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Energy transfer and conformational dynamics in Zn–porphyrin dendrimers
Chemical Physics Letters 403 (2005) 205–210 www.elsevier.com/locate/cplett
Energy transfer and conformational dynamics in Zn–porphyrin dendrimers Jane Larsen
a,*
a
, Johan Andersson a, Toma´sˇ Polı´vka a, Joseph Sly b, Maxwell J. Crossley b, ˚ kesson a Villy Sundstro¨m a, Eva A
Department of Chemical Physics, Lund University, Chemical Center, Box 124, SE-221 00 Lund, Sweden b School of Chemistry, The University of Sydney, NSW 2006, Australia Received 15 October 2004; in final form 14 December 2004 Available online 19 January 2005
Abstract The energy transfer within a series of Zn–porphyrin appended dendrimers was studied by means of time-resolved fluorescence anisotropy. We show that the energy transfer process between the Zn–porphyrin units in the dendrimers is limited to a maximum of four porphyrin units. At 200 K, the energy transfer process takes place on a 100-ps time scale, and can be modeled by Fo¨rster theory. Our results at room temperature further show that the porphyrin units are very mobile within the dendrimer, exhibiting rotational dynamics similar to that of a monomeric building block. 2005 Elsevier B.V. All rights reserved.
1. Introduction The study of dendrimers has attracted much attention in recent years due the wide range of possible applications in e.g. guest–host chemistry [1], optical data storage [2,3], catalytic chemistry [4], environmental chemistry [5,6], and biology [7–12]. Porphyrin appended dendrimers have also been suggested as potential mimics of the natural photosynthetic light-harvesting (LH) antenna systems due to their structural consistency, and ability to transfer absorbed energy between subunits within the molecule. The dendrimers investigated in this Letter are designed for LH. The processes occurring in the LH antennas of photosynthetic systems can be illustrated by those in the photosynthetic unit of purple bacteria, which consists of two ring-shaped LH pigment protein complexes – a peripheral LH2 antenna and a core LH1 antenna *
Corresponding author. E-mail address: [email protected] (J. Larsen).
0009-2614/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2004.12.109
surrounding the reaction center [13]. Solar energy absorbed by the bacteriochlorophyll (BChl) molecules in the LH2 antenna migrates by sub-picosecond excitation energy transfer (ET) steps within the antenna complex before it is transferred to the LH1 ring (5 ps) and finally to the reaction center (35 ps) [14]. The efficient ET in the LH2 antenna system and the large absorption cross-section of the BChls are characteristics also desirable for an artificial photosynthetic system. Previous findings have illustrated efficient ET within peryleneimide dendrimers [15–20], transition metal complexes [14,21–24], a novel bichromophoric system [2], Zn–porphyrin centered dendrimers [25–27], and free-base-porphyrin appended dendrimers [28]. The Zn–porphyrin dendrimers studied in this work are chosen as a mimic of the LH2 antenna system in purple bacteria due to their chemical stability and large absorption cross-sections. From studies of the free-base analog to G3P16 at 77 K, Yeow et al. [28] concluded that the ET is restricted to a dendron
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Fig. 1. The dendrimers, G1P4, G3P16, and G5P64 are constructed from a carbon–nitrogen skeleton with the monomeric porphyrin unit, G0P1 attached at the ends. Each grey circle in G3P16 and G5P64 symbolizes a monomeric unit attached as shown for G1P4.
consisting of four porphyrin units (see Fig. 1). In this Letter we study the ET process using fluorescence anisotropy measurements at both room temperature and at 200 K, and compare the ET efficiency of the Zn–porphyrin appended dendrimers to that of their free-base analogs.
2. Experimental The synthesis and purification of the Zn–porphyrin dendrimers is described elsewhere [29]. Solutions in THF were prepared with concentration such that the optical density at 430 nm was 0.1 mm1 in the steadystate absorption measurements, less than 0.01 mm1 in the steady state fluorescence measurements, and 0.1 mm1 in the time-resolved fluorescence measurements (the specific concentrations are listed in Table 2). A 1-mm glass cuvette was used for time-resolved measurements. Fresh samples were prepared prior to each measurement to avoid possible degradation, even though absorption spectra measured before and after experiments showed no signs of degradation. In the time-resolved fluorescence measurements, the samples were excited in the Soret band at 430 nm with the frequency-doubled output of a Ti:Sapphire oscillator (Spectra Physics, FWHM = 100 fs). The pulse repetition rate of 82 MHz was reduced to 4 MHz with a pulse picker. The photon density of the excitation light in the sample was 1012 cm2/pulse, resulting in excitation of approximately 1 out of 1000 porphyrin chromophores per pulse. The fluorescence spectra were measured within a time window of 2 ns and a spectral window of 100 nm using a streak camera (Hamamatsu Photonics). A polarizer in front of the streak camera insured detection of vertically polarized light only. The samples were excited with horizontally or vertically polarized light, thus enabling detection of the perpendicular or parallel components of the fluorescence, respectively.
Measurements were performed at both room temperature and 200 K. The low temperature measurements were performed with the sample inside a temperaturecontrolled cryostat (Oxford Instruments). At 200 K (±1 K), about 35 K above the freezing point of THF, the sample was still fluid and transparent; further cooling of the sample was not possible since THF does not form a transparent glass upon freezing. By using the same solvent at both temperatures, specific solvent effects were avoided and a direct comparison between the 200 K and room temperature measurements was possible. The time resolution in all measurements was 20 ps (FWHM). The data presented in this Letter were collected at the maximum of the 602 nm fluorescence band with a spectral resolution of 5 nm.
3. Structural and spectral characteristics of the Zn–porphyrin dendrimers The dendrimers are characterized by a single-bonded nitrogen/carbon skeleton with the monomer G0P1 (0th generation, 1 porphyrin) attached at the ends. In this study the monomer, 1st, 3rd and 5th generation dendrimer with 1, 4, 16, and 64 porphyrin units, respectively, were used (see Fig. 1). The single-bonded skeleton is flexible, resulting in approximately spherically shaped dendrimers [28]. Gas-phase molecular-dynamics simulations show that the size of the porphyrin units prevents back-folding of the individual arms [29]. Fig. 2 shows the steady-state absorption and fluorescence spectra of the compounds. The absorption spectra (Fig. 2a) are characterized by the strongly-absorbing Soret band at 429 nm [S2 S0], and two weaker Qbands at 558 nm [S1(1) S0(0)] and 598 nm [S1(0) S0(0)] [30]. The only difference between the absorption spectra of the different dendrimer generations is the increasing value of the extinction coefficient, which scales with the number of porphyrin units in the molecule (see Table 2). The emission spectra (Fig. 2b)
J. Larsen et al. / Chemical Physics Letters 403 (2005) 205–210 .
(a)
207
(b) .
. .
.
.
.
Fig. 2. (a) The steady-state absorption spectra of the monomer and the three dendrimers in a THF solution. (b) The steady-state fluorescence spectra of the compounds measured after excitation in the Soret band at 430 nm. The spectra are normalized at 602 nm. The insert shows an energy level scheme with corresponding transitions.
are characterized by two strong bands at 602 nm [S1(0) ! S0(0)] and 655 nm [S1(0) ! S0(1)]. The relative intensity of the low-energy emission band increases slightly with the number of porphyrin units, but the central wavelength of the fluorescence peaks is independent of the number of porphyrin units.
.
.
. .
4. Results and discussion .
4.1. Fluorescence anisotropy at room temperature The fluorescence anisotropy is defined as I k ðtÞ I ? ðtÞ rðtÞ ¼ ; I k ðtÞ þ 2I ? ðtÞ
ð1Þ
where Ii and I^ are the intensities of the fluorescence light polarized parallel and perpendicular to the excitation light, respectively. Directly after excitation, the anisotropy will be 0.4 if the transition dipole moments of the excited and the fluorescing states have the same orientation. The value of the anisotropy decreases if the porphyrin units change orientation or if ET between them takes place. The latter process depends on the orientation and the strength of the transition dipole moments, the distance between donor and acceptor unit, and the number of nearby porphyrin units. This implies that the ET rate will be faster in the larger dendrimers if the distance between porphyrin units is smaller or if the number of nearby porphyrin units is increased. The two contributions to the fluorescence–anisotropy decay could result in a double exponential decay of the anisotropy provided that the two processes occur on sufficiently different time scales. All fluorescence anisotropy decay curves at room temperature (see Fig. 3) have similar initial anisotropy of 0.1, which is well below the limiting value of 0.4. This is attributed to different transition dipole moment orientations for excitation (Soret band) and fluorescence
. .
.
. .
.
.
Fig. 3. Fluorescence–anisotropy decay experiments and fits at room temperature and 200 K. The blue lines in all the graphs represents single-exponential decay functions, r(t) = r0 + A1exp(t/s1) and the green lines double-exponential decay functions, r(t) = r0 + A1exp(t/ s1) + A2exp(t/s2).
(Q-band) [31]. Similar values have been reported for related compounds [28,31,32]. The fluorescence anisotropy decay of the monomer can only originate from rotation, since inter-molecular ET can be excluded at the concentration used here. This results in an anisotropy decay
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well reproduced by a single-exponential function with a time constant of 330 ± 20 ps (standard deviation), which agrees well with previous results for related compounds [32]. The fluorescence anisotropy decays of the dendrimers are very similar to that of the monomer. Again, the decays are well described by a single-exponential function with the same time constant as in the monomer, which shows that there is a large degree of structural flexibility within the different dendrimers, allowing the individual porphyrin units to rotate almost as freely as the monomer [28]. This implies that the distance between the porphyrin units must be sufficiently large to allow reorientation. To estimate the distance to the nearestneighbor unit dnn, the hydrodynamic radius Rhydro of free-base porphyrin dendrimers dissolved in THF (see Table 2) has been used [29]. The upper limit for dnn in G1P4 can be estimated by assuming a two-dimensional disc structure, yielding dnn = 2.45 nm. For G3P16 and G5P64, dnn can be estimated by assuming a homogenous distribution of the porphyrin units on the surface of a sphere and that the area occupied by each porphyrin unit is approximated by a circle:
form distribution over the hole surface [28]. The monomer, without the connecting chain, has a diameter of only 1.81 nm [29], which means that there is considerable space between the porphyrin units to allow for free rotation. The anisotropy decay at room temperature provides clear information about the flexibility of the dendrimers, but does not offer any information about the ET between the porphyrin units in the dendrimers. However, it does not rule out the possibility of ET. If the ET and reorientation take place on similar time scales, it would be difficult to distinguish the two processes. In order to resolve the ET, the samples were cooled to 200 K. At this temperature the solvent is still liquid, but the rotational motion is significantly slowed down due to the increase in viscosity. The ET is also expected to be affected by the cooling, since line narrowing will change the spectral overlap between the absorption spectrum and the fluorescence spectrum. However, this effect is smaller than the effect on the rotational motion due to cooling.
4Rhydro d nn pffiffiffiffi ; N
4.2. Fluorescence anisotropy at 200 K ð2Þ The fluorescence anisotropy data measured at 200 K are displayed in Fig. 3. An initial anisotropy of 0.1 is observed for all the compounds in agreement with the room temperature measurements. For both the monomeric building block and the dendrimers, an anisotropy decay component of 1.7 ns is found. This
where N is the number of porphyrin units. This yields an average dnn = 2.71 nm and dnn = 1.85 nm for G3P16 and G5P64, respectively. These values of dnn should be considered as estimates, in particular for G3P16 where it is not likely that the porphyrin units are forced into a uni-
Table 1 The values of the single- and the double-exponential fit functions, r(t) = r0 + A1exp(t/s1) and r(t) = r0 + A1exp(t/s1) + A2exp(t/s2) used to reproduce the fluorescence anisotropy decay at room temperature and 200 K Room temperaturea
G0P1 G1P4 G3P16 G5P64 a b
200 Ka b
r0
A1(s1 = 330 ± 20 ps)
r0
A1(s1 = 1.7 ± 0.1 ns)b
A2(s2 = 100 ± 25 ps)b
0.013 0.013 0.016 0.010
0.086 0.077 0.079 0.082
0.013 0.013 0.016 0.010
0.091 0.069 0.067 0.063
– 0.016 0.010 0.011
The uncertainties of r0, A1, and A2 are 10%, 2%, and 20%, respectively. The decay constants are fitted globally to all the anisotropy decay curves at the given temperature.
Table 2 The hydrodynamic radius, Rhydro, and the distance to the nearest-neighbor porphyrin unit, dnn, are estimated as described in Section 4.1 e (M1 cm1)a
G0P1 G1P4 G3P16 G5P64 a b c
5.77
10 106.34 106.89 107.49
c (M)b
Distance estimates 6
1.7 · 10 4.6 · 107 1.3 · 107 3.2 · 108
Rhydro (nm)c
dnn (nm)
0.95 ± 0.01 1.73 ± 0.01 2.71 ± 0.09 3.69 ± 0.16
– 2.45 ± 0.02 2.71 ± 0.09 1.85 ± 0.08
Measured in a CHCl3 solution [29]. The concentration used in the time-resolved fluorescence measurements. The hydrodynamic radius of the free-base porphyrin dendrimers has been determined in NMR experiments [29].
J. Larsen et al. / Chemical Physics Letters 403 (2005) 205–210
component is, similarly as for room temperature, assigned to the rotational motion of the porphyrin units, which has been slowed down by the cooling. However, to obtain satisfactory fits of the anisotropy decay of the dendrimers, an additional decay component with a time constant of 100 ± 25 ps and amplitudes varying from 0.010 to 0.016 (see Table 1) is needed. Since this component is missing for the monomeric unit, it is assigned to the ET between the different porphyrin units in the dendrimer. An interesting observation is that the ET time constant is independent of the number of porphyrin units in the dendrimer. If the number of porphyrin units involved in the ET had increased with increasing dendrimer size, the observed ET time constant would have decreased accordingly because more ET pathways become available. Since we observe the same ET time constant in all the dendrimers, the ET must be limited to at most four porphyrin units as this is the number of porphyrins in the smallest dendrimer, G1P4. Furthermore, the distances between the porphyrin units involved in the ET have to remain approximately the same since small changes in distance will have a large effect on the ET rate. It can therefore be concluded that the porphyrin units are arranged in groups of maximum four units, most likely in a dendron as illustrated in Fig. 1, and that the distance between the porphyrin units in a dendron is largely independent of the dendrimer size. The dendrons in G3P16 and G5P64 occupy 41% and 88% of the total surface area, respectively. To estimate the distances between the porphyrin units in a dendron, Fo¨rster ET theory has been applied. 4.3. Fo¨rster energy transfer Increasing the size of the dendrimers does not produce any significant changes in the absorption spectra, which implies that there are only weak interactions between the porphyrin units. This suggests that the ET between the porphyrin units can be described by Fo¨rster theory [33]. The Fo¨rster ET rate, kET (ps1), is related to the spectral overlap, H (cm3), between the acceptor absorption spectrum and the donor fluorescence spectrum according to [14,33,34] k ET ¼ 1:18V 2 H;
ð3Þ
where V is the dipole–dipole interaction between donor and acceptor. The spectral overlap integral calculated from the absorption and fluorescence spectra at room temperature yields H = 3.33 · 105 cm3. The dipole–dipole interaction, calculated from the transition dipole moment [34], and by assuming a random pffiffiffiffiffiffiffiffi orientation yielding an orientation factor of v ¼ 2=3, can be expressed as V ¼ 128:7R3 ;
ð4Þ
209
Fig. 4. The distances between the porphyrin units in G1P4 and a dendron in G3P16 or G5P64 are given by R1, R2, and R3. The gray circles mark the positions of the porphyrins units.
where R is the distance between two porphyrin units measured in nanometers. The dipole moment is calculated from the absorption spectra, which gives l = 5.59 D. Rearranging Eqs. (3) and (4) and assuming that the overlap integral is 20% smaller at 200 K than at room temperature (linear extrapolation of the overlap integral at room temperature and 200 K for the freebase analog gives a 20% decrease), R can be expressed as a function of the ET rate: pffiffiffiffiffiffiffiffiffiffiffiffi ð5Þ R ¼ 0:89 6 1=k ET : The four porphyrin units in G1P4 and in a dendron can be approximated by a non-uniform distribution on a disc (see Fig. 4) as proposed in [28] for G1P4. For the Obs 1 observed ET time constant of sObs ¼ ET ¼ ðk ET Þ G1P4 100 25 ps and R3 ¼ Rhydro ¼ 3:46 0:02 nm [29] this gives R1 = 1.98 ± 0.10 nm and R2 = 2.84 ± 0.07 nm. These distances are somewhat larger than those measured in the free-base porphyrin dendrimers [28] since the hydrodynamic radius is slightly larger than the actual distance R3. The individual ET times are sET = 116 ± 40, 1010 ± 150, and 3300 ± 120 ps for R1, R2, and R3, respectively. The main contribution to the observed ET time originates therefore from ET between the two nearest-neighboring porphyrin units. ET between dendrons in G3P16 and G5P64 over distances larger than R2 will therefore only influence the observed ET rate to a very small extent, and the possibility of dendron–dendron ET on a much longer time scale can therefore not be excluded. The ET observed in the free-base analogs at 77 K takes place on a time scale approximately 10 times slower than observed here for the Zn–porphyrin dendrimers [28]. This seemingly large difference can be explained by the use of different solvents, difference in temperature, and in particular by the difference in the spectral overlap integral for the free-base porphyrins compared to the Zn–porphyrins. Despite the difference in ET time for the two porphyrin dendrimers, the same conclusion can be drawn: ET is essentially limited within a maximum of four porphyrin units regardless of whether it is free-base-porphyrin
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dendrimers [28] or Zn–porphyrin dendrimers as reported here.
Acknowledgements Financial support from the Swedish Research Council, the Swedish Energy Agency, the Knut and Alice Wallenberg Foundation, and the Magnus Bergwall Foundation is gratefully acknowledged. We further thank the Australian Research Council for a Discovery Research Grant (DP0208776) to M.J.C. Proofreading of the manuscript by Dr. Han-Kwang Nienhyus is highly appreciated. References [1] J.F.G.A. Jansen, E.M.M. de Brabander-van den Berg, E.W. Meijer, Science 266 (1994) 1226. [2] D.W. Brousmiche, J.M. Serin, J.M.J. Fre´chet, G.S. He, T.C. Lin, S.J. Chung, P.N. Prasad, J. Am. Chem. Soc. 125 (2003) 1448. [3] T.H. Ghaddar, J.F. Wishart, D.W. Thompson, J.K. Whitesell, M.A. Fox, J. Am. Chem. Soc. 124 (2002) 8285. [4] K.W. Pollak, J.W. Leon, J.M.J. Fre´chet, M. Maskus, H.D. Abrun˜a, Chem. Mater. 10 (1998) 30. [5] A. Bar-Haim, J. Klafter, J. Phys. Chem. B 102 (1998) 1662. [6] A. Bar-Haim, J. Klafter, R. Kopelman, J. Am. Chem. Soc. 119 (1997) 6197. [7] T.A. Betley, M.M.B. Holl, B.G. Orr, D.R. Swanson, D.A. Tomalia, J.R. Baker, Langmuir 17 (2001) 2768. [8] A. Bielinska, J.F. Kukowska-Latallo, J. Johnson, D.A. Tomalia, J.R. Baker, Nucleic Acids Res. 24 (1996) 2176. [9] M.F. Ottaviani, B. Sacchi, N.J. Turro, W. Chen, S. Jockusch, D.A. Tomalia, Macromolecules 32 (1999) 2275. [10] J.C. Roberts, M.K. Bhalgat, R.T. Zera, J. Biomed. Mater. Res. 30 (1996) 53. [11] P. Singh, Bioconjugate Chem. 9 (1998) 54. [12] D.A. Tomalia, Sci. Am. 272 (1995) 62. [13] A.W. Roszak, T.D. Howard, J. Southall, A.T. Gardiner, C.J. Law, N.W. Isaacs, R.J. Cogdell, Science 302 (2003) 1969. [14] V. Sundstro¨m, T. Pullerits, R. van Grondelle, J. Phys. Chem. B 103 (1999) 2327.
[15] G. De Belder, S. Jordens, M. Lor, G. Schweitzer, R. De, T. Weil, A. Herrmann, U.K. Wiesler, K. Mu¨llen, F.C. De Schryver, J. Photochem. Photobiol. A 145 (2001) 61. [16] J. Hofkens, L. Latterini, G. De Belder, T. Gensch, M. Maus, T. Vosch, Y. Karni, G. Schweitzer, F.C. De Schryver, A. Hermann, K. Mu¨llen, Chem. Phys. Lett. 304 (1999) 1. [17] Y. Karni, S. Jordens, G. De Belder, J. Hofkens, G. Schweitzer, F.C. De Schryver, A. Herrmann, K. Mu¨llen, J. Phys. Chem. B 103 (1999) 9378. [18] Y. Karni, S. Jordens, G. De Belder, G. Schweitzer, J. Hofkens, T. Gensch, M. Maus, F.C. De Schryver, A. Hermann, K. Mu¨llen, Chem. Phys. Lett. 310 (1999) 73. [19] M. Lor, R. De, S. Jordens, G. De Belder, G. Schweitzer, M. Cotlet, J. Hofkens, T. Weil, A. Herrmann, K. Mu¨llen, M. Van der Auweraer, F.C. De Schryver, J. Phys. Chem. A 106 (2002) 2083. [20] G. Schweitzer, R. Gronheid, S. Jordens, M. Lor, G. De Belder, T. Weil, E. Reuther, K. Mu¨llen, F.C. De Schryver, J. Phys. Chem. A 107 (2003) 3199. [21] V. Balzani, S. Campagna, G. Denti, A. Juris, S. Serroni, M. Venturi, Acc. Chem. Res. 31 (1998) 26. [22] N.H. Damrauer, G. Cerullo, A. Yeh, T.R. Boussie, C.V. Shank, J.K. McCusker, Science 275 (1997) 54. [23] J. Andersson, F. Puntoriero, S. Serroni, A. Yartsev, T. Pascher, T. Polivka, S. Campagna, V. Sundstro¨m, Faraday Discuss. 127 (2004) 295. [24] J. Andersson, F. Puntoriero, S. Serroni, A. Yartsev, T. Pascher, T. Polivka, S. Campagna, V. Sundstro¨m, Chem. Phys. Lett. 386 (2004) 336. [25] M.S. Choi, T. Aida, T. Yamazaki, I. Yamazaki, Chem. Eur. J. 8 (2002) 2668. [26] D.L. Jiang, T. Aida, J. Am. Chem. Soc. 120 (1998) 10895. [27] R. Sadamoto, N. Tomioka, T. Aida, J. Am. Chem. Soc. 118 (1996) 3978. [28] E.K.L. Yeow, K.P. Ghiggino, J.N.H. Reek, M.J. Crossley, A.W. Bosman, A.P.H.J. Schenning, E.W. Meijer, J. Phys. Chem. B 104 (2000) 2596. [29] J. Sly, Ph.D. Thesis, The University of Sydney, 2004. [30] D. Dolphin, The Porphyrins, Physical Chemistry, Part A, Academic Press, New York, 1978. [31] Y.H. Kim, D.H. Jeong, D. Kim, S.C. Jeoung, H.S. Cho, S.K. Kim, N. Aratani, A. Osuka, J. Am. Chem. Soc. 123 (2001) 76. [32] Y.H. Kim, H.S. Cho, D. Kim, S.K. Kim, N. Yoshida, A. Osuka, Synth. Mater. 117 (2001) 183. [33] T. Fo¨rster, Z. Naturforsch. B 2 (1947) 174. [34] G.D. Scholes, Ann. Rev. Phys. Chem. 54 (2003) 57.
Energy transfer and conformational dynamics in Zn–porphyrin dendrimers Jane Larsen
a,*
a
, Johan Andersson a, Toma´sˇ Polı´vka a, Joseph Sly b, Maxwell J. Crossley b, ˚ kesson a Villy Sundstro¨m a, Eva A
Department of Chemical Physics, Lund University, Chemical Center, Box 124, SE-221 00 Lund, Sweden b School of Chemistry, The University of Sydney, NSW 2006, Australia Received 15 October 2004; in final form 14 December 2004 Available online 19 January 2005
Abstract The energy transfer within a series of Zn–porphyrin appended dendrimers was studied by means of time-resolved fluorescence anisotropy. We show that the energy transfer process between the Zn–porphyrin units in the dendrimers is limited to a maximum of four porphyrin units. At 200 K, the energy transfer process takes place on a 100-ps time scale, and can be modeled by Fo¨rster theory. Our results at room temperature further show that the porphyrin units are very mobile within the dendrimer, exhibiting rotational dynamics similar to that of a monomeric building block. 2005 Elsevier B.V. All rights reserved.
1. Introduction The study of dendrimers has attracted much attention in recent years due the wide range of possible applications in e.g. guest–host chemistry [1], optical data storage [2,3], catalytic chemistry [4], environmental chemistry [5,6], and biology [7–12]. Porphyrin appended dendrimers have also been suggested as potential mimics of the natural photosynthetic light-harvesting (LH) antenna systems due to their structural consistency, and ability to transfer absorbed energy between subunits within the molecule. The dendrimers investigated in this Letter are designed for LH. The processes occurring in the LH antennas of photosynthetic systems can be illustrated by those in the photosynthetic unit of purple bacteria, which consists of two ring-shaped LH pigment protein complexes – a peripheral LH2 antenna and a core LH1 antenna *
Corresponding author. E-mail address: [email protected] (J. Larsen).
0009-2614/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2004.12.109
surrounding the reaction center [13]. Solar energy absorbed by the bacteriochlorophyll (BChl) molecules in the LH2 antenna migrates by sub-picosecond excitation energy transfer (ET) steps within the antenna complex before it is transferred to the LH1 ring (5 ps) and finally to the reaction center (35 ps) [14]. The efficient ET in the LH2 antenna system and the large absorption cross-section of the BChls are characteristics also desirable for an artificial photosynthetic system. Previous findings have illustrated efficient ET within peryleneimide dendrimers [15–20], transition metal complexes [14,21–24], a novel bichromophoric system [2], Zn–porphyrin centered dendrimers [25–27], and free-base-porphyrin appended dendrimers [28]. The Zn–porphyrin dendrimers studied in this work are chosen as a mimic of the LH2 antenna system in purple bacteria due to their chemical stability and large absorption cross-sections. From studies of the free-base analog to G3P16 at 77 K, Yeow et al. [28] concluded that the ET is restricted to a dendron
206
J. Larsen et al. / Chemical Physics Letters 403 (2005) 205–210
Fig. 1. The dendrimers, G1P4, G3P16, and G5P64 are constructed from a carbon–nitrogen skeleton with the monomeric porphyrin unit, G0P1 attached at the ends. Each grey circle in G3P16 and G5P64 symbolizes a monomeric unit attached as shown for G1P4.
consisting of four porphyrin units (see Fig. 1). In this Letter we study the ET process using fluorescence anisotropy measurements at both room temperature and at 200 K, and compare the ET efficiency of the Zn–porphyrin appended dendrimers to that of their free-base analogs.
2. Experimental The synthesis and purification of the Zn–porphyrin dendrimers is described elsewhere [29]. Solutions in THF were prepared with concentration such that the optical density at 430 nm was 0.1 mm1 in the steadystate absorption measurements, less than 0.01 mm1 in the steady state fluorescence measurements, and 0.1 mm1 in the time-resolved fluorescence measurements (the specific concentrations are listed in Table 2). A 1-mm glass cuvette was used for time-resolved measurements. Fresh samples were prepared prior to each measurement to avoid possible degradation, even though absorption spectra measured before and after experiments showed no signs of degradation. In the time-resolved fluorescence measurements, the samples were excited in the Soret band at 430 nm with the frequency-doubled output of a Ti:Sapphire oscillator (Spectra Physics, FWHM = 100 fs). The pulse repetition rate of 82 MHz was reduced to 4 MHz with a pulse picker. The photon density of the excitation light in the sample was 1012 cm2/pulse, resulting in excitation of approximately 1 out of 1000 porphyrin chromophores per pulse. The fluorescence spectra were measured within a time window of 2 ns and a spectral window of 100 nm using a streak camera (Hamamatsu Photonics). A polarizer in front of the streak camera insured detection of vertically polarized light only. The samples were excited with horizontally or vertically polarized light, thus enabling detection of the perpendicular or parallel components of the fluorescence, respectively.
Measurements were performed at both room temperature and 200 K. The low temperature measurements were performed with the sample inside a temperaturecontrolled cryostat (Oxford Instruments). At 200 K (±1 K), about 35 K above the freezing point of THF, the sample was still fluid and transparent; further cooling of the sample was not possible since THF does not form a transparent glass upon freezing. By using the same solvent at both temperatures, specific solvent effects were avoided and a direct comparison between the 200 K and room temperature measurements was possible. The time resolution in all measurements was 20 ps (FWHM). The data presented in this Letter were collected at the maximum of the 602 nm fluorescence band with a spectral resolution of 5 nm.
3. Structural and spectral characteristics of the Zn–porphyrin dendrimers The dendrimers are characterized by a single-bonded nitrogen/carbon skeleton with the monomer G0P1 (0th generation, 1 porphyrin) attached at the ends. In this study the monomer, 1st, 3rd and 5th generation dendrimer with 1, 4, 16, and 64 porphyrin units, respectively, were used (see Fig. 1). The single-bonded skeleton is flexible, resulting in approximately spherically shaped dendrimers [28]. Gas-phase molecular-dynamics simulations show that the size of the porphyrin units prevents back-folding of the individual arms [29]. Fig. 2 shows the steady-state absorption and fluorescence spectra of the compounds. The absorption spectra (Fig. 2a) are characterized by the strongly-absorbing Soret band at 429 nm [S2 S0], and two weaker Qbands at 558 nm [S1(1) S0(0)] and 598 nm [S1(0) S0(0)] [30]. The only difference between the absorption spectra of the different dendrimer generations is the increasing value of the extinction coefficient, which scales with the number of porphyrin units in the molecule (see Table 2). The emission spectra (Fig. 2b)
J. Larsen et al. / Chemical Physics Letters 403 (2005) 205–210 .
(a)
207
(b) .
. .
.
.
.
Fig. 2. (a) The steady-state absorption spectra of the monomer and the three dendrimers in a THF solution. (b) The steady-state fluorescence spectra of the compounds measured after excitation in the Soret band at 430 nm. The spectra are normalized at 602 nm. The insert shows an energy level scheme with corresponding transitions.
are characterized by two strong bands at 602 nm [S1(0) ! S0(0)] and 655 nm [S1(0) ! S0(1)]. The relative intensity of the low-energy emission band increases slightly with the number of porphyrin units, but the central wavelength of the fluorescence peaks is independent of the number of porphyrin units.
.
.
. .
4. Results and discussion .
4.1. Fluorescence anisotropy at room temperature The fluorescence anisotropy is defined as I k ðtÞ I ? ðtÞ rðtÞ ¼ ; I k ðtÞ þ 2I ? ðtÞ
ð1Þ
where Ii and I^ are the intensities of the fluorescence light polarized parallel and perpendicular to the excitation light, respectively. Directly after excitation, the anisotropy will be 0.4 if the transition dipole moments of the excited and the fluorescing states have the same orientation. The value of the anisotropy decreases if the porphyrin units change orientation or if ET between them takes place. The latter process depends on the orientation and the strength of the transition dipole moments, the distance between donor and acceptor unit, and the number of nearby porphyrin units. This implies that the ET rate will be faster in the larger dendrimers if the distance between porphyrin units is smaller or if the number of nearby porphyrin units is increased. The two contributions to the fluorescence–anisotropy decay could result in a double exponential decay of the anisotropy provided that the two processes occur on sufficiently different time scales. All fluorescence anisotropy decay curves at room temperature (see Fig. 3) have similar initial anisotropy of 0.1, which is well below the limiting value of 0.4. This is attributed to different transition dipole moment orientations for excitation (Soret band) and fluorescence
. .
.
. .
.
.
Fig. 3. Fluorescence–anisotropy decay experiments and fits at room temperature and 200 K. The blue lines in all the graphs represents single-exponential decay functions, r(t) = r0 + A1exp(t/s1) and the green lines double-exponential decay functions, r(t) = r0 + A1exp(t/ s1) + A2exp(t/s2).
(Q-band) [31]. Similar values have been reported for related compounds [28,31,32]. The fluorescence anisotropy decay of the monomer can only originate from rotation, since inter-molecular ET can be excluded at the concentration used here. This results in an anisotropy decay
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well reproduced by a single-exponential function with a time constant of 330 ± 20 ps (standard deviation), which agrees well with previous results for related compounds [32]. The fluorescence anisotropy decays of the dendrimers are very similar to that of the monomer. Again, the decays are well described by a single-exponential function with the same time constant as in the monomer, which shows that there is a large degree of structural flexibility within the different dendrimers, allowing the individual porphyrin units to rotate almost as freely as the monomer [28]. This implies that the distance between the porphyrin units must be sufficiently large to allow reorientation. To estimate the distance to the nearestneighbor unit dnn, the hydrodynamic radius Rhydro of free-base porphyrin dendrimers dissolved in THF (see Table 2) has been used [29]. The upper limit for dnn in G1P4 can be estimated by assuming a two-dimensional disc structure, yielding dnn = 2.45 nm. For G3P16 and G5P64, dnn can be estimated by assuming a homogenous distribution of the porphyrin units on the surface of a sphere and that the area occupied by each porphyrin unit is approximated by a circle:
form distribution over the hole surface [28]. The monomer, without the connecting chain, has a diameter of only 1.81 nm [29], which means that there is considerable space between the porphyrin units to allow for free rotation. The anisotropy decay at room temperature provides clear information about the flexibility of the dendrimers, but does not offer any information about the ET between the porphyrin units in the dendrimers. However, it does not rule out the possibility of ET. If the ET and reorientation take place on similar time scales, it would be difficult to distinguish the two processes. In order to resolve the ET, the samples were cooled to 200 K. At this temperature the solvent is still liquid, but the rotational motion is significantly slowed down due to the increase in viscosity. The ET is also expected to be affected by the cooling, since line narrowing will change the spectral overlap between the absorption spectrum and the fluorescence spectrum. However, this effect is smaller than the effect on the rotational motion due to cooling.
4Rhydro d nn pffiffiffiffi ; N
4.2. Fluorescence anisotropy at 200 K ð2Þ The fluorescence anisotropy data measured at 200 K are displayed in Fig. 3. An initial anisotropy of 0.1 is observed for all the compounds in agreement with the room temperature measurements. For both the monomeric building block and the dendrimers, an anisotropy decay component of 1.7 ns is found. This
where N is the number of porphyrin units. This yields an average dnn = 2.71 nm and dnn = 1.85 nm for G3P16 and G5P64, respectively. These values of dnn should be considered as estimates, in particular for G3P16 where it is not likely that the porphyrin units are forced into a uni-
Table 1 The values of the single- and the double-exponential fit functions, r(t) = r0 + A1exp(t/s1) and r(t) = r0 + A1exp(t/s1) + A2exp(t/s2) used to reproduce the fluorescence anisotropy decay at room temperature and 200 K Room temperaturea
G0P1 G1P4 G3P16 G5P64 a b
200 Ka b
r0
A1(s1 = 330 ± 20 ps)
r0
A1(s1 = 1.7 ± 0.1 ns)b
A2(s2 = 100 ± 25 ps)b
0.013 0.013 0.016 0.010
0.086 0.077 0.079 0.082
0.013 0.013 0.016 0.010
0.091 0.069 0.067 0.063
– 0.016 0.010 0.011
The uncertainties of r0, A1, and A2 are 10%, 2%, and 20%, respectively. The decay constants are fitted globally to all the anisotropy decay curves at the given temperature.
Table 2 The hydrodynamic radius, Rhydro, and the distance to the nearest-neighbor porphyrin unit, dnn, are estimated as described in Section 4.1 e (M1 cm1)a
G0P1 G1P4 G3P16 G5P64 a b c
5.77
10 106.34 106.89 107.49
c (M)b
Distance estimates 6
1.7 · 10 4.6 · 107 1.3 · 107 3.2 · 108
Rhydro (nm)c
dnn (nm)
0.95 ± 0.01 1.73 ± 0.01 2.71 ± 0.09 3.69 ± 0.16
– 2.45 ± 0.02 2.71 ± 0.09 1.85 ± 0.08
Measured in a CHCl3 solution [29]. The concentration used in the time-resolved fluorescence measurements. The hydrodynamic radius of the free-base porphyrin dendrimers has been determined in NMR experiments [29].
J. Larsen et al. / Chemical Physics Letters 403 (2005) 205–210
component is, similarly as for room temperature, assigned to the rotational motion of the porphyrin units, which has been slowed down by the cooling. However, to obtain satisfactory fits of the anisotropy decay of the dendrimers, an additional decay component with a time constant of 100 ± 25 ps and amplitudes varying from 0.010 to 0.016 (see Table 1) is needed. Since this component is missing for the monomeric unit, it is assigned to the ET between the different porphyrin units in the dendrimer. An interesting observation is that the ET time constant is independent of the number of porphyrin units in the dendrimer. If the number of porphyrin units involved in the ET had increased with increasing dendrimer size, the observed ET time constant would have decreased accordingly because more ET pathways become available. Since we observe the same ET time constant in all the dendrimers, the ET must be limited to at most four porphyrin units as this is the number of porphyrins in the smallest dendrimer, G1P4. Furthermore, the distances between the porphyrin units involved in the ET have to remain approximately the same since small changes in distance will have a large effect on the ET rate. It can therefore be concluded that the porphyrin units are arranged in groups of maximum four units, most likely in a dendron as illustrated in Fig. 1, and that the distance between the porphyrin units in a dendron is largely independent of the dendrimer size. The dendrons in G3P16 and G5P64 occupy 41% and 88% of the total surface area, respectively. To estimate the distances between the porphyrin units in a dendron, Fo¨rster ET theory has been applied. 4.3. Fo¨rster energy transfer Increasing the size of the dendrimers does not produce any significant changes in the absorption spectra, which implies that there are only weak interactions between the porphyrin units. This suggests that the ET between the porphyrin units can be described by Fo¨rster theory [33]. The Fo¨rster ET rate, kET (ps1), is related to the spectral overlap, H (cm3), between the acceptor absorption spectrum and the donor fluorescence spectrum according to [14,33,34] k ET ¼ 1:18V 2 H;
ð3Þ
where V is the dipole–dipole interaction between donor and acceptor. The spectral overlap integral calculated from the absorption and fluorescence spectra at room temperature yields H = 3.33 · 105 cm3. The dipole–dipole interaction, calculated from the transition dipole moment [34], and by assuming a random pffiffiffiffiffiffiffiffi orientation yielding an orientation factor of v ¼ 2=3, can be expressed as V ¼ 128:7R3 ;
ð4Þ
209
Fig. 4. The distances between the porphyrin units in G1P4 and a dendron in G3P16 or G5P64 are given by R1, R2, and R3. The gray circles mark the positions of the porphyrins units.
where R is the distance between two porphyrin units measured in nanometers. The dipole moment is calculated from the absorption spectra, which gives l = 5.59 D. Rearranging Eqs. (3) and (4) and assuming that the overlap integral is 20% smaller at 200 K than at room temperature (linear extrapolation of the overlap integral at room temperature and 200 K for the freebase analog gives a 20% decrease), R can be expressed as a function of the ET rate: pffiffiffiffiffiffiffiffiffiffiffiffi ð5Þ R ¼ 0:89 6 1=k ET : The four porphyrin units in G1P4 and in a dendron can be approximated by a non-uniform distribution on a disc (see Fig. 4) as proposed in [28] for G1P4. For the Obs 1 observed ET time constant of sObs ¼ ET ¼ ðk ET Þ G1P4 100 25 ps and R3 ¼ Rhydro ¼ 3:46 0:02 nm [29] this gives R1 = 1.98 ± 0.10 nm and R2 = 2.84 ± 0.07 nm. These distances are somewhat larger than those measured in the free-base porphyrin dendrimers [28] since the hydrodynamic radius is slightly larger than the actual distance R3. The individual ET times are sET = 116 ± 40, 1010 ± 150, and 3300 ± 120 ps for R1, R2, and R3, respectively. The main contribution to the observed ET time originates therefore from ET between the two nearest-neighboring porphyrin units. ET between dendrons in G3P16 and G5P64 over distances larger than R2 will therefore only influence the observed ET rate to a very small extent, and the possibility of dendron–dendron ET on a much longer time scale can therefore not be excluded. The ET observed in the free-base analogs at 77 K takes place on a time scale approximately 10 times slower than observed here for the Zn–porphyrin dendrimers [28]. This seemingly large difference can be explained by the use of different solvents, difference in temperature, and in particular by the difference in the spectral overlap integral for the free-base porphyrins compared to the Zn–porphyrins. Despite the difference in ET time for the two porphyrin dendrimers, the same conclusion can be drawn: ET is essentially limited within a maximum of four porphyrin units regardless of whether it is free-base-porphyrin
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dendrimers [28] or Zn–porphyrin dendrimers as reported here.
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