Experimental Spectroscopic Measurements




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Supplementary Material (ESI) for Chemical Communications



This journal is © The Royal Society of Chemistry 2000

Experimental

Spectroscopic Measurements


The fluorescence experiments were performed on a SPEX fluorolog 112 instrument (SPEX Industries, Metuchen, NJ, USA). The spectral bandwidths were for the excitation monochromator 5.6 nm, and for the emission monochromator 2.7 nm. All fluorescence spectra were corrected. A GBC 912 (GBC Scientific Equipment Pty, Ltd, Australia) was used for absorption measurements. Fluorescence lifetimes were determined using a PRA3000 instrument (PRA Inc., London, Ontario, Canada) equipped with Glan-Thompson polarisers. Light emitting diodes, NanoLED-01 and NanoLED-04 (IBH, Glasgow, Scotland, UK) were used for excitation. Excitation and emission wavelengths were selected by interference filters (Omega/Saven, Sweden) and long-pass filters (Schott Germany).
The Förster radii were calculated from molar absorptivity, of the acceptor and corrected fluorescence spectrum, of the donor according to1
(1)
where
(2)
and
(3)
Here, <2> denotes the average squared orientational part of a dipole-dipole interaction, d is the fluorescence quantum yield of the donor, n is the refractive index and NA is the Avogadro constant. In calculating the Förster radius (), <2> = 2/3 was chosen as a reference state. Refractive indices were measured and found to be n = 1.36 and 1.45 for ethanol and chloroform, respectively. For absorption measurements, the maximum absorbance was always about 0.6 while it for fluorescence measurements was less than 0.08. The distance, R, between donor and acceptor in the cassettes was estimated by using standard bond lengths. Calculated energy transfer efficiency based on Förster´s theory, Ecalc, was determined according to2
(4)
where
(5)
and
(6)
A detailed description of <2> and notations used is given elsewhere.3 In eqs. 4 and 5, denotes the fluorescence lifetime. The experimental energy transfer efficiency, Eexpt, was determined from steady-state fluorescence and absorption measurements as follows2
(7)
Here the molar absorptivity and the corrected fluorescence spectra for donor and donor-acceptor molecules are denoted by , and , , respectively. Due to spectral overlap between the donor and the acceptor absorption in compounds 1 and 3, was compensated for direct excitation of the acceptor. The fluorescence quantum yield was calculated from eq. 84,
(8)
where A is the absorbance recorded at the excitation wavelength and n is the refractive index of the medium. As references of fluorescence quantum yields were used; N,N´-bis(1-hexylheptyl)-3,4:9,10-perylene-bis(dicarboximide) in dichloromethane ( = 0.995), 3,3´,4,4´-difluoro-1,3,5,7-tetramethyl-4-borata-3a-azonia-4a-aza-s-indacene in methanol ( = 0.946) and Cresyl violet in methanol ( = 0.547). The refractive indices of dichloromethane and methanol are 1.42 and 1.33, respectively. Fluorescence quantum yields were also calculated using fluorescence lifetime measurements of the acceptor in the presence, , and absence, , of donor. It is easily shown that the calculated fluorescence quantum yield is given by
(9)
where stands for the fluorescence quantum yield of the acceptor molecule. For all systems studied, the fluorescence decays were bi-exponential with the average fluorescence lifetime calculated according to
(10)



Table texts


Table I. Absorption and fluorescence spectroscopic properties of 1 - 4 in ethanol and chloroform. Peak values of donor-absorption and acceptor-fluorescence spectra are denoted by and , respectively. The Förster radius, the distance between the D and A groups, the rate of energy transfer and the calculated and measured efficiencies are R0, R, Ecalc and Eexpt, respectively. The mean value of the square angular dependence of dipole-dipole coupling is . Uncertainties in transfer efficiencies are within  0.010 as based on three or more observations.
Table II. Fluorescence lifetimes and quantum yields of 1 - 4 in ethanol and chloroform. The fluorescence lifetime of the donor and the acceptor molecules are denoted and, while the lifetime of the acceptor in the presence the donor is . The quantum yields of fluorescence calculated from absorption and fluorescence spectra and from fluorescence lifetimes are denoted and , respectively. For some systems the fluorescence decays are not perfectly well described by a single exponential function. In such cases a bi-exponential fit was used and the lifetime thus reported is the average lifetime.
Table III. The average fluorescence lifetimes, , and its different lifetime components, 1 and 2, of the acceptor in compound 1 - 4. The fractions of each component are denoted f1 and f2, respectively.

References



1 T. Förster, Ann. Phys., 1948, 2, 55-75.

2 J. R. Lakovicz, ‘Principles of Fluorescence Spectroscopy’, Plenum, 1983.

3 L. B.-Å. Johansson, F. Bergström, P. Edman, I. V. Grechishnikova, and J. G. Molotkovsky, J. Chem. Soc., Faraday Trans., 1996, 92, 1563-1567.

4 F. R. Lipsett, Prog. Dielectr., 1967, 7, 217.

5 H. Langhals, J. Karolin, and L. B.-Å. Johansson, J. Chem. Soc. Faraday Trans., 1998, 94, 2919-2922.

6 I. A. Johnson, H. C. Kang, and R. P. Haugland, Anal. Biochem., 1991, 198, 228-237.

7 D. Magde, J. H. Brannon, T. L. Cremers, and J. Olmsted III, J. Phys. Chem., 1979, 83, 696-699.
Table I.




Compound






R0 ( Å )

R ( Å )







































C2H5OH

1

500

40

50.41.0

24.5

0.5

18.8

0.983

0.966




2

500

129

47.71.0

19.9

0.655

62.0

0.995

0.930




3

500

39

50.41.0

19.9

0.655

85.9

0.996

0.944




4

500

145

39.91.0

19.9

0.655

21.2

0.985

0.955































CHCl3

1

504

40

50.01.0

24.5

0.5

18.0

0.982

0.968




2

504

128

47.21.0

19.9

0.655

58.2

0.994

0.936




3

504

41

50.01.0

19.9

0.655

82.3

0.996

0.941




4

504

148

39.41.0

19.9

0.655

19.7

0.983

0.946

Table II.




Compound











































C2H5OH

1

3.00.2

5.30.2

3.50.2

0.75

0.66

0.48

0.470.02




2

3.00.2

2.40.2

0.90.2

0.33

0.41

0.13

0.120.01




3

3.00.2

5.30.2

4.20.2

0.75

0.79

0.56

0.450.03




4

3.00.2

5.00.2

3.20.2

0.68

0.63

0.41

0.360.02




























CHCl3

1

3.00.2

5.30.2

3.80.2

0.78

0.72

0.54

0.600.03




2

3.00.2

2.20.2

0.90.2

0.42

0.38

0.15

0.120.02




3

3.00.2

5.30.2

4.20.2

0.78

0.79

0.58

0.560.03




4

3.00.2

5.60.2

3.40.2

0.72

0.61

0.42

0.410.03


Table III.




Compound

































C2H5OH

1

3.50.2

3.8

0.70

2.8

0.30




2

0.90.2

0.85

0.99

3.3

0.01




3

4.20.2

4.5

0.86

1.5

0.14




4

3.20.2

3.0

0.94

5.8

0.06






















CHCl3

1

3.80.2

4.0

0.83

2.8

0.17




2

0.90.2

0.85

0.99

4.3

0.01




3

4.20.2

4.7

0.59

3.2

0.41




4

3.40.2

3.2

0.94

5.3

0.06


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