Energy transfer processes in electronically coupled porphyrin

hetero-dyads connected at the b position

Barbara Ventura,a Francesco Barigelletti,a Fabio Lodato,b David L. Officerbc

and Lucia Flamigni*a

Received 28th October 2008, Accepted 18th December 2008

First published as an Advance Article on the web 6th February 2009

DOI: 10.1039/b819138g

Energy transfer in a series of hetero-dyads of zinc porphyrin and free-base porphyrin connected

at the b position by p conjugated bridges has been determined. The dyads have been

characterized and compared with the homo dyads, excellent models for the donor and the

acceptor porphyrins in the electronically conjugated system. The homo dyads provide reliable

parameters for the determination of the energy transfer rate calculated according to the Forster

theory. This model was inadequate to account for the experimental findings and an electron

exchange mechanism was shown to contribute. A favorable coplanar arrangement of the bridge

and the tetrapyrroles facilitates the energy transfer process, which displays a very low distance

dependence and an efficiency 498%.

Introduction

Porphyrin-type components have been extensively used to

construct dyads or higher homologues designed to achieve useful

photoinduced intramolecular processes. In particular the study

of energy transfer has been often performed on arrays based on

these chromophores. In fact they are well characterized from the

spectroscopic/photophysical viewpoint and are known to dis-

play a wide tunability of energy levels which can be achieved by

simple substitution or metallation. The use of components with

different excited state energies has allowed us to detect, by means

of time-resolved techniques, the decay of the donor’s and/or the

rise of the acceptor’s excited state populations.1–17 This was

done by interrogating both the luminescence and the transient

absorbance signals typical of the excited state. This type of

methodology was extensively used to determine the rate of the

energy transfer processes as a function of thermodynamic para-

meters and, keeping the latter constant, as a function of distance,

type of connecting bridge, orientation, solvent etc. This method

has been mostly used for singlet energy migration, but examples

of triplet energy transfer studies are present in the literature.18–21

Knowledge of the reaction parameters has allowed a thorough

discussion on the mechanism of energy transfer in the frame of

current theories, and has shed light on this process which is of

very high relevance to the issues of conversion of light energy

and of molecular electronics.

Whereas photoinduced processes in a wide variety of

meso–meso bridged porphyrin dyads (or higher homologues)

are well documented in the literature, very few examples

of reports on photoinduced processes in porphyrin dyads

connected at the b-pyrrole are reported.22,23 Actually, the

b position of porphyrins has seldom been used as connection

point in the construction of porphyrin based photoactive

arrays.24–26 The position of the bridge connection is not a

trivial issue, since the electronic distribution at the periphery

of a porphyrin depends both on the nature and on the location

of the substituents. In fact, the a1u and a2u HOMO orbitals of

porphyrins have a different electron density at the b (-pyrrole)

and at the meso carbons: whereas the a1u orbital puts charge

on the b carbons and has nodes at the meso positions the a2uorbital has electron density at the meso positions and at the

central nitrogens and nodes at the pyrrole carbons. Substi-

tuents with different electron donating or withdrawing ability

can modulate in different ways the energetic distribution of the

porphyrin frontier orbitals (i.e. the a2u/a1u energies) according

to their position and can even reverse the normal ordering

a2u 4 a1u.27–29 Thus, in a porphyrin supramolecular structure

the connection point between components can greatly affect

the electronic coupling between the component units and

result in processes with different mechanisms and rates. The

effect of orbital inversion on the rates of energy transfer in a

porphyrin dyad was elegantly addressed by Lindsey, Holten,

Bocian et al. for a series of porphyrinic dyads connected at

either the b-pyrrole or the meso carbons position by a diaryl-

ethyne linker.22 In those systems the stabilization of the a2uorbital of the donor by the presence of meso strong electron

withdrawing groups induces the orbital ordering to reverse

and the electronic communication between the units is enhanced

in case of a b-linkage compared to a meso-linkage. For the

monomer models of the present dyads, however, simple MO

calculations could exclude an inversion of the orbital ordering of

the porphyrin core;30 therefore in the components of the dyads

a normal a1u o a2u ordering should be considered.

Our aim in this work is to study the photoinduced energy

transfer process in a series of porphyrinic dyads where the

individual components, a free-base and a zinc porphyrin, are

connected at the b-pyrrole position by highly conjugated

a Istituto per la Sintesi Organica e la Fotoreattivita (ISOF), CNR,Via P. Gobetti 101, 40129 Bologna, Italy.E-mail: flamigni@isof.cnr.it; Fax: +39 051 6399844;Tel: +39 051 6399812

bNanomaterials Research Centre, Massey University, PalmerstonNorth, Private Bag 11222, New Zealand

cMacDiarmid Institute for Advanced Materials and Nanotechnology,New Zealand

2166 | Phys. Chem. Chem. Phys., 2009, 11, 2166–2176 This journal is �c the Owner Societies 2009

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bridges, namely a divinyl, 1FB-PZn, and two p-phenyl-

enevinylene groups with increasing length, 2FB-PZn and

3FB-PZn in Scheme 1. The pertinent monomer models with

the conjugated substituent at the b-pyrrole positions, 1FB,

2FB, 3FB and 1PZn, 2PZn, 3PZn have been recently

reported30 and an electronic delocalization along the bridge

was shown. On this basis, a sizeable coupling between the

porphyrin components connected by such bridges is expected.

One of the problems which can arise in the determination of

photoinduced processes in electronically coupled arrays is the

difficulty to find suitable models for the individual components

to represent the unit in the specific chemical surroundings. In

fact, it is the comparison between the behavior of isolated

suitable models and the behavior of the corresponding unit in

the arrays which allows identification of the intra-molecular

reactivity. However, in a strongly coupled structure, the

components are expected to behave quite differently from

simple models because of the intrinsic perturbation of their

properties rather than because of intra-molecular reactivity. In

these conditions the identification of intramolecular processes

is rather difficult to assess. In order to overcome this difficulty

and compare the properties of the free-base porphyrin and the

zinc porphyrin units in an identical chemical surrounding, we

synthesized the homo-dyads with the same type of connections

1PZn2, 2PZn2, 3PZn2 and 1FB2, 2FB2, 3FB2 reported in

Scheme 1. They were found to be excellent models for the

above hetero-dyads 1FB-PZn, 2FB-PZn, and 3FB-PZn. This

allowed us to have reliable models which permitted the

determination of the rates of photoinduced intramolecular

processes with some accuracy.

Therefore, this paper reports on the spectroscopic and

photophysical characterization of a series of porphyrin

homo-dimers made of either aryl free-base porphyrin or aryl

zinc porphyrin connected at the b-pyrrole positions by diffe-

rent conjugated chains, 1PZn2, 2PZn2, 3PZn2 and 1FB2, 2FB2,

3FB2 and of the corresponding hetero-dyads 1FB-PZn,

2FB-PZn, and 3FB-PZn. The properties of the pertinent

monomer models with the conjugated substituent at the

b-pyrrole positions, 1FB, 2FB, 3FB and 1PZn, 2PZn, 3PZn

(Scheme 1) previously reported,30 will be here reviewed for

comparison purposes. A thorough discussion on the energy

transfer mechanism in the frame of current theories is con-

ducted and comparison with energy and related electron

transfer related parameters is performed.

Results and discussion

Absorption spectroscopy

The absorption spectra of 1FB-PZn, 2FB-PZn, and 3FB-PZn

in toluene are reported in Fig. 1, together with the absorption

spectra of the constituent monomer models, i.e. 1FB and 1PZn

for 1FB-PZn, 2FB and 2PZn for 2FB-PZn and 3FB and 3PZn

for 3FB-PZn. This comparison shows that the spectroscopic

properties of the dyads are far from being additive with respect

to the model components. Indeed, in the dyads a strong

perturbation of the property of the constituent components

appears from the absorption spectra, which is more evident

for the first term of the series connected by a divinyl chain,

1FB-PZn, and tends to decrease for the higher homologues

with p-phenylenevinylene groups. In 1FB-PZn, in fact, with

respect to the superposition of the component models there is

a decrease of the absorbance on the Soret band to about half,

the appearance of a wide shoulder around 480 nm and a

remarkable bathochromic shift on the Q bands, except the Qx

(0,0) of the free base component, which quite remarkably

keeps the same maximum at 659 nm as the model 1FBScheme 1 Homo- and hetero-dyads and monomer models.

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(Fig. 1, top panel). In 2FB-PZn and 3FB-PZn the behavior is

similar, except for the reduction in the absorption of the Soret

band and the shift to lower energies of the Q bands that,

compared to the models, is less important than in 1FB-PZn.

It should be noticed that in 3FB-PZn the bands of the bis-

para-phenylene vinylene unit, around 330–380 nm in the

models 3FB and 3PZn (already red-shifted by ca. 20 nm with

respect to the parent compounds30) is further broadened and

overlaps with the porphyrin Soret bands. As already

mentioned for 1FB-PZn, also for the other dyads the lowest

energy band of the free-base porphyrin, Qx (0,0), is unaltered

with respect to the model, at l= 659 nm. It is difficult to say if

it is the same for the zinc porphyrin component due to the

massive overlap with the free base porphyrin bands in the

dyads. The behavior of the lowest energy band is rather

important in determining the luminescence properties of the

system, which will be discussed below. The data are supportive

of some electronic coupling between the porphyrin termini, in

agreement with the nature of the electronically delocalized

connectors, in particular a stronger coupling reflected by a

larger perturbation is displayed by the dyad connected by the

shorter divinyl chain.

The absorption spectra of the homo-dyads, 1FB2, 2FB2,

3FB2 and 1PZn2, 2PZn2, 3FB2, reported in Fig. 2, display

features which are very similar to those of the hetero-dyads. In

fact, in this case the spectrum is also not a simple super-

position of the spectra of the relative monomer models

(comparison of Fig. 1 and 2) but displays a noticeable

perturbation. This is in agreement with previous reports on

several bis zinc porphyrins connected either at the b or at the

meso position by a vinyl or ethynyl spacer.31–33 It has been

shown that the electronic coupling between zinc porphyrins

linked by either ethyne or butadiyne groups decreases in the

ordermeso-to-meso4meso-to-b4 b-to-b and with increasing

bridge length for a specific linkage topology.25,31 Very similar

absorption spectra to the present ones have been reported for

Fig. 1 Absorption spectra of toluene solutions of: (upper panel)

1FB-PZn (dot), 1FB (dash) and 1PZn (line); (middle panel)

2FB-PZn (dot), 2FB (dash) and 2PZn (line); (lower panel) 3FB-PZn

(dot), 3FB (dash) and 3PZn (line).

Fig. 2 Absorption spectra of toluene solutions of dyads and the

relevant homo-dimers models: (upper panel) 1FB-PZn (dot), 1/2 1FB2

(dash) and 1/2 1PZn2 (line); (middle panel) 2FB-PZn (dot), 1/2

2FB2 (dash) and 1/2 2PZn2 (line); (lower panel) 3FB-PZn (dot), 1/2

3FB2 (dash) and 1/2 3PZn2 (line). As a gray line is also shown the

superposition of the models spectra.

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bis-zinc and bis free-base porphyrin dyads connected at the

meso position by a tetraenic chain.34 It is worth noticing that

the divinyl bridged dyad 1FB2, behaves quite differently from a

monovinyl meso linked free-base dyad, trans-1,2-bis(meso-

octaethylporphyrinyl). The latter, in fact, shows an absorption

band in the near IR region, 600–900 nm, assigned to a minor

conformer with peculiar luminescence and photophysical

properties.35,36 The existence of two conformations was also

put forward for a b–b vinylene linked chlorophyll dimer

serving as a model of the photosyntetic bacterial special pair.37

Indeed in the present case we did not have any evidence of

different conformations; whereas the connection point, either

at the meso carbon or at the b-pyrrole, does not seem to make

a difference, the increase in chain length, from mono-vinyl to

di-vinyl can make this dyad less liable to form conformational

isomers.

For homo-dyads 2FB2, 3FB2, 2PZn2, 3PZn2 and hetero-

dyads 2FB-PZn and 3FB-PZn, containing p-phenylenevinylene

linkers, the extension of the p-conjugation over the bridge is

predicted considering that the whole molecule can assume a

planar conformation. Reports on b-linked dyads with different

isomeric phenylenevinylene connectors, in fact, show that, in

the para isomer, a planar conformation and a consequent

conjugation between the porphyrins and the bridge occurs.23,38

The coplanar geometry of the phenylene rings with the

porphyrin ones is permitted by the fact that in this case

the b-pyrrole carbons are connected to the vinyl group of

the oligophenylenevinylene chain, rather than directly to the

phenyl unit as is the case in most of the previously reported

dyads containing both oligophenylenevinylene and oligo-

phenylethynyl spacers.2,22,39 The molar absorption coefficients

of 1FB-PZn, 2FB-PZn, and 3FB-PZn, Fig. 2, can be

reproduced perfectly by the superposition of half of the molar

absorption coefficients of the corresponding free-base homo-

dyad and half of the molar absorption coefficients of the

corresponding zinc homo-dyad (gray line in Fig. 2). This

means that the single units of 1FB2, 2FB2, 3FB2 are good

models of the free-base unit and that the single units of 1PZn2,

2PZn2, 3PZn2 are good models for the zinc porphyrin com-

ponents in 1FB-PZn, 2FB-PZn, and 3FB-PZn. Therefore the

homo-dyads, or better one unit of the homo-dyads, will be

used as model for the components of the hetero-dyads in order

to establish their reactivity upon light absorption. The maxima

of the bands and the molar absorption coefficients are

collected in Table 1.

Luminescence spectroscopy

Homo-dyads. The luminescence spectra of the series 1FB2,

2FB2, 3FB2 and 1PZn2, 2PZn2, 3PZn2, detected in toluene

after excitation at 565 nm are reported in Fig. 3. The lumines-

cence parameters are reported in Table 2 together with the

parameters of the parent TPP and ZnTPP and, for com-

parison purposes, also the properties of the model compounds

1FB, 2FB, 3FB and 1PZn, 2PZn, 3PZn previously reported30

are listed in the Table. As a general remark, the change in

luminescence properties of the homo dyads with respect to the

models 1FB, 2FB, 3FB and 1PZn, 2PZn, 3PZn is smaller than

the change in absorption properties, and this has to be related

to the above noticed small perturbation of the Qx (0,0) lowest

energy band in the absorption spectra. However, the lumines-

cence quantum yield in the homo-dimers decreases by increas-

ing the length of the bridge, i.e. by decreasing the coupling.

Whereas the free-base homo-dimers display an emission quan-

tum yield similar to the TPP simple compound, the zinc

porphyrin homo-dimers have higher emission intensities than

the model ZnTPP, Table 2. The shape of the spectra and the

value of the maximum is rather homogeneous within each

series and only slightly red shifted with respect to the parent

TPP and ZnTPP with only one noticeable exception, 1PZn2.

This in fact not only displays a remarkable red shift with

respect to the parent compound and the other members of the

series, 47 nm with respect to ZnTPP, but it also displays an

emission quantum yield of the order of 0.3, six times higher

than ZnTPP, on the whole quite remarkable luminescence

properties. To our knowledge, this is one of the highest

fluorescence yield ever reported for zinc porphyrin dimers;

Table 1 Absorption maxima and molar absorption coefficients intoluene at 295 K

lmax/nm e/105 M�1 cm�1

1FB2 422 2.19483 (sh) 0.55522 0.45581 0.23607 0.24661 (sh) 0.05

2FB2 427 2.20484 (sh) 0.57522 0.40573 0.29603 0.19658 0.05

3FB2 428 1.76526 0.43571 0.30606 0.20665 0.07

1PZn2 427 3.19505 (sh) 0.75566 0.49614 0.40

2PZn2 434 3.36498 (sh) 0.74568 0.49607 0.37

3PZn2 435 3.56495 (sh) 0.89566 0.52603 0.34

1FB-PZn 424 2.53493 (sh) 0.70522 (sh) 0.50566 0.36609 0.35661 (sh) 0.04

2FB-PZn 429 2.92492 (sh) 0.65523 (sh) 0.37566 0.37601 0.30659 0.02

3FB-PZn 432 2.46525 (sh) 0.29569 0.36603 0.21659 0.02

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published Ffl values range between 0.018 40 and 0.18 1 but in

general they are of the same order or lower than the corres-

ponding monomer.4,32,33,41,42 The luminescence lifetimes,

collected in Table 2, are rather homogeneous within each

series but slightly lower than the parent TPP and ZnTPP, of

the order of 7.6 ns vs. 9.3 ns for the free-base derivatives and of

ca. 1.5 ns vs. 1.9 ns for the zinc porphyrin derivatives. Taken

together with the luminescence yield data discussed above, this

implies an higher radiative rate constant kr (kr = Ffl/t) for

each member of the zinc porphyrin series with respect to

ZnTPP, which is particularly noticeable for 1PZn2. For the

latter a kr = 2 � 108 s�1 can be calculated, one order of

magnitude higher than that of the simple models.

The 77 K behavior is in line with the behavior at room

temperature, with the emission slightly shifted to lower energy

for the homo-dimers with respect to the parent models,

resulting in a lower energy of the singlet excited state, by

ca. 0.03 eV–0.05 eV for the free-bases, and from 0.06 eV to

0.13 eV for the zinc derivatives, with the highest difference for

1PZn2. Phosphorescence bands at 805 nm, a slightly lower

energy with respect to the parent models, could be detected for

2PZn2 and 3PZn2, but not for 1PZn2. No phosphorescence

could be detected for the free-base series, but this is quite

expected since the simple TPP has a phosphorescence in the

near IR region, at ca. 840 nm hardly detectable by conven-

tional spectrofluorometers.43 A further bathochromic shift

would make the emission undetectable. As far as the lifetimes

are concerned, at variance with room temperature data, the

fluorescence lifetimes are essentially the same as TPP and

ZnTPP, i.e. ca. 13 ns for the free-bases homo-dimers and

ca. 2.5 ns for the zinc porphyrin homo-dimers, except for

1PZn2 which is still shorter, 1.7 ns. This seems to indicate that

at 295 K a temperature activated process affects the lifetime

of the singlet excited states in all homo-dimers which is

suppressed at low temperatures except for 1PZn2, confirming

an anomalous behavior for this dyad.

Hetero-dyads. Steady state experiments have been carried

out in order to detect sensitization or quenching of the

component units in the dyads. Since selective excitation of

Fig. 3 Luminescence spectra in toluene of optically matched solu-

tions of 1FB2, 2FB2, 3FB2 and 1PZn2, 2PZn2, 3PZn2 (lexc = 565 nm,

A = 0.14).

Table 2 Luminescence properties of homo-dyads 1FB2, 2FB2, 3FB2, 1Zn2, 2Zn2, 3Zn2, of the models 1FB, 2FB, 3FB and 1Zn, 2Zn, 3Zn and ofthe parent molecules TPP and ZnTPP in toluene

295 K 77 K

lmax/nm Ffa tb/ns lmax/nm tb/ns Ec/eV

TPP 650, 715 0.11 9.3 643, 712 13.4 1.921FB 662, 726 0.091 8.5 652, 721 12.4 1.902FB 663, 729 0.090 8.1 655, 724 12.2 1.893FB 662, 730 0.091 9.0 657, 726 13.7 1.891FB2 667, 733 0.13 7.4 664, 733 12.7 1.872FB2 663, 731 0.10 7.8 660, 730 13.0 1.883FB2 663, 731 0.06 7.9 660, 730 13.8 1.88ZnTPP 594, 642 0.047 1.9 598, 654 2.7 2.07

780d 1.591PZn 605, 654 0.050 1.8 607, 663 2.6 2.04

785d 1.582PZn 603, 659 0.063 1.9 611, 667 2.6 2.03

789d 1.573PZn 603, 660 0.073 1.8 613, 669 2.4 2.02

792d 1.571PZn2 641, 691 0.29 1.4 640, 698 1.7 1.94

—2PZn2 615, 670 0.09 1.6 617, 675 2.5 2.01

805d 1.543PZn2 610, 667 0.07 1.7 618, 674 2.5 2.01

805d 1.54

a Excitation at 565 nm. Emission maxima derived from non-corrected emission spectra. Absolute quantum yields determined comparing corrected

emission spectra to the emission of TPP (tetraphenylporphyrin) in aerated toluene as a standard (ffl = 0.11).27 b Excitation at 465 nm. c Derived

from the emission maxima at 77 K. d Phosphorescence bands.

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each component is not possible, Fig. 2, a wavelength has been

chosen with equal absorbance of the zinc and free-base

component. Iso-absorbing wavelengths are 530 nm for

1FB-PZn, 520 nm for 2FB-PZn and 510 nm for 3FB-PZn.

The luminescence spectra upon excitation at these wavelength

of 1FB-PZn, 2FB-PZn and 3FB-PZn are reported in Fig. 4; in

these Figures are also shown the emission spectra of optically

matched solutions of the models 1PZn2 and 1FB2 (upper

panel), 2PZn2 and 2FB2 (middle panel) and 3PZn2 and 3FB2

(lower panel). It is evident that the emission spectrum of

the hetero-dyads is essentially identical within experimental

errors, both in shape and in intensity, to the emission spectrum

of the optically matched corresponding free-base homo-dyads,

except for a small luminescence residue around 620 nm–630 nm.

The latter, typical of zinc porphyrins, could be due both

to a residual luminescence of the zinc porphyrin component

after quenching in the hetero-dyads and/or to a minor

zinc porphyrin-type contaminant (see below). The results

indicate that a very efficient and complete transfer of energy

from the zinc porphyrin unit to the free-base unit takes place

and that the free-base unit in each dyad exhibit the same

luminescent energy of the free-base unit in the homo-dyad,

confirming the good choice of the models. A further confirma-

tion of the energy transfer process both at room temperature

and at 77 K comes from excitation spectra; the excitation

spectrum detected at lem = 730 nm, where only the free base

component emits is superimposable to the absorption spec-

trum of the dyad, confirming that the photons absorbed by the

dyad, irrespective of the absorbing unit, are re-emitted from

the free-base component (data not shown). The lumines-

cence properties of 1FB-PZn, 2FB-PZn and 3FB-PZn are

summarized in Table 3.

Nanosecond time resolved luminescence experiments after

excitation at 465 nm in toluene solutions indicate the existence

of a strong emission in the range 650–750 nm with lifetimes

of 8.5 ns for 1FB-PZn, 9.0 ns for 2FB-PZn and 8.7 ns for

3FB-PZn, respectively. This is ascribed to the fluorescence of

the free-base porphyrin component either formed directly by

light absorbance or upon energy transfer from the zinc

porphyrin unit of the dyad. Similar nanosecond experiments

performed at 77 K in a toluene glass indicate an increase of the

lifetimes with respect to room temperature, 13.6–14.8 ns for

the free-base porphyrin emission at 650–750 nm. The lifetime

of the free-base unit in 1FB-PZn, 2FB-PZn and 3FB-PZn

(Table 3) is slightly longer both at room temperature and at

77 K than that of the corresponding free-base homo-dimers

(Table 2), but within typical values for free-base porphyrins. A

very weak luminescence was detected in the range 610–630 nm

with lifetime of 1.4, 1.7 and 1.7 ns for 1FB-PZn, 2FB-PZn and

3FB-PZn, respectively, very likely due to some zinc porphyrin

homo-dimer present as a contaminant.

Further insight can be gained by an experiment with a

picosecond time-resolved streak camera after excitation with

a 35 ps laser at 532 nm, where excitation of both components

in the dyad takes place, see Fig. 2. These experiments evidence

spectral changes in the first 100 ps; the time-resolved spectra

taken at the end of the laser pulse and with a delay after the

end of the pulse for 1FB-PZn, 2FB-PZn and 3FB-PZn are

compared in Fig. 5. Whereas the spectral changes are hardly

noticeable in 1FB-PZn, in 2FB-PZn and 3FB-PZn a fast decay

around 620 nm, typical and exclusive range of the zinc

porphyrin emission can be detected with a concomitant small

but noticeable increase of the band around 730 nm, typical

and specific for the free-base component. This is the demon-

stration of a fast energy transfer process occurring within the

hetero-dyads from the zinc porphyrin moiety to the free-base

porphyrin unit. In the case of 1FB-PZn the evolution can

hardly be detected (Fig. 5, upper panel) because of a process

faster than the instrumental resolution (10 ps) and of an

extensive emission overlap between the zinc porphyrin com-

ponent and the free-base porphyrin component. This can be

easily seen from the comparison of the emission spectra of

models 1FB2 and 1Zn2 and is due to the anomalous shift of the

latter, see Fig. 3 and Table 2. The time evolution for 2FB-PZn

and 3FB-PZn at 620 nm are reported in Fig. 6. Both lumines-

cence decays can be fitted by two exponentials, a major

component (ca. 90%) with lifetimes of 14 ps for 2FB-PZn

Fig. 4 Luminescence spectra of optically matched solutions of

hetero-dyads 1FB-PZn (upper panel), 2FB-PZn (middle panel), and

3FB-PZn (lower panel) in toluene (line) compared to the luminescence

of the pertinent homo-dyads 1FB2, 2FB2, 3FB2 (dots) and 1PZn2,

2PZn2, 3PZn2 (dash). Excitation at the iso-absorbing point between

the zinc and the free-base porphyrin (see text).

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and 25 ps for 3FB-PZn and a slower, minor (ca. 10%)

component of ca. 1.7 ns for both 2FB-PZn and 3FB-PZn, in

agreement with the one detected in this spectral range by

nanosecond luminescence experiment and ascribed to zinc

porphyrin homo-dimer contaminant. Picosecond experiments

were also performed at 77 K in a toluene glass and the detected

lifetime at 615 nm is 80 ps for 2FB-PZn and 230 ps for

3FB-PZn, 3FB-PZn, in addition to a very minor component

of ca. 2.7 ns. As for the ambient temperature data, we interpret

the fast and predominant component as due to quenching by

energy transfer and the slow minor lifetime as due to traces of

zinc homo-dyad. Therefore energy transfer occurs still rather

efficiently at 77 K. The data at 295 K and 77 K discussed

above for 1FB-PZn, 2FB-PZn and 3FB-PZn are collected in

Table 3.

Photoinduced processes in the hetero-dyads

In order to discuss the photoinduced processes occurring in

1FB-PZn, 2FB-PZn and 3FB-PZn we introduce Scheme 2, an

energy level diagram for the dyads reporting the energy

levels for the excited states localized on either porphyrin

Table 3 Luminescence properties of hetero-dyads 1FB-PZn, 2FB-PZn and 3FB-PZn in toluene

295 K 77 K

State lmax/nm Ffla tb/ns lmax/nm tb/ns Ec/eV

1FB-PZn 1 FB-1PZn 640 (sh) o0.01d — o0.01d —11FB-PZn 666, 733 0.11 8.5 661, 730 14.7 1.87

2FB-PZn 2 FB-1PZn 615 0.014d 618 0.080d 2.0121FB-PZn 663, 731 0.10 9.0 660, 728 14.7 1.88

3FB-PZn 3 FB-1PZn 612 0.025d 620 0.230d 2.0031FB-PZn 663, 731 0.06 8.7 660, 728 13.6 1.88

a Excitation at the iso-absorbing point between the free-base and the zinc porphyrin component, i.e. at 530 nm for 1FB-PZn, at 520 nm for

2FB-PZn and at 510 nm and for 3FB-PZn. b Excitation at 465 nm for the nanosecond range and at 532 nm for the picosecond range. c Derived

from the maximum of the 77 K emission. d In the range 610 nm–630 nm a minor component (ca. 10%) with lifetimes of 1.4 ns, 1.7 ns, 1.7 ns at

295 K and of 1.6 ns, 2.8 ns and 2.7 ns at 77 K for 1FB-PZn, 2FB-PZn and 3FB-PZn, respectively, was found (see text).

Fig. 5 Time resolved spectra over a 30 ps time window at the end of

the pulse (black) and 160 ps after the end of pulse (gray). Excitation at

532 nm, 1 mJ/pulse.

Fig. 6 Time evolution of the luminescence in toluene (295 K) of

2FB-PZn and 3FB-PZn at 615 nm is reported with the bi-exponential

fitting. The exciting profile, 532 nm, 1 mJ/pulse, is also reported as a

continuous line.

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components. These are derived from the luminescence maxima

at 77 K of Table 3 and when data are not available, from the

corresponding model homo-dimers in Table 2. It has in fact

been shown that 1FB2, 2FB2, 3FB2 and 1PZn2, 2PZn2, 3PZn2are excellent models for the corresponding units in the hetero-

dyads. Energy transfer is predicted in 1FB-PZn, 2FB-PZn and

3FB-PZn from the singlet excited states localized on zinc

porphyrin (FB-1PZn) to the singlet excited state localized on

the free-base component (1FB-PZn). The DG0 for the reaction

step is �0.07 eV for 1FB-PZn, �0.13 eV for 2FB-PZn and

�0.12 eV for 3FB-PZn. A quenching of the zinc porphyrin

luminescence in 2FB-PZn and 3FB-PZn was determined, with

lifetimes of 14 ps and 25 ps, respectively. The rate constant ken,

with ken = 1/t-1/t0 where t is the experimental quenched

lifetime and t0 is the lifetime in absence of quenching, i.e. that

of the model homo-dimer (1PZn2-3PZn2), can be calculated.

They are, respectively 7.1 � 1010 s�1 for 2FB-PZn and

3.9 � 1010 s�1 for 3FB-PZn. The rates are in line with those

reported for similar porphyrinic dyads using phenylethynyl

spacers22,44 but higher than those recently reported by

Albinsson for the same phenylethynyl bridges.45 The diffe-

rence in the above literature data was ascribed to the different

nature of the HOMO orbital, respectively an a1u in the latter

case and an a2u in the former. As already discussed above,

whereas a2u is characterized by an electron density at the meso

carbon, the attach position of the bridge, a1u has a node at the

same carbon. However, in the present case where the HOMO

was shown to be an a2u30 and the bridge is attached at the

b pyrrole carbon, where a2u has a node, the rate is still very

fast. This seems to confirm the explanation proposed by

Albinsson and related to the role of the dihedral angle at the

connection point between the porphyrin and the bridge.45 The

perpendicular orientation decreases the electronic coupling

and consequently the rate whereas nearly coplanar conforma-

tion, as in the present case, enhances the coupling and the

rate.45 The energy transfer rate process within the components

in 1FB-PZn is very fast and occurs within the laser pulse, so we

can set a lower limit of 1011 s�1, i.e. a lifetime of 10 ps,

corresponding to the instrumental resolution. It should how-

ever be stressed that in 1FB-PZn the large overlap of the

emission of the zinc porphyrin donor unit (remarkably red-

shifted with respect to the other zinc porphyrin components)

with that of the free-base porphyrin acceptor makes this

determination liable to some uncertainty. The energy transfer

efficiency Z, Z = ken/(ken + 1/t0), is 100% for 1FB-PZn, 99%

for 2FB-PZn and 98% for 3FB-PZn, respectively. Energy

transfer takes also place at 77 K in a toluene glass with rates

of 1.2� 1010 s�1 and 3.9� 109 s�1 for 2FB-PZn and 3FB-PZn,

corresponding to an efficiency Z = 97% and Z = 91%,

respectively.

In order to discuss the process we have to take into account

that singlet-singlet energy transfer can take place via two

different mechanisms: a Coulombic interaction and an

electron exchange interaction. We will first discuss the

Coulombic interaction, also known in its simplest form as a

dipole–dipole interaction, described by Forster.46 The rate of

the process can be estimated from eqn (1).

kFen ¼8:8� 10�25k2F

n4td6DA

JF ð1Þ

F and t are the emission quantum yield and lifetime of the

donor, dDA is the donor–acceptor center-to-center distance, n

is the refractive index of toluene (1.4961) and JF is the Forster

overlap integral which can be calculated from the emission

spectrum of the donor, 1PZn2, 2PZn2, 3PZn2, and the absorp-

tion spectrum of the acceptor, i.e. half of the absorption

spectrum of 1FB2, 2FB2, 3FB2, according to eqn (2).

JF ¼RFð�uÞeð�uÞ=�u4d�uRFð�uÞd�u

ð2Þ

An assumption regarding the geometrical factor K2, which

depends on the relative orientation of the transition dipoles of

the donor and acceptor, is necessary in order to use eqn (1).47

K2, which is 2/3 when there is a random orientation of the

transition dipoles of the units as occurs in freely diffusing

components, can assume quite different values for units

engaged in supramolecular structures, where the orientation

depends on the architecture of the array. In addition, a

detailed knowledge of the minimized geometry of the array

should be known. K2 has been calculated for similar systems

and was found to range between 5/6 and 1.25, therefore we use

an average value of 1.1.22,44,45 The parameters used to derive

the Forster energy transfer rate constant, kFen, are reported in

Table 4 with the results and the experimental values ken. The

calculated values are lower than the experimental data, at least

in the cases where the rate could be experimentally deter-

mined. This indicate that dipole–dipole mechanism by itself is

inadequate to describe in full the energy transfer occurring in

the present case. Similar unsatisfactory results by a dipole–

dipole interpretation for dyads based on a Zn porphyrin donor

and a free-base porphyrin acceptor connected at either the

meso or the b positions by phenylethyne bridges have been

reported previously by other groups.22,44,45

We will therefore consider the exchange mechanism: this has

been introduced by Dexter48 and can be described as a double

electron exchange involving the transfer of the electron from

the LUMO of the excited donor to the empty LUMO of the

acceptor with a concomitant transfer of an electron from the

HOMO of the acceptor to the HOMO of the donor. Since it

requires contact of the orbitals of the donor and of the

acceptor it is considered a short distance mechanism. Sub-

sequent development of the superexchange theory49 has allowed

to extend the validity of the Dexter theory to the exchange of

electrons mediated by the connecting bridge. The rate of

Scheme 2 Energy level diagram of 1FB-PZn, 2FB-PZn, and

3FB-PZn.

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energy transfer by electron exchange mechanism in a classical

formulation can be expressed by the following equation:48

kDen ¼4p2H2

hJD ð3Þ

where JD, the Dexter integral overlap, can be evaluated

from the absorption spectrum of the acceptor and from

the luminescence spectrum of the donors according to the

following equation:

JD ¼RFð�uÞeð�uÞd�u

RFð�uÞd�u

Reð�uÞd�u

ð4Þ

The value of JD for the various cases are collected with the

other parameters used for the calculation of the energy

transfer according to the Dexter theory in Table 5. The

contribution of the exchange mechanism to the overall,

experimentally determined rate constant ken, was determined

by the difference with the calculated Forster energy transfer,

i.e. kDen = ken � kFen. By introducing the kDen calculated values

(Table 4) and JD in eqn (3), an inter-component electronic

interaction H of the order of 10.0 cm�1 for 2FB-PZn and

3FB-PZn can be derived. The order of magnitude of such

parameters indicate a moderate coupling between the model

components, in agreement with previously discussed

spectroscopic data.

It could be interesting to address the issue of the distance

dependence of energy transfer even if only two terms of the

series can be considered. In the frame of the superexchange

mechanism, H follows an approximate exponential law with

increasing distance of the donor and the acceptor and can be

expressed as:50

H = H0 exp [�0.5br] (5)

where H0 is the intercomponent electronic interaction at the

contact distance, b is the attenuation factor and r can be taken

as the edge to edge distance between the donor and acceptor.

From eqn (3) and (5), the rate constant can be expressed as a

function of distance between reacting partners, with A being a

pre-exponential factor:51,52

kDen = A exp(�br) (6)

In the present case, where two p-phenylenevinylene bridges of

increasing length are present in 2FB-PZn and 3FB-PZn, an

attenuation factor b of the order of 0.06 A�1 can be calculated.

This treatment is highly simplified; in fact not only uses only two

terms of a series, but also regards b as a constant. It has actually

been shown by Albinsson and coworkers40,47,48 that the pre-

exponential factor A is a function of the energy gap between the

donor and the bridge and therefore b, in addition to be a system

specific parameter (and not a bridge specific parameter) might be

non exponential for conjugated bridges. For these type of con-

nectors, in fact, the energy is strongly affected by the bridge length.

In spite of this approximation, the extracted b can be compared

with literature values for other types of bridges. For singlet energy

transfer attenuation factors b = 1.33 A�1 for rigid hydrocarbon

bridges53 and b = 0.2 A�1 for poly-phenyleneethynylene45 have

been reported and values of the order of 0.06–0.08 A�1 can be

calculated from the data reported for polyene and poly-yne

linkages.39 For triplet energy transfer were reported values of

b = 0.17 A�1 for poly-yne bridges,54 and of b = 0.33 A�1 or

b = 0.32 A�1 or b = 0.65 A�1 for oligophenylene bridges.55,56

In conclusion the present b value places these p-phenylene-

vinylene bridged porphyrin systems as extremely effective

energy transducers, and this fact should be ascribed not only

to the intrinsic high electronic conductance of the p-phenylene-

vinylene bridge but also to the favorable coplanar arrangement

of the bridge and the chromophores.

Conclusions

Determination of energy transfer in a series of free-base

porphyrin/zinc porphyrin dyads connected by p-delocalizedbridges has been accurately performed with respect to models,

the corresponding free-base porphyrin and zinc porphyrin

homo-dyads, which perfectly represent the units in the same

molecular environment of the dyads. This has allowed us to

compare the experimental rates with those calculated accord-

ing to current models with more confidence than has been ever

done before. The mechanism cannot be accounted for entirely

by a dipole–dipole (Forster) scheme, and the inclusion of a

contribution by electron exchange (Dexter) in the p-phenylene-

vinylene containing dyads has led us to derive a very weak

distance-dependence of the rate of the exchange energy trans-

fer. This is assigned to the nature of the bridge and to the

highly coplanar arrangements of the bridge with respect to the

donor, which enhances the coupling. The overall result is a

very high (498%) energy transfer efficiency.

Experimental

The synthesis of dyads 1FB2 and 1FB-Zn were performed

according to the procedures described by Bonfantini and

Table 4 Energy transfer; parameters used for the calculation (according to the Forster theory) and experimental data

dDA/Aa K2 F t/ns JF/cm3 M�1 kFen/s

�1b ken/s�1c

1FB-PZn 14.0 1.1 0.29 1.4 5.69 � 10�14 3.0 � 1011 41011

2FB-PZn 18.3 1.1 0.09 1.6 6.07 � 10�14 1.8 � 1010 7.1 � 1010

3FB-PZn 24.5 1.1 0.07 1.7 8.43 � 10�14 3.1 � 109 3.9 � 1010

a Center to center donor–acceptor distance. b Calculated value. c Experimental value.

Table 5 Energy transfer; parameters used for the calculation (accordingto the electron exchange mechanism) and experimental data

r/Aa JD/cm ken/s�1 kDen/s

�1

1FB-PZn 6.3 2.42 � 10�4 41011 —2FB-PZn 10.5 4.35 � 10�4 7.1 � 1010 5.3 � 1010

3FB-PZn 17.1 2.97 � 10�4 3.9 � 1010 3.6 � 1010

a Edge-to-edge donor–acceptor distance.

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Officer.57 The synthesis of other dyads will be reported

separately.58 The solvent used was spectroscopic grade toluene

(C. Erba). Spectroscopic grade tetrahydrofurane (C. Erba)

was added in concentration ranging from 5% to 10% to the

free-base and zinc homo-dyads exhibiting some difficulty to

dissolve. Absorption spectra were recorded with a Perkin-

Elmer Lambda 9 spectrophotometer and emission spectra,

uncorrected if not otherwise specified, were detected by a Spex

Fluorolog II spectrofluorimeter equipped with a Hamamatsu

R928 photomultiplier. Relative luminescence intensities were

evaluated from the area of the luminescence spectra corrected

for the photomultiplier response. Emission quantum yields

were determined with reference to TPP in aerated toluene

(ffl = 0.1127). Luminescence lifetimes in the nanosecond range

were obtained with an IBH single photon counting equipment

with excitation at 465 nm from a pulsed diode source

(0.5 ns resolution). Luminescence lifetimes and spectra in the

picosecond range were determined by an apparatus based

on a Nd:YAG laser (Continuum PY62-10) with a 35 ps

pulse duration, 532 nm, 1 mJ/pulse and a Streak Camera

(Hamamatsu C1587 equipped with M1952). The luminescence

images from 500 laser shots were averaged and the time profile

was measured from the resulting streak image in a wavelength

range of ca. 20 nm around the selected wavelength. The fitting

of the luminescence decays were performed by standard

iterative non linear programs taking into consideration the

instrumental response. Time resolved spectra were taken over

a time window of 30 ps at the specified delays. More details for

the streak camera experiment can be found elsewhere.59

Standard 10 mm fluorescence cells were used at 295 K

whereas experiments at 77 K made use of capillary tubes

dipped in an home made quartz Dewar filled with liquid

nitrogen. Estimated errors are 10% on lifetimes and 20%

on quantum yields. Working temperature, if not otherwise

specified, was 295 � 2 K.

Computation of the integral overlap and of the rate for the

energy transfer processes according to Forster mechanism

were performed with the use of Matlab 5.2.60 Molecular

dimensions and distances were estimated after minimization

(MM2) by CS Chem 3D Ultra 6.0.61

Acknowledgements

We thank CNR of Italy (PM.P04.010 MACOL) for financial

support.

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