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The very low ionization potentials of porphyrins and the possible role of rydberg states in photosynthesis
Volume
75, number 3
THE VERY
CHEMICAL
LOW IONIZATION
AND THE POSSIBLE P- DUPUIS.
POTENTIALS
ROLE OF RYDBERG
R ROBERGE
14 July 1980;
LETTERS
1 November
1980
OF PORPHYRINS STATES
IN PHOTOSYNTHESIS
and C_ SANDORFY
DGpartemcnt de Cinmle. Unwerslt~ de Mont&al, Received
PHYSICS
Mont&al. Quibec H3C 3 VI, Chttada
m final form 5 August 1980
The very low iomzatlon potentials of porphyrms lead to the prcdlctlon that m adduon to bands due to (a, n*) transitions, bands due to Rydberg transItIons should exist in their wile spectra- The suggestion IS made that Rydberg excited states could be unportant m photosynthesis.
1.
Introduction
The fist porphynn iomzatlon potentials (II’s) have been reported in 1971 by Ridyard [ 11. Khandelwal and Roebber [2] measured the PE spectra of tetraphenylporphin and a number of its metalloporphyrins in the vapor phase. In all cases, they found the (vertical) IP between 6.39 and 6.50 eV unth the second PE band at 6.62-6.72 eV. Subsequently, Tsubomura and co-workers [3] determined the PE spectra of tetraphenylporphin and some metallotetraphenylporphyrins by photocurrent measurements in nonpolar solvents [4] _ The photocurrent thresholds which they obtained (m isooctane) range between 4.72 and 5.13 eV_ From these they inferred the gas-phase values under certain assumptions [3] and arrived at values from 5.9-6.3 eV_ Recently Kitagawa et al. [5] published the (gas-phase) PE spectra of a number of metallooctaethylporphyrins. They found the first band between 6.06 and 6.39 eV_ Now, withsuch low II’s porphyrins should have Rydberg bands in the vlslble. Furthermore, since chlorophylls can be assumed to have IPs sin&r to those of porphyr!! this might be important for the mechanism of photosynthesis_ In order to mvestigate this problem we have measured the He1 photoelectron (PE)
spectra
of a number
of simple
porphynn
deriva-
tives_ The following are presented in this paper: porphin, etioporphyrins-I, -II and -III, Cu-etioporphyrin-III, 1~,5,7-tetramethyl-2,6-diethyl4,8-di-n434
propylporphin, trans-octaethylc’hlonn, Fe(III)-r-octaethylchlorin, and Mn(III)-t-octaethylchlorin. The PE spectra have been determmed with a Perkin-Elmer PS-16 spectrometer with a HPIIWCURH heated probe attachment and a He1 source. The analyser and the main chamber were thoroughly cleaned after each measurement v&h acetone_ The probe was cleaned each time with either Contrad-70 soap (Scientific Products) or Extran-300 (BDH Chemicals), dissolved in distilled water and washed successively in deiomzed water, isopropanol and acetone (Perkin-Elmer Operator’s Manual 5900.8587A). The instrument was calibrated wth the ovgen and the argon lines. The positions of the latter were checked several times durmg the measurements_ The spectra were measured m the vapor phase at temperatures ranging between 230 and 250°C_ The PE spectra are given in counts/s versus eV.
2. Results and discussion The PE spectra are given in figs. 1-X The values of the band maxima up to 10 eV are summarized in table 1. AU the spectra are similar and there is no need to discuss them one by one. There
is little
doubt
(and
general
agreement)
that
the IP of porphylms corresponds to ‘IT ionization. Accordmg to Gouterman’s “four orbital theory” [6-g] the highest ftied orbitals in the ground state are (under
Volume 75, number 3
-. irJ
CHEMICAL PHYSICS LETTERS
1 November
2980
S
,,=---__ a’ ,:
I-- \,
-.
‘.
..
,._1’
,: :-..,
‘?
*.
--__
:
=-__
--_
._’
=._
_;
6
, /
Fig. 1. The HeI photoelectron
spectrum
5
of porphin.
7
9
I
I
II
I3
Fig. 3. The He1 photoelectron (-) and Cuetloporphynn-III
15
17
19 eV
spectra of etloporphyrbr-III (---).
1
7
8
Tlg. 2. The He1 photoelectron
9 spectrum
IO
II eV
of etloporphyrin-II.
/
D4h symmetry) 3a2, and lalu. They usually do not differ by more than 0.5 eV. The observed PE spectra are in conformity with Gouterman’s theory [6,7]. Up to about 8 eV they are in excellent agreement v&h the results of advanced quantum chemical calculations by Christoffersen and co-workers [IO,1 I]. The less good agreement at higher energies can be explained by the increasing difffculey of applying Koopmans’ theorem at these lugher energies [5,10,1 l]. According to Table 1 The maxima of the bands (in ev) m the He1 photoelectrbn POrphin etloporphyrin-I etioporphyrin-II etioporphyrin-III Cu-etroporphyrin-III :,3,5,7-tetramethyI-2,6ðyI4,8-d1+z-propyl porphm a) trans-octaethylchlorin Fe(LU)-transactaethylchlorin &I(W)-_ SxziethYlchIo&
a) Thrs spectrum
was recorded
6.9 6.3 6.3 6.3 6.2 6.3 S:Z 5.9
7.1 sh
6.4 sh 6.2sh 6.1 sh
7
9
II
13
15
3
17eV
Fig. 4. The He1 photoelectron spectra of (A) trans-octaethylchlonn, (B) Fe(W)-trans-octaethylchlorm; (0 Mn(iiI)-trane octaethylchlorm.
these authors the four PE bands of lowest energy correspond to n ionization for both free base porphin and free base chlorin. For porphin the estimated theoretical IPs are, under D,, symmetry: 6.8 (a& 7.2 (b,,),
spectra of nine porphyrm
derivauves
(up to IO eV) 9.1
7.2 sh 6.5 6.6 6.5 6.6
8.4 7.4 7.6 7.5 7.5
8.8 sh 8.3 8.3 8.0 sh 7.9
6.6 6.7 6.6 6.5
7.6 7.6 sh 7.7 7.4
8.1 8.4 sh 8.Osh
up to 8 eV only since we had msufficient
mateti
8.2 8.3
10.0 9.6 sh 9.5 9.5 9.6
8.5
8.6
to measure the complete
LO.3
10.2
9.5 zh
9.4
spectn~m
435
Volume 75, number 3
CHEMICAL PHYSICS LE-ITERS
9.2 (b3g) and 9.4 (bl,,). Chrr experunental vahre area 6.9,7.1 (shoulder) 7.2 sh, 8.4,8.8 sh and 9.1 eV. The first band has two shoulders of which one (7.2 eV) can be assigned to the b,, level the other is probably due to unresolved vibrational fiie structure. For the other porphyrms two well defmed bands have been found (figs. 2-4) with for the Cu complexes and the cNorins a pronounced shoulder between them. Our value for the band of lowest energy for free base porphin (6.9 eV) is much htgher than Ridyard’s [l] (5.75 ev). YIP et al. [ 121 have shown, however, that Ridyard’s spectrum belonged to PH- rather than to the nelltid porphm molecule. A!I known mterpretations of the electronic spectra of porphyrins are rn terms of (n, a*) transrtions. However, in view of the low IPs discussed above, we must ask if Rydberg bands could appear in the visible spectrum of these molecules. A great deal of knowledge has been accumulated on Rydberg term values m recent years. In the formula
;r=C&-/(r~-Aa)~, where R IS the Rydberg constant in cm-l, n IS a princrpal quantum number and A the quantum defect, both A and the term (the fraction) have characteristrc values for different types of Rydberg orbrtals. (For a comprehensive view of this problem see ref. [ 131. For 3s, 3p, 3d type Rydberg orbit& A has values of the order of 1.0,0.6 and 0.1 respectrvely whereas the corresponding term values are about 27000-22000, 20000-18000 and 14000--12000 cm-l.) Let us look at the 3s -+ 3a2, Rydberg transition which should have the lowest energy. If we take a low term value of 22000 cm-l expected for large molecules (it is about 24000 for pyrrole [13]) and an IP of 48000 cm-l (about 6 eV) this would place the 3s band near 26000 cm-l,just about the limrt of the vrsible in coincidence with the intense Soret band and it might contnbute to its apparent intensity. Edwards et al. [14,15] measured the absorptron spectra of porphin, octaethylporphm and a number of their metal complexes. Whereas they found no Rydberg series some of the bands they observed at higher frequencies might have Rydberg or mixed character. These values are, of course. vertical gas-phase II%. In a condensed medmm they are expected to be appreciably lower, especially in a polar environment hke in the presence of water molecules. (As a general reference see Jortner and Gaathon (16,171.) 436
1 November 1980
As mentroned above Tsubomura found values around 5 eV for a number of porphyrin derivatrves even in non-polar solvents_ In a recent paper Wallace et al. [ 181 reported results on pyrene using twophoton ronizatron techniques and stressed the importance of Rydberg states for the photoionization process m polar media. In methanol the tP was 1 .l eV lower than in the gas phase. In the aniomc micelle sodmm dodecyl sulfate m aqueous solution (SDS) the lowering amounted to 2.3 eV. Thus it can be estimated that m a condensed medium porphyrins have IPs near 5 eV. The higher Rydberg states would be expected to “follow” the IP. The lower, more penetratmg Rydberg states (3s, 3p) would probably shift by less, yielding low term values. With an IP of 40000 cm-l (about 5 eV) and a (pessinustrcally) low term value of 20000 cm-l the 3s Rydberg band would be around 20000 cm-l, right in the middle of the vrsrble where the Q bands start. Wrth a slightly lower JP and/or slightly higher term value the 14000 cm-l region could be attained (~700 run). Smce the top ground-state orbitals 3a2u and la,, he very close to each other a second 3s band m&t closely follow. Even the 3p with an expected term value of 18000-16000 cm-l could fall into the vrsible. One could object that Rydberg bands usually “disappear” in condensed media. This criterion has been successfully used to distmguish Rydberg from valence-shell transitions [19]. This, however, only means that the bands are broadened, sometunes to the point of disappearing m the background. It is believed that Rydberg exerted states might play a role in photosynthesis. Indeed, chlorophylls have spectra simrlar to those of porphyrins. Unfortunately our attempts to measure the PE spectra of chlorophylls a or b or of the pheorphorbides farled. We have been unable to put these compounds into the gas phase. The PE spectra of the three chlorms (in whrch one of the pyrrole rings is partly saturated like in the chlorophylls) are very simrhu to those of porphyrms, m particular therr IP (the first PE band) 1s between 6.2 and 5.9 eV. Whde the effect of the additron of ring V (the cyclopentanone ring) is not known, there is no chemrcal or spectroscopic reason why it would heighten the IJ?. Therefore, in our opinion, the extrapolation from porphyrins and chlorins to chlorophylls is justified. The low (3s and 3p) Rydberg states m&t be populated partly by direct absorption and partly by radia-
CHEMICAL PHYSICS LE’ITERS
Volume 75, number 3
honless transfer from the excited state of the intense Soret band. Because of their relatively large size these Rydberg states ~-III be strongly affected by intermolecular interactions which could push the electron onto higher Rydberg orbitals and from there to lonization. These low-lying Rydberg states are the only Rydberg states that can be reached by visible light absorption.
Acknowledgement We are indebted to Dr. S.F. MacDonald, to Professors H.H. Inhoffen, Kevm M. Smith and J.H. Furhrhop and to Dr. B.F. Bumham for tidly supplying us with samples. W&out their help his investlgation would not have been possible. Our thanks are due to Professors D.W. Turner, G_A. Kenney-Wallace, J.R. Bolton and R.E. Christoffersen for helpful discussions. Fmancial assistance from the Natural Science and Engineering Research Council of Canada, the MinistBre.de 1’EducatIon du Quebec and the grant of Killam Memonal scholarship from the Canada Councd (C.S.) is gratefully acknowledged.
References [l]
J.N.A.
RIdyard,
m
Molecular
spectroscopy 1971, ed. London, 1972)
P. Hepple (The Institute of Petroleum,
1 November
1980
[2] S.C. Khandelwaf [3]
and JL. Roebber, Chem Phys. Letters 34 (1975) 355. Y. Nakato. K. Abe and H. Tsubomura, Chem Phys.
Letters 39 (1976) 358. [4] Y. Nakato, T. Chiyoda and H. Tsubomura, Bull. Chem. Sot. Japan 47 (1974) 3001. [ 5 ] S. Kitagawa, 1. bfonshnna, T. Yonezawa and N. Sato, Inorg. Chem. 18 (1979) 1345. 161 M. Gouterman. J. Mol. Spectry. 6 (1961) 138. [7] Ch. Weiss, H. Kobayashi and ICI. Gouterman, J. hfoL Spectry. 16 (1965) 415. [S] YJ. Aronowitz and hf. Gouterman, J. hloL Spectry. 64 (1977) 267. [9] Ch. Weiss, J. lcfoL Spectry. 44 (1972) 37: [lo] D. Spangler, GAf. bfaggiora, L.L. ShIpman and RE. Chnstoffersen, J. Am. Chem. Sot. 99 (1977) 7470,7478. [ 111 J.D. Petke, GM. hfaggiora, L.L. Shipman and R_E. Chnstoffersen, J. Mot Spcctry. 71 (1978) 64,311. [12] RL. Ylp,C.B. Duke, W.R. Safaneck, E.W. Pfununerand G. Loubnel, Chem. Phyr Letters 49 (1977) 530. [ 131 hf.B. Robm, Higher excited states of polyatomfc molecules, Vofs. 1 and 2 (Academrc Press, New York, 1974, 1975). J. hfoL 1141 L. Edwards, D.H. Dophin and hf. Goutenran, Spectry. 35 (1970) 90. and A.D. 1151 L. Edwards, D.H. r)olphin, hf. Gouterman Adler, J. MoL Spectry. 38 (1971) 16. 1161 J. Jortner and A. Gaathon, Can. J. Chem. 55 (1977) 1801. r171 J. Jortner and A. Gaathon, in: Electrons in fluids, eds. J. Jortner and N.R. Kesmer (Springer, Berlin. 1973). (181 S.C. WaIface, G.E. Half and GA. Kenney-Wallace, Chem Phys. 49 (1980) 279. [I91 MB. Robm and N.A. Kuebler, J. BfoL Spectsy. 33 (1970) 247.
p. 96.
437
75, number 3
THE VERY
CHEMICAL
LOW IONIZATION
AND THE POSSIBLE P- DUPUIS.
POTENTIALS
ROLE OF RYDBERG
R ROBERGE
14 July 1980;
LETTERS
1 November
1980
OF PORPHYRINS STATES
IN PHOTOSYNTHESIS
and C_ SANDORFY
DGpartemcnt de Cinmle. Unwerslt~ de Mont&al, Received
PHYSICS
Mont&al. Quibec H3C 3 VI, Chttada
m final form 5 August 1980
The very low iomzatlon potentials of porphyrms lead to the prcdlctlon that m adduon to bands due to (a, n*) transitions, bands due to Rydberg transItIons should exist in their wile spectra- The suggestion IS made that Rydberg excited states could be unportant m photosynthesis.
1.
Introduction
The fist porphynn iomzatlon potentials (II’s) have been reported in 1971 by Ridyard [ 11. Khandelwal and Roebber [2] measured the PE spectra of tetraphenylporphin and a number of its metalloporphyrins in the vapor phase. In all cases, they found the (vertical) IP between 6.39 and 6.50 eV unth the second PE band at 6.62-6.72 eV. Subsequently, Tsubomura and co-workers [3] determined the PE spectra of tetraphenylporphin and some metallotetraphenylporphyrins by photocurrent measurements in nonpolar solvents [4] _ The photocurrent thresholds which they obtained (m isooctane) range between 4.72 and 5.13 eV_ From these they inferred the gas-phase values under certain assumptions [3] and arrived at values from 5.9-6.3 eV_ Recently Kitagawa et al. [5] published the (gas-phase) PE spectra of a number of metallooctaethylporphyrins. They found the first band between 6.06 and 6.39 eV_ Now, withsuch low II’s porphyrins should have Rydberg bands in the vlslble. Furthermore, since chlorophylls can be assumed to have IPs sin&r to those of porphyr!! this might be important for the mechanism of photosynthesis_ In order to mvestigate this problem we have measured the He1 photoelectron (PE)
spectra
of a number
of simple
porphynn
deriva-
tives_ The following are presented in this paper: porphin, etioporphyrins-I, -II and -III, Cu-etioporphyrin-III, 1~,5,7-tetramethyl-2,6-diethyl4,8-di-n434
propylporphin, trans-octaethylc’hlonn, Fe(III)-r-octaethylchlorin, and Mn(III)-t-octaethylchlorin. The PE spectra have been determmed with a Perkin-Elmer PS-16 spectrometer with a HPIIWCURH heated probe attachment and a He1 source. The analyser and the main chamber were thoroughly cleaned after each measurement v&h acetone_ The probe was cleaned each time with either Contrad-70 soap (Scientific Products) or Extran-300 (BDH Chemicals), dissolved in distilled water and washed successively in deiomzed water, isopropanol and acetone (Perkin-Elmer Operator’s Manual 5900.8587A). The instrument was calibrated wth the ovgen and the argon lines. The positions of the latter were checked several times durmg the measurements_ The spectra were measured m the vapor phase at temperatures ranging between 230 and 250°C_ The PE spectra are given in counts/s versus eV.
2. Results and discussion The PE spectra are given in figs. 1-X The values of the band maxima up to 10 eV are summarized in table 1. AU the spectra are similar and there is no need to discuss them one by one. There
is little
doubt
(and
general
agreement)
that
the IP of porphylms corresponds to ‘IT ionization. Accordmg to Gouterman’s “four orbital theory” [6-g] the highest ftied orbitals in the ground state are (under
Volume 75, number 3
-. irJ
CHEMICAL PHYSICS LETTERS
1 November
2980
S
,,=---__ a’ ,:
I-- \,
-.
‘.
..
,._1’
,: :-..,
‘?
*.
--__
:
=-__
--_
._’
=._
_;
6
, /
Fig. 1. The HeI photoelectron
spectrum
5
of porphin.
7
9
I
I
II
I3
Fig. 3. The He1 photoelectron (-) and Cuetloporphynn-III
15
17
19 eV
spectra of etloporphyrbr-III (---).
1
7
8
Tlg. 2. The He1 photoelectron
9 spectrum
IO
II eV
of etloporphyrin-II.
/
D4h symmetry) 3a2, and lalu. They usually do not differ by more than 0.5 eV. The observed PE spectra are in conformity with Gouterman’s theory [6,7]. Up to about 8 eV they are in excellent agreement v&h the results of advanced quantum chemical calculations by Christoffersen and co-workers [IO,1 I]. The less good agreement at higher energies can be explained by the increasing difffculey of applying Koopmans’ theorem at these lugher energies [5,10,1 l]. According to Table 1 The maxima of the bands (in ev) m the He1 photoelectrbn POrphin etloporphyrin-I etioporphyrin-II etioporphyrin-III Cu-etroporphyrin-III :,3,5,7-tetramethyI-2,6ðyI4,8-d1+z-propyl porphm a) trans-octaethylchlorin Fe(LU)-transactaethylchlorin &I(W)-_ SxziethYlchIo&
a) Thrs spectrum
was recorded
6.9 6.3 6.3 6.3 6.2 6.3 S:Z 5.9
7.1 sh
6.4 sh 6.2sh 6.1 sh
7
9
II
13
15
3
17eV
Fig. 4. The He1 photoelectron spectra of (A) trans-octaethylchlonn, (B) Fe(W)-trans-octaethylchlorm; (0 Mn(iiI)-trane octaethylchlorm.
these authors the four PE bands of lowest energy correspond to n ionization for both free base porphin and free base chlorin. For porphin the estimated theoretical IPs are, under D,, symmetry: 6.8 (a& 7.2 (b,,),
spectra of nine porphyrm
derivauves
(up to IO eV) 9.1
7.2 sh 6.5 6.6 6.5 6.6
8.4 7.4 7.6 7.5 7.5
8.8 sh 8.3 8.3 8.0 sh 7.9
6.6 6.7 6.6 6.5
7.6 7.6 sh 7.7 7.4
8.1 8.4 sh 8.Osh
up to 8 eV only since we had msufficient
mateti
8.2 8.3
10.0 9.6 sh 9.5 9.5 9.6
8.5
8.6
to measure the complete
LO.3
10.2
9.5 zh
9.4
spectn~m
435
Volume 75, number 3
CHEMICAL PHYSICS LE-ITERS
9.2 (b3g) and 9.4 (bl,,). Chrr experunental vahre area 6.9,7.1 (shoulder) 7.2 sh, 8.4,8.8 sh and 9.1 eV. The first band has two shoulders of which one (7.2 eV) can be assigned to the b,, level the other is probably due to unresolved vibrational fiie structure. For the other porphyrms two well defmed bands have been found (figs. 2-4) with for the Cu complexes and the cNorins a pronounced shoulder between them. Our value for the band of lowest energy for free base porphin (6.9 eV) is much htgher than Ridyard’s [l] (5.75 ev). YIP et al. [ 121 have shown, however, that Ridyard’s spectrum belonged to PH- rather than to the nelltid porphm molecule. A!I known mterpretations of the electronic spectra of porphyrins are rn terms of (n, a*) transrtions. However, in view of the low IPs discussed above, we must ask if Rydberg bands could appear in the visible spectrum of these molecules. A great deal of knowledge has been accumulated on Rydberg term values m recent years. In the formula
;r=C&-/(r~-Aa)~, where R IS the Rydberg constant in cm-l, n IS a princrpal quantum number and A the quantum defect, both A and the term (the fraction) have characteristrc values for different types of Rydberg orbrtals. (For a comprehensive view of this problem see ref. [ 131. For 3s, 3p, 3d type Rydberg orbit& A has values of the order of 1.0,0.6 and 0.1 respectrvely whereas the corresponding term values are about 27000-22000, 20000-18000 and 14000--12000 cm-l.) Let us look at the 3s -+ 3a2, Rydberg transition which should have the lowest energy. If we take a low term value of 22000 cm-l expected for large molecules (it is about 24000 for pyrrole [13]) and an IP of 48000 cm-l (about 6 eV) this would place the 3s band near 26000 cm-l,just about the limrt of the vrsible in coincidence with the intense Soret band and it might contnbute to its apparent intensity. Edwards et al. [14,15] measured the absorptron spectra of porphin, octaethylporphm and a number of their metal complexes. Whereas they found no Rydberg series some of the bands they observed at higher frequencies might have Rydberg or mixed character. These values are, of course. vertical gas-phase II%. In a condensed medmm they are expected to be appreciably lower, especially in a polar environment hke in the presence of water molecules. (As a general reference see Jortner and Gaathon (16,171.) 436
1 November 1980
As mentroned above Tsubomura found values around 5 eV for a number of porphyrin derivatrves even in non-polar solvents_ In a recent paper Wallace et al. [ 181 reported results on pyrene using twophoton ronizatron techniques and stressed the importance of Rydberg states for the photoionization process m polar media. In methanol the tP was 1 .l eV lower than in the gas phase. In the aniomc micelle sodmm dodecyl sulfate m aqueous solution (SDS) the lowering amounted to 2.3 eV. Thus it can be estimated that m a condensed medium porphyrins have IPs near 5 eV. The higher Rydberg states would be expected to “follow” the IP. The lower, more penetratmg Rydberg states (3s, 3p) would probably shift by less, yielding low term values. With an IP of 40000 cm-l (about 5 eV) and a (pessinustrcally) low term value of 20000 cm-l the 3s Rydberg band would be around 20000 cm-l, right in the middle of the vrsrble where the Q bands start. Wrth a slightly lower JP and/or slightly higher term value the 14000 cm-l region could be attained (~700 run). Smce the top ground-state orbitals 3a2u and la,, he very close to each other a second 3s band m&t closely follow. Even the 3p with an expected term value of 18000-16000 cm-l could fall into the vrsible. One could object that Rydberg bands usually “disappear” in condensed media. This criterion has been successfully used to distmguish Rydberg from valence-shell transitions [19]. This, however, only means that the bands are broadened, sometunes to the point of disappearing m the background. It is believed that Rydberg exerted states might play a role in photosynthesis. Indeed, chlorophylls have spectra simrlar to those of porphyrins. Unfortunately our attempts to measure the PE spectra of chlorophylls a or b or of the pheorphorbides farled. We have been unable to put these compounds into the gas phase. The PE spectra of the three chlorms (in whrch one of the pyrrole rings is partly saturated like in the chlorophylls) are very simrhu to those of porphyrms, m particular therr IP (the first PE band) 1s between 6.2 and 5.9 eV. Whde the effect of the additron of ring V (the cyclopentanone ring) is not known, there is no chemrcal or spectroscopic reason why it would heighten the IJ?. Therefore, in our opinion, the extrapolation from porphyrins and chlorins to chlorophylls is justified. The low (3s and 3p) Rydberg states m&t be populated partly by direct absorption and partly by radia-
CHEMICAL PHYSICS LE’ITERS
Volume 75, number 3
honless transfer from the excited state of the intense Soret band. Because of their relatively large size these Rydberg states ~-III be strongly affected by intermolecular interactions which could push the electron onto higher Rydberg orbitals and from there to lonization. These low-lying Rydberg states are the only Rydberg states that can be reached by visible light absorption.
Acknowledgement We are indebted to Dr. S.F. MacDonald, to Professors H.H. Inhoffen, Kevm M. Smith and J.H. Furhrhop and to Dr. B.F. Bumham for tidly supplying us with samples. W&out their help his investlgation would not have been possible. Our thanks are due to Professors D.W. Turner, G_A. Kenney-Wallace, J.R. Bolton and R.E. Christoffersen for helpful discussions. Fmancial assistance from the Natural Science and Engineering Research Council of Canada, the MinistBre.de 1’EducatIon du Quebec and the grant of Killam Memonal scholarship from the Canada Councd (C.S.) is gratefully acknowledged.
References [l]
J.N.A.
RIdyard,
m
Molecular
spectroscopy 1971, ed. London, 1972)
P. Hepple (The Institute of Petroleum,
1 November
1980
[2] S.C. Khandelwaf [3]
and JL. Roebber, Chem Phys. Letters 34 (1975) 355. Y. Nakato. K. Abe and H. Tsubomura, Chem Phys.
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