Resonance Raman characterization of excitonically coupled meso,meso-linked porphyrin arrays

Article abstract of DOI:10.1021/jp000306s

Resonance Raman (RR) spectra are reported for dimeric, trimeric, and tetrameric porphyrin arrays in which the porphyrins are directly linked at the meso position. The RR spectra of two monomeric building blocks of the arrays are also examined. The close proximity and orthogonal orientation of adjacent porphyrins in the arrays result in exceptionally strong excitonic interactions along the axis defined by the meso,meso-linkage-(s) and negligible interactions along the orthogonal axis. The coupling scheme breaks the degeneracy of the B excited state and leads to two B(0,0) absorption features. One feature, designated B_x(0,0), is the supermolecule absorption that is strongly red-shifted, due to exciton coupling. The other feature, designated B_y(0,0), is the superposition of absorptions of the orthogonal, individual porphyrins. This latter absorption occurs at approximately the same energy as the B(0,0) band of a monomeric porphyrin, due to the absence of excitonic interactions. The exciton coupling in the Q state is much weaker than that in the B state, due to the smaller oscillator strength of the former transition. The B-state excitation RR spectra of the meso,meso-linked arrays exhibit a complex and unusual scattering pattern. The most striking features are (1) the appearance of only polarized and anomalously polarized modes in the RR spectrum, (2) the intensity enhancement of anomalously polarized vibrations with B-state excitation, and (3) the large differential enhancement of symmetric versus nontotally symmetric vibrations with excitation. across the B-state absorptions. All of these scattering characteristics are due to the effects of symmetry lowering. The asymmetric meso substitution pattern inherent to the meso,meso-linked arrays contributes to symmetry lowering in both the ground and excited electronic states. The strong uniaxial excitonic interactions make an additional contribution to symmetry lowering in the excited state(s). This latter characteristic promotes novel Franck-Condon and vibronic scattering mechanisms in the B state(s) of the arrays. Collectively, the studies of the meso,meso-linked arrays provide insight into the type of RR scattering that might be anticipated for other types of systems that exhibit strong excitonic interactions among the constituents.

Full text of DOI:10.1021/jp000306s

J. Phys. Chem. B 2000, 104, 10757-10764
10757
Resonance Raman Characterization of Excitonically Coupled meso,meso-Linked Porphyrin
Arrays†
Anwar A. Bhuiyan,‡ Jyoti Seth,‡ Naoya Yoshida,§ Atsuhiro Osuka,§ and David F. Bocian*,‡
Department of Chemistry, UniVersity of California, RiVerside, California 92521-0403, and Department of
Chemistry, Graduate School of Science, Kyoto UniVersity, Kyoto 606-8502, Japan
ReceiVed: January 25, 2000; In Final Form: May 1, 2000
Resonance Raman (RR) spectra are reported for dimeric, trimeric, and tetrameric porphyrin arrays in which
the porphyrins are directly linked at the meso position. The RR spectra of two monomeric building blocks of
the arrays are also examined. The close proximity and orthogonal orientation of adjacent porphyrins in the
arrays result in exceptionally strong excitonic interactions along the axis defined by the meso,meso-linkage-
(s) and negligible interactions along the orthogonal axis. The coupling scheme breaks the degeneracy of the
B excited state and leads to two B(0,0) absorption features. One feature, designated BX(0,0), is the supermolecule
absorption that is strongly red-shifted, due to exciton coupling. The other feature, designated By(0,0), is the
superposition of absorptions of the orthogonal, individual porphyrins. This latter absorption occurs at
approximately the same energy as the B(0,0) band of a monomeric porphyrin, due to the absence of excitonic
interactions. The exciton coupling in the Q state is much weaker than that in the B state, due to the smaller
oscillator strength of the former transition. The B-state excitation RR spectra of the meso,meso-linked arrays
exhibit a complex and unusual scattering pattern. The most striking features are (1) the appearance of only
polarized and anomalously polarized modes in the RR spectrum, (2) the intensity enhancement of anomalously
polarized vibrations with B-state excitation, and (3) the large differential enhancement of symmetric versus
nontotally symmetric vibrations with excitation across the B-state absorptions. All of these scattering
characteristics are due to the effects of symmetry lowering. The asymmetric meso substitution pattern inherent
to the meso,meso-linked arrays contributes to symmetry lowering in both the ground and excited electronic
states. The strong uniaxial excitonic interactions make an additional contribution to symmetry lowering in
the excited state(s). This latter characteristic promotes novel Franck-Condon and vibronic scattering
mechanisms in the B state(s) of the arrays. Collectively, the studies of the meso,meso-linked arrays provide
insight into the type of RR scattering that might be anticipated for other types of systems that exhibit strong
excitonic interactions among the constituents.
Introduction
with phenylene linkers14 or 1,3,5-triazine units joining aniline
groups,15 (6) cyclic arrays with diphenylbutadiyne linkers16 or
diphenylethyne linkers,17 and (7) backbone polymeric arrays
with oligophenylenevinylene linkers18,19 or phenylethyne link-
ers.20 The extent of electronic interaction between the porphyrin
constituents in these different classes of arrays varies consider-
ably, depending on both the length and type of linker (the latter
of which controls the relative orientation of the porphyrins).
The largest electronic interactions are observed in architectures
that contain direct meso,meso-linked porphyrins. The close
proximity of the porphyrins in these assemblies results in
excitonic interactions that are several tenths of an electronvolt.21
Natural photosynthetic systems employ elaborate light-
harvesting complexes to capture dilute sunlight and funnel the
captured energy to the reaction center through rapid and efficient
transfer processes.1 The light-harvesting complexes in the natural
system are generally composed of electronically coupled
chlorophyll (or bacteriochlorophyll) molecules assembled in
protein matrixes that absorb over a wide spectral range. The
challenge of creating artificial mimics of the light-harvesting
complexes has led to the design and synthesis of a diverse
collection of multiporphyrin arrays. Examples of covalently
linked architectures comprised of five or more porphyrins
include the following: (1) dendritic arrays with stilbene linkers,2
diphenylethyne linkers,3 or a combination of diphenylethyne and
ester linkers,4 (2) windmill arrays with direct meso,meso-
linkages,5 (3) sheetlike arrays with Pd-coordinated pyridyl
linkages,6 (4) star-shaped arrays with diphenylethyne linkers,7,8
oligophenylethyne linkers,9 phenylene linkers,10,11 phenylenevi-
nylene linkers,12 or benzoxyphenyl linkers,13 (5) linear arrays
Resonance Raman (RR) spectroscopy is a particularly sensi-
tive probe of the structural and electronic properties of metal-
loporphyrin complexes. Metalloporphyrins produce extremely
intense and detailed RR spectra that have been interpreted
successfully by using models for vibronically induced scattering
from the porphyrin B (Soret) and Q states.22 RR techniques were
used early on in attempts to characterize interporphyrin interac-
tions in a variety of covalently linked dimeric assemblies,
including the following: (1) axially bridged iron complexes
(TPPFe)2X, where (TPP ) tetraphenylporphyrin, X ) O,23,24
N,25 C26), (OEPFe)2X, where (OEP ) octaethylporphyrin, X )
† Part of the special issue “Thomas Spiro Festschrift”. Dedicated to
Thomas G. Spiro on the occasion of his 65th birthday.
‡ University of California.
§ Kyoto University.
10.1021/jp000306s CCC: $19.00 © 2000 American Chemical Society
Published on Web 06/28/2000

10758 J. Phys. Chem. B, Vol. 104, No. 46, 2000
Bhuiyan et al.
Experimental Section
Sample Preparation. The various porphyrin complexes were
prepared as previously described.21 The porphyrins were initially
prepared as Zn(II) complexes. Zinc was removed from the
complexes by treatment with aqueous HCl. Cu(II) was inserted
into the complexes using the standard procedure with Cu-
(OAc)2.34 To a solution of free base porphyrin (20-100 mg) in
CH2Cl2 (50-200 mL) was added a saturated solution of Cu-
(OAc)2 in methanol (1-3 mL). The resulting solution was
refluxed for 2 h. After the usual workup, the Cu(II) complexes
were purified by silica gel chromatography and recrystallized
from CH2Cl2 and methanol. The molecular weights were
measured by the MALDI-TOF MS method. Monomer m/e )
747, calcd for C48H52N4Cu ) 747.3; dimer m/e ) 1495, calcd
for C96H102N8Cu2 ) 1493; trimer m/e ) 2242, calcd for
C144H152N12Cu3 ) 2241; tetramer m/e ) 2988, calcd for
C
192H202N16Cu4 ) 2987.
Absorption Spectra. The absorption spectra of the porphyrin
complexes were obtained with a Hewlett-Packard Model 8452A
diode array spectrometer. The spectra were obtained in the 300-
800 nm range in the absorbance mode. All spectra were obtained
at room temperature in CH2Cl2 solutions; the sample concentra-
tion was typically ∼0.01 mM.
RR Spectra. The RR spectra of the complexes were obtained
with a triple spectrograph (Spex 1877) equipped with either a
1800 groove/mm (Q-excitation) or 2400 groove/mm (B-excita-
tion) holographically etched grating in the final stage. A liquid
nitrogen cooled, UV-enhanced 1152 × 298 pixel charge-coupled
device (Princeton Instruments, LN/CCD equipped with an EEV
1152-UV chip) was used as the detector. The excitation
wavelengths were obtained from a Kr ion laser (Coherent Innova
200-K3), an Ar ion laser (Coherent Innova 400-15UV), and a
dye laser (Coherent CR 590) using Stilbene 3 (Lamda Physik,
Inc.) pumped by the multiline UV output of an Ar ion laser
(Coherent Innova 400-15UV). All spectra were obtained at room
temperature in CH2Cl2 solutions; the sample concentrations were
in the 0.05-0.5 mM range. To mitigate photodecomposition,
the sample was placed in a rotating NMR tube. The NMR tube
was illuminated by a laser beam focused through a lens, and
the scattering was collected at an angle 90° to the incident laser
beam by using a camera lens (Canon 50 mm) collection system.
The laser power at the sample was typically in the 10-20 mW
range. Absorption spectra of the samples obtained before and
after the RR measurements were found to be identical, indicating
that no significant photodecomposition occurred during the
course of the RR experiments. The spectra were calibrated
by using the known frequencies of indene and are accurate to
(2 cm-1. The relative intensities for the polarization measure-
ments were obtained from the peak heights in the recorded
spectra using the 1423 cm-1 band of CH2Cl2 as the internal
standard.
Figure 1. Structures of the meso,meso-linked porphyrin arrays.
O, N),27 and (OECFe)2O,28 where (OEC ) octaethylchlorin);
(2) lanthanide sandwich complexes (TPP)2Ln, where (Ln ) Ce,
La),29a,b (OEP)2Ln, where (Ln ) Ce, La, Eu, Nd),29b and (TPP)-
(OEP)Ce;29c and (3) transition metal-metal-bonded complexes
(OEPM)2, where (M ) Os, Re, Mo).30 The common structural
feature shared by all of these dimers is that the porphyrin rings
are face-to-face. The electronic coupling between the porphyrins
ranges from quite strong in the lanthanide sandwich dimers to
relatively weak in the axially bridged iron and transition metal-
metal-bonded dimers. Interestingly, even the most strongly
electronically coupled dimers do not exhibit any evidence for
intradimer vibrational coupling in the ground electronic state.29
The same is also the case for the extended strongly coupled
face-to-face assembly, the (OEP)3Eu2 triple decker.31 More
recently, RR techniques have been used to investigate other
types of covalently linked assemblies, including star-shaped
arrays,7,8b,c a molecular square,32 and their dimeric and trimeric
(both linear and right-angle) building blocks.33 All of these latter
arrays are joined via diarylethyne linkers, and the interporphyrin
electronic communication is relatively weak. As a consequence,
the vibrational spectra of these arrays resemble those of the
isolated constituent porphyrins.
IR Spectra. The IR spectra were measured on a Bruker
Equinox 55 Fourier transform IR spectrophotometer. The spectra
were obtained at room temperature on polyethylene film IR
cards in the region 900-1600 cm-1. The complexes were
dissolved in CH2Cl2, and the concentrated solutions were spread
on the film. The solvent was evaporated by blowing air over
the film. The film was then placed in the sample compartment
of the spectrophotometer and purged with dry air for 5 min. A
polyethylene film IR card from which neat CH2Cl2 had been
evaporated was used as the background. The reproducibility of
the spectra was checked by multiple scans.
At this time, the only strongly (electronically) coupled
porphyrinic assembly that has been subjected to detailed
vibrational scrutiny is (OEP)3Eu2. To further investigate the
structural and electronic properties of strongly coupled multi-
porphyrin arrays, we have examined the RR scattering charac-
teristics of the dimeric, trimeric, and tetrameric meso,meso-
linked assemblies shown in Figure 1. Both the diaryl- and triaryl-
substituted (aryl ) 3,5-di-tert-butylphenyl) monomeric building
blocks were also examined. The studies of the meso,meso-linked
arrays reveal novel RR scattering characteristics that have not
been previously observed for metalloporphyrins.

Excitonically Coupled Porphyrin Arrays
J. Phys. Chem. B, Vol. 104, No. 46, 2000 10759
Figure 3. Schematic energy level diagram for the meso,meso-linked
dimer. The transitions to the excitonic states BX and QX are allowed,
whereas those to the excitonic states BX′ and QX′ are forbidden (see
text). The transitions to the monomer-like states By,y′ and Qy,y′ are
allowed. The energy splittings shown in the diagram are not to scale.
Figure 2. Absorption spectra (295 K) of the meso,meso-linked
porphyrin arrays in CH2Cl2. The arrows indicate the excitation
wavelengths used in the RR studies.
Results and Discussion
the porphyrins, as has been previously discussed.21 A coupling
scheme that accounts for the spectral features is as follows: the
transitions to the Q- and B-excited states of metalloporphyrins
are in-plane (x, y) and (approximately) degenerate (Eu in D4h
symmetry).35 Thus, the choice of axes for the electronic
transitions is arbitrary (see Figure 1). If the lines through
opposite methine carbons are chosen as the axes, one axis of
the array (X) is along the meso,meso-linkage(s). Strong excitonic
interactions are expected along this axis because the transition
dipoles of the individual porphyrins are in-line (note that the
dipole-dipole interactions along this axis are independent of
the torsional angle between adjacent porphyrins).36 On the other
hand, the close proximity of the porphyrins in the arrays dictates
that the planes of adjacent rings and, accordingly, the y-axes of
nearest neighbor porphyrins are (nearly) orthogonal (due to steric
constraints). The dipolar exciton coupling in the orthogonal
orientation vanishes.36 This feature obviates the utility of
defining a Y-axis for the arrays. It should also be noted that the
excitonic interactions between non-nearest neighbor, coplanar
porphyrins in the trimer and tetramer are expected to be
negligible, owing to the substantial distance between these
constituents. The orthogonal orientation of adjacent porphyrins
in the arrays also precludes any conjugation between these
constituents. This situation can be contrasted with that of
porphyrin arrays with linkers such as butadiyne wherein strong
conjugative effects are observed.37
Absorption Spectra. The absorption spectra of the diaryl-
substituted monomer and the three meso,meso-linked arrays are
shown in Figure 2. The spectrum of the triaryl-substituted
monomer (not shown) is similar to that of the diaryl-substituted
monomer. The spectra of both monomers are also similar to
that of CuTPP (not shown).35 However, the number of aryl
substituents does affect the positions of the band maxima (diaryl-
substituted monomer, B(0,0) ) 406 nm, Q(1,0) ) 528 nm;
triaryl-substituted monomer, B(0,0) ) 412 nm, Q(1,0) ) 534
nm; CuTPP, B(0,0) ) 416 nm, Q(1,0) ) 539 nm). The optical
spectra of the Cu(II) complexes of the meso,meso-linked
porphyrins investigated here exhibit the same general charac-
teristics as previously reported for the Zn(II) analogues.21 In
particular, all three meso,meso-linked arrays exhibit two B(0,0)
bands. The maximum of one B(0,0) band of each array occurs
at approximately the same wavelength as the B(0,0) band of
the monomeric meso-aryl-substituted porphyrins (406-416 nm).
The maximum of the other B(0,0) band is appreciably red-
shifted and moves systematically to lower energy as the size of
the array increases (dimer, 444 nm; trimer, 462 nm; tetramer,
471 nm). The behavior of the Q(1,0) bands of the meso,meso-
linked arrays is qualitatively similar to that of the B(0,0) bands.
For the Q(1,0) bands, a shoulder is observed at approximately
531 nm for each array, which is in the same wavelength regime
as the Q(1,0) bands of monomeric meso-aryl-substituted por-
phyrins (528-537 nm). The second Q(1,0) band of the arrays
is red-shifted (dimer, 547 nm; trimer, 552 nm; tetramer, 558
nm), but to a much lesser extent than the analogous B(0,0) band.
The extent of the red-shift of the second Q(1,0) band of the
arrays as a function of array size appears to be less systematic
than that of the second B(0,0) band. However, the Q(1,0) bands
are quite broad and much weaker, which precludes an accurate
identification of the band maximum.
The strong excitonic interactions between the in-line transition
dipoles along the X-axis of the arrays combined with the null
interactions between the transition dipoles along the y-axes of
the constituents formally break the degeneracy of the Q- and
B-excited states of the individual porphyrins in the array. A
schematic energy level diagram indicating the excitonic split-
tings of Q- and B-excited states is shown in Figure 3. The in-
line orientation of the transition dipoles along the X-axis dictates
that the transition to the lowest energy excitonic state (BX or
QX) is strongly allowed, whereas the transition to the highest
The trends observed in the optical spectra of the meso,meso-
linked arrays are consistent with excitonic interactions between

10760 J. Phys. Chem. B, Vol. 104, No. 46, 2000
Bhuiyan et al.
Figure 4. RR spectra of the diaryl-substituted monomer in CH2Cl2.
The bands marked by the asterisks are due to solvent.
Figure 5. RR spectra of the dimer in CH2Cl2. The bands marked by
the asterisks are due to solvent.
energy state (BX′ or QX′) is forbidden.36 The transitions to the
other excitonic states that occur at intermediate energies for the
higher order aggregates (trimer and tetramer) are also only
weakly allowed. The transition to the lowest energy B excitonic
state, designated BX(0,0), is a supermolecular transition that
gives rise to the highly red-shifted B(0,0) band of the arrays.
The exciton coupling energy for the arrays can be extracted
from plots of the exciton splitting, ∆E, versus 2 cos[π/(N +
1)], where N ) array size.36 Previous studies of the Zn(II)
complexes of the meso,meso-linked arrays have shown that this
plot is linear with a slope of ∼4200 cm-1 21,38
.
This is also the
case for the Cu(II) complexes studied here. The exciton coupling
energy between adjacent porphyrins in the arrays is half the
value of the slope,36 or ∼2100 cm-1. The transition to the lowest
energy Q excitonic state gives rise to the (less) red-shifted Q(1,0)
band. The red shifts of the Q(1,0) bands are smaller than those
of the analogous B bands, owing to the smaller transition dipole
moment of the former state. This feature, combined with the
breadth of the absorption band, precludes an accurate determi-
nation of the Q-state exciton coupling energy. The electronic
transitions polarized along the y-axes of the individual porphy-
rins are energetically nearly coincident due to the general
structural similarity of the porphyrins in the arrays and the
absence of excitonic interactions between the orthogonal transi-
tion dipoles. This results in B(0,0) and Q(1,0) absorption features
that are at approximately the same energy as a monomer. For
convenience, this B-state absorption is designated By(0,0), even
though it represents multiple transitions.
Figure 6. RR spectra of the trimer in CH2Cl2. The bands marked by
the asterisks are due to solvent.
Vibrational Spectra. The high-frequency regions (1000-
1650 cm-1) of the RR spectra of the diaryl-substituted monomer
and the three meso,meso-linked arrays obtained with selected
exciting lines are shown in Figures 4-7. The IR spectra of the
various complexes are shown in Figure 8. The RR spectra of
the triaryl-substituted monomer (not shown) are similar to those
of the diaryl-substituted monomer. The spectra of both mono-
mers are also similar to those of CuTPP (not shown).39 The
frequencies of most of the RR bands of the three types of
monomers are very similar. Accordingly, the spectra were
assigned by analogy to the assignments previously reported for
CuTPP39 (and NiTPP40). The frequencies of the RR bands of
all three meso,meso-linked arrays are also similar to those of
the monomers. Accordingly, the RR bands for the arrays were
also assigned by analogy. The assignments for selected RR
bands attributable to porphyrin ring skeletal modes are sum-
marized in Table 1. Detailed assignments of the IR bands are

Excitonically Coupled Porphyrin Arrays
J. Phys. Chem. B, Vol. 104, No. 46, 2000 10761
polarization characteristics of certain modes of the meso,meso-
linked arrays and the monomeric building blocks versus CuTPP
and (3) the B-state RR enhancement patterns for the meso,-
meso-linked arrays versus the various monomeric meso-aryl-
substituted porphyrins, including CuTPP. As will be discussed
below, all of these scattering characteristics are due to the effects
of symmetry lowering. The asymmetric meso substitution pattern
inherent in the meso,meso-linked arrays (and diaryl- and triaryl-
substituted monomers) contributes to symmetry lowering in both
the ground and excited electronic states. The strong uniaxial
excitonic interactions make an additional contribution to sym-
metry lowering in the excited state(s). This latter characteristic
promotes novel Franck-Condon and vibronic scattering mech-
anisms in the B state(s) of the arrays.
Vibrational Characteristics of the meso,meso-Linked Arrays
and the Monomeric Building Blocks. The meso substituents and
substitution pattern in the diaryl- and triaryl-substituted mono-
mers and the meso,meso-linked arrays differ from that of TPP
(Figure 1). All of the complexes contain meso-3,5-di-tert-butyl
rather than phenyl groups. At first approximation, the structural
differences between these two types of meso substituents would
not be expected to perturb the vibrational characteristics of the
porphyrin rings. On the other hand, the meso substitution pattern
in both the diaryl- and triaryl-substituted monomers and all three
arrays is highly asymmetric and does alter the vibrational
characteristics of the porphyrins. The asymmetry in the mono-
mers is due to the presence of meso hydrogen atoms. Both
porphyrins in the dimer and the two terminal porphyrins in the
trimer and tetramer also have a hydrogen atom at the outer meso
position. However, these porphyrins have a porphyrin at the
other meso position rather than an aryl group. The central
porphyrin(s) of the trimer and tetramer have two porphyrin
substituents at the meso positions.
Figure 7. RR spectra of tetramer in CH2Cl2. The bands marked by
the asterisks are due to solvent.
The asymmetric meso substitution pattern in the diaryl- and
triaryl-substituted monomers and the constituent porphyrins in
the arrays breaks the (approximate) 4-fold symmetry of the
ground electronic state of the individual porphyrins. The
substitution pattern formally mixes the A1g and B2g (RR active)
vibrations and the A2g and B1g vibrations (RR active). The
substitution pattern also formally splits the Eu modes (IR active)
and mixes one member of the Eu pair with the A1g/B2g set and
the other member of the Eu pair with the A2g/B1g set. These
perturbations, along with mass effects (hydrogen atoms versus
aryl groups or porphyrins), most appreciably affect vibrations
involving atoms at the site of the perturbation, in particular,
the methine bridge stretches, νCaCm, and the porphyrin-aryl
group stretches, νCaCaryl. The behavior of these modes is
discussed in more detail below.
The νCaCm vibrations of MTPP complexes include ν10(B1g),
ν19(A2g), and ν37(Eu). The ν10, ν19, and ν37 modes of CuTPP
are observed at 1583, 1531, and 1574 cm-1, respectively.36,41
Comparison of the RR spectra of the diaryl- and triaryl-
substituted monomers and CuTPP shows that both ν10 and ν19
systematically upshift as the number of meso-aryl groups
decreases (ν10, 1585, 1612, and 1623 cm-1; ν19, 1531, 1540,
and 1547 cm-1). This trend is also consistent with the vibrational
characteristics reported for NiTPP versus NiP (P ) porphine,
which contains hydrogen atoms at all four meso positions). The
presence of four meso hydrogens in NiP upshifts ν10 and ν19
even more substantially (50-60 cm-1 relative to those of
NiTPP).40 The vibrational studies of NiTPP and NiP indicate
that ν37 should also follow this same trend. However, the only
IR bands for the diaryl- and triaryl-substituted monomers that
appear to be candidates for ν37 are observed at 1557 and 1565
Figure 8. IR spectra of the thin films of porphyrin complexes on
polyethylene.
not included in the table. These data proved less useful because
comparison of the IR spectra of the various porphyrin complexes
with that of the 1,3-ditertbutylbenzene (the aryl substituent of
the arrays) (not shown) revealed that most of the IR features
observed for the porphyrin complexes are attributable to the
aryl substituents.
Although many similarities exist between the RR spectra of
the meso,meso-linked arrays and the various monomeric por-
phyrins, including CuTPP, there are certain notable differences.
These differences include (1) the vibrational and (2) the

10762 J. Phys. Chem. B, Vol. 104, No. 46, 2000
Bhuiyan et al.
assignmentb
TABLE 1: Selected RR Bands (cm-1) of the Complexes Assignable to Porphyrin Skeletal Modes
monomer
dimer
trimer
tetramer
polarizationa
1623, 1612, 1585c
1560
1612
1561
1547
1535
1529
1501h
1367
1357
1337
1302
1253
1232
1610, 1583d
1562
1551
1536
1531
1611, 1582d
1562
1553
1536
1531
ap(dp)e
p
p
p
ν10
ν2
1557, 1565, 1574c,f
1535, 1535, 1538c,g
1547, 1540, 1531c
1501
ν37
ν38
ap
ν19
1501
1366
1501
1366
ap(dp)e
ν11
1368
1361
1339
1306
p
p
ν4
ν29
1337
1337
ap
ν20
1303
1303
p(dp)e
ν12
1253, 1253, 1232c
1232
1253, 1232d
1234
1253, 1236d
1236
ap
p
p
p
ν26
ν1
1215
1203
1175
1080
1072
1005
1212
1205
1175
1080
1072
1005
ν(CmCm)
ν(CmCm)
ν34
1197
1172
1082
1079
1008
1185
1079
1070
1005
p
p(dp)e
ν17
p
p
ν47
ν6
a Abbreviations: p, polarized; ap, anomalously polarized; dp, depolarized. b Taken from refs 39-41. c Observed for the diaryl-substituted monomer,
triaryl-substituted monomer, and CuTPP, respectively (see text). d These bands are assigned to the terminal and central rings, respectively (see
text). e The polarization listed in parentheses is that observed for CuTPP and /or NiTPP (see text). f These frequencies are taken from the IR
spectra. g The frequency for CuTPP was taken from the IR spectrum. h This band is ap at all excitation wavelengths, with the exception of 568 nm,
for which the band is p (see text).
cm-1, respectively (versus 1574 cm-1 for CuTPP). Accordingly,
the trend in frequency versus number of meso-aryl groups is
opposite that observed for ν10 and ν19. This reversal most likely
occurs because ν37 splits in the lower symmetry environment
of the diaryl- and triaryl-substituted complexes. The trends
observed in the frequencies of the ν37 modes suggest that the
magnitude of the splitting is largest for the diaryl-substituted
monomer. The effects of symmetry lowering apparently out-
weigh the effects of changing the mass of the meso substituent.
The νCaCm vibrations of the meso,meso-linked arrays were
assigned using the above-noted trends as a reference point. For
vibrations occur in place of some of the νCmCaryl vibrations,
are formally C-C single bond stretches due to the (approxi-
mately) orthogonal orientation of the porphyrins, and should
occur in a spectral region similar to that of the νCmCaryl modes.
Candidates for νCmCm vibrations are RR bands at 1197 (dimer),
1203 (trimer), and 1205 cm-1 (tetramer), that have no analogues
in the RR spectrum of the monomers. The trimer and tetramer
also each exhibit another band in the 1212-1215-cm-1 region
that has no analogue in the RR spectrum of the monomer and
is a candidate for a second νCmCm mode.
RR Polarization Characteristics of the meso,meso-Linked
Arrays and the Monomeric Building Blocks. The polarization
characteristics of the RR bands of the diaryl- and triaryl-
substituted monomers and all three meso,meso-linked arrays are
similar to one another. These RR polarization characteristics
are in turn distinctly different from those of CuTPP. The RR
polarization characteristics of symmetrically substituted por-
phyrins, such as CuTPP, are typically as follows: A1g, polarized
(p); A2g, anomalously polarized (ap); B1g and B2g, depolarized
(dp).22 Inspection of Table 1 reveals that the only p and ap RR
bands are observed for both the diaryl- and triaryl-substituted
monomers and all three meso,meso-linked arrays. No dp bands
are observed. The depolarization ratios (F) of the p bands are
in the range 0.3 e F e 0.6; those of the ap bands are in the
range 1 e F e 2. The dispersion in F-values for individual bands
does not fall outside these ranges, with one exception. The ν11
band of the dimer is p (F ∼ 0.3) with Q-state excitation and ap
(F ∼ 2) with B-state excitation (Figure 5).
example, the ν10 mode of the dimer is observed at 1612 cm-1
,
a frequency identical to that observed for the analogous mode
of the triaryl-substituted monomer. A ν10 mode of the trimer
and the tetramer is also observed at ∼1612 cm-1. A second ν10
mode is observed for both of these arrays at ∼1583 cm-1. This
latter frequency is nearly identical to that observed for the ν10
mode of CuTPP. Accordingly, the ∼1612 and ∼1583-cm-1
bands of the trimer and tetramer are attributed to the ν10
vibrations of the terminal and central porphyrins, respectively.
This same strategy was used to assign the ν19 and ν37 modes of
the porphyrins in the meso,meso-linked arrays. Interestingly, the
latter mode is not clearly visible in the IR spectra but is activated
in the RR spectra (due to symmetry lowering). For both the ν19
and ν37 modes, the frequency matching with modes of the
monomeric porphyrins is not as good as that for ν10. In addition,
for the trimer and tetramer, two distinct bands assignable to
the ν19 and ν37 vibrations of the terminal and central porphyrins
could not be identified. However, the RR bands due to both of
these vibrations lie in very congested spectral regions and may
be obscured by other stronger bands.
The νCmCaryl vibrations of MTPP complexes include ν1(A1g),
ν27(B2g), and ν36(Eu). Both ν1 and ν27 are RR active and are
observed for CuTPP at 1238 and 1269 cm-1, respectively.35
The ν3b mode is IR active but not observed.40 The RR spectra
of the diaryl- and triaryl-substituted monomers both exhibit a
band similar to ν1 at ∼1232 cm-1; a band similar to ν27 could
not be identified. A ν1-like feature is also observed at ∼1232
cm-1 for all three meso,meso-linked arrays. A more interesting
question for the arrays is whether the interporphyrin CmCm
stretches, νCmCm, are observed in the RR spectrum. The νCmCm
The polarization characteristics of the RR bands are deter-
mined by the excited-state properties of the molecules.22 The
observation that the diaryl- and triaryl-substituted monomers
and the meso,meso-linked arrays exhibit only p and ap modes
is attributed to the effects of symmetry lowering in the excited
state. The fact that the altered polarization occurs in the
monomers indicates that the meso substitution pattern plays a
key role in the excited-state symmetry lowering. In the case of
the arrays, the symmetry lowering resulting from the different
excitonic interactions along the two molecular axes may also
contribute to the unusual polarization characteristics. However,
the complexity of the scattering patterns precludes any accurate
separation of these contributions. The ap characteristics exhibited

Excitonically Coupled Porphyrin Arrays
J. Phys. Chem. B, Vol. 104, No. 46, 2000 10763
by modes such as ν10(B1g) and ν11(B1g), which are dp in CuTPP,
are attributed to the A2g/B1g mixing that occurs because of the
nature of the meso substitution pattern (vide supra). Likewise,
it would be expected that A1g/B2g mixing would lead to p
character for the normally dp B2g modes. The only B2g vibration
observed is ν29 of the dimer at 1367 cm-1, which is p, as
predicted (Figure 5, Table 1).
different from those of the dimer. For these arrays, By(0,0)
excitation results in the expected scattering from totally sym-
metric modes (Figures 6 and 7, top traces). However, certain
nontotally symmetric modes are also enhanced, including the
ap modes ν11 at 1501 cm-1 and ν19 at 1531 cm-1. This scattering
pattern prevails with excitation between BX(0,0) and By(0,0)
(Figures 6 and 7, second traces); however, the RR enhancement
of the nontotally symmetric modes is somewhat larger than those
with By(0,0) excitation. With BX(0,0) excitation, the scattering
pattern changes and only totally symmetric modes are observed.
However, unlike the case of the dimer, BX(0,0) excitation of
the trimer and tetramer only gives rise to scattering from A1g-
derived vibrations. No enhancement is observed for vibrations
that become totally symmetric due to symmetry lowering, in
particular ν37(Eu) (1547 cm-1) and ν29(B2g) (1367 cm-1). Indeed,
the BX(0,0) excitation RR spectra of the trimer and tetramer
look like typical B(0,0) excitation spectra of symmetrical,
monomeric metalloporphyrins.
A plausible explanation for the B-state RR enhancement
patterns of the meso,meso-linked arrays is as follows: as the
number of porphyrins in the array increases, the oscillator
strength of the BX(0,0) absorption systematically increases. This
spectral characteristic is obscured by the fact that the intensities
of the BX(0,0) and By(0,0) bands remain comparable as the array
size increases. This occurs because the By(0,0) band is a
superposition of features, due to the N independent absorptions.
In the case of the trimer and tetramer, the oscillator strength of
the BX(0,0) transition is apparently sufficiently large, such that
it effectively becomes a “super” BX(0,0) transition. Excitation
into this “super” BX(0,0) transition promotes exceptionally strong
Franck-Condon RR scattering. The intensity enhancement for
modes derived from truly symmetric vibrations (e.g., A1g modes)
is apparently much larger than that for modes that only become
symmetric by symmetry lowering (e.g., ν37(Eu) and ν29(B2g)).
Franck-Condon scattering is also strong with By(0,0) excitation,
but not as strong as with BX(0,0) excitation. In this case, the
RR enhancement of A1g-derived modes does not completely
overpower that of modes that become totally symmetric by
symmetry lowering. In addition, the energetic proximity of the
single “super” BX(0,0) state and multiple “normal” By(0,0) states
affords significant vibronic coupling between these states. This
vibronic coupling results in Herzberg-Teller scattering from
nontotally symmetric modes with By(0,0) excitation. This model
also explains the RR scattering characteristics of the dimer. For
this smaller array, the oscillator strength of the BX(0,0) transition
is apparently not sufficient to move it into the “super” BX(0,0)
limit. Hence, the RR enhancement of A1g-derived modes does
not completely overpower those of the modes that become
totally symmetric by symmetry lowering. The lower oscillator
strength of the BX(0,0) band of the dimer (relative to that of
the trimer or tetramer) also decreases the magnitude of the
Herzberg-Teller coupling between the BX(0,0) state and the
By(0,0) states. Accordingly, the RR intensity enhancement of
nontotally symmetric vibrations is relatively weak, with By(0,0)
excitation.
Finally, it should be noted that the dispersion in the F-values
observed for all of the RR bands indicates that the extent of
symmetry lowering is different in the different excited states
of the complexes. This is most strikingly illustrated by the
behavior of the ν11 band of the dimer, which is ap with B-state
excitation and p with Q-state excitation. The vibronic contribu-
tion to symmetry lowering, such as that which occurs in the
Q(1,0) states, is also undoubtedly different along different
normal coordinates. The symmetry lowering along the ν11
coordinate in the Q-state of the dimer is apparently sufficiently
large, such that all symmetry is lost (hence, the p character of
the RR band). The extent of symmetry lowering appears to be
smaller along other normal modes, and ap character is retained
in the RR polarization. Together, these features indicate that
the vibronic character of the excited states of all of the
complexes is quite complicated.
RR Enhancement Patterns of the meso,meso-Linked Arrays.
Despite the unusual polarization characteristics observed for the
RR bands of the diaryl- and triaryl-substituted monomers, the
RR enhancement patterns exhibited by these monomers are
similar to that of CuTPP and other more symmetrical porphyrins.
In particular, for both monomers, totally symmetric modes are
enhanced with B-state excitation and nontotally symmetric
modes are predominantly enhanced with Q-state excitation
(Figure 4). In addition, all of the totally symmetric modes
enhanced with B-state excitation are derived from A1g-like
vibrations. Modes that become totally symmetric due to sym-
metry lowering are not enhanced. This general RR enhancement
pattern is well understood and occurs because Franck-Condon
scattering dominates with excitation into the strong B-state
absorption, whereas Herzberg-Teller scattering dominates with
excitation into the relatively weak Q-state absorption.22
The RR enhancement patterns observed for the meso,meso-
linked arrays are more complicated. As is the case for the
monomers, Q-state excitation predominantly enhances nontotally
symmetric modes (Figures 5-7, bottom traces). On the other
hand, the RR enhancement pattern observed with B-state
excitation is quite unusual and changes with the size of the array.
For the dimer, By(0,0), excitation results in a standard RR
scattering pattern, wherein predominantly totally symmetric
modes are enhanced (Figure 5, top trace). These totally
symmetric modes are primarily derived from A1g-like vibrations.
Totally symmetric modes also dominate the RR spectra, with
BX(0,0) excitation and at excitation energies between By(0,0))
and BX(0,0) (Figure 5, second and third traces). However, BX-
(0,0) excitation also elicits strong scattering from vibrations that
have become totally symmetric in the low-symmetry environ-
ment, in particular ν37(Eu) at 1547 cm-1 and ν29(B2g) at 1367
cm-1. Neither of these vibrations is typically observed with
B-state excitation. Indeed, ν37(Eu) is not generally RR active.
The ν37(Eu) and ν29(B2g) vibrations are most likely enhanced
via excited-state mixing with A1g-derived vibrations. Another
feature worthy of comment is the enhancement (albeit weak)
of the ap ν11 mode at 1501 cm-1, with excitation between By-
(0,0) and BX(0,0) (Figure 5, second trace).
Summary and Conclusions
The electronic and vibrational characteristics of the
meso,meso-linked arrays are unusual from a number of respects.
The B-excited states experience extremely strong uniaxial ex-
citonic interactions. These interactions break the degeneracy of
the B-excited state and give rise to a “super” BX(0,0) transition
and a series of isoenergetic “normal” By(0,0) transitions. The RR
spectra of the arrays are characterized by features including (1)
The B-state excitation RR enhancement patterns for the trimer
and the tetramer are similar to one another and distinctly

10764 J. Phys. Chem. B, Vol. 104, No. 46, 2000
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Acknowledgment. This work was supported by grants from
the National Institutes of Health (GM 36243 to D.F.B.), by
Grant-in-Aids for Scientific Research from the Ministry of
Education, Science, Sports and Culture of Japan (A.O.), and
by CREST (Core Research for Evolutional Science and Tech-
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