Exciton coupling and charge resonance in the lowest excited states of lutetium phthalocyanine dimer and trimer

14October 1994

CHEMICAL PHYSICS LETTERS

ELSEVIER

Chemical PhysicsLetters 228 (1994) 625-632

Exciton coupling and charge resonance in the lowest excited states of lutetium phthalocyanine dimer and trimer N a o t o I s h i k a w a 1, Y o u k o h K a i z u Department of Chemistry, TokyoInstituteof Technology, O-okayama,Meguro-ku, Tokyo 152,Japan Received 31 May 1994;in final form 9 August 1994

Abstract

The measurement of the electronic spectrum of triple-decker phthalocyanine trimer in solution showing the existence of lowlying excited states in the near-IR region is reported. The spectroscopic measurement in solution was made possible by using a newly synthesized trimer ( [ (Pc)Lu (CRPc)Lu(Pc) ]: Pc = phthalocyanine; CRPc= 15-crown-5substituted Pc) having high solubility and purifiability. The lowest excited states caused by intra- and inter-macrocyclicHOMO-LUMO transitions are studied in terms of exciton coupling and charge resonance. The inherent differences between the trimer and dimer are elucidated. The assignment of absorption bands below 20 × 103 cm- 1is discussed. 1. Introduction

Molecular stacking structures composed of a small number of phthalocyanines and porphydns have been subjected to extensive investigations as models of higher dimensional structures such as molecular semiconductors or the photosynthetic reaction center. Phthalocyanine, a n-conjugated macrocycle, forms several kinds of face-to-face dimer structures: [M(III)(Pc)2], [ M ( I V ) ( P c ) 2 ] , [(PcSi)20], etc. (Pc: phthalocyanine; M(III): lanthanide, scandium and yttrium; M(IV): tin and zirconium) [1-7]. In particular, the lanthanide phthalocyanine dimers have been intensively studied from the standpoint of electrochemistry because of their electrochromic property [8-11 ] and intrinsic semiconductivity [12-16]. Similar structures are seen in porphyrins and have been studied as analogs of the photosynthetic reaction center [ 17-19 ]. Present address: Departmentof Chemistry,Universityof California, Berkeley,CA 94720, USA.

Spectroscopic properties of molecular assemblies consisting of discrete chemical parts are governed by two factors: the local electronic structure and the collective electronic structure. To see each contribution in the closed-shell dimers, we proposed to describe the lowest excited states with linear combinations of intra- and inter-macrocyclic (n-n*) transitions, i.e. exciton coupling and charge resonance configurations [20,21 ]. This description was also introduced in semi-empirical CI calculations by using localized orbitals (LOs) that have maximum population on each macrocycle, and thereby the lowest excited states of the dimers are studied theoretically [ 21,22 ]. For the sake of verification of these ideas, studies of larger systems, such as the trimer, are necessary. A synthesis ofphthalocyanine trimer, [ ( P c ) L u ( P c ) L u ( P c ) ], has been reported by Kasuga [23 ]. However, due to its low solubility in organic solvents, no detailed spectroscopic data has been reported. Triple-decker trimers have also been reported for porphyrin: [Ce2(OEP)3], [Laz(OEP)3], and [Eu2(OEP)3], (OEP=octaethylporphyrin) [24-

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N. Ishikawa, Y. Kaizu / Chemical Physics Letters 228 (1994) 625-632

Spectroscopic and electrochemical measurements on these trimers have been made and their electronic structure has been discussed in MO terms [24-27 ]. Nevertheless, the inherent differences between the dimer and the trimer in their excited states have not necessarily been elucidated because of their complicated absorption spectra and the presence of two nearly degenerate HOMOs of porphyrin. In the case of phthalocyanine, the HOMO and LUMO are separated in energy from the rest of the MOs, and its lowest excited state is described predominantly by a single HOMO-LUMO excited configuration, 16es,-2alu), while that of porphyrin involves two configurations, 14eg,- la~u) and 14eg,--3a2u) [28]. Therefore, the phthalocyanine trimer is also important as a preliminary case before considering the more complicated case of porphyrin trimers. Previously, we reported the synthesis and spectroscopic properties of a new stacking structure, phthalocyanine ( 2 + 2 ) tetramer [29]. The tetramer is composed of two Pc dimer radicals [Lu(CRPc)(Pc)] (CRPc: crown ether substituted phthalocyanine), bound to each other by means of the complex formation between potassium cations and crown ethers in the dimers. In the synthetic preparation of [Lu(CRPc) (Pc)], we found that trimer-type compounds are obtained as by-products. We sought a modified synthesis to obtain those trimers as the main products. These trimers also have crown ether moieties, and consequently show high solubility and purifiability. The introduction of crown ethers to the periphery of phthalocyanine is known to drastically increase solubility and to cause little change in the lowest absorption band profile [ 30-32 ]. The synthesis of the soluble trimer allowed us to obtain detailed spectroscopic data for the phthalocyanine trimer. The absorption spectrum of [(Pc)Lu(CRPc)Lu(Pc)] (Fig. 1 ), the symmetric isomer, shows some intrinsic differences from that of the corresponding dimer, [ Lu (Pc) 2] -. In this Letter, we present experimental data and discuss the lowest excited states of the trimer in terms of exciton coupling and charge resonance. A qualitative description of the essential difference between the lowest excited states in the trimer and dimer is given.

e:N o:0

[(Pc)Lu(CRPc)Lu(Pc)] Fig. 1. Schematic diagram of crown ether substituted lutetium phthaioeyanine trimer, [ (Pc)Lu( CRPc )Lu(Pc ) ].

2. Experimental

2.1. Synthesis [(Pc)Lu(CRPc)Lu(Pc)] was synthesized by a similar method to that used for [Lu(CRPc)(Pc) [27 ] modifying the molar ratio of two starting materials. H2CRPc (0.12 g) and three times the molar amount of [Lu(Pc) (CH3COO)(H20)2] (0.22 g) were put into dried l-chloronaphthalene ( l0 ml), and the mixture was refluxed for 6 h. The mixture was allowed to cool to room temperature, and the products were precipitated through the addition of 50 ml ofhexane. The filtered precipitate was extracted with chloroform, and then was chromatographed on alumina (Merck alumina 90). Following the initial green band with chloroform as eluent, a blue fraction of [ (Pc)Lu (CRPc)Lu(Pc) ] was obtained by use of 1% methanol/chloroform. Repeating the chromatographic procedure and recrystallizing from chloroform/hexane gave a dark blue microcrystalline powder of [ (Pc)Lu (CRPc)Lu (Pc) ]. The compound was identified by elemental analysis, mass spectrum (FAB

N. Ishikawa, Y. Kaizu / Chemical Physics Letters 228 (1994) 625-632

method on JEOL JMS-HX 110/HX110 utilizing MS1 only) and IH NMR spectrum.

627

2.5

2.0-

2.2. Analysis Calculated for CI28HIo402oN24Lu2:C , 58.05; H, 3.96; N, 12.69. Found: C, 57.95; H, 3.73; N, 12.54. MS: m / e 2647.6 (mol. wt. 2647.6). IH NMR (CDC13): J 8.52 (2H s), 8.26 (1H m), 7.91 (2H s), 5.16 (2H, s), 4.67 (2H m), 4.31 (2H m), 4.37 (2H m). H2CRPc was prepared according to Kobayashi and Lever [33]. [Lu(Pc) (CH3COO) (H20)2] was prepared by the method of De Clan et al. [ 1 ]. The chloroform used for the synthesis was purified by distillation and passed through an alumina column just before use.

"7 ~

E

=o

1.5-

1.0-

0.5-

./ i

i

1.0 0.5 "7

--0.5 -1.0

2. 3. Measurement

5

10

15

20

25

30

35

40

Wavenumber / lOacrn -~

The absorption spectrum was measured on a Hitachi spectrophotometer 330. The magnetic circular dichroism (MCD) spectrum was taken on a JASCO spectropolarimeter J-500C in an external magnetic field set at 1 T. The solvent used for the spectral measurement was purified by distillation.

3. Results Fig. 2 shows the absorption and MCD spectra of [ (Pc)Lu(CRPc)Lu(Pc) ] in chloroform solution. In the Q-band region, two bands are observed at 15.8× 103cm -~ (633 nm) and 14.0× 103cm -~ (716 nm) similarly to [ L u ( P c ) 2 ] - . Both bands show MCD A-term dispersion, which indicates that the bands are doubly degenerate. Two shoulder bands are observed at 17.2 × 103 c m - 1 ( 580 nm) and 18.2 × 103 c m - 1 ( 550 nm). The former can be assigned to a vibronic band of the intense 15.8 × 10a cm- ~band since it has common characteristics with the vibronic bands of the Q band of monomer [ M (Pc) ] (M = divalent metal) and dimer [ L u ( P c ) 2 ] - : they are located at l A X 103 cm -1 higher than the '0-0' band and exhibits a MCD B-term maximum which coincides in position with the absorption maximum. The MCD maximum of the latter, however, does not coincide with the absorption maximum, which indicates a

Fig. 2. Absorption (top) and MCD (bottom) spectra of [ ( P c ) L u ( C R P e ) L u ( P c ) ] in chloroform.

considerable A-term contribution and suggests that the band is attributable to another electronic excitation. In the lower energy region, two shoulder bands at 10.5× 103 cm -1 (950 nm) and 12× 103 cm -1 (830 nm) are observed. A contribution of the A-term in the MCD band is appreciable in the former but unclear in the latter. In the near-IR region, a broad band whose maximum is at 7.4× 103 c m - ' (1360 nm) is observed. This lowest energy band is apparently different in its profile from the near-IR band at 1400 nm of dimer radicals, [ Lu (Pc) 2 ], [ Lu (CRPc) (Pc) ] and [ Lu (CRPc) 2 ] which may be by-products. All the bands below 12X 10 3 cm -1 obey the Beer-Lambert law. Their relative strengths are invariant during the purification procedure. Thus we conclude that they are intrinsic to [ ( P c ) L u ( C R P c ) L u ( P c ) ] . In the Bband region, a broad band is observed at 30.3× 10a cm -1 (330 nm) with a shoulder band at 27×103 e r a - 1.

N. lshikawa,

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Y. Kaizu / Chemical Physics Letters 228 (I994) 625-632

4. Discussion

ICR- ) = ~

4. I. The lowest excited states of the closed shell dimer, [Lu(Pc)2]-

1 (ll(k,_a)B~x)+ll(j,_b)A~B)).

Before discussing the electronic excited states of the trimer, we will briefly mention the simpler system, the closed shell dimer [Lu(Pc)2]-, for the sake of comparison. The method of treatment follows that of our previous paper [ 21 ]. The ground state and two locally excited singlet configurations of the dimer may be described as follows: IG ) = l aabbl,

(1)

ILEA) = (IjabGI- Ijabbl ) / x / ~ ,

(2)

ILEB) = (I aa/d~l- l a a ~ l ) / x / ~ ,

(3)

where a and b are the HOMOs of component Pc rings A and B respectively, and j and k are the respective LUMOs of A and B. The exciton coupling between them,

(LEAIfllLEB)=2(jalbk)-(jklba)-e~mer

(4)

gives excited states IEC+_ ) - - ~

1

ILEA) + ~

1

ILEB).

(5)

De Clan et al. showed by X-ray study that the two Pc rings are rotated ~ 45 ° with respect to each other, and thus the dimer has Dgd symmetry [ 1,2 ]. In this case, the plus and minus combinations correspond to the allowed and forbidden excited states respectively. The interaction between charge transfer configurations, lET B'A)

----

(I kabbl - Ikabbl ) / v / 2 ,

ICTA-B) = ( l a a j b l - laafbl ) / x / ~ ,

(6) (7)

is given by (CTB~A I/~rlCTA~B)

= -- (kBjAIbBaA) +2(kBaAI bBjA) .

1 (ICT B~A) + [CT A~B) )

(8)

Since both sides of each two-electron integral contain orbitals belonging to different sites, this interaction is much smaller then the exciton coupling. This interaction leads to a pair of charge resonance states

(9)

The transition intensity of IC R + ) is given by the interaction with the exciton component IEC + ). By the previous work [21 ], the higher-energy band in the Q-band region of [Lu(Pc)2]- and [Sn(IV) (Pc)2] has been assigned to the IE C + ) state, and the lower band to the ICR + ) state. The calculation [21 ] also showed that the forbidden IE C - ) state is located at the lowest energy. Lowering of the molecular symmetry from D4a can provide the transition intensity to this forbidden state. The weak and broad band spreading over 7 X 103.12 × 103 cm-I reported by Shirk et al. for the dimer anions [ 31 may correspond to the IE C - ) state.

4.2. The lowest excited states of the trimer The lowest excited states of the lutetium phthalocyanine trimer can be treated similarly. The ground state and locally excited singlet configurations of the trimer are described by IG) = laab6c~l,

(10)

ILEA) = ( Ijabbcgl - Ijabbcgl ) / x / ~ ,

( 11 )

ILE B) = ( l aald~c~l - l aakbcel ) / x / ~ ,

(12)

ILEc ) = ( l aab6l~l - l aabbl--cl )/v/-2,

(13)

where a, b and c are the HOMOs of component Pc rings A, B and C respectively, and j, k and l are the respective LUMOs of A, B and C as illustrated in Fig. 3. The ring B is assumed to be at the center. Interactions among the three excited configurations are:

(LEAlt:IltE~)=2(jalbk)-(jklba)=-e,

(14)

(LE~llCllLEC)=2(kblcl)-(kllcb)---e ' ,

(15)

(LEAItZIILEC)=2(jalcl)-(jllca)=--e " .

(16)

In the case of the triple-decker phthalocyanine trimer, the last interaction is smaller than the other two (e, e ' > e" ). If the interplanar distance and orientational angle are equal to those of the dimer, the exciton couplings between neighboring rings, e and e', are

N. Ishikawa, Y. Kaizu / ChemicalPhysicsLetters228 (1994)625-632

P~

k ~

pc c

Pd

629

The IEC + - + ) and IEC +++ ) states are allowed excited states. The ratio of squares of their transition dipole moments is predicted as ( v / 2 - 1 ) 2 / ( ~ + 1 ) 2 ~ 1/ 34 in this model. The 15.8 X 10acm band can be assigned to the IEC + + + ) state because

ILE')

o f its energy and intensity. A substantially weaker

band that corresponds to the IE C + - + ) state should be located at lower energy then the IEC +++ ) band since e > 0 in the case of a face-to-face configuration. The [EC + ' - ) is a forbidden state. Charge transfer configurations provide three allowed excited states as follows. Singlet charge transfer configurations are given by

Local ExcitonConfigurations

pc A

j

pc B

pcc

/

~,~. ~¢ ~ f,4 ICT"-") "', " " - . -'" " , . - " " ,'" ICT~ " " ' : ' " " " " ~ 2 / " ""C'" .

"~'""..'",JCT ~'-c)

[CT " ) . . - " . ,

a ~CT~-"b._H._

c._H__

Charge TransferConfigurations Fig. 3. Schematic diagram of local exciton configurationsand chargetransfer configurationsin the trimer. expected to be close to that of the dimer. They are equal if the monomer MOs are chosen as a basis set. Assuming that e" is negligible compared with e, and that the three diagonal energy terms are equal to that of the monomer, Era, the energy matrix of excited states is given by: Htrimer=

Em

e

.

( 17 )

Em

This gives the excited states

(24)

ICTB~C) =

(laabbkel- laabb-£cl )/,v/2,

(25)

ICT c-A> =

(llabbcel- Ilabbcc'l )/,,/~,

(26)

ICTA'-C) = (I aabGjel-

ladbbjcl )/v/2,

(27)

ICTA~B> = ( l a a j b c e l -

laafbc,3l )/V/2,

(28)

ICTC~e>=(laal6cel-laaTbcel)/v/2.

(19)

IEC+++) = ½lEE A) + (1/x/~) lEE a) + ½1LEC), (20) and their excitation energies E(EC ÷ - ÷ ) = E ~ - x/~e,

(21)

E(EC+'-) =Em,

(22)

E ( E C + + + ) =Em + V/2e •

(23)

(29)

The ICT e~A ) and ICT B~c ) states are equivalent in energy. The configuration interaction between them (CTB~AI/~ICTB'C) = 2 ( k a l c k ) - (kklca)

(30)

causes charge resonance configurations [CRA~B~C + ) = ( l E T B~A) + [CT B~c) )/~/'2. (31) Similarly, each of the energy-equivalent pairs, (ICTC-A), ICT A ' c ) ) and (ICTA~B>, ICTC~a) ), yields charge resonance configurations ICRA-C__.)=(IC~-A)_+ I C T A - C ) ) / x / ~ ,

IEC+-+ ) =½ ILE A ) - (I/v/2)ILE e ) + ½ ILEC), (18) [EC+'+) = (1/x/~) I L E A ) - (1/x/~) ILEC),

= ([kabSce[- [kabbcC'l )/~/2 ,

ICT e~A )

(32)

ICRA~B'C_+ ) = (ICT A'B) _ ICT c~B ) ) / x / ~ . (33) The three plus combinations are allowed by symmetry but practically forbidden. They borrow intensity mainly from the IEC + + ÷ ) state by configuration interaction. By the above consideration, five bands are predicted from nine 'HOMO-LUMO' excited configurations. The most intense band at 15.8X 103 c m - 1 corresponds to the IEC+ + + ) state. The IE C + - + ) band should be positioned at 2x/~ e lower than the

630

N. Ishikawa, Y. Kaizu / Chemical Physics Letters 228 (1994) 625-632

IEC + + + ) state. The exciton coupling term e has been estimated for the dimer at 2× 103-3× 103 cm -1 by our previous work [ 20,21 ]. Applying it to the trimer, the IEC + - + ) state is predicted to be lower by about 5 × 103-9 X 103 era- ~than the IEC + + + ) state, hence may contribute to either or both of the near IR bands at 10.5× 103 or/and 7.4× 103 cm -~. The rest of the bands at 14.0× 103 and 18.2X 103 cm -~ can be assigned to charge resonance states. Further detailed assignment, including determination of the ordering of the three charge resonance states, have been carried out with a LO-CI (configuration interaction in LO basis) calculation and will be reported elsewhere.

['ldimer=(Era/tE Erne'~-,)= ({~e ~)"

Therefore the energy of the allowed exciton coupling state is:

D==-E(EC+ )=~+e.

(35)

Provided the diagonal energy change in the trimer ILE B) configuration is ~' while those of ILEA) and ILEc) are the same as ~ of the dimer, the corresponding energy matrix is given by: Htrimer =

(Eo+~

Em

e+ ~' e

4.3. Treatment including orbital energy shifts The exciton coupling term e gives face-to-face dimer and triple-decker trimer blue-shifts of the absorption band of I e and x/~ e respectively. However, these are not consistent with the observed energy shifts of the Q band; the observed energies of the Q band of the monomer ([Lu(Pc)(CHaCOO)(H20)2]), the exciton band in the Q region of the dimer ( [ L u ( P c ) 2 ] - ) and that of the trimer are 14.9× 103, 16.2×103 and 1 5 . 8 × 10 3 cm -1, respectively. So far, we have neglected the difference in diagonal energies of the local exciton configurations due to the variation in the environments of the respective Pc rings. Rings A and C in the trimer mainly interact with the neighboring ring B. Energy shifts of the orbitals belonging to A and C can be close to that of the dimer since the environments in which A and C are placed are similar to that of the component Pc rings in the dimer. In the LO study of [Lu(Pc)2] -, it was shown that the energy gap between the highest occupied LO and the lowest unoccupied LO is diminished compared with that of the monomer [21 ]. On the other hand, ring B in the trimer interacts with both Pc rings and two lutetium ions. The energy gap offing B is expected to be different from those of rings A and C. Assuming that the diagonal energies ~ of the local exciton configurations of the dimer, (2) and ( 3 ), are different by E from the monomer excitation energy Em, the energy matrix of the exciton coupling of the dimer is written as

(34)

6t

0e

)

Em+~

.

(36)

e

The energies of the allowed exciton coupling states are: T+ - E ( E C +++) = ½[~+6' + x/(3-c~' )2+ 8e2 ] ,

(37)

T_ - E ( E C +- + ) = ½[6+8' - x / ( 8 - 8 ' )2+8e2 ] .

(38)

Solving the simultaneous equations ( 35 ), (37) and (38), the parameters, c~, 6' and e, are expressed in terms of D, T÷ and T_ as follows: 6=~ [T+ + T _ +4D

+x/(T+ - T _ ) 2 - 8 ( D - T ÷ ) ( D - T _ ) ]

,

(39)

6' =T+ + T _ - 6 ,

(40)

e=D-8.

(41)

The parameters can be determined by substituting experimental values in ( 39 ), (40) and ( 41 ). Table i shows the result obtained by using the above data (Em=14.9Xl03 cm -1, D=16.2×103 cm -~ and T+=15.8×103 cm -~) for two cases in which the IEC + - ÷ ) state is attributed to 10.5×103 and 7.4 × 103 cm-l. The roots in which e takes a negative value are shown in the table. In both cases, the decrease in energy of the ILEB) configuration is larger than those of ILEA) and ILEc): - ~' > - ~. This can be interpreted as follows: the central Pc is subjected

N. Ishikawa, Y. Kaizu / Chemical Physics Letters 228 (1994) 625-632

Table 1 The parameters (in 103 cm -~ ) determined by the spectroscopic data of [ (Pc)Lu(CRPc)Lu(Pc) ] and [Lu(Pc)2] -

T_ = 10.5 × 103 cm -~ 14.66 11.64

1.54

-0.24

- 3.26

T_ =7.4X 10acm -~ 13.58 9.62

2.62

- 1.32

-5.28

to a larger potential t h a n the Pcs on b o t h sides a n d thereby the excitation energy o f the central Pc is lowered.

5. Conclusions The synthesis a n d spectroscopic m e a s u r e m e n t o f a soluble lutetium p h t h a l o c y a n i n e trimer, [ ( P c ) L u ( C R P c ) L u ( P c ) ], were reported. The absorption and M C D spectra show two electronic transition b a n d s at 15.8 × 103 a n d 14.0 × 103 c m - 1, which resemble those o f [ L u ( P c ) 2 ] - , a n d weak bands at 7.4×103, 10.5X103, 12X103 and 18.2×103 cm - t . The increase in the b a n d s is caused b y the increase o f possible intra- a n d inter-macrocyclic H O M O - L U M O transitions. F r o m the three local excitation configurations, two allowed exciton coupling states, IEC+++> and [EC+-+), result. Six charge transfer configurations give three allowed charge resonance states, I C R A ~ B ' - c + ) , I C R A " c + ) a n d [CRA--B~C+ ) . The most intense b a n d at 15.8 × 103 cm -1 is assigned to the I E C + + + ) state. The I EC + - + ) state m a y contribute to both or either o f the two lowest bands. The b a n d s at 14.0× 103 a n d 18.2×103 c m -1 have a character o f charge resonance. A simple exciton coupling consideration resuits in a blue-shift o f the t r i m e r [EC + + + ) b a n d with respect to the d i m e r I E C + > band. However, this consequence contradicts the experimental result. The actual red-shift o f the IEC + + + ) b a n d is caused because the central Pc ring is subjected to a different potential field from that o f the two sides. Using exp e r i m e n t a l d a t a for the t r i m e r a n d dimer, the exciton coupling t e r m a n d the diagonal energy terms o f local exciton configurations were estimated. T h e local excitation energy o f the central Pc ring was found to be lower than that o f the sides.

631

Acknowledgement The authors wish to express their gratitude to Professor M a r t i n H e a d - G o r d o n a n d Mr. Chris W h i t e for helpful discussions.

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