Two-photon absorption in butadiyne-linked porphyrin dimers: Torsional and substituent effects

Article abstract of DOI:10.1039/c4tc01120a

Dyes exhibiting efficient two-photon absorption (2PA) are in demand for a wide range of applications, and meso-meso butadiyne-linked porphyrin dimers (bis-porphyrins) are promising chromophores in this area. As part of an investigation of the structure-property relationships controlling the performance of these dyes, we present the synthesis of eight π-extended dimers, with substituents providing diverse push and pull electronic effects, and high solubility. We show that the peak 2PA cross sections can be increased from 3000 GM to 20000 GM by attaching terminal electron-withdrawing or -donating groups, and by converting the free-base dimers to zinc complexes. The two-photon excited fluorescence spectra of porphyrin dimers in viscous media, under conditions such that excited states do not planarize prior to emission, reveal that dimers in planar conformations dominate the two-photon absorption. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4tc01120a

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DOI: 10.1039/C4TC01120A
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Two-Photon Absorption in Butadiyne-Linked
Porphyrin Dimers: Torsional and Substituent Effects
Cite this: DOI: 10.1039/x0xx00000x
James D. Wilkinson,a Geoffrey Wicks,b Agnieszka Nowak-Król,c Łukasz G.
Łukasiewicz,c Craig J. Wilson,a Mikhail Drobizhev,b Aleksander Rebane,*b,d
Daniel T. Gryko*c,e and Harry L. Anderson*a
Received 00th January 2012,
Accepted 00th January 2012
Dyes exhibiting efficient two-photon absorption (2PA) are in demand for a wide range of
applications, and meso-meso butadiyne-linked porphyrin dimers (bis-porphyrins) are
promising chromophores in this area. As part of an investigation of the structure-property
relationships controlling the performance of these dyes, we present the synthesis of eight
π-extended dimers, with substituents providing diverse push and pull electronic effects, and
high solubility. We show that the peak 2PA cross sections can be increased from 3,000 GM
to 20,000 GM by attaching terminal electron-withdrawing or -donating groups, and by
converting the free-base dimers to zinc complexes. The two-photon excited fluorescence
spectra of porphyrin dimers in viscous media, under conditions such that excited states do
not planarize prior to emission, reveal that dimers in planar conformations dominate the
two-photon absorption.
DOI: 10.1039/x0xx00000x
www.rsc.org/
Introduction
Ar
Materials with strong two-photon absorption (2PA) are
increasingly important at the frontiers of microscopy,
photodynamic therapy (PDT), telecommunications, 3D data
storage, light-activated drugs and optical power limiting.1-6
There have been significant advances in the elucidation of
molecular design principles for maximizing the 2PA cross
section (σ2),1,2,7-9 however structure-property relationships are
still poorly understood for all but the simplest chromophores.
The nonlinear optical properties of porphyrins have attracted
much interest over the past 20 years.2,10-13 Monomeric
porphyrins exhibit only modest 2PA cross-sections,14,15 even
with optimized substitution patterns,16,17 but dimers and higher
oligomers have been found to offer substantial increases in
σ2.11,15,18-22 Several varieties of conjugated porphyrin oligomers
have been explored, resulting in 102–103 fold enhancements in
σ2 per porphyrin unit.2,11 Butadiyne-linked porphyrin dimers
offer a good compromise between high 2PA cross section, long
excited state lifetimes, high solubility and synthetic
accessibility.24-28
N
N
N
N
R1
M
R2
2
Ar
A-P2C8(M)-A (R1 = R2 = NO2; M = Zn, 2H)
A-P2C8(M)-D (R1 = NO2, R2 = NBu2; M = Zn)
A-P2C8(M)-R (R1 = NO2, R2 = Bu; M = Zn)
D-P2C8(M)-D (R1 = R2 = NBu2; M = Zn, 2H)
D-P2C8(M)-R (R1 = NBu2, R2 = Bu; M = Zn)
R-P2C8(M)-R (R1 = R2 = Bu; M = Zn, 2H)
OC8H17
Ar =
OC8H17
OC10H21
D-P2C10(M)-D (R1 = R2 = NMe2; M = Zn, 2H)
A'-P2C10(M)-A' (R1 = R2 = CF3; M = Zn, 2H)
Ar =
OC10H21
OC10H21
(OCH2CH2)3OCH3
Cl–
Cl–
N
N
N
N
+
N
+
Me
Zn
N Me
A common design strategy for optimizing σ2 is the use of
centrosymmetric quadrupolar architectures, of the type A-D-A
or D-A-D, featuring strong π-conjugated “push” electron-
donating (D) and “pull” electron-accepting (A) groups.1,2,7-9,29
Previously we have reported a butadiyne-linked dimer, Oxdime
(Fig. 1) with terminal pyridinium electron-acceptor groups
which exhibits enhanced 2PA (σ2 =17,000 GM
Oxdime
2
(OCH2CH2)3OCH3
Fig 1. Structure of the compounds studied in this work and the previously reported
two-photon PDT sensitiser Oxdime.5,24-26,30
This journal is © The Royal Society of Chemistry 2013
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Journal of Materials Chemistry C
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Ar
Zn
Ar
Ar
View Article Online
OC H
DOI: 10.10398/C147TC01120A
O
O
N
N
N
N
N
N
N
N
Ar =
R1
R2
(C6H13)3Si
Zn
Ar
Si(C6H13)3
Pd(PPh3)2Cl2,
OC8H17
CuI, i-Pr2NH, 70%
2
3
1 R1 = R2 = Si(C6H13 3
2 R1 = H, R2 = Si(C6H13 3
)
TBAF
TBAF
Ar
Zn
Ar
)
Ar
N
N
N
16%
31%
N
N
N
N
H
Si(C6H13)3
H
Zn
Ar
H
2
N
2
5
4
(1) Pd2(dba)3, PPh3,
(2) TBAF, Pd2(dba)3, PPh3,
CuI, i-Pr2NH
CuI, i-Pr2NH
Pd2(dba)3, PPh3,
O2N
I
CuI, i-Pr2NH
O2N
I
I
NBu2
Ar
M
Ar
M
N
N
N
N
N
N
N
N
O2N
NO2
O2N
NBu2
2
2
Ar
Ar
A-P2C8(Zn)-A (M = Zn)
A-P2C8(2H)-A (M = 2H)
A-P2C8(Zn)-D (M = Zn)
A-P2C8(2H)-D (M = 2H)
CF3CO2H
CF3CO2H
Scheme 1. Synthesis of A-P2C8(Zn)-A, A-P2C8(Zn)-D, A-P2C8(2H)-A and A-P2C8(2H)-D.
Ar
Ar
Ar
NaOH
OH
toluene reflux
NH
N
NH
N
N
NH
N
N
N
Br
OH
HN
Pd2(dba)3, AsPh3,
Et3N, toluene, THF
64%
HN
79%
HN
Ar
Ar
Ar
6
7
8
Ar
Ar
Ar
Ar
Pd2(dba)3, AsPh3
Et3N, 78%
NH
N
NH
N
NH
N
N
N
NH
N
N
NBS
23%
Br
Br
N
HN
HN
HN
HN
9
10
Ar
Ar
Ar
Ar
Ar
Ar
M
R
N
N
N
N
N
N
N
R
M
R
Pd2(dba)3, AsPh3, Et3N
toluene, THF
N
Ar
Ar
11 R = NMe2
12 R = CF3
OC10H21
OC10H21
OC10H21
A'-P2C10(2H)-A' M = H2, R = CF3 (77%)
A'-P2C10(Zn)-A' M = Zn, R = CF3 (42%)
Zn(OAc)2
Zn(OAc)2
Ar =
D-P2C10(2H)-D M = H2, R = NMe2 (27%)
D-P2C10(Zn)-D M = Zn, R = NMe2 (38%)
Scheme 2. Synthesis of A'-P2C10(2H)-A', A'-P2C10(Zn)-A', D-P2C10(2H)-D and D-P2C10(Zn)-D.
2 | J. Name., 2012, 00, 1-3
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DOI: 10.1039/C4TC01120A
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at 916 nm),5,24-26,30 but the effect of terminal electron acceptors the protecting group, to give intermediate 8. Palladium-
and donors has not been systematically tested.31 Here we catalyzed homocoupling, under copper-free conditions, gave
present
and -withdrawing groups on σ2 in butadiyne-linked porphyrin remaining meso-positions followed by Sonogashira coupling
dimers. with alkynes 11 and 12 gave dimers A'-P2C10(2H)-A' and D-
a
study of the effect of electron-donating dimer 9 in 78% yield. Subsequent bromination at the two
Efficient 2PA requires strong electronic coupling over a P2C10(2H)-D. In this approach, zinc was inserted at the end of
large π-system, making it sensitive to conformational changes the synthesis.
which alter π-orbital overlap. Various strategies have been used
to enhance 2PA by constraining the conformations of
π-systems, including the synthesis of multiply-linked porphyrin
One-photon absorption spectra
tapes (e.g. β-to-β 1,3-butadiyne doubly-linked,32 meso-β The absorption spectra of the bis-porphyrins show the typical
doubly-linked,33 and triply-linked tapes34), ladder-complex features of butadiyne-linked porphyrin dimers (Table 1 and Fig.
formation,35,36 straps and host-guest chemistry.37 Butadiyne- 2).21,47-52 For the zinc family, the peak of the Soret (B) band is
linked porphyrin dimers consist of mixtures of conformations shifted bathochromically by up to 5 nm to 469 nm in the “pull-
due to rotation about the central butadiyne link, and the push” chromophore A-P2C8(Zn)-D, compared to the “push”
different conformations have characteristic absorption and chromophore D-P2C8(Zn)-R. The trend in peak wavelength of
fluorescence spectra.38-43 Here we use this feature to compare this band [A'-P2C10(Zn)-A' ≈ D-P2C8(Zn)-R ≈ R-P2C8(Zn)-R <
the 2PA cross sections of planar and twisted conformations.
A-P2C8(Zn)-R ≈ D-P2C10(Zn)-D ≈ D-P2C8(Zn)-D ≈ A-
P2C8(Zn)-A < A-P2C8(Zn)-D] suggests that the electron-
accepting nitro group has a greater influence on the π-system,
than the electron-donor. The CF3 acceptor has less influence on
the absorption spectra, as seen by comparing A-P2C8(Zn)-A
with A'-P2C10(Zn)-A'. The Soret bands of the free-base family
also span a 7 nm range, but the electron-donating group has the
greatest effect on the π-system, dramatically broadening the
Soret band in D-P2C8(2H)-D and D-P2C10(2H)-D. The
extinction coefficients of the Soret transition are large and
correlate well with the inverse of the bandwidth for the zinc
dyes; the oscillator strengths (fB) are all similar as expected.
The Q bands of all the free-base dimers show an extra peak
at 620–650 nm, as expected from the lower symmetry of the
free-base chromophore. The Q band transitions are more
sensitive to changes in the electronic substitution, with a
maximum red shift in the lowest-energy band of 12 nm from R-
P2C8(Zn)-R to A-P2C8(Zn)-D, and 19 nm from R-P2C8(2H)-R
to D-P2C10(2H)-D. There is again distortion in this region for
the D-P2C8(2H)-D free-base dye, with significant broadening
and loss of structure in the Q band accompanied by increased
oscillator strength (Fig. 2b).
Results and discussion
Synthesis
Porphyrin dimers sometimes exhibit poor solubility, which can
make it difficult to investigate and apply of their photophyical
properties, so in this work we have tested two different types of
solubilizing groups: 3,5-dioctyloxyphenyl and 3,4,5-
tridecyloxyphenyl. The family of six P2C8 octyloxy-solubilized
zinc porphyrin dimers shown in Figure 1 was synthesized from
porphyrin monomer 1 using a combination of statistical
desilylation and Sonogashira coupling steps, as illustrated in
Scheme 1.38,44 Desilylation of 1 yielded a mixture from which
the singly desilylated monomer 2 was separated and subjected
to a Pd-catalyst in the presence of 1,4-benzoquinone as oxidant
to give butadiyne-linked dimer 3. This dimer was then similarly
desilylated, and the doubly- and singly-desilylated dimers 4 and
5 separated. Coupling 4 and 5 to 4-iodonitrobenzene yielded A-
P2C8(Zn)-A and a silyl/nitro dimer, respectively; the latter was
desilylated and in situ coupled to N,N-dibutyl-4-iodoaniline
yielding A-P2C8(Zn)-D. The synthesis can also be performed
without separating 4 and 5, by isolating A-P2C8(Zn)-A and the
silyl/nitro dimer precursor to A-P2C8(Zn)-D after the next step.
The free-base porphyrin dimers were prepared either by
demetalation of the final zinc compound, or by demetalation of
intermediate 3, followed by the application of Cu-free
Sonogashira coupling steps45 (see ESI Fig. S1 and S2).
Another series of porphyrin dimers (P2C10, Fig. 1) with
electron-withdrawing CF3 terminals, or NMe2 electron-donors,
was prepared as shown in Scheme 2. Bromoporphyrin 617,46
was subjected to Sonogashira coupling followed by removal of
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Journal of Materials Chemistry C
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intermediate state.15,20 Resonance enhancement due to the
Table 1. One- and two-photon absorption data (in CHCl3, plus 1% pyridine for M = Zn)
presence of this state is in part responsible for the large σ2
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a
a
dimer
λmax,1PA (nm)
fB
fQ
λmax,2PA
σ2
observed for porphyrin dimers comparDeOdI:t1o0.1m03o9n/Co4mTCe0rs1.11250,2A0
[ε (µM–1 cm–1)]
(nm) (103 GM)
Besides the centrosymmetry supporting the role of the Qx(0-0)
transition, the close match between the difference in the
shortest and longest wavelengths of the 2PA maxima and the
difference in the wavelengths of the lowest energy Qx(0-0)
transition of the corresponding compounds further implies this
state is significant in 2PA [comparing R-P2C8(Zn)-R with A-
P2C8(Zn)-A, and R-P2C8(2H)-R with D-P2C10(2H)-D]. It is also
observed that the oscillator strength of the Q band is strongest
in A-P2C8(Zn)-A and D-P2C8(2H)-D, contributing to their large
σ2 values.
The 2PA cross section of the butadiyne-linked porphyrin
dimer core is found to be quite insensitive to substitution. This
is emphasized by comparing the factor of ~2-fold difference
between σ2 of A-P2C8(Zn)-A and R-P2C8(Zn)-R, and between
D-P2C8(2H)-D and R-P2C8(2H)-R, and the factor of ~20
between analogously modified porphyrin monomers.16
Commonly held design principles, such as the advantage of a
quadrupolar system over a dipolar system, are thus strongly
dependent upon the core π-system. The tetrapyrrole core has
elsewhere been found to be surprisingly unperturbed by
peripheral substitutions; in a recent study of metal-free
tribenzo-tetraazachlorin, distinct gerade-gerade 2PA transitions
are maintained, despite the formal lack of a center of
inversion.15
A-P2C8(Zn)-A
468 [0.34], 494 [0.16]
694 [0.10], 748 [0.10]
2.96
1.79
2.68
2.67
2.54
2.50
2.67
2.82
2.88
0.67 904
0.35 920
0.65 902
0.58 900
0.57 902
0.52 920
0.58 900
0.55 890
0.58 920
15
6
A'-P2C10(Zn)-A' 464 [0.24], 499 [0.08],
686 [0.05], 749 [0.05]
A-P2C8(Zn)-D
A-P2C8(Zn)-R
D-P2C8(Zn)-D
D-P2C10(Zn)-D
D-P2C8(Zn)-R
R-P2C8(Zn)-R
A-P2C8(2H)-A
469 [0.30], 495 [0.19]
702 [0.09], 756 [0.10]
466 [0.35], 498 [0.15]
690 [0.09], 748 [0.09]
468 [0.25], 496 [0.17]
704 [0.08], 752 [0.08]
466 [0.22], 496 [0.14],
703 [0.06], 744 [0.06]
464 [0.32], 497 [0.18]
697 [0.08], 745 [0.09]
465 [0.46], 499 [0.16]
681 [0.09], 744 [0.10]
458 [0.27], 484 [0.11]
622 [0.06], 708 [0.06]
738 [0.07]
12
13
13
21
12
9.1
10
A'-P2C10(2H)-A' 455 [0.31], 483 [0.09]
625 [0.05], 709 [0.05]
2.55
0.47 915
3.4
740 [0.07]
460 [0.22], 646 [0.08]
747 [0.10]
463 [0.23], 644 [0.07],
725 [0.08], 754 [0.09]
456 [0.29], 481 [0.09]
623 [0.05], 706 [0.05]
736 [0.06]
D-P2C8(2H)-D
D-P2C10(2H)-D
R-P2C8(2H)-R
3.42
2.75
2.39
0.85 925
0.66 926
0.47 915
17
10
7.7
aCalculated using f = 4.319 × 10–9 A/n where n is the solvent refractive index
(1.45 for CHCl3) and A is the integrated absorption band (units: mol–1 L cm–2;
integration range for M = Zn fB 18,020–26,670 cm–1/375–555 nm; range for
fQ 11,830–16,450 cm–1/608–845 nm; integration range for M = 2H fB 17,699–
30,030 cm–1/333–565nm; range for fQ 11,111–17,699 cm–1/565–900 nm).
Table 2. Electrochemical potential data for selected dimers
dimer
Eox
Eox – Eox,ref
Ered
Ered – Ered,ref
Eox – Ered
Two-photon absorption spectra
A-P2C8(Zn)-A
D-P2C8(Zn)-D
R-P2C8(Zn)-R
A-P2C8(2H)-A
0.43
0.23
0.39
0.61
0.04
-0.16
0
-1.41
-1.53
-1.47
-1.18
-1.23
-1.31
-1.32
-1.28
0.06
-0.06
0
1.84
1.76
1.86
1.79
1.86
1.58
1.59
1.83
The 2PA cross-sections and spectra were measured by two-
photon excited fluorescence (2PEF) under dilute conditions
(concentration ca. 10–6
M) in chloroform, in the presence of 1%
pyridine in the case of the zinc complexes, to suppress
aggregation. We can be confident that there is no aggregation
under these conditions, because aggregation was not detected
0.06
0.08
-0.28
-0.28
0
0.10
0.05
-0.03
-0.04
0
A'-P2C10(2H)-A' 0.63
D-P2C8(2H)-D
D-P2C10(2H)-D
R-P2C8(2H)-R
0.27
0.27
0.55
1
when the same compounds were examined by H NMR in the
same solvents at much higher concentrations (ca. 10–3
M). The
entire series of dimers was found to have large σ2 (~104 GM at
~900 nm; Table 1) with maxima lying between 890 and 926
nm, in accordance with literature values5,15,19-21,36,51,52 and
representing an hundred-fold enhancement compared to typical
porphyrin monomers.14-17 The 2PA spectra feature a second,
broader peak at lower intensity at 1050–1150 nm. The strongly
quadrupolar A-P2C8(Zn)-A dimer exhibits a peak cross section
of 1.5 × 104 GM, which is 1.7-fold larger than that of the R-
P2C8(Zn)-R reference system, and similar to that of Oxdime.5,30
However this enhanced 2PA is not observed in A'-P2C8(Zn)-A',
illustrating the weaker effect of the CF3 acceptor. D-P2C8(2H)-
D shows a 2.2-fold increase in peak cross section over the free-
base reference compound R-P2C8(2H)-R.
All data are in volts against internal ferrocene (Fc/Fc+) and were acquired
using square wave voltammetry in THF containing Bu4NPF6 (0.1 M). Eox,ref
and Ered,ref refer to the oxidation and reduction potentials respectively for the
relevant R-P2C8(M)-R reference compound. Square wave frequency 8 Hz;
glassy carbon working electrode, Pt counter electrode, Ag/AgNO3 reference
electrode.
The largest 2PA cross sections are found when two acceptor
groups are attached to the electron-rich zinc porphyrin dimers,
as in A-P2C8(Zn)-A, or when two donor groups are attached to
the electron-poor free-base dimers, as in D-P2C8(2H)-D. The
effects of electron accepting and donating substituents can be
quantified by the changes in redox potentials of the porphyrin
dimers (Table 2). The quantity Eox – Eox,ref is the difference
between the first oxidation potential of a specific dimer and that
of the reference dimer R-P2C8(M)-R. Eox – Eox,ref acts as a
A three-level model has previously been invoked to
describe the 2PA spectra of centrosymmetric porphyrin dimers,
with the lowest-energy Qx(0-0) transition assigned to the
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measure of the electronic effect of the acceptor or donor groups
D-P2C8(2H)-D and D-P2C10(2H)-D are indistinguishable by
upon the porphyrin dimer core, and reveals that the aniline electrochemistry, suggesting little difference in “push” between
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donor group induces the greatest electronic perturbation on the the dibutyl and dimethyl aniline groups, but these dimers
DOI: 10.1039/C4TC01120A
dimer core, both in D-P2C8(Zn)-D and D-P2C8/10(2H)-D, but exhibit different cross sections. The P2C10 variant does not
this perturbation is only translated into the one- and two-photon display as heightened Q band oscillator strength as the P2C8
absorption profiles for the free-base core. Relative to the zinc variant, and thus resonance enhancement is weaker, suggesting
core, the free-base core is electron poor, and D-P2C8(2H)-D is subtle effects upon the optical properties originating from the
thus a stronger quadrupolar “push-pull-push” system than A- bulkier side groups.
P2C8(2H)-A; as a result, an enhanced σ2 is observed. However,
The effects of the π-core and the quadrupolar nature of the
based on this electrochemical analysis of the “push” and “pull” structure upon σ2 are consistent with other studies,34 but do not
effect alone, the bis-donor dimer would also be expected to account for all of the subtle variation within the series. The
exhibit the highest σ2 in the zinc series, as the magnitude of Eox design principles derived for simple organic moieties cannot be
– Eox,ref is again greater for D-P2C8(Zn)-D than A-P2C8(Zn)-A. simply applied to more complex π-cores, and do not play such
Instead A-P2C8(Zn)-A dominates the σ2 trend, itself also highly an obvious role in butadiyne-linked porphyrin dimers.
polarizable and benefitting from the longest π-system due to
Conformation effects
complete conjugation of the strong “pull-pull” nitro groups to
the phenyl rings. The importance of a long conjugation pathway Previously it has been found that there is a poor match between
is also seen from the low value of σ2 of A'-P2C10(2H)-A' the one- and two-photon transition energies in butadiyne-linked
(with -CF3 σ-acceptors) compared with A-P2C8(2H)-A porphyrin dimer, as is confirmed by the data in Table 1 and
(with -NO2 π-acceptors).
Figure 2; for example values of λmax,2PA do not correlate well
with λmax,1PA. This implies that 2PA involves transitions to a
gerade final state,15,20 and that the chromophores are effectively
centrosymmetric, which is surprising because these dimers exist
in solution as broad distributions of conformations, due to free
rotations about the butadiyne link,38 and only the conformation
in which both porphyrins are co-planar is centrosymmetric.
The different torsional conformations of a butadiyne-linked
porphyrin dimer have different fluorescence spectra. In the
ground state, the barrier to rotation about the butadiyne link is
less than kT at room temperature, whereas this barrier becomes
higher in the singlet excited state.38 In low-viscosity solvents,
the excited states undergo planarization faster than radiative
decay, so that most of the emission originates from the planar
conformation, regardless of which conformation was excited,
but in viscous media torsional rotation becomes slow on the
time-scale of emission and the shape of the observed
fluorescence spectrum provides information on the
conformations that were excited.39 Although there is
a
continuous distribution of conformations, the spectra can be
adequately analyzed in terms of just two extreme conformations
with twisted and planar geometries.
We analyzed the one- and two-photon excited fluorescence
spectra of R-P2C8(Zn)-R in a viscous chloroform/polymer
medium, in order to test which conformations contribute most
to the 2PA. The normalized one-photon excitation and
fluorescence spectra of R-P2C8(Zn)-R in Figure 3 show that
twisted and planar conformations fluoresce at 685 and 755 nm
respectively. One photon excitation at 500 nm generates
exclusively the planar excited state, whereas excitation at 460
nm gives predominantly the twisted excited state, with also
some planar excited state due to competing absorption by both
conformations at 460 nm, and possibly due to partial
planarization in the excited state.
Fig 2. One- and two-photon absorption spectra of a) A-P2C8(Zn)-A and R-P2C8(Zn)-
R in CHCl3 containing 1% pyridine and b) D-P2C8(2H)-D and R-P2C8(2H)-R in CHCl3.
This viscous solution was then used to acquire a 2D 2PEF
emission-excitation spectrum (Fig. 4). The two-photon
excitation emission map reveals that upon excitation at the 2PA
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maximum (890 nm), emission is seen exclusively from the
planar form (755 nm), implying that this conformation is the
stronger two-photon absorber. The tail of the contour reveals a
second 2PA peak clearly emitting at the wavelength of the
twisted conformation (685 nm) with excitation shifted
bathochromically by 15 nm, as expected for a twisted
conformation.53
View Article Online
DOI: 10.1039/C4TC01120A
Whereas the one-photon excited emission spectra at various
excitation wavelengths demonstrate that both conformations
absorb strongly, in ratios reflected in the varying emission ratio
of the two peaks (685:755), under two-photon excitation there
is very little variation in the emission ratio, as the planar form is
the dominant 2P absorber (Fig. 5). Plotting the individual two-
photon excitation spectra confirms the dominance of the planar
form over the twisted form in σ2 (Fig. 6).
Fig 5. Emission spectra of R-P2C8(Zn)-R in viscous solution under 1P (solid) and 2P
(dots) excitation at the energy indicated.
Fig 3. Normalized one-photon excitation spectra (solid lines, detection at 685
and 755 nm) and fluorescence spectra (dashed lines, excitation at 460 and 500
nm) spectra of R-P2C8(Zn)-R in viscous solution; twisted conformation in black
and planar conformation in grey.
Fig 6. Two-photon excitation spectra of the planar and twisted conformations of
R-P2C8(Zn)-R acquired in viscous solution (values normalized to σ2 measured in
solution), compared to the total 2PA spectrum (integrated across all emission
wavelengths) in normal solution.
Conclusions
This systematic study of donor- and acceptor-substituted
butadiyne-linked porphyrin dimers shows that the attachment of
electron acceptors and donors can increase the 2PA cross
section by about a factor of three in zinc and free-base dimers.
However the nonlinear optical properties of these dimers are
much less sensitive to donor/acceptor substituents than has been
found for the corresponding porphyrin monomers.16,17,54-56
These insights will be useful in the design of applied two-
photon absorbers, as optimizing features such as
biocompatibility can be prioritized. We have also provided
direct spectroscopic evidence that the 2PA of these dimers is
dominated by the contribution from the planar form. This result
is in accord both with inferences made from the 2PA spectra
regarding the planarity of related dimers,15 theoretical work on
butadiyne-linked dimers,51 and with systems modified to
enhance planarity.29,33-37 This work indicates that the σ2 of
Fig 4. 3D two-photon excitation-emission plot of R-P2C8(Zn)-R in viscous
chloroform/polymer solution.
6 | J. Name., 2012, 00, 1-3
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butadiyne-linked dimers can be increased by synthetic
modifications to suppress twisted conformations.
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