phototransformation of spiropyran VI thatis adsorbed on the surface of silver film with Ramanspectroscopy (RS). The photocoloration reaction of VIcould be presented as a sequence of structural trans-formations [7].

hν1(t1)

hν2(t2)OO

CH3H3C

VI

O +

CH3H3CO− O

O

CH3H3C

UV irradiation of the solution of VI results in con-version from colorless to the colored form; that hasthe absorption bands in the visible region with maximaaround 382, 404, 562, and 582 nm.

On the first step of our investigation we have com-pared RS spectra of the adsorbed layers of compoundVI and the perchlorate VII.

Perchlorate VII is the model compound of the col-ored (merocyanine) form of spiropyran VI as a non sub-stituted model of the parent spiropyran IV. Thin lay-

400 500 600 700

λ, nm

1,0

0,8

0,6

0,4

0,2

0,0

D

32 28 24 20 16

ν , 103 cm−1

24

7163

5

5

6

74

3

21

Figure 4. The effect of the time of UV-irradiation on the ab-

sorption spectra of the thin film of spiropyran III (1) 0, (2)30, (3) 120, (4) 240, (5) 270 sec, (6) after heating at 70 ◦C–

“fixing”, (7) UV irradiation of fixed film during 60 sec.

ers of VI and VII adsorbed on silver films were obtainby evaporation of solvent from acetonitrile solutions.

O +

CH3H3CHO

ClO4−

VII

Figure 8 presents the Raman spectra of spiropyranVI and perchlorate VII adsorbed on silver films. In ourexperiments with application of the RS only the col-ored (with an open pyran cycle) forms of the spiropyranmolecules have been detected.

There are no typical peaks for closed spiropyranmolecule bond at the 1640 cm−1. Dissociation of theCspiro-O bond in the adsorbed spiropyran moleculescould be associated with the fact that the interaction ofthe heteroatom O and the sorbent surface results in adecrease of the energy of the Cspiro-O bond. The impor-tant role of the surface is well seen from the compar-ison of the Raman spectra of spiropyran VI adsorbedon an unannealed (curve 1) and annealed (curve 3)


Table 1. Asymptotic slope if t→0 and the ratio of rate constants for spiropyrans I–V in polysterene matrix and thin solid

film.

Compound Structure

Brutto constants of Rate ratio vs. V

photocoloration rates in

Abs. s−1

Polystyrene Thin solid Polystyrene Thin solid

matrix film matrix film

CH3

CH3H3C

CH3

N O

HOC

I 0,056 0,042 0,22 21

CH3HOC

CH3N

O

O

OII 0,04 0,006 0,16 3

OCH3

HOC

CH3H3C

N O

CH2Ph

III 0,047 0,01 0,19 5

OCH3

HOC

CH3H3C

O

OIV 0,03 0,004 0,12 2

NO2

CH3H3C

N O

CH3

V 0,25 0,002 1 1

silver films. On the annealed silver film the intensityof the RS signal is several times smaller (Figure 7). Thiscould be linked with the fact that the efficiency of theinteraction of the heteroatom O with the surface is inthis case smaller and therefore a smaller number of thespiropyran open molecules have been formed.

Since the investigated spiropyran is showing boththe photochromic and the thermochromic behavior,the structure study of the adsorbed spiropyran VImolecules and of their photochromism with the tem-perature is of interest. Cooling of the compound VI filmto 77 K results in a noticeable change in the reflectance

Vol. 07 Solid state photochromism of spiropyrans 21

0,5

0,0

Ab

sorb

ance

0 50 100

t, sec

I

IIIII

VIV

Figure 5. Evolution of the maximum visible absorption

band during the Photocoloration of 1,3,3-trimethyl-6′-nitro-indolinospiropyran V and spiropyrans I–IV (thin solid

films) under 365 nm irradiation.

0,5

0,0

Ab

sorb

ance

0 5 10

t, sec

V

I

IIIIIIV

Figure 6. Evolution of the maximum visible absorption band

during the Photocoloration of 1,3,3-trimethyl-6′-nitro-

indolinospiropyran V and spiropyrans I–IV (in polystyrene

matrix) under 365 nm irradiation.

600 900 1200 1500

ν , cm−1

1

2

3

52

4

65

3

10

52

10

93 1

23

31

33

7

15

09 1

58

71

60

2

Figure 7. The reflectance Raman spectra (exc = 514.5 nm)

of spiropyran VI (1,3) and colored form VII (2), adsorbed

from acetonitrile solutions on the unannealed (1,2) and

annealed (3) silver films.

600 900 1200 1500

ν , cm−1

1

2

3

4

5

1247

1297

1146

980

1181

1227

1504

655 1050

1120

1146

1247

1297

1380

1486

Figure 8. The reflectance Raman spectra (exc = 514.5 nm)

of spiropyran VI adsorbed on silver film:

1 - before irradiation at T = 293;

2 - before irradiation at T = 77 K;

3 - after UV irradiation for 20 min at T = 77 K;

4 - after irradiation by visible light through a UV light filter

for 20 min at T = 77 K (4);5 - the effect of raising the temperature to 263 K of sam-

ple 4.


Raman spectra (Figure 8). The comparison of curves1 and 2 is shows that freezing of spiropyran VI re-sults in the formation of spiro form (curve 2). It couldbe concluded that freezing leads to the reorganiza-tion of the structure of the adsorbed merocyanine lay-ers, which results in attenuation of its interactionswith the surface. This could be explained by the in-termolecular interactions which, with decrease in thethermal energy, induce the formation of the aggregatesof spiro form molecules on the surface. These aggre-gates at low concentration could be exist also at roomtemperature, as indicated by the large width of theband of the reflectance Raman spectra, whose intensityincreases considerably with decrease in temperature(see Figure 8, curves 1 and 2). However, the concentra-tion of the aggregates is relatively small at room tem-perature.

UV irradiation of the adsorbed spiropyran VI layerat 77 K does not lead to fundamental changes in the gi-ant Raman spectrum (Figure 8, curve 3). It should benoted that only a certain decrease in the Raman sig-nal as a whole was observed, which could be due tothe photodestruction of the adsorbed molecules andtheir desorption. As is evident from the comparison ofcurves 1 and 4 (Figure 8), the irradiation with visiblelight favors further structural changes in the adsorbedlayer of the spiropyran VI molecules. This could be at-tributed to the formation of associated forms of the me-rocyanine molecules which were already manifested bythe decrease in the temperature. Just as for spiropyransolutions the effect of visible light favors the cycliza-tion of the merocyanine molecules. For the moleculesadsorbed on the silver surface this effect favors theirassociation. In heating of the irradiated speciment, theshape of the Raman spectrum is completely recovered(Figure 8, curve 5).

4. CONCLUSION

A new series of spiropyrans molecules that exhibit pho-tochromic behavior in solid state were studied.

Photochromic behavior in thin solid spiropyransfilms were compared with the well known 1,3,3-trimethyl-6′-nitro-indolinospiropyran ones in solidfilms and polystyrene matrix.

Using Raman spectroscopy the influence of themetallic surface properties on photochromic behaviorof the adsorbed spiropyran layers was studies. It hasbeen shown that interactions with the surface could beresulted in dissociation of the Cspiro-O bond.

ACKNOWLEDGMENT

This work was kindly supported by Russian Foundationof Basic Research (grants 02-03-81011 Bel 2002, 04-03-32485), CRDF–Ministry of Education of Russian Federa-tion (grant REC-004) as well as grant NSh-945.2003.3.A.V.M. and N.A.V. gratefully acknowledges a Interna-tional Scientific Technical Center endowment (grantISTC-2117).

REFERENCES

[1] IUPAC Technical Report, Pure Appl. Chem. 73(2001), 639.

[2] D. H. Duerr and H. Bouas-Laurent, Photochromism.Molecules and system, Amsterdam Elsever, 1990.

[3] M. Irie, Chem. Rev. 100 (2000), 1685.[4] B. S. Lukyanov, Y. I. Ryabukhin, G. N. Dorofeenko,

L. E. Nivorozhkin, and V. I. Minkin, Chemistry of Het-erocyclic Compounds (Issued in Latvia) N2 (1978),161.

[5] B. S. Lukyanov, L. E. Nivorozhkin, V. I. Minkin, andA. V. Metelitsa, Chemistry of Heterocyclic Com-pounds (Issued in Latvia) N12 (1990), 1700.

[6] B. S. Lukyanov, N. V. Volbushko, N. A. Voloshin, T. V.Kutsenko, L. E. Nivorozhkin, A. V. Metelitsa, andV. I. Minkin, Inventors certificate N 1385562 (USSR),1987.

[7] G. T. Vasilyuk, S. A. Maskevich, S. G. Podtunchenko,V. I. Stepuro, B. S. Lukyanov, and Yu. S. Alexeenko,Journal of applied spectroscopy 69 (2002), 301.

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