B1OEVOoRGANICCNEMSlXY 9,281-298 (1978) 281

Porphyrins_ XXXVII.* Absorption and Emission of Weak

Complexes with Acids, Bases, and Salts

EVERETT AUSTIN and MARTIN GOUTERMAW Department of Chemistry, University of Washington. Seattle, Washington, 98195

ABSTRACT

Absorption and emission data are reported for species resulting from addi- tion of acids, alkali, and metal salts to Ha(Etio) (etioporphyrin) and Hz(TPP) (tetraphenyiporphyrin) in various organic solvents_ rVfr/r acid addition. di- cations of both TPP and Etio are observed in equilibrium with free base, and a monocation of Etio is also observed in acetone, but not in benzene. The fluorescence yield of He(TPP)Clz was approximately 0.1, and that of Hg(TPP)Br2. about 0.01 in acetone. The fluorescence yield of Hq(Etio)Q was approximately twice that of Hq(Etio)Brg in THF. IVith alkali addition.

absorption spectra show a predicted variation with metal, and all species show strong fluorescence and weak phosphorescence, which is consistent with only a small heavy atom effect. In addition, three distinct species were observed in the fluorescence of Naz(TPP) compleses in THF at 77 K. With meral salt

addition. spectra closely resembling monocation and dication spectra are produced in acetone for Etio and TPP, respectively. For all cases studied, except perhaps uranyl salts with TPP, fluorescence yields agreed to within about 25% of that of Ha(Etio) acetate for Etio complexes and about 25% of

that of H4(TPP)CI2 for TPP complexes, indicating shielding of the metal salt from the porphyrin H cloud by solvent molecules.

INTRODUCTION

It is weIl known that free-base porphyrins, H,(P), generally shown dramatic changes in absorption spectra on addition of either strong acids or strong bases [l] . With acid addition the spectra are usually characterized as the acid dication,

&(PY2 [l, 2]_ The change from H,(P) to H4(P)+2 is accompanied by a change of the visible absorption spectrum from the characteristic four-banded “free base type” to the characteristic two-banded “metal type.” However, in some cases a three-banded absorption spectrum is obtained, and this has been

* Paper 36. -4. Antipas, J. W. Buchler, M. Gouterman, and P. D. Smith, J. Am Chem. sot- 100,3015 (1978).

*To whom correspondence should be addressed.

@ Elsevier North-Holland, Inc., 1978 0006-306 l/78/0009-028 lSO1.75

282 EVERETT AUSTIN AND MARTIN GOUTERMAN

identified as the acid monocation, Ha(P)’ [3, 4]_ On addition of alkali base (e.g., NaOH), a two-banded “metal type” spectrum is also observed [5] _ In this case the species has been identified as the dialkali complex Na2(P) or as the bare dianion (P)-2; however, the species Na(P)- is not easily ruled out. Indeed, this raises the general question of the degree of association of any such ionic por- phyrin with their counterions

Addition of a salt (e.g., FeCla or UO,Cl,) to free-base protoporphyrin dimethylester [6] leads to a three-banded absorption spectrum similar to that observed on formation of Ha(P)‘. In this case the species was originally identi- fied as a “sitting-atop” (SAT) complex [6]. Later work on titration with metal salts suggested formation of the acid dication and anions of the solvated metal [7]. This work caused some doubts about the existence of a SAT complex, an issue that has involved some controversy.

All the species responsible for the spectral changes described above can be generally described as weak complexes. where the spectral changes: (1) occur instantaneously, (2) appear to be due to an equilibrium between the porphyrin free base and the added reagent, and (3) are readily reversible. This is in contrast to metal-insertion reactions, which are generally stro22g complexes, -violating one or more of these conditions.

The purpose of this paper is to investigate the nature of these weak com- plexes. While absorption spectroscopy is the customary method for investigating these species, there has been little study of their emission spectra. In general, porphyrin emission spectra are quite sensitive to the presence of heavy atoms near the n cloud. Heavy atoms cause quenching of fluorescence, often with the appearance of phosphorescence [S-l 0] _ A similar effect would be expected if a counterion containing a paramagnetic metal [e.g., Fe(III)] were closely associ- ated with the porphyrin ‘IT cloud [lo] _ In this paper we report on the absorption and emission spectra of weak complexes between free-base porphyrins and various acids, bases, and salts. As we show later, these spectra provide consider- able insight into the extent of interaction between the various ionic porphyrins and their counterions. The data may be helpful in understanding the role of the weak complexes in metallation; moreover, in the case of the alkali metal com- plexes, systematic study of their emission is necessary to complete under- standing cf the electronic properties of the “Periodic Table of the Porphyrins”

DOI-

ACID ADDITION

Experimental

The soIvents used were spectroquality benzene from Eastman, reagent-grade acetone from Matheson, Coleman, and Bell (MCB), and analytical reagent-grade tetrahydrofuran (THF) from Mallinckrodt. The acids used were reagent-grade

PORPHYRIN COMPLEXES WITH ACIDS, BASES, SALTS 283

HCl, HBr, and HI from Baker, reagent grade HNOa and glacial acetic acid from Fischer, and 99% trifluoracetic acid (TFA) from Aldrich. The etioporp!tyrin-I (Etio) was kindly supplied by Dr. David Dolphin of UBC, and th: tetraphenyl- porphin (TPP) had earlier been prepared in our laboratory using standard

methods. Absorption spectra were taken on a Cary 14 spectrophotometer. The emis-

sion apparatus has been described [ 11, 121. For these particular esperiments we used an RCA C-8852 photomultiplier tube cooied with a dry-ice/isopropanol bath. The amplified emission signals were fed directly into a PDP8/e computer

and corrected for the wavelength sensitivity of the detector_ Excitation specrra were not corrected.

Quantum yields were calculated from absorption and emission data by the formula:

* A(sample)

f(sample) =

X OD^(standard)

A(standard) ODh(sample) x @f(standard) 7

where A is the area under the corrected emission spectral curve, X is the wave- length of the esciting light, and where the geometry of the apparatus and the intensity of the exciting light are held constant. We took (Pf<sta*dard) x 0.1 using either H,(Etio) or Ha(TPP) for the standard_ as indicated below- (The value of 0.1 is a rough value suitable for the accuracy of our measurements;see Refs. 9 and IO.)

For H,(TPP)Cla in acetone. epi was determined relative to Hz(TPP). The yield for H,(TPP)Bra was determined relative to the chloride complex. Only the ratio of the quantum yields of Ha(Etio)Cla and Ha(Etio)Bra was deter- mined_

The acid dications H,(Etio)+* and H1(TPP)** were prepared for optical study by adding acid to a solution of the porphyrin in about 15 ml of acetone, tetrahydrofuran, or benzene, until the absorption spectrum showed only the acid dication to be present. A few drops of 50% HI or HBr diluted by equal volume of solvent or a few drops of 1 M HCI were sufficient to generate t!le dihydrohalide species free of either unprotonated free base or the monopro- tonated cation.

The acid monocation HafEtio)+ was generated in two ways. It was made by the method of Corwin et al. [4], who added 6 parts acetic acid to 4 parts acetone containing Hz(Etio). It could also be made by titration. For the titra- tion experiments we used a stock solution of approximately 2.5 X 10m6 M Ha(Etio) in acetone and a stock solution of about 1.2 X 10-l hl trifluoroacetic acid (TFA) in acetone. Equal parts of porphyrin solution and a suitable dilution of the TFA stock were mixed and the absorption spectrum recorded_ Thus the total porphyrin concentration was about 1.2 X 10m6 M in every case. When [TFA] s 6 X lo-” M, free base and monocation are present in roughly equal

284 EVERETT AUSTIN AND MARTIN GOUTERMAN

TABLE 1

Absorption and Emission Peaks for H,(Etio)X, Species

A. Absorption: Wavelength in nm (T = 298 K)

x- Solvent Q(l,O) Q(0, 0) dQo.oY4Ql.o)

Cl- THFb 555 597 0.45 Br- THF 560 602 0.40 Cl- Acetone 549 592 0.38 Br- Acetone 555 597 0.32 TFA- Acetone 546 589 0.31 NOa- Benzene 552 594 0.5

B. Emission: Wavelength in nm (2’ = 298 K)

X- Solvent QCO, 0) Q(O, 1) @f

Cl- Br-

THF 599 658 n

THF 604 663 0

= af [H4(Etio)CIz I _ 2

- qf[ H4(Etio)Br2 1

b Tetrahydrofuran.

amounts, and when [TFA] E 6 X 10v3 M, nearly pure monocation is ob- tained. When [TFA] reaches approximately 6 X 10s2 M, the dication pre- dominates_ The experiment was carried out under similar conditions for TPP with TFA and acetone. In benzene, titrations were carried out on Ha(TPP) and on H,(Etio), using HNOa.

Results

Absorption and emission parameters for H,(Etio)*2 are given in Table 1 and for H,(TPP)+ 2 in Table 2. The spectra with several anions and several soiqents were studied. In all cases, there is a change in the absorption spectrum with change in counterion, showing that in these solvent systems there must be association between the acid dication and at least one of its counterions. The association was also evident from the emission studies. In acetone, Hd(TPP)& had a fluorescence yield of about 0.1. Relative measurements in THF also showed that H&TPP)Bra had a fluorescence yield about 10 times smaller than H,(TPP)Cl,. Similarly, H*(Etio)Brz showed a fluorescence yield approximately

PORPHYRIN COMPLEXES WITH ACIDS, BASES, SALTS 285

TABLE 2

Absorption and Emission Peaks for H4(TPP)X2 Species

A. Absorption: Wavelength in nm fT = 298 K)

X- Solvent QO, 0) Qtl, 0) Q(o, 0)

Cf THF” -558 608 658 Br- THF -562 614 665 cl- Acetone -560 609 660 Br- Acetone -56.5 615 662 I- Acetone -567 625 674

B. Emission: Wavelength in nm

X-

cl-

Br- Br- Cl- Br-

Solvent

THF THF THF Acetone Acetone

T(K) QW, 0) Q(o, 1) @f

298 694b c

298 701b c

77 698 754 298 690b 0.1 Id 298 695’ O-Old

a Tetrahydrofuran. b Broad.

c cPr[ H,(TPP)Cl, ] - 10.

*f[H,(TPP)Brz 1 d tZO%.

2 times smaller than H,(Etio)Cia, also in THF. At low temperatuares none of

these four systems showed clear phosphorescence. However, it should be noted that weak phosphorescence at 743 nm for H,(Etio)Cla in EPA has been re- ported previously [ 13]_

As mentioned above, titration of H,(Etio) in acetone showed that protona-

tion occurs in two well-separated steps. On acidification with trilkoracetic acid, the four-banded visibIe spectrum smoothly converts to the characteristic three- banded spectrum of the acid monocation shown in Fig. 1. This monocation spectrum was identical to that reported by Corwin et al. [4] obtained in the acetic acid:acetone ratio (6r4). Since the spectrum is essentially like that of the salt complexes, the peak wavelen$hs are given below in Table 7.

An interesting point is that if benzene is used as a solvent, acidification of

286 EVERETT AUSTIN AND hlARTiN GOUTERMAN

!E:d OAc

;?” (r,*! ’

0: ; p

!z

&’ I iQJ

1 :2

, I ‘2

08’ , \ 7p \

h ‘.

p

, ‘@=7--o--l 650 700

V.'cvelenc:h ( ;! m )

FIG. 1. Absorption (solid line) and emission (dashed line) excited at 530 nm of Ha(Etio)+ [+I.2 X 10v6 M]. Absorption with 6 X lo-” M trifluoroacetic acid in acetone (some residual free base is evident at -490 and -620 nm). Emission in acetic acid: acetone is 6:4. Both are at room temperature_

H,(Etio) with TFA or HNOa showed apparently direct conversion from free base to dication; no monoprotonated species could be seen in the series of absorption spectra. The Hx(TPP) was similarly titrated with TFA in acetone and in benzene and with mOa in benzene; in these titrations only the free base and acid dication were evident in the absorption spectrum.

The fluorescence of Ha(Etio)’ was studied in acetic acid:acetone (6:4). The fluorescence yield was 0.064 (+20%) (spectral data are given in Table 6). We attempted to look for the phosphorescence using ethyliodide for heavy atom enhancement, known to work with the free base [13] _ We studied the system in a low-temperature glass (2 acetone:2 methanol:4 acetic acid:1 ethyl iodide) and a snow (10 acetic acid:7 acetone:3 ethyliodide), but found that on cooling to 77 K the solutions contained only Ha(Etio)‘2.

BASE ADDITION

Experimental

The solvents used were spectrograde methanol from Eastman and reagent- grade THF from Mallinckrodt or MCB. In our first experiments the solvents were used only after distilling first from LiAlH, and then from a refluxing solution with metallic Na and benzophenone. Later these solvents were used without special treatment, which proved unnecessary. The lithium was from Alpha Inorganic the sodium from J. T. Baker, and the potassium from Mallinckrodt.

The absorption spectra and ordinary emission spectra were taken as above. The weak low-temperature phosphorescence of these compounds was detected

PORPHYRIN COMPLEXES WITH ACIDS, BASES, SALTS 287

using a modification of our usual emission apparatus. In this arrangement a PAR mechanical chopper was used to chop the exciting beam into light and dark periods of either 6 ms or 33 ms each. A signal from the chopper could rhen be used to trigger a Techtronls Type 545B oscilloscope, whose delayed trigger was

then used to gate the photon counter, which counted for a variable length of

time at any point in the light or dark periods. By photon counting only in the dark period, the weak phosphorescence of these compounds was no longer

masked by the tail of the fluorescence. Additionally. phosphorescence excita- tion spectra could be done to verify the identity of the phosphorescing species.

Lifetime measurements were carried out using for exciting light a General

Radio 1538 strobotac flash lamp, with the flash rate controiled by the oscillo- scope. Filters were used to isolate the exciting flash from the detection. After amplification the photomultiplier output was signal averaged by a PAR model

TDH-9 waveform eductor.

The alkali metal complexes of Etio and of TPP were prepared by treating approximately 15 ml of a soIution of the porphyrin in pyridine, in THF, or in a pyridine:THF (I : 1) mixture with about 0.5 ml of methanolic alkali methoxide

in large excess. The methoxide solutions were prepared by adding metallic Li, Na, or K to about 10 ml of methanol_ The method of preparing the alkali metal porphyrins is essentially that of Erdman and Corwin [5] and Barnes and Dorough

P41Y except we did not find it of great importance to exclude atmospheric moisture or use unusually dry solvents in order to maintain the solutions for an afternoon’s work at room temperature. It was necessary, however, to protect the solutions from UV light, as irreversible decompositions then cccurred within an hour or two, especially in the Na compleses. The solutions were stable indef-

initely at liquid N2 temperature.

Results

The absorption and emission parameters for alkali metal complexes of Etio are given in Table 3. Table 4 gives 298 K absorption and emission parameters

for alkali metal complexes of TPP; Table 5 gives emission and excitation data for

alkali metal TPP complexes in 77 K THF. A typical absorption and emission spectrum is shown in Fig. 2.

In both THF and pyridine there was a regular change in absorption spectrum of alkali metal porphyrin complexes along the series Li, Na, K; the spectrum red shifts and the ratio fo,Jfi,-, (where fofo.-, and five are the oscillator strengths of the Q(O,O) and Q( 1 ,O) bands respectively) changes as previously described for octalkylporphyrins and tetraphenylporphyrins [ 15]_ In contrast. when CsOH or NaOH are added to Ha(Etio) in dimethylsulfoxide (DMSO)-as Clarke et al_ did with NaOH [16] -the absorption spectra of the two solutions agree to within one nanometer_ Furthermore, the solution treated with CsOH show3 no clear quenching of fluorescence under a long-wave UV lamp, as compared visually to

288 EVERETT AUSTIN AND MARTIN GOUTERMAN

TABLE 3

Absorption and Emission Peaks for Alkali Metal Complexes of Etioporphyrin

A. Absorption: WaveIength in nm (T = 298 K)

Metal Solvent Q(1 I 0) mAa E(&O.OMQ1.0)

Li Pyr” 553 588 0.53 - Na Pyr 554 590 0.40

K Pyr 557 593 0.3

B. Emission: Wavelength in nm

Metal Solvent T WI Q<O, 0) Q<O, 1) T(O, 0) rp(ms)

Li Pyr 298 593 646

Li 2THFb : 1 Pyr 77 -740 Na Pyr 298 594 647 Na 1THF:IPyr 77 593 642,650 -750 50 + 20 K Pyr 298 598 650 K ITHF: 1Pyr 77 -760 50 f 20

a Pyridirre. b Tetrahydrofuran.

the NaOH-generated solution, and shows stron g fluorescence and no clear phosphorescence at 77 R. This suggests complete dissociation into the metal free &anion (Etio)-2 f 2Na+ or 2Cs+ in DMSO, in contrast to the association of one or more atoms with the porphyrin ring in THF and in pyridine.

AU the alkali metal porphyrins generated in THF or pyridine possess strong fluorescence and a weak phosphorescence. To locate the phosphorescence, it was necessary to separate long-Iived from short-lived emission by gating the photon counter (see above). The ratio of phosphorescence to fluorescence was approximately 500-l.

Since the complexes phosphoresced weakly, an attempt was made to estimate the observed phosphorescence lifetime of the Na and K Etio complexes. Baseline

drift due to the long eduction times necessitated by the low signalnoise ratio reduced the usual accuracy of the technique, but we were able to estimate lifetimes of 50 k 20 ms for both Na and K complexes of Etio in 1 pyridine:l TI-IF cracked glass.

PORPHYRIN COMPLEXES WITH ACIDS, BASES, SALTS 289

TABLE 4

Room-temperature Absorption and Emission Peaks for Alkali Metal Complexes of TPP

A. Absorption: Wavelength in nm

Metal Solvent Q(l, 0) Q(o, 0) l (Qo.oMQ1.0)

Li THP 575 617 0.75

Na THF 577 620 1.0

K THF 584 629 I.5 Li Pyrb 573 614 0.73

Na Pyr 578 622 1 .o K Pyr 584 629 1.3

B. Emission: Wavelength in nm

Metal

Li Na K

Solvent

Pyr Pyr Pyr

O(O, 0) Q(o. 1)

629 678 642 690’ 65Sd

a Tetrahydrofuran. b Pyridine. c Shoulder. d Broad.

At 77 K in THF, the alkali metal TPP complexes showed three distinct species, in addition to residual free base (see Table 5). On freezing solutions that contained enough alkali methoxide so that their room-temperature spectra show complete conversion of the free base to the alkali complex, two distinct emitting species, designated I and II, are observed. Species I corresponds to what is expected for the species observed in the room temperature absorption; species II is red-shifted by 35-65 nm depending on the metal. Rough absorption spectra for the two species were determined by low-temperature excitation spectra_

Species I and II both showed a clear and systematic variation in emission and

absorption along the series Li, Na, K, which indicates that one or more metals are associated with the porphyrin in each case. In an effort to determine which might have a higher number of metal atoms, low-temperature emission and excitation spectra were taken using a constant concentration of total porphyrin

290 EVERETT AUSTIN AND MARTIN GOUTERMAN

TABLE 5

Emission and Excitation Peaks for Alkali Metal Complexes of TPP in THFa at 77 K

A. Excitation: Wavelength in nm (Uncorrected)

Species designation Jletal

I I I

II II II

III

Li Na K Li Na K Na

B(O.0) Q(l,O) Q(O, 0)

-440 550 -615-630= -440 5S5 -620-630’ -440 590 -630-640=

450 595 640 460 620 672 465 632 682 435 53sb ) 575 610’, -665

B. Emission: Wavelength in nm

Species designation Metal O@, 0) O(O, 11 T(O, 01

I Li I Ka I K

II Li II Na II K

III Na

625 67s 635 690 -!350 639 699 -860

662 720b 68’7 75s 703 770

-680 750

a Tetrahydrofuran. b Shoulder. ’ Could not be determined accurately because of scattered light.

while varying the concentration of Na methoxide added in a constant volume of

methanol. With reduced Na methoxide the ratio [II]/[I] slightly decreased. However, with lower Na methoxide two complications occurred; the free-base Ha(TPP) became the predominant species and another species, III, was regularly observed. Although the emission spectrum of III and II were rather similar, the excitation spectrum of III showed, in addition to an origin band at appro.xi- mately 665 nm, a second strong band at 575 nm,that is absent from the excita-

PORPHYRIN COMPLEXES WITH ACIDS, BASES, SALTS !9I

3

750 mm

! -\__ ---_ 12

600 650 70G -750 800

Wavelength (nm)

FIG. 2. Absorption (solid Iine) in pyridine at 300 K and emission (dashed line) excited at 555 nm in pyridine:tetrahydrofuran (1:l) at 77 R of Na,(Etio). The phosphorescence (inset) is expanded by a factor of 10.

tion spectrum of II. In addition, the excitation spectrum of species III showed hints of two other bands at 538 nm and 610 nm, thus giving the appearance of four-banded free-base type of spectrum_ However, it was red shifted at about 25 nm from the excitation spectrum of H,(TPP) in the same solvent.

SALT ADDITION

Experimental

The acetone used was reagent-grade from MCB. The metal salts used were: (I) analytical reagent-grade chromic chloride from Mallinckrodr, (2) reagent- grade ferrous chloride tetrahydrate, ferric chloride hexahydrate, zinc chloride, and stannous chloride dihydrate from J. T. Baker, and (3) 98% uranylchloride from Research Organic/Inorganic Chemical Corp. The spectroscopic apparatus used is described above.

292 EVERETT AUSTIN AND MARTIN GOUTERMAN

Hz (Et101 + FeCI,-(ti,O), m Acetone

200 K

,QK’.o) 1

/ : 1 P 9 I t _s

I tr I $

d : ,’

.L : -5

? s I L \ 4

-a 1

550 600 650 700 Waeiength (nm 1

FIG. 3. Absorption (solid line) and emission (dashed line) excited at 530 nm produced by adding FeCla*(HzO)e to Ha(Etio) in acetone at room temperature.

The species were prepared by treating an acetone solution of porphyrin with enough acetone solution of metal salt to cause the four-banded free-base spec- trum to be replaced by that of the weak salt complex. The resulting metal concentration was 1W3 IM or less.

The fluorescence yield of H3(Etio)OAc was measured relative to H*(Etio) by the method outlined above. The fluorescence yields of the salt complexes of Etio were then determined relative to H3(Etio)OAc and those of TPP, relative to H4(TPP)C12.

Results

Absorption and emission spectra for the salt complexes with H,(Etio) are shown in Fig. 3, with parameters given in Table 6.

When metal salts such as FeC13(H20)e or U02C12 are added to H,(Etio) in acetone, a three-banded absorption spectrum nearly identical to the monopro- tonated cation H3(Etio)+ (see above) results. If the metal salt is sufficiently soluble [e.g., FeCIa(IIaO)s] , increasing its concentration produces eventualIy a two-banded spectrum nearly identical to H4(Etio)t2. However, when ZnClz is added to Hz(Etio) in acetone, the absorption spectra taken within approximately 5 min shows the strong complex Zn(Etio).

Room- and low-temperature emission studies were also carried out. At room temperature, ‘Lhe species studied have emission spectra identical to within 2 nm of the Ha(Etio)+ species prepared from TFA. Solutions containing Fe(III), Sn(II), or U(W) salts had fluorescence yields less than, but within about 25% of, the value for the Ha(Etio)+ species, indicating little, if any, quenching of fluores- cence by the heavy atom. At 77 K the solutions converted mostly to species with two-banded visible spectra. These species were nearly identical to H4(Etio)+2 in both absorption and emission, showing strong fluorescence and no obvious

PORPWRIN COMPLEXES WITH ACIDS, BASES, SALTS 293

TABLE 6

Absorption and Emission Peaks for Species Generazed by Addition of Metal Salts to Ha(Etio) in Acetone and for Acid Monocations at 298 K

A. Absorption: Wavelength in nm

Metal salt Q,U, 0) Q,(C), 0); Qx(l,W Qx(O, 0)

FeCla - (HzO)s 528 355 599 FeC1a*(Hz0)4 527 555 599 Sr1Cls*(H~0)~ 527 355 598 uo2c1, 527 355 599

Ha(Etio)OAc= 527 555 598 Ha(Etio)TFA 528 555 598

B. Emission: Wavelength in nm

Metal salt Qx<O, 0) Q,(o, 1) +‘fb

FeCl, -(H20)e 605 664 0.058 Fe& -(H*O)* 605 664 SnCla=(HaO)a 604 664 0.057 uo2c12 604 666 0.048

Hs(Etio)OAca 603 665 0.064

o Ha(Etio) in 6 acetic acid:4 acetone. b &20%.

phosphorescence. A weak phosphorescence was detected at about 745-8 nm for the U(V1) complex using gated photon counting, which is comparable to that observed for the dication [ 13]-

The procedure used above for preparing weak complexes of salts with etio- porphyrin follows that used by Fleischer and Wang [6] on protoporphyrin dimethyl ester, with rather similar results. We also tried the same procedure with Ha(TPP). The results are given in Fig. 4 and Table 7. On addition of metal s&s such as FeCla-(H20)a to H,(TPP) in acetone, an absorption spectrum resem- bling H4(TPP)‘2 was obtained. In emission the species showed a single broad band that sharpened into two broad peaks at 77 K, behavior shown by the emission of H,(TPP)’ 2 _ Room-temperature fluorescence yields of solutions with Fe(III) and Sn(I1) were measured and showed essentially no quenching; a definite but small quenching is shown by uranyl (Table 7).

294 EVERETT AUSTIN AND MARTIN GOUTERMAN

H2 (TPP! + S~CIZ-(H~O)~ tn Acetone

FIG..4. Abso@ion (solid line) and emission (dashed line) excited at 600 nm produced by adding SnC12.(H20)2 to H2(TPP) in acetone at room temperature.

DISCUSSiON

For acid and base addition, the combined absorption and emission data

show rather general association between the porphyrin species and the count- er-ion in organic solvents (although not in DMSO), since change of counterion causes shifts in absorption bands and changes in emission yields. Thus acid com-

plexes H,(P)Xa show absorption shifts with change of halide X from Cl to Br; similarly, the base complexes M,(P) show absorption shifts with change of

alkali metal M among the series Li, Na, K. The acid halide salts show marked quenching of fluorescence between Cl and Br. However, the alkali meral com-

plexes show little fluorescence quenching. but they do show a heavy atom

effect in the phosphorescence quantum yield, which, though small. is substan-

tially larger than that of the free base [ 13]_ The contrast between the heavy atom effect observed in the acid halide com-

plexes and that in the alkali complexes deserves some comment. To obtain a

heavy atom effect, the orbitals of the (rr,a *) transition must mix with orbitals on the perturbing atom so as to introduce angular momentum about the perturbing nucleus [lo]. With halide ions the principal valence orbitals are the filled np, which have angular momentum; these can mix with the empty ring rr* orbitals to produce a strong heavy atom effect. Such an effect was earlier discussed for complexes SnXa(Etio), which showed dramatic heavy atom effects along the series X = F, Cl, Br, I [17]. On the other hand, with alkali metals the principal valence orbitals are the empty ns, which have no angular momentum. Thus any mixing of these orbitals with the filled rin g orbitals gives little heavy atom

effect. A similar pattern of orbital interaction is the likely cause of the small

PORPHYRIN COMPLEXES WITH ACIDS, BASES, SALTS 295

TABLE 7

Absorption and Emission Peaks for Species Generated by Addition of Metal Salts to H2(TPP) in Acetone

A. Absorption: Wavelength in nm (T = 298 K)

Metal salt QO, 0) Q(l,O) Qio, 0)

FeCla .(HaO)s 560 610 659 Fe& -(HzO)~~ 558 608 660 SnC12*(H20)2 560 610 658 uo2 c1.2= -550 598 658 CrCla -(HaO)a - 610 660

B. Emission: Wavelength in nm

Metal salt T W) QW, 0) Qio, 1) @f

FeCla .(HaO)a FeC13 -(HzO)e SnC12.(H20)2 SnC12 -(Hz 0)~ UOaCla uo,c12 CrCla .(HaO)a CrCla -(Ha 0)a

298 701b 77 683

298 697’ 77 683

298 70jb 77 707

298 700b 77 692

0.09= 743

o.osc 747

0.05Jd 755

742

D Incomplete conversion of free base. b Broad. = +20%7. ’ Rough.

heavy atom effect observed in SC(W) porphyrins, which have a remarkably

strong fluorescence and long phosphorescence lifetime [ 1 l] _ Our data for interaction with salts suggest a fairly clear picture of the nature

of the complex species. The near identity of observed absorption spectra for the various salt complexes of Ha(Etio) with that of the species known to be Ha(Etio)’ and for the various salt complexes of Ha(TPP) with that of the dication H,(TPP)‘2 suggests that these acid species are responsible for the spectra. On the other hand, the slight shifts in absorption wavelengths among the various cases and the slight quenching of fluorescence in some cases suggest a close association of the

metal species with the porphyrins. The question arises as to whether this associa- tion is in the form of SAT complex models, which show a near-neighbor reIation

296 EVERETT AUSTIN AND MARTiN COUTERMAN

of the metal and the nitrogens of the ring [6, 18, 191, or whether there is a solvation sphere around the metal [7] -

We would like to argue that only the solvation-sphere model is consistent with the emission data. It is well known that in Cr(III) and Fe(M) metallo- porphyrins, the effect of the odd electron structure is to totally quench ring fluorescence [lo] _ The mechanism for odd-electron quenching [20] would be inconsistent with a SAT geometry and the observed minimal fluorescence quenching. As further support for this view we note that open-shell lanthanide porphyrins, where the metal is far out of plane [21], are observed to show strong fluorescence quenching [22] _ The same general arguments hold for the diamagnetic metais Sn(lI) and U(VI), where the mechanism of quenching is spin- orbit effect rather than odd-electron. These atoms are like the halide atoms in having valence orbltals with angular momentum, rather than like the alkali metals whose weak spin-orbit effect was discussed above. Thus a quenching of fluorescence comparable to that observed for the bromide counterion (Table 2) is qualitatively what would be exp_- m-ted for either Sn(I1) or U(V1) in a SAT geometry.

In this regard we might note the properties of metal complexes ofN-methyl porphyrins studied by Lavallee and co-workers [23-25]_ in these complexes one of the four central nitrogens is covalently bound to CH3, while a divalent metal halide unit, M(II)X, is complexed to the ring in a five-coordinate structure [23] _ The geometry thus resembles that proposed for a SAT complex. It is found that there are only minor shifts in the absorption spectra with change of metal [24], but there is a marked quenching of fluorescence wivith heavy atom and with paramagnetic metal comprexes [25] _ Thus our argument on the importance of emission quenching as a monitor for a SAT geometry has strong empirical support.

While the arguments given here tend to rule out a SAT complex as the species producing the optical spectra in our experiments on salt addition lo porphyrin solutions, they do not rule out the possible existence of a SAT complex in other circumstances. Evidence for a SAT complex based on elemental analysis, IR, and

NhiR are usually (and kinetic arguments are often) taken on samples rather different frcm the dilute solutions of optical spectrcscopy, where trace amounts of water could be playing a crucial role. A more serious question is whether any

of the optical spectra reported as SAT complexes are actually SAT complexes rather than protonated mono- or dications perhaps associaied with a solvated metal ion. Since earlier studies did not report on emission properties, the results of this crucial test for a SAT model are not clear.

Our studies further highlight the lability of these weak complexes, in partic- ular their dependence on solvent, temperature, and the nature of the porphyrin ring. Thus we easily form the monoprotona:ed H,(Etio)+ in acetone but not in benzene; and the monocation species in acetone fails to survive at 77 K. A com-

PORPHYRIN COMPLEXES WITH ACIDS, BASES, SALTS 297

parable species Ha(TPP)* was not observed under the same conditions used for Ha(Etio)+, although Rau and Longo [26] report such a species for trreso- tetrapyridylporphyrin in water. This probably reflects a delicate equilibrium for

disproportionation

3Ha(P)+ = H,(P) + H,(P)*s,

which is strongly dependent on the porphyrin, the solvent, and the counterion. Similar lability among the alkali metal complexes of TPP are indicated by our

low-temperature studies on the emisision. Here we observe, in addition to an emission spectrum corresponding to the observed room-temperature absorption

species (I in Table 5), a second species (II), which seemed to increase with increase in Na methoxide, and a third species (III), which appeared at very low Na methoxide. We are tempted to assign I to the species Na(TPP)- and II to

Na,(TPP), while III is perhaps H(TPP)- or NaH(TPP). Of course. more detailed studies would be needed to establish these species.

CONCLUSIONS

The main emphasis of this paper is the usefulness of emission spectra in establishing the nature of weak porphyrin complexes with acids. bases; and salts. Furthermore, we have shown the difference between the marked heavy atom effect of a halide counterion (Cl. Br), where the heavier counterion strongly quenches fluorescence and the small heavy atom effect of alkali counterions (Li, Na, K), which have little effect on fluorescence and whose main heavy atom effect is to produce a weak phosphorescence. We reproduce the earlier reported absorption spectral changes observed on adding metal salts to porphyrins. The lack of significant fluorescence quenching. particularly for the paramagnetic metals, suggest that under the conditions of our experiments the optical species

is either a protonated mono or dication, depending on the porphyrin, solvent. temperature, and salt concentration; the protonated cation in some (perhaps in

all) cases is closeIy associated with the solvated metal salt. Finally, we point out that a crucial test for a SAT complex with paramagnetic and many heavy atom diamagnetic salts (as opposed to a weak complex with a solvated meta! ion) is a strong quenching of fluorescence. In this regard, see also Lavailee and Bain- Ackerman [25].

77tese studies began with a series of oxperirnertrs on ttrattyl cotttpkves carried out by James Kaufman, Carl Schottaker. and Steve MacFarlane over severai years. A saktable discussiott was held with Professor Xorntan Rose of ottr department. Dr. E 8. Fleisclrer of rite University of CaIifornia at Irvine. settf tts a helpful letter and preprints_ I%‘e obtained

298 EVERETT AUSTIN AND MARTIN GOUTEmlAN

considerable detailed advice from Dr. Alan D- Adler of Western Connecticrct State CoJIege. ataries R ConneIl gave advice on tlte apparatu- Tne researcJ1 was supported in part by

US_ Public HeaitJl Services Research Grant AM I6.508.

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Received 24 October I9 77