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Electronic states of oxalic acid and dimethyloxalate. Luminescence studies
JOURNAL
OF
MOLECULAR
SPECTROSCOPY
43,
296-311 (1972)
Electronic States of Oxalic Acid and Dimethyloxaiate. Luminescence Studies’ H. J. MARIA AND S. P. MCGLPNN Coates Chemical Laboratories,
Louisiana State University,
Baton Rouge,
Louisiana
70805
A short-lived phosphorescence is observed from oxalic acid, dimethyloxalate ester, and covalent oxalate salts such as those of zinc and cadmium. The phosphorescence is attributed to the transition 13A, -+ llA,(?r* + n) of the Cqh trans-planar oxalate moiety or to the transition 23A + 1lA (C,) with which it correlates when the molecule is not planar. The phosphorescence of glassy solutions refers to emissive relaxation of an excited oxalate entity which is believed to be nonplanar. The phosphorescence of crystalline oxalate entities originates largely from defect centers which might retain planar oxalate geometries. The intersystem (Sly3 Tr) crossing event in crystalline dimethyloxalate exhibits a striking vibronic effect which is based on coupling of b, modes for which i; = 400 cm-l. I. INTRODUCTION
There are some literature allusions (1-S) to a luminescence of oxalic acid. Unfortunately, these are only available to us in abstract form. In one case (1): an emission-referred to as a “fluorescence”-was monitored in the region 400606 nm; in another (8), a very weak emission-referred to as a “phosphorescence” -was observed upon irradiating a sample for 4 set, closing an excitation shutter, and opening a second shutter to record luminescence; and, finally, a “sky-blue phosphorescence” of mean lifetime 7 ‘v 1.7 set was observed (3) at liquid oxygen temperature. Such reports do not provide a satisfactory overview of the luminescence characteristics of oxalic acid. We have observed a short-lived phosphorescence from oxalic acid, dimethyloxalate, and zinc and cadmium oxalates. This report is concerned with the nature of this luminescence process and with the strong vibronic coupling effects which are evident in its excitation spectrum in crystalline dimethyloxalate. II. EXPERIMENTAL
PROCEDURE
Materials Oxalic acid and dimethyloxalate were purified as described in paper I. Zinc and cadmium oxalates were prepared by metathesis of the appropriate Reagent Grade nitrate salt with sodium oxalate. 1 This work was supported by research contract between the United States Atomic Energy Commission, Biology Branch, and the Louisiana State University. 2 Paper I of this series of two papers is entitled “I. Absorption Studies” and is referenced as H. J. Maria and S. P. McGlynn, J. Mol. Spectrosc., 43, 177 (1972). 296 Copyright
9
1972 by Academic
Press, Inc.
OXALIC
ACID
AND
297
DIMETHYLOXALATE
All solvents were fluorimetric grade (Hartman-Leddon used as received.
Company)
and were
Excitation and emission spectra were recorded at 77°K on an instrument consisting of a Cary 15 excitation monochromator and a half-meter Jarrell-Ash scanning emission monochromator. The signal was chopped at 660 cps and detected by an EMI 9558&B phototube coupled with a Princeton Applied Research 220 lock-in amplifier. Excitation was of front-surface nature. The polarization of phosphorescence was measured on an Aminco-Kiers Spectrophosphorimeter. Decay curves were photographed on a Tektronix oscilloscope in conjunction with the Aminco-Kiers Spectrophosphorimeter. All samples were degassed by repeating three times a cycle of freezing (77”K), pumping (~1 ,u> and thawing (300°K). III.
RESULTS
1. Oxalic Acid
The phosphorescence emission and phosphorescence excitation spectra of a low3 M solution of oxalic acid in EPA are shown in Fig. 1. The excitation spectrum contains a quite weak band at 3270 A-; this band is well separated from the main excitation system and is identified here as the origin of the T1 + So absorption process. The 0’ t 0” band is the only vibrational structure of TI c- So which we expect to resolve because of the overlap of the 1’ t 0” excitation and the tail of the & t SOabsorption region. The band system whose first member occurs at
A
B
u,i;,Iill r ._ if f
250
290
330
320
Wavelength
360
340
280
400
(nm)
FIG. 1. Phosphorescence and phosphorescence excitation Concentration: lO+ M in EPA. A. Excitation spectrum for &,, = 360 nm B. Phosphorescence spectrum for hex = 275 nm
spectra
of oxalic
acid at 77°K.
MARIA AND McGLYNN
298
3060 8 corresponds to the lowest energy (81 c So) spin-allowed absorption illustrated in Fig. 1 of paper I. The phosphorescence spectrum of Fig. 1 consists of four bands with an average separation of -1560 cm-‘. The first prominent maximum occurs at 3400 i and lies at lower ecergy than does the supposed T1 c So(0'c ON) excitation maximum at 3270 A. However, close examination of the first phosphorescence band shows it to be highly asymmetrical; indeed, one can surmise the presence of perhaps two shoulders on its blue side. These shoulders are more evident in the spectrum of a lop4 molar solution; however, in this instance, the signal-to-noise ratio is generally unfavorable to the recording of a good overall spectrum. Similar remarks apply also to the second broad band in the emission spectrum of Fig. 1. We attribute the blue degradation effects observed within each band to the existence in solution of oxalic acid molecules in more than one geometrical configuration. The different possible geometrical configurations of the ground statelentity are discriminated by the angle of t.wist of the two carboxyl groups about the C-C bond (as inferred in paper I) ; the excited state geometries may be discriminated by the possible changes in this dihedral angle which are caused by excitation of the ground state entity. TABLE I VIBRATIONALSTRUCTUREIN THE PHOSPHORESENCE OF DIYETHYLOXALATE IN EPAa AT 77°K Concentratedsolution (some microcrystallinecontent)
Dilute solution (no microcrystallinecontent) x(A)
t (cm-l)
3248 3438 3653 3880
30790 29090 27380 25770
F1 = 1700 cm-l
Analysis
x(A)
F(cm-l)
0’ 0’ 0’ 0’
3248 3290 3339 3392 3492 3547 3617 3721 3775 3857 3992 4290
30790 Ot’ 30390 Of’ 29950 01’ 29480 0,’ 28640 0,’ 28190 Of’ 27650 Of’ 26870 0,’ 26490 Of’ 25930 Of’ 25050 Of’ 23310 0,’ & = 1780 cm+ li, = 420 cm-l 53 = 950 cm-l
- 0” - “1 - 2v, - 3v,
Analysisb 0” 0” (unresolved) Y2 YZ YI (v* + Y1) (1.3+ Y1) 2v, (Y2+ 2Vl) (va + 2Vl) 3Vl 4v*
* EPA is a 5:5:2 volume mixture of ethylether, isopentane and ethanol. b The subscripts t and f indicate, respectively, “true” and “false” origins for the phosphorescence.
OXALIC
ACID
AND
DIMETHYLOXALATE
299
The lack of resolution evident within each band is responsible for the smallness of the observed interval (~1560 cm-‘). This frequency interval is considerably smaller than that of the totally symmetric > C = 0 stretching mode (4) (1762 cm-’ in the So state); nonetheless, we identify the observed frequency with this vibrational mode. The assertion that the smallness of the measured interval is associable wit.h experimental inaccuracies is borne out in Tables I & II. The lifetime of phosphorescence of 1O-3 M oxalic acid in ethanol is 1.5 ms. From the excitation spect’rum of Fig. 1, we obtain an 81 - T1 splitting of 2100 cm-‘. We have investigated the effect of irradiation time on the phosphorescence of lop3 M oxalic acid in EPA. The sample was subjected to the following treatment: (i) It was irradiated for 45 minutes at 77°K using the 275 output of a 150-W xenon arc lamp. (ii) It was irradiated at 77°K with the full output of a G. E. AH6 mercury arc lamp for one hour. (iii) Same as step (ii) but sample was allowed to warm to 300°K during irradiation. Total irradiation time in this step was 85 min. (iv) Same as step (ii) but sample was maintained at 300°K. Total irradiation time in this step was 30 min. TABLE
II
VIBRATIONAL STRUCTURE IN THE PHOSPHORESCENCE OF DIEJETHYLOXALATE IN 3-METHYLPENTANE
AND AS A SINGLE CRYSTAL AT 77°K
3-Methylpentane (extensive mycrocrystalline content) _ .
xm
8(cm-r)
3265 3297 3345 3490 3455 3498
30630 30330 29895 29410 28940 28590
3555 3613 3668 3729
28130 27680 27260 26820
3785
26420 Of’ i;r = 1720 cm-r ijl = 430 cm-r
Single Crystal
Analysis”
x(A)
t (cm-r)
01’ 0,’ 0,’ 01’ Ot’ 0,’
-
0” 0” Y2 P1 Y1 YI
3260 3309 3368 3417 3469 3517
30675 30229 29690 29265 28900 28430
Ot’ 0,’ 0,’ Of’ 01’ 0,’
-
0” 0” Y2 Ya Y1 Y,
0,’ 0,’ 04’ 0,’
-
(Y2 + Y1) (0 + Yl) 2vr 2vr
3577 3640 3692 3743
27960 27470 27090 26726
Of’ 01’ 01’ 0,’
-
(Y2 -t Y1) (0 -t YI) 2vr 2v,
(Y2 + 2ur) :r = 1770 cm-r 12 = 500 cm-r F3 = 955 cm+
F3 = 915 cm-r a The subscripts phorescence.
Analysisa
t and f indicate,
respectively,
“true”
and “false”
origins for the phos-
300
MARIA AND McGLYNN
The phosphorescence was measured at 77°K at the end of each step and periodically during the course of the first step. The phosphorescence spectrum remained unchanged in all instances. We conclude, therefore, that t’he luminescence is intrinsic to oxalic acid. 2. Dhneth~yloxalate Phosphorescence and phosphorescence excitation spectra of dimethyloxalate in EPA are shown in Fig. 2 along with the phosphorescence from a frozen (i.e., microcry&alline) solution in 3-methylpentane and from a single crystal grown by evaporation of an n-hexane solution. Band positions and vibrational analyses are given in Tables I and II. For a dilute0 solut8ion in EPA, the highest-energy maximum in phosphorescence at 3250 A coincides within experimental error (for measurement zf the excitation spectrum) with the lowest-energy exckation maximum at 3230 A. It is concluded that the band at 3250 8 represents the true 0’ -+ 0” origin of phosphorescence of dimethyloxalate. Of course, in view of the conclusions of paper I concerning the existence of nonplanar molecules in solution, this origin must be composite. For all other experimental conditions represented in Fig. 2, most of the phosphorescence originates from defect centers which are located -300 cm-’ below the t’rue origin in the 3-methylpentane microcrystalline system and -455 cm-’ below the true origin in the single crystal. It is possible, in all instances, to analyze the phosphorescence in terms of progressions in a frequency whose average value is 1740 cm-‘. This is in good agreement with the frequency of the > C = 0 totally symmetric stretching mode (6) whose value is 1757 cm-’ in the crystal and 1764 cm-’ in the liquid in the SO state. An average X1 - T1 interval of 2275 cm-’ is obtained from absorption (see paper I), excitation, and emission spectra. The polarization of dimethyloxalate phosphorescence in glassy solution has been measured relative to an excitation at 280 nm. The degree of polarization at 77°K is -0.16 in mixed alcohol (methyl-, isopropyl-, ethyl-) solvent (concentration = 7 = low3 M) and is -0.14 in EPA4 (concentration = 2 X 10e3 M). The lifetime of dimethyloxalate phosphorescence in these same solvents is -1 ms and is 1.4 ms in a crystalline sample at 77°K. S. Covalent Oxalate Salts The phosphorescence of microcrystalline powder samples of cadmium and zinc oxalates is shown in Fig. 3. The principal maxima are listed in Table III. The phosphorescence spectrum of cadmium oxalate consists mainly of two progressions in a frequency of ii1 = 1460 cm-‘. The first members of the two progressions are at 302720cm-’ and 31900 cm-‘, respectively. However, the band at 32720 cm-’ (3056 A) is probably not the true origin of the molecular phosphorescence since there is a suggestion of a further incompletely resolved band at 3030 8. The separation between corresponding members of the A and B progrrs-
355
Wavelength
(nm) I--
B
.-,” z a z
-_
A. 335
355
Wavelength
210
375
(n ml
250
290
330
Wavelength FIG. 2. Phosphorescence
and phosphorescence
77°K. A. (1) Phosphorescence
B. C.
excitation
spectra
of dimethyloxalate
at
of a dilute solution in EPA for LX = 250 nm. (2) Phosphorescence of a concentrated solution in EPA for LX = 265 nm. (3) Phosphorescence of a saturated solution (C N 1OW M) in 3-methylpentane for &,, = 265 nm. Phosphorescence of a crystal grown by slow evaporation of a solution in ?A-hexane for LX = 305 nm. Excitation spectrum obtained from solution A(2) for X,, = 355 nm. 301
302
MARIA AND McGLYNN
if5gTc2 300
340
360
Wavelength
380
400
420
(nm)
FIG. 3. Phosphorescence of cadmium and zinc oxalate powders. (1) Cadmium oxalate for X,= = 250 nm (2) Zinc oxalate for X,, = 265 nm TABLE III VIBRATIONAL STRUCTURE IN THE PHOSPHORESCENCE OF CADMIUM AND ZINC OXALATEPOWDERSAT 77°K
3056 3135
3197
3842
Analysisa
3052
32770
A
~1 ~1
3196
31290
A -
v,
2v, 2v, 3v, 3~1
3352
29830
A -
2v,
3525
28370
A - 3v,
Analysisa
32720
A B A B A B A B A B
31900 31280
3290
3355 3460 3530 3640 3715
; (cm+)
C(cm-‘)
29800 28900 28330 27470 26920 26030 ?I = 1460cm-l
-
4~1 4v, il = 1470cm-l
n The symbols A and B identify assumed origin bands which we think refer to emission from different defect oxalatic centers.
sions is roughly constant at -870 cm-‘. This interval may be associated with a totally symmetric molecular vibrational mode, v (C-O) + 6 (0-C=O), which has a frequency in the range 890-900 cm-’ in 1: 1 oxalato complexes (6 >. Alternatively, the interval may represent the energy difference of two dominant types of defect and it is this interpretation which is preferred here. The phosphorescence spectrum of the zinc salt is not as well resolved as that of c,admjum; only the more prominent maxima are listed in Table III. They form a progression in a frequency of 1470 cm-’ which compares well with the frequency of 1433 cm-’ observed in the ir spectrum of &[ZnOXz] *2HzO and which is assigned (6) to the totally symmekie mode, v (C-O) + v (C-C). It is obvious, in
OXALIC
ACID
AND
DIMETHYLOXALATE
303
this case, that there are weaker unresolved emission bands at higher energies than the first prominent band at 32770 cm-’ (3052 8). Thus, the 32770 cm-’ band represents the 0’ * 0” band of a dominant defect oxalate site. The phosphorescence decay times of cadmium and zinc oxalates were observed oscilloscopically. The lifetime, in both instances, is somewhat less than one millisecond. We have not attempted to measure the phosphorescence spectra of other covalent oxalate salts; we assume they will be similar to those of cadmium and zinc. On the other hand, simple ionic oxalate salts such as sodium and potassium have an entirely different behavior; work on the ionic salts is now in progress. IV. DISCUSSION
The discussion assumes that oxalate molecules in true solution sample a large variety of dihedral angles whereas those in single or microcrystalline regions remain trans-planar. Much of the true-solution phosphorescence is assumed to originate in nonplanar excited state molecules; almost all of the crystalline emission is of defect nature and originates from oxalate entities which, in spite of their defect character, are assumed to retain planarity. We will first attempt derivation of conclusions which we think independent of planarity, nonplanarity or defect nature of the emitting species. Thereafter, we will discuss those facets of the spectroscopy which we deem to require consideration of the molecule geometry. 1. General
The observed luminescence is intrinsic to covalent oxalate molecules. The intrinsic nature is evidenced by a number of properties : observation of a more or less identical emission from un-ionized acid, ester and covalent salts; the null irradiation time) on either the intensity of lueffect of photochemistry (i.e., minescence or its spectrum; the coincidence, within experimental error, of the 0’ -+ 0” emission band of dimethyloxalate with the corresponding 0’ +- 0” band of the T1 +- SO process in both absorption and excitation spectra; and the more by or less direct replication of the SI c S0 absorption spectrum of dimethyloxalate the phosphorescence excitation spectrum. The observed luminescence is a phosphorescence of TI -+ So type. The long emissive lifetimes (~1 ms) and the intrinsic molecular nature of the process dictate this identification. The observed luminescence is of ?I’,+ --+ ?‘I type. A number of observations validate this assignment and enable extension to a more specific 13A, -+ I’A, (C,,) designation for the truns-planar molecule : First, the observed phosphorescence in solution is polarized in a direction which polarization direction. The obis closely perpendicular to the XI +- So absorption served relative degree of polarization is ~0.15, or -50 % of the expected theoreti-
MARIA AND McGLYNN
304
cal limit of -0.33 for nrecise perpendicularity of the 81 t So and 7’1~ So traneition oscillators. Since the corresponding limit for exact parallelism of the same two oscillators is +0.50, we may conclude rough perpendicularit,y, as specified. Sincae spin-orbit coupling selection rules usually dictate perpendicularity of transition moments ‘pi -‘I1 and 3Pa - ‘pl where ‘I’i and 31’i derive from identical electronconfigurational excitations, we can conclude that the observed S1 and T1 states may be of similar configuration nature. Since the & state of the planar molecules is identified in paper I as kn,* (l’A,) we might accept a similar designat,ion for the observed T1 state. It is worth emphasizing here t.hat the polarization measurements refer to glassy solutions and, thus, to nonplanar entities. The relative polarization ratio for the 31’n,* + ‘p& nT* c ‘I’1 events in such a molecule has been computed (see TableVI and the discussion relating to it) as - 0,22, in good agreement with experiment. Polarization work was not extended to single crystalline entities because of the defect nature of the phosphorescence of such systems. Second, the observed emission lifetime of the crystalline systems is consistent with a 31’n,t + ‘I1 process and too short by a factor of 10’ to 10’ for a 31’,,* + ‘I’1 decay event (see, for example, Table IV). Third, the observed energy interval E (l’A,) - E (T1) = 2275 cm-’ is consistent with expectation for 7’1 = 13A,. Indeed, we evaluate the exchange energy energy as 2K,,. N 2[z (0.6)’ (0.5)‘K,,,, + 2 (0.2)2(0.54)2k,,,J N 4200 cm-‘; where kpZp, and kpyp, are atomic exchange integrals on oxygen; where 0.6 and 0.2 are the coefficients of 2,= - and 2,1 - AO’s of oxygen in the W-MO whereas 0.54 is the coefficient of the 2,, - A0 of oxygen in the ?r* - RIO; where the extra fact’or of 2 is caused by the presence of two carbonyl oxygens in the molecule; and where the MO’s cited are those for a tr@wplanar oxalate framework. The computed value of 2K,,* is not significantly dependent on the presence or absence of planarity because of the localization of dominant integrals on the > C=O group. On the other hand, the supposition that T1 = Z3A, provides an observed XZ - 7’1split E (2lA,) - E (Z3A.) = 12155 cm-’ whereas the computcad exchange energy in this instance is ~3200 cm-‘. Thus, we may disregard the g3A,, assignment where the 2A,, state derives from a ?r* t G rxcited configurat8ion, TABLE COMPUTED
OSCILLATOR
STRENGTHS
IV
.IND LIFETIMES OF SOME TRIPLET H 11.4,
PROCESSES IN trans PL.INAR (OXALIC ACID (C2h) -
Configurationn1r* n** u** 7rTr* UT* e (r as used here denotes
Symmetry 13Az1 13B, 23A, 13B, 23A, an in-plane
Oscillator strength 1.1 x 3.3 x 5.7 x 6.3 x 9.3 x
10-b 10-14 10-4 10-g 10-s
orbital which is more delocalized
Lifetime (s) 7.7 9.2 3.0 2.6 1.7
x x x x x
10-A 101 10-c 10-z 10-s
than the n orbit ala.
OXALIC
ACID AND DIMETHYLOXALATE
305
u being an in-plane orbital which is more delocalized than the 7~ orbitals. A Ti = 13B, assignment may not be eliminated on the basis of observed S-T intervals; it is, however, eliminated by the polarization result’s and by the observed decay lifetimes. The only other states of reasonably low energy which might conceivably be considered as assignment possibilities for the TI state are of even (y) orbital parity in Dhe planar molecule. The chances of observing an emission forbidden by orbital and spin selection rules and with decay lifetime of -1 ms seem too remote to merit consideration. Since we suppose that the emission of the single crystal systems is attributable to planar oxalatic entities, we must consider the assignment’ 13A, (3rnr*) -+ l’A, (C,,) to be fully substantiated. 2. Phosphorescence of a Planar
(Czh) molecule
The T, state of the planar molecule (i.e., the presumed emitting species in the crystal) has been assigned above as 13A, (3rnr*, CS). As further validation of this assignment, we now present the results of spin-orbit coupling calculations for all reasonable triplet designations in Table IV. The MO wavefunctions used were those generated by the Mulliken-WolfsbergHelmholz procedure described in paper I; the spin-orbit perturbational computations followed along previous lines (7) and refer, in the present instance, to a trans planar oxalic acid molecule. The agreement between the calculated emissive lifetime (0.8 X lop3 s) of a 13A, state and the observed value for the crystal, 1.4 X lop3 s, is excellent. To some extent, however, this coincidence must be considered fortuitous: The MWH calculations underestimate all transition energies, and this makes the energy denominators in the perturbation expressions too small and the spin-orbit effects too severe; on the other hand, vibronic coupling effects are not included in Table IV and this neglect renders the effective perturbation theory numerators smaller than they otherwise should be. Both effects tend to cancel and, as a consequence, the results of Table IV attain some validity. As seen, the only other reasonable assignment is 13B,(r,,.) for which the observed lifet’ime is shorter than that calculated by a factor of ~18 and which is invalidated anyway by the polarization results for the glassy solutions. A dissection of the computational spin-orbital coupling results for the 13A, + llA, transition is given in Table V. This includes the effects of triplet mixing into the llA, ground state as well as singlet mixing into the 13A, state; it is predicted that both types of mixing are roughly comparable with regard to intensity induction in the transition under consideration. It is also predicted that the polarization of phosphorescence should not’ be entirely perpendicular to the transition moment direction of the l’A, +- llA, excitation. However, the contribution of z-polarization (i.e., 1) intensity, while real, is marginal at only 0.1 % of the in-plane polarization intensity. Another source of out-of-plane intensity in t*he 13A, +- l’A, transition might
306
MARIA
AND
McGLYNN
TABLE SPIN ORBIT COUPLING IN 13A, +
Spin orbit coupling Matrix Element (cm-l)
Contributing state Orbital symmetry ____
?T*
I
llA,
e
H’(x)
3.67 3.77 6.25 6.33 2.80 4.70 5.75
-10.3 0 -21.8 48.7 37.6 17.0
0
_I
H'(Y) 0 -49.3 0 -23.7 33.9 -44.3 36.4
V TR.~NSITION OF OXALIC ACID (C2,+)
T
Conferred transition moment (41
.Y
H'(z) ~__ 57.5 0 8.6 0 0 0 0
~
Y
~2 _i___-
0 -2.2
0 x
10-z
-4.4
X 10-a
,7.0 X 10-t 0
X x x X
lo+ 10-a 10-4 lo+
0 1.5 -2.6 -9.0 -2.6
x 10-d x 10-a x 10-s X lO+
3.0 x 10-j 0 0 0 0
0 -9.8 -4.25 -1.0 -1.2
_~_
* The energies are those obtained from the MWH calculations of paper I. Since electron correlation is not included, the singlet and triplet states arising from a given configuration are degenerate. Axes are defined in paper I. derive from spin-orbit mixing of 13A, and l’A, states. This mixing is not considered in Table V because of the inability of the computer program to handle the mixing of states which, because of MWH inadequacies, are erroneously supposed to be degenerate. However, it is not too difficult to show that the matrix element (13A,I H’ IllA,) w h ere H’ is the spin-orbit coupling Hamiltonian, contains no one-center atomic spin-orbital integrals and that, in fact, its magnitude is dominated by three-center atomic terms. Thus, the element (13A,( H’ 11'S,) iu small, as is also the intensity llA, t I’A, which is being “stolen” in this perturbation process. As a result, and despite the smallness (2275 cm-‘) of the perturbation denominator, it is concluded that such mixing cannot contribute more out-ofplane intensity than ~5 % of the total in-plane contribution displayed in Table V. Thus, even in a point group where the triplet spin states contain a component which transforms as the totally symmetric representation, we find that the mixing of singlet and triplet states arising from the same orbital configuration is forbidden in a first approximation-even though group-theoretically allowed. In the present instance, this result implies an in-plane nature of the !!‘, + X0 event. In general, this same result implies a ready generalization of the “no-mixing” maxim which obtains for point groups for which none of Rz,y,a transforms as r1 .
3. Phosphorescence
of a Twisted
(C,) Molecule
The discussion of the previous section assumed molecule planarity. In the case of solution species, it is probable that some dimethyloxalate molecules remain planar (see paper I) whereas the great majority exhibits dihedral angles different from zero. In particular, lifetime data and relative polarization values are thought to be available for such an ensemble. We now select one molecule
OXALIC
ACID
AND
DIMETHYLOYALATE
307
from such an ensemble-onr for which the two carboxyl planes intersect at an angle of 45”-and cquirc whether this “average molrrule” provides satisfactory explanation of the experiment’al observations. Both MWH and CNDO/s molecular orbital calculations, as discussed in the paper I, have shown that the first excited singlet’ state of the twisted molecule (i.e., the 2lA state) correlates with the l’A, (lI’?,+) st’ate of the planar molecule. It was also found that both this state and the 2’B state of the twisted molecule (see Table III of paper I), which correlates wit’h the llB, (I’,,*) state in CZ,,, are not mixed to any appreciable extent with any other state by configuration interact,ion. The spin-orbit coupling computation, therefore, was restricted to one configuration in each of these states and wavefunctions used were those obtained from the T\IWH processing. The results are given in Table VI. It is now. seen that the Z3B --f llA transition retains a polarization which is parallel to the 2’d +- llA transition-moment. The calculated polarization ratio is, in fact,, approximately +0.30. Therefore, because of the observed ncgat’ive polarization rat’io, the Z3B state cannot be the moment is now phosphorescing st’ate. By cont’rast, the 23L4 -+ llA transition polarized in a manner which is only partially perpendicular to the 2lA +- 1’A absorption oscillator. Indeed, in this inst’ance, the calculated polarization ratio -0.22. This number is rrmarkably close to the observed is approximately polarization ratio of -0.15. Thus, the assumption of twisting provides wady interpretation of lifetime and relative polarization da,ta for dissolved dimethyloxalate. It also, of course, rat’ionalizes the lack of resolution evident in the solution and gaseous absorption spectra as well as differences between crystal absorption spectra on the one hand and the gas and solution phase spectra on the other (see paper I),
4. Vibronic Considerations Vibronic perturbations can cause 3rn,* - “I’=** mixing and thus induce perpendicular intensity int’o the nominal 3rnr* + ‘rl event’ of the planar molecule Since the observed lifetime of the solution phase molecules are readily accounted for on the basis of a planar emitting species (we Table IV), it follows that the TABLE
VI
CALCULATED OSCILL.YTORSTRENGTHS FOR TR.INSITIONSTO SOME STATES OF OXALIC ACID WITH .\I)IHEDR.\L ANGLE OF 45" (C, POINT GROUP) Oscillator strength f .Y
State 21/l 238 2lB 2”B
1.2 x 5.7 x 4.9 x 3.7 x
10-10 10-6 10-Z 10-g
1.9 3.2 1.9 1.4
x X x X
10-4 1OP 10-l 10-C
Lifetime (secj
.If 1.1 x 5.5 x 3.2 X 8.3 x
10-z 10-7 1OP 10-G
3.9 5.5 4.9 1.4
x x x x
10-e 10-4 10-g 10-d
MARIA
30s
AND McC;I,YXK
polarization ratio is the only observable IThich--in this paper, at least-forces us to w~wludr nonplanarity of thcx emitting solution wt,ity. We now cnquirr whcthcr vibronic perturbations in a planar entity can lead to a rationalization of the relative polarization observed in the glassy solution phase. We will conclude that it cannot and, hence, further justify our assertion of nonplanarity in such phases. Alternatively, we also seek explanation of a number of observations pertaining to the microcrystalline and single-crystalline systems in which me have posited, per crystallographic dict’ates, molecule planarity. These observations arc: 110 ncwssity to invoke vibronic coupling insofar as vibrational analysis of the phosphorescence is concerned and a striking vibronic effect which is displayed in the phosphorescence excitation spectrum. We believe that the vibronic coupling considerations indulged here provide ready rationalization of both of t’hrw observations. (i) Vib~onic cou@i~~g in the sillylet l?za?zif&l. A nontotally symmetric b, vibrational mode may vibronically mix the I’A, and l’B, states and thus introduce vibronic intensity into the l’A, +- I’A, absorption process. Such vibronic mixing is known to occur in t’he isoelectronic molecules oxalyl chloridr and glyoxal. It was shown in paper I that this same vibronic mixing also occurs in dimethyloxalate and that the vibronic intensity in the lowest energy & +-- & absorption band may equal as much as 45 SC,of t#he t,ot,al observed int,rnsit,y of this nominal Yn,* +- ‘l?l transition. If we describe the observed ‘F + stat’e as a (llA,) + b (I’&) and the observed 31’n,: state as a’(13AU) + b’(13B,), then t,he intersystem crossing event can bc decomposed into the four sub-events depicted in Pig. 4. The spin-orbit matrix elcmcnts (1’B,I H’ / 13B,) and (l’i1,l H’ 113A,) cont,ain no one-center coupling t,erms and, in fact, as previously specified, obtain most of their magnitudes from three-center terms; these elements, therefore, are very small and are assumed to bc safely neglectable. F’urt’hermorr>, t,he cAmissivc> 13R,, + 1’,4, channcbl in the NOMINAL STATES I ‘A,
REAL atI
‘A,,
STATES
+
bdB,)
=
S,
+
b’(13EI) /”
=
T I’
1x1 o’(13A
x”
i 1
FIG. 4. A decomposition of the S1 -U-J T1 intersystem crossing event into four subevents. The decay event T1 -+ SOis also schematized, relative probabilities and polarizations of t,he emissive components being indicated.
OXALIC
ACID
AND
DIMETHYLOXALATE
309
!!‘I-+ SOphosphorescence act is relatively improbable compared to the 13A, + A, channel; indeed, when proper weighting is introduced via the coefficients a’ and b’ of Fig. 4, we find from Table IV that the 13A, + llA, channel is ~150 times more probable. Thus, if spin-polarization exists in the T1 state, we conclude that the emission channel being monitored at short times is the 13A, -+ llA, route and, therefore, that the ASIw TI intersystem crossing channel dominantly effective in mediating this emissive route in b (llB,) ~3 a’ (13A,). Consequently, we expect the phosphorescence excitation spectrum to be dominated by a vibronic component in the b, mode-exactly as is observed (see Fig. 2C where the origin band at 3040 A appears only as a shoulder on the much stronger vibronic band at 3000 8). The supposition of spin polarization is not one for which we have adduced any experimental proof here. It is perhaps as well then to emphasize that the only consideration of relevance to our discussion is that the nonemissive relaxation time of the perpendicularly polarized spin component maintains the sequence: ~(b’~& ?n) ‘Ag) < 7 (spin relaxation). That spin polarization obtains in this system is not unexpected in view of the rather short phosphorescence lifetimes under consideration. (ii) Vibronic coupling i,n the triplet manifold. Other things being equal, the vibronic mixing of the 13A, and 13B, states will be considerably larger than that between the singlet states; indeed, based upon energy denominator considerations drawn from the energy scheme of Fig. 5, the percentage of 13BU in 13A, should be ~10 times larger than the percentage of llB, in llA,. Thus, the 8070-
I ‘8”-
6050-
I
40-
2 A,-
30-
I ‘Au-
..-.,38u -13A,
20 IO
1
0 L 1’AsDIMETHYLOXALATE FIG. 5. Electronic energy-level diagram for transplanar dimethyloxalate. The position of the llB, state is approximate; that of the 13B, state is computational and follows from an estimated exchange energy of -3.5 eV.
310
MARIA AND M&LYNN
can aquire transition probability which is polarized 13A, -+ llA, transition perpendicular to the molecule plane (i.e., parallel to the l’A,, + l’A, transition) by three rout,es. These routes have been discussed previously; they are, in summary : (a) Spin-orbit mixing of the 13A, state with ?A,. Using experimental energies (Fig. 5) and matrix elements given in Table V, we calculate the maximum transition moment’ length that may be “stolen” as M, = 6 X lo-” 8. (b) Spin-orbit mixing of the 13A, state with the l’A, state (i.e., self-mixing of configurations). This process, as discussed in the text, leads to zero onpcrnt.cr contributions and is quite negligible. (c) The 13B, component which is vibronically introduced into the l”A,, state can mix via spin-orbit coupling with the (1 and 2)‘A, states. If the luminescing state is pure 13B,, we calculate a stolen transition moment length M, = s x 1o-4 8. The states that are most important in conferring intensity on the 13A, component are I’& and 13B,, where the latter state mixes via spin orbit routes with the ground state. Again using experimental energies (Fig. 5) and assuming h’ (13B,) E 30 kK, we calculate M, = 3.9 X lop3 s and M, = 3.4 X lo-” A. All stolen transition probabilities must be multiplied by a factor corresponding to the fraction of 13B, or 13A, in the nominal 31’+ state. Thus if we assume that the 13B, admixture into the nominal 13A, state is as high as 20%, we obtain a transition probability. value of -150 for the ratio of the in-plane to out-of-plane Thus, we do not expect to be able to observe the effect of vibronic mixing in the triplet manifold in either the emission from or absorption to the first triplet, state. This discussion-insofar as emission is concerned-also assumes retention of spin polarization in the T1 state. This is not unlikely because of the short lifetimch of this state. Our results do not lead to any conclusions concerning the presenccb or absence of spin polarization in glassy solution media (in which our observations arc’ fully interpretable by an assumed nonplanarity of the molecule). Furthermore, if our assertion that’ the crystalline luminescence is mostly of defect origins is correct, little if anything can be learned from polarization studies on thci luminescence of such crystalline systems. V. CONCLUSIONS
The conclusions
of this work are as follows:
(i) Covalent oxalates emit a phosphorescence of 3rn,* + ‘I’,+ type. Specific designations are 13A, --$ l’A, in truns-planar oxalatrs and 23A --$ llA in nonplannr twisted oxalates. (ii) The phosphorescence of glassy solutions initiates in a nonplanar configuration; that of the crystals initiates in defects which are assumed to (‘orltain planar oxalate molecules.
OXALIC
ACID AND DIMETHYLOXALATE
311
(iii) The phosphorescence excitation spectrum of the single-crystal exhibits a striking vibronic effect. It appears that b, modes coupled to the l’A, st’ates provide the primary excitation preparation for efficient intersystem crossing. (iv) The observation of (iii) above and the observed phosphorescence lifetime of the crystal are readily interpreted on the basis of vibronic-spin-orbit computations. (v) The observed phosphorescence lifetime and relative polarizations of dissolved oxalate entities are readily interpretable 011 the basis of a spin-orbit model for twisted oxalate geometries. It is shown, conversely, that a vibronicspin-orbit model applied to a planar oxalate geometry can not explain the polarization results. RECEIVED: September
27, 1971 REFERENCES
1. S. A. BRUNS, Nauchn. Soobsch. Ilast. Gorrr. Dela, Akad. Nauk SSSR, 16, 114 (1962). 3. V. V. TRUSOV, Nauch?A. Zap., Odessk. Gos. Ped. Inst., Kafedry Mat., Fiz. i Estestvozn 26, 86 (1961). 3. A. P. VISHNEVSKII, Doklady Akad. Nauk. SSSR 63, 503 (1948). 4. H. MURATA AND K. KAWAI, J. Chem. Phys. 26, 589 (1956). 5. J. K. WILMSHURST AND J. F. HORWOOD, J. Mol. Spectrosc. 21, 48 (1966). 6. J. FUJITA, A. E. MARTELL, AND K. NBKAMOTO, J. Chem. Phys. 36,324 (1962). 7. D. G. CARROLL, L. G. VANQUICKENBORNE, SND S. P. MCGLYNN, J. Chem. Phys. 46,2777 (1966).
OF
MOLECULAR
SPECTROSCOPY
43,
296-311 (1972)
Electronic States of Oxalic Acid and Dimethyloxaiate. Luminescence Studies’ H. J. MARIA AND S. P. MCGLPNN Coates Chemical Laboratories,
Louisiana State University,
Baton Rouge,
Louisiana
70805
A short-lived phosphorescence is observed from oxalic acid, dimethyloxalate ester, and covalent oxalate salts such as those of zinc and cadmium. The phosphorescence is attributed to the transition 13A, -+ llA,(?r* + n) of the Cqh trans-planar oxalate moiety or to the transition 23A + 1lA (C,) with which it correlates when the molecule is not planar. The phosphorescence of glassy solutions refers to emissive relaxation of an excited oxalate entity which is believed to be nonplanar. The phosphorescence of crystalline oxalate entities originates largely from defect centers which might retain planar oxalate geometries. The intersystem (Sly3 Tr) crossing event in crystalline dimethyloxalate exhibits a striking vibronic effect which is based on coupling of b, modes for which i; = 400 cm-l. I. INTRODUCTION
There are some literature allusions (1-S) to a luminescence of oxalic acid. Unfortunately, these are only available to us in abstract form. In one case (1): an emission-referred to as a “fluorescence”-was monitored in the region 400606 nm; in another (8), a very weak emission-referred to as a “phosphorescence” -was observed upon irradiating a sample for 4 set, closing an excitation shutter, and opening a second shutter to record luminescence; and, finally, a “sky-blue phosphorescence” of mean lifetime 7 ‘v 1.7 set was observed (3) at liquid oxygen temperature. Such reports do not provide a satisfactory overview of the luminescence characteristics of oxalic acid. We have observed a short-lived phosphorescence from oxalic acid, dimethyloxalate, and zinc and cadmium oxalates. This report is concerned with the nature of this luminescence process and with the strong vibronic coupling effects which are evident in its excitation spectrum in crystalline dimethyloxalate. II. EXPERIMENTAL
PROCEDURE
Materials Oxalic acid and dimethyloxalate were purified as described in paper I. Zinc and cadmium oxalates were prepared by metathesis of the appropriate Reagent Grade nitrate salt with sodium oxalate. 1 This work was supported by research contract between the United States Atomic Energy Commission, Biology Branch, and the Louisiana State University. 2 Paper I of this series of two papers is entitled “I. Absorption Studies” and is referenced as H. J. Maria and S. P. McGlynn, J. Mol. Spectrosc., 43, 177 (1972). 296 Copyright
9
1972 by Academic
Press, Inc.
OXALIC
ACID
AND
297
DIMETHYLOXALATE
All solvents were fluorimetric grade (Hartman-Leddon used as received.
Company)
and were
Excitation and emission spectra were recorded at 77°K on an instrument consisting of a Cary 15 excitation monochromator and a half-meter Jarrell-Ash scanning emission monochromator. The signal was chopped at 660 cps and detected by an EMI 9558&B phototube coupled with a Princeton Applied Research 220 lock-in amplifier. Excitation was of front-surface nature. The polarization of phosphorescence was measured on an Aminco-Kiers Spectrophosphorimeter. Decay curves were photographed on a Tektronix oscilloscope in conjunction with the Aminco-Kiers Spectrophosphorimeter. All samples were degassed by repeating three times a cycle of freezing (77”K), pumping (~1 ,u> and thawing (300°K). III.
RESULTS
1. Oxalic Acid
The phosphorescence emission and phosphorescence excitation spectra of a low3 M solution of oxalic acid in EPA are shown in Fig. 1. The excitation spectrum contains a quite weak band at 3270 A-; this band is well separated from the main excitation system and is identified here as the origin of the T1 + So absorption process. The 0’ t 0” band is the only vibrational structure of TI c- So which we expect to resolve because of the overlap of the 1’ t 0” excitation and the tail of the & t SOabsorption region. The band system whose first member occurs at
A
B
u,i;,Iill r ._ if f
250
290
330
320
Wavelength
360
340
280
400
(nm)
FIG. 1. Phosphorescence and phosphorescence excitation Concentration: lO+ M in EPA. A. Excitation spectrum for &,, = 360 nm B. Phosphorescence spectrum for hex = 275 nm
spectra
of oxalic
acid at 77°K.
MARIA AND McGLYNN
298
3060 8 corresponds to the lowest energy (81 c So) spin-allowed absorption illustrated in Fig. 1 of paper I. The phosphorescence spectrum of Fig. 1 consists of four bands with an average separation of -1560 cm-‘. The first prominent maximum occurs at 3400 i and lies at lower ecergy than does the supposed T1 c So(0'c ON) excitation maximum at 3270 A. However, close examination of the first phosphorescence band shows it to be highly asymmetrical; indeed, one can surmise the presence of perhaps two shoulders on its blue side. These shoulders are more evident in the spectrum of a lop4 molar solution; however, in this instance, the signal-to-noise ratio is generally unfavorable to the recording of a good overall spectrum. Similar remarks apply also to the second broad band in the emission spectrum of Fig. 1. We attribute the blue degradation effects observed within each band to the existence in solution of oxalic acid molecules in more than one geometrical configuration. The different possible geometrical configurations of the ground statelentity are discriminated by the angle of t.wist of the two carboxyl groups about the C-C bond (as inferred in paper I) ; the excited state geometries may be discriminated by the possible changes in this dihedral angle which are caused by excitation of the ground state entity. TABLE I VIBRATIONALSTRUCTUREIN THE PHOSPHORESENCE OF DIYETHYLOXALATE IN EPAa AT 77°K Concentratedsolution (some microcrystallinecontent)
Dilute solution (no microcrystallinecontent) x(A)
t (cm-l)
3248 3438 3653 3880
30790 29090 27380 25770
F1 = 1700 cm-l
Analysis
x(A)
F(cm-l)
0’ 0’ 0’ 0’
3248 3290 3339 3392 3492 3547 3617 3721 3775 3857 3992 4290
30790 Ot’ 30390 Of’ 29950 01’ 29480 0,’ 28640 0,’ 28190 Of’ 27650 Of’ 26870 0,’ 26490 Of’ 25930 Of’ 25050 Of’ 23310 0,’ & = 1780 cm+ li, = 420 cm-l 53 = 950 cm-l
- 0” - “1 - 2v, - 3v,
Analysisb 0” 0” (unresolved) Y2 YZ YI (v* + Y1) (1.3+ Y1) 2v, (Y2+ 2Vl) (va + 2Vl) 3Vl 4v*
* EPA is a 5:5:2 volume mixture of ethylether, isopentane and ethanol. b The subscripts t and f indicate, respectively, “true” and “false” origins for the phosphorescence.
OXALIC
ACID
AND
DIMETHYLOXALATE
299
The lack of resolution evident within each band is responsible for the smallness of the observed interval (~1560 cm-‘). This frequency interval is considerably smaller than that of the totally symmetric > C = 0 stretching mode (4) (1762 cm-’ in the So state); nonetheless, we identify the observed frequency with this vibrational mode. The assertion that the smallness of the measured interval is associable wit.h experimental inaccuracies is borne out in Tables I & II. The lifetime of phosphorescence of 1O-3 M oxalic acid in ethanol is 1.5 ms. From the excitation spect’rum of Fig. 1, we obtain an 81 - T1 splitting of 2100 cm-‘. We have investigated the effect of irradiation time on the phosphorescence of lop3 M oxalic acid in EPA. The sample was subjected to the following treatment: (i) It was irradiated for 45 minutes at 77°K using the 275 output of a 150-W xenon arc lamp. (ii) It was irradiated at 77°K with the full output of a G. E. AH6 mercury arc lamp for one hour. (iii) Same as step (ii) but sample was allowed to warm to 300°K during irradiation. Total irradiation time in this step was 85 min. (iv) Same as step (ii) but sample was maintained at 300°K. Total irradiation time in this step was 30 min. TABLE
II
VIBRATIONAL STRUCTURE IN THE PHOSPHORESCENCE OF DIEJETHYLOXALATE IN 3-METHYLPENTANE
AND AS A SINGLE CRYSTAL AT 77°K
3-Methylpentane (extensive mycrocrystalline content) _ .
xm
8(cm-r)
3265 3297 3345 3490 3455 3498
30630 30330 29895 29410 28940 28590
3555 3613 3668 3729
28130 27680 27260 26820
3785
26420 Of’ i;r = 1720 cm-r ijl = 430 cm-r
Single Crystal
Analysis”
x(A)
t (cm-r)
01’ 0,’ 0,’ 01’ Ot’ 0,’
-
0” 0” Y2 P1 Y1 YI
3260 3309 3368 3417 3469 3517
30675 30229 29690 29265 28900 28430
Ot’ 0,’ 0,’ Of’ 01’ 0,’
-
0” 0” Y2 Ya Y1 Y,
0,’ 0,’ 04’ 0,’
-
(Y2 + Y1) (0 + Yl) 2vr 2vr
3577 3640 3692 3743
27960 27470 27090 26726
Of’ 01’ 01’ 0,’
-
(Y2 -t Y1) (0 -t YI) 2vr 2v,
(Y2 + 2ur) :r = 1770 cm-r 12 = 500 cm-r F3 = 955 cm+
F3 = 915 cm-r a The subscripts phorescence.
Analysisa
t and f indicate,
respectively,
“true”
and “false”
origins for the phos-
300
MARIA AND McGLYNN
The phosphorescence was measured at 77°K at the end of each step and periodically during the course of the first step. The phosphorescence spectrum remained unchanged in all instances. We conclude, therefore, that t’he luminescence is intrinsic to oxalic acid. 2. Dhneth~yloxalate Phosphorescence and phosphorescence excitation spectra of dimethyloxalate in EPA are shown in Fig. 2 along with the phosphorescence from a frozen (i.e., microcry&alline) solution in 3-methylpentane and from a single crystal grown by evaporation of an n-hexane solution. Band positions and vibrational analyses are given in Tables I and II. For a dilute0 solut8ion in EPA, the highest-energy maximum in phosphorescence at 3250 A coincides within experimental error (for measurement zf the excitation spectrum) with the lowest-energy exckation maximum at 3230 A. It is concluded that the band at 3250 8 represents the true 0’ -+ 0” origin of phosphorescence of dimethyloxalate. Of course, in view of the conclusions of paper I concerning the existence of nonplanar molecules in solution, this origin must be composite. For all other experimental conditions represented in Fig. 2, most of the phosphorescence originates from defect centers which are located -300 cm-’ below the t’rue origin in the 3-methylpentane microcrystalline system and -455 cm-’ below the true origin in the single crystal. It is possible, in all instances, to analyze the phosphorescence in terms of progressions in a frequency whose average value is 1740 cm-‘. This is in good agreement with the frequency of the > C = 0 totally symmetric stretching mode (6) whose value is 1757 cm-’ in the crystal and 1764 cm-’ in the liquid in the SO state. An average X1 - T1 interval of 2275 cm-’ is obtained from absorption (see paper I), excitation, and emission spectra. The polarization of dimethyloxalate phosphorescence in glassy solution has been measured relative to an excitation at 280 nm. The degree of polarization at 77°K is -0.16 in mixed alcohol (methyl-, isopropyl-, ethyl-) solvent (concentration = 7 = low3 M) and is -0.14 in EPA4 (concentration = 2 X 10e3 M). The lifetime of dimethyloxalate phosphorescence in these same solvents is -1 ms and is 1.4 ms in a crystalline sample at 77°K. S. Covalent Oxalate Salts The phosphorescence of microcrystalline powder samples of cadmium and zinc oxalates is shown in Fig. 3. The principal maxima are listed in Table III. The phosphorescence spectrum of cadmium oxalate consists mainly of two progressions in a frequency of ii1 = 1460 cm-‘. The first members of the two progressions are at 302720cm-’ and 31900 cm-‘, respectively. However, the band at 32720 cm-’ (3056 A) is probably not the true origin of the molecular phosphorescence since there is a suggestion of a further incompletely resolved band at 3030 8. The separation between corresponding members of the A and B progrrs-
355
Wavelength
(nm) I--
B
.-,” z a z
-_
A. 335
355
Wavelength
210
375
(n ml
250
290
330
Wavelength FIG. 2. Phosphorescence
and phosphorescence
77°K. A. (1) Phosphorescence
B. C.
excitation
spectra
of dimethyloxalate
at
of a dilute solution in EPA for LX = 250 nm. (2) Phosphorescence of a concentrated solution in EPA for LX = 265 nm. (3) Phosphorescence of a saturated solution (C N 1OW M) in 3-methylpentane for &,, = 265 nm. Phosphorescence of a crystal grown by slow evaporation of a solution in ?A-hexane for LX = 305 nm. Excitation spectrum obtained from solution A(2) for X,, = 355 nm. 301
302
MARIA AND McGLYNN
if5gTc2 300
340
360
Wavelength
380
400
420
(nm)
FIG. 3. Phosphorescence of cadmium and zinc oxalate powders. (1) Cadmium oxalate for X,= = 250 nm (2) Zinc oxalate for X,, = 265 nm TABLE III VIBRATIONAL STRUCTURE IN THE PHOSPHORESCENCE OF CADMIUM AND ZINC OXALATEPOWDERSAT 77°K
3056 3135
3197
3842
Analysisa
3052
32770
A
~1 ~1
3196
31290
A -
v,
2v, 2v, 3v, 3~1
3352
29830
A -
2v,
3525
28370
A - 3v,
Analysisa
32720
A B A B A B A B A B
31900 31280
3290
3355 3460 3530 3640 3715
; (cm+)
C(cm-‘)
29800 28900 28330 27470 26920 26030 ?I = 1460cm-l
-
4~1 4v, il = 1470cm-l
n The symbols A and B identify assumed origin bands which we think refer to emission from different defect oxalatic centers.
sions is roughly constant at -870 cm-‘. This interval may be associated with a totally symmetric molecular vibrational mode, v (C-O) + 6 (0-C=O), which has a frequency in the range 890-900 cm-’ in 1: 1 oxalato complexes (6 >. Alternatively, the interval may represent the energy difference of two dominant types of defect and it is this interpretation which is preferred here. The phosphorescence spectrum of the zinc salt is not as well resolved as that of c,admjum; only the more prominent maxima are listed in Table III. They form a progression in a frequency of 1470 cm-’ which compares well with the frequency of 1433 cm-’ observed in the ir spectrum of &[ZnOXz] *2HzO and which is assigned (6) to the totally symmekie mode, v (C-O) + v (C-C). It is obvious, in
OXALIC
ACID
AND
DIMETHYLOXALATE
303
this case, that there are weaker unresolved emission bands at higher energies than the first prominent band at 32770 cm-’ (3052 8). Thus, the 32770 cm-’ band represents the 0’ * 0” band of a dominant defect oxalate site. The phosphorescence decay times of cadmium and zinc oxalates were observed oscilloscopically. The lifetime, in both instances, is somewhat less than one millisecond. We have not attempted to measure the phosphorescence spectra of other covalent oxalate salts; we assume they will be similar to those of cadmium and zinc. On the other hand, simple ionic oxalate salts such as sodium and potassium have an entirely different behavior; work on the ionic salts is now in progress. IV. DISCUSSION
The discussion assumes that oxalate molecules in true solution sample a large variety of dihedral angles whereas those in single or microcrystalline regions remain trans-planar. Much of the true-solution phosphorescence is assumed to originate in nonplanar excited state molecules; almost all of the crystalline emission is of defect nature and originates from oxalate entities which, in spite of their defect character, are assumed to retain planarity. We will first attempt derivation of conclusions which we think independent of planarity, nonplanarity or defect nature of the emitting species. Thereafter, we will discuss those facets of the spectroscopy which we deem to require consideration of the molecule geometry. 1. General
The observed luminescence is intrinsic to covalent oxalate molecules. The intrinsic nature is evidenced by a number of properties : observation of a more or less identical emission from un-ionized acid, ester and covalent salts; the null irradiation time) on either the intensity of lueffect of photochemistry (i.e., minescence or its spectrum; the coincidence, within experimental error, of the 0’ -+ 0” emission band of dimethyloxalate with the corresponding 0’ +- 0” band of the T1 +- SO process in both absorption and excitation spectra; and the more by or less direct replication of the SI c S0 absorption spectrum of dimethyloxalate the phosphorescence excitation spectrum. The observed luminescence is a phosphorescence of TI -+ So type. The long emissive lifetimes (~1 ms) and the intrinsic molecular nature of the process dictate this identification. The observed luminescence is of ?I’,+ --+ ?‘I type. A number of observations validate this assignment and enable extension to a more specific 13A, -+ I’A, (C,,) designation for the truns-planar molecule : First, the observed phosphorescence in solution is polarized in a direction which polarization direction. The obis closely perpendicular to the XI +- So absorption served relative degree of polarization is ~0.15, or -50 % of the expected theoreti-
MARIA AND McGLYNN
304
cal limit of -0.33 for nrecise perpendicularity of the 81 t So and 7’1~ So traneition oscillators. Since the corresponding limit for exact parallelism of the same two oscillators is +0.50, we may conclude rough perpendicularit,y, as specified. Sincae spin-orbit coupling selection rules usually dictate perpendicularity of transition moments ‘pi -‘I1 and 3Pa - ‘pl where ‘I’i and 31’i derive from identical electronconfigurational excitations, we can conclude that the observed S1 and T1 states may be of similar configuration nature. Since the & state of the planar molecules is identified in paper I as kn,* (l’A,) we might accept a similar designat,ion for the observed T1 state. It is worth emphasizing here t.hat the polarization measurements refer to glassy solutions and, thus, to nonplanar entities. The relative polarization ratio for the 31’n,* + ‘p& nT* c ‘I’1 events in such a molecule has been computed (see TableVI and the discussion relating to it) as - 0,22, in good agreement with experiment. Polarization work was not extended to single crystalline entities because of the defect nature of the phosphorescence of such systems. Second, the observed emission lifetime of the crystalline systems is consistent with a 31’n,t + ‘I1 process and too short by a factor of 10’ to 10’ for a 31’,,* + ‘I’1 decay event (see, for example, Table IV). Third, the observed energy interval E (l’A,) - E (T1) = 2275 cm-’ is consistent with expectation for 7’1 = 13A,. Indeed, we evaluate the exchange energy energy as 2K,,. N 2[z (0.6)’ (0.5)‘K,,,, + 2 (0.2)2(0.54)2k,,,J N 4200 cm-‘; where kpZp, and kpyp, are atomic exchange integrals on oxygen; where 0.6 and 0.2 are the coefficients of 2,= - and 2,1 - AO’s of oxygen in the W-MO whereas 0.54 is the coefficient of the 2,, - A0 of oxygen in the ?r* - RIO; where the extra fact’or of 2 is caused by the presence of two carbonyl oxygens in the molecule; and where the MO’s cited are those for a tr@wplanar oxalate framework. The computed value of 2K,,* is not significantly dependent on the presence or absence of planarity because of the localization of dominant integrals on the > C=O group. On the other hand, the supposition that T1 = Z3A, provides an observed XZ - 7’1split E (2lA,) - E (Z3A.) = 12155 cm-’ whereas the computcad exchange energy in this instance is ~3200 cm-‘. Thus, we may disregard the g3A,, assignment where the 2A,, state derives from a ?r* t G rxcited configurat8ion, TABLE COMPUTED
OSCILLATOR
STRENGTHS
IV
.IND LIFETIMES OF SOME TRIPLET H 11.4,
PROCESSES IN trans PL.INAR (OXALIC ACID (C2h) -
Configurationn1r* n** u** 7rTr* UT* e (r as used here denotes
Symmetry 13Az1 13B, 23A, 13B, 23A, an in-plane
Oscillator strength 1.1 x 3.3 x 5.7 x 6.3 x 9.3 x
10-b 10-14 10-4 10-g 10-s
orbital which is more delocalized
Lifetime (s) 7.7 9.2 3.0 2.6 1.7
x x x x x
10-A 101 10-c 10-z 10-s
than the n orbit ala.
OXALIC
ACID AND DIMETHYLOXALATE
305
u being an in-plane orbital which is more delocalized than the 7~ orbitals. A Ti = 13B, assignment may not be eliminated on the basis of observed S-T intervals; it is, however, eliminated by the polarization result’s and by the observed decay lifetimes. The only other states of reasonably low energy which might conceivably be considered as assignment possibilities for the TI state are of even (y) orbital parity in Dhe planar molecule. The chances of observing an emission forbidden by orbital and spin selection rules and with decay lifetime of -1 ms seem too remote to merit consideration. Since we suppose that the emission of the single crystal systems is attributable to planar oxalatic entities, we must consider the assignment’ 13A, (3rnr*) -+ l’A, (C,,) to be fully substantiated. 2. Phosphorescence of a Planar
(Czh) molecule
The T, state of the planar molecule (i.e., the presumed emitting species in the crystal) has been assigned above as 13A, (3rnr*, CS). As further validation of this assignment, we now present the results of spin-orbit coupling calculations for all reasonable triplet designations in Table IV. The MO wavefunctions used were those generated by the Mulliken-WolfsbergHelmholz procedure described in paper I; the spin-orbit perturbational computations followed along previous lines (7) and refer, in the present instance, to a trans planar oxalic acid molecule. The agreement between the calculated emissive lifetime (0.8 X lop3 s) of a 13A, state and the observed value for the crystal, 1.4 X lop3 s, is excellent. To some extent, however, this coincidence must be considered fortuitous: The MWH calculations underestimate all transition energies, and this makes the energy denominators in the perturbation expressions too small and the spin-orbit effects too severe; on the other hand, vibronic coupling effects are not included in Table IV and this neglect renders the effective perturbation theory numerators smaller than they otherwise should be. Both effects tend to cancel and, as a consequence, the results of Table IV attain some validity. As seen, the only other reasonable assignment is 13B,(r,,.) for which the observed lifet’ime is shorter than that calculated by a factor of ~18 and which is invalidated anyway by the polarization results for the glassy solutions. A dissection of the computational spin-orbital coupling results for the 13A, + llA, transition is given in Table V. This includes the effects of triplet mixing into the llA, ground state as well as singlet mixing into the 13A, state; it is predicted that both types of mixing are roughly comparable with regard to intensity induction in the transition under consideration. It is also predicted that the polarization of phosphorescence should not’ be entirely perpendicular to the transition moment direction of the l’A, +- llA, excitation. However, the contribution of z-polarization (i.e., 1) intensity, while real, is marginal at only 0.1 % of the in-plane polarization intensity. Another source of out-of-plane intensity in t*he 13A, +- l’A, transition might
306
MARIA
AND
McGLYNN
TABLE SPIN ORBIT COUPLING IN 13A, +
Spin orbit coupling Matrix Element (cm-l)
Contributing state Orbital symmetry ____
?T*
I
llA,
e
H’(x)
3.67 3.77 6.25 6.33 2.80 4.70 5.75
-10.3 0 -21.8 48.7 37.6 17.0
0
_I
H'(Y) 0 -49.3 0 -23.7 33.9 -44.3 36.4
V TR.~NSITION OF OXALIC ACID (C2,+)
T
Conferred transition moment (41
.Y
H'(z) ~__ 57.5 0 8.6 0 0 0 0
~
Y
~2 _i___-
0 -2.2
0 x
10-z
-4.4
X 10-a
,7.0 X 10-t 0
X x x X
lo+ 10-a 10-4 lo+
0 1.5 -2.6 -9.0 -2.6
x 10-d x 10-a x 10-s X lO+
3.0 x 10-j 0 0 0 0
0 -9.8 -4.25 -1.0 -1.2
_~_
* The energies are those obtained from the MWH calculations of paper I. Since electron correlation is not included, the singlet and triplet states arising from a given configuration are degenerate. Axes are defined in paper I. derive from spin-orbit mixing of 13A, and l’A, states. This mixing is not considered in Table V because of the inability of the computer program to handle the mixing of states which, because of MWH inadequacies, are erroneously supposed to be degenerate. However, it is not too difficult to show that the matrix element (13A,I H’ IllA,) w h ere H’ is the spin-orbit coupling Hamiltonian, contains no one-center atomic spin-orbital integrals and that, in fact, its magnitude is dominated by three-center atomic terms. Thus, the element (13A,( H’ 11'S,) iu small, as is also the intensity llA, t I’A, which is being “stolen” in this perturbation process. As a result, and despite the smallness (2275 cm-‘) of the perturbation denominator, it is concluded that such mixing cannot contribute more out-ofplane intensity than ~5 % of the total in-plane contribution displayed in Table V. Thus, even in a point group where the triplet spin states contain a component which transforms as the totally symmetric representation, we find that the mixing of singlet and triplet states arising from the same orbital configuration is forbidden in a first approximation-even though group-theoretically allowed. In the present instance, this result implies an in-plane nature of the !!‘, + X0 event. In general, this same result implies a ready generalization of the “no-mixing” maxim which obtains for point groups for which none of Rz,y,a transforms as r1 .
3. Phosphorescence
of a Twisted
(C,) Molecule
The discussion of the previous section assumed molecule planarity. In the case of solution species, it is probable that some dimethyloxalate molecules remain planar (see paper I) whereas the great majority exhibits dihedral angles different from zero. In particular, lifetime data and relative polarization values are thought to be available for such an ensemble. We now select one molecule
OXALIC
ACID
AND
DIMETHYLOYALATE
307
from such an ensemble-onr for which the two carboxyl planes intersect at an angle of 45”-and cquirc whether this “average molrrule” provides satisfactory explanation of the experiment’al observations. Both MWH and CNDO/s molecular orbital calculations, as discussed in the paper I, have shown that the first excited singlet’ state of the twisted molecule (i.e., the 2lA state) correlates with the l’A, (lI’?,+) st’ate of the planar molecule. It was also found that both this state and the 2’B state of the twisted molecule (see Table III of paper I), which correlates wit’h the llB, (I’,,*) state in CZ,,, are not mixed to any appreciable extent with any other state by configuration interact,ion. The spin-orbit coupling computation, therefore, was restricted to one configuration in each of these states and wavefunctions used were those obtained from the T\IWH processing. The results are given in Table VI. It is now. seen that the Z3B --f llA transition retains a polarization which is parallel to the 2’d +- llA transition-moment. The calculated polarization ratio is, in fact,, approximately +0.30. Therefore, because of the observed ncgat’ive polarization rat’io, the Z3B state cannot be the moment is now phosphorescing st’ate. By cont’rast, the 23L4 -+ llA transition polarized in a manner which is only partially perpendicular to the 2lA +- 1’A absorption oscillator. Indeed, in this inst’ance, the calculated polarization ratio -0.22. This number is rrmarkably close to the observed is approximately polarization ratio of -0.15. Thus, the assumption of twisting provides wady interpretation of lifetime and relative polarization da,ta for dissolved dimethyloxalate. It also, of course, rat’ionalizes the lack of resolution evident in the solution and gaseous absorption spectra as well as differences between crystal absorption spectra on the one hand and the gas and solution phase spectra on the other (see paper I),
4. Vibronic Considerations Vibronic perturbations can cause 3rn,* - “I’=** mixing and thus induce perpendicular intensity int’o the nominal 3rnr* + ‘rl event’ of the planar molecule Since the observed lifetime of the solution phase molecules are readily accounted for on the basis of a planar emitting species (we Table IV), it follows that the TABLE
VI
CALCULATED OSCILL.YTORSTRENGTHS FOR TR.INSITIONSTO SOME STATES OF OXALIC ACID WITH .\I)IHEDR.\L ANGLE OF 45" (C, POINT GROUP) Oscillator strength f .Y
State 21/l 238 2lB 2”B
1.2 x 5.7 x 4.9 x 3.7 x
10-10 10-6 10-Z 10-g
1.9 3.2 1.9 1.4
x X x X
10-4 1OP 10-l 10-C
Lifetime (secj
.If 1.1 x 5.5 x 3.2 X 8.3 x
10-z 10-7 1OP 10-G
3.9 5.5 4.9 1.4
x x x x
10-e 10-4 10-g 10-d
MARIA
30s
AND McC;I,YXK
polarization ratio is the only observable IThich--in this paper, at least-forces us to w~wludr nonplanarity of thcx emitting solution wt,ity. We now cnquirr whcthcr vibronic perturbations in a planar entity can lead to a rationalization of the relative polarization observed in the glassy solution phase. We will conclude that it cannot and, hence, further justify our assertion of nonplanarity in such phases. Alternatively, we also seek explanation of a number of observations pertaining to the microcrystalline and single-crystalline systems in which me have posited, per crystallographic dict’ates, molecule planarity. These observations arc: 110 ncwssity to invoke vibronic coupling insofar as vibrational analysis of the phosphorescence is concerned and a striking vibronic effect which is displayed in the phosphorescence excitation spectrum. We believe that the vibronic coupling considerations indulged here provide ready rationalization of both of t’hrw observations. (i) Vib~onic cou@i~~g in the sillylet l?za?zif&l. A nontotally symmetric b, vibrational mode may vibronically mix the I’A, and l’B, states and thus introduce vibronic intensity into the l’A, +- I’A, absorption process. Such vibronic mixing is known to occur in t’he isoelectronic molecules oxalyl chloridr and glyoxal. It was shown in paper I that this same vibronic mixing also occurs in dimethyloxalate and that the vibronic intensity in the lowest energy & +-- & absorption band may equal as much as 45 SC,of t#he t,ot,al observed int,rnsit,y of this nominal Yn,* +- ‘l?l transition. If we describe the observed ‘F + stat’e as a (llA,) + b (I’&) and the observed 31’n,: state as a’(13AU) + b’(13B,), then t,he intersystem crossing event can bc decomposed into the four sub-events depicted in Pig. 4. The spin-orbit matrix elcmcnts (1’B,I H’ / 13B,) and (l’i1,l H’ 113A,) cont,ain no one-center coupling t,erms and, in fact, as previously specified, obtain most of their magnitudes from three-center terms; these elements, therefore, are very small and are assumed to bc safely neglectable. F’urt’hermorr>, t,he cAmissivc> 13R,, + 1’,4, channcbl in the NOMINAL STATES I ‘A,
REAL atI
‘A,,
STATES
+
bdB,)
=
S,
+
b’(13EI) /”
=
T I’
1x1 o’(13A
x”
i 1
FIG. 4. A decomposition of the S1 -U-J T1 intersystem crossing event into four subevents. The decay event T1 -+ SOis also schematized, relative probabilities and polarizations of t,he emissive components being indicated.
OXALIC
ACID
AND
DIMETHYLOXALATE
309
!!‘I-+ SOphosphorescence act is relatively improbable compared to the 13A, + A, channel; indeed, when proper weighting is introduced via the coefficients a’ and b’ of Fig. 4, we find from Table IV that the 13A, + llA, channel is ~150 times more probable. Thus, if spin-polarization exists in the T1 state, we conclude that the emission channel being monitored at short times is the 13A, -+ llA, route and, therefore, that the ASIw TI intersystem crossing channel dominantly effective in mediating this emissive route in b (llB,) ~3 a’ (13A,). Consequently, we expect the phosphorescence excitation spectrum to be dominated by a vibronic component in the b, mode-exactly as is observed (see Fig. 2C where the origin band at 3040 A appears only as a shoulder on the much stronger vibronic band at 3000 8). The supposition of spin polarization is not one for which we have adduced any experimental proof here. It is perhaps as well then to emphasize that the only consideration of relevance to our discussion is that the nonemissive relaxation time of the perpendicularly polarized spin component maintains the sequence: ~(b’~& ?n) ‘Ag) < 7 (spin relaxation). That spin polarization obtains in this system is not unexpected in view of the rather short phosphorescence lifetimes under consideration. (ii) Vibronic coupling i,n the triplet manifold. Other things being equal, the vibronic mixing of the 13A, and 13B, states will be considerably larger than that between the singlet states; indeed, based upon energy denominator considerations drawn from the energy scheme of Fig. 5, the percentage of 13BU in 13A, should be ~10 times larger than the percentage of llB, in llA,. Thus, the 8070-
I ‘8”-
6050-
I
40-
2 A,-
30-
I ‘Au-
..-.,38u -13A,
20 IO
1
0 L 1’AsDIMETHYLOXALATE FIG. 5. Electronic energy-level diagram for transplanar dimethyloxalate. The position of the llB, state is approximate; that of the 13B, state is computational and follows from an estimated exchange energy of -3.5 eV.
310
MARIA AND M&LYNN
can aquire transition probability which is polarized 13A, -+ llA, transition perpendicular to the molecule plane (i.e., parallel to the l’A,, + l’A, transition) by three rout,es. These routes have been discussed previously; they are, in summary : (a) Spin-orbit mixing of the 13A, state with ?A,. Using experimental energies (Fig. 5) and matrix elements given in Table V, we calculate the maximum transition moment’ length that may be “stolen” as M, = 6 X lo-” 8. (b) Spin-orbit mixing of the 13A, state with the l’A, state (i.e., self-mixing of configurations). This process, as discussed in the text, leads to zero onpcrnt.cr contributions and is quite negligible. (c) The 13B, component which is vibronically introduced into the l”A,, state can mix via spin-orbit coupling with the (1 and 2)‘A, states. If the luminescing state is pure 13B,, we calculate a stolen transition moment length M, = s x 1o-4 8. The states that are most important in conferring intensity on the 13A, component are I’& and 13B,, where the latter state mixes via spin orbit routes with the ground state. Again using experimental energies (Fig. 5) and assuming h’ (13B,) E 30 kK, we calculate M, = 3.9 X lop3 s and M, = 3.4 X lo-” A. All stolen transition probabilities must be multiplied by a factor corresponding to the fraction of 13B, or 13A, in the nominal 31’+ state. Thus if we assume that the 13B, admixture into the nominal 13A, state is as high as 20%, we obtain a transition probability. value of -150 for the ratio of the in-plane to out-of-plane Thus, we do not expect to be able to observe the effect of vibronic mixing in the triplet manifold in either the emission from or absorption to the first triplet, state. This discussion-insofar as emission is concerned-also assumes retention of spin polarization in the T1 state. This is not unlikely because of the short lifetimch of this state. Our results do not lead to any conclusions concerning the presenccb or absence of spin polarization in glassy solution media (in which our observations arc’ fully interpretable by an assumed nonplanarity of the molecule). Furthermore, if our assertion that’ the crystalline luminescence is mostly of defect origins is correct, little if anything can be learned from polarization studies on thci luminescence of such crystalline systems. V. CONCLUSIONS
The conclusions
of this work are as follows:
(i) Covalent oxalates emit a phosphorescence of 3rn,* + ‘I’,+ type. Specific designations are 13A, --$ l’A, in truns-planar oxalatrs and 23A --$ llA in nonplannr twisted oxalates. (ii) The phosphorescence of glassy solutions initiates in a nonplanar configuration; that of the crystals initiates in defects which are assumed to (‘orltain planar oxalate molecules.
OXALIC
ACID AND DIMETHYLOXALATE
311
(iii) The phosphorescence excitation spectrum of the single-crystal exhibits a striking vibronic effect. It appears that b, modes coupled to the l’A, st’ates provide the primary excitation preparation for efficient intersystem crossing. (iv) The observation of (iii) above and the observed phosphorescence lifetime of the crystal are readily interpreted on the basis of vibronic-spin-orbit computations. (v) The observed phosphorescence lifetime and relative polarizations of dissolved oxalate entities are readily interpretable 011 the basis of a spin-orbit model for twisted oxalate geometries. It is shown, conversely, that a vibronicspin-orbit model applied to a planar oxalate geometry can not explain the polarization results. RECEIVED: September
27, 1971 REFERENCES
1. S. A. BRUNS, Nauchn. Soobsch. Ilast. Gorrr. Dela, Akad. Nauk SSSR, 16, 114 (1962). 3. V. V. TRUSOV, Nauch?A. Zap., Odessk. Gos. Ped. Inst., Kafedry Mat., Fiz. i Estestvozn 26, 86 (1961). 3. A. P. VISHNEVSKII, Doklady Akad. Nauk. SSSR 63, 503 (1948). 4. H. MURATA AND K. KAWAI, J. Chem. Phys. 26, 589 (1956). 5. J. K. WILMSHURST AND J. F. HORWOOD, J. Mol. Spectrosc. 21, 48 (1966). 6. J. FUJITA, A. E. MARTELL, AND K. NBKAMOTO, J. Chem. Phys. 36,324 (1962). 7. D. G. CARROLL, L. G. VANQUICKENBORNE, SND S. P. MCGLYNN, J. Chem. Phys. 46,2777 (1966).