Studies of the phosphorescent state of tetraphenyl group IV compounds

CHEMICAL PHYSICS6 (1974) 235-252.0

NORTH-HOLLANDPUBLISHINGCOMPANY

STUDIESOFTHEPHOSPHORESCENTSTATEOF TETRAPHENYLGROUPIVCOMPOUNDS Tien-Sung LIN Depamenr

of Chemimy.

Washington University, St. Louis, Missouri 63130. USA

Received 1I July 1974 Revised manusaipt received 15 August 1974 Detailed studies resolution emission (1) The internal atom effect appears

of the phosphorescent state of tetraphcnyl Group IV compounds arc presented. Analysis of high spectra of mixed crystal systems at 4.2 K indicates the following: heavy atom etTect arises through,both electronic and vibronic interactions while rhe external heavy largely through electronic interactions. (2) Both exciton- trap interactions and dispersive forces ate responsible for the mixed crystal solvent effects which produce spectral shifts up to -. 80 cm-*. (3) Translationally equivalent interactions control triplet energy transfer. (4) The Tt ++ So origin of tetnphenylmethane has been located at 27474 cm-’ by impurity-induced emission_ EPR studies of the phosphorescent state of tettaphenylmethane dispersed in tetraphenyltin show that the magnitudes of the spin dipolax interactions and the inter-ring interactions are comparable (-0.1 cm-‘).

I. Introduction The spectroscopic properties of tetraphenyl group IV compounds (Ph,X, X = C, Si, Ge, Sn, Pb) have been investigated previously by many workers: Hochstrasser and Marchetti presented the absorption spectra of the lowest triplet states of Ph,X solids (except Ph,C) at liquid heIium temperatures [I]. They also reported the absorption and emission spectra of the lowest singlet state of Ph4Ge and of Ph4Sn solids. C;outerman and Sayer indicated that the luminescences of Ph,X are of excimer type at room temperature as a result of sigma bond formation among phenyl groups [2]. They also pointed out that the emission spectra ob-

served by LaPaglia [3] were nor of normal luminescences. So far no detailed analyses of the normal phosphorescence spectra of Ph,X series have been reported. Here we would like to report a mixed crystal study of the system at low temperatures. The study is concerned with the vibronic activities of the phosporescent state of Ph4X compounds. We are interested in the environmental effect on the spectral features and phosphorescence intensity, such as the spin-orbit Coupling effect, spectral shifts and the interactions

between impurity sites. Furthermore the vibrational analyses of the phosphorescence spectra can supplement the assignment of infrared and Raman spectra in some cases, especially the assignments of totally symmetric modes. We have also performed EPR studies on the phosphorescent state of the Ph,X mixed crystals. These studies were designed to map the spin dipolar interaction and the inter-ring interaction in this system.

2. Experimental Tetraphenylmethane,

tetraphenyltin

and tetrapheayl-

from AIdrich Chemical CO., tztraphenylgermane from Strem Chemical Inc., and tctraphenylsilane from Alfa Chemical Co. All of the cornpounds used in the experiment were recrystallized from benzene solution and zone refied (at least ICI0zones passage) except Ph,Pb. hlixed crystals of Ph,X were prepared from melt in a Bridgman furnace, except Ph,Pb which was prepared from toluene solutions. Crystals of W,X are tetragonal [4-61. two molecules per unit cell, space group P;?21c. lead were purchased

236

T.-S. Lin. Phosphorescen? srafe of (C6H5)4X

The crystallographic axes were determined by the conoscopic technique. The concentrations of the mixed crystals given in the text arc the amounts origtnally present before the crystal growth. Oriented mixed crystals were mounted on a copper plate with optical access and immersedin liquid hehum in a double-walleddouble dewar with quartz tail section. The spectra were taken photographically with a 3.4 m Jan&Ash spectrograph(Mark IV). The grating has a rtding of 15000 linesjinch, and it yields a dispersion of 5.02 &nm in first order at 3500 A. The photographic plates used were Kodak spectroscopic plates 103-Oand 103.a0. The excitation source was a 150 W high pressure xenon lamp (Hanovia 9OlCl). The long wavelengthexcitation light was filtered out by a 5 cm quartz cell containing NC304solution. A WoIlastonprism was employed in tha polarization study. The measuredwavelengthof spectral lines was calibrated againstFe-Ne tines from a hollow-cathode tube. Kay&s Tabelleder Schwingmgszahlen [7] was used to convert measured spectral wavelengthsto wavenumbers.The uncertainty in the reported wavenumbers is about ?; 3 cm-l. The photographic exposure time ranged from 10 to 40 min. The spectra shown below are microphoto-

meter tracingsof the photographic plates.

3. spectral analysesof diluted mixed crystals The singtet-triplet transition of Ph,X arises from an excitation of the phenyl chromophore (3Bt, +t A,, for benzene). From the neat crystal studies the energiesof the O-O bands of the Tt + So transitions are in the followingorder [ 1J: Ph,Pb > PhqSn > Ph&e > Ph$i. Thus we prepared the mixed crystals by havinglighter molecules dispersedin heavier host crystals. The phosphorescences reported below are of trap emissions arisingfrom the impurity center. The concentration of the diluted mixed crystals is less than 0.5 mol %. The vibrational anafysesof the phosphorescence spectra are b&sedon the Czv local symmetry of the phenyl ring.

3.1. Tetmphenylmethane The phosphorescence spectra of Ph,C in three different host crystals(Ph,,Sn, Ph,Ge and Ph,Si) are shown in fig. I. The general features of the Ph,C phosphorescence in different host crystals are about the same, except for the following: (1) the phosphorescenceyield is higher in Ph$n host than in lighter host crystals, and (2) totally symmetric modes - 1005 cm-t (Y,, al) and 1578 (usa. at) - are intensified in Ph,Sn relative to the Ph4Si host. The O-O bands of the Tt -) So spectra of Ph,C are located at different position in different hosts: 28249 cm-1 in Ph$n, 28293 cm-t in Ph,Ge, and 28327 cm-t in Ph$i. The nature of these spectral shifts will be discussedin section 4. The analystsof the Ph,C phosphorescence in Ph4Sn hosts is presented in table 1. The vibronic origins are 619 cm-t (v,, aI orb, mode in the local C,, symmetry), 1005 (vt, at), 1578,1583 (Q, ugb). Progressionsbuilt on these vibronic origins are 1620 (it t us) and 2582 (ut + usa). Some low-frequency bands, 39 and 74 cm-‘, are assrgnedas lattice modes. These phonon emissionband: are quite broad (-5 cm-t) and are observed only for the prominent vibronic bands: 1038,1077; and 1645,1656 cm-l. The frequencies of the participating phonon modes are different in different hosts. We notice that the O-O band and the totally symmetric modes of 1005 cm-t (ring-breathing)and 1578 cm-1 (CC in-plane) are the most intense ones in the spectrum of Ph& in Ph&in. We also notice that the O-O band of the Ph,C phosphorescence in Ph,Si is the most intense band - accounting for about 80% of the total emission intensity. This differs from the vibronic activity of the benzene phosphorescence where the most intense bands are ezg modes (~g and vs), which account for 90% of the total intensity, while O-O and u1 bands account for only about 2% of the intensity [S]. The participation of uBaand u8b modes in the Ph,C phosphorescence may still be essential even though the phenyl ring has a local symmetry of C,,. The enhancement of the totally symmetric modes (it and vBa)must arise from the electrostatic interaction with the environment. The discussion of this will be given in section 5. Wehave not been able to record the T, + S, ab-

7X.

Lin,

Phosphorescent

stateof (C~HS)~X Table 1 tiysisof

h(A)

acme’)

S

W

3538.9 3543.8 3548.2 3557.2 3569.4 3573.0 3576.0 3577.3 3581.3 3601.7 3604.3 3618.2 3627.6 3629.3 3631.3 36545 3661.0 3668.8 3669.5 3674.0 3679.1 3689.8 3690.9 3694.4

28249 28210 28175 281O4 28008 27980 27956 27946 27915 27757 27737 27630 27559 27546 27485 27356 27307 21249 21244 27211 27173 27094 27086 27060

39 74 145 241 269 293 303 334 492 512 619 690 703 764 893 942 1000 LOOS 1038 1077 1155 1163 1189

s

3748.3

26671

1578

m

3749.0 3154.2

26666 26629

1583 1620

3757.8 3759.3 3809.4 3893.9 3986.2

26604 26593 26243 25667 25079

1645 1656 2006 2582 3170

w, b W VW W W W W W W W W W

w m m m s

Ph,c

m,b w,b

in Pll,SI

W W

W

3800

3600

h,h

VW VW different

sorption spectrum of Ph,C by usinga neat crystal of 22 mm thickness. This indicates the spin-orbit coup ling is very weak in Ph,C. The phosphorescence intensity may be induced by external spin-orbit coupling and by the triplet-triplet energy transfer process. However,we have tentatively assignedthe O-O band of T, + So at 27474 cm-l. The assignment is based on the impurity induced emission(exciton-trap emission)from the nearly pure crystal of Ph,C dope,d with 0.2% Ph4Si(See section 7). A phosphorescence excitation spectrum is neededto confirm our assign ment.

A%n”)

4000

-

Fig. 1. Phosphorescence spectra of (CbH5)4C in three host crystals at 4.2 K. Top: in (C6H5)&. middle: in (C6H&Ge, bottom: in (C6Hs)4Si.

phosphorescence in (C6Hs)cSn crystals

Intensity

w,b

11

(Q.Hs)&

237

VW VW W

Assignment

IR or

o-o lattice

35 (RI

mice

143 (R) ?, al 260 (R) 292 (R)

v&al orbI u4. b2

480 513 610 700

ORI ([RI ([RI (IR)

705 CR)

76i.l ([RI “IX,* b2 vsnbz

‘X OR) 101O (R) vL + 39 -G lM2 (R) vI + 74 - 2 1070 (IRI 1158 (RI v9;1v al 1165 (R) V9b. b I 116.5 (IR) 1580 (R) ~8~. aI Vab. bl vlt&,-40r vaa+39t 1 vp,tN7 v&+74+ 1 2u,-4 VI +vgat 1 2us+14.2vb+4 “ln31

3.2. TetraphenyWane The phosphorescencespectra of Ph4Si in three different hosts (Ph,Pb,Ph,Sn and PhACc)are shown in fig. 2. ‘Tie O-O bands are located at 28590 cm” in Ph,Pb, 28585 cm-1 in Ph4Sn, and 28558 cm-l in Ph,Ce. The vibrationalstructure of the Ph,Si phosphorescence is similarto the spectra of Ph4C. We have not noticed any differences in the vibrational frequencies for the Ph$i phosphorescence in different host caystals. Table 2 givesthe analysisof the Ph,Si phosphorescence in Ph4Sn. The prominent vibrational bands are 240,702,707,993,1000,1031,1098 and 1587 cm-l. These bands can be assignedwithout much dif-

T.-S.

238

Lin,

Phosphorescent

state of (C&)4X Table 2 hEdySiS

.

Ph,St

in

Of

(C&kai

phosphorescenceill (CtjHS)4Sn

Inten- h(A)

F(c~-’

)

d+i*)

Asignmmt

SilY

vs W W W

m

m s Ph4Si 8

in

P$Sn

W W

B

W W

W S 3 W W

Ph4Si in P$Ge

W

vs vs

th w

m S

W

m W

m Fig. 2. Phosphorescence spectra of (CsHs)4ai in different CMtals at 4.2 K. Top: in (C6Hs),#b, middle: in (C6&)4% batIOm:

‘Xk’StiS

Ph4Pb

ill (C,jtiS)4&

ficulty except the 702 and 707 cm-l bands. These two bands also appear together in the spectra where Ph,Pb and Ph4Ce arc the hosts. The infrared spectrum of crystalline Ph,Si in KBr pellet [9, lo] shows a strong doublet in this region: 700 and 707 cm-t. The 702-707 doublet could arise from the following three possibilities: (a) split of a degenerate mode due to the lower local symmetry (De,, + C,,), (2) crystalline effect, and (3) accidental overlapping We could rule out the first possibility: first there is no degenerate mode of benzene [8] around 700 cm-r; second we observe the 702-707 doublet only in the spectra of Ph,Si. To test the other possibilities, we have synthesized the (C,D,),Si compound to measure

w,b m 5 vs w, b m W W W

m W W W W W W W

m W W

w,b W W s

IR or Raman

3491.3 3502.6 3505.5 3509.1 3519.2 3525.0 3526.9 3550.2 3555.1 3556.9 3574.1

28585 28542 28518 28489 28407 28361 28345 28159 28121 28106 27966

43

67 96 178 224 240 426 464 479 619

3580.9 3585.4 3586.1 3605.7 3615.0 3616.8 3623.2 3624.1 3628.2 3629.7 . 3632.8 3637.0 3638.3

279la 21883 27878 27726 27655 27641 27592 27585 27554 27543 27519 27487 27478

667 702 707 a59 930 944 993 1000 1031 1042 1066 1098 1107

3647.5 3649.3 3654.1 3656.0 3679.0 3681.4 3700.3 3703.0 3708.8 3718.8

27408 27395 27359 27345 27174 27156 27017 26998 26955 26883

1177 1190 1226 1240 1411 1429’ 1568 1587 1630 1702

3727.4 3733.9 3736.3 3760.0 3764.6 3775.7 3787.0 3801.6 38025 3821.5 3841.9 3844.7 3849.6 3861.2 3872.7 3878.1 3928.3 3934.2

26821 26774 26757 26588 26556 26478 26399 26297 26291 26160 26021 26003 25969 25891 25815 25779 25449 25411

1764 1811 1828 1997 2029 2107 2186 2288 2294 2425 2564 2582 2616 2694 2770 2806 3136 3174

O-O

lattice lattice

248 (R)

?,a1

462 (R) “6, al

01 bS

620 (IR), 625 (RI 663 (R)

v4.bz vIt(?), b2 850 (IR) *to. bz vl7ar aa 945 (R) vttb.bz *ssb 998 (R) *lral vt~,at;vtz, bt1030 (IR) i,+43-1 1040 (R) 1060 (IR) v1+671 v;s.bl

1110(R)

q+178-1

(IN 1170 (R) 1180 (IR)

*9ar

al

vt +?40

1245 (R)

*twat;*t9b.bt “8bv bt vSa*al Qa+43;V, +U6 vsa+67+2; *1

+V4

2v1+3

*4+*8a

V, +Vg,,-4

v,+vsa-5 2vt + v6 2V,+v4a

2”eb zve.a

1750 (R)

T.-S Lfn, Phosphorescent

state o,C(C&)rX

(C6W,Si

I

I

in

(C,H&n

I

1

3900

3700

3500 AJ

239

-

Fig. 3. Phosphorescence spectrum of (CeDs)4Si in (C6Hs)4Sn crystals at 4.2 K. the isotopic effec. The spectrum of the perdeuterated l%,Si would also allow us to confii the spectral Table 3 Analysis of (CbDs)aSi phosphorescence in (CbHs)4Sn c~stals Intensity

h(A)

$xi-‘)

vs w

28723 28681 28651 285.58 28513 28495 28114 28127

42 66 165 210 228 549 596

o-o lattice lattice

m m s s m

3480.6 3485.7 3488.4 3500.7 3506.2 3508.4 3548.4 3554.3

W

3564.1

28050

673

vll(?), b2

m

3585.2 3600.3 3613.4 3619.0 3621.1 3630.0 3672.8 3675.9 3619.0 3698.2 3710.2 3720.3 3801.5 3812.9 3894.5 3901.6

21884 21768 21667 27624 27608 27540 27220 21197 27114 27033 26945 26814 26298 26219 25670 25623

872 955 1056 1099 1115 1183 1503 1526 1549 1690 1778 1909 2425 2504 3053 3100

W

vs s m W

m W

m vs W W W W W W W

A;(cni’)

Assignment

?.a‘ v4rb2 v6,al

orb1

vsa bz Vll=l ~1st bt vsaval v, + 228 “ebv bl VBa*at vea+42+ 1 2Vl - 1 VI + %a 2”eb

%a+2

analyses on (C6H5)4Si. The phosphorescence spectrum of (C6D5)4Si in Ph,Sn is shown in fig. 3, and the spectral analysis is given in table 3. The O-O band is located at 28723 cm-t [a blue shift of 138 cm-l from the O-O band of (C,H,),Si]. There is no discernible splitting in any of the vibronit bands in the spectrum of perdeuterated compound. Based on the intensity pattern, the 702-707 doublet in (C6f-15)4Siis likely corresponding to the 549 and 673 cm- 1 bands in (CgDg)@i. The 549 cm-l can be assigned as b, mode (u4). We note that the intensity ratio of the 549 cm-t band to the 673 cm-l band is about 2: 1, while the ratio of 702-707 doublet is about 1: 1. This may imply the 702-707 doublet is in Fermi resonance. Therefore we can tentatively assign the 707 cm-l in (C61-i5)4Si as another b2 mode. The 707 cm-1 mode should be sensitive to the kind of group attached to the phenyl group, the so-called Xsensitive mode in Whiffen’s notation [ 10, II]. Low frequency modes of 43 and 67 cm-l appear in phosphorescences in both (C&)4Si and (C,D&Si Ph.$n hosts. They must therefore correspond to the phonon modes. These lattice modes also appear for the stronger molecular bands. Again the most intense bands are the O-O, and two totally symmetric modes (ul and u&) in both (C&&i

240

T.-s.m,

Phosphorescent 3rRteof (C&)4X

and (C6DJ)4Si spectra. The totally symmetric modes are intensified in the heavier host crystal. Wealso note that the 240 and 702-707 cm-t bands in (C6H5)4Si, and 228 and 549 cm-l bands in (C6D&,Si gain intensity in the heavier host lattice. The 240 cm-t (228 cm’*) should have a, symmetry. Wewill discuss this further in connection with the Ph,Ge phosphorescence, The phosphorescenceyield of Ph4Si differs in different hosts: the heavier the host crystal, the higher the phosphorescenceyield of the guest molecule. The enhancement of the singlet-triplet transition in heavier host crystals must arise from the external spin-orbit coupling(see section 5.2). A new broad band appears at 3680 A in the spectrum of Ph,$i in Ph4Ce crystals. This new bpnd may be attributed to the triplet excimer formation of the host lattice. A similarbroad band also shows up in the spectra 0fPh4C in Pi+i, and l%,C in Ph,Ce. 3.3. Tetraphenylgennane The phosphorescencespectra of Ph4Ce dispersedin Ph,Pb and Ph4St crystals are given in fig. 4. The spectral analysisof the Ph4Ge in Ph4Sn crystals is given in table 4. The O-O bands are located at 28927 cm-l in the Ph4Pb crystal, and 28924 cm-t in the Ph4Sn cry+ tal. The phosphorescencequantum yield is higher than Ph,C or Ph,Si in the same host. ‘the prominentvibronicbandsare 234,682,997,1027, 1089,1570,1583 cm-t modes.The e mode (vg) of benzene againsplits into b, (vgb) - 15$ 0 cm-r, and a, (uga) - 1583 cm-t in the C2vlocal symmetry. The splitting of 13 cm-t in Ph4Ce is slightlyless than the 19 cm-t splitting in Ph4Si. The assignmentof at mode to 1583 cm-t is based on the intensity enchangement criteria - a heavy-atom external spin-orbit coupling to be discussedbelow (section 5.2). Progressionsbuilt on the vibronic originsare pretty much the same as the previouscases. We note the 234 cm-t band appearsvery strongly in the spectrum (similarmode also appears in figs. 1-3, i.e., 241 cm-l for Ph4C in Ph$n; 240 cm-t for Ph& in Ph4Sn). As we pointed out in Ph&i spectra, this mode shonld be assigned as a, mode (in-plane ring

deformation). Thisisin contrastto thevibrational analysisof theRaman spectrum of Ph4Ge solids [ 121, where an intense band of

229 cm-t has been assigned

1

I

I

I

3700

3500

&A

I

3900 -

Fig. 4. Phosphorescence spectra of (C&,)4Ge in (CbH&Pb aystals (top), and in (CeHs)dSn crystals (bottom) at 4.2 K. as bt mode (antisymmetric in-planering deformation). The justification of our assignmentwill be discussedin section 5. The relative intensity of the totally symmetric fundamental Y, to the O-O band in the phosphorescence spectrum is much greater than that in the fluorescence of Ph4Ce solids [I]. This implies a greater environmental enhancement of the O-O transition in the phosphorescence than that in the fluorescence.The most intense vibronic band in the fluorescence is the Y6fundamental. This differs from the vibronic activity of the phosphorescence where it and yBaare the most intense ones. The different vibtonic activities in the singlet and the triplet manifolds are also noticeable in othertetra- , phenyi Group 1Vcompounds. However, the activity of ys mode in the fluorescence and vaa mode in the phosphorescence of ph4Xbear some resemblances to the emissionspectra of the benzene molecule [8). We must note that the activity of PI mode in the benzene phos-

T.-S. Lin, Phorphorercenr state ~f(C~lis)~X Table 4 hdYSisOf

(CaHs)&e phosphorescence in (CaHs)&r crystals

Inten- A(A)

i$a+

)

A&m+

)

Assignment

Sity

n s W W

s m VS W S W W

m W W

w

m vg M s m S

m m S

m m S VS W S S S m

m m m

3456.3 3461.3 3464.1 3467.1 3477.2 3482.9 3484.5 3489.7 3495.7 3504.3 3511.4 3531.8 3537.3 3538.7

28924 28883 28859 28834 28751 28704 28690 28648 28598 28561 28471 28306 28263 28251

41 65 90 173 220 234 276 326 363 453 618 661 673

3539.8 3547.3 3519.8 3583.6 3585.1 3588.3 3591.6 3600.1 3602.4 3610.2 3613.9 3622.0 3654.7 3656.5 3661.8 3712.9 3717.0 3725.6 3792.9 3194.8 3877.2 3881.3

28242 28182 21927 27897 27885 27860 27835 27769 27751 27691 27663 27601 27354 27341 27301 26926 26896 26834 26358 26344 25785 25757

682 742 997 1027 1039 1064 1089 115s 1173 1233 1261 1323 1570 1583 1623 1998 2028 2090 2566 2580 3139 3167

IRor Raman

o-o lattice lattice Lltticc 180 (R) 235 (R) 276 (1R) 330 (IR)

?,a1

orb1

war

615 (IR)

675 (1R) (RI 705 (JR) 740 (JR)

v4vbz

hIal vsn bz

1026 (IR) 1037 (ii) 1060 1090 1155 1170

V9aq al

O-O 11% 611

33

991

25

669 14 1609 17

(IR) UR) (1R) (R)

vt+234+2 1263 (IR) 1320 (R) 1580 OR) 1585 (R)

vt4, br usb, bt vaa+41

2vr +4 vr+u5+4 vt+ut5+4 “1 +VS,, -

1

“1 +VSa

&ab 2vaa

Table S Relative intensities of the O-O band to vibronic origins in Ph&e and Ph4Sn spectra

O-O 26% ye 618 7 u4 682 17 q 997 30 vB 1583 20

O-O 30% 701 io 995

1577

40

10

phorescence is almost negligible, while the yl made in the Ph,Ce phosphorescence accounts for about one third of the total intensity. The relative intensities estimated for the O-O band and the prominent vibronic origins in the PhqGe phosphorescence and tluorescence spectra are given in table 5. The differences in their intensity patterns indicate that, in addition to spin-orbit coupling (internal and external), the vibronic coupling should also play an important role irt the singlet-triplet trnusition For the same kind of vibrational modes, their frequencies are different in the phosphorescence and fluorescence. This could arise from the difference in the environmental effect (S1 + So from the neat crystal of Ph4Ge [ 11, and T1 + So from Ph,Ck in the Ph,Sn crystal), or possibly from the participation of low frequency modes upon excitation.

3.4. Temphenykin

1000 (IR)

vr+41+1 vt+65+2 vrsvbt Vabvbr

V.Salal Vl +v.$

241

O-O 8% 615 44 659 16 999 20

1615 12

Fig. 5 gives the phosphorescence spectrum of Ph4Sn dispersed in F’h,Pb crystals. The spectral analysis is given in table 6. The O-O band is located at 29059 cm-l (5 cm-t higher than the center of gravity of the Tt + So transition observed in the neat crystal of Ph4Sn[l]). The prominent vi’oronic origins are 701,995 and 1577 cm-l. A splitting of 10 cm-1 (1567- 1577) is observed for the e& (vg) mode of

benzene. Progressionsbuilt on vibronic origins are the same as previous cases. We note that some low frequency modes appear in the spectrum: 40, I3 1,201,209 and 267 cm-l. The totally symmetric mode u1 appears very strongly but not the vga mode. The next strong bands are the Us mode and the O-O line. The vibronic activity (intensity patterns) of the Ph4Sn phosphorescence in Ph,Pb crystals differs from that of the Ph,Si or Ph, Ge in the same host crystal; specifically the appearance of uL mode and the symmetric in-plane deformation mode (267 cm-t). We may take this as a sympton of the effectiveness of the guest-host interaction and of the triplet energy transfer. A comparison of the phosphorescence spectrum with absorption spectrum [l] shows that there is no mirror-image relation between them. This may be an indication of different geometry and/or equilibrium nuclear position upon electronic excitation.

The phosphorescence spectrum diffen greatly from

T.-S. Lb, Phosphcvexenr srarc of (C6HS)4X

242

Ph,Sn

in

Fig. 5. Phosphorescence spectrum of (C&)&

Table 6. Analysis of (C,&)&

phosphorescence in (CbHS)4h

h(A)

E(cm+)

29059

S

3440.3 3445.0 3464.3 3465.2 3472.2 3495.3 3525.3

vs

3562.3

28064

hen

AF(cni*)

Asdgnment

sity s

1, b m

m m

29019 38858

28850 28792 28602 28358

c0’stA

IR or Raman

o-o 40 201 209 267 457 701

lattice

s

35658

28036

vS,bt

m

3572.2

27986

1023

215 (R) 458 (IR) 700 (IR) 997 (IR) 1025 (IF!)

1073

“mba

1075 (IR)

W

3583.0

27902

1157

%b* bl

1150 (IR)

W

3636.4 3637.7 3653.7 3664.3 3693.8 36975 3772.8 3774.2 3795.7 3856.3 3859.1

27492 27482 27362 27283 27065 27038 26496 26488 26338 25924 25906

1567 1577 1697 1776 1994 2021 2561 2571 2721 3135 3153

“ab, bl

W

in m W W W W W W W W

995

‘?,a, ~4. bl “,.a1

ha,

1557

“1

ut+vq+

OR)

1

2q +4 q+vs+3 VI+U.gb+l ut+v~,+l 2”8b+ 1 2vaa+ 1

the fluorescence spectrum of the Ph4Sn crystal [I]. The relative intensities estimated for the O-O band and the

prominent vibronic originsin the eh,Sn phosphores-

Ph4Pb

in (&,Hs)&% crystals at 4.2 K.

cence and fluorescence spectra are givenin table 5. The intensity patterns in Ph4Sn are similarto those in Ph4Ce, except that the v6 mode is almost missing(extremely weak) in the phosphorescencespectrum of Ph4Sn, and the O-Oand v1 account for about 70% of the total phosphorescenceintensity of Ph4Sn. Within the limitation of the resolution of our spcctrograph and the breadth of the spectral lines, we have not observed any multiplet structure on the phosphorescence spectrum of Ph4Sn in the Ph,Pb crystal. This is in contrast to the Tl + So absorption spectrum of PII,+ [ 11,where the O-O band consists of three ex&on components with splittings of 4.0 and 3.4 am-1. ‘IItsstructurelessO-O band of the phosphorescence would imply that emission takes place from the lowest exciton state at liquid helium temperatures (see section 8).

4. Spectral shifts of the phospbo~scenceorigins in different hosts

The O-O bands of the T, + So transition have been located at different positions in different hosts as mentioned previously. Fig. 6 summarizesthe energy levels of various systems.We note that the energiesof the O-O band of Ph4C increase as the molecular weight of the host increases:Ph,Si < ph4Ge < Ph4Sn; while the

7X.

Lin, Phosphorescenr stale of (C&)4X

Fig. 6. A summary ol energylevelsof the O-O bandsof in different host crystals.

((3,5)4X

ordering is inverted for Ph,Si: Ph,Pb < Ph,,Sn < Ph,Ge, We also note that none of the phosphorescenceorigins are coincident with the absorption originsobserved in neat crystals [I]. These spectral shifts of the O-O bands may arise from (1) an electrostatic interaction between the guest molecule and the host crystal, and (2) the dispersiveinteraction. The former factor would be of an exciton-impurity interaction in the deep trap limit [13]. The depression of the trap level can be evaluated by the second order perturbation, namely, AW=AE-11($,~lYI~~~~12, where AE is the energy difference between the exciton band and the free state of the impurity level, V is the interaction operator between the excited guest and the host which depends upon the wave functions of the concerned site. and 4; refers to the excited wave. function at the guest site. The dispersive interactions for nonpolar states in

nonpolar media has been formulated as follows [ 141: Red shift = i agZR-6(1E~A + m2) , 4 where aA and ccgare the molecular polarizabilitiesof the solute and solvent, respectively,m is the transition moment, E is the transition energy, and each solute molecule is surrounded by 2 solvent molecules at a mean distanceR. In Ph,C, one might rule out the possibleexcitonimpurity interaction. This is justified by (1) the energy gap between the triplet exciton band of the host and the tentative O-O band of Ph,C is large, e.g., 1100 cm-i for Ph,Si, and

213

(2) the intermolecular interaction matrix element is small as evidenced by a lower quantum yield of Ph.& phosphorescenerelative to Ph,Si or Ph4Ge in the same host crystal - Ph,Sn. Thus the spectral shift in the Ph,C phosphoreence may arise mainly from the dispersivecontribution. Since the singlet-triplet transition probability for Ph,C isextremely low, we may neglect the m2 term altogether. So the main dispersivecontnbution to the spectral shift of Ph,C should come from the polarizability of the medium. It is known that the polarizability is a function of molecular size; this may infer that Ph$n is more polarizable than Ph4Ge and Ph,Si. Therefore, the O-O band of the phosphorescencespectrum of _Ph,Cin the Ph,,Sn crystal should have the biggestred shift, i.e., it lies at the lowest of the three hosts, while Ph,C in Ph,Si should lie at the highest of the three. If the red shift is just proportional to the polatizability of the host media, one would expect the ordering of the O-O band of the Ph,Si phosphorescencein different hosts to be the same as that of Ph,C, namely, Ph4Ge> Ph$n > Ph,Pb. Howeverthe observed ordering of the O-O bands is Ph,Ge < Ph4Sn < Ph,Pb. Thus the electrostatic interaction between the guest molecule and the exciton band must becomeimportant enough to upset the ordering, cqecially in the heavierhosts. This is to say the exciton-impurity interaction may be the dominant factor when Ph4Sn and

P$Pb are used as hosts. We may estimate the re-

lative magnitudeof the interactionmatrixelement fromthe OSciMOl strengthof the host transition when the impurity molecule is the same.Wewould then expect Ph$i-PhqCe

has a smaller interaction matrix

element than Ph,Si-Ph4Sn or Ph,Si-Ph,Pb does. But the smallermatrix element of Ph4Si-Ph4Ge is compensated by the smallerenergy separation between free Ph,Si state and the host exciton state. This yields the depressionof the Ph,Si more in Ph,Ge than in Ph$!in or Ph,Pb. We should emphasize that the excitonimpurity interaction is signilkant in Ph$i, becausethe oscillator strength of the Ph,Si triplet is about one order of magnitudegreater than that of Ph,C. The effect of exciton-impurity interaction.on the electronic transition originsis seen more clearly in the Ph,Ge phosphorescencein different hosts: Ph.+& <

T.-S.Lin Phosphoremnr state of (C~HS)~X

244

Ph&Gn
5. Vibronic st~cture and spin-orbit couplings The presence of heavy atoms in polyatomic molecules often changesthe structure and the intensity of the phosphorescencespectra [ 151.Similarly by dis persingthe ium~es~nt center in the media of heavyatom perturber modifies the phosphorescencespectral features [I 5- 1S]. These spectral changesare due to the internal and external spin-orbit coupling effects. 5.1. Intern1

spin-orbit

coupling

The singlet-triplet transition is enhanced greatly by the presence of heavy atoms in the molecule. The So +cT, !ransition of Ph,X is of the benzenelike transition. One may consider Ph,X as a tetramer consisting of four phenyi ringsconnected to a central atom of Group IV element [l]. As the central atom changes from carbon to heavier elements: Si, Ge, Sn and Pb, the So *T, transition probabiiity increasesgradually [ 1, 191.Thus one is able to observe the direct T, + So absorption spectra in a 0.1 mm Ph,Pb crystal (fIO--S). The So *T, tr~ition in benzene is both orbital and spin forbidden (3Blu). The introduction of heavier Group 1Velements to connect the four phenyl rings lowers the symmetry of the chromophore (phenyl) to a C2, local symmetry. It also admixes the singletand triplet character dour the spa-orbit coup~g. Thus the So ++Tr transition in heavier Ph4X is greatly enhanced. The interaction between pheny! groups gives rise to excitonlike states (cyclic tetramer) which have been establishedby Hochstrasserand Marchetti in their neat cry&I studies 111.However we have not observed any multiple band structure in the phosphorescence spect:a of the mixed crystal ex~~ents. This could be due to the fact that only the lowest substate of the four exciton states (12 states if spin states are included) is responsiblefor the emission. The spectral analysesof the mixed crystal systems givenin section 3 may allow us to study the effect of internal heavy atoms on the mechanismof the singlettriplet transition. As we mentioned there, the phos-

photescence structure of Ph& changes when the host is changed from ph4Si to Pb4Sn. Similarchangesoccur in the Ph4Si phosphorescencein different hosts. These changesare due to the external spin-orbit coupling (see below). To avoid the comp~cation due to the external spin-orbit coupling in the present discussion,we shallexamine the spectral features when the molecuIar weightsof the guest molecule and the host molecule are the closest, namely, Ph,C!in Ph.& Ph,Si in Ph,Ge, PhqGe in Ph4Sn, end Ph$n in Ph,Pb: (1) Ph,&‘in Ph$i: TIte O-O band account for about 80%of the phosphorescenceintensity. Two of the to tally symmetric modes (vr and Q) and the 942 cm-l band (ys , b2) account for the remainingintensity. No noticeable vibronic progression appearsin the spectrum. The strong appease of the O-O band indicates the equilibriumnuclear position remainsthe same in the electronic excitation. (2) Ph$i in Ph&e: The O-O band is the most intense line in the spectrum. The vi and vsa appear very strongly with comparableintensity to the O-O band. The 241,701-707.1029 and 1107 cm-r bands ap pear with moderate intensity. Progressionsbuiit on these vibronic originsspread throughout the entire spectrum. Vibronic coupling plays a minor role in the Ph4Si phosphorescence. (3) PhqGe in Ph&c Ihe O-O band is no longer the strongestline in the spectrum. in thjs system the y1 mode is the most intense one. The decrease in the intensity of the O-O band and the intensification of the r~rmode imply the eq~b~um positions are different in the two electronic states. The strong bands are the O-O, 1089,1583,1027,234,682 and 1570 cm-t (givenin the order of decreasingintensity). Progressions built on vibronic originsappear with moderate intensity. The appearanceof the nontotally symmetric modes with moderate intensity indicates intensity borrowingvia the ~bronic coup~g. (4) Pk&z In PhqPb: Ihe strongest band is the v1 mode. The strong bands are the O-O, 701, 1023, 1073,1577, and 1697 cm-l. The nontotally symmetric mode 701 cm-1 (vq, bz) appears strongly, even stronger than the vsa mode of 1577 cm-l. The pho+ phorescence intensity of Ph$n comes from the spin-

T.-S. Lin, Phosphorescenrstareof (&H&X

245

orbit coupling effect and vibronic borrowing mechanism.

and 707 cm-1 modes are observed in both Pb4Sn and Ph,Pb hosts, but not in the Ph4Ge host_

The effects of internal heavy atom on the phosphorescence spectra of Ph,X given above are summarized as follows: (1) The overall phosphorescence intensity increases as the central atom X becomes heavier. This is due to the internal spin-orbit coupling. (2) The intensity of the O-O band decreases as X becomes heavier. This may be attributed to the greater change in the equilibrium positions upon excitation in a heavier central atom. (3) The u1 mode and the v4 mode gain intensity, but the yL mode loses intensity, as X becomes heavier. The progression built on u4 becomes a pattern in heavier X. This may be due to the effective coupling between the electronic and vibrational state in the singlet-triplet transition in the heavier central atom. We may say that the internal heavy atom effect is electronic and vibronic in nature. The enhancement of the nontotally symmetric vibrations in the heavier central atoms could arise from the stronger mixing of the p-orbitals or d-orbitals of the heavier central atoms with the n-cloud of the phenyl rings; while in the lighter molecules, the mixing of u- and x-orbitals could be more important,

(3) P/qGe: The spectrum of Pb,Ge in the Ph4Pb host shows: (a) the y1 mode is the most intense band; (b) greater intensity for 707,235,1030 and 1092 cm-t modes: and(c) no intensification for the v& mode.

5.2. Extend

spin-orbit coupling

As we mentioned previoudy, the vibronic structure

and the intensity of the O-O band of’ the impurity phosphorescence change in different hosts. This is an environmental effect, more likely an effect of external spin-orbit coupling. A summary of the spectral chang es of Ph4X in heavier media is given below:

In summary the effects of heavy atom media on the phosphorescence spectra are (1) the enhancement of the overall phosphorescence intensity, (2) the intensification of the totally symmetric modes, especially the vl and yBa modes, and (3) the slightenhancement of the nontotaliy symmetric modes, such as v4 and LJ~. This allows us to assign unambiguously many totally symmetric modes, especially the us, mode and one low frequency mode (-240 cm-*).The assignments of these bands are inconclusive in the IR and Raman studies [IO- 121. The heavy media effect is thus mostly electronic interaction in nature with some extent of vibronic coupling. The enhancement of the singlet-triplet transition in the heavy atom medium should arise from the external spin-orbit coupling; the guest triplet is populated by the coupling with the more allowed host excitation aa a result of electrostatic interaction. The coupling mechanism could be either the mixing of several guest singlet states with rhe guest Tr state caused by the presence of the heavy perturber, or the mixing of several host states with rhe guest T, state. Since the singlet-triplet transition is greatly enhanced in the heavier Ph,X due to the internal spin--orbit coupling, the coupling between the triplet manifold of the host with the guest T, state should be important in the triplet-triplet energy transfer process.

(1) P/r& The intensities of the q and vB modes increase gradually when the host crystal is changed from

One might also expect external spin-orbit coupling affects on the phosphorescencelifetime of E’b4Xin

Ph,Si to Ph4Ce to Ph4Sn. When Pb4C is dispersed in the heavier medium, it yields greater overall phosphorescence intensity. It also shows more vibronic structure in heavier media.

different media. Detailed lifetime measurements are in progress.

6. Interaction between impurity moleculea (2) Ph,$i: The intensification of the q , vga and u4 modes is clearly noticeable in heavier media - Ph4Sn and Ph,Pb. Also the 240 cm-l band gains intensity in the Ph,Pb crystal. Progressions built on the 240

So far we have dealt only with the phosphorescence spectra of diluted mixed crystals (fess than OS mol % in every case). We may consider the phosphorescence

T.-S. Lin. Phosphorescetu

246

spectra of diluted mixed crystals arisingfrom the isolated moleculesimbedded in rigid lattices. When the concentration of impurity moleculesis increased, we would expect some changesin the spectral features. These spectral changesmay allow us to measure the interaction between impurity moleculesand to track down the mechanismof triplet-triplet energy transfer. 6.1. Spectra of concentrated mixed crystals The mixed crystal systemswe have studied are (1) A.

Ph,Si 00

sfuteof(C5H5)4X

ph,$i in the Ph4Sn crystal, (2) Ph$i in Fh4Ge,and (3) Ph4Gein Ph,Sn. The comparativestudy of (1) and (2) aims to examine how the guest-guest interaction is affected in different host crystals. System (1) and (3) should yield information about whether the guestguest interaction is different for a different impurity in the same host crystal. We shall focus our attention on the spectral changes of the O-O band and its vicinity in the followinganalyscs. C.Ph&in

1. Ph,si in f%,Ge

in Ph,Sn 0.03 x

0.05x

Ph& 0.1x

00

04

II 41

4) 67

b

_-IL

I 3%

2%

b II

L 18

1

: P\ 5%

10%

‘\

IO %

J

-

Wauenumber( cm’

Fii. 7. Concentration dependentspectrain the vicinityof phosphoasancc ~(CsHd&,~ (C)(CgHs)4Cein(CsHs)~Sn.

origins: (A) (C&)aSi

in

(C~HS)&,(8)(C6Hs)aSi

T.-S. Lin, Phosphorescent state of (C&),X

241

originalintensity. The broad 68 cm-t band forms the of the phosphorescence spectrum (fig. 8). new origin (O”4”)

6.1.2. Ph$i in Ph4Ge The spectra are shown in fig. 7B. The 40 and 68 cm-t bands are the phonon modes. For 2% of Pb,Si in PhqGe, a new band appears at the lower frequency side: 18 cm-l from the O-O band. This new band grows at the expense of the O-O band as the concentration of the impurity is increased.A weak broad band appears at 61 cm-1 when the concentration is 10%. The spectral pattern of the most concentrated Ph,Si in Ph,Cc is different from that of Ph4Si in Ph,,Sn. The 18 cm-t (O’-0’) band forms as the new origin for the phosphorescence spectrum at 10%.‘the d-0 band has all of the phosphorescence intensiWin contrast to Ph,Si in Ph,Sn where the 0”-0” has most of the intensity.

Fig.8. Concentration dependent spectra of the (CeH&Si phosphorescence in (C6Hs)&.

6.1.1. Ph4Si in Ph$n The O-O bands of the Pb4Si phosphorescence for

different Ph4Si concentrations in the Ph4Sn crystal are shown in fig. 7A. The top spectrum is for 0.03% of Ph4Si in Ph$n, and the bottom one is for a 10% sample.For diluted mixed crystals, the three bands shown are the O-O band and two phonon modes at 43 and 67 cm-t. For 2%Ph,Si in Ph,Sn, a new band appears at the lower energy side: 17 cm-t from the O-O band. This new band is a new origin (Of-O’) for the sub-spectrumsuperimposedon the spectrum of the diluted system (fig. 8). The 17 cm-t new band grows at the expense of the O-O band when the concentration is increased to 5.5%.At 5.5%a new broad band at 55 cm-t begins to grow and overlapswith the lattice modes. At 10% the intensity of the O-O band of the diluted crystal diminishesgreatly, the Of-O’band also loses intensity and becomes broadened, and a broad band, centered at 68 cm-1 from the O-O band, gainsnearly all of the

61.3. Ph,Ge in PhJn The spectra are shown in fig. 7C. For a 2%impurity concentration, a new band (O’-O’)again appears at the lower energy side: 12 cm-l from the O-O band. The third broad band at 57 cm-l (0”-0”) gains almost all of the phosphorescence intensity at high concentration. The spectral changesof this system are sirni!arto Ph4Si in Ph,Sn, except that the spectral shift of the 0’-0’ band is 12 cm-1 in this case. 6.2. Intermolecular interactions The above spectral changesin the concentrated mixed crystal should arise from the interaction between guest molecules. If the interaction is between interc~nge equivalent (translationally inequivalent) molecules, one should observe two bands: one displaced above and the other below the isolated guest band: these two bands should have different pofarization [20]. If the interaction is between tms&tiondIy equivalent nearest.neighborsites, one should observe a new band system born at the lower energy side of the isolated guest band. Experimentally we observed the appearance of a new red-shifted band system, but not a splitting, in the concentrated mixed crystals. This would indicate that the interaction may be due to trandationahy equivalent molecules.This may infer that the triplet

240

T.-S.

Lin.

Phosphorescent

energy transfer takes place predominantly via the interaction between translationallyequivalent moleculesin this system, Possibly the low energy emission band systems could also arise from conglomeratestates. The enew separation between the O-Oband(isolated guest band) and the O’-0’ band (new band systerns) is about the same for Ph,Si in the two different hosts, Ph.@ and Ph4Ge crystals. This is understandable because the crystal structure is the same for both Ph4Sn and Ph,Ge except for a slightdifference in the intermolecular distance. On the other hand, the shift of the O’-0’ band from the O-O band for Ph4Ge in Ph,Sn is 5 cm-1 less than that for Ph4Si in the same host crystal-PhqSn. Therefore the Of-O band system should arise from the pairwiseinteraction between impurities. The 6-O band should mainly depend on the type of the concerned impurity, and little on the kind of host crystal chosen for the study as long as the crystal structures of the host lattices are similar. The position of the 0”-0” bandis differentfor different guests in the samehost crystal, e.g., Ph4Si in Ph$n and Ph,Ge in Ph,Sn. It is also different for different hosts containing the same guest molecule, e.g., Ph,Si in Ph4Ge and P$Si in Ph4Sn. The intensity of the 0”-0”band is host dependent: broad and very intense for Ph,Si inPh$n and Ph,Ge in Ph,Sn, but only moderate for Ph4Si in Ph4Ce. The O”-0” band may be attributed to an emissionfrom the conglomerate states as the concentration of the impurity is increased to its maximum(-20 mol %).

stare of(C&)4X

Ph,.Si in Ph,C

Fig. 9. impurityinduced emissionspectra.Top: 0.2%of (C6Hs)aSi iu (C6Hs)4C crystals,bottom: 0.15% of (QH&Sn in KsH5).&c crystals.

Z 1. Ph.@ in Ph,C

7. Impurity induced emissions The impurity spectra we have reported so far are of the trap type, i.e., the triplet energy of the impurity is alwayslower than the energy of the host. Here we would like to describe an impurity induced emission, in which the triplet energy of the impurity is higher than that of the host. The purpose of this study was to locate the triplet state of pure Ph,C; the T, c-S, of PhdC is too weak to be observed directly, and the neat crystal does not phosphoresce.However, by introducing an impurity havinghigher triplet energy into the crystal lattice to scramble the periodic@ of the caystal, we should thereby observe an emission from the defect site, or more appropriately from the detached exciton level. The systemswe have. studied ~ZE(1) ptbSi in Ph.,C and (2) ph,Sn in Ph,Ce.

We used diluted mixed crystals for the study: 0.2% of Ph4Si in the Ph4C host. The emissionspectrum is givenin fig. 9 (top). The first band appears strongly at 27474 cm-l. This band accounts for about 90%of the total emissionintensity. The rest of the spectrum is weak. One may take this strong band as the O-O band of the singlet-triplet transition of Ph& in Ph,C crys. tals. This should be 2 goodapproximationas can be seen from the spectrum of Ph,$n in Ph,Ge to be dip cussed next. 7.2.Ph& in Ph4Ge The concentration of the mixed crystal studied was 0.15% of Ph.,Sn in the QGe host. ‘lhe spectrum is displayed in fig. 9 (bottom). The O-0 baud of the Tr +S,

absorptionof the hGe crystalalsoappearsin the spec-

T.-S. tin, Phosphorescenrstareof

trum. The O-O band of the emission is 33 cm-l redshifted from the 60 band of the triplet absorption of Ph&e crystals. ‘fhe prominent vibrational modes are 293,559 (v6, al orb ), 999 (q, al), 1292(q, b,), 1860 and 1881 cm -1 . The us modes are likely

buried under the broad and intense band centered at 1701 cm-l. The vibrational frequencies of these modes differ from the systems of PhqGe in Ph,Sn, and Ph,Sn in Ph,Pb. The observed err&on of Ph4Sn in Ph4Ge must arise from the defect sites of the host lattice. The proximity of the O-O band of the absorption

and the O-O band of the impurity induced emission for Ph,Sn in Ph,Ge would support our previous assignment of the singlet-triplet transiton of pure Ph4C. An accurate measurement would have to rely on the excitation spectrum. A very broad and intense emission band at 3682 A was observed (half bandwidth: -200 cm-l). Another broad but not so intense band appears at 3742 A(half bandwidth: -100 cm-l). These bands may be attributed to the triplet excimer formation. Similar broad bands were observed in the previous mixed crystal studies.

8. EPR measurements of the tetraphenylmethane phosphorescent state Hochstrasserand Marchetti have measured the exciton splittings for Ph,X solids (except Ph,C) [ 11. The splittingsarise from the interaction between phenyl rings(intramolecular interaction). The smallest triplet exciton splittingsof the series occur in Ph,Si (-2 cm-l); this splitting is still greater than the zero-field splittings(IFS) of the chromophore (D= 0.15 cm- t for benzene [2 1I). The spin should therefore quantize along the exciton principal axes as was demonstrated in the Zeeman study [l]. However the exciton splittingsof PhqC have not been reported. Thus the spin quantization axes and the ZFS of the triplet state of Ph,C are not known. We therefore undertook an electron paramagnetic resonance measurement of the Ph,C phosphorescent state in order to obtain the followinginformation: spin quantization axes, ZFS, and inter-ring interaction.

(C6HS),X

249

8.1. Pmiicted spectral patterns The EPR spectra of the triplet state of PhqX should be quite different in different spin coupling limits. The spin coupling schemes are determined by the relative magnitude of the spin dipolar interaction and the interring interaction. We shalldescribe the EPR spectral patterns in the following three coupling schemes. 8.1.1. Tetramer limit (exciton) In this limit, the inter-ring(intramolecular) interaction is stronger than the molecular spin dipolar interaction. The exciton interaction will tend to aIign the spin principal axes along the exciton framework. The exciton spin states in the molecular S, symmetry are $ (transforms as A representation), and a degenerate pair tx and t,, (E representation). The exciton spin principal valueswill then be modulated by the interring interaction to smallervalues [22-241. The &value should be zero in the S, symmetry. If Ph,X has tetrahedral symmetry (TJ: two phenyl rings lie in a plane which is perpendicular to the second plane containtig the other phenyl rings;a simple calculation yields 0.075 cm-t for the exciton D value (take D = 0.15 cm-1 for ihe chromophores). In the real structure, none of the phenyl rings lies in the same plane (4-6). so the D value in the tetramer case would be less than 0.075 cm-l ; one may take this as the upper limit for the tettamer model. As we mentioned earlier, the Ph,X crystal contains two molecules per unit cell. The two inequivalent molecules are related by the two-fold screw axis, c. Therefore the external field should see only one type of molecule whenever it is parallel or perpendicular to the c axis. The system we shall be concerned with is Ph,C in Ph4Sn. The predicted spectral patterns are given as follows: (a) Rotating axis: c(ff!k y). Since tx and 5 are a degenerate pair, one should observe one resonance signalbelow g = 2, and the other one coincident with g = 2 (because of the degeneracy). The resonance spectral patterns should be the same at all orientations as long as the magnetic field is perpendicular to the c axis.

(b) Rotatingaxis: 1 c. When the magnetic field is parallel to the c-axis (H(lz), one should observe two resonance signals,one above the g = 2 line and the other below it. These two signalsarise from one type

250

T.-S.

Lin.

Phosphorescenr

of molecule. When the magnetic field is off the crystallographic axis, one should observe two inequivalent tetrarners, and therefore two pairs of resonance signals (two below and two above the g = 2 line). Thus one should observe: at most four resonance signals in this orientation. When the field is 90” away from the caxis (HUx,y), one should again observe the same pattern as in (a) orientation.

stare

of(CgH5)4X

ZFS is complicated by the inter-ring interaction [23]. A 12 X 12 secular determinant is needed to map the ZFS and the exciton splittings.

If the propagation of the excitation wave is static, one would expect a modulation of ZFS. If it is dynamic as in the case of triptycene [24), the EPR line shape should be temperature dependent. 8.2. Observed spectral patterns

8.1.2. Monomer limit (chromophore) In this limit, the spin-spin interaction of the chromophore is greater than the exciton interaction. The phenyl rings preserve their identity and each of the phenyl rings will be treated in the frame of local Czv symmetry. The EPR signals should correspond to that of an individual phenyl ring. The predicted spectral patterns are as follows: (a) Routing oxis: c. The magnetic field is always perpendicular to the c-axis. In this magnetic orientation, the two molecules are inequivalent in the unit cell, while the phenyl rings of a molecule are only

pairwiseequivalent magnetically.Therefore one should observe two pairs of resonance signalsfor the two groups

of phenyl rings on one molecule, or Four pairs of signals for the two molecules in the unit cell. The resonance spectra would be orientational dependent because the phenyls should see different magnetic fields at different orientations. Thus a maximum of eight orientation-dependent resonance signals could appear in the spectrum. (b) Rotating axis: 1 c. When the magnetic field is parallel to the c-axis, the two molecules in the unit cell become equivalent; so are the four phenyl rings in each molecule. The resonance spectrum should show only one pair of signals, one above the g = 2 line and the other below it. This is the same as the exciton case. When the magnetic field is not parallel to the c-axis, one could observe as many as eight lines. Note that one cannot evaluate the ZFS parameters from the extrema of the above resonance signals because the magnetic fields are not parallel to any of the principal axes of the phenyl rings. 8.1.3. Intermediate case When the spin dipolar splitting is comparable to the exciton splitting, we are in the intermediate region. The overall spectral patterns would be similar to the individual chromophore case. The evaluation of

The EPR spectra of Ph,C dispersed in Ph$n crystals (0.05%) were obtained at liquid helium temperatures using an X-band EPR spectrometer. Two crystal orientations were used in the experiment: the rotating axis of the crystal was either parallel to or perpendicular to the c-axis. The spectral patterns are summarized as follows. (a) Rotating axis:c. The external field is always perpendicualr to the crystallographic c-axis. A plot of resonance fields versus crystal orientations is displayed in fig. 10 (top). The resonance fields changed as the crystal was rotated. We observed as many as four pairs of resonance lines for each orientation. (b) Rotutinguxis; 1 c. We observed only one pair of resonance signals (one above and one below the g = 2 line) when the external field was parallel to the c-axis (fig. 10 (bottom)). The spectral patterns changed as the crystal was rotated. Four pairs of signals appeared when the field was perpendicular to the c-axis. 8.3. ZFS and inter-ring interaction As discussedin section 8.1 thereshould be no change in the resonance fields as the crystal is rotated

around the c-axis(If is alwaysperpendicular to c) if the spin coupling scheme tends to the tetramer limit. Tbis is not what was observed,which implies that the inter-ringinteraction in the phosphorescent state of Ph,C is smaller than, or comparable to, the ZFS. From an order of magnitude calculation based on the latest X-ray data for the Ph&t crystal structure (host) [S], we obtain a D value of 0.10 f 0.01 cm-t for the phosphorescent state of Ph&. We have assumed the E value to be zero in the calculation. The estimated uncertainty for the D value arises from uncertainties in (1) the magnetic field readouts using a Bell 240 Incremental Gaussmeter. (2) the microwave frequency, and (3) the alignment of the crystal. The magnitude of the above D value is greater than

T.-S. fin, Phosphorescenrstare of(c6HS)4X

251

other systems - Ph4Si in Ph,Sn, perdeutero-Ph@i in Ph,Sn, and PhaGe in Ph,Sn - in which the inter-ring

interactions are known from optical studies [ 11.Unfortunately, we failed to observe any EPR sighs from these systems. The difficulty may be due to the short phosphorescent lifetimes in these systems. Application of the technique of optical detection of magnetic resonance

to these systems may overcome this difficulty.

9. summary

0

2800

3200

3600

Resonance Field, Gauss Fig. 10. Plots of EPR signals vs. angles of rotation for the (C&)&phosphorescence in (C&).&l CfYStdSat4.2 K. Top: the totAng atis is the crystallographic c axis. Botrom: the rotating axis is the LIaxis (I E).

the uppermost tetramer value (0.075 cm-l), but smaller than the monomer value (0.15 cm-t). Apparently the spin splitting is modulated by the inter-ringinteraction. The magnitude of inter-ring interactions may be comparable to the ZFS. An exact evaluation of the inter-ring interaction would require more accurate measurements ofZFS and a complete diagonalization of the resulting 12 X 12 matrix. A temperature dependent study of the EPR line shape is still needed in order to determine the nature of interaction among the phenyl rings - static or dynamic. It may be worthwhile to mention that we have tried to obtain the EPR spectra of Ph,C in EPA or toluene glass samples at 4.2 K, but have failed to observe any triplet signals even using a signal averager. The difficulties may be due to (1) the extremely low solubility of

Ph4C in these solvents, and (2) the facile photodecomposition of Ph,C [2S]. We have also tried these EPR techniques on several

High resolution opticaI studiesof Ph,X phosphorescence in mixed crystal systems at low temperatures have yielded the following information: (1) The nature of the heavy atom effect: Both electronic interactions and vibronic interactions are responsible for $e internal effect on the enhancement of singlet-triplet transitions; it is mostly electronic interactions, with scme vibronic, which are responsible for the external effect. (2) The solvent effect: Dispersive forces and exciton-trap interactions are responsible for the specrcal shifts in these systems. (3) The intermolecular interaction: The interaction between translationally equivalent molecules accounts for the new sub-spectral systems in concentrated mixed crystals. The energy transfer process is likely controlled by this type of interaction. (4) Location of the singlet-triplet transition in the Ph,C neat crystal: The origin of this transition has been assignedto a band at 27474 cm-l in the Ph4C crystal by the technique of impurity induced (0.2% Ph,Si) emission. Finally, EPR studies of the phosphorescent state of Ph,C show that the magnitudes of the spin dipolar interaction and the inter-ring interaction are comparable (-0.I cm..‘).

Acknowledgement I am grateful to Professor S.I. Weissmanfor making the EPR spectrometer and the helium dewar available to me. I also wish to thankMr.J.R.Braun for the preparation of perdeuterated tetraphenylsilane. ‘Thiswork was supported by a grant-in-aidfrom the kesearch Cor. poration.

252

T.-S.

Lin.

Phosphorescent

References [ 11 R.M. Hochstrasser and A.P. Marchetii, J. Chcm. Phys. 52 (1970) 1360. (21 M. Gouterman and P. Sayer, Chem. Phys. Letters 8 (1971) 126. [3] S.R. LaPaglia, J. Mol. Spectry. 7 (1961) 427; Spectmchim. Acta 18 (1962) 1295. 141 A.1. Kifaiogorodskii, Organic chemical crystallography (Consullams Bureau Enterprises, Inc., New York, 1961) p. 120. [S] P.C. Chieh and J. Trotter, J. Chem. Sot. A (1970) 9: 1. 161 PC. Chieh. J. Chem. Sot. A (1971) 529; (1972) 1207. [7] H. Kayser,Tabelle der Schwingungzahlen. revised by W.F. Meggers (Edward Brothers. Inc.. Ann Arbor, Michigan, 1944). 18) E.R. Bernstein. S.D. Colson. D.S. Tiiti and C.W. Robinson, I. Chem. Phys. 48 (1968) 4632. 191 T.S. Lin. unpublished results. ilO] A.L. Smith, Spectrochim. Acta 23A (1967) 1075. [ll] D.H.WhBTen. J.Chem.Soc. (1965) 1350. 112) JR. During. C.W. Sink and J.B. Turner, Spectrochim. Acta 26A (1970) 557.

state

of

QH&X

1131 D.P. Craig, Advan. Chem. Phys. 8 (1965) 27; D.P. Draig and S.H. Walmsley, Excitons in molecular crystals (Benjamin, New York, 1968) Ch. 6. 1141 H.C. Loaguet-Higgins and J.A. Pople, J. Chem. Phys. 27 (1957) 192. T. Azumi and M. Kinoshita, Molecular I151 S.P. McClynn. spectmscopy of the triplet state (Prentice-Hall, New Jersey, 1969)Ch. 7-S. M. Kasha. J. Chem. Phys. 20 (1952) 7. G-W. Robinson, J. Mol. Spectry. 6 (1961) SS. B.W. Gashand S.D. Colson, J. Chem. Phys. 59 (1973) 3528. and references therein. J. Chem. Phys. 17 (1949) 905. I191 D.S. McClute, 1201D.M. Hanson, J. Chem. Phyr 52 (1970) 3409. WI M.S. de Groat, 1.A.M. Hesselmann and J.H. van der Waals, Mol. Phys. 16 (1969) 45. 1221Ph. Kottis, J. Chem. Phys. 47 (1967) 509. 1231 R.M. Hochstrasser and T.-S. Lin, J. Chem. Phys. 49 (1968) 4929. 1241 M.S. de Groat and J.H. MII der WaaIs, Mol. Phys. 6 (1963) 545. I251 T.-S. Lin, Chem. Phys. Letters 19 (1973) 410.