Triplet sensitization of anthracene photodimerization in γ-cyclodextrin

3. Photochem.

Photobid.

A:

Chem.,

65 (1992)

313-320

313

Triplet sensitization of anthracene photodimerization y-cyclodextrin Takashi

Tamaki.

Research Institute

Yuji

Kawanishi,

Takahiro

Seki and Masako

in

Sakuragi

for Pol’ymers and Textiles, I-I-4 Higashi, Tsukuba 305 (Japan)

Ahs tract The anthracene derivatives which formed a 2:l inclusion complex with y-cyclodextrin (‘yCyD) underwent photodimerization with a high quantum yield (approximately 0.5) in both solution and solid state. The photosensitization of this supramolecular system in solution was carried out successfully using xanthene dyes, such as eosin Y, erythrosln and rose bengal, as triplet sensitizers. The photodimerization is described in terms of the hostassisted photoreaction.

1. Introduction A few years ago, it was found that y-cyclodextrin (y_CyDj formed 2: 1 (guest : host) in&&on complexes with anthracene derivatives such as 2-ant’nracenesulphonate @AS) and 2-anthracenecarboxylate (2AC) in aqueous solutions ]l]_ The spectroscopic measurements indicated that two anthracene molecules in the host cavity were in a close configuration in which two v systems were significantly overlapped. This supramolecular system is remarkable because [4+4] photocycloaddition of the guest anthracenes takes place very efficiently. The quantum yield of photodimerization is an order of magnitude greater than that in the absence of the host molecule. It is of interest to determine how the triplet state can contribute to this photodimerization in the host. This paper is concerned with triplet sensitization experiments in aqueous solution. Xanthene dyes, such as eosin Y, erythrosin and rose bengal, which are as soluble in water as the inclusion complex, were employed as triplet sensitizers, Biacetyl has been successfully used for the triplet sensitization of anthracene dimerization 123. Saltiel and coworkers [3] have proposed the triplet-triplet annihilation (TI’A) mechanism to account for this triplet sensitization, The TTA process is consistent with the well-established mechanism of anthracene photodimerization in which a singlet excimer acts as an intermediate [4]. According to Saltiel et al. [5], the encounter between the triplet state and the ground state molecules only leads to self-quenching. In contrast, it has been reported that the sensitization of the intramolecular cycloaddition of 1,2-di(9-anthryl)ethanes by biacetyl gives [4+ 23 and [4+4] isomers depending on the ground state geometry [6], Obviously, it is difficult to explain the geometrical selectivity in this intramolecular cycloaddition in terms of the ‘ITA mechanism which is valid only for diffusion-controlled processes. The photodimerization of anthracene derivatives in inclusion complexes is related to the intramolecular cycloaddition of anthrylethanes. The xanthene dyes are better triplet sensitizers than biacetyl, because the absorption band of the former, which is located around 500 nm, is further from the anthracene band than that of the latter.

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314 2. Experimental

details

2.1. Materials 2Cyanoanthracene (2CNA) was prepared from 2-anthracenecarboxylic acid (2AC) [7]. 2AS was the same as used previously and the other chemicals were commercially available. The inclusion complexes of water-insoluble anthracene derivatives, such as anthracene, 2CNA, 2-chloroanthracene, 2-acetylanthracene and 2-benzoylanthracene, were obtained as precipitates by the addition of the guest compounds in acetone to an aqueous solution of +yD. The water-soluble anthracenes, such as 2AS, 2AC, anthracene-2,6-disulphonate (2,6-ADS) and l-anthracenesulphonate (IAS), formed inclusion complexes with /3- and r_CyDs when dissolved in water. Poly(viny1 alcohol) (PVA) films of the water-soluble inchrsion complexes were prepared by casting 4 wt.% aqueous polymer solutions containing 2 wt.% inclusion complexes on a glass plate. The reversible photodimerization only occurred when the film was prepared using the inclusion complex of the dimeric products instead of the inclusion complex of the original monomer compounds. The thermal stabilities of the dimerized anthracene derivatives were estimated by monitoring the absorption intensities at A_ of the monomer bands at constant temperature in a cryostat. The triplet energies of the xanthene dyes are 196 kJ mol- ’ for erythrosin and eosin Y and 187 kJ mol-’ for rose bengal [S], which are higher than those of the anthracene derivatives (approximately 176 kI mol-l). 2.2. Methods The photodimerization was carried out using a 500 W high pressure mercury lamp equipped with Corning 7-51 and Toshiba Y-48 glass filters to isolate light of 365 run (for direct photoreaction) and wavelengths longer than 480 nm (for photosensitization) respectively. The cycloreversion was obtained by irradiation with 280 nm light from a 1 kW xenon lamp. The aqueous sample solutions in cylindrical cuvettes (15 cm) were deoxygenated by flushing with nitrogen or argon gas for at least 15 ruin before irradiation. The laser photolysis was performed using the apparatus described previously [9]. A dye laser (coumarin 307; fluence, 5 mJ cme2; pulse width, 20 ns) was used to excite the xanthene dyes. The direct photolysis of anthracene derivatives was performed using an XeCl excimer laser (Lambda Physik EMG53MSC). The time evolution curves of the transient absorption bands obtained in the laser photolysis experiments were simulated using a biehonential function to give the rise and decay rates.

3. Results and discussion 3.1. Inci&on compkx formation The formation of inclusion complexes between anthracene derivatives and y-CyD has been confirmed by UV, induced circular dichroism (ICD) and nuclear magnetic resonance (NMR) spectroscopy measurements. The predominant formation of the 2: 1 (guest: host) complex is characteristic of y-CyD. &Cyd gives 1: 1 and/or 2:2 complexes and a+CyD does not form significant complexes. The cavity size of the host decreases in this order and the Corey-Pauling-Koltun molecular model can support these guest-host stoichiometries. An example of the ICD spectrum is shown in Fig. 1 for the 2CNA-r_ CyD complex.

315

zoo

300

400

III,, I I I I

15 10

t

I

5

1

I

WAVELEbKSTH/~ d/A Fig. 1. Absorption (top) and induced circular dichroism (bottom) spectra of the 2CNA-_rCyD complex in a KF3r pellet before (full line) and after (broken line) irradiation.

Fig. 2. X-ray

diffraction diagram of ZCNA,

-y-CyD and their inclusion complex

The strong electronic interaction of guest r systems is clearly illustrated by the Davydov splitting which appears at the absorption maxima. Furthermore, the inclusion complex with y_CyD shows considerable fluorescence quenching and no observable excimer emission. These spectroscopic properties are different from those of the 2:2 inclusion complex with &C!yD, which shows appreciable excimer emission and no Davydov splitting. This indicates that the two guest molecules are in a closer configuration in the 2:l complex with y-CyD than in the 2:2 complex with p-CyD. We attempted to obtain photodimerizable inclusion complexes of -y-CyD using several anthracene derivatives. The Z-substituted derivatives are ideal for this purpose if the substituent is polar, such as cyano, carboxylate and sulphonate. However, no dimerizable complexes were obtained for parent anthracene and the g-substituted derivatives and few for the l-substituted derivatives. The water-soluble anthracenes, such as 2AC, 2AS, 2,6-ADS and lAS, formed this type of inclusion complex with an equilibrium constant of the order of lo6 M-’ in water. The water-insoluble anthracene derivatives gave inclusion complexes as precipitates. The photodimerizable complex was obtained only for 2CNA for the water-insoluble derivatives examined. The X-ray diffraction pattern of the 2CNA-rCyD complex powder is shown in Fig. 2. The inclusion complex has a unique crystallinity different from those of the host and guest crystals. The structural analysis has not yet been carried out because it is difficult to obtain the monocrystalline solid.

317

300 (a)

300 (b)

400 WAVELENGTH/m

400

500

E

!500

600

WAVELENGTH/mn

Fig. 4. Absorption spectra of deoxygenated aqueous solutions of ZAS (2X lo-’ M) and erythrosin (I x 10m5 M) in the presence (a) and absence (b) of y-D (4x 10m3 M): (1) before irradiation; (2) after irradiation with visible light (A>480 nm); (3) after re-irradiation with 280 nm light.

band is observed after exposure of the sample solution under a nitrogen atmosphere to visible light (greater than 480 nm) for several tens of minutes. No change is observed in the absence of erythrosin. The sensitization of dimer formation is evident from the significant recovery of the monomer band on exposure to 280 nm light (Fig. 4(a)). The formation of the [4+4] cycloadducts was confirmed by high performance liquid chromatography (HPLC) analysis of the photoproducts obtained from the solution containing the inclusion complex and the sensitizer, although the quantum efficiency was very Low (less than 1%). As shown in Fig. 4(b), irreversible absorption changes prevail in the host-free solutions, yielding fewer dimers compared with the result shown in Fig. 4(a). The host is essential for promoting the dimerixation of the guest as in the case of direct photoexcitation. The significant loss of the sensitizer on irradiation with visible light indicates that it undergoes an irreversiblephotoreaction yielding unknown products. This unfavourable reaction becomes more prominent in the order eosin Y
318

Figure 5 shows the transient absorption spectra obtained by laser photolysis of an aqueous solution containing 2AS and eosin Y in nitrogen using a pulsed dye laser tuned at 500 nm. The bands around 425 nm and 600 nm can be ascribed to the triplet-triplet (T-T) absorption of 2AS and the sensitizer respectively. These bands are identical with those obtained from the direct photolysis of these compounds. The triplet energy transfer from the sensitizer to 2AS is shown by the fact that the decay time of the former is coincident with the rise time of the latter. The T-T absorption spectrum obtained in the presence of y-CyD is similar to that shown in Fig. 5, but less intense. No new band is detected. Figure 6 shows the time evolution curves for the 425 nm band of 2AS (2 x 10e3 M) in the presence of the sensitizer (1 X 10m5 M) and y-CyD (varying concentration). The rise (kr) and decay

Fig. 5. Transient absorption spectra of a deoxygenated aqueous solution containing 2AS (1 X 10m4 M) and eosin Y (1 X 10s5 M) at t - 1 /.LS(full line) and t= 20 ps (broken line) after laser pulse excitation (A = 500 nm).

-‘= .l

05

0

0

2

4

6

8

10

12

14

18

18

Fig. 6. Time evolution of the T-T absorption band of 2AS (2X lo-’ M) in deoxygenated aqueous solution after laser pulse excitation of eosin Y (1 x 10e5 M). Monitored at A = 425 run. Concentrations of y-CyD: (1) 5~10-~ M; (2) 1~10-~ M; (3) 2x1O-3 M; (4) 2x10-’ M.

319

(Q rates and the intensity of this band decrease with increasing concentration of the host. On the basis of this opposing relationship between the host and the 425 nm species, it can be supposed that the 425 nm band originates from free 2AS monomer in the solution, but not from guest monomer included in the host. If this is true, a definite relationship should be obtained between the kinetic rates and the concentration of the free monomer, which can be calculated using the equilibrium constant of inclusion complex formation (3 X 1C16 Mm2 for 2AS) [l]. The dependence of k1 and k2 on the free monomer concentration can be compared with the concentration dependence of these kinetic rates in the absence of the host. As shown in Fig. 7, the two curves separately obtained for kl are quite similar to each other, although there is considerable deviation above 10s3 M probably due to levelling off at high concentrations. This confirms that the 425 nm band can be ascribed to the free monomer. In contrast, the value of k2 is larger in the presence of the host than in the host-free solution. Trapping in the host may be responsible for this acceleration of triplet state quenching. The fact that there is no transient absorption band which can be ascribed to the guest monomer suggests the rapid deactivation of the triplet state through the photoreaction. It has been suggested that anthracene photodimerization occurs via a singlet excimer intermediate [4]. In order to account for dianthracene formation following triplet excitation transfer from biacetyl to anthracene in benzene [2], Saltiel and coworkers [3] have proposed the mechanism of TIA which can give rise to the singlet excimer intermediate with a spin-statistical factor of l/9. They have also proposed that the collisional encounter between triplet state and ground state molecules results only in self-quenching [5]_ In contrast, a separate report, on the intramolecular cycloaddition of 1,2-di(9-anthryl)ethanes sensitized by biacetyl [6], indicates that the triplet state reactivity is remarkably dependent on the ground state geoxnetry: the [4 + 21 cycloadducts are exclusively formed for derivatives having negligibly overlapping w systems, whereas the [4 + 41 cycloadduct is efficiently formed for the derivative having partially overlapping r-systems. These triplet sensitization experiments suggest different reaction mechanisms : the TTA mechanism which is valid for diffusion-controlled

<

s’

% * 5XlCF

Fig. 7. Plots of the rise &)

5XlW

and decay &) rates of the T-T absorptionof 2AS sensitizedby eosin Y (1 X 10e5 M) in deoxygenated aqueous solutions vs. the concentration of free ZAS in the presence (full line) and absence (broken line) of ?QD. The values in the presence of the host were obtained from the data shown in Fig. 6.

320

processes and the geometry-assisted mechanism which operates for an intimate pair formed prior to photoexcitation. The photosensitization of dimerization in our supramolecular system may be related to that in the intramolecular system. The sensitization reaction can be written ShY-

1s” -

3S*+A-

+j*

(1)

S+3A*

(2)

3A*+A+y-CyD3S*+

(=),q~

(A2)~D -

S+(J+~)~-C~D

(3) (4)

where A, AZ and S indicate anthracene derivatives, dianthracenes and triplet sensitizers respectively, the superscripts denote the excited states and the subscript (r_CyD) indicates the inclusion complexes. At the time of this investigation, it is uncertain how the triplet energy can be transferred to the dimerizable pair in the host cavity. Possibilities include energy transfer to the free monomer followed by complex formation (eqns. (2) and (3)) and direct energy transfer to the inclusion complex (eqn. (4)). In an attempt to illustrate direct energy transfer to the guest in the host cavity, chemical modification of the host compound with the sensitizer is now under investigation.

Acknowledgments

We are grateful to S. Kimura of the Tottori Kougyo Shikenjo for considerable assistance with the evaluation of the thermal stability.

References 1 T. Tamaki, Chem Leti., (1984) 53. T. Tamaki and T. Kokubu, L InclusionPhenom, 2 (1984) 815. T. Tarn&i, T. Kokubu and K. Ichimura, Temhedron,43 (1987) 1485. 2 H. L. J. Backstromand K. Sandros,Acra Chem Scund., 12 (1958) 823. 3 J. L. Charlton, R. Dabestani and J. SaItiel.J. Am. Chem Sot., 105 (1983) 3473. 4 B. Stevens, Adv. Photo&em., 171 (1971) 8. 5 J. Saltiel, G. R. Marchard, R. Dabestani and J. M. Pecha, Chem. Phys. Left., 100 (1983) 219. 6 H.-D.

Becker and K. Andersson, Tetmhedron L&f., 26 (1985) 6129. 7 T. Tamaki, Bull. Chem. Sot. Jpn., 51 (1978) 1145. 8 D. 0. Cowan and R. L. E. Drisko, I. Am. Chem. Sot., 91 (1970) 6286. 9 T. Tamaki, M. Sakuragi, K. Ichimura and K. Aoki, Chem. Whys Leti., 261 (1989) 23.