The optical properties of linear polyenes terminally substituted with different molecules (mainly anthryl- and TPP-substituents) have been studied. Bicyclic spacers within the polyene chain affect the quantum yields but do not interrupt intramolecular energy transfer between the endgroups

Blessing, Gerd

Effenberger, Franz

Holl, Norbert

Kesmarszky, Thomas

Port, Helmut

Schlosser, Hubert

Wolf, Hans Christoph

Online Publikationen der Universität Stuttgart

Mol. Cryst. Liq. Cryst.. 1990, vol. 183. pp. 21-30 
Reprints available directly from the publisher 
Photocopying permitted by license only 
© 1990 Gordon and Breach Science Publishers S.A. 
Printed in the United States of America 
INTRAMOLECULAR ENERGY TRANSFER mROUGH 
LINEAR POLYENES. EFFECT OF SPACERS. 
GERD BLESSING, NORBERT HOlL, HELMUT PORT, 
HANS CHRISTOPH WOLF 
3. PhysikaJisches lnstitut, Universiliit Stuttgart, D-7000 Stuttgart BO, W. Germany 
and 
FRANZ EFFENBERGER, TIIOMAS KESMARZKY, 
HUBERT SCHLOSSER 
lnstitut fUr Organische ehemie, Universitiit Stuttgart, D-7000 Stuttgart 80, W. Germany 
Abstract The optical properties of linear polyenes terminally substituted with 
different mokcules (mainly anthryl- and TPP-substituents) have been studied. 
Bicyclic spacers within the polyene chain affect the quantum yields but do not 
interrupt intramokcular ene~ transfer between the endgroups. 
1. INTRODUCfION 
In today's molecular chemical physics there is growing interest in investigating not 
only single organic molecules or the molecular solid state, but supermolecules or 
molecular functional units with specific physical, chemical or biological functions. 
On this line of research, we tried to find supermolecules in which energy absorbed 
in one specific component of the molecule can be transferred to an other one. 
There is hope that, in future, elements can be designed which perform logical 
functions on molecular electronic level. 
We have already shownl that in linear polyene molecules with two different 
substituents at both ends an intramolecular transfer of energy is possible and 
detectable. The present paper continues this research. In particular, the quantum 
yields of the different emissions observed from these supermolecules are 
determined and the question concerning the role of spacers incorporated into the 
polyene chain is addressed. 
21 
22 
A 
G. BLESSING, N. HOLL, H. PORT, H.C. WOLF et al. 
SY"I 9 ~I 1 ~I 4 
R, - 9A. 1.4-0M9A, CH~: R:z - CH~. TPP 
9A~CHO 
TPP 
(0) (b) 
FIGURE 1 Molecular structure of 
(a) the substituents anthracene (A), l,4-dimethyl-anthracene (DMA), 
tetraphenylporphyrin (TPP) and of 
(b) the supermolecules R(P n -R2 (n = 5 or 9), 9A-P 2-Bic-P I-CHO, 
9A-P 2-Bic-P n-TPP (n=6 or 8). 
2. EXPERIMENTAL 
TPP 
We have studied the optical properties of two types of supermolecules consisting of 
linear polyene chains terminally substituted with different molecules (mainly 
anthracenes and tetraphenylporphyrin). 
The first type (same as studied beforeI,2) contains a single polyene chain 
stabilized by methyl side groups in linear configuration. The second type 
additionally contains a spacer molecule, bicyclo[2.2.2]octane ("Bie"), which 
interrupts the conjugated polyene chain but does not absorb or emit light in the 
spectral regions of interest. Keeping within the schemes indicated in Fig. 1 the 
INTRAMOL. ENERGY TRANSFER, EFFECf OF SPACERS 23 
following nomenclature for the different molecules is used throughout this paper: 
A-P -TPP for polyene chains containing n double bonds (n=5 or 9) without spacer 
n 
and A-P2-Bic-Pn-TPP for molecules with spacer (Bic), differing in chain length on 
the TPP-substituted side (n=6 or 8). 
The molecules usually were dissolved in D.-hexane, in case of low solubility in 
chloroform (absorption spectrum of ~C-P 9-TPP in Fig. 2(a». The concentrations 
used were below 5 .10-6 mole/l in order to avoid concentration effects and to 
maintain low optical densities. The solutions were degassed from oxygene by 
repeated freeze-and-thaw cycles. 
Optical spectra were measured by standard techniques, absorption at 295K, 
emission and excitation at 180K, while staying above the solidification temperature 
of the solvent Emission spectra were corrected with respect to the spectral 
sensitivity of the detection system, excitation spectra were normalized to equal 
numbers of exciting photons. Quantum yields were measured relative to common 
standards: anthracene in n-hexane (30%)3, perylene in cyclohexane (78%)4 and 
TPP in benzene (11%/. 
3. MOLECULES WITIIOUT SPACER 
The absorption spectrum of the molecule 9A-P9-TPP is shown in Fig. 2(a) in 
comparison with those of the reference molecules H3C-P9-TPP and 9A-P9-CH3, 
respectively, and of the substituents anthracene and TPP. The spectrum clearly 
exhibits features characteristic for all three partner molecules: the S3°"SO anthryl 
absorption at about 256nm, the S2 ..... S0 and S1 ..... S0 TPP absorption (B- and Q-bands, 
respectively) with its most prominent maximum at about 425nm (BO'O) and the 
x 
S2 ..... S0 polyene (P 9) absorption contributing shoulders on both sides of the main 
TPP band. With respect to the individual molecules a loss of structural details and 
a slight red-shift is observed generally. While the extinction coefficients of the 
main A and TPP bands are strongly reduced, increasing absorption is observed in 
the spectral regions of TPP Q-bands and polyene. Both of these effects must be 
due to coupling between the substituent molecules and the polyene, which is more 
distinct for TPP substitution according to Fig. 2(a) (spectra of the reference 
molecules). Qualitatively the same behaviour is found for the corresponding 
compounds with shorter chain length (P 5) and for DMA substitution. The P 5 
absorption is decreased in strength relative to P 9 and blue-shifted, the absorption 
with DMA substituents is comparable to molecules with 9A substituents. 
24 G. BLESSING, N. HaLL, H. PORT, H.C. WOlF et aI. 
v/cm-1 v/cm-1 
16000 24000 40000 16000 24000 
200 
.... 150 I 
0 
.... 
)( 100 
.., 
50 
0 
200 
.... 150 I 
0 
.... 
I ,. 
: \ , , 
I \ 
t: 
;' \. I 
I ., • 
. ' ~.t 
: ). , . 
)( 100 
~ 
'iii 
c 
Q) 
..... 
c 
'. ~ ~' " '. f I w 50 , .: . 'J ~ ,: 
. ~ 
0 ~:"'".,. ..... 
700 600 500 400 JOO 200 800 700 600 500 400 300 
Wnm Wnm 
(0) (b) 
FIGURE 2 
(a) Absorption spectra 
top: --9A-P9-TPP,- - -H3C-P9-TPP, .- .. - A 
bottom: --9A-P 9-TPP, - - - - - 9A-P 9-CH3' - - - TPP 
(b) Emission spectra 
top: --9A-P9-TPP, 
bottom: - - - ~C-P 9-TPP, _ .. - - 9A-P 9-C~ 
Examples for the fluorescence emission spectra of 9A-P 9-TPP, 9A-P 9-CH3 
and H3C-P 9-TPP are given in Fig. 2(b). All spectra were obtained during 
excitation at ).exc = 256nm and nonnalized to the same maximum intensity. Dual, 
blue and red, fluorescence emissions are observed from the supermolecule, but 
single, blue or red, fluorescence components from the reference molecules. 
However, the respective emissions show the same spectral features, are located in 
the anthracene and TPP fluorescence regions (for Sl-+S0 transitions) and therefore 
are clearly determined by the substituents. Remarkably, in the 9A-P 9-CH3 
reference molecule the A-like fluorescence is maintained whereas in 9A-P s-CHO 
INTRAMOL ENERGY TRANSFER, EFFECf OF SPACERS 25 
it is totally quenched6. The relative intensity of red versus blue emission in the 
supermolecule is increased at shorter chain length (P 5) and also with excitation at 
longer wavelength (due to more direct TPP excitation, cf. Fig. 2( a». 
A quantitative evaluation of the fluorescence quantum yields was performed 
for blue and red emissions of the different compounds. Values obtained during 
excitation at ).~c =256nm are summarized in Table 1. In general they are very low 
in the composite molecules. Compared to the single substituent molecules the 
quantum yield of the blue emission is more heavily reduced for short than for long 
polyene chains. Red emission quantum yields are higher with DMA than with A 
substituents, blue emission quantum yields are lower. 
Table 1 Quantum yields in %, measured for ).exc =256nm. The given 
values belong to the red and blue emissions of the molecules, respectively. 
The experimental errors are within ±20%. 
molecule red emission blue emission 
A ------- 4S 
1,4DMA ------- 31 
TPP 1.6 --------
H3C-PS-TPP 0.9 --------
9A-PS-TPP 0.1 0.4 
1,4DM9A-PS-TPP 0.2 0.2 
H3C-P9-TPP 0.2 --------
9A-P9-TPP 0.03 1.7 
1,4DM9A-P9-TPP 0.2 0.4 
9A-P2-Bic-P1-CHO ------- 49 
OHC-P6-TPP 0.4 -------
9A-P2-Bic-P6-TPP 1.7 9 
OHC-Pa-TPP 0.2 -------
9A-P2-Bic-Pa-TPP 0.1 3 
26 
16000 
G. BLESSING, N. HOLL, H. PORT, H.C. WOLF et al. 
ii/cm-1 
24000 40000 16000 
, 
, 
I 
, 
,-,1 
, 
ii/cm-1 
2<4000 <40000 
I 
I I j I 
,-' , 
, I 
700 600 500 <400 Joo 200 700 600 500 <400 JOO 200 
Nnm Nnm 
(a) (b) 
FIGURE 3 Excitation and absorption spectra 
(a)-- excitation of 1,4-DM9A-Ps-TPP (Ad = 73Onm), 
- - - absorption of 1,4-DM9A-P s-TPP, 
- - - - - excitation of ~C-P s-TPP (Ad = 73Onrn) 
(b)-- excitation of 1,4-DM9A-P9-TPP (Ad = 73Onm), 
- - - absorption of 1,4-DM9A-P 9-TPP 
Excitation spectra have been measured with selective detection of the red 
(TPP) emission at Ad=73Onm. Examples are given in Fig. 3 for the molecules 
1,4-DM9A-PS-TPP and 1,4-DM9A-P9-TPP together with the corresponding 
absorption spectra. In the case of the shorter chain length the excitation spectrum 
follows the features of the absorption spectrum quite well. Apparently the 
TPP-emission is strongly excited not only in the spectral regions being typical for 
TPP absorption but also in all other regions, particularly in that of anthracene 
(S3+-S0) absorption. This excitation band is missing (Fig. 3(a» for H 3C-PS-TPP, a 
compound without A-substituent. Our results provide important evidence for an 
energy transfer within the supermolecule, which was already discussed in the 
1· 1 Iterature . 
A distinct difference between excitation and absorption spectra is observed 
at )'>425nm, below the TPP B-Band, for the molecule with longer chain length. 
Such excitation is not effective for populating the TPP St-state that the emission 
emerges from. Consequently the coupling between polyene and TPP in that energy 
region must be strongly dependent on chain length. On the other hand energy 
transfer from anthracene to TPP is still possible, even though the A S3 -band of the 
excitation spectrum in Fig. 3(b) has reduced intensity with respect to Fig. 3(a). 
INfRAMOL ENERGY TRANSFER, EFFECf OF SPACERS 27 
4. MOLECULES wrm SPACER 
In Fig. 4 the absorption spectra of the supermolecules with spacer 
9A-P2-Bic-Pn-TPP (n=6 or 8) are compared with those of the corresponding 
reference molecules, 9A-P 2-Bic-P (CHO and OHC-P n -TPP (n = 6 or 8). The 
synthetic spectra are constructed by normalizing the component spectra at the A 
S3-band and the TPP Q-bands, respectively. With respect to the supermolecules 
without spacer the essential spectroscopic features are maintained and determined 
by the partner molecules anthracene, polyene (P 6' P g) and TPP. Similarly the 
shoulders in the polyene spectral region are red-shifted for the P g relative to the 
P 6 compound. Moreover, additional distinct bands at 220nm, 280nm and 290nm 
appear in the spectra of both reference and supermolecules. 
iI/cm-1 'V/cm-1 
16000 24000 40000 16000 24000 40000 
150 
0'1 
, 
6100 
' . . . 
.: ~ 
..... _. : 
)( 
:,0' 
.., 50 
)( 
.. ' 
................ : .. 
0+-~.~···-·.-----.-----L,~~-----1 
150 
.--" 
....•. .: 
.~., 
.., 50 
O~~-.-----.--------r-----.-----~ 
700 600 500 400 300 200 700 600 500 400 300 200 
Nnm Nnm 
(0) (b) 
FIGURE 4 Absorption spectra of molecules with spacer 
(a) top: --9A-P 2-Bic-P fCHO, - .... OHC-P 6-TPP 
bottom: --9A-P 2-Bic-P 6-TPP measured, - - - synthetic (see text) 
(b) top: --9A-P2-Bic-P f CHO, ----- OHC-Pg-TPP 
bottom: -------- 9A-P 2-Bic-P g-TPP measured, - - - synthetic (see text) 
28 G. BLESSING, N. HOLL, H. PORT, H.C. WOLF et al. 
Whereas the 220nm band can be attributed to the short polyene chains and 
the CHO-group, the other two bands probably are due to mixed states originating 
from coupling between anthracene and polyene P 2. The comparative results of Fig. 
4 provide no indication for coupling between the polyene partners P 2 and P n 
across the spacer within the supermolecules. 
v/cm-1 
16000 24000 
800 700 600 500 400 300 
,vnm 
FIGURE 5 Emission spectra of molecules with spacer 
top: --9A-P 2-Bic-P 6-TPP, .... - 9A-P 2-Bic-P g-TPP 
bottom· --9A-P -Bic-P -CHO - - - - - OHC-P -TPP 
. 2 l' 6 
Examples for the fluorescence emission spectra of the same supermolecules 
and reference molecules as above are compiled in Fig. 5. All spectra were obtained 
during excitation at Aexc = 256nm and normalized to the same maximum intensity. 
As in section 3 dual fluorescence emissions, blue and red, are observed from the 
supermolecule, single blue or red emissions from the reference molecules. In 
contrast to Fig. 2(b) the blue emission is further red-shifted and has completely 
lost its vibronic sub-structure. Variation of the polyene P n chain length does not 
affect band positions and shapes, but the relative intensity of the red emission in 
the spectra is higher for P 6- than for P 8-compounds. 
INTRAMOL. ENERGY TRANSFER, EFFECT OF SPACERS 29 
A quantitative evaluation of the fluorescence quantum yields of different 
compounds with spacer was performed in the same manner as in section 3, for blue 
and red emissions with .\exc = 256nm. The obtained values listed in Tab. 1 (see 
section 3) show distinct variations in absolute numbers. Whereas for 
9A-P 2-Bic-P l-CHO the observed quantum yield of blue emission is as high as for 
the anthracene molecule, it is decreased for the supermolecule. The reduction is 
stronger for the P 8- than for the P 6-compound with spacer, but still essentially 
smaller than for supermolecules without spacer. The quantum yield of the red 
emission is about four times smaller for OHC-P6-TPP than for the TPP molecule 
and about as high as in TPP for the P 6-supermolecule. In the supermolecule and 
reference molecule with P 8' however, the quantum yield is drastically reduced and 
comparable to those of corresponding molecules without spacer. 
16000 
.. ' 
...... 
. , , 
..... .... 
v/cm-1 
24000 40000 
.; 
'. 
'. 
· . 
· . 
· . , 
: ~ 
'\'\ , 
.~ .. 
16000 
... 
.. ........ ! 
.. ...... 
v/cm-1 
24000 40000 
" 
" , , 
, , 
, ' , , 
, , 
, , 
: .... : \ 
, , 
700 600 SOO 400 300 200 700 600 500 400 300 200 
A/nm A/nm 
(a) (b) 
FIGURE 6 Excitation spectra of molecules with spacer in comparison with 
absorption spectra 
(a) 9A-P 2-Bic-P 6-TPP 
top: ..... absorption, --excitation ().d = 466.5nm) 
bottom: ..... absorption, --excitation ().d= 667.5nm) 
(b)9A-P 2-Bic-P 8-TPP 
top: ..... absorption, --excitation ().d=466.5nm) 
bottom: ..... absorption, --excitation ().d = 663.5nm) 
30 G. BLESSING, N. HOLL, H. PORT, H.C. WOLF et al. 
Excitation spectra of the supermolecules 9A-P2-Bic-P6-TPP and 
9A-P 2-Bic-P 8-TPP have been measured with selective detection on red and blue 
emission (red at Ad = 667.5nm and 663.5nm, respectively, blue for both molecules at 
).d=466.5nm). In Fig. 6 they are compared with the corresponding absorption 
spectra. The excitation spectra of both molecules completely reflect the structure of 
the absorption spectrum of the reference molecule 9A-P2-Bic-PI-CHO (Fig. 4 top) 
when detected in the blue spectral region. On the other hand, in case of red light 
detection for only the P 6-compound the excitation spectrum follows all relevant 
features of the absorption. Similar to the case without spacer (Fig. 3) the excitation 
spectrum of the P g-compound is strongly narrowed in the polyene absorption 
regions. The specific A-S3 absorption band is preserved for both supermolecules, 
carrying less contribution for the longer chain in agreement with the observations in 
section 3. The observed spectroscopic results provide evidence for existing 
intramolecular energy transfer between the endgroups of the supermolecules with 
spacer (with energy flow from the anthracene to the TPP side). 
Sf CONCLUSION 
Comparative studies on optical spectra and quantum yields of substituted polyenes 
with and without spacers have been performed. In molecules without spacer only a 
small fraction of the excitation energy is emitted radiatively. When introducing a 
bicyclic spacer, the fluorescence quantum yields are enhanced and intramolecular 
transfer of energy from the A- to the TPP-endgroup is still observed. 
ACKNOWLEDGEMENT 
Financial support by the Deutsche Forschungsgemeinschaft (SFB 329) is gratefully 
acknowledged. 
REFERENCES 
1. 
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4. 
5. 
6. 
St. Maier, H. Port, H.C. Wolf, F. Effenberger and H. Schlosser, 
Synthetic Metals, 22. ES17 (1989) 
F. Effenberger, H. Schlosser, P. Bauerle, St. Maier, H. Port and 
H.C. Wolf, Angewandte Chemie, lOOa, 274 (1988) 
S. Ates and A Yildiz, 
J. Chern. Soc., Faraday Trans. I, 79, 2853 (1983) 
J.B. Birks, 
Photophysics of Aromatic Molecules (Wiley & Sons, 1970), Tab. 4.3, p.122 
P.G. Seybold and M. Gouterman, 
J. Mol. Spectra J1,1 (1969) 
N.Holl, 
Diplomarbeit, UniversiUit Stuttgart (1988) 


Intramolecular energy transfer through linear polyenes : effect of spacers

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Intramolecular energy transfer through linear polyenes : effect of spacers

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