Linear dichroism of free base tetraphenyl porphin

Volume 37, number 3

LtNEAK

CHEMICAL PHYSICS LETTERS

DICHROISM

Bengt NORDeN

OF FREE

BASE TETRAPHENYL

1 FCbrEaiy 1976

PORPHIN

and ,&ke DAVIDSSON

horgarlic Chemistry 1. Chemical Center, Universify of Lund, S220 07 Lund 7, Sweden

Rcccived 18 September

1975

Linear dicluoism (LI>) spectra 0: tetraphcnyl matrixes were studied. A negative LD component

porphin in oriented polyethylene, polypropylene and polyvinylchloridc at 400 nm is su ggested to be due to an out-of-plant pohrised transition. From evidence of changed polarisations within the (0,O) visible bands it is swgcstcd further that the matrix may play an active role in perturbing the electronic spectrum by stimulating vibronic transitions with differing pohrisations.

Ei, i =x, y, z of the diagonal orientation sorption coefficient tensbr, respectively,

1. Introduction Recently the behaviour of tetraphenyl porphin (TPP) in oriented polyethylene matrix observed by linear dichroisnt (LD) [I] has been reinvestigated and compared to the effects in liquid crystals as orienting solvents by Gale et al. [2]. The conclusion about inplane (.uy) polarisations of the Q band, as well as of the Soret (B) band, *&as thereby confirmed, but the authox claimed on the basis of an observed constant dichroic ratio throughout the entire spectrum that all electronic transitions have the same poIarisation and that no component with negative LD contribution is present at 400 nm (such a component was earlier proposed due to a z-polarised n + x* transition [l]). The authors ,also claimed that the oriented polyethylene matrix technique yields a lower degree of orientation than any of the liquid crystal methods and that it might be less reliable due to surface xd crystallisation effects. We here comment on these statements by reference to LD measurements on TPP employing some different glassy polymer matrices.

2. Qualifications

LD/A = LD/{A -!oglo =

tensor and abaccoiding to

[2(2 f IO-LD + IOLD)-‘l’]}

6P/(2Q + P),

where P = CIU_,E, + fyEy + fzEz>,

(1) Q = CI(E,

f Ed f

EJ.

If in a given matrix the orientation OF a solute molecule does not depend on its concentration C (this is for instance not expected if different sites are SUCMSsively occupied) the ratio LD/_4 is also essentially invariant.

Y

of the method

Assuming utiiaxial orientation the observables LD (=_A,, - A,) andA (for notations see refs. [3,4]) are related to the elements fi (G SMXiXi in refs. [2,5]) and 433

Volume 37, number 3

CHEMICAL

PHYSICS

Is it now under any circumstances possible to determine three different non-zero components ei by the present method’? Let the molecule have C,, (or D& symmetry and assume that two mutually perpendicularly polarised n 4 n* transitions with pure polarisations x andy (O-O transitions can be used) are identified with confidence. All orientation parameters are then determined in eq. (2) [3], let us say that the order fx> 4,> fz is obtained, i.e. x is a “main orientation axis”. Hence it is realised [eq. (I)] that the absorption suspected to be z-po!arised (E_~= 0, LD/A negative) can be equahy welly-polarised unless

LDl-4 < 6f;,/(2

‘I--_.)-

(2)

The inequality (2) can thus theoretically be used to prove the presence of an out-of-plane polarisation. Unfortunately the situation when x andy polarised transitions can be found both with pure polarisation is very.rare, indeed the common procedure is to start with an assumption that any transition is either x ory polarised, then the maximum LDI.4 is picked out to determine e.g. fx and the minimum value is referred to the y-transition. With this background the possibility of using two or more different matrices [3] yielding different (nonproportional) sets (_KX ,f,, fz) seems a promising alternative. The idea is sketchily described in terms of solving the equation system of eq. (1) due to thz different matrix experiments with respect to ei. The objectivity might be questioned and perhaps a more safe form-ulation is generated in eq. (3)

(3)

A,;- A, (A, = absorbance

of random solution) is generally not determined with the same precision and accuracy as a differentially measured LD. On the other hand it can be shown that eq. (3) does not assume uniaxial orientation and may thus be expected to be useful e.g., in the case of a Couette flow, i.e., when the

distribution of the third Euler ande is far from uniform and where A, is easily determined

(stopped flow).

434,

as A ,,

LiZTITERS

1 February

1976

3. Experimental Two commercial polyethylene sheet qualities were used, PE Labrock and PE DFDS 033 1 (Unifos Kerni AB, Sweden). They differed only slightly with respect to degrees of crystallisation and oxidation and the main difference in orienting property may be referred to as a higher preorientation in PE Labrock. Isotactic polypropylene (PP, Perstorp Homo, Sweden) was used. Polyvinylcliloride sheets were manufactured by dissolving the sample (typically 0.1-2 mg) in 10 ml of a solution 10 g PVC (high MW, BDH, England) in 75 ml tetrahydrofurane (distilled over anhydrous MgS04 + FeSO,). This syrup was transferred to a glass plate (20 cm2) and the tetrahydrofurane was allowed to

evaporate at a controlled partial pressure, so that the sheet was dry after three days. The sheets were stretched

to R times their original length (PE, PP at room temperature, PVC at 60°C). LD spectra were recorded using a Jasco J-40 circular dichroism spectrometer supplemented with an Oxley prism. LD was corrected for the chromaticity of the prism and for non-linearity of instrument to be accurate within a few percent [6]. Absorption spectra were recorded on a Cary 118-C spectrophotometer. The monochromators of the two instruments were matched by using some 20 lines of a mercury arc. TpP was a gift from Dr. L.Y. Johansson.

4. Results Representative LD spectra are given in fig. 1 for TPP in various matrices. Characteristic details, as positions of extrema etc., of LD andA spectra are collected in table 1 together with some considerations about 5 and the possible polarisations. In PE DFDS a fairly constant LDjA over the range 430-700 run indicates that the x and y molecular axis are practically degenerate in an orientational sense, i.e., that fX =f,. However, at about 400 nm an LD/A is observed which is definitely lower than any of the values associated with the Q, and Q, O-O bands. Tftis fact may be taken as evidence fcr a z polarised transition at 400 nm. In PE Labrock, which was the matrix in ref. [l] , as well as in PE DFDS at R = 5 the pres-

ence of a negative LD component in this region is further pronounced by the direct observation of a nega-

Volume 3?, number 3

CHEhPICAL PHYSICS LETTERS

L February t976

J, L+___ :I jr&, -331

Wovelength

4m

5m

700

600

LCO

500

@Ilo

polymer

sheets:

700

fnm

Fi+ 1. Limzar djckojsm (abovs) and absorption (below) spectra of tetraphenyl (a) PE DFDS,R = 3.0, (b) PP. R = 4.0, (c) PVC, R = 5.0.

porphin

in thice stretched

Table 1 Spectral

observations

Band notation

Q.&O) Q,CO, 01

b

a)

c

(645)

653 645

if

590 548

592 550

+ +

650

various matrices PE DFDS 10 pm, R = $0

PE J_,abrock 20 I-rm, R = 3.0

u

Qx@,O)

for TPP in

u 6.50

b 651

c 0.29

646s 590 545

591 547

0.20 0.25

PP

20 pm, R =3.0 b

c(R = 5) a 650 0.47 0.38 0.31 0.31

655

595

597 562 551 54.55

550

Q+dL 0) Long0 B

517 f 480 f 430s f 420 f (407s -) 391 290 (--)

515 480 (430s) 416 3915

Orient. I Orient. II

416 400s

515 481 430s 420 400s 392 290 fx=

0.27 0.30 0.16 0.15 (+) O.l--fv

0.37 0.35 0.80 0.34 -0.01 -0.11 +0.1

f- = 0.3

516 417 39.5

522 485 430 420 394

0.17

0.06 0.03 0.26 0.28 0.59 0.37 -0.49

Possible polarisetions

=5

Q

b

c

644

656 640

0.096 -0.13

590 552

590 553

0.24 0.31

515 480 420

514 481 420

0.19 0.21 0.29

400

400

0.18

0.05

-0.1 290 fz = -0.2 &= 0.22 > fy

--~_

af u = ~(~~~)~nffl, tive deflection.

516 480

c

PVC ZOOpm,R

[,=0-l

>fx

-

b = h(LD ma~~~nrn, c = L!l/A (dinlensionless).

However

at higher concentrations

in

PE Labrock, a split B band and variations in LDIA might suggest the possibility of dimer formation [7]. In PP and in PVC the negative LD contribution at 400 nm is exhibited as a negative and a decreased LDfA, respectively, but in these matrices the main attention is drawn to two new negative LD components, nameiy at 55 1 and 641 nm, respectiveiy. With the background of the FE DFDS cafe and assuming the identical set (ex,, ev, Q, an obvious explanation (Alt II, table 1) would be preferred orientations,& >&

+ fz FrrPP &dfy >Ix >“fi in PVC. The orientational degeneracy lift& between x and y could be understood remembering the symmetry lowering from D4,, to Dzh by the NH hydrogens, the orientation being effected by e.g. a molecular quadrupole moment. In fig. 2 the variation in LD with concentration is indirectly studied by the dependence on A for some f3ms of equal thicknesses (it was checked in PVC so %hat A corresponded linearly to Q. Since the formation of polymeric species or of crystallites would modify the orientation, the observation of a fairly con43.5

Volume 37, number 3

CHEMICAL PHYSICS LETTERS

1 February 1976

From simple LCAO MO theory the transitions responsible for the visibte-UV regic I of porphins may be derived from one-electron 7 + rr* transitions .leading to a ground state 1At, = (a:,as,) and two excited states i E$ = (a~a~ue~): Igi = (aia$,e,‘) both of which are doubly degenerate. Configuration interac-

tion b-_ween the excited states is claimed [8] to be the reason for transitions to the lower state (Q-,,) to

Fig. 3,. LD and A for TPP in some films of equal thickness: APE,R=5.0,1=20~m,h=515nm;0PVC,R=2.8,1=20 gm, k=515 nm. stant LD/A is taken as evidence against any important conta~ation in PVC and PE DFDS.

5. Discussion The application of eq. (2) may be illustrated by observing that LD/A = -0.5 at 394 nm (and -0.1 at 290 nm) < 0, and assuming that [\, is determined by LD/A a 0 from the Qy band, this provides the existence of z polarisation in the 394 m-n band (and in the 290 nm band). However, in what concerns an application of eq. (3) the requirement of an invariant (eX, ey, E=) seems indeed not fulfilled. Thus a conclusion about solvent sensitive absorption coefficients is reached both from the observation of band shifts in the absorption as well as from. the appearance of new bands in LD virtually due to splittings of the 650 (PVC) and 550 (PP) nm bands. The new LD bands at these wavelengths may be explained by either of two mecha nisms: firstIy, if a solvent shift is cxmbited by a defiled fraction of mcIecules which are subject to orientation via association to the solvent polymer, xy-polarised transitions can contribute preferentially to Au, while the r~rn~~~g random fraction is equally represented in A,, and A,. Hence an increase in LDM on the so!vent shift side of the absorption band is observed, virtually ~~cat~g the presence OF a z-polarised transition on the opposite side. The second alternative which we here fmd the more plausible one, is then simply the change of (eX, eyT eZ) due to the solvent interaction in such a way that new transitions with poiarisations perpendicular to the main orientation axis (or plane) gain intensity. 436

‘:

‘.

be forbidden and have much lower intensity than for the W band (B,,). In the free base TPP (D,, symmetry) the QqrJ bands are split into x and y components. In table 1 (?l, 0) refers to the electronic origin and (I, 0) is the fundan~ent~i in some vibrational mode. The directions are identified so that x = NH . .. HN line according to fluorescence polarisation observations by Weigl [9] and ~gurneRts by Gouterm~ is]. A recent suggestion that the visible bands are due to n + rr* transitions [lo] is ruled out by the observation of in-plane polarisation fl] and also by other considerations [ll] . However, the present fnidings of negative LD contributions in the visible imply that we cannot eliminate t!le possibility of existing weak n (aza ione pair) -+ rr* transitions. Such a possibility has also been suggested from the observation of a line broadening of 9, when passing to higher vibrational energy [l I]. The shoulder (observable in PE, PP) in the absorption on the low-energy side of the B band and the corresponding larger LD/A seems interesting in the light of recent discussions [ 1 l] about a possible splitting of B_ry into B, and By bands in Dzh. The shoulder might thus be associated with B,. It seems not surprising that the solvent dependent polarisations are found at the O-O bands. Substitution in porphins usually strengthen the Q-JO, 0), Qy(O, 0) bands leaving almost unchanged the (L , 0) bands. Thus also in glasses forbidden O-O bands are known to be weakly allowed [ 121. The presence of the polymer chain oriented p~ralIe1 to some direction in the xyplane may yield a perturbation (of BLg symmetry in Ddh when parallel to x or y, BZg when crossing diametral phenyl groups) which can mix B and Q states (giving increased intensity to Q,, and QX, respectively) or effect a local symmetry in which the transition in question is allowed. However, the negative LD at 644 nm in PVC and the simultaneously low LDIA at 650 and 550 nm may rather indicate that new vibrations are stimulated, transferring the intensity into another

Volume 37, number

3

CHEMICAL

PHYSIC’S LETTERS

poltisation. Depending on how the molecule is oriented (orientation alternative I or II) the observations may be explained by either z-polarised components (effected by e.g. a Bzs ~b~at~o~ in Q,, ES, in Qy) or (II) by y (Bls vibratipn) or x (Big vibrationf polarisations, respectively. In view of the fact that the molecule is at first hand oriented by the xy-plane par-

allel to the polymer axis, at second hand only some direction

in the plane is preferred, it appears tentatively that the complex formation will attenuate E&, B3g vibrations. This idea is also supported by the expected larger LD sensitivity to a changed Ed than to a changed E_, in the case of a planar (xy) molecule, reasonable

since 4 is weighted by -U; ff,). In a calculation by McHugh and Gouterman

[I31 it was concluded that the Longo band is due to either a Q(2, Oj or an n -+ 5~* transition. The latter altcmative seems ruled out by the large positive LD at 480 run. To summarise, the present results disagree in the following aspects with those of Gale et al. Firstly we find that LD/A (or the dichroic ratio) is not constant

~~~r~~~ou~ the entire spectral range (as was claimed 121) in any of the investigated cases. On the contrary, a negative LD band is frequently obsenied at about 400 nm with an LD/.4 magnitade that strongly suggests that it is derived from an out-of-plane pblarised transition. The reason for the discrepancy seems to be the fact that Gale et al. have employed a less well defined matrix order (R = 2) with possibly non-uniaxial symmetry. The statements that the stretched sheet method is intrinsically sensitive to crystallisation effects and gives overall lower molecular orientation than in liquid crystal methods, have further been found ir-

1 I‘cbruzry

1976

relevant. Actually, the orientations we have obtained are considerably higher than those observed for TPP in either of the liquid crystals [2] _ From different LDJ.4 behaviour in different mztrices in the visible region we conclude .tha t the matrix plays an active role by perturbing the electronic spectrum. Negative LD contributions can be interpreted in terms of z-po~a~~tions (II + z* transitions or out-of-plane vibronic 7~+ X* components) but we cannot rule out the possibility that they may still be in-plane polarised.

Referents [ 1: A. Davidsson, Xl.Gouterman, L.Y. Johansson,

R. Lnrsson, B. Nordc’n and hi. Sundbom, Acra Chcm. Stand. 26 (1972) 840. [2 ] R. G&T, R.D. Peacock znd B. Samori, Chcm. Phys. Letters 37 (1976) 430. [3] .&.Davidsson and B. NordCn, Chcm. Phys. Letters 28 (1974) 221. [4] A. Davidsson and B. NordEn, Chcm.&ripls (1975) to

hc publis~cd_ [S] A. Su~pe, Mol. Cryst. Liquid Cryst. I (1966) 527. [6] .&. Davidsson and 8. NordCn, Chem. Scripta, to bc published. [?] R-R. f)as, J. Inorg. NM. them. 37 (1975) 153. 18) hi. Couterman. J. Mol. Spcctry. 6 (1961) 138. 191 J.W. We& J. Mol. Spcctry. 1 (1957) 133. [IO] A.H. Convin, A.B. Chivis. R.W. Poor, D.G. Whitten and E.W. Baker, J. Am. Chem. Sot. 90 (1968) 6577. [I 11 R. Gale, A.J. hfcC3ffcry and Tk1.D.Rowe, J. Chcm. SOC. Dalton (i972) 596. ,[ 121 H. Shull, J. Chcm. Phys. 17 (1949) 295. [ 13) A.J. Xictlugh and 5f. Gouterman, Theorct. Chim. Acta 24 (1972) 346.

437