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Model mechanisms for the thermal cis—trans isomerization of cyanines
Volume 172, number 2
CHEMICAL PHYSICS LETTERS
31 August 1990
Model mechanisms for the thermal cis-trans isomerization of cyanines Klaus Schijffel Institutfir Physikulische und Theoretische Chemie, Freie UniversitiitBerlin, Takustrawe 3, &lo00 Berlin 33, Germany
Fritz Dietz and Thomas Krossner Sektion Chemie, Karl-Marx-Universiriil Leipzig, Talstrasse 35, DDR-7010 Leipzig, German Democratic Republic
Received 5 March 1990; in final form 15 June 1990
Quantumshemical calculations have been performedfor rotations around the different CC bonds of streptocyanine eations and of various ion pairs of TMC+ and PM? with Cl- as the gegenion (counterion) in order to explain the experimentally well known temperature dependence of the activation energy for the thermal isomerization.
1. Introduction The photochemical tram-cis isomerization is a very effective deactivation process for the first-cxcited singlet state of most cyanines [ l-31. This deactivation channel diminishes the efficiency of spectral sensitization and the laser activity of the dyes. Whereas the thermal trans-cis isomerization of cyanines is an exceptional case [ 4 1, the back-reaction cis-trans proceeds mainly thermally induced from a short-lived photoisomer to the stable trans configuration. Activation energies for the thermal back reaction have been determined experimentally by several authors. Depending on the special cyanine molecular structure and on the reaction conditions for one group of compounds, Arrhenius activation energies of the order of magnitude of 60 kJ/mol have been obtained [ 5- 10 1.Activation energies within the range from 25 to 45 kJ/mol for the back isomerization have been found for other compounds [ 1l161. Different reaction mechanisms for the thermal cistrans back isomerization in different temperature regions were assumed by D&r, Kotschy and Kausen [ 17,181. Below a critical temperature T, a significantly lower activation energy (25.1 kJ/mol between 215 and 252 K) is needed than at higher tem-
peratures (64.8 kJ/mol between 252 and 314 K). The activation energy at higher temperatures is independent of the polymethine chain length [ 181. Such a bend in the Arrhenius plot was later also bound by Knudtson and Eyring [ 19 1. An alternative mechanism through the first-excited triplet state has been excluded. For another type of polar compound with conjugated double bonds, the protonated Schiff bases of retinal, a catalytic effect of the gegenion (counterion) on the thermal cis-trans isomerization has been concluded [20-253. We have calculated the potential-energy curves for different isomerization paths using the MNDO procedure with geometry optimization to clarify the different isomerization mechanisms for the thermal cistrans back reaction of cyanines. The polyrnetbinestreptocyanincs trimethinecyanine TMC+ and pentamethinecyanine PMC+ have been used as model compounds. The ion pairs were modeled by TMC+ and PMC+ with the chloride anion as gegenion at various positions in a distance of 350 pm from the main atoms of the polymethine chain. The results were used to propose model mechanisms with different activation energies.
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IZC
TMC+ H
H
100 H
PMC+ 2. Results and discussion The potentiaLenergy curves in the S,, state for the thermal cis-trans isomerization by rotation of the molecular parts about the C& bond of TMC+ and the C2-C3 and Cs-CI bonds of PMC+ have been obtained using the MNDO approximation [ 26 ] with full geometry optimization (see figs. 1 and 2). Fig. 1 also shows the potential-energy curve for the rotation about the Cl-Cs bond of TMC+ calculated by the ab initio method with an ST0 3G basis set [ 27 ] using the MNDO optimized geometries. While the
trons
60”
3cP
90'
120’
Fig. 1. Potential-energy curves for the thermal isomerization of TMC+ by C&bond rotation, ( l ) MNDO; ( x ) STO-3G.
E(kJ/mo/)
0
b
Fig. 2. Potential-energy curves of thermal isomerization of PMC+ by (a) C&3 and (b) C3-C4 bond rotation.
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CHEMICAL PHYSICS LETTERS
Table 1 Activation barriers forthe rotation around CC bonds A& (in kJ/mol), and energydifferences between the tram and cis isomers AE,, (in kJ/mol) of the streptocyanine cations TMC+ and PMC+ Compound
Rotation about bond
A&<
A&e, MNDO
ab initio
exp.
MNDO
ab initio 18.0 a’ 20.5 ‘) 22.6 c,
TMC+
c*-c3
56.0
130.4 ‘) 115.4b’ 112.8 =’
9.1
PMC+
C&3
57.6
114.1 =’ 112.7 =’
8.1 19.1 e’ 25.1 ” 64.8 0’
C,-C4
53.2
‘) STO-3G. b, 4/31G. =)CISD/4-3lG//HF/4-3lG. ‘) Bispiperidinopentamethine perchlorate in ethanol.
112.7 =)
4.6
13.9 c’
d, CISD/6-3lG”//HF/4-31G. ‘) T: 215-252 K. I) T: 252-314 K.
MNDO calculated barrier heights are in good agreement with the experimentally measured values, the activation energies and the energy differences between the trans and cis isomers obtained by the ab initio method are too high by a factor of 2 (see also refs. [ 28,291). In table 1, the values for the barrier heights for the thermal cis-trans isomerization and the energy differences between the trans and cis isomers are compared. The activation energies for both model compounds are of the order of magnitude of about 50 kJ/mol. Tables 2 and 3 summarize for TMC+ and PMC+
the optimized-geometry parameters for the planar trans and cis configurations and also for the perpendicular conformation (pet-p) which corresponds to the energy maximum of the potential-energy curves. The geometry data for the planar trans and cis isomers express the typical features of the electronic structure of polymethines: equalization of the CC bond lengths and alternation of the bond angles [ 301. The perpendicular conformations are characterized by an elongated bond at which the isomerization takes place. With increased distortion angles, a charge separation of the positive charge of the streptocyanine cations is observed with an accumulation of
Table 2 Experimental and optimized geometries for the TMC+ trans and cis configurations and the perpendicular conformation in the ground state (bond lengths in pm, bond angles in deg) Bond angle
Exp. *’
Trans MNDO
Perpendicular ab initio
MNDG
STG-3G
4-31G
Cis ab initio STG-3G
4-31G
MNDG
ab initio 4-31G
f
137.9 132.5
140.8 134.3
139.0 133.7
137.9 131.1
145.9 132.1
130.5 149.2
146.1 127.9
140.6 134.4
138.0 131.0
:
131.4 139.8 124.7 117.1 122.7
134.3 140.8 123.5 123.2 123.5
133.7 139.0 124.8 119.4 124.8
131.1 137.9 125.4 119.8 125.4
136.3 137.4 122.5 126.0 123.7
137.1 133.4 122.8 122.1 125.1
134.1 134.0 122.7 122.8 126.0
134.3 140.7 127.7 129.3 123.1
138.2 131.5 127.7 125.0 129.5
A
B c
‘) Bisdimethylaminotrimethine perchlorate, average values from ref. [ 301.
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Table 3 Geometry data (experimental and optimized bond lengths in pm and bond angles in deg ) for PMC+ trans, 2-3 cis, and 3-4 cis co&urations, and for the 2-3 and 3-4 perpendicular conformations Bond angle
Experiment *) Ref. [30]
131.2 136.5 142.7 138.8 139.9 132.3 124.8 117.8 122.6 118.6 126.6 Bond angle
Tram Ref. [32]
131.0 139.4 137.8 139.9 137.6 133.0 126.5 121.0 124.5 121.0 126.1
MNDO
134.7 140.5 140.7 140.7 140.5 134.7 123.6 123.9 124.8 123.9 123.6
2-3 cis ab initio STG-3G
4-31G
134.5 138.2 139.6 139.6 138.2 134.5 125.0 120.4 125.1 120.4 125.0
131.7 137.4 138.5 138.5 137.4 131.7 125.6 120.7 125.7 120.7 125.6
2-3 perp MNDG
132.2 145.5 136.8 143.3 138.4 136.0 122.6 126.5 124.9 123.9 123.8
3-4 cis
MNDO
ab initio 431G
MNDO
ab initio 4-31G
134.8 140.3 140.5 140.7 140.6 134.7 123.3 127.8 129.4 123.2 123.7
132.2 137.7 138.7 138.3 137.6 131.6 129.1 125.5 124.8 120.9 125.4
134.7 140.5 140.4 140.8 140.5 134.7 123.3 127.8 129.4 123.2 123.7
131.6 137.1 138.8 139.0 137.4 131.8 124.9 124.1 129.9 119.8 125.8
3-4 perp ab initio
MNDO
STG-3G
4-31G
130.4 149.7 133.0 145.5 134.1 137.5 122.8 122.6 124.4 121.5 125.5
127.9 146.1 133.9 142.7 134.8 134.0 122.7 123.3 125.4 121.3 126.2
139.1 136.1 146.1 137.5 143.6 133.1 124.1 125.5 124.5 124.0 123.2
ab initio STO-3G
4-31G
137.4 134.1 145.4 133.0 149.9 130.5 125.6 121.3 124.6 123.8 122.8
134.0 134.8 142.7 133.9 146.1 128.0 126.2 121.3 125.4 123.3 122.7
‘r Bidimenthylaminopentamethine perchlorate.
the main part at the polyenic fragment (fig. 3). The maximum of the charge separation is accomplished at the perpendicular conformation. Therefore, this structure should be especially stabilized by polar solvents. A chloride anion is placed at various positions at the main atoms of the polymethine chain in a distance of 350 pm to study the influence of the gegenions of the cationic cyanines on the thermal isomerization of cyanines. The activation barriers for rotation about the CC bonds of TMC+ and PMC+ were calculated for such ion pairs. These barrier heights for the thermal isomerization of ion pairs are 190
given in table 4. A catalytic effect by lowering the activation barrier can be assumed especially if the gegenion is located at a main atom (N or C) of the positively charged polyenic structural fragment of the perpendicular conformation. The barrier height is diminished in these cases by a factor of 1.5 to 3. Such a localization of the gegenion can be realized in ion pairs. It is assumed that these ion pairs are stable only at low temperatures. According to the Gibbs-Helmholtz equation, the dissociation equilibrium of the ion pairs should be shifted at higher temperatures than a critical temperature to the free solvated ions. Now, the free movable counterion cannot operate to
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CHEMICAL PHYSICS LETTERS
Volume 172, number 2
cycle pedal or a hula twist motion) can be excluded [ 3 11. The influence of the counterion on the S, potential curve and, therefore, on the photoisomerization has not been investigated. We believe that a similar effect is active which was found in the photoisomerization in rhodopsin [ 241.
3. Conclusions Based on the quantum-chemically calculated barriers for the rotations about the different CC bonds of the streptocyanine cations and of various ion pairs of TMC+ and PMC+ with Cl- as the gegenion, we can explain the different activation energies for the thermal isomerization of cyanines depending on the temperature. This fact was first established experimentally by D&r, Kotschy and Kausen [ 171. At lower temperatures than a critical temperature T,, the activation barrier of the thermal cis-trans isomerization is diminished by a factor of 2 to 3 relative to the activation energies at temperatures higher than T,. The lower activation energies could be caused by a catalytic effect of the gegenion via a frozen-ion-pair structure at low temperatures. An increase of the temperature raises the mobility of the ions. The anions are no longer arranged at a fixed position at the
Fig. 3. Charge separation Aq= 14, - Iq2 from the polyenic fragment (q, ) to the polymethinic fragment (q2) with respect to a rotation around the C2& bond for TMC+, and a rotation around the Cz-C3 and C,-Cd bond for PMC+.
lower the isomerization barrier. Isornerization by a coupled mechanism realized by a concerted rotation about two bonds (like a bi-
Table 4 Relative energies (in relation to the most stable ion pair, in U/mol) of cis- and trans-contigurations and of the perpendicular conformations of TMC+Cl- and PMC+Cl-, and barrier heights for the thermal trans-cis and cis-tram isomerizations Ion pair
TMC+CIbarrier barrier PMC+Clbarrier barrier
barrier barrier
Config. / conform.
trans t pcrp cis trans-cis cis-tram tram 2-3 perp 2-3 cis trans-cis cis-trans 3-4 perp 3-4 cis trans-cis cis-touts
Cation
56.0 9.1 56.0 46.9 57.6 8.1 57.6 49.5 53.2 4.6 53.2 48.6
Cl- in a distance of 350 pm from the atoms N,
C,
C3
C,
G(W)
C6
N7
11.2 47.9 10.4 36.7 37.5 8.6 29.1 9.1 20.5 20.0 121.3 11.6 112.7 109.7
0.0 45.7 9.7 45.7 36.0 0.0 26.9 8.5 26.9 18.4 89.6 3.1 89.6 86.5
1.5 66.7 10.5 65.2 56.2 7.5 47.5 15.4 40.0 32. I 77.0 Il.8 69.5 65.2
0.0 72.4
11.2 65.6 17.1 54.4 48.5 7.5 81.3 13.2 73.8 68.1 31.1 4.8 23.6 26.3
0.0 99.8 6.0 99.8 93.8 15.1 1.4 15.1 13.7
8.6 116.9 14.1 108.3 102.8 22.9 a.4 14.3 14.5
72.4 3.7 42:2 8.3 38.5 33.9 54.4 8.8 50.7 45.6
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cyanine cation and, therefore, the catalytic effect cannot act further.
Acknowledgement
Two of us (FD and TK) thank Dr. P. Birner and Professor Dr. H.-J. Kijhler (Organization and Computer Center of the Karl-Marx Universittit Leipzig) for the possibility to use the MNDO program system.
References [I ] G. Scheibe, J. Heiss and K. Feldmann, Angew. Chem. 77 (1965) 545. [2] G. Scheibe, J. Heiss and K. Feldmann, Ber. Bunsenges. Physik. Chem. 70 (1966) 52. [ 31 F. Dietz and S. Rentsch, Chem. Phys. 96 (1985 ) 145. [4] W. Biiumler and A. Penzkofer, Chem. Phys. Letters 150 (1988) 315. [ 51 C. Rullitre, Chem. Phys. Letters 43 ( 1976) 303. [ 61 J.P. Fouassier, D.-J. Lougnot and J. Fame, Opt. Commun. 23 (1977) 393. [ 71 J.P. Fouassier, D.-J. Lougnot and J. Faure, J. Chim. Phys. 74 (1977) 23. [ 81 D.-J. Lougnot, P. Brunero, J.P. Fouassier and J. Faure, J. Chim. Phys. 79 (1982) 343. [ 91 S.P. Velsko, D.H. Waldeck and G.R. Flemming, J. Chem. Phys. 78 (1983) 249. [IO] S. Rentsch, U.W. Grummt and D. Khetchinashwili, Laser Chem. 7 (1987) 261. [ I I ] E. Akesson, H. Bergsttim, V. Sundstriim and T. Gillbro, Chem. Phys. Letters 126 ( 1986) 385. [ 12] E. Akesson, V. Sundstrijm and T. Gillbro, Chem. Phys. Letters 121 (1985) 513. [ I3 ] V.A. Kuzmin and A.P. Darmanyan, Chem. Phys. Letters 54 (1978) 159.
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[ 141A.P. Darmanyan, A.M. Vinogradov, V.A. Kuzmin, N.N. Romanov and F.S. Babichev, Izv. Akad. Nauk. USSR, Ser. Kbim. (1978) 1785. [ I5 1F. Baumgiirtner, E. Gunther and G. Seheibe, Z. Elektroehem. 60 (1956) 570. [16]G.Scheibe,Chimia 15 (1961) 10. [ 171F. D&T, J. Kotschy and H. Kausen, Ber. Bunsenges. Physik. Chem. 69 (1965) 11. [ 181 G. Sbheibe, in: Opt&he anregung organ&her systeme, Vol. 2. Intern. Farbensymposium 1964, S&loss Elmau (Verlag Chemie, Weinheim, 1966). [ 191J.T. Knudtson and E.M. Eyring, J. Chem. Phys. 78 (1974) 2355. [20] A. Warshel, Proc. Natl. Aead. Sei. US 75 (1978) 2558. [21] A. Warshel and C. Deakyne, Chem. Phys. Letters 55 (1978) 459. [22] A. Warshel and M. Gttolenghi, Photoehem. Photobiol. 30 (1979) 291. [23] P. Tavan, K. Schulten and D. Oesterhelt, Biophys. J 47 (1985) 415. [24] R.R. Birge and L.M. Hubbard, J. Am. Chem. Sot. 102 ( 1980) 2195,794s. [25] S. Seltzer, J. Am. Chem. Sot. 109 (1987) 1627; V. BonaEiGKoutecky, K. Sehiiffel and M. Michl, Theoret. Chim. Acta 72 ( 1987) 459. [26] M.J.S. Dewar and W. Thiel, J. Am. Chem. Sot. 99 (1977) 4899,4907. [27 ] M. Peterson and P. Poirier, Programmsystem MONSTER GAUSS ( 1985). [ 28 ] S. Saebs and J. Almlaf Aeta Chem. Stand. ( 1980) 65 1. [29] J. Baudet and L. Grajcar, Compt. Rend Acad. Sci. (Paris) 299 (1984) 149. [ 301 S. Diihne and S. Kulpe, Structural principles of unsaturated organic compounds, Abhandl. d. Akad. d. Wiss. d. DDR, Abt. Math. Nat. Tech. ( Akademie-Verlag,Berlin, 1977). [ 3 11T. Krossner and F. Dietz, in preparation. [ 32 ] M. Honda, C. Katayama and J. Tanaka, Acta Cryst. B 42 (1986)90.
CHEMICAL PHYSICS LETTERS
31 August 1990
Model mechanisms for the thermal cis-trans isomerization of cyanines Klaus Schijffel Institutfir Physikulische und Theoretische Chemie, Freie UniversitiitBerlin, Takustrawe 3, &lo00 Berlin 33, Germany
Fritz Dietz and Thomas Krossner Sektion Chemie, Karl-Marx-Universiriil Leipzig, Talstrasse 35, DDR-7010 Leipzig, German Democratic Republic
Received 5 March 1990; in final form 15 June 1990
Quantumshemical calculations have been performedfor rotations around the different CC bonds of streptocyanine eations and of various ion pairs of TMC+ and PM? with Cl- as the gegenion (counterion) in order to explain the experimentally well known temperature dependence of the activation energy for the thermal isomerization.
1. Introduction The photochemical tram-cis isomerization is a very effective deactivation process for the first-cxcited singlet state of most cyanines [ l-31. This deactivation channel diminishes the efficiency of spectral sensitization and the laser activity of the dyes. Whereas the thermal trans-cis isomerization of cyanines is an exceptional case [ 4 1, the back-reaction cis-trans proceeds mainly thermally induced from a short-lived photoisomer to the stable trans configuration. Activation energies for the thermal back reaction have been determined experimentally by several authors. Depending on the special cyanine molecular structure and on the reaction conditions for one group of compounds, Arrhenius activation energies of the order of magnitude of 60 kJ/mol have been obtained [ 5- 10 1.Activation energies within the range from 25 to 45 kJ/mol for the back isomerization have been found for other compounds [ 1l161. Different reaction mechanisms for the thermal cistrans back isomerization in different temperature regions were assumed by D&r, Kotschy and Kausen [ 17,181. Below a critical temperature T, a significantly lower activation energy (25.1 kJ/mol between 215 and 252 K) is needed than at higher tem-
peratures (64.8 kJ/mol between 252 and 314 K). The activation energy at higher temperatures is independent of the polymethine chain length [ 181. Such a bend in the Arrhenius plot was later also bound by Knudtson and Eyring [ 19 1. An alternative mechanism through the first-excited triplet state has been excluded. For another type of polar compound with conjugated double bonds, the protonated Schiff bases of retinal, a catalytic effect of the gegenion (counterion) on the thermal cis-trans isomerization has been concluded [20-253. We have calculated the potential-energy curves for different isomerization paths using the MNDO procedure with geometry optimization to clarify the different isomerization mechanisms for the thermal cistrans back reaction of cyanines. The polyrnetbinestreptocyanincs trimethinecyanine TMC+ and pentamethinecyanine PMC+ have been used as model compounds. The ion pairs were modeled by TMC+ and PMC+ with the chloride anion as gegenion at various positions in a distance of 350 pm from the main atoms of the polymethine chain. The results were used to propose model mechanisms with different activation energies.
0009-26 14/90/S 03.50 0 1990 - Elsevier Science Publishers B.V. ( North-Holland )
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IZC
TMC+ H
H
100 H
PMC+ 2. Results and discussion The potentiaLenergy curves in the S,, state for the thermal cis-trans isomerization by rotation of the molecular parts about the C& bond of TMC+ and the C2-C3 and Cs-CI bonds of PMC+ have been obtained using the MNDO approximation [ 26 ] with full geometry optimization (see figs. 1 and 2). Fig. 1 also shows the potential-energy curve for the rotation about the Cl-Cs bond of TMC+ calculated by the ab initio method with an ST0 3G basis set [ 27 ] using the MNDO optimized geometries. While the
trons
60”
3cP
90'
120’
Fig. 1. Potential-energy curves for the thermal isomerization of TMC+ by C&bond rotation, ( l ) MNDO; ( x ) STO-3G.
E(kJ/mo/)
0
b
Fig. 2. Potential-energy curves of thermal isomerization of PMC+ by (a) C&3 and (b) C3-C4 bond rotation.
188
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Table 1 Activation barriers forthe rotation around CC bonds A& (in kJ/mol), and energydifferences between the tram and cis isomers AE,, (in kJ/mol) of the streptocyanine cations TMC+ and PMC+ Compound
Rotation about bond
A&<
A&e, MNDO
ab initio
exp.
MNDO
ab initio 18.0 a’ 20.5 ‘) 22.6 c,
TMC+
c*-c3
56.0
130.4 ‘) 115.4b’ 112.8 =’
9.1
PMC+
C&3
57.6
114.1 =’ 112.7 =’
8.1 19.1 e’ 25.1 ” 64.8 0’
C,-C4
53.2
‘) STO-3G. b, 4/31G. =)CISD/4-3lG//HF/4-3lG. ‘) Bispiperidinopentamethine perchlorate in ethanol.
112.7 =)
4.6
13.9 c’
d, CISD/6-3lG”//HF/4-31G. ‘) T: 215-252 K. I) T: 252-314 K.
MNDO calculated barrier heights are in good agreement with the experimentally measured values, the activation energies and the energy differences between the trans and cis isomers obtained by the ab initio method are too high by a factor of 2 (see also refs. [ 28,291). In table 1, the values for the barrier heights for the thermal cis-trans isomerization and the energy differences between the trans and cis isomers are compared. The activation energies for both model compounds are of the order of magnitude of about 50 kJ/mol. Tables 2 and 3 summarize for TMC+ and PMC+
the optimized-geometry parameters for the planar trans and cis configurations and also for the perpendicular conformation (pet-p) which corresponds to the energy maximum of the potential-energy curves. The geometry data for the planar trans and cis isomers express the typical features of the electronic structure of polymethines: equalization of the CC bond lengths and alternation of the bond angles [ 301. The perpendicular conformations are characterized by an elongated bond at which the isomerization takes place. With increased distortion angles, a charge separation of the positive charge of the streptocyanine cations is observed with an accumulation of
Table 2 Experimental and optimized geometries for the TMC+ trans and cis configurations and the perpendicular conformation in the ground state (bond lengths in pm, bond angles in deg) Bond angle
Exp. *’
Trans MNDO
Perpendicular ab initio
MNDG
STG-3G
4-31G
Cis ab initio STG-3G
4-31G
MNDG
ab initio 4-31G
f
137.9 132.5
140.8 134.3
139.0 133.7
137.9 131.1
145.9 132.1
130.5 149.2
146.1 127.9
140.6 134.4
138.0 131.0
:
131.4 139.8 124.7 117.1 122.7
134.3 140.8 123.5 123.2 123.5
133.7 139.0 124.8 119.4 124.8
131.1 137.9 125.4 119.8 125.4
136.3 137.4 122.5 126.0 123.7
137.1 133.4 122.8 122.1 125.1
134.1 134.0 122.7 122.8 126.0
134.3 140.7 127.7 129.3 123.1
138.2 131.5 127.7 125.0 129.5
A
B c
‘) Bisdimethylaminotrimethine perchlorate, average values from ref. [ 301.
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Table 3 Geometry data (experimental and optimized bond lengths in pm and bond angles in deg ) for PMC+ trans, 2-3 cis, and 3-4 cis co&urations, and for the 2-3 and 3-4 perpendicular conformations Bond angle
Experiment *) Ref. [30]
131.2 136.5 142.7 138.8 139.9 132.3 124.8 117.8 122.6 118.6 126.6 Bond angle
Tram Ref. [32]
131.0 139.4 137.8 139.9 137.6 133.0 126.5 121.0 124.5 121.0 126.1
MNDO
134.7 140.5 140.7 140.7 140.5 134.7 123.6 123.9 124.8 123.9 123.6
2-3 cis ab initio STG-3G
4-31G
134.5 138.2 139.6 139.6 138.2 134.5 125.0 120.4 125.1 120.4 125.0
131.7 137.4 138.5 138.5 137.4 131.7 125.6 120.7 125.7 120.7 125.6
2-3 perp MNDG
132.2 145.5 136.8 143.3 138.4 136.0 122.6 126.5 124.9 123.9 123.8
3-4 cis
MNDO
ab initio 431G
MNDO
ab initio 4-31G
134.8 140.3 140.5 140.7 140.6 134.7 123.3 127.8 129.4 123.2 123.7
132.2 137.7 138.7 138.3 137.6 131.6 129.1 125.5 124.8 120.9 125.4
134.7 140.5 140.4 140.8 140.5 134.7 123.3 127.8 129.4 123.2 123.7
131.6 137.1 138.8 139.0 137.4 131.8 124.9 124.1 129.9 119.8 125.8
3-4 perp ab initio
MNDO
STG-3G
4-31G
130.4 149.7 133.0 145.5 134.1 137.5 122.8 122.6 124.4 121.5 125.5
127.9 146.1 133.9 142.7 134.8 134.0 122.7 123.3 125.4 121.3 126.2
139.1 136.1 146.1 137.5 143.6 133.1 124.1 125.5 124.5 124.0 123.2
ab initio STO-3G
4-31G
137.4 134.1 145.4 133.0 149.9 130.5 125.6 121.3 124.6 123.8 122.8
134.0 134.8 142.7 133.9 146.1 128.0 126.2 121.3 125.4 123.3 122.7
‘r Bidimenthylaminopentamethine perchlorate.
the main part at the polyenic fragment (fig. 3). The maximum of the charge separation is accomplished at the perpendicular conformation. Therefore, this structure should be especially stabilized by polar solvents. A chloride anion is placed at various positions at the main atoms of the polymethine chain in a distance of 350 pm to study the influence of the gegenions of the cationic cyanines on the thermal isomerization of cyanines. The activation barriers for rotation about the CC bonds of TMC+ and PMC+ were calculated for such ion pairs. These barrier heights for the thermal isomerization of ion pairs are 190
given in table 4. A catalytic effect by lowering the activation barrier can be assumed especially if the gegenion is located at a main atom (N or C) of the positively charged polyenic structural fragment of the perpendicular conformation. The barrier height is diminished in these cases by a factor of 1.5 to 3. Such a localization of the gegenion can be realized in ion pairs. It is assumed that these ion pairs are stable only at low temperatures. According to the Gibbs-Helmholtz equation, the dissociation equilibrium of the ion pairs should be shifted at higher temperatures than a critical temperature to the free solvated ions. Now, the free movable counterion cannot operate to
3 1August 1990
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cycle pedal or a hula twist motion) can be excluded [ 3 11. The influence of the counterion on the S, potential curve and, therefore, on the photoisomerization has not been investigated. We believe that a similar effect is active which was found in the photoisomerization in rhodopsin [ 241.
3. Conclusions Based on the quantum-chemically calculated barriers for the rotations about the different CC bonds of the streptocyanine cations and of various ion pairs of TMC+ and PMC+ with Cl- as the gegenion, we can explain the different activation energies for the thermal isomerization of cyanines depending on the temperature. This fact was first established experimentally by D&r, Kotschy and Kausen [ 171. At lower temperatures than a critical temperature T,, the activation barrier of the thermal cis-trans isomerization is diminished by a factor of 2 to 3 relative to the activation energies at temperatures higher than T,. The lower activation energies could be caused by a catalytic effect of the gegenion via a frozen-ion-pair structure at low temperatures. An increase of the temperature raises the mobility of the ions. The anions are no longer arranged at a fixed position at the
Fig. 3. Charge separation Aq= 14, - Iq2 from the polyenic fragment (q, ) to the polymethinic fragment (q2) with respect to a rotation around the C2& bond for TMC+, and a rotation around the Cz-C3 and C,-Cd bond for PMC+.
lower the isomerization barrier. Isornerization by a coupled mechanism realized by a concerted rotation about two bonds (like a bi-
Table 4 Relative energies (in relation to the most stable ion pair, in U/mol) of cis- and trans-contigurations and of the perpendicular conformations of TMC+Cl- and PMC+Cl-, and barrier heights for the thermal trans-cis and cis-tram isomerizations Ion pair
TMC+CIbarrier barrier PMC+Clbarrier barrier
barrier barrier
Config. / conform.
trans t pcrp cis trans-cis cis-tram tram 2-3 perp 2-3 cis trans-cis cis-trans 3-4 perp 3-4 cis trans-cis cis-touts
Cation
56.0 9.1 56.0 46.9 57.6 8.1 57.6 49.5 53.2 4.6 53.2 48.6
Cl- in a distance of 350 pm from the atoms N,
C,
C3
C,
G(W)
C6
N7
11.2 47.9 10.4 36.7 37.5 8.6 29.1 9.1 20.5 20.0 121.3 11.6 112.7 109.7
0.0 45.7 9.7 45.7 36.0 0.0 26.9 8.5 26.9 18.4 89.6 3.1 89.6 86.5
1.5 66.7 10.5 65.2 56.2 7.5 47.5 15.4 40.0 32. I 77.0 Il.8 69.5 65.2
0.0 72.4
11.2 65.6 17.1 54.4 48.5 7.5 81.3 13.2 73.8 68.1 31.1 4.8 23.6 26.3
0.0 99.8 6.0 99.8 93.8 15.1 1.4 15.1 13.7
8.6 116.9 14.1 108.3 102.8 22.9 a.4 14.3 14.5
72.4 3.7 42:2 8.3 38.5 33.9 54.4 8.8 50.7 45.6
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cyanine cation and, therefore, the catalytic effect cannot act further.
Acknowledgement
Two of us (FD and TK) thank Dr. P. Birner and Professor Dr. H.-J. Kijhler (Organization and Computer Center of the Karl-Marx Universittit Leipzig) for the possibility to use the MNDO program system.
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