Photoinduced electron transfer in a directly linked meso-triphenylamine zinc porphyrin-quinone dyad

Article abstract of DOI:10.1142/S108842461100329X

A multimodular donor-acceptor system composed of three triphenylamine entities at the meso-positions of a zinc porphyrin macrocycle and a quinone at the fourth meso-position was newly synthesized and characterized. The triphenylamine entities acted as ene

Full text of DOI:10.1142/S108842461100329X

Journal of Porphyrins and Phthalocyanines
J. Porphyrins Phthalocyanines 2011; 15: 391–400
DOI: 10.1142/S108842461100329X
Published at http://www.worldscinet.com/jpp/
Photoinduced electron transfer in a directly linked
meso-triphenylamine zinc porphyrin-quinone dyad
Channa A. Wijesinghea, Marja Niemib, Nikolai V. Tkachenko*b,
Navaneetha K. Subbaiyana, Melvin E. Zandlera, Helge Lemmetyinenb and
Francis D’Souza*a◊
a Department of Chemistry, Wichita State University, 1845 Fairmount, Wichita, KS 67260-0051, USA
b Department of Chemistry and Bioengineering, Tampere University of Technology, P.O. Box 541,
33101 Tampere, Finland
Dedicated to Professor John A. Shelnutt on the occasion of his 65th birthday
Received 25 February 2011
Accepted 25 April 2011
ABSTRACT: A multimodular donor-acceptor system composed of three triphenylamine entities
at the meso-positions of a zinc porphyrin macrocycle and a quinone at the fourth meso-position was
newly synthesized and characterized. The triphenylamine entities acted as energy transferring antenna
units in addition of improving the electron donor ability of the zinc porphyrin. Appreciable electronic
interactions of the triphenylamine and quinone entities with the porphyrin π-system were observed. In
agreement with the spectral and electrochemical results, the computational studies performed by the
DFT B3LYP/3-21G(*) method revealed delocalization of the frontier HOMO over the triphenylamine
and the porphyrin macrocycle while the LUMO to be fully localized over the quinone entity. Free-energy
calculations suggested photoinduced electron transfer from the singlet excited zinc porphyrin to the
directly linked quinone to be exothermic and this was experimentally confirmed by the time-resolved
pump probe and up-conversion techniques. In the investigated system, the ET reaction path was found to
depend upon the excitation wavelength. That is, when Zn porphyrin was predominantly excited, a rapid
charge separation followed by equally fast charge recombination was observed. However, excitation of
the peripheral TPA substituents resulted in an extremely long-lived CS state with triplet spin character
via the TPA triplet and Zn porphyrin triplet states.
KEYWORDS: photoinduced electron transfer, triphenylamine, porphyrin, quinone.
electron transfer events in the reaction center producing
INTRODUCTION
long-lived charge separated states [1, 2]. The initial step
Electron transfer is ubiquitous in both biological and
involves transfer of an electron from the so-called ‘spe-
synthetic systems. Among different aspects, understand-
cial pair’ chlorophyll dimer to the quinone located within
ing the mechanism and control of electron transfer paths
the reaction center. The distant location of radical ion-
in natural and synthetic models are the widely investigated
topics. The fundamental process of primary photosynthe-
sis, for example, is characterized by a series of sequential
pair species avoiding the energy wasting back electron
transfer and a small overall reorganization energy (λreorg
~ 0.2 eV) along with a well-balanced electronic coupling
between different donor and acceptor entities results in
high efficiency of charge separation [1, 2]. In order to
SPP full member in good standing
further improve the understanding of the mechanistic
*Correspondence to: Nikolai V. Tkachenko, email: nikolai.
details and the associated technological developments in
the area of optoelectronics and solar energy harvesting, a
tkachenko@tut.fi and Francis D’Souza, email: Francis.
DSouza@wichita.edu, fax: +1 316-978-3431
Copyright © 2011 World Scientific Publishing Company

392
C.A. WIJeSInghe et al.
N
O
O
N
N
N
N
Zn
N
Zn
N
N
N
N
O
O
1
2
N
Scheme 1. Structure of the porphyrin-quinone systems investigated in the present study
plethora of molecular structures comprised of donor and
the present study, we have extended the study by replac-
ing the fullerene entity by the well-known two-dimen-
sional electron acceptor, quinone directly linked to the
zinc porphyrin macrocycle, 1 (Scheme 1). Photoinduced
electron transfer in 1 has been investigated using tran-
sient flash photolysis, pump-probe and up-conversion
techniques, and compared with the results of a directly
linked zinc porphyrin-quinone, 2 lacking the TPA meso-
substituents [16].
acceptor entities have been designed and photoinduced
electron transfer relevant to early events of photosynthe-
sis have been studied [3–11]. From these studies the role
of free-energy of reaction, the magnitude of reorganiza-
tion energy and the extent of electron coupling between
the initial and final states have been established and theo-
retically modeled primarily by Marcus theory [12].
Among the different electron donor-acceptor systems
reported to-date, the systems built on porphyrin (or other
tetrapyrroles) as donor and the three-dimensional electron
acceptor, fullerene have attracted wide attention [7–9,
11]. The relatively easy reduction [13] and low solvent
and internal reorganization energies of fullerene in elec-
tron transfer reactions [14] along with the low suscepti-
bility to solvent stabilization of the radical anion make
it to be a superior electron acceptor. As a consequence,
fullerenes in donor-acceptor dyads accelerate forward
electron transfer (charge-separation; kCS) and slow down
the backward electron transfer (charge-recombination;
kCR), thus generating the much desired long-lived charge-
separated (CS) states [7–9, 11], a result often next to
impossible to achieve in the case of donor-acceptor sys-
tems having simple two-dimensional electron acceptors
[3–5]. Further improvements in charge stabilization in
porphyrin-fullerene systems have been accomplished
by introducing one or more electron donor or acceptor
entities to initiate sequential electron transfer for distant
location of the radical ion-pairs [3–9].
RESULTS AND DISCUSSION
Optical absorption and fluorescence studies
Figure 1a shows the optical absorption spectra
of 1 along with a control compound, meso-tetrakis-
(triphenylamine)porphyrinatozinc, (TPA)4-ZnP, that is,
a compound having four TPA entities at the four meso-
positions and lacking the quinone entity, in benzonitrile.
The Soret band of 1 was located at 444 nm while the
Q-bands were located at 565 and 611 nm, respectively.
The absorption due to TPA entities was located at 309
nm. These peak maxima of porphyrin macrocycle were
found to be red shifted by up to 6 nm compared to the
control compound lacking the quinone. These results
indicate substantial interactions between the porphyrin
π-system with the directly linked quinone entity. Addi-
tionally, the peak maxima for compound 2 were located
at 418, 549 and 618 nm, respectively, for the Soret and
Q-band transitions with much narrower bands compared
to 1 (data not shown) [16, 17]. The large red shift of up to
26 nm for 1 along with broader bands as compared to 2
suggests strong electronic interactions between the TPA
entities and the porphyrin π-system. These results suggest
that both the triphenylamine and quinone substituents of
1 interact with the porphyrin π-system and modulate the
electronic structure.
Recently, we reported on a zinc porphyrin-fullerene
system in which the donor, zinc porphyrin was function-
alized at the meso-positions with triphenylamine (TPA)
entities [15]. In this molecule, the TPA entities in the
dyad played two important roles; first by promoting exci-
tation transfer to the donor, and second, by stabilizing
the charge separated states on the order of a few micro-
seconds. DFT calculations suggested that the improved
charge stabilization is due to the extensive delocalization
of the highest occupied molecular orbital (HOMO) on
the entire donor entity. Intrigued with these findings, in
Figure 1b shows the fluorescence spectra of 1 along
with the control compound. The emission band of 1 was
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PhOtOInDuCeD eleCtrOn trAnSfer In A DIreCtly lInkeD meso-trIPhenylAmIne
393
of TPA entities [15]. These results indicate occurrence of
excitation transfer from the TPA entities to the zincpor-
phyrin of (TPA)4-ZnP [18]. Such a photophysical pro-
cess is also possible in 1 due to structural similarities,
although the emission bands were almost completely
quenched due to the presence of the additional quinone
entity. These observations suggest the occurrence of
sequential energy transfer followed by electron transfer
in 1 when the peripheral TPA entities are directly excited
in polar benzonitrile solvent.
Electrochemistry and computational studies
Electrochemical studies using differential pulse vol-
tammetric (DPV) technique were performed to evaluate
the redox potentials of 1 in benzonitrile containing 0.1 M
(n-Bu4N)ClO4, as shown in Fig. 2. The electrochemis-
try of 2 has earlier been reported by us [17, 19]. During
anodic scan, compound 1 revealed oxidation processes
corresponding to both the porphyrin macrocycle and
the TPA entities while during cathodic scan reduction
processes corresponding to both quinone and porphyrin
were observed. The site of electron transfer was assigned
based on comparative electrochemistry involving the
control compounds and spectroelectrochemical stud-
ies [15, 20, 21]. The two one-electron anodic processes
corresponding to the oxidation of the porphyrin macro-
cycle were located at 0.29 and 0.49 V vs. Fc/Fc+ while
the third anodic process involving at least four-electrons
was located at 0.72 V vs. Fc/Fc+ corresponding the oxida-
tion of the peripheral TPA entities. The two one-electron
cathodic processes corresponding to the quinone entity
were located at -0.81 and -0.93 V vs. Fc/Fc+ while the
porphyrin ring based one-electron reductions appeared at
-1.80 and -2.01 V vs. Fc/Fc+, respectively. The reduction
of the quinone entity of 1 was positively shifted com-
pared to the first reduction of dyad 2 located at -0.85 V vs.
Fc/Fc+ in benzonitrile. The electrochemically measured
Fig. 1. (a) Absorption spectra normalized to the Soret band
and (b) fluorescence spectra of (i) 1 and (ii) the control,
meso-tetrakis(triphenylamine)porphyrinatozinc in PhCN. λex
564 nm
=
found to be red shifted by about 9 nm compared with the
control compound, (TPA)4-ZnP, in addition to quenching
of its intensity by more than 80%. Such quenching (more
than 80%), accompanied with a small red shift, was also
observed for 2 compared to its control compound, meso-
tetraphenylporphyrinatozinc(II), ZnTPP. In agreement
with the absorption behavior, the emission maximum of
1 was found to be red shifted by ~25 nm compared with
the emission maximum of 2 [17]. These results suggest
occurrence of excited state events in 1 from the singlet
excited zinc porphyrin entity. As reported earlier, when
(TPA)4-ZnP was excited at 318 nm corresponding to
the absorption of TPA entities, a new emission at 480
nm was observed [15]. Control experiments performed
using triphenylamine confirmed that this band is due to
the emission of peripheral substituents. However, when
compared with the emission intensity of triphenylamine,
the emission of (TPA)4-ZnP was found to be substantially
quenched. Further scanning the emission wavelength into
the red region revealed additional bands corresponding to
porphyrin emission, which was not observed for ZnTPP
lacking TPA entities. Excitation spectrum recorded by
fixing the emission monochromator to the porphyrin
emission maxima at 632 nm revealed absorption peaks
Fig. 2. Differential pulse voltammograms of 1 (~0.5 mM) in
benzonitrile containing 0.1 M (TBAP)ClO4. DPV conditions:
scan rate = 20 mV/s, pulse width = 50 ms, step time = 100 ms
and pulse height = 0.025 V
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C.A. WIJeSInghe et al.
HOMO–LUMO gap for 1, calculated from the first
oxidation potential of porphyrin and the first reduction
potential of quinone, was found to be 1.10 V PhCN, and
is smaller by about 130 mV compared to 2 which could
be attributed to the influence of the peripheral electron
donating TPA entities.
In order to gain insights into the geometry and elec-
tronic structure, computational studies were performed
using density functional methods (DFT) at the B3LYP/3-
21G(*) level on 1 [22]. The B3LYP/3-21G(*) methods
have recently been successfully used to predict the geom-
etry and electronic structure of molecular and supramo-
lecular donor-acceptor assemblies [23]. Compound 1 was
optimized to a stationary point on the Born-Oppenheimer
potential energy surface whose structure is shown in
Fig. 3a. The porphyrin ring was found to be almost flat
with no appreciable ring puckering due to the presence of
the meso-substituents. The quinone entity attached at the
meso-position of the porphyrin macrocycle was tilted at
an angle of 78°. The molecular electrostatic potential map
(MEP) for compound 1 in Fig. 3b clearly shows electron
deficient site on the quinone entity. In the optimized struc-
ture, the HOMO was localized on the porphyrin macrocy-
cle with considerable contribution also on the peripheral
TPA entities, while the LUMO was fully localized on
the quinone entity (Figs 3c,d). These results suggest that
the triphenylamine-substituted porphyrin is an electron
donor and quinone an electron acceptor in electron trans-
fer reactions. The computed gas phase HOMO–LUMO
gap was found to be 1.56 eV which was smaller than the
HOMO–LUMO gap of 2 being 1.82 eV [19]. Although
the electrochemical studies indicate involvement of ZnP
entity in the first oxidation process, the appearance of
part of the HOMO over the peripheral TPA substituents
Fig. 3. B3LYP/3-21G(*) optimized (a) geometry, (b) molecular electrostatic potential map, (c) highest occupied molecular orbital
and (d) lowest unoccupied molecular orbital of porphyrin-quinone 1
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PhOtOInDuCeD eleCtrOn trAnSfer In A DIreCtly lInkeD meso-trIPhenylAmIne
395
is expected to stabilize the radical cation by delocaliza-
the measurements were repeated after approximately two
weeks and the longer-living component was found to
be stronger in amplitude than in the first measurements.
Thus it seems that the samples were not quite stable in
time, even though the absorption spectrum measured
before and directly after the up-conversion measurements
showed no signs of sample degradation.
tion, ultimately slowing down the charge recombination
during the light induced electron transfer process.
The energy levels of the CS states (∆GRIP) were evalu-
ated using the Weller-type approach [24] utilizing the
redox potentials, and the dielectric constant of benzo-
nitrile. By comparing these energy levels of the CS states
with the energy levels of the excited states, the driv-
ing forces (∆GCS) were also evaluated, according to the
following equations.
Transient absorption of the compounds was moni-
tored with two systems in two different time scales. In
the femtosecond pump-probe measurements mainly Zn
porphyrin was excited at 410 nm. The global fitting of
the pump-probe absorption decay curves of 1 in PhCN
required four exponentials (Fig. 5). The component with
a lifetime 1.1 ps shows characteristic cation absorption
of TPA substituted Zn porphyrin above 700 nm [15] and
can be attributed to the CS state. The 0.3 ps component
is a mirror image of the 1.1 ps component and corre-
sponds to the formation of the CS state from the singlet
excited state of Zn porphyrin. The lifetime of the singlet
excited state (0.3 ps) is in agreement with the shorter
fluorescence decay component (0.6 0.3 ps) detected in
the up-conversion measurements. Similar behavior was
observed for compound 2 (not shown): the CS state was
formed in 0.37 ps and it decayed in 2 ps. The spectrum
of the CS state in 2 corresponds well with that of ZnTPP
radical cation, i.e. the broad radical cation band is at a
lower wavelength range compared to that seen in 1.
The time constants obtained for charge separation
and charge recombination and the energies of the states
can be used to estimate the reorganization energy, λ, of
the electron transfer reaction. According to Marcus ET
theory, the rate constants for charge separation, kcs, and
charge recombination, kcr, are;
∆GRIP = Eox - Ered - ∆GS
(1)
where ∆GS = -e2/(4πε0εRRCt-Ct) and ε0 and εR refer to
vacuum permittivity and dielectric constant of the sol-
vent benzonitrile. This gives 1.26 eV and 1.39 eV as
the energies of the CS states of 1 and 2, respectively, in
benzonitrile. The term RCt-Ct refers to the center-to-center
distance (estimated as 3.5 Å) of the optimized structure
shown in Fig. 3a. Furthermore, the driving force for
charge separation can be calculated as
-∆GCS = ∆E0-0 - ∆GRIP
(2)
where ∆E0-0 is the energy of the lowest excited state (esti-
mated from the first Q-band in the absorption spectrum
as 2.03 eV and 2.01 eV for 1 and 2, respectively).
Suchcalculationsrevealthatthegenerationof((Ph2N)3-
ZnP)▪+-Q▪– in 1 and ZnP▪+-Q▪– in 2 are exothermic via the
singlet excited state of the porphyrin in PhCN [24].
Time-resolved spectroscopy studies
Femtosecond up-conversion measurements were car-
ried out to determine the fluorescence lifetime of the first
excited singlet state of Zn porphyrin in 1 and 2 in PhCN.
A fast component with a lifetime close to the instrument
time resolution was observed for both dyads (Fig. 4).
In addition, another longer-living component was seen,
although not very accurately resolved with the measured
time range. For 2 this longer-living component is a minor
one compared to the fast component (Fig. 4a), but for 1
the situation is reversed (Fig. 4b) [26]. There is no direct
explanation for this longer-living fluorescence. However,
(∆Gcs - λ)2
4λkBT
(∆Gcr - λ)2


kcs = k0exp -
and






kcr = k0exp -
(3)



4λkBT

where k0 is the rate for the passage through the transi-
tion state from the reactant state (the singlet excited
Fig. 4. Fluorescence decay curves of (a) 2 and (b) 1 in PhCN. The excitation wavelength was 410 nm and the signal was monitored
at 620 nm. The lifetimes and the amplitudes (in brackets) obtained from the bi-exponential fitting are indicated in the figure
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C.A. WIJeSInghe et al.
to Zn porphyrin was expected, because it was observed
earlier for a similar dyad with fullerene as the acceptor
instead of quinone [15], however the efficiency of the
singlet-singlet energy transfer was estimated to be 80%,
which indicates that a considerable part of excited TPA
chromophores relax to the triplet excited state. One can
also notice that through the multiple excitation of one and
the same molecule by relatively long (15 ns) laser pulse
at 355 nm the actual yield of the triplet state after the
excitation may exceed 20%, thus generating relatively
strong transient absorption response of the samples.
In the time domain of flash photolysis measurements
(>20 ns) and selective excitation of TPA at 355 nm, one
can expect to observe triplet excited states of TPA and
ZnP chromophores, one formed as the result of inter-
system crossing and another resulting from singlet state
energy transfer from TPA to ZnP, respectively. The tran-
sient absorption spectrum of (TPA)4-ZnP measured right
after excitation (Fig. 6a, spectrum denoted as “at t = 0”)
has a strong absorption band around 1000 nm, which is
in agreement with previously published results [15]. In
this wavelength range triplet state of regular ZnTPP has
only weak and featureless absorption, whereas TPA is
Fig. 5. Transient absorption decay component spectra of 1 in
PhCN measured with the femtosecond pump-probe method
(420 nm excitation). The spectra with time constants 0.3 and
1.1 ps correspond to the charge separation and recombination
reactions, respectively, and the longer-lived components origi-
nates from the solvent response (see text for more details)
state or the charge-separated state for charge separation
and charge recombination, respectively) to the product
state (charge-separated state or ground state for charge
separation and charge recombination, respectively), kB is
the Boltzmann constant and T is the temperature. If k0 is
assumed to be the same for both charge separation and
charge recombination, dividing the equations 3 one by
another and solving for λ gives:
Eex (2Ecs - Eex )
(4)
λ =
kcs
4kBT ln + 2(2Ecs - Eex )
k cr
where Eex and Ecs are the energies of the singlet excited
state and the CS state, respectively. The calculated values
for λ are 0.89 eV and 0.90 eV for 1 and 2, respectively,
and they are quite reasonable for porphyrin-quinone
dyads [25].
The two longer-living components needed for the fit-
ting of the pump-probe results are more difficult to inter-
pret, because the solvent, benzonitrile, also participates
in the signal. The measurements were carried out also for
pure PhCN, and a long-living signal with a broad 600 nm
centered absorption was observed. It was not possible to
get rid of this solvent signal completely by reducing the
excitation intensity, but it was taken into account while
analyzing the data. Thus, the 6 ps component and the
longest-living component, with a lifetime too long to be
resolved with the instrument (time limit ~1 ns), originate
most probably from the solvent, though some typical
Q-band features of Zn porphyrin can be noticed in the
spectra of the components around 560 nm and 600 nm.
The flash-photolysis measurements in the microsec-
ond time-scale were carried out to investigate possible
contribution of the triplet excited state to photoinduced
electron transfer. The measurements were carried out
under nitrogen flow to reduce triplet state quenching by
molecular oxygen dissolved in the solvent. The excitation
wavelength was 355 nm, which excites the TPA moieties
predominantly. Energy transfer from the triphenylamines
Fig. 6. Transient absorption decay component spectra and cal-
culated spectrum at 0 delay time of (a) (TPA)4-ZnP and (b) 1 in
PhCN measured with the microsecond flash-photolysis method
(355 nm excitation)
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397
expected to have spectrum similar to that presented in
Fig. 6a at t = 0. The spectrum changes drastically with
time constant of 50 µs, converting to less intense and
virtually flat absorption over the measured near infra red
region. This change can be attributed to the triplet-triplet
energy transfer to the Zn porphyrin chromophore [15].
Comparison with the spectrum detected at 0 delay
time for 1 (Fig. 6b) shows clearly that the first detect-
able state is the triplet state of the TPA chromophore also
in 1. Global fitting of the transient absorption decays of
1 requires at least three exponential components which
resulted in decay associated spectra presented in Fig. 6b.
The triplet state of TPA chromophores relaxes by yield-
ing two other intermediate states. At first a state with
rather broad and featureless absorption appears (the
spectrum at t = 30 µs in Fig. 6b), and then a state with
a broad but clear band around 1300 nm is formed (the
longest-lived component, 840 µs, in Fig. 6b). The latter
can be attributed to the Zn porphyrin cation with triplet
character, i.e. to the CS triplet state [15, 28]. Based on
this observation three step relaxation of TPA triplet state
can be proposed. The first step is intramolecular triplet-
triplet energy transfer from TPA chromophore to ZnP
moiety with time constant 15 µs. Second is charge sepa-
ration between the porphyrin and quinone moieties with
81 µs time constant, and finally the charge recombination
with time constant 840 µs. Further studies involving time
resolved EPR may be needed to confirm this long-lived
charge separated state.
Fig. 8. Suggested reaction scheme for 1
one may argue that presence of quinone at the meso-
position may affect electron density distribution along the
porphyrin ring, and thus change electronic interactions
of the TPA and porphyrin moieties, which is confirmed
indirectly by the difference in steady state absorption and
emission of the compounds (see Fig. 1).
Based on the time-resolved absorption measurements,
reaction scheme presented in Fig. 8 can be suggested.
When exciting the TPA units predominantly (at 355 nm),
the intersystem crossing is followed by the triplet-triplet
energy transfer to yield the triplet state of TPA substi-
tuted Zn porphyrin. The long-living CS state with trip-
let spin character is then formed from the Zn porphyrin
triplet state. Alternatively, if the Zn porphyrin is directly
excited (at 410 nm), a rapid ET reaction occurs from the
singlet excited state and the formed CS state with singlet
spin character relaxes quickly to the ground state. A dis-
tinct difference between the charge separated states with
singlet and triplet spin characters can be attributed to a
short (single carbon bond) linker between the donor, por-
phyrin, and acceptor, quinone, moieties, in which case
some degree of electronic coupling between the donor
and acceptor remains after the electron transfer, thus
preserving multiplicity of the whole dyad.
To confirm triplet nature of the CS, the measurements
were repeated for air saturated sample and comparison of
the transient absorption decays in air and under nitrogen
is presented in Fig. 7 at the monitoring wavelength cor-
responding to the absorption of the CS state, 1480 nm.
Atmospheric oxygen is known to quench efficiently the
triplet state, which reduces gradually the yield of the
CS state, but lifetime of CS state formed in air saturated
sample remains virtually identical to that in deoxygen-
ated sample.
Apparently the TPA triplet state relaxes faster in 1 than
in (TPA)4-ZnP, which has no clear explanation. Though
EXPERIMENTAL
Chemicals
Benzonitrile (in sure seal bottle under nitrogen) was
from Aldrich Chemicals (Milwaukee, WI). Tetra-n-
butylammonium perchlorate, (TBA)ClO4 was from
Fluka Chemicals. All the chromatographic materials and
solvents were procured from Fisher Scientific and were
used as received.
Synthesis of 1
5-(2,5-dimethoxyphenyl)-10,15,20-tri-N,N-diphenyl-
porphyrin. This compound was synthesized by reacting
2,5-dimethoxybenzaldehyde (12 mmol), 4-formyltriphe-
nylamine (37 mmol), and pyrrole (49 mmol) in refluxing
propionic acid. The crude product was purified on a basic
Fig. 7. Transient absorption decays of 1 in PhCN at 1480 nm
measured with the microsecond flash-photolysis method (355 nm
excitation) in open air and under nitrogen
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C.A. WIJeSInghe et al.
1
alumina column. H NMR (300 MHz; CDCl3): δH, ppm
while the MEP and frontier orbitals were generated using
GaussView program. The mass spectra of the newly syn-
thesized compounds were recorded on a Varian 1200L
Quadrupole MS using APCI mode in dry CH2Cl2.
-2.69 (2H, s (br), pyrrole-NH), 3.5 (3H, s, 5-OCH3), 3.9
(3H, s, 2-OCH3), 7.0–7.16 (12H, m, N-ph), 7.26–7.36
(2H, m, ph), 7.38–7.56 (24H, m, m-ph and N-ph), 7.61
(1H, m, ph), 8.1 (6H, m, o-ph), 8.80–9.0 (8H, m, pyr-
role-H). MS (ESI): m/z 1175.49 (calcd. 1176.41).
Flash-photolysis method was used to study time-
resolved absorption in microsecond time-scale. The
experiments were carried out with a modified Luzchem
laser flash system (mLFP111 prototype from Luzchem
Co.) using the third harmonic of Nd:YAG laser providing
30 ns pulses at 355 nm for excitation. Excitation power
density was roughly 6 mJcm-2. A continuous Xe-lamp
(Oriel Simplesity Arc Source) was used to provide moni-
toring light and the signal was recorded with a digitiz-
ing oscilloscope (Tektronix, TDS3032B, 300 MHz). The
system was controlled with a PC computer. The samples
were deoxygenated by nitrogen bubbling for 20 min prior
to the measurements and a nitrogen flow was maintained
on the surface of the solution during the measurements.
All measurements were carried out at room temperature.
Pump-probe technique for time-resolved absorp-
tion was used to detect the fast processes with a time
resolution shorter than 0.2 ps. The fundamental of the
Ti:sapphire laser (TiF-50, CDP Corp.) pumped by Nd-
YAG CW laser (Verdi-6, Coherent Inc.) was split in two:
A part of the beam went through a second harmonic
generator to obtain excitation at 410 nm (pump), and the
other part was passed through a cuvette filled with water
generating a white continuum for monitoring (probe) the
changes in absorption. Before focusing on the rotating
sample cuvette, the pump beam was passed through an
adjustable delay line, which was scanned to obtain tran-
sient absorption signals up to approximately 1 ns after
the excitation. A CCD camera was used to detect the dif-
ferential absorption spectra at different delay times, and
from these the transient absorption decay curves were
drawn at different wavelengths. Exponential fitting of the
absorption decay curves was done globally for the mea-
sured wavelength range for both pump-probe and flash-
photolysis results. The data analysis procedure has been
described in more detail earlier [27].
Up-conversion instrument (FOG-100, CDP Corp.) for
time-resolved fluorescence was used to detect the fast
processes with a time resolution of ~200 fs. The same
Ti:sapphire generator used for pump-probe was used
to excite (~410 nm) the sample solution in a rotating
cuvette [27]. Emission from the sample was collected to
a nonlinear crystal (NLC), where it was mixed with the
so-called gate pulse, which was the laser fundamental.
The signal was measured at a sum frequency of the gate
pulse and the selected emission maximum of the sam-
ple. The gate pulses were passed through a delay line
so that it arrived at NLC at a desired time after sample
excitation. Scanning through the delay line the emission
decay curve of the sample was detected. Exponential fit-
ting of the fluorescence decay curve was carried out in
a similar manner as for the transient absorption decay
curves [27].
5-[phenyl(2,5-dione)]-10,15,20-tri-N,N-diphenyl-
porphyrin. A 9 mL solution of BBr3 (1 M in CH2Cl2) was
drop wise added to a solution of 5-[2,2′-(2,5-dimethoxy)]-
10,15,20-tri-N,N-diphenylporphyrin (1.0 mmol) in CH2Cl2
at -78 °C. The solution was maintained at this temper-
ature until the addition was completed and stirred at
room temperature under oxygen for 12 h in dark. Then
the mixture was brought to below 5 °C and 100 mL of
cold water was added followed by addition of saturated
sodium bicarbonate. After stirring 1 h at room tempera-
ture the organic layer was separated using CH2Cl2 and
dried over anhydrous Na2SO4. The solvent was evapo-
rated and the crude product was purified on silica col-
umn. 1H NMR (300 MHz; CDCl3): δH, ppm -2.70 (2H, s
(br), pyrrole-NH), 7.26–7.31 (12H, m, N-ph), 7.39–7.48
(2H, m, ph), 7.48–7.55 (24H, m, m-ph and N-ph), 7.60
(1H, m, ph), 8.05–8.15 (6H, m, o-ph), 8.90–9.05 (8H, m,
pyrrole-H). MS (ESI): m/z 1145.44 (calcd. 1146.34).
Zinc 5-[phenyl(2,5-dione)]-10,15,20-tri-N,N-diphenyl-
porphyrin, 1. A 0.0125 mmol of free base porphyrin
was dissolved in 30 mL of CHCl3, and an excess of zinc
acetate (50 equiv.) in methanol was added. The course
of the reaction was monitored spectroscopically. At the
end of the reaction (1 h), the solvent was evaporated and
1
the product was purified on silica gel column. H NMR
(300 MHz; CDCl3): δH, ppm 7.26–7.31 (12H, m, N-ph),
7.39–7.48 (2H, m, ph), 7.48–7.55 (24H, m, m-ph and
N-ph), 7.60 (1H, m, ph), 8.05–8.15 (6H, m, o-ph), 8.90–
9.05 (8H, m, pyrrole-H). MS (ESI): m/z 1207.36 (calcd.
1209.73).
Instrumentation
The UV-visible spectral measurements were carried
out with a Shimadzu Model 1600 UV-visible spectropho-
tometer. The fluorescence emission was monitored by
using a Varian Eclipse spectrometer. A right angle detec-
tion method was used. The 1H NMR studies were carried
out on a Varian 400 MHz spectrometer. Tetramethylsi-
lane (TMS) was used as an internal standard. Differen-
tial pulse voltammetry was recorded on a EG&G model
263A Potentiostat/Galvanostat using a three electrode
system. A platinum button electrode was used as the
working electrode. A platinum wire served as the coun-
ter electrode and a Ag/AgCl electrode was used as the
reference electrode. Ferrocene/ferrocenium redox couple
was used as an internal standard. All the solutions were
purged prior to electrochemical and spectral measure-
ments using argon gas. The computational calculations
were performed by DFT B3LYP/3-21G(*) methods with
GAUSSIAN 03[22] software package on high speed PCs
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PhOtOInDuCeD eleCtrOn trAnSfer In A DIreCtly lInkeD meso-trIPhenylAmIne
399
and Gomez-Lopez M. Chem. Rev. Soc. 1999; 28:
263–277. g) Kavarnos GJ and Turro NJ. Chem. Rev.
CONCLUSION
A novel multi-modular donor-acceptor system, 1
composed of three triphenylamine entities, porphyrin
and quinone, was designed, synthesized and studied to
investigate photoinduced processes by time-resolved
spectroscopic techniques including pump probe and up-
conversion methods. The spectral and computational
studies revealed appreciable electronic interactions
between porphyrin π-system and the meso-substituents.
The free-energy change for charge-separation and charge-
recombination were evaluated using electrochemical and
absorption data. The charge-separation processes were
monitored by steady-state and time-resolved emission
studies, which revealed occurrence of efficient electron
transfer processes in 1. The ET reaction path was found
to be dependent on the excitation wavelength, i.e. whether
the Zn porphyrin was excited directly or via the triph-
enylamine moieties. When Zn porphyrin was predomi-
nantly excited, a rapid charge separation followed by
equally fast charge recombination was observed. Instead,
excitation of the TPA substituents lead to an extremely
long-living CS state with triplet spin character via the
TPA triplet and Zn porphyrin triplet states.
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Acknowledgements
This work was financially supported by the National
Science Foundation (Grant Nos. 0804015 and EPS-
0903806) and matching support from the State of Kansas
through Kansas Technology Enterprise Corporation, and
Academy of Finland.
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