Site-selection spectroscopy of magnesium and cadmium

Site-selection spectroscopy of magnesium and cadmium tetrabenzoporphines in n-octane ... Note: In lieu of an abstract, this is the article's first pag...
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56

J. Phys. Chem. 1981, 85,56-63

Site-Selection Spectroscopy of Magnesium and Cadmium Tetrabenzoporphines In n-Octane R. J. Platenkamp” and G. W. Canters Center for the Study of the Excited States of Molecules, Gorlaeus Laboratory, Uriiversm of LeMen, The Netherlands (Received: August 19, 1980)

Site-selection spectroscopy of MgTBP and CdTBP in n-octane yields highly resolved fluorescence, excitation, and phosphorescence spectra from which the vibrational frequencies in ground and excited states have been determined. The degeneracy of the first and second excited singlet states (lEUin D4h) is lifted by the crystal field. Values of crystal-field splittings for various sites are reported. Zero field magnetic resonance studies in the triplet state of CdTBP reveal that the zero-field splitting parameters D and E vary strongly with the site studied. Zeeman experiments on the lowest excited singlet state of CdTBP provide a lower limit for the orbital angular momentum Ah in this state of 2.3h. When assuming that the low-frequency mode of 150 cm-’ is Jahn-Teller active, one finds A = 3.7 and a value of 1.27 for the deformation parameter CY.

Introduction The metallotetrabenzoporphyrins (MeTBPs) are symmetrically substituted metalloporphines (MePs), point group D4,, (Figure 1). According to Gouterman one may expect their electronic structure to be similar to that of the MePs, and he has shown that the four-orbital model which fairly well describes the MePs also holds for the MeTBPs.l A degenerate S1 So transition (Q band) and a degenerate S2 Sotransition (Soret band) are predicted. Both transitions are polarized in the molecular plane. The large component of the electronic orbital angular momentum along the z axis in the SI state of MePs, measured in units of h by the parameter A, should only be quenched to a small extent by the presence of the benzo groups. The calculated value for A is 4.35 for the MePs2 (experimental values range from 4.2 to 4.4334),and it amounts to 3.7 for the MeTBPs.2 Spectroscopic studies on the MePs have shown that the degeneracy of the lowest excited singlet state is removed when the MePs are incorporated in a Shpolskii host such as n - o ~ t a n e . ~ ?This ~ ? ~lifting of the degeneracy is induced by the action of the crystal field on the Jahn-Teller unstable S1 state. Values for the crystal-field splitting (6) between the two orbital components of the lE, state range from a few to more than 100 cm-l, depending on the central metal and the host. By convention we label the orbital components by Q, and Q,,, the x state being the lower in energy. Analysis of the Zeeman effect showed that there is a weak Jahn-Teller interaction in the case of ZnPM and MgP,’ whereas until now no effect of a possible Jahn-Teller coupling could be detected in the orbital Zeeman spectra of PdP. To gain insight into the effect of benzo groups on the orbital angular momentum, the Jahn-Teller coupling, and

-

-

(1)M. Gouterman, J. Mol. Spectrosc. 6, 138-63 (1961); “The Porphyrins”, Vol. 111, D. Dolphin, Ed., Academic Press, London, 1978, Chapter 1, pp 1-166. (2)A. J. McHugh, M. Gouterman, and C. Weiss, Jr., Theor. Chim. Acta, 24,346-70 (1972). (3)G.W. Canters and J. H. van der Waals in “The Porphyrins”, Vol 111, D. Dolphin, Ed., Academic Press, 1978,pp 531-82; G. W. Canters, J. van Egmond, T. J. Schaafsma, and J. H. van der Waals, Mol. Phys., 24, 1203-15 (1972);G.W. Canters, G. Jansen, M. Noort, and J. H. van der Waals, J. Phys. Chem., 80, 2253-9 (1976). (4)E. C. M. Kielman, H. P. J. M. Dekkers, and G. W. Canters, Mol. P h y ~ . ,32,899-919 (1976). (5) G. Jansen, M. Noort, G. W. Canters, and J. H. van der Waals, Mol. Phys., 35,283-94(1978);G. Jansen, M. Noort, N, van Dijk, and J. H. van der Waals, Mol. Phys., 39,865-80 (1980). (6)M. Noort, G . Jansen, G . W. Canters, and J. H. van der Waals, Spectrochim. Acta, Part A, 32, 1371-5 (1976). (7) R. J. Platenkamp et al., to be submitted for publication. 0022-3654/81/2085-0056$01 .OO/O

the crystal-field splitting in the SI state of the TBPs, we performed a spectroscopic study of Mg- and CdTBP. Several reports appeared during the last decade about high-resolution spectroscopy of TBPs, but the occurrence of a crystal-field splitting and of Jahn-Teller coupling does not seem to have been investigated. Work by Gradyusko et al. has shown that high-resolution optical spectra can be obtained when the MeTBPs are incorporated in n-octane at low temperatures.8 The spectra always show a number of multiplets ascribed by the authors to different sites occupied by the molecules in the lattice. Fielding and Mau showed that in the case of ZnTBP one can improve on this situation by the use of site-selection technique^.^ They obtained single-site absorption and emission spectra of high quality, yet no doublet structure in absorption was observed. In this study we report the optical spectra of Mg- and CdTBP incorporated in a single crystal of n-octane at 4.2 K. When one uses site-selection techniques, the expected doublet structure is observed in the absorption spectra, even in the Soret band. The Zeeman effect in CdTBP reveals that a possible Jahn-Teller coupling is very weak and that the large oribtal momentum present in the MePs is conserved to a considerable extent in the TBPs in nice agreement with theoretical prediction of McHugh et ala2

Materials and Methods MgTBP and CdTBP were a gift from Professor T. J. Schaafsma. Details of the crystal preparation can be found elsewhere.1° Speed of crystal growth was 5 cm per 4 h. For the optical experiments in the case of broad-band excitation, a 900-W XBO lamp was used as an excitation source. The light passed through a CuS04-5H20filter (25 g/L, path length 5 cm) and a small monochromator with a bandwidth of 20 nm. In the site-selection experiments a Molectron DL-200 dye laser (bandwidth 0.3 cm-l) was used for the excitation; this laser was pumped by a Molectron UV 22 nitrogen laser. Rhodamine 6G and B were used as dyes for excitation into the Q band, and Coumarine 120 for excitation into the Soret band. The emission was dispersed with a 0.85-m Spex double monochromator (bandwidth 1 cm-’) equipped with either an EM1 9558 or (8)A. T.Gradyusko, A. N. Sevchenko, K. N. Solov’ev, and S. F. Shkirman, Sou. Phys.-Dokl. (Engl. Transl.), 11, 587-90 (1967). (9)P. E. Fielding and A. W. H. Mau, Aust. J. Chem., 29, 933-40 (1976). (10)G. Jansen and M. Noort, Spectrochim. Acta, Part A, 747-53 (1976).

0 1981 American Chemical Society

The Journal of Physical Chemistty, Vol. 85,No. 1, 1981 57

Site Selection of Tetrabenzoporphines

TABLE I : Band Positions in the Single-Site Emission Spectra of M g (Fluorescence), Cd- (Phosphorescence), and ZnTBP (Fluorescence) in n-Octane at 4.2 K” ZnTBP MgTBP 15962

C dTBP 12335 23 9 252

485 4 91

484

ref 9

ref 8

15928 131 24 2

137 244

264 482

486 51 2

Flgure 1. Structure formula of a metailotetrabenzoporphine molecule. M represents the metal atom.

555 586 703 71 2 727 739 74 8 827

MgTBP in n-CB Emission 4.2K

54 8 552 6 51 6 99 719

568 703

703

74 5 827

740 827

842, 846 955 1017 1158 1237 1250 1321 1331 1340

958 1074 1125 1160 1238 1251 1324

1066 1123 1159 1253

1334 1341 1351 1450 1457 1456 1565 1573 1570 1619 1626 1624 Data for ZnTBP are taken from ref 8 and 9. Peak positions are given in cm-’with respect to the 0-0 transition mentioned at the head of the columns and have an accuracy of + 3 cm-’.

15962

”

620

’

630

650 hnm

6iO

’

Figure 2. Fluorescence spectra of MgTBP in n-C8 at 4.2 K under broad-band excitation in the Soret band (upper trace) and under monochromatic (bandwidth 0.5 cm-’) laser excitation in a vibronic absorption band (588.30 nm) of site I. The positions of the 0-0 transition of sites I and I1 are indicated in cm-’ in the upper trace.

513

650

660

670

580

690

A nrn

Figure 3. Single-site fluorescence spectrum of MgTBP in n G 8 at 4.2 K under laser excitation in the 0-0 band of site I(628.49 nm). The positions of the bands are indicated by their distance from the 0-0 band in cm-’. At wavelengths below 640 nm, scattered light prevented the recording of the spectrum.

an RCA 31034 A cooled photomultiplier. On excitation with the pulsed laser, the detected signals were averaged in a dual-channel boxcar integrator (PAR 162) operating in the ratio mode to calibrate intensities. The signal of a photodiode receiving a fraction of the laser light was used for this calibration.

In the Zeeman experiments the same optical equipment was used. Magnetic fields (up to 10 T) are generated by a split pair superconducting magnet (Thor Co.). The magnet is incorporated in a special cryostat which allows for horizonal optical access to the sample space through four side windows. The crystal was mounted in a holder with a special gear system such that all orientations of the magnetic field with respect to the molecular axes were attainable. The shifts of the spectral lines were calibrated with an air-spaced FPA etalon with a free spectral range of 9.05 cm-I (Technical Optics). The experiments were performed at 4.2 K. In the ODMR experiments (performed at 1.3 K) a CW dye laser (Spectra Physics 375) working with Rhodamine 6G and pumped by an argon laser (Spectra Physics 171-17) was used for excitation. Phosphorescence was detected via the previously mentioned Spex monochromator, now with a bandwidth of 2 cm-l. Microwaves were obtained from a H P 8690 B sweep oscillator with a HP 8699 B plug-in unit (0.1-4 GHz). Use was made of a H P 5480 A signal averager. The dye laser was tuned to a particular site, and magnetic resonance observed as a change of the intensity of the 0-0 transitions in the phosphorescence.ll Results Absorption and Fluorescence of MgTBP. The fluorescence spectrum of a single crystal of MgTBP in n-octane, excited into the Soret band with a bandwidth (11) J. Schmidt, Thesis, University of Leiden, The Netherlands, 1971.

58

The Journal of Physical Chemistty, Vol. 85,No. 1, 1981

17081

831

I

620L

578

15781

I

I 59L

576

123L1 235

I 606

600

I

-

I

6163

707

18321

I 570

Platenkamp and Canters

I 582

I 612

I 618

I

588

hlnrnl

Figure 4. Single-site SI So absorption spectrum of MgTBP in n-C, at 4.2 K of site I. The spectrum was recorded with the monochromator set at the wavelength of the 0-0, transition (626.49 nm) and by scanning the wavelength of the dye laser. The peak at 626.49 nm therefore represents the signal observed when scanning the laser across the spectral window of the monochromator under low gain. Positions of the 0-0, and 0-0, transitions are indicated in cm-I (15 962 and 15 992 cm-I). Numbers above the vibronic bands denote their positions in cm-l with respect to the 0-0, transition or, if they are in parentheses, to the 0-0, transition. The detailed spectrum of the origin In the insert of the figure has been recorded with the monochromator set at a vibronic emission band (655.35 nm).

of 20 nm, is shown in Figure 2 (upper trace). In a separate field splitting of 30 cm-’. This crystal-field splitting is also (conventional) absorption experiment we observed that the seen in the vibronic lines. We have carefully checked lines seen in this region of the fluorescence spectrum occur whether the absorption and emission spectra really belong at the same positions in the absorption spectrum, and we to one species, by looking at the emission spectra when therefore attribute the lines in Figure 2 to origins of exciting in the strongest lines of Figure 4 and, conversely, MgTBP molecules differing in local environment. Reby measuring the excitation spectra of the strongest lines in Figure 3. The results are consistent with the assignment garding the source of these local differences, one could think of variations in site, solvation, or a g g r e g a t i ~ n ; ~ l ~ J ~ of J ~the spectra to a single site. The vibronic frequencies in the rest of this paper we shall use the term “site” to refer appearing in the absorption spectrum are listed in Table to a particular origin. The lower trace of Figure 2 is the 11. fluorescence spectrum one obtains via selective excitation With the monochromator set at the 0-0 transition of the of a vibronic level of the site labeled I. Apparently the Q band, we scanned the excitation spectrum of the Soret vibronic structure is too weak compared to the strong 0-0 band. It shows four sharp lines, superimposed on a broad background (Figure 5). Only part of the Soret band was transition to be observable. Selective excitation into the scanned. We attribute the two lines at lower energy to the 0-0 band of site I led to the emission shown in Figure 3, two origins of the S2 state. This implies a crystal-field in which now the ground-state vibrational frequencies splitting of 35 cm-l, close to the value of 6 in the Q band. appear (Table I). Because of the extreme weakness of the The two lines at higher energy are thought to be vibronic vibronic features in the emission spectrum and the overlap lines, with a separation of 237 and 243 cm-l from their of bands belonging to different sites, finding a vibronic origins. A similar frequency is observed in the Q-band level isolated enough to perform the initial site selection absorption spectrum (see Table 11). (Figure 2) proved rather hard. Finally, we tried to confirm literature reports about Sz The &-band absorption spectrum of site I is shown in fluorescence for MgTBP,13 but when exciting into the Figure 4. As shown in the insert one observes a crystal(12) (a) A. M. Merle, W. M. Pitts, and M. A. El-Sayed, Chern. Phys. Lett., 54, 211-6 (1978); (b) E. C. M. Keilman and G. W. Canters, Spectrochirn. Acta, Part A, 35, 1089-99 (1979).

(13) J. E. Zaleskii, V. N. Kotlo, A. V. Sevchenko, K. N. Solov’ev, and S. F. Shkirman, Dokl. Akad. Nauk. SSSR,Engl. Transl., 18,3204 (1973); 19, 589-91 (1975); J. F. Kleibeuker, Thesis, Agricultural University, Wageningen, The Netherlands, 1976.

The Journal of Physical Chemistty, Vol. 85, No. 1, 1981 59

Site Selection of Tetrabenzoporphines

TABLE I1 : Band Positions in the Single-Site S, So Absorption Spectra of Mg- and CdTBP and in the S, So Absorption Spectrum of CdTBP in n-Octane at 4.2 Ka

Mg T B P / n - C 8 Absorption 4.2 K

f

f

CdTBP (Q band) site I1 site IV

MgTBP (Q band) site I X

Y

X

Y

X

y

CdTBP (Soret band) site IV

15962 15992 16018 16036 15739 15775 22381 30 46 44 66 62 115 157 157 150 150 140 141 160 197 189 191 197 208 228 235 234 245 245 238 258 258 2561259 335 309 309 418 420 455 478 477 470 471 480 507 539 538 581 581 578 578 580 578 581 602 596 651 660 651 682 686 676 717 708 707 708 721 723 717 778 814 813 820 823 825 831 832 828 830 843 847 847 851 848 862 1015 1009 1008 1018 1036 1035 1073 1070 1080 1094 1096 1096 1093 1095 1103 1106 1107 1106 1115 1117 1110 1113 1123 1135 1146 1143 1150 1155 1155 1169 1167 1173 1178 1179 1185 1185 1196 1199 1199 1206 1235 1256 1268 1270 1274 1294 1283 1288 1306 1310 1299 1322 1339 1342 1337 1335 1348 1351 1346 1353 1364 1373 1382 1391 1432 1412 1480 1495 a

23181

23525

1

1

A.nm

L~'O

125

Figure 5. Single-site absorption spectrum of the Soret band of MgTBP in n-C at 4.2 K. The monochromator in the detection path (bandwidth: 2 cm- 9) was set at the 0-0, emlsslon band of slte 1. The numbers above the peaks denote their positlons in wavenumbers. Cd T B P / n - C 8 FLUORESCENCE

,

,

630

620

1

1

610

h.nm

650

1

Peak positions are given in cm" with respect t o the

0-0 transitions of the x and y manifolds mentioned at the

head of the columns, except for the last column where only one origin is given. The accuracy of the data is f 3 cm-'.

upper member of the doublet (A = 4250.8 A in Figure 5) we did not observe emission from the second excited state. The same Q-band experiments as reported above for site I (0-0, at 6265 A) were performed on another site (0-0, transition at 6287 A). The vibrational frequencies appeared to be the same within a few cm-l, but the value of 6 in the Q band is larger (84 cm-l). Absorption, Fluorescence, and Phosphorescence of

I I 1 800 810 820

I

'

830

h.nm Figure 6. Fluorescence (upper trace) and phosphorescence (lower trace) of CdTBP in n-C, at 4.2 K under broad-band excitation in the Soret band. Roman numerals refer to the sites studled in this work.

CdTBP. Excitation (bandwidth 20 nm) of a single crystal of CdTBP in n-octane into the Soret band gives the emission spectra shown in Figure 6. Contrary to the case of MgTBP, one observes both fluorescence and phosphorescence emission. It is apparent that a large number

60

The Journal of Physical Chemistry, Vol. 85, No. 1, 1981

of sites is present. Just as for MgTBP most lines can be attributed to origins. The sites labeled I-IV have been studied with site-selection techniques. Site selection in the fluorescence proved possible for a great number of sites as illustrated in Figure 7 for the sites I and IV (upper trace). In both cases the 0-0 transition only is observed, and emissions from other sites are absent. After scanning the fluorescence spectrum of a particular site, we let the monochromator run on in a number of cases to record the corresponding phosphorescence spectrum. When exciting site I, which has the highest 0-0 frequency in both fluorescence and phosphorescence,we surprisingly enough observed phosphorescence not only from this site but also from many other sites, although the 0-0 band of site I is still more pronounced than in the wide-band excited phosphorescence spectrum (comparethe lower traces of Figure 6 and Figure 7). This phenomenon was also observed for sites I1 and 111. Only for site IV was the selection made in the singlet Q band conserved in the triplet state (Figure 7 lower trace). To gain some insight into the loss of selection in the phosphorescence on selective excitation in the Q band, we performed time-resolved measurements. We found that the loss of selection occurs within 10 ns after the exciting laser flash. This makes it probable that the selection disappears during the intersystem crossing from S1 to To. As in the case of MgTBP the vibronic lines in emission are very weak compared to the 0-0 lines, and we could not observe vibrational structure in fluorescence, nor in phosphorescence except for the phosphorescence of site IV. The vibronic spacings observed in the phosphorescence of this site have been gathered in Table I. Measuring the single site SI Soabsorption spectra by monitoring the 0-0 transition either in the phosphorescence or the fluorescence proved to be no problem. As an example, the absorption spectra of sites I1 and IV are shown in Figure 8, a and b; the vibronic spacings of these sites are reported in Table 11. Detailed spectra of origins are shown in Figure 9 for sites I1 and IV. Again we observed two origins, &oxand Qoy, separated by a crystal-field splitting of 18 cm-l for site I1 and 36 cm-l for site IV. As in the case of MgTBP, the doublet structure is repeated throughout a large part of the spectra (see, for instance, Figure 8). However, site IV is somewhat peculiar. The Qoxis a normal Shpolskii line, but the Qoy is very broad (14 cm-’) and seems to be structured. Lowering the temperature to 1.3 K had no effect on the appearance of this line. Moreover, all of the y components appear to be broadened just as the corresponding 0-0, origin. Absorption spectra of the Soret band of site IV were taken with the detection monochromator set at the 0-0, of the Q band. We could see some vibronic structure (Table 11) but did not observe a constant frequency difference between various pairs of bands and could not assign a definite crystal-field splitting. The absorption lines in the Soret bands are relatively broad (12-15 cm-l), which points to a lifetime of less than 1ps for this state. As in the case of MgTBP, we did not observe any fluorescence from Sz. Triplet-State Parameters of CdTBP. The phosphorescence lifetimes and zero-field ESR transitions of CdTBP in n-C8were measured in the hope to obtain additional criteria for distinguishing between the various sites. It turned out that for all sites the lifetimes are in the range from 3.5 to 4.5 ms. The zero-field splittings measured in an ODMR experiment, however, vary strongly from site to site (see Table 111). All resonances studied at 1.3 K correspond to a decrease in phosphorescence.

-

Piatenkamp and Canters Cd TBP/n-C8 42K

, I ,

IOSCr

16099

‘,i“ FLUORESCENCE

FLUORESCENCE islleII

615

635 A

625

m

635

12529

655hnm

645

=

123321

PHOSPHORESCENCE

PHOSPHORESCENCE

1511811

790

800

1511.

# I r 810 A.nm 810

820

IPI

hnm

Flgure 7. Fluorescence (upper two traces) and phosphorescence (lower two traces) spectra of CdTBP in n-C, at 4.2 K under laser So vibronic band of site I (irradiation at 613.20 irradiation in a SI nm, left-hand side of flgure) and of slte I V (irradiation at 633.91 nm, right-hand side of figure). The positions of the laser ilne have been marked in the upper two spectra. The positions of the origins in the four spectra have been indicated in cm-’.

-

Zeeman Experiments. Zeeman experiments on CdTBP in n-C8 were performed at 4.2 K in the origin of the Q band of site 11. This site is relatively isolated (see Figure 6) and has a small 6 (18 cm-’). Hence one expects a relatively large Zeeman effect. The magnetic field was orientated perpendicular to the molecular plane by maximizing the Zeeman shifts of the two 0-0 transitions at constant field. It turned out that in this orientation the crystallographic a axis makes an angle of 25O with the field axis. This makes it plausible that the site is a so-called “A” site.5 With fi1l.z the shifts of the Qo, and QOy were measured as a function of H, between 0 and 9.5 T (see Figure 10). Considerable broadening of the Qoy transition with increasing magnetic field causes a large uncertainty in the position of the QOyat high fields. The Zeeman shifts appear to be larger for Qo, than for QOy. Discussion Optical Spectra. Assignment of the vibronic lines for the first 900-1000 cm-l in the absorption spectra to the x or y origins is rather simple in the TBPs. Beyond that point the crowded appearance of the spectra make detailed assignments questionable.lZbThe spectra appear as a series of doublets with a nearly constant crystal-field splitting over the Q band. The vibrational frequencies of Mg-, Zn-? and CdTBP are similar in many respects. The central metal obviously does not have much influence on the normal modes, except for the low-frequencyones. A similar situation is encountered for the MePs, where it was found that the frequencies are sensitive toward alterations in the porphine skeleton, but not toward changes in the central metaL6J4 On comparing the optical spectra of the MeTBPs with those of the MePs one notices that the MePs exhibit a weak Q band whereas for the MeTBPs the Q band is more intense. Gouterman has explained why the oscillator strength of the Q band is enhanced when going from the MePs to the MeTBPs.l In terms of the four-orbital model, one has a strong configuration interaction between the near degenerate al, eJ and 8% egtransitions in the MePs. This leads to an intense Soret (summing of transition

-

-

(14) K. N. Solov’ev, N. M. Ksenofontova, S. F. Shkirman, and T. F. Kachura, Spectrosc. Lett., 6, 466-67 (1973).

The Journal of Physical Chemistry, Vol, 85, No. 1, 1981 61

Site Selection of Tetrabenzoporphines

CdTBPln - C8 Absorption 4.2 K

-

CdT BPln CB Absorption 4.2K

b

0-oy 0-0, 157741

115739

SiteIP

Figure 8. Single-site SI So absorption spectra of site I1 (a) and site I V (b) of CdTBP In n-C, at 4.2 K. The wavelength of the detection monochromator (band-pass 2 cm-') was set at the 0-0 phosphorescence transition in the case of site IV, whereas in the case of site I1 it was set at the 0-0 phosphorescence for the first 250 cm-I only. At higher energies it was set at the 0-0, fluorescence transition. Different parts of the spectrum had to be scanned with different dyes and orders of the dye laser. Otherwise the same remarks apply as in Figure 4. +

02

Platenkamp and Canters

The Journal of Physical Chemistty, Vol. 85, No. 1, 1981

TABLE 111: Zero-Field ODMR Transitions and

CdTBPln-Ce Absorption 4.2 K

16036

Zero-Field Parameters of CdTBP in n-Octane in Four Different Sitesa

15739

0-ox

transitions,

MHz

MHz

-

CdTBP/n- C8. L.2K

1

2

ID1

0-Ophos [El ( A det),cm-'

I1 I11 IV V

788 605 920 1202

705 302 608 775

747 454 764 989

42 152 156 214

12482 12289 12332 12480

is not expected since only weak 0-1 bands are seen in the absorption spectra. In order to obtain measurable quantities of TBP in solution, one usually measures the solution spectra in a solvent to which a strong solvating agent has been added, or which is strongly solvating itself. It is conceivable that solvation influences the intensity ratio between the origin and the vibronic bands in the spectrum. This offers no explanation for the intensity differences between the emission and absorption spectra here observed in a Shpolskii host at low temperatures. Several authors1' have pointed out that the inclusion of nonadiabatic terms in the description of excited states may have a bearing on vibronic intensity borrowing mechanisms. They predict that the transition moment due to BO breakdown adds to the transition moment due to HT coupling in the S1 So absorption but subtracts from the latter in emission. This causes a deviation of the mirror symmetry in the intensity pattern between absorption and emission. In our case the vibrational quanta are too small (7000 cm-') for this mechanism to count as the sole source for the observed intensity difference. A second cause, as pointed out by Craig and Small,l* derives from interference between Frank-Condon allowed parts and vibronically induced parts of the transition dipole moment of a 0-1 vibronic band. If the two moments interfere constructively in absorption, for instance, they will do so destructively in emission. For this to explain the intensity difference between emission and absorption observed for the TBPs, the two contributions should be about equal in magnitude for a great number of vibronic transitions, and it is difficult to imagine why no sizable variations in relative magnitude of the two contributions should occur for a series of bands. As yet no clear explanation for the observed difference between emission and absorption is at hand, therefore. The observation of sharp lines in the Soret band of MgTBP is interesting. It indicates that the Sz lifetime is rather long, which implies the possibility that emission from Sz may be observed. Contrary to a number of reports in the literature,13J6 however, we did not see S2 fluorescence, in accordance with the results of Fielding and Mau? It should be noted though, that the occurrence of Sz fluorescence in the TBPs seems restricted to systems in which a strongly solvating agent is present.13J6

-

1

I \o-ox

-a

site

a The bands from site V vary strongly in intensity with preparation and thermal history of the sample and do not occur in Figure 6, for instance. The phosphorescence 0-0 band of this site is located at 801.3 nm. The accuracy of the data i s 1 5 MHz.

Flgure 9, Single-site spectra of the 0-0 region of the S1 Soabsorption band of CdTBP in n-C, at 4.2 K with detection at the 0-0 phosphorescence bands (801.14 and 810.90 nm) of site I1 (left) and site I V (right). The positions of the origins are denoted in crn-l. Monochromator bandwidth 2 cm-'. SHIFT 1crn-l)

zero-field parameters,

\ I

I

1

I

I

20

40

60

80

100

* H Z (TESLA']

-

Flgure 10. Shifts of the 0-Oxand 0-0 bands in the single-site S1 So absorption spectrum of CdTBP in n-d, (sRe 11) at 4.2 K as a function of the field strength squared of an external magnetic field applied along the molecular (out-of-plane) z axis. Accurate determination of the position of the 0-0, peak at high field was prevented by the field-induced broadening of this band. No data points for the 0-0, band have therefore been given in the figure for fields greater than 7.5 T.

dipoles) and weak Q band (cancellation of dipoles).' Vibronic intensity in the Q band, subsequently, derives from vibronic coupling with the Soret band. In MeTBPs the presence of the benzo groups increases the energy separation between the alu and aZuMOs. This leads to less CI and hence a stronger Q band.' As for the vibronic borrowing, the lack of intensity of the vibronic lines in emission is conspicuous in our experiments; the bands are at least 50 times weaker than the fluorescence origin. Similarobservations were made by Fielding and Mau? who report a difference by a factor of more than 30. Gouter(15)M.Gouterman, Grad. Stud.-Tex. Tech. Uniu., 2, 1-174 (1973). man15J6has attributed the lack of vibronic intensity in (16)L.Bajema, M.Gouterman, and C. B. Rose, J. Mol. Spectrosc., 39, emission to weakness of vibronic coupling between the SI 421-31 (1971);L.Edwards, M.Gouterman, and C. B. Rose, J. Am. Chem. SOC.,98,7638-41 (1976). and S2states in the TRPs. Surprisingly enough, however, (17)P.A. Geldof, R. H. P. Rettschnick, and G. J. Hoytink, Chem. there is a pronounced vibronic structure in the absorption Phys. Lett., 10,549-58(1971);G.Orlandi and W. Siebrand, Chem. Phys., spectra of MgTBP and CdTBP (see Figures 4 and 8). On 58,4513-23(1973). the basis of the room-temperatureoptical ~ p e c t r a , ' ,this ~ ~ , ~ ~ (18)D.P.Craig and G. J. Small, J. Chem. Phys., 50,3827-34 (1969).

The Journal of Physical Chemistry, Vol. 85, No. 1, 1981 63

Site Selection of Tetrabenzoporphines

The spread in 0-4energies of the various sites amounts to 1000,400, and 250 cm-l for the S2,S1,and Tostates of CdTBP, respectively. From this we conclude that the solvent-solute interaction decreases in the order S2> S1 > To, in agreement with the results obtained by Fielding and Mau for ZnTBP.g ODMR Spectra. We assume that the populating of the triplet state of CdTBP occurs mainly via the T~ sublevel as in ZnP and PdP and that radiative decay in the 0-0 band occurs mainly from T ~ The. experimentally ~ ~ ~observed ODMR signals then correspond to the 2-Y and 2-X transitions and yield the results of Table 111. The zero-field parameter IEl varies from site to site. Similar variations of IEI have also been observed in different sites for ZnP.20 As yet no explanation has been offered. The difference in the ID1 parameters for the various sites is striking. One might explain this by assuming that aggregates of various form are but one would then expect a somewhat larger spread in the phosphorescence wavelength for the various sites. Another explanation is that we see the effect of spin-orbit coupling (SOC) on the zero-field splittings. Kooter et al. in a study of the lowest triplet state of PdP showed that the contribution DLs of the SOC to D is presented by eq 1>3in which Z is the SOC

2DLs = 6 -(S2

+ Z2)'I2

-Z2/(26)

(1)

matrix element connecting the T~ and rYstates of the lower orbital component of the originally degenerate 3E, state with those of the upper one (the approximate equality in eq 1is valid for Z 5) and a (>2). We therefore conclude that, if there is a Jahr-Teller coupling in the SI state of CdTBP, it must be weak and it must occur along a low-frequency mode.

Acknowledgment. We thank Professor Dr. T. J. Schaafsma for providing us with Mg- and CdTBP, Mr. M. Noort, Mr. J. A. Pardoen, and Mr. H.D. van Osnabrugge for their assistance in the experiments, and Professor Dr. J. H van der Waals for critically reading the manuscript. The investigations were supported by the Netherlands Foundation for Chemical Research (S.O.N.) with financial aid from the Netherland Organization for the advancement of Pure Research (Z.W.0). ~

(22) R. P. H. Kooyman, T. J. Schaafsma, and J. F. Kleibeuker, Photochern. Photobiol., 26, 235-40 (1977). (23) J. A. Kooter, G. W. Canters, and J. H. van der Waals, Mol. Phys., 33, 1545-64 (1977). (24) R. L. Ake and M. Gouterman, Theor. Chirn. Acta, 15, 2E42 (1969).