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Fluorescence of the helicenes
Volume
2, number 6
CHEMICAL
PHYSICS
FLUORESCENCE
E. VANDER Organic
Clremistry
OF DONCKT
Department,
Free
J. R. GREENLEAF
Atomic
and Molecular
P?tysics
Group.
The Schuster
Received
LETTERS
THE
October
HELICENES
and J. NASIELSKX
of Brussels,
University
Belgium
and J. B. BIRKS Laboratory,
II September
University
of Manchester,
in fig.
1.
The fluorescence quantum efficiency ((I) of each compound was obtained by comparison of its integrated fluorescence spectrum with that of a reference solution of 9, lo-diphenylanthracene in cyclohemne, a correction being applied for the difference in the solvent refractive indices [I]. A value of 4 = 0.8 was assumed for tpe reference solution [2]. The fluorescence lifetime (T) of eacfi compo~d was observed in deoxygenated 1, 4-dioxan solution at room temperature. Solute concentrations of about 5 X 10’4M were used, ‘and the observations were made in reflect on using a pulsed light fluorometer [3]. The experimental values of q and ‘i; and OPthe radiative lifetime (7o), calculated from the relation, r. = 714
(1)
are listed in table 1. The absorption spectra of the compounds from 200 to 500 nm wavelength have been published [4,5]. Fig. 2 plots the low-energy end of the absorption spectra. For hexaheficene the first absorption band consists of two low-intensity maxima separated by about 1300 cm-‘. From a de-
UK
1968
The ffuorescence spectra. quantum efficiencies and lifetimes of hexa-, hepta-. o&awere observed in I,$-diovan solution. The fluorescence is assigned to the ILb state. the absorption spectra of the higher compounds.
The fluorescence spectra of hexa-, h&a-, octa- and nona-helicene were observed in 1,4dioxan solution at room temperature. Concentrations of < 10’5M were used to avoid self-absorption, and the solutions were deoxygenated by nitrogen bubbling. The solutes were excited by radiation of 328 nm wavelen~h, and the emission was observed at SO0 to the incident beam. The fluorescence spectra, corrected for the spectral response of the lP28 photomultiplier, are plotted
1968
and nona-helicene which is obscured
in
I 70 60 50 LO 30 -
20 ” ‘104
26000
2iuOo
Fig. 1. Fluorescence quantum I (arbitrary 6 = hexa,
22000 3 km’
2oooo 2
185W
spectra of helicenes. Relative _z units) against xavenumbec is (cm 1
7 = hepta,
8 = octa, 9 = nona.
tailed study of the low-temperature absorption and fluorescence spectra of hexahelicene, this band has been assigned to a IL:, - IA transition [6]. For hepta- and octa-helicene, the shouLders at 23 600 cm-l and 23 000 cm-l, respectively, on the edges of the adjacent more intense absorption bands might also be attributed to x&b - LA transitions. For non~~~i~ene a slight inflexion or2 the absorption edge is the only h+t OEthe presence of a 1Lb - 1~ transition. The fluorescence spectra (fsg. 1) of the folir 408
CHEMICAL
Volume 2, number 6
Fluorescence
properties
Table 1 of helicenes in 1.4-dioxan lution r(ns)
PHYSICS LETTERS
Q
Hexahelicene
0.041
14.5
Heptahelicene
0.021
13.8
0.66
0.57
Octahelicene
0.014
10.0
0.71
0.59
Nonahelicene
0.014
9.6
0.68
‘L . \ . \ . I .
I ; 8 . ,
i.
(2)
where 71is the refractive index of 1,4-dioxan, and the integral is taken over the calculated ‘Lb - IA absorption band, which has a molar extinction coefficient E( 8) at wavenumber D, and a mean wavenumber va. The theoretical values of io from (2) are listed in table 1. They agree satisfactorily with the experimental values of To from (l), considering the approximations involved in the caicuiaQons. It. is concluded that the fluorescence of hexa-, hepta-, octa- and nona-helicene corresponds to a lLb-lA transition. This applies even to nonaheiicene where the ‘Lb - ?A absorption is masked
410
\.
‘;b(/.Ls) from IrOrn (2) (1) 0.30 =_0.35
compounds have a similar structure with two maxima separated by about 1000 cm-l, which are shifted towards the red T.vith increase in moiecular size. The value of r. for each compound is - 1A transition, e.g. for benzene typical of a IL there is reasonF. = 0.52 ps [7‘i . For hexahellcene able mirror symmetry between the 1Lb - lA absorption band and the fluorescence spectrum, confirming the assignment of the latter to a lLb - IA transition, but no such mirror symmetry is apparent for the other compounds. bands of The “hidden” IL b - IA absorption hexa-, hepta- and o&a-heiicene were calculated on the following assumptions: (i) the shape of the band is mirror-symmetrical with respect to the corresponding fluorescence spectrum; and (ii) the intensity of the first band (in hexaheiicene) or shoulder (in hepta.. or octa-heiicene) is not enhanced by the underlying tail of the adjacent absorption band. The theoretical radiative lifetime was then evaluated from the relation [8] = 2.88 x 10-%$2j-@)dF
1968
so-
Compound
l,j;,
October
27om
26OW
25000 akfti'l 7h00
23000
22cul
Fig. 2. Absorption edges of helicenes. Molar extinction coefficient E against wavenumber P(cm-1).
by the adjacent intense absorption may be assigned to IL,IA.
band,
which
One of us thanks the Fonds National de la Recherche Scientifique Beige (F.N.R.S.) for financial support. The authors wish to thank Professor R. H. Martin for the generous gift of the samples of hexa-, hepta-, octa- and aonahelicenes.
REFERENCES [l] J. 3. Hernans and S. Levinson. J. Opt. Sec. Amer. 44 (1931) 4GJ. [2] T. Medinger and F. Wiitiinson. Trans. Faraday Sot. 61 (1965)
620.
[3] J. B. Birks. T. A.King and I. H. Mum-o. Proc. Phys. Sot. 80 (19G2) 355. 143 31. Flammang-Barbieus. J. Nnsielski and R.H. Martin. Tetrahedron Letters 8 (1967) 743. (51 R. H. Mark. &I. Flammang-Barhiew. J. P. Cosyn and hi. Gelbcke. Tetrahedron Letters 31 (1968) 3507. [6] 0. E. Weigang Jr., J. A-Turner and P. A.Trouard. J. Chem. Phys.45 (1966) 1126. [7] J. B. Birks and 1. H. Munro. Progress in Reaction Kinetics 4 (1967) 239. [8] G.N. Lewis and M.Kasha. J. Am. Chem. Sot. 67 (1945) 994.
2, number 6
CHEMICAL
PHYSICS
FLUORESCENCE
E. VANDER Organic
Clremistry
OF DONCKT
Department,
Free
J. R. GREENLEAF
Atomic
and Molecular
P?tysics
Group.
The Schuster
Received
LETTERS
THE
October
HELICENES
and J. NASIELSKX
of Brussels,
University
Belgium
and J. B. BIRKS Laboratory,
II September
University
of Manchester,
in fig.
1.
The fluorescence quantum efficiency ((I) of each compound was obtained by comparison of its integrated fluorescence spectrum with that of a reference solution of 9, lo-diphenylanthracene in cyclohemne, a correction being applied for the difference in the solvent refractive indices [I]. A value of 4 = 0.8 was assumed for tpe reference solution [2]. The fluorescence lifetime (T) of eacfi compo~d was observed in deoxygenated 1, 4-dioxan solution at room temperature. Solute concentrations of about 5 X 10’4M were used, ‘and the observations were made in reflect on using a pulsed light fluorometer [3]. The experimental values of q and ‘i; and OPthe radiative lifetime (7o), calculated from the relation, r. = 714
(1)
are listed in table 1. The absorption spectra of the compounds from 200 to 500 nm wavelength have been published [4,5]. Fig. 2 plots the low-energy end of the absorption spectra. For hexaheficene the first absorption band consists of two low-intensity maxima separated by about 1300 cm-‘. From a de-
UK
1968
The ffuorescence spectra. quantum efficiencies and lifetimes of hexa-, hepta-. o&awere observed in I,$-diovan solution. The fluorescence is assigned to the ILb state. the absorption spectra of the higher compounds.
The fluorescence spectra of hexa-, h&a-, octa- and nona-helicene were observed in 1,4dioxan solution at room temperature. Concentrations of < 10’5M were used to avoid self-absorption, and the solutions were deoxygenated by nitrogen bubbling. The solutes were excited by radiation of 328 nm wavelen~h, and the emission was observed at SO0 to the incident beam. The fluorescence spectra, corrected for the spectral response of the lP28 photomultiplier, are plotted
1968
and nona-helicene which is obscured
in
I 70 60 50 LO 30 -
20 ” ‘104
26000
2iuOo
Fig. 1. Fluorescence quantum I (arbitrary 6 = hexa,
22000 3 km’
2oooo 2
185W
spectra of helicenes. Relative _z units) against xavenumbec is (cm 1
7 = hepta,
8 = octa, 9 = nona.
tailed study of the low-temperature absorption and fluorescence spectra of hexahelicene, this band has been assigned to a IL:, - IA transition [6]. For hepta- and octa-helicene, the shouLders at 23 600 cm-l and 23 000 cm-l, respectively, on the edges of the adjacent more intense absorption bands might also be attributed to x&b - LA transitions. For non~~~i~ene a slight inflexion or2 the absorption edge is the only h+t OEthe presence of a 1Lb - 1~ transition. The fluorescence spectra (fsg. 1) of the folir 408
CHEMICAL
Volume 2, number 6
Fluorescence
properties
Table 1 of helicenes in 1.4-dioxan lution r(ns)
PHYSICS LETTERS
Q
Hexahelicene
0.041
14.5
Heptahelicene
0.021
13.8
0.66
0.57
Octahelicene
0.014
10.0
0.71
0.59
Nonahelicene
0.014
9.6
0.68
‘L . \ . \ . I .
I ; 8 . ,
i.
(2)
where 71is the refractive index of 1,4-dioxan, and the integral is taken over the calculated ‘Lb - IA absorption band, which has a molar extinction coefficient E( 8) at wavenumber D, and a mean wavenumber va. The theoretical values of io from (2) are listed in table 1. They agree satisfactorily with the experimental values of To from (l), considering the approximations involved in the caicuiaQons. It. is concluded that the fluorescence of hexa-, hepta-, octa- and nona-helicene corresponds to a lLb-lA transition. This applies even to nonaheiicene where the ‘Lb - ?A absorption is masked
410
\.
‘;b(/.Ls) from IrOrn (2) (1) 0.30 =_0.35
compounds have a similar structure with two maxima separated by about 1000 cm-l, which are shifted towards the red T.vith increase in moiecular size. The value of r. for each compound is - 1A transition, e.g. for benzene typical of a IL there is reasonF. = 0.52 ps [7‘i . For hexahellcene able mirror symmetry between the 1Lb - lA absorption band and the fluorescence spectrum, confirming the assignment of the latter to a lLb - IA transition, but no such mirror symmetry is apparent for the other compounds. bands of The “hidden” IL b - IA absorption hexa-, hepta- and o&a-heiicene were calculated on the following assumptions: (i) the shape of the band is mirror-symmetrical with respect to the corresponding fluorescence spectrum; and (ii) the intensity of the first band (in hexaheiicene) or shoulder (in hepta.. or octa-heiicene) is not enhanced by the underlying tail of the adjacent absorption band. The theoretical radiative lifetime was then evaluated from the relation [8] = 2.88 x 10-%$2j-@)dF
1968
so-
Compound
l,j;,
October
27om
26OW
25000 akfti'l 7h00
23000
22cul
Fig. 2. Absorption edges of helicenes. Molar extinction coefficient E against wavenumber P(cm-1).
by the adjacent intense absorption may be assigned to IL,IA.
band,
which
One of us thanks the Fonds National de la Recherche Scientifique Beige (F.N.R.S.) for financial support. The authors wish to thank Professor R. H. Martin for the generous gift of the samples of hexa-, hepta-, octa- and aonahelicenes.
REFERENCES [l] J. 3. Hernans and S. Levinson. J. Opt. Sec. Amer. 44 (1931) 4GJ. [2] T. Medinger and F. Wiitiinson. Trans. Faraday Sot. 61 (1965)
620.
[3] J. B. Birks. T. A.King and I. H. Mum-o. Proc. Phys. Sot. 80 (19G2) 355. 143 31. Flammang-Barbieus. J. Nnsielski and R.H. Martin. Tetrahedron Letters 8 (1967) 743. (51 R. H. Mark. &I. Flammang-Barhiew. J. P. Cosyn and hi. Gelbcke. Tetrahedron Letters 31 (1968) 3507. [6] 0. E. Weigang Jr., J. A-Turner and P. A.Trouard. J. Chem. Phys.45 (1966) 1126. [7] J. B. Birks and 1. H. Munro. Progress in Reaction Kinetics 4 (1967) 239. [8] G.N. Lewis and M.Kasha. J. Am. Chem. Sot. 67 (1945) 994.