Stereoelectronic properties of photosynthetic and related systems

JOURNAL

OF MOLECULAR

SPECTROSCOPY

Stereoelectronic A6 lnitio Configuration

73, 3 11-33 1 (1978)

Properties Interaction

Singlet and Triplet

of Photosynthetic Calculations

and Related

on the Ground and Lower Excited

States of Magnesium

Chlorin

J. D. PETKE,] GERALD M. MAGGIORA,~LESTER L. AND RALPH

E.

Systems

and Chlorin SHIPMAN,~

CHRISTOPFERSEN’

Departments of Chemistry and Biochemistry, University of Kansas, Lawrence, Kansas 66045 and tire Chemistry LJiwision, Argonne National Laboratory, Argonne, Illinois 60439 Ab initio configuration interaction wavefunctions and energies are reported for the ground state and many low-lying singlet and triplet states of magnesium chlorin and chlorin, and are employed in an analysis of the electronic absorption spectra of these systems. In chlorin, the calculated visible spectrum consists of two i(?r, r*) states, the lower energy, y-polarized state exhibiting moderate absorption intensity in contrast to the very weak absorption of the higher energy z-polarized state. The configurational composition of both states is well described by the four-orbital model. Five i(r, r*) states are responsible for the Soret band envelope. A moderately intense y-state lies under the low energy edge of the band envelope, while two x-polarized states of moderate and strong intensity, respectively, are responsible for the band maximum. The final two ‘(w, r*) states lie at the high energy edge of the Soret band and introduce a measure of asymmetry into the band envelope. Two ‘(n, r*) states of very weak oscillator strength are also found in this region of the spectrum. All the Soret states are of complex configurational composition, and several of the higher lying states contain contributions from doubly excited configurations. The calculated visible spectrum of magnesium chlorin also consists of two ‘(a, z*) states. with the weakly absorbing x-polarized state lying approximately 200 cm-i lower in energy than the moderately intense y-polarized state. The configurational composition of both states is well described by the four-orbital model. Four i(?r, r*) states constitute the bulk of the intensity in the Soret band envelope. In distinction to chlorin, the moderately intense ‘(a, s*) state at the low energy edge of the band envelope is z-polarized. Two intense I(*, x*) states of y- and z-polarization, respectively, constitute the band maximum region, and a single z-polarized state of moderately strong intensity can be assigned to the high energy shoulder of the band envelope. Two other weakly absorbing i( a, r*) states are also found in this region, along with another weakly absorbing state of mixed in-plane and out-of-plane polarization. No clearly defined ‘(n, ?r*) states are observed. As was the case for chlorin, all the Soret states are of complex configurational composition, and some of the higher energy states contain significant contributions from doubly excited configurations. Chlorin and magnesium chlorin both possess three 3(a, T*) states which lie below Si and a single J(?r, r*) which lies slightly above Sp. All four of the low-lying “(x, r*) states in each molecule are well described by the four-orbital model, with TI being essentially a single configuration in each case. The remainder of the 3(r, x*) states are clustered in the same i Department of Chemistry, University of Kansas. * Department of Biochemistry, University of Kansas. 3 Chemistry Division, Argonne National Laboratory.

311 0022-2852/78/0732-0311$02.00/0 Copyright @ 978 by Academic Press. Inc. AU rights of reproductionin any form reserved.

312

PETKE

ET AL.

energetic region as the comparable i( r, s*) Soret states, with comparably complex configurational compositions. Dipole moments and charge distributions for low-lying singlet and triplet states are also reported, and are used to rationalize chemical reactivity characteristics.

I. INTRODUCTION

This paper is one of a series concerning the theoretical study of the ground and low-lying excited states of several molecules related to the chlorophylls, by means of ab i&o configuration interaction (CI) techniques. The present work deals with the electronic states and spectra of two reduced porphyrins, chlorin, and magnesium chlorin, and is a companion to a previous study on porphine and magnesium porphine (1). Although the chlorins as a class of molecules have been studied less extensively than the porphyrins, they are perhaps more important as simple analogs of chlorophyll systems, since the intense red absorption band characteristic of the chlorins is also a distinct feature of the spectra of the chlorophylls. In the current paper a thorough characterization of many low-lying singlet and triplet states of chlorin and magnesium chlorin is presented and compared to experimental data. The analysis includes the configurational composition of each state and associated transition energies, oscillator strengths, and electron densities. Underlying factors responsible for the various spectral features are analyzed, and appropriate comparisons are given between the two chlorins and their respective porphyrins. II.

CALCULATIONS

The configuration interaction method used here has been described in detail elsewhere (Z-3), and only a brief outline is given below. Configurations for a given state are constructed from an orthonormal basis of molecular orbitals which in the present studies are taken from ground state self-consistent-field (SCF) wavefunctions for chlorin and magnesium chlorin. The generation of configurations ($1 consists of performing single and double excitations from the major contributing configurations of an initial CT expansion \kk(0) which serves to approximate the state of interest. The final CI expansion for state k,

(1) COnSiStS of the configurations contained which have sufficiently large interaction

in \kk co) plus those of the generated with \kk(O)according to the relation

I(~k(“)l~lk)12 @“,I~I@,,,) - (\kk(“)IAl\kk(o))> where the threshold

set {a,,,)

(2)

*’

value 6 is taken as 2 X 10d4 atomic units4 in the present

study.5

4The definition of atomic units recommended in H. Shull and G. G. Hall, Nature, 184, 1559-1560 (1959) are adopted herein. 6 The value of 6 was chosen on the basis of exploratory CI expansions. The present value results in sufficiently large CI expansion for each state such that inclusion of additional configurations have negligible effect on the present results.

EXCITED

STATES

313

OF CHLORINS

Details of the ground state ab in&o SCF calculations, which utilize a basis of floating spherical Gaussian orbitals (FSGO) obtained using the molecular fragment technique (4, S), have been presented in the first two papers of this series (6, 7). The molecular geometries utilized here are those described in the ground state SCF studies. The coordinate axes employed in these studies are pictured in Fig. 1. In the region of the reduced double bond, carbon atoms 7 and 8 are given a slight out-of-plane twist ; therefore the molecular symmetry of chlorin is formally Cz, with the z-axis as the two-fold axis. In magnesium chlorin placement of the magnesium atom approximately 0.4 Angstroms above the xy-plane results in an asymmetric (Cl) molecular geometry. In chlorin the proton configuration used here, in which the NH-NH axis is perpendicular to the x-axis in Fig. 1, is found to be the most stable configuration in CND0/2 studies on the ground state (8), and also corresponds to the probable geometry of the lowest triplet state, as determined by fluorescence-detected magnetic resonance (9). The molecular orbitals allowed to have variable occupancy in the CI expansions include the nine highest-lying occupied molecular orbitals and the 11 lowest-lying unoccupied orbitals in the ground state SCF wavefunction of each molecule, with the remaining occupied molecular orbitals constituting a fixed core. Table I presents the variable occupancy molecular orbitals of the two molecules for comparison, with occupied orbitals labeled 1, 2, . . . , etc. and virtual orbitals labeled l*, 2*, etc., each set taken in order of increasing magnitude of orbital energy. A measure of correlation of a TABLE Comparison

of Molecular

Orbitals

I Used in CI Expansions” MAGNESIUM

CHLORIN ORBITALb

I

I”(0”’

IO"

SYMMETRY

ORBITAL ENERGYC

b b

0.685 0.585

$ 7” 6”

0.503 0.434 0.420 0.395 0.311 0.204 0.177 -0.058 -0.094 -0.168 -0.219

5(o)

a

-0.226

6

a

-0.229

;

b b a

-0.229 -0.235 -0.266

a

-0.297

9 (0) l,(d)

2

'ij

0.923 0.683 0.256 0.933

0.904 0.952

z: 0:919 0.992 0.982 0.982

CHLORIN

0R61TALb

12*(e)(o*) II" lo* 9* a* t 67; 5" 4" 3* 2" I*

b) C) d) e)

0.590 0.571

-0.047 -0.093 -0.186 -0.218 -0.193 -0.256 -0.302

0.991 0.926

4 6 : II(d)(o) 12(d)(o) 9 2

a)

0.676

z:: 0:522 0.446 0.436 0.397 0.321 0.216 0.183

0.948 0.688 0.297 0.437 0.183 0.679 0.308 0.941 0.980 0.444 0.121 0.953

ORBITAt ENERGY

chlorin wbitals are listed with magnesium chlorin orbifals for which r..>O.lO. composition of orbital5 listed is Drimarily r or J*. unless otherwise s&ified. in atomic units:1 a.u.=27.21 ev. fixed double occupancy in Cl. not included in Cl.

-0.199 -0.224 -0.366 -0.287

314

PETKE

ET AL.

Moqnerium Chlorin

Chlorin

FIG. 1. Molecular

molecular orbital value Of 7ij2,

of magnesium

geometries

and coordinate

axes.

chlorin with one of chlorin is given in Table

7ij2 =

)C

(C8i>A

8

(C8j)B

12

I by the

(3)

where (C,JA is the sth coefficient of the ith molecular orbital of molecule A in a symmetrically orthonormalized FSGO basis (1, 6, 7, 10). A value Tij2 II 1 indicates a close correlation between molecular orbital i of chlorin and molecular orbital j of magnesium chlorin. Among the virtual orbitals, which may be qualitatively characterized as ?r* orbitals despite the absence of rigorous u-x symmetry, one-to-one correlations are found generally, as shown in Table I. Certain occupied molecular orbitals of the two molecules exhibit similar one-to-one correlations as well, particularly the r-type orbitals 1, 2, 3, 7, and 8 of chlorin, which correlate, respectively, with orbitals 1, 2, 3, 5, and 7 of magnesium chlorin. A general absence of correlation among u-type orbitals is found, due primarily to the differences in nitrogen environment of the two systems. The inclusion of only the nine highest occupied and 11 lowest unoccupied molecular orbitals as a basis for the CI expansions in the present studies closely parallels the choice of orbitals utilized in similar studies on porphine and magnesium porphine (I), in which the excited states responsible for the visible and Soret absorption bands were found to be adequately described.6 A comparison of tabulated Tij2 for occupied molecular orbitals of porphine and chlorin (6), and of magnesium porphine and magnesium chlorin (7) reveals that all molecular orbitals which correlate strongly with those used in the porphyrin CI studies (1) are included in the current CL Important strong correlations exist involving molecular orbitals which constitute a four-orbital model : 6A more complete discussion of both the limitations of the present treatment and the type and quality of information which may be obtained from the FSGO-based CI procedure may be found in (I), and references therein.

EXCITED

STATES

OF CHLORINS

315

orbitals 1, 2, l* and 2* of porphine (or magnesium porphine) correlate, respectively, with orbitals 1, 2, 2*, and l* of chlorin (or magnesium chlorin). Four orbital models have been widely employed to rationalize the general spectral features of porphyrins and related systems (11-13). In the previous CI study of porphine and magnesium porphine (1) a four-orbital model was found to account for the configurational composition of excited states associated with the visible absorption spectra, but did not adequately describe states responsible for ultraviolet transitions, although “fourorbital” configurations were shown to account for much of the intensity of the uv bands. III. PREVIOUS

EXPERIMENTAL

AND THEORETICAL

STUDIES

General overviews of chlorin and metallochlorin spectroscopy have been given in several reviews (12-14). Considering unsubstituted and alkyl substituted chlorins specifically, the visible spectrum of chlorin in benzene (15) and of etiochlorin in hexane and dioxane (16) have been reported, along with two low temperature studies of the spectrum of octaethylchlorin (17, 18). These studies show a characteristic visible absorption spectrum consisting of four bands as shown in Fig. 2. The low energy band I [Q,(O-0) in Platt’s notation (19)] at approximately 1.5 600 cm-’ is considerably more intense and slightly red shifted compared to the corresponding band in porphine (15, ld), and is attributed to a transition to a l(?r, r*) state. The remainder of the visible spectrum has been interpreted in two ways. Solov’ev et al., on the basis of fluorescence polarization studies on chlorin (20) and octaethylchlorin (18), assign band III at as a second 7r --) ?r* electronic transition, and approximately 19 200 cm-’ [Q=(O-0)] bands II and IV as vibronically induced Qy(O-1) and Qr(O-1) bands, respectively.’ However, Knop and Stichtenoth (27) interpret similar data as showing band IV at 20 200 cm-l to be due to a third electronic transition polarized parallel to band I. A general discussion of the assignment of this band has been given by Weiss (13). The Soret band of octaethylchlorin is found to be slightly less intense than the Soret band of porphine (17, 18), but nevertheless dominates the absorption spectrum (see Fig. 2). This band has traditionally been assigned to two transitions to ‘(?r, 7r*) states (13, 18) but its structure remains largely unresolved. Fluorescence polarization reported in the region of the Soret band (18) implies that there are two perpendicularly polarized transitions, B, and B,, in order of increasing transition energy. The resolution of the polarization spectrum in the Soret region, however, is not sufficient to prove the existence of only two electronic transitions. In comparing the various experimental spectra cited above with the present calculations, it should be noted that the effect on spectral peak position of methyl or ethyl substitution at the pyirole carbon atoms has been shown to be minimal (15). Moreover, peak positions of spectra taken in a variety of solvents at room temperature compare closely with those obtained at low temperature (16, 18). A vapor-phase spectrum of chlorin, which is appropriate for direct comparison with results of the present calculations, is not available. However, comparisons of vapor and condensed phase spectra of several porphyrins (21) reveal a significant difference in the position of the Soret maximum, with the solvent spectra red-shifted approximately 800 to 1000 cm-’ with respect to the vapor-phase Soret peak. 7The designations Qz and Qy utilized herein are consistent with the coordinate system and differ from the notation given in (18) due to an interchange of n and y axes.

given in Fig. 1,

316

PETKE

ET AL.

1.0

4

2’Bty) 3’A (x1 0.5

f

I’B (y)

; 0

15

1

2!(x)

, 20 V(cm”

x I0’3)

FIG. 2. Comparison of the experimental spectrum of octaethylchlorin transitions of chlorin (below). Computed transition energies are estimates calculated oscillator strength.

(18) (top) with computed from Eq. (4). j represents the

Low temperature phosphorescence studies on octaethylchlorin (18, 22) place the lowest triplet state at approximately 12 260 cm-l. The visible absorption spectra of magnesium chlorin (15) and magnesium tetraphenylchlorin (23) both exhibit a typical four-band pattern as shown in Fig. 3. In contrast to chlorin, band IV of these metallo derivatives shows the lowest intensity. Polarization studies on zinc tetraphenylchlorin (24), which has a visible spectrum similar to its magnesium analog, show that bands I and II belong to two separate electronic transitions of mutually perpendicular polarization. The splitting of these two bands is only about 700 cm-l.

EXCITED

STATES

317

OF CHLORINS

Details of the Soret band of unsubsituted magnesium chlorin are not available. In magnesium tetraphenylchlorin (23), the Soret band shown in Fig. 3 exhibits the usual high intensity, and features a distinct shoulder on the high energy side. Polarization studies on zinc tetraphenylchlorin in the Soret region (24) indicate the presence of two electronic transitions, although the resolution is low. In comparing the spectrum of magnesium tetraphenylchlorin with results of the present studies it should be noted that the effect of tetraphenyl substitution on the spectrum of magnesium chlorin is observed as a general red-shifting of the entire spectrum, particularly in the Soret band, where the shift is approximately 1500 cm-’ (1.5, 23). Absorption associated with

40(

30(

5’A(y)

6’A(x)

I.(

9’A(x)

f 4’Nx)

O.! 3’A(y)

Z’A(x) C 15

7’4

\ 20

25

*IA(‘)

I()‘A(y) 30

71 (cm-r x lO’3) FIG. 3. Comparison of the experimental spectrum of magnesium tetraphenylchlorin (23) (top) with computed transitions of magnesium chlorin (below). Computed transition energies are estimates from Eq. (4). j represents the calculated oscillator strength.

318

PETKE ET AL.

‘(n -+ ?r*) transitions of the phenyl groups is not expected to contribute to either the visible or Soret regions of the spectrum, since the band origin of the lowest l(.lr -+ ?r*) transition in benzene vapor is observed at 38 000 cm-’ (25). The (O-O) band of phosphorescence in magnesium octaethylchlorin is found at approximately 12 135 cm-’ (22). A number of theoretical studies of chlorin excited states have been reported, all of In conjunction with their experimental which employ semi-empirical techniques. studies, Knop and Stichtenoth (17) have reported Pariser-Parr-Pople (PPP) calculations on chlorin, and assign the three lowest energy singlet-singlet transitions to bands I, III, and IV, as is consistent with their interpretation of the spectrum. However, the calculated transition energy and oscillator strength of the third electronic state, 25 746 cm+ and 0.593, respectively, suggests that the third transition is part of the Soret band. Transitions to the next five higher-lying l(?r, r*) states, with computed transition energies ranging from 26 883 to 30 447 cm-‘, are assigned to the Soret band. The lowest four triplet states of chlorin are predicted from these studies to be 5615, 10 071, 13 503, and 14 446 cm-’ above the ground state. A different picture is given by other PPP calculations (26, 27), by CNDO/S allvalence-electron studies (8), and by CNDO/S3 studies (28), which predict two visible bands QY and QZ of generally low oscillator strength, and a Soret band composed of two high intensity bands B, and B,. In terms of the axis system shown in Fig. 1, the ordering of transitions given in these studies is QU, QZ, B,, B,; all are due to transitions to ‘(7r, ?r*) states. Additional high-lying ‘(a, K*) states which give rise to N and L bands are also listed (8, 26), along with a ‘(n, ?r*) state located at 32 577 cm-’ (8). None of these higher lying states has been assigned specifically to the Soret band, although Maggiora and Weimann (8) indicate a possible assignment of the computed N, and N, bands with the high energy region of the Soret. In contrast to the results given in (17), PPP calculations listed by Weiss (13) give the energies of the four lowest triplet states of chlorin as 11070, 11440, 15 850, and 16 970 cm-’ above the ground state. No calculations dealing explicitly with magnesium chlorin have been reported. IV. DISCUSSION OF EXCITED

STATES AND COMPARISON WITH PREVIOUS RESULTS

A. Singlet States A summary of computed data on the excited singlet states of chlorin and magnesium chlorin obtained in this study is listed in Table II. Due to the low symmetry of these two systems, transitions from the ground state to all excited singlet states are allowed according to electric dipole selection rules. Among the data listed in Table II are estimates of excited state energies relative to the ground state, which were obtained from the equation AE(eet) = 0.610AE(c”1c~ - 441. (4) where AE(est) and AEccalc) are the estimated and computed transition energies, respectively, in units of cm-l. Equation 4 was obtained in the previous study on porphine and magnesium porphine (1) from a plot of calculated vs. experimental transition energies, and is expected, on the basis of their similarity to the porphyrins, to be useful in estimating experimental Franck-Condon transition energies for chlorin and mag-

EXCITED

STATES OF CHLORINS

319

TABLE II Singlet States’ CHLORIN STATEb ENERGY ABOVE GROUNO STATEd (cm-') l'A

OSCILLATOR STRENGTHe

(ground state)

MAGNESIUM MAJOR CONFIGURATIONSf

STATE‘

CHLORIN

ENERGY ABOVE~ OSCILLATOR GROUND STATE STRENGTHe (cm-')

MAJOR CONFIG"RATIONSf

0.94(SCF)

1'A

0.223(y)

0.77(1+1") 0.52(2+2*)

2'A

27 660 (16 430)

0.015(x)

-0.63(1+2") 0.64(2'1")

29 630 (17 630)

0.018(x)

0.66(1+2') -0.64(2+1*')

3lA

28 050 (16 670)

0.348(y)

0.81(1+1*) 0.46(2'2")

Z'B

39 600 (23 720)

0.621(y)

-0.3g(i+I") 0.56@2*) -0.550+2")

4'A

40 650 (24 360)

0.498(x)

31.4

41 860 (25 090)

0.563(x)

-0.40(1+2") -0.37(2+1*) 0.65&l*) -o.z7(3,l+i*,2*)

5lA

4'A

44 190 (26 510)

0.898(x)

-0.32(1+2") -0.43(2+1*j -0.36(3+1") -0.21(5+1") 0.46(1~+1"*) 0.37(1+3')

43 590 (26 150)

1.185(y) -0.38(1+1') 0.69(2*2") 0.24(3+Zt') -0 3O(i 1+2">) -0:23(l'+i",2*)

6'~

43 990 (26 390)

0.982(x)

7lA

46 I60 (27 720)

o.oo~(~) -0.82(4-r") -0.2O(i l+l*'2) 0.23(1*+i'. 2") o.30(5,i-l'2)

8'~

47 040 (28 250)

0.103(x)

9lA

49 810 (29 940)

1.018(x) -0.27(1+2") -0.27(2+l") 0.5l(3-I") 0.50(5+1") o.Z8(j,l+i",2")

IO'A

50 790 (30 540)

0.013(y)

IlB

26 760 (I5 880)

2'A

46 820 (28 120)

31s

0.507(Y)

o.lg(l'i") -0.28(2+2*) -0.57(3+2*) -0.23(&l*) -0.39(i,l+l"2) -0.24(2.kl?"2) -0.24(2-*3")

41Bcg)

46 910 (28 170)

0.86(5+1") -0.23(j,2+1**)

S'A

48 510 (29 150)

-0.36(1+2") -0.29(2+1") -0.35(3+1") 0.26(&l*) -0.45(12-1"2) 0.23(j,l+i*,2:~) -0.35(1+3")

GIA (g) 48 930 (29 410)

lo-'(x)

50 600 t:o 430)

0.645(y)

5lB

0.89(5-2")

(ground state)

-0.29(2+2*)

-0.30(3+2*)

0.94(SCF)

o.29(1+2') -0.41(1+3") 0.39(2+1") -0.21(12+2~~2) 0.56(12+l"2) 0.25(7+1")

0.52(1+2") 0.22(l-r3'~) 0.40(2-tl~) 0.20(3+1") 0.35(5+1") -0.39(12-l"2)

-0.53(3-l>? 0.47(5-t") -0.28(2 i-i* 2“') -0.24(2;1+1 i*) -0.24(4,1+1"~) 0.25(2,1+1",2")

0.26(3+2") 0.57(6+1") 0.43(2 1+1"2) -o.38(li-i" 2") D.21(5,i+l~2)

0.50(4-tl") _~:;;~$~;:~:1) 6'8

51 670

0.277(v)

(31 080)

0.20(2+2") 0.45(6+1") 0.20(8+2"~ _ 0.21(~2+1~~;") :;:;;{;l$

2k)

-o.21(7,i+lA2)

f)

Number of configurations: chlorin l~(782). 'B(890); (n,n*) unless specified. Mgnesium chlorin 'A(Il48). Designations in approximate Czv symmetry: Listed in terms of C symmetry. A(n,n')zAl, A(",'*):&, B(n.a')=B2, B(n,a*)=B1: Designations in approximate C, symnetry: Listed in terms of Cl symnetry. x-polarized states *A', y-polarized states:l\". Calculated values with estimated values from Eq. (4) in parentheses. Calculated from Cl expansions truncated to 200 configurations, and using fz(2/3)AE/ti1', whew p and AE are the computed transition dipole and transition energy, respectiveiy, in ato;;lieunits. Configurations with coefficients of magnitude >0.20 are listed. A bar denotes beta spin.

g) h)

(".?I") in-plane y-polarized.

?I) b) C)

d) e)

but also contains

significant

z component.

320

PETKE

ET AL.

nesium chlorin. The possibility of obtaining a separate relationship utilizing the computed data for chlorin and magnesium chlorin will be discussed at the end of this section. The transition energies obtained directly from the computed CI wavefunctions for chlorin and magnesium chlorin are found to be high compared with experimental values, as has been found generally for studies of this type in which the current FSGO basis is employed (1, 29). The qualitative features of the visible absorption spectrum of chlorin are well reproduced by the present results. This is illustrated clearly in Fig. 2, which compares the low temperature experimental spectrum of octaethylchlorin (18) with the present results, given in terms of estimated transition energies and computed oscillator strengths.8 A point of interest concerns the location of the higher energy 2lA visible band, which is suggested by the present work to lie somewhat closer to band I (1’B) than has been experimentally assigned. The smaller QU - QZ splitting predicted here may be a manifestation of uncertainties in the theoretical treatment, or it may be an actual feature of the gas-phase spectrum of chlorin, which is not available for comparison. The possibility that band II corresponds to the 2lA state seems less likely than band III based on polarization data, although the degree of polarization does drop to a negative value in the region of band II (18). The possibility of a third electronic transition in the visible spectrum is not supported by the present calculations. The computed visible spectrum of magnesium chlorin also consists of two electronic transitions as shown in Fig. 3, but its general features deviate somewhat from what has been suggested from studies on similar systems (23, 24). The present results predict that in the gas phase spectrum of magnesium chlorin the first two electronic transitions are essentially accidentally degenerate and correspond to band I (see Fig. 3), while bands II, III, and IV are of vibronic origin. The calculated order of the two visible states places the 2lA with low oscillator strength at a lower relative energy than the higher intensity 3lA, in contrast to the corresponding states in chlorin and in contrast to polarization studies on zinc tetraphenylchlorin (24). Although there were no changes in the order of these two states at various levels of the CI treatment, a definitive statement on the actual ordering of these two states is likely beyond the capabilities of the present calculations. However, the conclusion concerning the near degeneracy of the 2lA and 3lA states is thought to be reliable. Additional experimental work is needed to clarify these points further. In comparing the present results regarding visible absorption with the previous study of porphine and magnesium porphine (I), it is found that, qualitatively, the relative peak positions of the visible bands of the four molecules are in general agreement with experimental findings (IS), with the exception that the present study fails to place the high intensity Qy band of chlorin to the red of Qg in porphineSg The configurational composition of the states responsible for visible absorption in all four molecules is basically given in terms of a four orbital model which includes molecular orbitals 1, 2, 1*, and 2* in each case. Relative oscillator strengths predicted here and in (1) are in excellent agreement with the qualitative pattern of intensities for the four molecules 8 The computed oscillator strengths are given to provide relative values of the integrated extinction coefficients of the various states.‘Absolute comparison of calculated oscillator strengths with experimental values is not intended. 9 Note, however, that the observed porphine-chlorin QY splitting is only 540 cm-l (15).

EXCITED

321

STATES OF CHLORINS TABLE III

Oscillator Strength Analysis for l(?r, T*) States MAGNESIUM

CHLORIN STATE

116 ZIA

CONFIGURATION

STATE

3' A

(l+l*) f&z*)

-3.709 I.371

0.466(y)

41A

(l-z*)

-1.120 0.075 -1.375 -0.129

0.802 (XI

5'A

(PI")

(I-rZ")

2.694 -2.296

0.014(x)

-1.788 -1.756 0.704

0.970(Y)

-1.637 -1.307 0.522

0.746(x)

w; (P2*) (kl") (Z-+3*) (1+2*) (2+1")

I.ZOl(x)

0.881 0.882 0.737 -0.085 0.305

1.052(y)

-1.491 -1.053 -0.277 -0.114 -0.052

1.315(x)

0.806

0.802(Y)

7lA

9'A

b)

(I?*)

0.283(v)

(I?*) (2‘2") (3+2") (6-l")

I a.u.-2.54,debYe., ,~ in atomic units: computed from the relation f=(2/3jAElfpijI ‘, where AE is the computed in atomic units.

C)

The sumnation

in-pl,ne Y-polarized,

includes only

but also contains

1.747(Y)

-2.145 -0.040 -1.419 0.085 0.342

1.348(x)

0.060 0.069

0.002(c)

0.243 -0.023 0.486 -0.219 0.466

0.130(x)

I.124 0.026

1.193(x)

0.947 0.212 0.499 IO'A

a)

1::;:; 0.173

0.907 0.383 0.188 -0.634 -0.640 0.069 -0.082 -0.057

OSClLLATgR STRENGTH O.Oll(X)

21 A

-1.310 -1.542 -0.285 0.054 0.092

(a) 'ij 2.641 -2.276

0.294(Y)

(3+2*)

CONFIGURATION

g::;

3.547 -1.644

o+z*,

i2+2*j

5'A

OSCILLATBR STRENGTH

(I-tl”)

@+I")

31B

pij(a)

CHLORIN

-0.056 -0.150 0.187 -0.244

O.Oll(y)

transition energy

listed values of pij.

small L component.

(1.5), particularly in the prediction of a relatively high oscillator strength for the llH and 3lA states of chlorin and magnesium chlorin, respectively. Underlying factors responsible for these increased oscillator strength values are illustrated in an analysis of oscillator strengths for chlorin and magnesium chlorin given in Table III. The analysis is similar to that used previously for porphine and magnesium porphine (I), in which the ground state wavefunction 9, is taken to include only the SCF configuration *0 = CSCF@SCF and the transition

dipole Pi for excitation

to state

(5)

PETKE

322

ET AL.

is therefore Pi

=

C

CSCFcji(%CF

1r (*j>

(6) (7)

Considering the llB state of chlorin as an example, Table III shows that the two main contributors to the transition dipole are dipole matrix elements involving +scF and the single excitation configurations (1 + l*) and (2 + 2*), which give values of pii of opposite sign which tend to cancel. This observation applies generally to transition dipoles of visible absorption both here and in (I), but in the case of the llB state of chlorin and the 3lA state of magnesium chlorin the pii value due to the configuration (1 -+ l*) dominates, and the resultant oscillator strength is relatively large. The above two states show a CI composition more heavily weighted toward one configuration, (1 --f l*), than for other visible states [see Table II here and in (I)]. Moreover, the dipole matrix element ((1 --f 1*>I rl%x) is significantly

larger in magnitude

than ((2 + 2*)

( r(*SCF),

resulting in relatively large transition dipoles for the Qa, states.‘O One of the major points presented in the previous study on porphine (I) was that, in contrast to previous conceptions, the Soret band consists of several (i.e., more than two) ?r + ?r* electronic transitions of substantial oscillator strength. The same feature is found in both chlorin and magnesium chlorin, as indicated by the simulated spectra shown in Figs. 2 and 3. In chlorin five l(n, ?r*) states of generally complex CI composition appear in the Soret region as well as the weakly-allowed 4lB and 6lA states, which may be qualitatively characterized as l(n, x*) type states, where the “n” molecular orbital contains large contributions from the N2 lone pair. In magnesium chlorin at least three l(g, r*) states, 4lA-6lA, comprise the major portion of the Soret intensity, while the 9lA state may be associated tentatively with the high energy shoulder present in the experimental spectra of zinc and magnesium tetraphenylchlorin (23). Additionally, three low intensity l(r, ?r*) Soret states, 7lA, 8lA, and 10IA, are found. None of the Soret states of chlorin or magnesium chlorin is described by a four-orbital model. Comparisons of the various l(?r, r*) Soret states of the two molecules in terms of the configurational composition are given in Table II. Generally, no definite one-to-one correspondences between respective states are found, although certain similarities between states are seen, such as between the 2’B and SA, and the 4lA and 6lA states of chlorin and magnesium chlorin, respectively. Furthermore, when the Soret states of the two systems are compared on the basis of increasing magnitude of the transition energy, there are notable differences in the polarizations of transitions. It is important to note that, in several of these states, such as the 4lA and SA states of chlorin and the 4lA and 6lA states of magnesium chlorin, there are large contributions from doubly excited configurations such as (12 -+ 1*2). The presence of this type of configuration in certain of these states of the chlorins implies a fundamental difference between them loIn chlorin, for example, dipole matrix elements between &c~ and the configurations (1 + l*), (2 + 2*), (1 ---f 29, and (2 + l*) are, respectively, -3.47, 2.36, -3.07, and -2.68 at.omic units.

EXCITED

STATES

OF CHLORINS

323

and corresponding excited states of porphine and metalloporphyrins, in which closed shell doubly excited configurations are forbidden by symmetry to appear. Furthermore, the presence of these configurations emphasizes the inadequacy of previous theoretical treatments (8, 17, 26-28), in which only singly excited configurations were employed. The analysis of oscillator strengths given in Table III provides a different and perhaps more useful means of comparing the various l(?r, r*) Soret states of both the present systems and the previously studied porphyrins [see Table III in (I) for relevant data on porphine and magnesium porphine]. It is seen, for example, that the composition of the transition dipole of the Y-polarized 2lB state of chlorin is similar to that of the .!?A, 2lE(y), and 21B2U states of magnesium chlorin, magnesium porphine, and porphine, respectively: the transition dipole consists of a pair of large additive dipole matrix elements

and @SCF

1y 1 (2

-+

2*)

>,

plus a smaller contribution of opposite sign from a matrix element involving a third single excitation. l1 A similar analysis applied to other Soret states reveals other relations, including the correlation of the 3lA, 3lB, and 4rA states of chlorin, respectively with the 2lE(x), 3lE(y), and 3lE(x) states of magnesium porphine and the 2lB3,, 3rBzu, and 3lB3, states of porphine. Either the 4lA or 6lA state of magnesium chlorin may be considered as correlating approximately with the 3lA state of chlorin, while other magnesium chlorin Soret states are found not to correlate on this basis with states of the other three molecules. A general characteristic these and other Soret states which carry significant oscillator strength is the presence of substantial contributions from dipole matrix elements involving “four orbital” configurations, even in states where these configurations are not major contributors in terms of CI composition. As discussed previously, the estimated transition energies employed in Figs. 2 and 3 and Tables II and V were obtained from Eq. (4), a linear relation between calculated and experimental transition energies for porphine and magnesium porphine. For the present systems the development of a similar equation is complicated by several factors including ambiguities in assigning computed transitions to unresolved spectral peaks, and the necessary use of experimental data obtained from condensed phase rather than vapor phase spectra. Nevertheless, the development of a relation between available experimental data and the present calculations was attempted, in which the following assignments were used. For magnesium chlorin, computed transition energies for the 3lA, PA, and 13A states were assigned, respectively, to the experimental transition energies of Q1/ (15), the Soret maximum (15), and phosphorescence emission (22). For chlorin, computed l’B, llA, and 13B transitions were assigned to experimental peaks for Qv U5), Qz W, and phosphorescence emission (18), respectively. Assignment of either the chlorin 3lA or 4rA computed transitions to the Soret maximum at 25 775 cm-’ (15) is reasonable assuming the presence of an x-polarized transition (18). If the 3lA state is assigned, a least-squares fit of these data results in the relation (in units of cm-l) AE(est) = 0.594AE’“a1c’ + 76.9, I1Note that spectively,

chlorin and magnesium with porphine and magnesium

chlorin configurations porphine configurations

(1 -+ l*) and (2 + 2*) correlate, (1 + 2;) and (2 + l*).

(8) re-

324

PETKE

ET AL.

A:

&ST.)

6:

AE(EST*) I

0.594AE(C”‘c’) + 76.9

AE(EST’) =

0.%4AE(C*LC’) + 795.

10

I

0.610AE(c*LC*) - 441.

20 AE(ca’c.)

30

LO

(CM-’ x ~0-3)

FIG. 4. A comparison of linear relationships giving estimates of transition energies, AEcest), from were obtained from least-squares fits calculated energies above the ground state, AE( osle). Equations of experimental and calculated transition energies for A: porphine and magnesium porphine, and B, C : chlorin and magnesium chlorin, as detailed in (I) and the text.

with a linear correlation coefficient of 0.985 and a root-mean-square Similarly, if the chlorin 4lA state is employed, a relation AEcest) = 0.564AE(Cs’c) + 795,

error of 883 cm-‘.

(9)

with a linear correlation coefficient of 0.988 and a root-mean-square error of 787 cm-‘, is obtained. Plots of these two relations along with Eq. (4) are given in Fig. 4. Clearly, utilization of any one of the three equations to obtain estimates of Franck-Condon transition energies will yield basically the same qualitative picture of the absorption spectrum, particularly in the visible region. Equation 4 was obtained from fitting calculated data for porphine and magnesium porphine (1) with primarily vapor phase experimental data, and assignments were made without ambiguity. The estimated transition energies for chlorin and magnesium chlorin obtained from Eq. (4) are therefore an appropriate choice for comparisons of the vapor phase spectra of the four systems. Computed oscillator strengths and estimated transition energies for absorption from the lowest excited singlet state (S1 -+ S,) are given in Table IV. Very low intensity is found for all of the transitions in either molecule, implying that the visible region of the Sl+ S, absorption spectra of these two systems would appear to be devoid of outstanding features. B. Triplet States

A tabulation of data on the computed triplet states of chlorin and magnesium chlorin is given in Table V. A comparison of the estimated transition energies [from Eq. (4)]

EXCITED

325

STATES OF CHLORINS TABLE IV

Computed Oscillator Strengths and Estimated Transition Energies of Excited Singlet-Singlet Transitions MAGNESIUM

CHLORIN TRANSITION

ESTIMATED TRANSATION ENERGY (cm-')

C)

d)

TRANSITION

ESTIMATED TRANSATION ENERGY (cm-')

OSCILLATOR STRENGTHb

lo-5(Y)

I 750

lo-s(Y)

2'A+3'A

240

7 840

lo-'*(x)

2lA+4lA

7 930

lo-~(x)

9 720

0.012(y)

9 210

0.003(Y)

2'A+5lA

IO 630

O.OOl(Y)

Z'M.'A

12 240

0.021(x)

2lA+7'A

II 290

0.014(y)

I2 290

0.001(x)

2lA+E'A

II 820

10-5(Y)

I3 270

0.007(Y) ,,-4(c)

2'M'A

I3 510

lo-"(x)

2'A+lO'A

I4 II0

0.032(y)

13 530

a) b)

OSCILLATOR STRENGTHb

CHLORIN

I4 550

0.003(x)

I5 200

0.018(x)

v 960

,0-h(d)

computed from estimated energies above ground state. calculated from Cl expansions truncated to 200 configurations, and using fz(2/3)AE1y12, where p and BE are the computed transition dipole and transition, energy. respectively. in atomic units. largely z-polarized, with a small in-plane y component. largely x-polarized, with small out-of-plane z component.

of the respective singlet and triplet states of the two systems pictured in Fig. 5 reveals a basic pattern also characteristic of the porphyrins (I), namely four low-lying triplet states (AE < 20 000 cm-l) along with a band of higher-lying triplets between 2.5 000 and 30 000 cm-’ above the ground state, SO. The lowest triplet state, Ti, of magnesium chlorin is estimated to be only 11 660 cm-’ above So compared with an experimental value of 12 135 cm-’ (ZZ), and is significantly lower-lying than Tz and Tz. The lowest triplet of chlorin is estimated to lie 12 960 cm-l above So, in reasonable agreement with phosphorescence data (18). In each system TI-TS lie below the lowest excited singlet state 31. In terms of configurational composition there is a very close correlation between TI-TJ of the two systems; the composition of T1 in each case is essentially described by the single configuration (1 -+ l*), while Tz-T* are also generally dominated by single “four-orbital” configurations. The present calculations predict that in chlorin the spatial symmetry of T1 is the same as that of S1, in contrast to arguments presented by Solv’ev et al., based on an analysis of phosphorescence polarization data (18), and in contrast to the case of porphine in which similar theoretical and experimental evidence shows that the spatial symmetries of TI and S1 differ (I, 18). In the present calculations on chlorin the prediction of the 1’B and 13B states as S1 and T1, respectively, appears to be unambiguous, since computed S1-S2 and Tl-Tt splittings are relatively large, and the order of the states did not change at various levels of the CI treatment. The computed llB state of chlorin clearly corresponds to the experimentally observed Sr state on the basis of a comparison of computed and observed oscillator strengths, while the prediction of the chlorin 13B state as T1 appears strengthened in view of the similarity of its configurational composition to T1 in magnesium chlorin. Moreover, as mentioned earlier, the basic

326

PETKE

ET AL.

NH proton conformation used here is in agreement with the observed conformation for T1 (9) ; therefore only minor geometry differences exist between the actual geometry and that used here. Such differences are unlikely to result in a change in identity of the computed Tr state. On the basis of this evidence, the current predictions of the spatial symmetries of the chlorin Sr and T1 states are thought to be quite reliable for the chosen geometry. Additionally, CNDO/S studies on chlorin singlet (8) and triplet (30) states in which CzU symmetry was employed show that the spatial symmetries of Sr and T1 are the same. The data presented by Solov’ev et al. shows that the (O-O) band of phosphorescence is polarized predominantly along the y-axis (in the present coordinate system), and also TABLE V Triplet States CHLORIN

HACNESIUH

ELECTRONIC STATEa

ENERGY ABOVE GROUND STATEb

I38

21 970 (12 960)

13A

24 170 (14 300)

23~

25 240 (14 960)

238

33B

MAJOR CONFIGURATIONSC

ELECTRONIC STATEa

ENERGY ABOVEb GROUND STATE

13A

19 a40 (II 660)

0.95(1+1”)

0.85(2+1*) -0.38(1+2*)

23~

25 290 (14 990)

o.a4(1+2*) 0.44(2++1")

o.a6(1+2") 0.39(2+1*)

33A

26 600 (15 790)

o.a3ci+l*) -0.43(1+2*)

31 210 (la 600)

43A

32 400 (I9 320)

0.90(2+2")

38 960 (23 320)

53A

42 250 (25 330)

-o.a1(4+1*) o.20(i2+i* -0.21&+2*) 0.20(5+3")

-o.as(j+P) 0.31(3,1+1*,2*)

41 420 (24 aso)

53A(d) 638

a)

(~.n*)

unless

chlorin b)

Calculated

c)

Configurations

d)

(n,n*)

47 700 (28 660)

0.27(6+2*) -;.;w+;:;

47 740 (28 680)

0.36(4+1*)

48 400 (29 080)

-o.a9(5+2*) 0.20(5,2'1*,2*)

48 990 (29 440)

-0.46(4-d*) -0.44(b+lft)

specified.

3A(a92). values

Number of

FOT approximate with

with

MAJOR CONFIGURATIONSC

2")

63~

42 470 (25 470)

73A

44 a70 (26 930)

a3A

45 260 (27 170)

Y3A

46 290 (27 800)

0.26(1+3") 0.77(3+1") 0.23('j'l*1 0.32(3,1+1*.2*)

IO'A

46 710 (28 050)

0.26(3+2*) 0.49(6+1:) -0 64(12+1* 2*) -0:30(7+2*)'

o.aaw*)

45 350 (27 220)

53B

CHLORIN

estimated

coefficients

configurations:

Cly

state

values of

chlorin

designations

from

magnitude

Eq.(4) >0.20

0.79(3+2") -0.22(6+1"3 0.22(F2") 0.20(3'6*)

3A(726), 3B(754); magnesium

see Table

11.

in parentheses. are

listed.

A bar

denotes

beta

spin.

EXCITED

STATES

327

OF CHLORINS

MG CHLORIN

-1dA 9’A

30-

=g -4’A

6’A

-3’A

;;; 25,o x

=5’A

-

-2’8

=i 5 2 20. ?I

-43A -

-2% -

2’A

-1’B 15-

-33A -23A

-2% -1% -1%

-13A

FIG. 5. Estimated

energies

(from Eq. (4)) relative

to the ground

state for chlorin

and magnesium

chlorin.

contains an out-of-plane z-component. On the basis of a value for the degree of polarization of - 18% upon So + Sz excitation it is implied that the phosphorescence is perpendicular to the x-axis. In view of these observations and assuming that spin-orbit coupling of T1 with the singlet manifold is the primary mechanism responsible for the observed phosphorescence, it is shown by Solov’ev et al. that, for a molecule of CZ,, symmetry, the symmetry of the spatial part of TI of octaethylchlorin must be fully symmetric (and therefore different from the symmetry of Sr) in order to account for the observed phosphorescence polarization. Such rigorous CZ,, symmetry is not expected to apply in the actual physical situation, however, and group theoretic arguments become inconclusive regarding the spatial symmetry of T1 when the molecular symmetry is lower than CZv. For example, in the C2 geometry employed in the present study, a T1 state of either A or B spatial symmetry may lead to the observed polarization; in each case the absence of an x-component of phosphorescence emission must arise due to negligibly small spinorbit coupling between T1 and ‘il (?r, ?r*) excited states. The ratio of y and z components of phosphorescence is estimated by Solov’ev et al. to be Assuming that the phosphorescence of chlorin is PlJ/PZ = 1.7 for octaethylchlorin. similar, spinorbit coupling of T1 with the computed 4lN (II, 7r*) state appears to be the only means by which an out-of-plane =-component may arise. The 4lH state gives both

328

PETKE

ET AL.

TABLE VI Computed Oscillator Strengths and Estimated Transition Energies of Triplet-Triplet

Transitions

CHLORIN TRANSITION

ESTIHATEO TRANSITION ENERGYa(cm-'1

MAGNESIUM OSCILLATOk STRENGTH

TRANSITION

CHLORIN

ESTIMATED TWINSITIDN ENERCYa(cm-')

oSCILLATO[ STRENGTH

13gtl3A

I 340

0.002(y)

3 330

0.008(y)

13F++Z3A

2 000

0.003(Y)

4 130

0.014(y)

5 640

lo-'(x)

13P+z3El

7 660

0.002(x)

135+33s

IO 360

lo+(x)

13 670

0.002(x)

133+33A

II 870

lo-b(y)

13 810

0.002(y)

138+43B

I4 260

10-6(x)

I5 270

0.138(Y)

13&43A

15 700

10-6(v)

I5 510

l3&53S

I5 720

0.007(x)

16 140

0.033(y)

16 390

0.006(x)

13LI+53A

16 120

10-5(c)

l3Ew638

16 480

0.008(x)

a)

b)

c)

IO-'*(x)

computed from estimated energies above ground state. calculated from Cl expansions truncated to 200 configurations. and using f:(2/3)AE1p)', where @ and AE are the computed transition dipole and transition energy, respectively, in atomic units. transition dipole canposed of approximately equal y and z components.

FIG. 6. r-electron populations and bond orders listed (top to bottom) for the SO, S1, and TZ states of chlorin, respectively.

EXCITED

STATES

OF CHLORINS

329

y- and r- polarized components of the transition dipole upon excitation from So, and carries a very low oscillator strength in comparison with lower lying ‘A(?r, r*) and lB(r, r*) states (see Table II). This implies that spin-orbit interaction between T1 and the ‘A (r, ?r*) states may be extremely weak, resulting in the observed absence of an x-component of phosphorescence emission. Further experimental and theoretical work is needed before the T1 state can be unequivocally assigned. In Table VI calculated data on triplet-triplet (T1 --f T,) transitions are presented. These data show that the visible triplet-triplet absorption spectrum of chlorin is expected to be extremely weak. For magnesium chlorin, however, the 13A + 73A excitation is likely to contribute a prominent peak at approximately 1.5 270 cm-‘, accompanied by a series of weak but observable transitions in the region 13 000 to 16 000 cm-‘. C. Charge Distribution Analysis

A comparison of the A electron populations and bond orders for the ground and several low-lying excited states of chlorin and magnesium chlorin is given in Figs. 6 and 7. The values listed were obtained from full CI expansions for each state and are given in terms of a symmetrically orthonormalized FSGO basis (10). In chlorin the highest carbon r-electron populations in each state (SO, Si, TI) appear at the methine carbon atoms which are positioned adjacent to the reduced ring. This

FIG. 7. T-electron populations and bond orders listed of magnesium chlorin, respectively.

(top to bottom)

for the SO, S1, Sz, and T, states

330

PETKE TABLE Electric

ET AL. VII

Dipole Moments&

CHLORIN STATE

a) b)

c)

MAGNESIUM id

(b)

CHLORIN

STATE

so

-1.54

so

-3.86

-1.65

3.48

51

-1.80

51

-4.18

-2.31

3.48

Tl

-1.17

52

-4.00

-1.96

3.48

Tl

-3.64

-0.97

3.51

Given in Oebye units. Directed along the x-axis defined in Figure I. The positive end of the vector is directed toward the reduced ring, relative to the origin of the coordinate system. The positive end of pz is directed toward the magnesium atom relative to the macrocyclic ring.

observation is consistent with previous ground state studies (6, 27)) and correlates well with experimental evidence showing that in the ground state, these carbon atoms are particularly susceptible to electrophilic attack (31). Another feature of the r-electron distribution which is common to all three states is the relatively low ?r population of the nitrogen atom in the reduced ring. In comparing the r-electron distributions of the three states it is found that the ?r distributions in So and S1 are generally similar, while the distribution in T1 differs from that of So and .Sl in having increased ?r populations at the methine carbons and nitrogen atoms situated along the Ct axis, at the expense of the carbon atoms which connect these two sites. For magnesium chlorin the identity of the lowest excited singlet state was not unequivocally established by the present calculations ; therefore K charge density data for the computed Sz state, which is similar to Sr in chlorin, is given in Fig. 7 along with data for So, Sr, and T1 states. This data shows that there are essentially no distinctive differences between the r-electron distributions for the Sr and Sz states, and that the salient features of the 7r distributions of the reported magnesium chlorin states are basically the same as for chlorin. Electric dipole moments for several low-lying states of both molecules are listed in Table VII, for the purpose of qualitative comparison. The dipole moments for the SO, Sr, and T1 states of chlorin indicate that the reduced ring contains net positive charge in each case, and that the T1 state is somewhat less polar than either Sr or So. The presence of the four-coordinated magnesium ion in magnesium chlorin results in a large out-of-plane component ‘CL,which dominates the total dipole moment in each reported state, while the dipole components ~1~illustrate in-plane charge distributions for magnesium chlorin similar to the corresponding states of chlorin. ACKNOWLEDGMENTS This work was supported in part by the Division of Physical Research, United States Energy Research and Development Administration, and the Upjohn Company, Kalamazoo, Michigan. Services and computer time made available by the Argonne National Laboratory Computer Center have been invaluable in this study.

RECEIVED: December

8. 1977

EXCITED

STATES

OF CHLORINS

33 1

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15. U. EISNLR ANDR. P. LINSTEAD,J. Chem. Sot. 3742-3754 (195.5). 16. F. PRUCKNER,Z. Pkys. Chem. 187a, 257-275 (1940). 17. J. V. KNOP AND H. STICHTENOTH, Z. Naturforsch 27a, 639-644 (1972). 18. Ii. N. SOLOV’EV,A. T. GRADYUSHKO,M. P. TSVIRKO, APEDV. N. KNYUKSHTO,J. Luminescence 14, 365-374 (1976). 19. J. R. PLATT, “Radiation York, 1956.

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New

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