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J. Phys. Chem. C 2007, 111, 7192-7199
Photophysics of a Butadiyne-Linked Porphyrin Dimer: Influence of Conformational Flexibility in the Ground and First Singlet Excited State Mikael U. Winters,† Joakim Ka1 rnbratt,† Mattias Eng,† Craig J. Wilson,‡ Harry L. Anderson,*,‡ and Bo Albinsson*,† Department of Chemical and Biological Engineering, Physical Chemistry, KemiVa¨gen 3, SE - 412 96 Go¨teborg, Sweden, and Department of Chemistry, UniVersity of Oxford, Chemistry Research Laboratory, Mansfield Road, Oxford OX1 3TA, United Kingdom ReceiVed: December 5, 2006; In Final Form: March 14, 2007
The photophysics of a butadiyne-linked porphyrin dimer has been investigated by spectroscopy and quantum mechanical calculations. Primarily, the influence of conformation on the ground and first singlet excited states was studied, and two spectroscopically distinct limiting cases were identified. Experiments show that the twisted and planar conformers are separate spectroscopic species that can be selectively excited and have unique absorption and emission spectra. Calculated ground-state spectra compare well with experimental spectra of the two species. A spectrum of the planar conformer was obtained by the addition of a dipyridyl pyrrole ligand, which forms a 1:1 complex with the dimer and thus forces it to stay planar. The absorption spectrum of the twisted conformer could be deduced from the excitation spectrum of its emission. The interpretation of the ground-state spectrum of the free noncomplexed dimer is that it represents an average of a broad distribution of conformations. Calculations support this conclusion by indicating that the barrier for rotation is relatively small in the ground state (0.7 kcal/mol). Studies of the temperature dependence of the fluorescence spectrum of the dimer indicate a mother-daughter relationship between the twisted and planar conformations in the excited state, where the former has approximately 3.9 kcal/mol higher energy. Furthermore, time-correlated single-photon counting experiments also suggest that the twisted population adopts a planar configuration in the first singlet excited state with a rate constant of krot ) 8.8 × 109 s-1 in 2-MTHF at room temperature. The temperature dependence of the fluorescence lifetimes indicated that an activation energy barrier of approximately 2 kcal/mol, in part related to solvent viscosity, is associated with this rate constant.
Introduction Porphyrin oligomers show much promise in many areas of molecular engineering and have been suggested as building blocks for many diverse applications such as artificial photosynthesis,1 novel optical materials,2-6 and molecular-scale electronics.7 Metalloporphyrin oligomers such as zinc(II) complexes are especially versatile, as coordination of ligands such as pyridine and imidazole to the zinc atom facilitates the selfassembly of supramolecular structures, such as boxes and grids.8-12 The resulting structures can be utilized by addressing individual parts, for instance, by photoexcitation. Various porphyrin oligomers have also been accommodated into donoracceptor systems to control charge separation,13-17 particularly with artificial photosynthesis and solar cell applications in mind. Conjugated porphyrin oligomers are attractive from many perspectives, because of their unique electronic properties.1,7,18 For instance, some butadiyne-linked porphyrin oligomers exhibit large two-photon cross sections19,20 and hyperpolarizabilities,2,3 making them interesting candidates for photodynamic therapy and nonlinear refraction. Other potential uses are in the field of molecular electronics, where conjugated porphyrin oligomers are candidate molecular wires.7,21,22 The conformation of butadiyne-linked porphyrin oligomers is critical for electronic coupling between the porphyrin mac* To whom correspondence should be addressed. E-mail: balb@ chalmers.se (B.A.),
[email protected] (H.L.A.). † Chalmers University of Technology. ‡ University of Oxford.
rocycles and, thus, for interporphyrin conjugation. The coupling between porphyrin molecules is strongest when they are coplanar and gradually decreases to a minimum when the units are perpendicular. However, the triple bond potentially allows free rotation, and in principle, a continuous distribution of dihedral angles between adjacent porphyrins is possible for alkyne-linked porphyrin oligomers. It is thus interesting to consider the extent to which this distribution is biased toward the planar conformation, as there is, on one hand, the potential for rotational freedom but, on the other hand, a significant gain in energy to be made from obtaining maximum conjugation. Calculations performed by Stranger et al. on a butadiyne-linked porphyrin dimer indicated essentially barrierless rotation between the dihedral angles 0° and 60° but also pointed to the existence of a significant barrier for rotation above 60°.23 At 90°, the total energy of the system had increased by approximately 14 kcal/ mol relative to that at 0°, indicating that free rotation was not possible at ambient temperature. Therien and co-workers found interesting photophysical properties when describing the ultrafast dynamics of ethyne-linked porphyrin oligomers.24 In their work, a 30-35-ps component associated with a red shift of the transient signal was found for a meso-meso-linked dimer, and it was hypothesized that this spectral evolution was due to a planarization of nonplanar conformers in the excited state. When time-resolved and steady-state measurements were compared, it was concluded that, although nonplanar conformers were present, the population of maximally conjugated structures was
10.1021/jp0683519 CCC: $37.00 © 2007 American Chemical Society Published on Web 04/26/2007
Photophysics of a Butadiyne-Linked Porphyrin Dimer
Figure 1. Porphyrin oligomers studied in this work (Pn, n ) 1, 2, 4, 8) and the dipyridyl pyrrole ligand26 L used to force the dimer into a planar conformation in the P2‚L complex. The aryl substituents, Ar, are 3,5-di(octyloxy)phenyl. The bent structure corresponds to a geometry optimized at the PM3 level.
dominant. The group of Aida investigated butadiyne-linked porphyrin dimers with meso-pyridyl functionalities that allowed them to form tetrameric boxes. When the resulting structures were characterized spectroscopically, it was found that, surprisingly, the boxes built of perpendicular conformers were favored over the ones built of planar conformers.25 Apparently, in this configuration, the dipole moments of the pyridyl groups cancelled each other, and the net gain that this provided was enough to overcome any barrier that might have existed. For an ethyne-linked dimer, the boxes were formed exclusively by planar conformers.11 This result seems to indicate that the barrier for rotation in butadiyne-linked dimers is small. The issue of conformational heterogeneity is thus an interesting one. The butadiyne-linked zinc porphyrin oligomers (Pn) presented in Figure 1 have previously been incorporated into donor-bridge-acceptor systems (n ) 1, 2, 4), and their merit as “molecular wires” was studied.22 In that work, the oligomers served as the electron-mediating structure, and the question of the impact of conformational heterogeneity, in the ground state as well as in the first singlet excited state, is critical, particularly for longer oligomers. Kim, Osuka, and co-workers explored the effects on energy transfer that arise from conformational flexibility in extended, directly linked porphyrin oligomers (up to 512 units)27 and drew the conclusion that conformational heterogeneities can rule out the use of such structures as energy mediators over long distances. The present work explores the impact of the conformational dynamics described above on the photophysical properties of the simplest member of the oligomer series that exhibits dihedral conformers, the porphyrin dimer in Figure 1 (n ) 2), and, in particular, the coupling and decoupling of the porphyrin units that occurs during rotation. The basic spectroscopic properties have been described in previous publications8 and are qualitatively explained by Kasha’s point-dipole model for exciton coupling.28 Results Ground-State Absorption. The ground-state absorption spectra of Pn (n ) 1, 2, 4, 8) are shown in Figure 2. As a consequence of the stabilization of their excited states by the larger π systems, the B and Q bands of the longer oligomers are red-shifted relative to those of P1. The interaction between the porphyrin macrocycles results in a splitting of the absorption peaks, which is most clear for P2 but is largely concealed for P4 and P8. For P4, the spectrum is still structured, and the Q
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Figure 2. Ground-state absorption spectra of P1 (solid line, 2-MTHF), P2 (dashed line, 2-MTHF), P4 (dash-dotted line, 2-MTHF), and P8 (dotted line, DCM/pyridine).
Figure 3. Titration of a solution of P2 in dichloromethane (approximately 0.8 µM) containing a small quantity of pyridine (0.1 vol %) with the dipyridyl pyrrole ligand L (added in excess, cfinal g 10 µM) to form the P2‚L complex. Isosbestic points are located at 484 and 723 nm.
band is formed by several overlapping peaks. The spectral evolution of conjugated porphyrin oligomers is quite interesting, and the features of their ground-state absorption spectra as well as the impact of conformation on these spectra will be discussed in this article. To test how the absorption spectrum of the porphyrin dimer depends on its conformation (or rather, on its distribution of conformations), a concentrated solution of the bidentate dipyridyl pyrrole ligand L (Figure 1) was used to titrate a sample of P2 dissolved in dichloromethane (Figure 3). A small quantity of pyridine was added before the titration to avoid shifts in the spectrum due to complexation. The dipyridyl pyrrole ligand L forms a strong 1:1 complex with P2 (K ≈ 107-108 mol-1 dm3, Supporting Information) that effectively forces the dimer to stay planar or very nearly planar and was added in sufficient excess so that the competition from pyridine was of no consequence. The intensity of the B band peak at 457 nm decreases upon titration, whereas the intensity of the peak at 491 nm increases. In the Q band, the intensity of the peak farthest to the red increases, and that of the peak on the blue side decreases. The absorption of the ligand itself is in the UV, with a maximum at 340 nm. Thus, the final spectrum above 400 nm is almost exclusively that of the planar dimer, which exhibits two peaks in the B band and one strong peak in the Q band. It is similar to the spectrum of an aggregated dimer,8 as expected. The bending of the porphyrin dimer upon complexation has minor effects on the absorption spectra, as shown in Figure 3, and this is also supported by quantum mechanical calculations (vide
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Figure 4. Emission and excitation spectra of P2 recorded in 2-MTHF. Emission spectra were recorded using the excitation wavelengths 457 nm (solid line) and 491 nm (dotted line). The emission wavelengths (indicated by arrows) used for the excitation spectra were 666 nm (dashdotted line) and 727 nm (dashed line). The Q band is slightly distorted due to scattering. Note that the excitation spectra in the right panel are scaled by a factor of 2.
infra). In contrast, the ground-state absorption spectrum of disaggregated P2, free to rotate, is a mixture of several spectroscopic species. UV-vis titrations of P2 with the dipyridyl pyrrole ligand L were also performed in dichloromethane and chloroform in the absence of pyridine (Figures S1 and S2, Supporting Information), and the spectral changes were found to be very similar to those shown in Figure 3. Fluorescence Spectroscopy. When P2 was excited at the four main peaks, the fluorescence spectrum of P2 exhibited a dependence on excitation wavelength (Figure 4). When the dimer was excited at either of the B or Q band peaks of higher energy (457 nm or 661 nm), a weak fluorescence peak was present at 666 nm (indicated by an arrow in Figure 4) that did not appear when the sample was excited at any of the peaks of lower energy (491 or 721 nm). This peak arose irrespective of the solvent used [dichloromethane (DCM)/pyridine, tetrahydrofuran (THF), and 2-methyltetrahydrofuran (2-MTHF) were tested]. Moreover, excitation spectra were recorded for emissions at 666 and 727 nm (Figure 4, dashed curves). Whereas recording the excitation spectra of the emission at 727 nm reproduced a spectrum resembling the full ground-state absorption spectrum, the emission at 666 nm did not reproduce the full spectrum but rather produced a spectrum that exhibits only two of its peaks. This spectrum is much more like that of a porphyrin monomer, albeit shifted to the red. This result indicates that the origin of this emission peak in the steady-state spectrum is due to a distinctive spectroscopic species. As it was established from ground-state absorption that the planar conformation has a unique ground-state absorption spectrum, it thus seems plausible that there is a spectroscopic difference between the planar and twisted conformations of P2. The excitation spectrum obtained from the emission at 666 nm well mirrors the absorption spectrum of a twisted conformation. The emission at 666 nm was further investigated by measuring the steady-state spectrum at lower temperatures. For excitation at 457 or 661 nm, a strong temperature dependence was found, as demonstrated in Figure 5. At room temperature, the strongest emission is at 727 nm, whereas the peak at 666 nm is only a small shoulder on the blue side of the main peak. As the temperature is lowered, the main emission is gradually weakened, and the intensity of the emission at 666 nm grows strongly. This seems to exclude the possibility that the low emission shoulder is due to an impurity, but rather indicates that the two emitting species stand in a mother-daughter
Winters et al.
Figure 5. Steady-state fluorescence spectra of P2 recorded in 2-MTHF at 295 K (black), 255 K (red), 215 K (green), 175 K (blue), 135 K (cyan), and 95 K (magenta). The sample was excited at 457 nm.
Figure 6. Fluorescence decay traces for P2 in 2-MTHF at 295 K. The sample was excited at 457 nm, and the emission was monitored at 666 nm (dark gray trace) and 727 nm (black trace). The instrument response function is represented by the light gray trace. Note that the scale of the abscissa is logarithmic.
relation to one another that is strongly temperature-dependent. This suggests that the population of twisted conformers, which were assigned to the emission at 666 nm, adopt a planar configuration in the excited state. When the corresponding measurements were made by exciting at 491 nm, no large temperature effects were observed in the fluorescence spectrum. To further test the idea that the twisted and planar conformations are separate spectroscopic species with unique absorption and emission spectra, time-correlated single-photon counting (TCSPC) was used to measure the fluorescence lifetimes of the porphyrin dimer at the two emission peaks. The temperature dependence of the fluorescence lifetimes at these wavelengths was then studied. The fluorescence lifetime decay was recorded at 5-nm intervals over the entire emission spectrum at room temperature to visualize the spectral evolution. It was found that, shortly after excitation at 457 nm, the spectrum resembled that found by steady-state fluorescence measurements at 95 K, but after approximately 500 ps, the spectrum had evolved into that found at 295 K (Figure S3, Supporting Information). Moreover, when the sample was excited at 457 nm, two fluorescence lifetimes were necessary to fit the resulting fluorescence decay curves (Figure 6). The fluorescence at 666 nm was dominated by a fast decay, whereas the fluorescence at 727 nm exhibited a corresponding rise time in the emission and a subsequent slow decay (Table 1). This again suggests that the species emitting at short and long wavelengths stand in a mother-daughter relationship. At both emission wavelengths, the long lifetime was approximately 1.2 ns at room
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TABLE 1: Temperature Dependence of the Fluorescence Lifetimes for P2 in 2-MTHF, Excited at 457 nm emission wavelength ) 666 nm
emission wavelength ) 727 nm
T (K)
τ1 (ps)
R1a,b
f1a,b (%)
χ2
τ1 (ps)
R1a
τ2 (ns)
R2a
χ2
295 255 215 175 135 95
100 160 290 480 730 820
0.96 0.97 1 1 0.82 0.99
80 90 100 100 77 97
2.8c 2.4c 1.7 1.5 1.4 1.4
120 300 340 720 850 -
-0.47 -0.48 -0.43 -0.32 0.16 -
1.21 1.27 1.33 1.35 1.38 1.44
0.53 0.52 0.57 0.68 0.84 1
1.6 1.6 2.0 1.6 1.5 1.3
a R is the normalized preexponential factor, and f1 is the fractional fluorescence intensity b Because of a small spectral overlap, a fraction of the longer emission lifetime is also present at this wavelength. c Measurements in a cryostat lead to higher χ2 values than normal. See Experimental Section.
Figure 8. (Bottom) Calculated ground-state stick spectra at different dihedral angles: 0° (black, solid) and 90° (red, solid) represent the border cases and the intermediate dihedral angles are represented by hatched bars [10° (red), 20° (green), 30° (royal blue), 40° (cyan), 50° (magenta), 60° (violet), 70° (dark green), 80° (navy blue)]. (Top) Calculated spectra fitted to Gaussian components with half-widths at half-maximum arbitrarily set to 1000 cm-1: 0° (blue, dot-dashed line), 90° (red, dashed line), and a simulated room-temperature spectrum (black, solid line) based on a Boltzmann-weighted combination of the 10 (0-90°) conformers using the calculated ground-state potential energy surface (Figure 9).
Figure 7. Temperature dependence of the fluorescence lifetime data of P2 in 2-MTHF, plotted as ln(1/τ1) (squares) and ln(1/τ1 - 1/τ10) (circles); see Table 1. The straight solid line is a linear regression to the ln(1/τ1 - 1/τ10) data, yielding Ea ) 2 kcal/mol.
temperature. When the sample was excited at either 491 or 721 nm, the fluorescence decay was monoexponential and exhibited no emission at 666 nm. As the temperature was lowered, it was found that both the short fluorescence decay and the corresponding rise time became progressively longer, whereas the longer decay lifetime was not much affected (Table 1). This shows that the process connecting the two excited states is sensitive to temperature, making a rotation around the butadiyne axis a plausible explanation, and that the planar conformation is preferred in the first excited singlet state. Furthermore, it appears that this rotation is almost completely hindered at 95 K (Figure 7), and therefore, it was assumed that the fluorescence lifetime measured at 666 nm is approximately equal to the intrinsic fluorescence lifetime of the twisted conformation (τ10). The rate constant for rotation was calculated as krot ) [1/τ1 - 1/τ10], and in Figure 7, ln(krot) is presented together with ln(1/τ1). By using the Arrhenius equation, an apparent activation energy of 2 kcal/mol was obtained by linear regression of ln(krot), indicating the possibility of an activation energy for rotation in the excited state. To clarify the extent to which the determined activation energy is related to solvent viscosity effects, the temperature dependence of the viscosity of 2-MTHF was checked. The Andrade equation, η(T) ) Aη exp(Eη/RT), can be used to check the viscous contribution to the rotational barrier, and this was done for 2-MTHF using data from ref 29, which yielded Eη ≈ 2 kcal/mol for the linear region. As 2-MTHF is a glass-forming solvent, its viscosity exhibits a
very strong temperature dependence at low temperatures, and thus, the Andrade equation applies only down to moderately low temperatures for this solvent. Young et al. used the Williams-Landel-Ferry equation, η(T) ) Aη exp[Eη/R(T T0)], for 2-MTHF (characteristic temperature T0 ) 81 K) and obtained a viscous barrier of Eη ) 0.74 kcal/mol.29 This also suggests that a significant part of the activation energy is due to viscous effects, but as this equation is more appropriate for glass-forming solvents, the estimated contribution of viscosity to Ea is probably more accurate. Quantum Mechanical Calculations. Calculated electronic transitions of 10 different conformations of P2 are shown in Figure 8. It is evident from these results that increasing the dihedral angle from 0° to 90° results in a blue shift of the Q band and an intensified B band transition at 420 nm. At a dihedral angle of 0°, the B band consists of two strong transitions, and the Q band consists of only one. This is in qualitative agreement with the spectrum of the planar conformation displayed in Figure 3. At a dihedral angle of 90°, a blueshifted Q band is present, and only one major transition in the B band remains. This, in turn, reproduces the excitation spectrum in Figure 4 that is believed to resemble the absorption spectrum of the twisted conformation. The bending of P2 that is induced by the addition of the ligand L results in a small red shift of the transitions, but the effect is minor compared to that of twisting. The tendency to form the planar conformation in the excited state can be understood by comparing the potential energy surfaces of the S0 and S1 states. In Figure 9, the potential energy is plotted as a function of the torsion angle. The ground-state energies were obtained by full geometry optimization of P2 at several torsion angles between 0° and 90°, whereas the excitation energies of the planar and twisted conformers were taken from
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Figure 9. S0 and S1 potential energy surfaces of P2. The S0 surface was calculated at the BLYP/6-31G(d) level. The E00 values for the twisted and planar conformation were determined from spectra, and the dashed black line indicates an approximate PES for S1. The calculated ground-state barrier for rotation (0.67 kcal/mol) is indicated by a double-headed arrow.
absorption and emission spectra of P2. The calculations indicate that the ground state exhibits a very low barrier for rotation (0.67 kcal/mol) and should thus have a broad distribution of dihedral angles at room temperature. Nevertheless, the groundstate absorption spectrum of P2 is quite structured, as is evident in Figure 2, and this is also supported by the calculated transitions (vide infra). The value found for the barrier to rotation is similar to the estimated 1 kcal/mol obtained by Lin et al. from AM1 semiempirical calculations on a similar porphyrin dimer30 and also similar to experimental values reported for diphenylethyne- and diphenylbutadiyne-linked porphyrins (0.79 and
