7072
J . Am. Chem. SOC. 1989, I 1 1 , 7072-7078
Supramolecular Photochemistry and Photophysics. Adducts of Pt(bpy)(NH3)22+with Aromatic Crown Ethers Roberto Ballardini,**laMaria Teresa Gandolfi,* , l b Luca Prodi,lb Mauro Ciano," Vincenzo Balzani,**laqb Franz H. Kohnke,lc Hooshang Shahriari-Zavareh,ldNeil Spencer,ld and J. Fraser Stoddadd Contribution from the Istituto FRAE- CNR Bologna, Italy, "G. Ciamician" Department of Chemistry of the University, Bologna, Italy, Department of Organic a n d Biological Chemistry of the University, Messina, Italy, a n d Department of Chemistry of the University, Sheffield, UK. Received February 22, 1989
Abstract: We report results of an investigation concerning absorption spectra, emission spectra, luminescence lifetimes, electrochemical properties, and photochemical behavior of Pt(bpy)(NH3)22+(bpy = 2,2'-bipyridine) and its adducts with the following crown ethers in CH2CI2and CH3CN solution: 18-crown-6 (1 8C6), dibenzc-18-crown-6 (DB18C6), dibenzc-24-crown-8 (DB24C8), dibenzo-30-crown- 10 (DB30C lo), dibenzo-36-crown- 12 (DB36C 12), bis(m-phenylene)-32-crown-10 (BMP32C IO), bis@-phenylene)-34-crown-l0 (BPP34C10), and dinaphtho-30-crown-10 (DN30C10). Adduct formation with the aliphatic crown ether 18C6 does not change the photochemical and photophysical properties of the Pt(bpy)(NHJ?+ complex. By contrast, adduct formation with the aromatic crown ethers causes (i) a strong decrease of the crown ether absorption band and of the ligand (bpy)-centered absorption bands of Pt(bpy)(NH312*+in the 260-330-nm region, (ii) the appearance of a new broad absorption band in the 340-450-nm region, (iii) the complete or partial quenching of the crown ether fluorescence and of the ligand-centered phosphorescence of p t ( b ~ y ) ( N H ~ )(iv) ~ ~the + , appearance of a new, broad, and short-lived luminescence band in the 550-630-nm region, (v) the quenching of the photoreaction of Pt(bpy)(NH3)22t in CH2CI2,and (vi) a perturbation in the electrochemical reduction potentials. These results are attributed to an electronic interaction, in the ground and excited state, between the bpy ligand of the Pt complex and the aromatic rings of the crowns. The intensity of such electronic interaction depends on the size of the crown ring and on the nature and substitution positions of the aromatic rings present in the crown. The results obtained show that the assembly of a coordination compound into an appropriate supramolecular structure can protect the compound from photoreaction and can profoundly change its spectroscopic, photophysical, and electrochemical properties.
In the field of photochemistry, which is a t the crossroads of chemistry, physics, and biology and a t the interface between matter and light, the quantitative growth of the past 20 years has been accompanied by profound qualitative changes. The interest of the research workers has progressively moved from photoreactions taking place inside molecules (intramolecular p h o t o ~ h e m i s t r y ) ~ - ~ to processes taking place between long-lived excited states and suitable reaction partners (intermolecular electron- and energytransfer proces~es).~-ll In the past few years a trend to study artificial assemblies of two or more molecular components (supramolecular photochemi~try)l~,'~ has also emerged with the dual aim of making progress toward the understanding of photobiological processes and the design of artificial systems capable of performing useful functions. In the area of supramolecular photochemistry one can distinguish (Figure 1) systems made by (i) covalently linked molecular building blocks (e.g., diads, triads, tetrads, polynuclear com( I ) (a) lstituto FRAE-CNR, Bologna, Italy. (b) 'G. Ciamician" Department of Chemistry of the University, Bologna, Italy. (c) Department of Organic Chemistry of the University, Messina, Italy. (d) Department of Chemistry of the University, Sheffield, UK. (2) Turro, N. J. Molecular Photochemistry; Benjamin; New York, 1965. (3) Balzani, V.; Carassiti, V. Photochemistry of Coordination Compounds; Academic Press: London, 1970. (4) Turro, N. J. Modern Molecular Photochemistry: Benjamin: Melno Park, CA, 1978. ( 5 ) Balzani, V.; Moggi, L.; Manfrin, M. F.; Bolletta, F.; Laurence, G. S . Coord. Chem. Rev. 1975, 15, 321. (6) Balzani, V.; Bolletta, F.; Gandolfi, M. T.; Manfrin, M. F. Top. Curr. Chem. 1978, 75, 1. (7) Karvanos, G. J.; Turro, N. J. Chem. Rev. 1986, 86, 401. (8) Juris, A,; Balzani, V.; Barigelletti, F.; Campagna, S.; Belser, P.; von Zelewsky, A. Coord. Chem. Rev. 1988, 84, 8 5 . (9) Meyer, T. J. Prog. Inorg. Chem. 1983, 30, 389. (IO) Sutin, N.; Creutz, C. J . Chem. Educ. 1983, 60, 809. ( 1 I ) Fox, M. A., Chanon, M., Eds. Photoinduced Electron Transfer; Elsevier: Amsterdam, 1988; parts C and D. (12) Balzani, V . ; Sabbatini, N.; Scandola, F. Chem. Rev. 1986, 86, 319. (13) Balzani, V., Ed. Supramolecular Photochemistry; D. Reidel: Dordrecht, Holland, 1987.
0002-7863/89/1511-7072$01 S O / O
p l e ~ e s ) ' ~ -and ~ * (ii) molecular entities associated by electrostatic interaction, hydrogen bonds, or other intermolecular forces (host-guest systems, ion-pair species, second-sphere coordination compIexes).12J8-2' Continuing our investigations on systems of the latter type,22 we have now examined in a systematic way the photochemical and photophysical behavior of the host-guest species originating from p t ( b ~ y ) ( N H ~ and ) ~ ~crown + ethers of different size and structure (Figure 2).23 Experimental Section Chemicals. [ p t ( b p ~ ) ( N H ~ ) ~ l ((bpy P F ~=) ~2,2'-bipyridine) has been prepared from Pt(bpy)C12 following a slightly modified literature proc e d ~ r e . ~ ~The " ? ~purity of the compound was checked by elemental analysis and by comparison with the known absorption spectrum.2" The crown ethers 18-crown-6 (18C6), dibenzo-18-crown-6 (DB18C6), and dibenzo-24-crown-8 (DB24C8) were obtained from Aldrich. The other
For recent examples, see ref 15-18. (15) Gust, D.; Moore, T. A,; Moore, A. L.; Makings, L. R.; Seely, G.R.; Ma, X.; Trier, T. T.; Gao, F. J . A m . Chem. SOC.1988, 110, 7567. (16) Hopfield, J. J.; Onuchic, J. N.; Beratan, D. N. Science 1988, 241, 817. (17) Bignozzi, C. A,; Paradisi, C.; Roffia, S . ; Scandola, F. Inorg. Chem. 1988, 27, 408. (18) Lehn, J.-M. Angew. Chem., In?. Ed. Engl. 1988, 27, 89. (19) Hennig, H.; Rehorek, D.; Archer, R. D. Coord. Chem. Reu. 1985,61, (14)
1.
(20) Ramamurthy, V.; Eaton, D. F. Acc. Chem. Res. 1988, 21, 300. (21) Shinkai, S . Pure Appl. Chem. 1986, 58, 1253. (22) Ballardini, R.; Gandolfi, M. T.; Balzani, V.; Kohnke, F. H.; Stoddart, J. F. Angew. Chem., Int. Ed. Engl. 1988, 27, 692. (23) (a) Colquhoun, H. M.; Stoddart, J. F.; Williams, D. J. Angew. Chem., In?. Ed. Engl. 1986, 25, 487. (b) Stoddart, J. F.; Zarzycki, R. In Cation Binding by Macrocycles: Complexation of Cationic Species by Crown Ethers; Inoue, Y., Gokel, G. W., Eds.; Marcel Dekker: New York, in press. (24) (a) Morgan, G. T.; Burstall, F. H. J . Chem. SOC.1934, 965. (b) Colquhoun, H. M.; Doughty, S. M.; Maud, J. M.; Stoddart, J. F.; Williams, D. J.; Wolstenholme, J. B. Isr. J . Chem. 1985, 25, 15. (c) Colquhoun, H. M.; Stoddart, J. F.; Williams, D. J.; Wolstenholme, J. B.; Zarzycki, R. Angew. Chem., In?. Ed. Engl. 1981, 20, 1051.
0 1989 American Chemical Society
J . Am. Chem. SOC.,Vol. I l l , No. 18, 1989 7073
Supramolecular Photochemistry and Photophysics
AE or e'
AE or e-
nn
I
hJ
U Figure 1. Schematic representation of two types of supramolecular systems: covalently linked molecular building blocks (A); host-guest systems (B).
0 18C6
DB18C6
DB24C8
DB30C10
Table I. Absorption, Emission, and Lifetime Data of the Aromatic Crown Ethers in CH2C12 absorption" emission 298 K _______ K b 77 KC 298 aromatic 2, M-I crown ethers A,nx3 nm cm-' A,, nm I," ns Amax9 nm I, s 277 5650 310 0.4 DB18C6 277 5200 308 0.5 460 >lo-" DB24C8 277 4400 308 0.4 DB30C10 277 4700 308 0.7 460 >lo-" DB36C12 275 3700 300 1.0 410 >lo-'' BMP32C10 28 1 3300 420 290 5300 320 0.6 BPP34C12 485 1 .6d 268 8900 339 9.6 DN30C10 274 8100 310 4400 315 3800 "Only the absorption band above 250 nm are reported. bFluorescence. 'Phosphorescence. dEstimated uncertainty &lo%. 'See text.
2.c
P
0 X
0 1.0
DB36C12
BMP32C 10
O.? BPP34C10
DN30C10
D
G
A,
Figure 2. Structures of the crown ethers and abbreviations used.
nm
Figure 3. Absorption spectra in CH2CI2: DB30C10 (NH3)Z2+.18C6 (-- -); Pt(bpy)(NH3),2+*DB30ClO (-).
(-e-);
crowns, i.e., dibenzo-30-crown-10 (DB30C10), dibenzo-36-crown-12 (DB36C12), dinaphtho-30-crown-10 (DN30C10), bis(m-phenylene)-32crown- 10 (BMP32C IO), and bis(p-phenylene)-34-crown-10 (BPP34ClO) were prepared following literature p r o c e d ~ r e s . ~Tetrabutylammonium ~ tetrafluoroborate and tetrabutylammonium perchlorate were Fluka products. Acetonitrile ( A N ) was an R P E Carlo Erba product. Dichloromethane (CH2C12) was a Merck product. Both were distilled before use on P205. Butyronitrile (Fluka) was distilled together with CH2CI2 to remove water traces. The solid 1:1 adducts between [Pt( b ~ y ) ( N H ~ ) ~ ] ( p F and 6 ) 2 DB24C8 or DB30C10 were prepared as previously reported.'* The other adducts were obtained in solution by adding [ P ~ ( ~ ~ Y ) ( N H ~ ) to ~ ]a (stoichiometric PF~)~ amount or an excess of crown ether dissolved in the selected solvent. The concentration of the platinum complex was 1 .O X M unless otherwise noted. Equipment. Absorption spectra were recorded with a Kontron Uvikon 860 spectrophotometer. Uncorrected emission spectra were recorded with a Perkin Elmer 650-40 spectrofluorimeter. Corrected excitation spectra were obtained with a Perkin Elmer LS 5 spectrofluorimeter that was also used to obtain luminescence lifetimes in the 5 1s-10 ms time region. A Nd:YAG DLPY4 JK laser and an Edinburgh single-photon-counting apparatus were used for longer and shorter lifetimes, respectively. In all cases the experimental error on the lifetime values is estimated to be &lo%. In the Edinburgh system the flash lamp was filled with HI, and a monochromator was used to select the excitation wavelength. The instrument response function was deconvoluted from the emission data to obtain an undisturbed decay that was then fitted by a least-squares (25) (a) Colquhoun, H . M.; Goodings, E. P.; Maud, J. M.; Stoddart, J. F.; Wolstenholme, J. B.; Williams, D. J. J . Chem. Soc., Chem. Commun. 1983, 1140; J . Chem. Soc.. Perkin Trans. 2 1985, 607. (b) Allwood, B. L.; Shahriari-Zavareh, H.; Stoddart, J. F.; Williams, D. J. J . Chem. SOC.,Chem. Commun. 1987, 1058. (c) Allwood, B. L.; Spencer, N.; Shahriari-Zavareh, H.; Stoddart, J. F.; Williams, D. J. J . Chem. Soc., Chem. Commun. 1987, 1061. (d) Stoddart, J. F. Pure Appl. Chem. 1988, 60, 467.
Pt(bpy)-
method on an IBM PC. Each sample gave rise to single-exponential decays. For very low intensity emissions taking place with I 5 10 ns lifetimes, values could not be obtained because it was impossible to separate the luminescence from the scattering of the exciting light. Luminescence quantum yields were measured by using R ~ ( b p y ) , ~in + aerated aqueous solution (@em = 0.02826) as a standard for room-temperature experiments and were corrected for different refractive index of the solvents. At 77 K, the standard was Rh(bpy),'+ (ae, = 1.02') in butyronitrile. In both cases the estimated error is f20%. Photochemical experiments were carried out with a medium-pressure Q 400 Hanau mercury lamp whose light was filtered with a Oriel interference filter (A, = 313 nm, T,,, = 16%). The solutions were irradiated in a spectrophotometric cell (capacity, 3 mL; optical path, 1 cm) housed in a thermostated cell holder and stirred with a magnetic stirrer. The incident light intensity (1.6 X lo-' Nhvlmin) was measured by the ferric oxalate actinometer.28 Cyclic voltammetry was performed with an Amel 448/XA oscillographic polarograph on Ar-purged acetonitrile and dichloromethane solutions containing 0.1 M tetrabutylammonium tetrafluoborate and tetrabutylammonium perchlorate, respectively, as supporting electrolyte. The working electrode was a platinum microelectrode, the counter electrode was a platinum wire, and a standard calomel electrode (SCE) was the reference electrode. All the experiments were performed at room temperature (-25 "C), unless otherwise noted.
Results T h e experiments were carried o u t in CH2C1,, acetonitrile, or butyronitrile, [Pt(bpy)(NH,),] (PF,), is soluble in acetonitrile and (26) Van Houten, J.; Watts, R. J. J . Am. Chem. SOC.1975, 97, 3843. (27) Halpern, W.; DeArmond, M. K. J . Luminesr. 1972, 5 , 225. (28) (a) Hatchard, C. G.; Parker, C. A. Proc. R. SOC.London, A 1956, 235, 518. (b) Fisher, E. EPA Newsletters 1984, July 33.
1074 J . Am. Chem. SOC.,Vol. 111, No, 18, 1989
Ballardini et al.
Table 11. Absorption, Emission, and Lifetime Data of [Pt(bpy)(NH3)z](PF6)z and Its Adducts with Crown Etherso absorptionb emission 298 K 298 K A,,
[Pt(b~y)(NHd21 (PF&z Pt (bpy) (N H3)22’. 18C6 Pt(bpy)(NH3)F*DB18C6
nm 319e 307e 319 308 319 308 319 3 20 309 3 20
e,
M-’ cm-’ 18 000 12 500 21 000
488(f
