1338
DANIEL
w.T f T O M A S 4 N D ARTHURE. MARTELL
of the mother liquor. A 180-mg. sample of the recrystallized product was chromatographed and crystallized as described above to give 51 mg. for spectrophotometric studv. Anel. Calcd. for C18H38S101: C , 78.47; H, 5.18; S,7.63. Found: C, 77.41; H, 4.77; S,7.95. Spectrophotometric analysis given in Table I of the various colored impurity bands separated from the chromatographic column gave, in addition t o the usual etio-type spectra, a rhodo-type spectrum, which indicated t h a t perhaps a phenyl ring is missing or that a pyrrole ring may be substituted in one of the fractions. Tetra-(p-chloropheny1)-porphine.--A reaction mixture consisting of 47.7 g. of pyrrole, 100 g. of p-chlorobenzaldehyde (m.p. 47.8") and 98 rnl. of pyridine Lvas treated as outlined above for 4'8hours a t 17fj3. Direct filtration of the reaction mixture resulted in the isolation of 3.2 g . of purple crystals. Dilution of the mother liquor with ether-acetone resulted in the crystallization of 0.5 g. of additional porphyrin. Further dilution with ether resulted in the isolation of a quantity of p-chlorobenzoic acid. The porphyrin was extracted with ch1rm)form; the chloroform solution evaporated to a small volume, and diluted with four times its volume of methanol. T h e crystalline material thus formed weighed 3.6 g., nhich represents 2.657, of the theoretical amount. A 174-mg. sample of this material dissolved inaless polar trichloroethylene solvent than previouFly used !vas chromatographed as ticscribed above and recrystallized from chloroform-methanc~lt o yield ij7 mg. nf purified porphine. .4m/. Calcd. for CIIH~SS1C14: C , 70.21; H , 3.46; S , 7.45; C1, 18.88. Found: C, 70.34; H, 3.38; K, 7.30; C1, 18.65. Separation and analysis of the isolated bands gave, in addition t o some relatively pure porphine which had been adsorbed on the lower region of the talc, three fractions containing appreciable t o large quantities of chlorin. It can be observed in Table I t h a t the presence of a greater amount of chlorin impurity is indicated by an increase of wave length of the first absorption peak near 650 mp. Tetra-(0-nitropheny1)-porphine.-A number of smallscale refluxing reactions were run a t temperatures between
[COKTRIBUTION O F
THE
VOl. 78
100 and 130" for lengths of time which varied from 24 t o 48 hours. A number of successful reactions are summarized below: 4-Nitrobenzaldehyde, 6.
Pyrrole, ml.
2.5 2.5 2.5 5.0 5 0 2.5
2.0 2.0 2.0 4 0 4.0 1.1
Pyridine, ml .
1.0 1.0 1.0 1 5 2.0 1.0
3Iethanol, ml.
3.1 3.0 2.6 4 0 3.3 2 8
t,
O C .
105 112 120 130 130 120
Time, hr.
Yield, mg.
24 24 40 40
7 30 13 33 50 81
44
21
The last of the runs listed above represents about 2.6% of the theoretical yield. The combined yields obtained above (-0.20 g.) was extracted with 200 ml. of chloroform in a Soxhlet apparatus. Eight days were necessary t o dissolve most of t h e porphine, which began to crystallize out of the solution before the extraction was complete. The product was recrystallized by the addition of methanol to a chliiroform solution. The amount of purified material thus obtained weighed 138 mg. The compound was chromatographed on an 8-cni. by 3cm. talc column. A 25-mg. sample was dissolved in 400 ml. of hot chloroform t o give a deep orange-red solution which appeared green on the talc. Development of the chromatogram with more chloroform resulted in the recovery of the compound in solution and the formation of a 1 2 m m . green-browfl layer a t the top of the colunin, which was coated with tar impurities. Below this was found a 12-mm. tan-brown-green band. Spectrophotometric analysis of both bands in acetone solution gave spectra very t o t h a t of tetra-(p-nitrophen).I)-porphine itself. Anal. Calcd. for C1rHleKaOs: C, 71.55; H , 3.,5'7; N , 15.15. Found: C, 67.12; H , 3.03; S,12.60.
Acknowledgment.-The authors are indebted to Dr. Richard B. Martin for assistance with, and supervision over, the initial phases of this work. [VOKCESTER, MASS.
DEPARTMENT O F CHEMISTRY O F CLARK
UNIVERSITY]
Absorption Spectra of para-Substituted Tetraphenylporphines1s2 BY
DANIELm'. THOMAS AX9
L4RTIlUR E. -\IARTEI,L
RECEIVEDSEPTEMBER 29, I955 Absorption spectra of tetraphenylporphine, tetra-(p-methoxypheny1)-porphine, tetra-p-tolylporphine, tetra-(p-chlorophenyl)-porphine and tetra-(p-nitropheny1)-porphinein the ultraviolet, visible and infrared regions are reported. T h e visible and ultraviolet spectra indicate t h a t the para-substituents exert only a small effect on the electronic transitions of the porphine ring system. Assignments of infrared frequencies are made where possible.
The syntheses of tetraphenylporphine and its pmethyl, methoxy, chloro and nitro derivatives have been described in a recent p ~ b l i c a t i o n . ~I n the present paper are described the ultraviolet, visible and infrared spectra of these compounds, with a view t o determining the influence, if any, of the p phenyl substituents on the porphine ring and on the electron-donor properties of the central nitrogen atoms. A small amount of work has been reported on the infrared spectra of the porphines, b u t nothing has yet appeared on the tetraphenylporphines. Of the. ( 1 ) This research
infrared studies made on pyrrole-substituted prophyrins, assignments have been suggested chiefly for the substituent g r o ~ p s , ~and J few for the porphine structure itself. However, a number of investigations of N-H bonding have been reported by Falk and Urillis4 and Vestling and Downing.6 Visible and ultraviolet spectra have been reported for tetraphenylp~rphine.~-lONo spectra have (4) J. B. Falk and J. B. Willis, A u s t . 579 (1951).
J , S r i r n l . Research, 4, No. 4 .
(5) C. W. Craven, K. R . Reissmann and H . I. Chinn, J. A n a l . C h r m . , 84, No. 7, 1214 (1952). (S) C. S. Vestling and J. R . Downing, THISJ O U R N A L , 61, 3511 (1939).
(7) V. M. hlbers and H . V. Knorr, J. Chem. P h y s . , 9, No. 7, 497
was supported b y the National Institutes of Health of the U.S . Public Health Service under G r a n t No. G-3819(c). ( 2 ) Abstracted from a dissertation submitted by Daniel W. Thomas t o t h e Faculty of Clark University in partial frlfillment of t h e requirements for t h e degree of Master of Arts (3) I3 W. Thomas and A. E. Martell, THIS JOURNAL,78, 1335
(8) R. H.Ball, G. D. Dorough and M . Calvin, THIS J O U R N A L , 689, 2278 (1936). (9) G. D. Dorough and K. T. Shen, ibid., 78, 3939 (1950). (10) G. D. Dorough, J. R. Miller and F. M. Huennekens, ;bid., 73,
(1856).
4315 (1951).
(1941).
April 5 , 1956
1339
SPECTRA OF P-SUBSTITUTED TETRAPIIENYLPORPIIINES
been described for the remaining para-substituted porphines described above; they are new compounds with the exception of the p-methoxy derivative.“ Experimental The five tetraphenylporphines used in this investigation were prepared a s described p r e v i ~ u s l y . ~All compounds were recrystallized and separated chromatographically from chlorins and porphine-type impurities. Infrared Spectra.-Infrared absorption measurements were carried out with a Perkin-Elmer model 21 double beam spectrophotometer equipped with sodium chloride optics. All samples were run as both Nujol and hexachlorobutadiene mulls. Since Nujol absorbs strongly a t 2915,1460 and 1375 an.-’, and hexachlorobutadiene does not, the latter mulling agent was used t o determine the absorption bands overlapped by Nujol. Potassium bromide pellets were also prepared but the absorption bands were not a s sharp or as distinct in this medium as in the mulls. Visible and Ultraviolet Spectra.-Absorption spectra of benzene solutions of tetraphenylporphine and its p-CH3, P-OCHa, and p-C1 derivatives were measured in the 700-350 mp region in l-cm. quartz cells with a Beckman DU quartz spectrophotometer. Tetraphenylporphine and its p-NOn derivative were measured a s pyridine solutions. Readings of the optical densities were taken a t 5 mp intervals and a t more frequent intervals near the absorption peaks. The absorption curves repiesent smooth lines drawn through the plots of the corresponding extinction coefficients, which were calculated from the optical densities and the molar concentration of the solution by Beer-Lambert’s law. Measurements of tetraphenylporphine and its derivatives were obtained a t concentrations of M by weighing out the appropriate quantity of porphyrin and dissolving to 100 ml. in a volumetric flask. Tetraphenylporphine and its nitro derivative were dissolved by pyridine in 50-ml. volumetric
flasks. For regions below 450 mp, solutions were diluted t o 10-7 M .
Discussion Ultraviolet and Visible Regions.-The ultraviolet and visible absorption spectra of tetraphenylporphine and its p-methyl, methoxy and chloro derivatives in benzene are given in Fig. 1. The spectrum of the corresponding p-nitro compound in pyridine is illustrated in Fig. 2, along with tetraphenylporphine for comparison. Extinction values and wave lengths of the absorption maxima are listed in Table I. T h e spectra have been divided into two groups, the first in the region of 700 to about 450 mp, and the second from 450 to 350 mb. The absorption bands in the 700-450 mp region can be regarded as vibrational terms of a common electronic transition, while the intense band in the near ultraviolet region, the so-called “soret” band,
251 20
300 200
TABLE I ABSORPTION CHARACTERISTICS OF THE PORPHYRINS Porphine
Tetraphenyl-
Tetra-(p-methoxyphenyl)-
Tetra-p-tolyl-
Tetra-( p-chlorophenyl)-
Tetra-(p-nitro: phenyl).
Absorption maxima Benzene Pyridine Acetone e X 10-1 X, m p e X 10-8 X, mp
100
A, mp
647 592 548 515 485 419 653 595 555 519 488 424 650 592 550 516 485 420 647 592 550 515 485 421
3.4 5.3 8.1 18.7 3.4 478 4.5 5.5 11.9 17.0 4.3 485 4.1 5.4 8.2 18.9 3.7 490 3.7 6.0 9.0 21.0 4.0 515
647 592 550 515 485 420
3.9 5.5 8.6 18.7 3.8 464
647 590 547 513 480 6 50 593 553 517 483 647 590 547 513 480
650
600
550
.
A,
500
450
400
mr.
Fig. 1.-Visible and ultraviolet absorption bands in benzene solution of tetraphenylporphine --, and its parasubstituted derivatives: - - -, methoxy; -.-.- , chloro; and . . . ., methyl.
25
I
645 589 546 513 480 647 593 554 518 487 428
3.4 6.5 9.6 19.3 5’.0
250
(11) P. Rothemund, TEISJOURNAL, 61, 2912 (1939).
645 590 552 515 480
700
650
600
550
500
450
400
350
A, mp.
Fig. 2.-Visible and ultraviolet absorption bands in pyand its p-nitro ridine solution of tetraphenylporphine -, derivative -.-.-.
can correspond t o a different electronic excita-0 0 \ ++ tion. In,'? T h e intensities of the four main absorpN tiori bantls of tctra~)henylj)orl)lii~ie in the visible region arc itleiitical t o those previously published b y Dorough, et crl.'" However, the 419 nip band was found in the present work t o have x i extinction coefficient 20';; higher than thc value previously reported by Dorough and co-workers. This increased intensity was observed for all the derivatives of tetraphenylporphine illustrated in Fig. 1 and is probably due to increased purity o f these substances. The &nitro compound, which appears to be unique, does not show this increased absorption. It is apparent from the absorption bands illustrated in Figs. 1 and 2 , and listcd in Table I , t h a t small shifts in the absorption rnaxiiiia occur as the result of substitution of various polar groups in the paru-positions of tetraI)hen!.lporphine. I t is well known t h a t the substituent groups used in this investigation iiicreasc or clccre:ise the electron density in the ortho ant1 para 1)ositioiis in the benzene ring. For exainplc, thc nitro grouIi woultl be expected t o undergo resonmcc interactions with the porphine ring i n the nixniier indicated b y formulas I and I I , which invol\-e greater conjugation within the molecule. I n addition to the forms shom7n, other energetically reasoii:~hlc.reson:ince structures may be shown to coritributc tciwartl a hybrid in which the electron densities about the nitrogen atoms are lowered, mid the potential energies of ited states arc also reduced. T h e inductive effect illustrated for the nitro group would be re\-ersed for thc other substituents, which usually exert ;i reson:mce electron release t o an aromatic system in the order -OCHa > --CHa > -C1. In the latter case the inducti\-ts electron attractive effect of thc halogen would be expected to more than balance the resoiiance-tl;pe of electron release exI I cept under crrtain conditions favoring such electron release. Many of the alisorptioii bands of the para-substituted derivatives exhibit siiiall shifts t o longer wave lengths as coinpared t o the etio-type spectrum of tetraphenylpori)hine, while other bands showed no change in position. I n no case, however, was the maxiiiiuiri of an absorption band displaced t o lower wave length than t h a t found in the parent sorption bands of tetraphenylporphine decrease in compound. the order -OCH3, -NOa >> -C1 > -CH3. If one The most pronounced bathochrorllic shift occurs neglects the direction of the effects of these groups in the niethoxy derivative. In this case the main this is also the relative order of interactions of these absorption peak in t h e Visible region occurs at 519 groups with aromatic systems, such as the benzene mfi, and its intensity is decreased somewhat, while ring. Apparently the intensification and bathothe intensities of most of the weaker bands in this chromic shifts in the absorption bands take place group are considerably increased, and are shifted t o regardless of t h e directional effect of the polar a greater extent than the niain band. Tetra-(P- group, and this is in accord with the influence of chloropheny1)-porphine, o n the other hantl, shows such groups on the K-type absorption bands of only minor shifts, but the intensities of all the JIeakS other resonating systems such as are higher than those of the purent tetrapheiiylporI 1 phine. Like the niethoxy derivative in benzene, I-(C=C)-R the spectra of t c t r a - ( ~ - ~ i i t r o p h c ~ i ~ l ~ - ~ ~ion r p hTihnee introduction of a resonating group X at one pyridine also shows ;L proiiouiiced shift t o the red end of a general system of this type usually profor a majority of bands, including an h rnp displace- duces a bathochromic shift of the electronic transiment of the much less intense sorct peak. tions, with a corresponding intensification of the I n general, i t may be concluded t h a t the extent absorption bands. of the influence of substituent groups on the abX factor which must be considered in the interpretation of the spectra in the visible and ultravio(12) F. H a ~ ~ r o w i at nz d ly. Klernrn, Bev., 68, 2312 (10331
April 5 , 19.56
SPECTRA OF P-SUBSTITUTED 'rETRAPIIENYLPORPIIINES
1341
let regions is t h a t the phenyl and substituted vibrations, which usually occur near I (io0 C I H . I, phenyl rings cannot be coplanar with the porphine have been reported by Bellaniy13to be frequently nucleus. Examination of molecular models indi- shifted to slightly higher frequency by pau-sul)sticates t h a t the four benzene rings have partial rota- tution. The most pronouncetl shift of this t!.pe tions which cannot bring them within 60" of being may be found in the P-niethosyphenyl band, whirl1 coplanar with the resonating porphine system. is found a t l(i10 cm-1 compared to 159i in tetraSince the average angle of the attached rings is phenylporphine. Although listed in Table I I ;is probably considerably greater than this, it is ap- tnediuni, the intensities of these baiitls in the / I parent t h a t resonance interactions between the nitro-, methyl- and inethoxy coinpountls ;ire colitwo aromatic systems must be greatly reduced from siderably enhaiiccd over those of tetr~tl)Iieiiyl~)i~rwhat would be expected if they were coplanar. phine. The skeletal iii-plaiic conjuga tctl phenyl Hence, the bathochrornic shifts and spectral inC tensifications observed are considerably less than C- vibrations appear near 15SO ~ 1 1 1 . -I . 'I'hese what would be expected in simpler aromatic sys- absorptions usually appear whciie\.er the ~ ) h e n y l tems with greater freedom of rotation. ring is further conjugated i n so111eiiianiicr, accortlT h e small shift in the maxima of all bands to ing to Bellainy'"" antl Fuson.I4 111 this c':ise, a t lower wave length when the solvent was changed least partial conjugation niay be statetl to take from benzene to acetone, as indicated in 'Table I , is place with the porphiiie riiig. a normal occurrence for such a variation of solvent. The weak antl iiiccliuni lxtiitls assignetl to I)yrroIe In the compounds of the present investigation, or niethine C C stretching vibrations ha\.e ;t this shift or solvent effect is not large enough to be soinewhat lower frcyuency ( 1 3 i O - I .iCi.ic'iii. I ) t l i a i i considered significant. t h a t usually found (L I(i00 c'iii. ' i for stretching Infrared Spectra.-The frequencies, approximate vibrations. T h e lower frequeiie!. i i i this c';isc is intensities and some assignments of the infrared probably due to conjugatioii with atljacent siiixlc absorption bands of tetraphenylporphine antl its bonds. Phcnyl rings also exlii1)it alisorptioii iie;tr para-substituted products are listed in Table 11. 1.700 c i i i . ~ - ' ( I N l > l 2 C I I I : - ' J wliicli is ofteii In addition to the frequencies given, the spectra of shifted to higher frcqwncies with /Iui.o-sulistitupyrrole, benzaldehyde and its p-methyl, methoxy, tion. chloro and nitro derivatives were also determined T h e strong frequencies iiear I ;L-)O cni. I ;ire to aid in the assignment of frequencies. tentatively assigned to the C S stretching vibraThe N-H . . . . N stretching frequencies were tion. Observation of the iioii-ioiiic resoiiance found t o be very weak in Nujol mulls, but to be forms of porphine indicates two forills with C much sharper and about twice as intense when hexa- S-- antl six with -C N boiitls. Hen chlorobutadiene was used as the mulling agent. vibration would be expected to h:t\.e eoiisitleral)le Therefore, the frequencies listed for this absorp- single bond character nntl to occur a t a longer iv;tve tion band in Table I1 were measured in the latter length (;.e., lower frequency ) . Thus the aYsigiic(1 medium. The K-H stretching frequency, which frequency is much lower thaii t h e v;dw u s u a l l ~ ~ occurs at 3300 cm.-I in pyrrole, occurs a t 33.50 found (l(i00-1650 cni.-') for -- C S groups. cm.-' in tetraphenylporphine, an indication of The absorption bands found in all porpliyriiis stronger hydrogen bonamg in t h e condensed ring 1ic;tr 1440 c t n - l havk-been assigned to the C I3 systems of the porphines. I t is t o be noted also bending vibration of pyrrole. These absorptions t h a t replacement of the p-hydrogen by an element were absent in the :ildehytles but were present i i i more electronegative than carbon (;.e., b y substi- pyrrole. X C--H bending vibration i n this rcgioii tution of methoxyl, chloro and nitro groups) there (147'0 cni.-l) also has been assignetl to pyrrole in is a slight increase in the N-H stretching fre- pyrrole-substituted porphyrins by Cra\ e n , Iieissquency. Thus, i t appears that hydrogen bonding is mann and Chinn." somewhat decreased by the substitution of these T h e C-H bending vibrations of 111ethy1jiroul)s electronegative elements. The decrease in hydro- give rise t o absorptions near I450 c111.--~.'I'hus gen bond strength indicates t h a t the influence of frequencies of 1452 and 1457 C I T I . - ~ have heen giveii the substituents is to decrease the electron density the assignments of antisymmetric - C-13 vibratioiis about the central nitrogen atoms. Hence, the influ- of the methyl and methoxy groups, respectively ence of the para-groups is due to their inductive Absorption bands also were found i n this region of effects rather than their resonance interactions. the spectrum for the corresponding aldehydes. This conclusion is reasonable in view of the non- Symmetric deformations of the methyl derivative codlanarity of the porphine and benzene rings de- occur at 1375 cm.-l. scribed above. T h e shift t o lower frequency in the A strong absorption a t 124i mi. has been idenP-tolyl derivative indicates t h a t hydrogen bonding tified as due t o the -C-0- stretching vibration of is perhaps somewhat stronger in this compound. the methoxy substituent b y comparing the specT h e frequencies listed from 28.70-3140 cm. - 1 are trum of the corresponding porphyrin with that of due to =C-H stretching vibrations of the phenyl anisaldehyde. Strong absorption at 1173 ~ 1 1 1 .- 1 and substituted phenyl rings, and of the pyrrole has also been attributed t o the methoxy group. rings. All of these probably occur at t h e higher (13) L. J. Bellamy, "The Infrared Spectra o f Complex . \ I o l e c u l c i , " part of this frequency range, while the -C-H London, hfethuen; J o h n Wile? a n d Scms. Inc , SCW York. S . Y,, I, stretch of the methyl groups would be found in the p. G O ; (a) p. 00-61; (b) p . l j 9 ; icl p. 271 (14) N Fusnn. R . G . F i w k r , H 31. I
