Photoisomerization of thioindigo adsorbed on alumina

Voldme 73, number 1

CHEMICAL

PHOTOlSOMERlZATlON

OF THIOINDIGO

H-D. BREUER and H. JACOB FR 13.2 PhpkaIlsclze Chemre, Unwersit~t O-6600

Saarbriicken.

PHYSICS LE-l-l-ERS

ADSORBED

1

July 1980

ON ALUMlNA

des Saarlandes.

West Germany

Recewed 21 March 1980, III final form 14 Apnl 1980

The photolsomenzation of thomdlgo adsorbed on alumina has been mveaated. We trans- CISaomenzatton can be mduced by uradmuon, us-tiuomdIgo 1s stabtied at the surface and cannot be back converted. Adsorption from a solution contammg trans- and c&ttiomdlgo 1s stereospeclfic for the CISLsomer.

1. Introduction The mechamsm of the photolsomenzatlon of tiein&go m solution has been mvestrgated m recent pa-

pers [l-3]. It now seems to be well estabhshed that the tram + CIS and the CIS + tram rsomenzatlon proceeds vra a common tnplet state [ 11. Smce the photochemrcal behavrour of the first-exerted smglet state of tram-throimbgo. whrch 1s mvolved tn the rsomenzation, IS very sensrttve to solvent propertres, one should expect some mfluence of a solid surface onto whtch thiomdtgo IS adsorbed. If the molecule, whtch 1s known to be planar m its ground state, IS adsorbed as a whole parallel to the surface, the torstonal degree of freedom about the central double bond may be lost due to the furatton at the surface. In this paper, we present some expertmental results whtch demonstrate that photoisomerizatton of thtomdrgo IS possible III the adsorbed state. Furthermore we show that the adsorptton from a solution contammg a rmxture of tram+ and cts-thromdrgo IS stereospeclfic for the CIS configuration.

2. Experimental For the adsorptron expenments, thromdrgo was dissolved m benzene (Merck, uvasol) and adsorbed on AlZ03 (Woelm, 200 m’/g). In all expenments surface coverages were about 0.1 monolayer. When the 172

adsorptron was completed the samples were dned and kept in the dark untd they were used. Spectra of the adsorbed thtomdrgo and of the photoproduct were recorded m a photoacousttc spectrometer. Photoacoustic spectroscopy (PAS) and some of its apphcatrons have been described m detail recently [4]. When deahng wrth powdered samples PAS has some important advantages over conventional spectroscoprc methods because of its hrgh sensitlvlty and the fact that no stray hght problems can arise. The double-beam spectrometer used for thts work was butlt in our laboratory. For the photoisomertzatron experiments we used a xenon arc lamp (450 W) and an OG-550 fdter. Adsorbed thiomdigo was uradrated m the photoacoustrc cell. Cts-nch solutions were prepared m a water bath to ehrnmate heat effects.

3. Results and discussion Curve (a) m fig. 1 is the absorption spectrum of thioindigo adsorbed on Al,O,. The peak near 470 nm was neither observed in the spectrum of a freshly prepared solution from which thioindigo was adsorbed, nor in a dark-adapted solution. Smce it coinctdes wrth the absorption maxtmum of the CIS isomer, it can be concluded that part of the trans-thiomdrgo is converted tnto crs-thioindigo by adsorption. If the sample of curve (a) is reradiated for 2 h, spectrum (b) is obtained. Here clearly the cts isomer is in

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CHEMICAL

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PHYSICS

700 nm

Fig. 1. (a) Photoacoustic spectrum of tians-Uuoindlgo adsorbed on alumina. (b) Sample of (a) after 2 h irradiation (A > 550 nm).

excess of the tram configuration. At present we cannot determme exactly the quantum yield for the tram-cis isomerrzatron, srnce we do not know the absorpuon coefficients of the adsorbed throindigo. However, takmg the absorptron coeffrcrents rn benzene [S] as an approxrmation, it is evrdent that in the photostatronary state more than 50% of the adsorbed thiomcligo 1s rn the cis form. Siiar results have been observed by irradratrng crsstilbene adsorbed at an alumma surface [6]. Performmg the same experiments at a surface coverage higher than 0.5 monolayer, we did not observe photouomerrzation. The correspondmg spectra have the same shape as curve (a) m fig. 1. In solutron the crs isomer 1s not stable over a longer per-rod of time, and without rllumrnation no CISabsorptron can be detected. At the surface, however, no CIS+ tram isomenzation could be observed. Even two hours illumination with the unfiltered xenon lamp drd not produce any changes rn the spectrum. This indrcates that the crs isomer is energetically favoured at the surface. If thioindrgo is adsorbed from a solution containing about equal amounts of transand cis-tluoindrgo we obtain the spectrum shown m fig. 2. Here only cis-thioindigo is adsorbed while the

LETTERS

I IuIy i98Q

Fig. 2. Photoacoustx spe~%um of cis-tiomdigo on alumina obtained by adsorption from a solution containing &a.~+ and cls-thiomdlgo.

concentration of the trans form in the solution remained unchanged. Thermal conversion of adsorbed trans-thioindigo into cis can be excluded since in a number of control experiments no thermal reaction rn erther drrectron has been observed in the adsorbed state. Srnce the absorption intensity of the cis isomer 111fig. 2 is comparable to that of curve (b) in fig. I, the surface coverage must be in the region of 0.0% 0.1 monolayer. As in the case of photoisomerization at the surface, the adsorbed crs-thioindigo is stable and cannot be converted photochemically into the trans configuratron. The fact that photoisomerization is observed in the adsorbed state indicates that only one part of the trans-thioindigo molecule can be coplanar with the surface. The other ring system can rotate about the central double bond. This is in agreement with the observation that at higher surface coverages no photorsomerization occurs. In thrs case the rotation is hindered by neighbouring molecuIes. Since the cis isomer is stabdized at the surface and cannot be converted back into the trans form, both ring system now seem to have very similar adsorption bonds and the molecule no longer has a torsional degree of freedom. On the other hand, the deformation of the morecule at the surface can only be very small. The absorpE73

Volume 73, number 1

CHEMICAL PHYSICS LETTERS

tion maxima of both isomers in the adsorbed state agree very well with those observed m solution. Any htortion or twistmg of the C=C double bond, however, would result in a sigmficant change m the adsorption spectra. For the photolsomenzatlon in solution at room temperature, two sunilar mechanisms have been suggested: (1) A common tnplet can be populated both from the excited smglet state of the trans and cis isomer. In the tnplet state the molecule 1s twisted out of plane by about 90” [I J. (2) Excitation of trans-thiomlgo leads to the formation of a planar trans tnplet whch IS in thermal equrlrbnum with a twsted tnptet configuratlon [2]. The results of our adsorption studes cannot rea&ly be compared with the observations III solution. No tnplet-triplet absorption spectra are avadable whch could yield informatlon on the configuration of the tnplet state involved III the trans + CIS lsomenzation. There is, however, a significant difference with respect to soiutlon expenments m that no cis + trans IsomerIzation is possibIe at an alumina surface. The fact that

174

1 July 1980

only the cis isomer is adsorbed

from a solution contauung trans- and cls-thiomdigo, and the dependence of surface coverage, support the mterpretation of our results as surface effects.

Acknowledgement The authors whish Chernkchen Industne

to thank the Fonds der for financial support.

References [ 1] K.H. Grellmann and P. Hentzschel, Chem. Phys. Letters 53 (1978) 545. [Z j H. Chimer and D. Schulte-Fro&de, Chem. Phys. Letters 66 (1979) 363. [3] T. Karstens, K. Kobs and R. Memming, Ber. Bunserges. physlk. Chem. 83 (1979) 504. [4] Y.H. Pao, ed , Ophcal spectroscopy and detectlon (Academic Press, New York, 1977). [S] Ghl. Wyman and B.hl. Zamegar, J. Phys. Chem. 77 (1973) 831. [6] H. hloesta, DiscussIons Faraday Sot. 58 (1974) 244.