Organometallic multiads of zinc(ii) porphyrins with interchromophoric cooperativity in S1 and T1 energy transfers

Article abstract of DOI:10.1039/c2cc16804a

Three different S₁ and T₁ energy donors are linked onto a central zinc(ii) porphyrin acceptor and the rates for energy transfers show evidence for cooperativity. The Royal Society of Chemistry 2012.

Full text of DOI:10.1039/c2cc16804a

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COMMUNICATION
Organometallic multiads of zinc(II) porphyrins with interchromophoric
cooperativity in S1 and T1 energy transferswz
Bin Du and Pierre D. Harvey*
Received 2nd November 2011, Accepted 11th January 2012
DOI: 10.1039/c2cc16804a
Three different S1 and T1 energy donors are linked onto a central
zinc(II) porphyrin acceptor and the rates for energy transfers
show evidence for cooperativity.
porphyrin (see 5, 8 and 9 in Chart 1). The energy transfer rates
are significantly dependent upon the number of donors (2 vs. 4).
The general procedure for the syntheses of the target multiads
involves a converging approach of the Tru/zinc(porphyrin)-
containing core with the flanking trans-bis(phosphine)-
(ethynylaromatic)s (Scheme 1; see the ESIz for experimental
details). The main precursor 2-carbaldehyde-5,50,10,100,15,150-
hexahexyltruxene,9 1, is condensed with 4-trimethylsilylethynyl-
benzaldehyde (B3 : 1 ratio) and pyrrole, followed by a metallation
with zinc acetate providing the mono-ethynyl derivative 4 in
Platinum(II) acetylides have been widely used for the design of
photonic materials,1 and their connection to porphyrins led to the
designs of photocells,2 and molecular, oligomeric, polymeric and
dendrimeric devices exhibiting exciton coupling,3 red emission,4
singlet, S1, and triplet, T1, energy transfer,5 host–guest,6 self-
assembly,7 optical limited and nonlinear optical properties.8
Recently, we9 and others10 reported the synthesis and emission
properties of truxene-containing zinc(II) porphyrins. From the
study on the antenna properties of truxene (Tru) and tritruxene,9
it was deduced that despite modest rates of S1 and T1 energy
transfers, the fluorescence and phosphorescence intensity of Tru
donor was strongly quenched and that the fluorescence and
phosphorescence quantum yields of the central zinc(II) porphyrin
or free base were enhanced compared to the tetraphenyl-porphyrin
analogues, even at 298 K. Moreover, the rates were found
independent of the nature of the antenna, Tru or tritruxene,
indicating that the closest unit played the main role in the
energy transfer rate. We now wish to report multiads containing
3 different S1 and T1 energy donors around a central zinc(II)
Chart 1 Structures of the multiads 5, 8 and 9.
De´partement de Chimie, Universite´ de Sherbrooke,
´
E-mail: pierre.harvey@uhsebrooke.ca; Fax: +1 819-821-8017
w This article is part of the ChemComm ‘Porphyrins and phthalo-
cyanines’ web themed issue.
2500 Boul. de l’Universite, Sherbrooke, QC, Canada J1K 2R1.
Scheme 1 Synthesis paths for 5, 8 and 9: (i) (a) CH2Cl2, BF3ꢀOEt2,
DDQ; (b) Zn(OAc)2, THF/MeOH; (c) TBAF, THF. (ii) CH2Cl2/
iPr2NH, CuI. (iii) CH2Cl2/iPr2NH, CuI, trans-[Pt(P(n-Bu)3)2Cl2].
(iv) (a) CHCl3, BF3ꢀOEt2, DDQ; (b) Zn(OAc)2, THF/MeOH.
(v) THF/MeOH, K2CO3, RT. (vi) CH2Cl2/iPr2NH, CuI.
z Electronic supplementary information (ESI) available: Experimental
section and all photophysical data for 5–9. See DOI: 10.1039/c2cc16804a
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 2671–2673 2671

10% yield. In parallel, the monoethynyl-zinc(II) porphyrin
species 25b is reacted with trans-[Pt(P(n-Bu)3)2Cl2] to afford
3 in 35% yield. The reaction of 3 and 4 in the presence
of iPr2NH and CuI provides target 5 in 30% yield. For
the preparation of targets 8 and 9, 1 is first condensed with
5-4-trimethylsilylethynylphenyldipyrromethane10 in the presence
of the oxidizing agent DDQ to generate 6 in 11% yield.
The latter is desilanated with potassium carbonate to form 7
in excellent yield (98%). The latter is then reacted with 3 or with
trans-[Pt(P(n-Bu)3)2Cl(CRCC6H5)],5b in the presence of CuI
and iPr2NH to give 8 and 9, in 10 and 75% yield, respectively.
The energy level for each component has been determined
using the position of the 0–0 peaks in the absorption, fluorescence
and phosphorescence spectra of truxene,9 trans-Pt(P(n-Bu)3)2-
(CRCC6H5)2 [Pt],11 7 (Fig. 1) and 2.5b The relative positions of
the 0–0 peaks (Table 1) indicate that the energy levels vary as
Tru 4 [Pt] 4 2 4 7 for S1 and as [Pt] 4 Tru 4 2 4 7 for T1.
Previous transient absorption and emission spectroscopic
studies from our group and others on dyads containing [Pt]
and meso-aryl substituted zinc(II) porphyrin,5c [Pt] and 2,5a
Tru and 7,9 and b-alkyl (Zn(P1)) and meso-aryl substituted
zinc(II) porphyrin (Zn(P2))12 provided evidence for S1 or T1
energy transfers, excluding the possibility of charge separated
states. Hence using these building blocks, any excited state
Fig. 2 Emission spectra of 5 and 8 in 2-MeTHF at 77 K.
quenching of the donor is a consequence of energy transfers.
The fluorescence (FF) or phosphorescence (FP) quantum
yields for these chromophores are truxene, FF = 0.07 in
toluene at 298 K,13a substituted truxenes 0.18 o FF o 0.56,
in THF at 298 K,13b,c substituted [Pt] complexes 0.17 o FP o
0.38, in 2-MeTHF at 77 K,11b 2 0.047 and 0.045 at 298 and
77 K, respectively, in 2-MeTHF,5a and 7 (FF = 0.019 in
2-MeTHF at 298 K; this work). The emission spectra of 5 and 8
exhibit strong fluorescence and phosphorescence bands of the
meso-aryl substituted zinc(II) porphyrin central acceptor but
those for Tru (350–450), [Pt] (450–550) and octa-alkyl-zinc(II)
porphyrin (575 nm) are very weak (Fig. 2). This anticipated
quenching is associated to efficient S1 and T1 energy transfers.
The rates for S1 and T1 energy transfers, kET, are obtained using
kET = (1/te) ꢁ (1/teo) where te and teo are the fluorescence (S1)
and phosphorescence (T1) lifetimes of the donor chromophore
(Tru, [Pt] or Zn(P1)) in the presence and absence of an energy
acceptor (Zn(P2)), respectively (Zn(P1) and Zn(P2) are defined in
Chart 1). The te and teo data, measured at the emission maximum,
indicate quenching in the presence of the energy acceptor by 1 to 2
orders of magnitude (Tables 2 and 3). In comparison with
compound 10 (Chart 2),9 kET(S1) for 5, 8 and 9 are all in the
same vicinity meaning that the replacement of Tru by [Pt] 1 or 2
times has little effect on kET at both temperatures.
In order to verify this hypothesis, kET(T1) Tru - Zn(P2) for 7
and 9 was measured and compared these to 10 and 11 (Table 3).
Fig. 1 (top) Absorption, fluorescence and phosphorescence spectra of
truxene (Tru) and trans-Pt(P(n-Bu)3)2(CRCC6H5)2 [Pt] in 2-MeTHF.
The arrows point the 0–0 transitions. (bottom) Absorption (blue),
excitation (red) and fluorescence (black) spectra of 7 in 2-MeTHF.
kET increases going from 7 to 10 by two orders of magnitude further
supporting the presence of cooperativity between the donors.
The replacement of two Tru in 10 by [Pt] gives 9 and an
increase is noted going from 7 to 9 as well but by one order of
magnitude meaning the cooperativity between Tru and [Pt] is
not as good as between Tru and Tru in the T1 state. To support
this, kET(T1) [Pt] - Zn(P2) is also compared for 9 and 11.
Both cases exhibit rates in the same range. We also note that
Table 1 Positions of the 0–0 peaks in the absorption, fluorescence
and phosphorescence spectra of the model compounds in 2-MeTHF
(tw = this work)
Abs./nm,
298 K
Fluo./nm,
298 K
Fluo./nm,
77 K
Phos./nm,
77 K
Ref.
k
ET(T1) for [Pt] - Zn(P2) is larger than that for Tru -
Zn(P2). This is explained by the fact that only the Dexter
Tru
[Pt]
2
306
340
575
605
354
346
597
610
356
—
575
610
457
445
703
797
9
11
5b
tw
mechanism operates in the T1 state,14 whereas both, Dexter15 and
7
Forster,16 operate in the S1 states. The former involves a double
¨
c
2672 Chem. Commun., 2012, 48, 2671–2673
This journal is The Royal Society of Chemistry 2012

Table 2
t
F data for Tru, 2, 5, 7, 8 and 9 in 2-MeTHF (tw = this work)
Zn(P1) - Zn(P2) between 5 (1 Zn(P1)) and 8 (2 Zn(P1)) is
noted, which may be due to the long separation between the
two Zn(P1) in 8, and may act independently. No Zn(P1)
phosphorescence (703 nm)5b was detected and no kET(T1)
Zn(P1) - Zn(P2) data are available.
kET
nsꢁ1
298 K 77 K
/
kET/
nsꢁ1
77 K Ref.
/
,
tF/ns,
Chromophore 298 K
,
tF/ns,
Tru Tru
56.3 ꢂ 0.2
—
—
64.6 ꢂ 0.5
—
—
9
5b
tw
This work showed an increase in kET(S1) and kET(T1) going
from 2 to 4 antennas around a central unit. This effect is
relevant to photosystems where excitation energy delocalization
(exciton) takes place efficiently bringing the collected light
energy to the special pair in the reaction center. The observed
cooperativity may well be due to an excitonic effect like in the
natural systems.
2
5
Zn(P1)
Tru
1.45 ꢂ 0.06
1.71 ꢂ 0.03
0.54 ꢂ 0.02 1.8
0.26 ꢂ 0.08 3.2
0.20 ꢂ 0.09 4.9
0.29 ꢂ 0.05 2.9
Zn(P1)
Zn(P2)
Tru
Zn(P2)
Tru
1.67 ꢂ 0.02
—
1.68 ꢂ 0.56 0.58
1.88 ꢂ 0.03
—
1.17 ꢂ 0.21 0.84
7
8
tw
tw
1.57 ꢂ 0.05
—
0.89 ꢂ 0.17 1.1
0.31 ꢂ 0.05 2.6
1.65 ꢂ 0.04
—
0.19 ꢂ 0.04 5.0
0.19 ꢂ 0.05 4.7
Zn(P1)
Zn(P2)
Tru
1.80 ꢂ 0.02
—
0.77 ꢂ 0.24 1.3
1.88 ꢂ 0.06
—
—
—
a
PDH thanks the Natural Sciences and Engineering Research
Council of Canada (NSERC) for funding.
9
tw
9
a
Zn(P2)
10 Tru
Zn(P2)
1.66 ꢂ 0.02
—
0.80 ꢂ 0.03 1.2
1.68 ꢂ 0.02
0.39 ꢂ 0.06 2.6
2.25 ꢂ 0.03
Notes and references
a
Not Measured
1 W. Y. Wong and P. D. Harvey, Macromol. Rapid Commun., 2010,
31, 671.
Table 3 tP data for Tru, [Pt], 7 and 9–11 in 2-MeTHF (tw = this work)
2 (a) H. M. Zhan, S. Lamare, A. Ng, T. Kenny, H. Guernon,
W. K. Chan, A. B. Djurisic, P. D. Harvey and W. Y. Wong,
Macromolecules, 2011, 44, 5155; (b) L. Liu, L. Hu, H. Fu,
Q. M. Fu, S. Z. Liu, Z. L. Du, W. Y. Wong and P. D. Harvey,
J. Organomet. Chem., 2011, 696, 1319.
Chromophore
tP, 77 K
kET/sꢁ1, 77 K
Ref.
Tru
[Pt]
7
Tru
[Pt]
Tru
Zn(P2)
Tru
[Pt]
Zn(P2)
Tru
620.2 ꢂ 0.3 ms
35.0 ꢂ 1.3 ms
12.6 ꢂ 1.0 ms
12.5 ꢂ 1.6 ms
2.1 ꢂ 0.2 ms
15.6 ꢂ 4.8 ms
11.2 ꢂ 0.5 ms
0.71 ꢂ 0.04 ms
25 ꢂ 0.4 ms
—
—
9
11b
tw
3 G. Langlois, S. M. Aly, C. P. Gros, J. M. Barbe and P. D. Harvey,
New J. Chem., 2011, 35, 1302.
7.8 ꢃ 101
—
4 (a) F. L. Jiang, D. Fortin and P. D. Harvey, Inorg. Chem., 2010,
49, 2614; (b) K. Onitsuka, H. Kitajima, M. Fujimoto, A. Iuchi,
F. Takei and S. Takahashi, Chem. Commun., 2002, 2576.
5 (a) D. Bellows, S. M. Aly, T. Goudreault, D. Fortin, C. P. Gros,
J. M. Barbe and P. D. Harvey, Organometallics, 2010, 29, 317;
(b) D. Bellows, S. M. Aly, C. P. Gros, M. El Ojaimi, J.-M. Barbe,
R. Guilard and P. D. Harvey, Inorg. Chem., 2009, 48, 7613;
(c) L. Liu, D. Fortin and P. D. Harvey, Inorg. Chem., 2009,
48, 5891; (d) A. Harriman, M. Hissler, O. Trompette and
R. Ziessel, J. Am. Chem. Soc., 1999, 121, 2516.
9
4.7 ꢃ 102
3.6 ꢃ 104
—
tw
10
11
1.3 ꢃ 103
9
Zn(P2)
[Pt]
Zn(P2)
—
12.6 ꢂ 0.8 ms
11.8 ꢂ 0.8 ms
5.1 ꢃ 104
—
5c
6 (a) L. G. Mackay, H. L. Anderson and J. K. M. Sanders, J. Chem.
Soc., Perkin Trans. 1, 1995, 2269; (b) L. G. Mackay, H. L. Anderson
and J. K. M. Sanders, J. Chem. Soc., Chem. Commun., 1992, 43.
7 (a) I. Fratoddi, C. Battocchio, R. D’Amato, G. P. Di Egidio,
L. Ugo, G. Polzonetti and M. V. Russo, Mater. Sci. Eng., C, 2003,
23, 867; (b) M. E. Amato, A. Licciardello, V. Torrisi, L. Ugo,
I. Venditti and M. V. Russo, Mater. Sci. Eng., C, 2009, 29, 1010.
8 (a) A. Baev, O. Rubio-Pons, F. Gel’mukhanov and H. Agren,
J. Phys. Chem. A, 2004, 108, 7406; (b) D. Beljonne, G. E. O’Keefe,
P. J. Hamer, R. H. Friend, H. L. Anderson and J. L. Bredas,
J. Chem. Phys., 1997, 106, 9439.
9 B. Du, D. Fortin and P. D. Harvey, Inorg. Chem., 2011, 50, 11493.
10 X. F. Duan, J. L. Wang and J. Pei, Org. Lett., 2005, 7, 4071.
11 (a) L. A. Emmert, W. Choi, J. A. Marshall, J. Yang, L. A. Meyer
and J. A. Brozik, J. Phys. Chem. A, 2003, 107, 11340;
(b) K. Gagnon, S. M. Aly, A. Brisach-Wittmeyer, D. Bellows,
J.-F. Berube, L. Caron, A. S. Abd-El-Aziz, D. Fortin and
P. D. Harvey, Organometallics, 2008, 27, 2201.
12 (a) K. M. Kadish, N. Guo, E. Van Caemelkbecke, A. Froiio,
R. Paolesse, D. Monti, P. Tagilatesta, T. Boschi, L. Prodi, F. Bolleta
and N. Zaccheroni, Inorg. Chem., 1998, 37, 2358; (b) J. M. Camus,
S. M. Aly, C. Stern, R. Guilard and P. D. Harvey, Chem. Commun.,
2011, 47, 8817.
13 (a) M. S. Yuan, Q. Fang, Z. Q. Liu, J. P. Guo, H.-Y. Chen,
W. T. Yu, G. Xue and D.-S. Liu, J. Org. Chem., 2006, 71, 7858;
(b) M. S. Yuan, Z.-Q. Liu and Q. Fang, J. Org. Chem., 2007,
72, 7915; (c) J. L. Wang, Z.-M. Tang, Q. Xiao, Q. F. Zhou, Y. Ma
and J. Pei, Org. Lett., 2008, 10, 17.
14 (a) S. Faure, C. Stern, R. Guilard and P. D. Harvey, J. Am. Chem.
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15 D. L. Dexter, J. Chem. Phys., 1953, 21, 836.
Chart 2 Structures of 10–12.
electron exchange stressing the need for a good donor–acceptor
orbital overlap and is bound to be sensitive to the dihedral angle
between the truxene and porphyrin planes. The n-hexyl groups
on the Tru unit render this aryl bulky and n-hexyl/n-hexyl
steric interactions increase the dihedral angle towards a poorer
orbital overlap as previously demonstrated using computer
modeling for 10.9 Such a steric situation does not exist with
[Pt] and the smaller dihedral angle is driven by the b- and
ortho-hydrogen contacts of the phorphyrin and phenyl groups.
Therefore, [Pt] is most likely to exhibit a better donor–acceptor
orbital overlap, and a faster kET(T1) [Pt] - Zn(P2) value.
The kET(S1) Zn(P1) - Zn(P2) data in 5 and 8 are similar to
each other (2.6 o kET o 4.7 (ns)ꢁ1), but are slower than that
found in 12 (25 (ns)ꢁ1).12 This is consistent with the shorter
donor–acceptor distance in 12. However, no change in kET(S1)
16 T. Forster, Ann. Phys., 1948, 437, 55.
¨
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 2671–2673 2673