EXCITED STATE PROCESSES IN CROWNED STYRYL DYES

Laser Photonics Laboratory

Dept. of Chemistry, Moscow State University, Vorob'evy gory, Moscow, 119899, Russia

e-mail: druzh@light.chem.msu.su

Photophysical and photochemical processes in crowned styryl dyes 1-3 and their metal complexes with alkali (Li⁺, Na⁺, and K⁺), alkaline-earth (Mg²⁺, Ca²⁺, Sr²⁺, and Ba²⁺), and Cd²⁺ cations were studied.

R₁-R₂ = C₄H₄ (1a, 1b, 2d, 3c, and 3e); CMe=CHCMe=CH (2a, 2b, 3a, 3b, and 3d); CH=CMeCMe=CH (2c); and CH=C(OMe)CH=CH (2e). R₃ = Me⁺ (2a-2c, 3a, 3c, and 3d) and Et⁺ (2d, 2e, 3b, and 3e). X = CMe₂ (2a-2c, 3a, 3c, and 3d) and S (1a, 1b, 2d, 2e, 3b, and 3e). n = 1 (1a, 1b, 2a-2e) and 2 (3a-3e). R₄ = Ph (2a), (2b), (3a and 3b), (1a, 2c, 2d, and 2e), (1b, 3c, and 3e), (3d).

PHOTOISOMERIZATION. High quantum yield of trans-cis-isomerization of sryryl dyes 1 and 2 series results in low fluorescence intensity in non-viscous solvent. The temperature fluorescence quenching of 1 and 2 are described by the following equation: I^-1 = I₀^-1 + kh ^-1(T)exp(-E_a/RT), where I and I₀ are fluorescence intensity, h (T) is friction coefficient, E_a is intrinsic activation energy of trans-cis-isomerization. The results obtained (Table 1) show that E_a depends on charge of dye molecule, macrocycle size, and solvent polariry.

ESRC REACTION. The complex formation of crowned dyes 1a, 1b, 2c-2e, and 3c-3e with alkali (Li⁺, Na⁺, and K⁺), alkaline-earth (Mg²⁺, Ca²⁺, Sr²⁺, and Ba²⁺), and Cd²⁺ cations leads to shortwavelength shift (800-7400 cm^-1) of absorption spectra. As this takes place, fluorescence spectra of cationic dyes 2c-2e and 3c-3e are shifted to shortwavelength only by 40-320 cm^-1 (Table 2). The fluorescence intensity of complexes is higher than that of non-bonding dyes 3c-3e (Table 2). The absorption and fluorescence spectra of non-crowned dye 2a, 2b, 3a, and 3b does not change in the presence of salt. This clearly shows that intramolecular excited state recoordination (ESRC) reaction takes place in metal complexes of cationic crowned dyes 2c-2e, and 3c-3e (Scheme). The reaction, in essence, is the breaking of bond of metal cation with macrocyclic nitrogen atom. As the result of reaction the size of p-system of dye is increased owing to incorporation of the long pair of oxygen or nitrogen atom. The fluorescence spectrum profile of ESRC reaction product is close to that of non-bonding crowned dye.

Table 2

LASING. The solution of dyes 3 in MeCN lase in spectral region 683.8-731.6 nm at the excitation by second harmonic of ruby laser (347 nm, 20 ns) in transversal geometry. The lasing threshold (t) of crowned styryl dyes 3c-3e and their metal complexes is much less than that of non-crowned dyes 3a and 3b (Table 2). It is caused predominantly by fluorescence quantum yield increasing as going from dyes 3a and 3b to crowned dyes 3c-3e and their metal complexes. The differential lasing efficiency (q ) changes only slightly on introducing of crown ether into dye 3a (Table 2) or on complex formation of dye 3c with Na⁺ and 3d with Mg²⁺, Ca²⁺, and Cd²⁺. It is concluded that that excited dyes 3a, 3c, and 3d and their complexes 3cNa⁺, 3dMg²⁺, and 3dCa²⁺ have a stimulated emission cross-section close to each other.

ESRC LASER. A novel type of photochemical laser is established. Laser radiation of metal complexes of dyes 3c, 3d, and 3e is described by electronic four-level scheme (Scheme). The absorption of the exciting light results in the transition of complex from the ground to excited state. Then the excited complex is converted into excited product with broken Me-N bond by ESRC reaction. The excited product lases passing to the ground state. The back recoordination reaction (RC) recovers the original metal complex of dye from the product with broken Me-N bond.

This work was supported by Russian Fund of Basic Researches (grant N 95-03-09482)