5794
J. Phys. Chem. B 2009, 113, 5794–5802
Conformational Changes of meso-Aryl Substituted Expanded Porphyrins upon Protonation: Effects on Photophysical Properties and Aromaticity Jae-Yoon Shin,† Jong Min Lim,† Zin Seok Yoon,† Kil Suk Kim,† Min-Chul Yoon,† Satoru Hiroto,‡ Hiroshi Shinokubo,‡ Soji Shimizu,‡ Atsuhiro Osuka,*,‡ and Dongho Kim*,† Spectroscopy Laboratory for Functional π-electronic Systems and Department of Chemistry, Yonsei UniVersity, Seoul 120-749, Korea, and Department of Chemistry, Graduate School of Science, Kyoto UniVersity, Sakyo-ku, Kyoto 606-8502, Japan ReceiVed: NoVember 19, 2008; ReVised Manuscript ReceiVed: March 2, 2009
meso-Hexakis(pentafluorophenyl) [30]heptaphyrin(1.1.1.1.1.1.0) and meso-hexakis(pentafluorophenyl) [38]nonaphyrin(1.1.0.1.1.0.1.1.0) have been investigated with a particular focus on their photophysical properties affected by protonation with acids using steady-state and time-resolved spectroscopic measurements along with femtosecond Z-scan method. It was found that the smaller Stokes shift and longer excited singlet/triplet state lifetimes of protonated [30]heptaphyrin and [38]nonaphyrin compared to their distorted neutral counterparts are strongly associated with the rigid and planar molecular structures. Much larger two photon absorption cross-section values of protonated [30]heptaphyrin and [38]nonaphyrin (6300 and 6040 GM) than those of their neutral forms (1350 and 1300 GM) also reflect the enhanced rigidity and planarity as well as aromaticity. In parallel with this, the nucleus-independent chemical shift (NICS) values of protonated forms exhibit large negative values, -14.3 and -11.5 ppm for [30]heptaphyrin and [38]nonaphyrin, respectively, at central positions. Thus we have demonstrated the structure-property relationships between molecular planarity, photophysical properties, and aromaticity of expanded porphyrins upon protonation based on our experimental and theoretical investigations. This study also promises a possibility of structural control of expanded porphyrins through protonation in which the molecular flexibilities of expanded porphyrins lead to distorted structures especially as the number of pyrrole rings increases. Introduction Expanded porphyrins containing more than four pyrrole rings with large cavity size and extended π-conjugation pathway have received much attention because of their potential applications, such as anion receptors,1 multimetallic chelates,2 photodynamic therapy (PDT) sensitizers,3 magnetic resonance imaging (MRI) contrasting agents,4 and nonlinear optical materials.5 In addition, the larger expanded porphyrins can provide an opportunity to investigate the structure-property relationship in conjunction with aromaticity due to their structural diversity and versatile π-conjugation networks depending upon the number of pyrrole rings.6 Notably, the π-conjugation in expanded porphyrins is easily controllable through facile two-electron oxidation or reduction process to provide a set of [4n]/[4n + 2] congeners in the same chemical environment.5b This unique feature of expanded porphyrins gives a definite advantage over π-conjugated homoannulenes as an ideal test bed to study the aromaticity defined by Hu¨ckel’s [4n + 2] rule. As the number of pyrrole rings increases in expanded porphyrins, the molecular structure becomes more distorted as a general trend, which causes a disruption in π-conjugation and reduces the aromaticity even for the cases that satisfy Hu¨ckel’s [4n + 2] rule.7 To ensure the overall molecular planarity for enhanced π-conjugation, various strategies have been envisaged, including replacing the pyrrolic nitrogen atoms by heavier heteroatoms,1c,d omitting meso-methine to implement direct pyrrole-pyrrole linkages,8 * To whom correspondence should be addressed. E-mail: (D.K.)
[email protected]; (A.O.)
[email protected]. † Yonsei University. ‡ Kyoto University.
changing the peripheral pyrrolic β-substituents or the mesosubstituents,9 or inserting an internal 1,4-phenylene bridge across the macrocycle.10 In line with these efforts, protonation with appropriate acids can be considered to be a simple method to control the molecular conformation of expanded porphyrins without any synthetic procedures.11 There are also several previous studies in which the protonation of porphyrinoids has been investigated in porphyrins,12 sapphyrins,13 and rosarins.14 In these cases, the attachment of protons to core-nitrogens interferes with molecular planarity because their relatively small core-sizes cannot bear the intrusion of protons. On the other hand, expanded porphyrins with more than six pyrrole rings have larger cavity rings to accommodate more protons easily accompanied by anion bindings. In this study, we have investigated the photophysical properties of free base and protonated meso-aryl substituted expanded porphyrins with a particular emphasis on the structure-property relationship between the planarity and photophysical properties in conjunction with the aromaticity of molecules. Moreover, to the best of our knowledge the photophysical properties of larger expanded porphyrins than hexapyrrolic-expanded porphyrin have rarely been studied especially in the effect of protonation with acids. With these objectives in our mind, we have chosen two expanded porphyrins which are meso-hexakis(pentafluorophenyl) [30]heptaphyrin(1.1.1.1.1.1.0) 115 and meso-hexakis(pentafluorophenyl) [38]nonaphyrin(1.1.0.1.1.0.1.1.0) 28b as benchmark molecules (Chart 1). They are larger expanded porphyrins containing seven and nine pyrroles, respectively, with the same peripheral substituents. At the same time, the molecular structures of free-base [30]heptaphyrin and [38]nonaphyrin are distorted although the number of π-electrons in two molecules
10.1021/jp8101699 CCC: $40.75 2009 American Chemical Society Published on Web 04/03/2009
meso-Aryl Substituted Expanded Porphyrins CHART 1: Molecular Structures of meso-Hexakis(pentafluorophenyl) [30]heptaphyrin(1.1.1.1.1.1.0) 1 and meso-Hexakis(pentafluorophenyl) [38]nonaphyrin(1.1.0.1.1.0.1.1.0) 2
satisfy Hu¨ckel’s [4n + 2] rule. In this context, [30]heptaphyrin and [38]nonaphyrin in neutral (1 and 2) and their protonated forms (1p and 2p) are ideally suited for the systematic investigation of the photophysical properties induced by structural changes upon protonation. To aim at our objectives in this study, we have utilized various spectroscopic techniques such as spectrophotometric titration, femtosecond and nanosecond transient absorption, and femtosecond Z-scan measurements along with the quantum mechanical and nucleus-independent chemical shift (NICS) calculations. On the basis of these measurements, we have observed that protonated [30]heptaphyrin 1p and [38]nonaphyrin 2p exhibit intense and well-resolved absorption and fluorescence bands in NIR region, larger two-photon absorption (TPA) cross-section values, highly negative NICS values, and relatively long excited singlet and triplet state lifetimes compared with their neutral counterparts. The structural changes of 1 and 2 induced by protonation give rise to a significant change in the photophysical properties as well as aromaticity. Therefore, the protonation of expanded porphyrins can allow us to scrutinize the structureproperty relationship between the molecular planarity and photophysical properties in conjunction with the aromaticity of molecules. Experimental Methods Sample Preparation. meso-Hexakis(pentafluorophenyl) [30]heptaphyrin(1.1.1.1.1.1.0) 1 and meso-hexakis(pentafluorophenyl) [38]nonaphyrin(1.1.0.1.1.0.1.1.0) 2 were synthesized according to previous reported papers.8b,15 Dichloromethane and chloroform were used as solvents (Sigma-Aldrich, spectrophotometric grade) without further purification. Commercial methanesulfonic acid, MSA and trifluoroacetic acid, TFA (SigmaAldrich, spectrophotometric grade, 99+ %) were used for protonation. UV-vis-NIR Spectrophotometric Titration. Two milliliters solution of samples in dichloromethane (or chloroform) solvent were titrated carefully in a 1 cm absorption cell by addition of 0.1, 0.5, and 1 M-MSA (or TFA) standard solutions which were made by mixing MSA (or TFA) and dichloromethane (or chloroform) with exact ratio of volume. Using a microliter syringe, MSA (or TFA) standard solutions were added in 0.5 to 10 µL aliquots. After addition of each aliquot, UV-vis-NIR absorption spectra were measured. The total volume change during the titration was corrected by each absorption spectrum multiplied by correction factor, f ) (original volume + volume of aliquots)/(original volume). Steady-State Absorption and Fluorescence. Steady-state absorption spectra were obtained by using an UV-vis-NIR spectrometer (Varian, Cary5000). For the observation of steadystate fluorescence spectra in NIR region, a photomultiplier tube (Hamamatsu, H9170-75), a lock-in amplifier (EG&G, 5210)
J. Phys. Chem. B, Vol. 113, No. 17, 2009 5795 combined with a mechanical chopper, and a CW He-Cd laser (Melles Griot, Omnichrome 74) for photoexcitation at 442 nm were used. Femtosecond Transient Absorption Measurements. The femtosecond time-resolved transient absorption (TA) spectrometer consisted of a homemade noncollinear optical parametric amplifier (NOPA) pumped by a Ti:sapphire regenerative amplifier system (Quantronix, Integra-C) operating at 1 kHz repetition rate and an optical detection system. The generated visible NOPA pulses had a pulse width of ∼100 fs and an average power of 1 mW in the range 500-700 nm which were used as pump pulses. White light continuum (WLC) probe pulses were generated using a sapphire window (2 mm of thickness) by focusing of small portion of the fundamental 800 nm pulses which was picked off by a quartz plate before entering to the NOPA. The time delay between pump and probe beams was carefully controlled by making the pump beam travel along a variable optical delay (Newport, ILS250). Intensities of the spectrally dispersed WLC probe pulses are monitored by miniature spectrograph (OceanOptics, USB2000+). To obtain the time-resolved transient absorption difference signal (∆A) at a specific time, the pump pulses were chopped at 25 Hz and absorption spectra intensities were saved alternately with or without pump pulse. Typically, 6000 pulses excite samples to obtain the TA spectra at a particular delay time. The polarization angle between pump and probe beam was set at the magic angle (54.7°) in order to prevent polarization-dependent signals. Crosscorrelation fwhm in pump-probe experiments was less than 200 fs and chirp of WLC probe pulses was measured to be 800 fs in the 400-800 nm region. To minimize chirp, all reflection optics in probe beam path and 2 mm path length of quartz cell were used. Nanosecond Transient Absorption Measurements. Nanosecond transient absorption spectra were obtained using nanosecond flash photolysis method. The tunable excitation pulse was obtained from an optical parametric oscillator system (Continuum, Surelite OPO) which was pumped by a Nd:YAG laser (Continuum, Surelite II-10). A CW Xe lamp (150 W) was used as a probe light source for the transient absorption measurement. After passing through the sample, the probe light was collimated and then spectrally resolved using a monochromator with a 15 cm f.1. (Acton Research, SP150) equipped with a 600 groove/mm grating. The light signal was detected via an avalanche photodiode (APD) (Hamamatsu, C5331-11). For the temporal profile measurements, the output signal from the APD was recorded using a 500 MHz digital storage oscilloscope (Lecroy, WaveRunner 6050A). The experiments were conducted under the argon-saturated condition. Computational Method. Quantum mechanical calculation were performed with the Gaussian 03 program suite.16 All calculations were carried out by the density functional theory (DFT) method with Becke’s three-parameter hybrid exchange functionals and the Lee-Yang-Parr correlation functional (B3LYP),17,18 employing the 6-31G* basis set. In the case of protonated [30]heptaphyrin 1p and neutral [38]nonaphyrin 2, the X-ray crystal structures were used as initial geometies for geometry optimization without any modification while geometry optimization of neutral [30]heptaphyrin 1 and protonated [38]nonaphyrin 2p were started from a possible geometry based on 1H NMR analysis. The NICS(0) values were obtained with the GIAO method at the B3LYP/6-311G** level based on geometry optimized structures modified by replacing mesosubstituents with hydrogen. The center of rings for the NICS(0) values was designated at the nonweighted means of π-conjuga-
5796 J. Phys. Chem. B, Vol. 113, No. 17, 2009 tion pathway of peripheral ring. Moreover other NICS(0) values were also calculated at additional points as the unweighted geometric mean of two interpyrrolic C-N bonds. The oscillator strength was calculated by performing time dependent (TD)DFT calculation. Two-Photon Absorption Cross-section (σ(2)) Measurements. The TPA measurements were performed using the openaperture Z-scan method with 130 fs pulses from an optical parametric amplifier (Light Conversion, TOPAS) operating at a 5 kHz repetition rate using a Ti:sapphire regenerative amplifier system (Spectra-Physics, Hurricane X). After passing through an f ) 10 cm lens, the laser beam was focused to 1 mm quartz cell. As the position of the sample cell was varied along the laser-beam direction (z-axis), the transmitted laser beam from the sample cell was then probed using a Ge/PN photodiode (New Focus, 2033) as used for reference monitoring. Assuming a Gaussian beam profile, the nonlinear absorption coefficient β can be obtained by curve fitting to the observed open aperture traces with the following equation19
T(z) ) 1 -
βI0(1 - e-R0l) 2R0(1 + (z/z0)2)
where R0 is a linear absorption coefficient, I0 is the on-axis peak intensity of the incident pulses at the focal point, l is a sample length, and z0 is the diffraction length of the incident beam. After obtaining the nonlinear absorption coefficient β, the TPA cross-section σ(2) (in unit of 1 GM ) 10-50 cm4 · s · photon-1molecule-1) of a single solute molecule can be determined by using the following relationship
σ(2)NAd × 10-3 β) hV where NA is the Avogadro constant, d is the concentration of the TPA compound in solution, h is Planck’s constant, and V is the frequency of the incident laser beam. The TPA cross-section value of AF-50 was measured as a reference compound, which was found to exhibit a TPA value of 50 GM at 800 nm.20 Results and Discussion Conformational Change by Protonation. Although all these molecules contain [4n + 2] π-electrons in their π-conjugation pathway to satisfy Hu¨ckel’s [4n + 2] rule, the weak-aromatic characters of [30]heptaphyrin 1 and [38]nonaphyrin 2 in 1H NMR spectra were reported in the previous studies.8b,15 It was expected that their weak-aromatic characters in 1H NMR spectra are attributable to their figure-eight molecular conformations, one of which is evidenced by X-ray crystal structure of freebase [38]nonaphyrin 2.8b The notable behavior of [30]heptaphyrin 1 is the spectral change in 1H NMR spectrum induced by addition of trifluoroacetic acid (TFA).15 The protonated [30]heptaphyrin 1p possesses a strong aromatic character with a large diatropic ring current in 1H NMR spectrum compared to its neutral form.15 Because the planar structure with three inverted pyrroles was observed in protonated [30]heptaphyrin 1p by X-ray crystallography, it is believed that its enhanced aromatic character is generated by conformational change from twisted to planar structures.15 We could also observe the strong diatropic ring current of protonated [38]nonaphyrin in 1H NMR spectrum alluding to a conformational change after the addition
Shin et al. of methanesulfonic acid (MSA) (Supporting Information, Figure S1). While the 1H NMR spectrum of a neutral form shows the β-CH proton peaks in 4∼8 ppm, upon addition of MSA,8b a set of peaks were observed in the range of -7∼18 ppm along with the small peaks due to the presence of minor conformations. The β-CH protons were assigned by using 1H1H-COSY technique (Supporting Information, Figure S2) and were observed in downfield region at 11.48, 10.63, 10.33, 10.30, 10.21, 9.90, 9.33, 9.13, 6.77, and 6.06 ppm and in upfield region at -1.31, -1.91, -4.53, -5.10, -5.58, -5.59, -5.76, and -6.95 ppm. This spectroscopic feature of 2p would indicate that five pyrrole rings pointed inside and four pyrrole rings were inverted. The other seven peaks were assigned as NH protons, but further confirmation by H-D exchange experiment was hampered by the precipitation upon addition of D2O. Furthermore, a peak due to three protons was observed at -2.25 ppm, which had no correlation with any other peaks. This peak could be assigned as the protons of methanesulfonate anion bound inside the cavity of the macrocycle being under paratropic ring current effect. Though further structural confirmation by X-ray single crystal analysis has been unsuccessful due to a lack of single crystal despite of repetitive trials, these NMR data suggest that the planarity and aromatic character of protonated [38]nonaphyrin 2p is enhanced as compared with its neutral counterpart. On the basis of these 1H NMR spectroscopic data, we found the optimized structures of twisted neutral [30]heptaphyrin 1 and relatively planar protonated [38]nonaphyrin 2p by quantum mechanical calculations, and observed that they exhibit a good correlation between experimental and theoretical data (vide infra). Spectrophotometric Titration. The steady-state absorption spectra were measured by a stepwise addition of acid to monitor the effect of protonation. At first, the spectrophotometric titration of [30]heptaphyrin 1 was performed with TFA in CHCl3 (Figure 1a,b). During the titration process, the spectral changes in the absorption spectra showed two-step processes with several isosbestic points. In the concentration range of TFA from 0 to 2.1 × 10-2 M, the Soret-like band at 634 nm and Q-like bands at 834 and 940 nm were shifted to red region accompanied by an appearance of the longer wavelength bands at 1013 and 1089 nm (Figure 1a). Upon increasing the concentration of TFA up to 2.3 × 10-1 M, the Soret-like band became more intense with a peak shift to blue region at 633 nm again (Figure 1b). Although the spectral changes in Q-like band region were complicated, the four peaks at 873, 907, 985, and 1037 nm became intensified and well resolved. It seems that these spectral changes in spectrophotometric titration are associated with the conformational changes of 1 induced by protonation.15 The spectrophotometric titration of 1 with MSA in CH2Cl2 also exhibited the same two-step spectral changes (Figure 1c,d). Thus, we can conclude that the addition of MSA to CH2Cl2 solution of 1 leads to the same conformational changes with addition of TFA. The only difference is the fact that the concentration of MSA required for protonation (6.6 × 10-5 M) is much lower than that of TFA. Since this result reflects the strong binding ability and higher acidity of MSA, the spectrophotometric titrations of [38]nonaphyrin 2 were carried out by adding MSA. In the spectrophotometric titration, the absorption spectrum of 2 exhibited the intensification of Soret- and Q-like bands followed by a small bathochromic shift (Figure 2). It should be noted that a large amount of MSA was added to 2 to observe the spectral change (2.9 × 10-1 M). This feature can be explained in terms of the fact that the highly twisted conformation of 2 makes the attachment of protons difficult due
meso-Aryl Substituted Expanded Porphyrins
J. Phys. Chem. B, Vol. 113, No. 17, 2009 5797
Figure 1. Spectrophotometric titration of [30]heptaphyrin 1 (a,b) with TFA in CHCl3 and (c,d) with MSA in CH2Cl2.
Figure 3. Steady-state absorption (black) and fluorescence (red) spectra of [30]heptaphyrin 1, [38]nonaphyrin 2, and their protonated forms (1p, and 2p, respectively) upon addition of MSA in CH2Cl2.
Figure 2. Spectrophotometric titration of [38]nonaphyrin 2 with MSA in CH2Cl2.
to the steric hindrance, and consequently lowers the proton affinity of the imine nitrogens.
Steady-State Absorption and Fluorescence. The absorption spectra of neutral and protonated expanded porphyrins represent large differences as shown in Figure 3. In both cases, the peaks in the range of 300∼500 nm disappeared by addition of MSA and the same phenomenon was also observed in the protonation of [30]heptaphyrin 1 with TFA (Figures 1 and 3). In addition, [30]heptaphyrin 1 and [38]nonaphyrin 2 exhibit narrower and intensified B-like bands upon protonation without large spectral shifts (Figure 3). The molar extinction coefficients are increased from 1.7 × 105 to 4.4 × 105 M-1cm-1 for 1 and from 1.4 × 105 to 6.1 × 105 M-1cm-1 for 2, respectively. At the same time, the full width
5798 J. Phys. Chem. B, Vol. 113, No. 17, 2009
Shin et al.
TABLE 1: Steady-State Absorption and Fluorescence Bands, Stokes Shifts, Singlet and Triplet Excited-State Lifetimes, Two-Photon Absorption Cross Sections σ(2), and NICS values of neutral and protonated 1 and 2 in CH2Cl2
1 1p 2 2p
λabs (nm)
λflu (nm)c
EStokes (cm-1)d
τS (ps)e
634a, 1106b 633a, 1037b 725a, 1280b 746a, 1260b
1141 1046 1328 1280
280 80 280 120
47 400 18 44
τT (µs)
σ(2) (GM) (λex /nm)j
NICS (ppm)k
3.1f 6.1g 0.2h 6.2i
1350 ( 200 (1290) 6300 ( 500 (1300) 1300 ( 200 (1390) 6040 ( 500 (1450)
-8.7 -14.3 -8.7 -11.5
a
Soret-like band. b Lowest energy Q-like band. c Fluorescence band maxima. d Energy gap between b and c. e The average value of τ in excited-state absorption and ground-state bleaching. f Pumped and probed at 630 and 670 nm, respectively. g Pumped and probed at 630 nm. h Pumped and probed at 720 and 600 nm, respectively. i Pumed and probed at 790 and 600 nm, respectively. j λex is the laser excitation wavelength. k NICS(0) values at the central position of macrocycles.
at half-maximum (fwhm) of Soret-like band is decreased from 1492 to 846 cm-1 for 1 and from 1294 to 1064 cm-1 for 2 (Figure 3). Because the narrow absorption band with a large extinction coefficient reflects a substantially symmetrical structure,21 we can estimate that the conformations of 1 and 2 are changed from twisted to rather planar structures upon protonation. While the distinct and strong Q-like bands appear in 1p and 2p, 1 and 2 exhibit the weak and smeared Q-like bands. In some cases, the aromatic character of expanded porphyrin can be identified by its absorption spectrum, especially by Q-like bands. Many aromatic expanded porphyrins exhibit well-defined Q-like bands in NIR region which is in a sharp contrast with nonaromatic and antiaromatic ones to show featureless Q-like bands or none.9a,22 According to this trend in the absorption spectra of expanded porphyrins, the weak and broad (but observable) Q-like bands of 1 and 2 reflect the weak aromatic character in agreement with their 1H NMR spectra although they should be Hu¨ckel aromatic molecules due to [4n + 2] π-electronic circuits. In other words, the distorted conformation makes 1 and 2 have the reduced aromaticity. The fluorescence spectra were detected in the near-infrared (NIR) region (Figure 3). Upon protonation, the fluorescence peaks were hypsochromically shifted from 1141 to 1046 nm for 1 and from 1328 to 1280 nm for 2, respectively, accompanied by a hypsochromic shift of the lowest energy Q-like bands. However, since the magnitudes of hypsochromic shifts in the lowest energy Q-like bands are smaller than those of fluorescence bands, the Stokes shifts of 1 and 2 are decreased from ca. 280 to ca. 80 cm-1 and from ca. 280 to ca. 120 cm-1 (Table 1). In the protonated form of 1 with TFA, the Stokes shift is also reduced from ca. 330 to ca. 100 cm-1 (Supporting Information, Figure S3 and Table S2). The decreased Stokes shifts in the protonated forms indicate that the reorganization energy by solvent is changed upon protonation. Judging from the former protonation study of porphyrins,12b the trend in this Stokes shift seems to reflect the conformational changes of 1 and 2 by protonation, especially from the twisted to planar conformations. Excited Singlet and Triplet State Dynamics. In order to obtain detailed information on the excited-state dynamics affected by protonation, we have carried out femtosecond and nanosecond transient absorption measurements. In both neutral and protonated expanded porphyrins, we have recorded a series of transient absorption spectra at various delay times between pump and white light continuum probe pulses. From a series of transient absorption spectra, the decay profiles were monitored at ground-state bleaching and excited-state absorption signals of each sample. The femtosecond transient absorption spectra of 1 and 1p were obtained with photoexcitation at 630 nm (Figure 4). The decay profiles of both 1 and 1p exhibit single exponential decay, and the singlet excited-state lifetimes were evaluated to
Figure 4. Femtosecond transient absorption spectra and decay profiles (inset) of (a) neutral [30]heptaphyrin 1 and (b) its protonated form 1p with MSA in CH2Cl2. For all cases, the pump excitation is at 630 nm.
be ca. 47 and 400 ps (the average values of ground-state bleaching and excited-state absorption signals, see Supporting Information, Table S1), respectively. The protonated form of 1 with TFA in chloroform also shows longer singlet excited-state lifetime than its neutral form (ca. 167 ps vs ca. 51 ps, see Supporting Information, Figure S4 and Table S1). The similar trend was observed in femtosecond transient absorption decay profiles of [38]nonaphyrin 2 (Figure 5). From the single exponential decay profiles, the singlet excitedstate lifetimes were determined to be ca. 18 ps for 2 and ca. 44 ps for 2p (the average values of ground-state bleaching and excited-state absorption signals, see Supporting Information, Table S1). This large increase in the lifetime of singlet excitedstate for protonated forms reflects the structural rigidification upon protonation, because the relatively flexible molecular framework of neutral forms provides efficient nonradiative decay channels from excited electronic state through many lowfrequency vibrational motions. This structural rigidification upon protonation would arise from the Coulombic repulsion among protonated pyrrole units as well as the interaction between the
meso-Aryl Substituted Expanded Porphyrins
Figure 5. Femtosecond transient absorption spectra and decay profiles (inset) of (a) neutral [38]nonaphyrin 2 and (b) its protonated form 2p with MSA in CH2Cl2. For all cases, the pump excitation is at 730nm.
counteranion of acid and the expanded porphyrin macrocycle through hydrogen bonding interactions. Actually, hydrogen bonding interactions of counteranions were observed in the X-ray crystal structure of 1p protonated with TFA.15 The triplet excited-state lifetimes were also measured by using nanosecond flash-photolysis technique in order to investigate the triplet excited-state dynamics, which turned out to be consistent with the singlet excited-state lifetimes (Supporting Information, Figure S5). The triplet excited-state lifetimes of 1 and 2 become longer upon protonation from 3.1 to 6.1 µs and from 0.2 to 6.2 µs, respectively (Table 1). For the case of 1p treated with TFA, the triplet excited-state lifetime was estimated to be 2.0 µs whereas neither bleaching nor excited-state absorption signal was detected in chloroform solution of 1 probably due to its low intersystem crossing yield and very fast triplet state decay less than a few nanosecond (Supporting Information, Figure S5 and Table S2). On the basis of these time-resolved spectroscopic measurements, we can argue that the excited singlet and triple state lifetimes of protonated [30]heptaphyrin 1p and [38]nonaphyrin 2p are much longer than their neutral counterparts (Table 1). Considering a large change in the Stokes shift and singlet/triplet excited-state lifetimes between neutral and protonated forms in 1 and 2, we could suggest that nonradiative decay channels are diminished significantly in the protonated forms due to enhanced molecular planarity and rigidity because the flexible motions of the overall molecular framework especially in neutral forms provide efficient nonradiative decay pathways such as internal conversion for energy relaxation processes of electronic excited states. In other words, upon protonation the structural rigidification accompanied by planarization of 1p and 2p produces a blockade in flexible molecular motions of twisted neutral forms, leading to slower energy relaxation processes of singlet and
J. Phys. Chem. B, Vol. 113, No. 17, 2009 5799 triplet excited states of 1p and 2p compared to their neutral ones. The similar explanation of the perturbed photophysical properties in nonplanar porphyrin diacids (diprotonated forms of free base tetraphenylporphyrin and octaethylporphyrin) was provided by Chirvony et al.12b In their investigation, porphyrin diacids exhibit broadened optical bands, larger Stokes shifts, and reduced fluorescence lifetimes compared with planar neutral forms. These features are the same as that of twisted neutral [30]heptaphyrin 1 and [38]nonaphyrin 2. Chirvony et al. explained that different properties of porphyrin diacids come from their nonplanar conformations and conformational flexibility. Because conformational flexibility can allow the molecule to access multiple conformations in excited electronic state, they suggested the funnel point where internal conversion is enhanced due to small energy gap between ground and excited state, and thus reduces S1 state lifetimes in porphyrin diacids. Accordingly, their investigation can support our interpretation that the change of photophysical properties in 1p and 2p is induced by their conformational changes. Two-Photon Absorption Cross-Section Values. The conformational change from twisted to planar structure causes a difference in the degree of p-orbital overlap, which leads to a change in the extent of π-electron delocalization. Since twophoton absorption phenomenon, third order nonlinear optical property, is well known to be sensitive to π-electron delocalization, we have measured the TPA cross-section values of neutral and protonated expanded porphyrins by using femtosecond open aperture Z-scan method in conjunction with molecular planarity (Supporting Information, Figure S6). In order to obtain the maximum TPA cross-section values without any linear absorption contribution, we have examined the twophoton excitation wavelengths in the wavelength-doubled region of B-like bands for neutral and protonated forms in which there is no one photon absorption. Consequently, the TPA crosssection values of neutral forms 1 and 2 were determined to be 1350 and 1300 GM, respectively. On the contrary, in protonated forms 1p and 2p, much larger TPA cross-section values of 6300 and 6040 GM were obtained (Table 1). The TPA cross-section value of 1 in chloroform was also increased by adding TFA from 1290 to 5680 GM (Supporting Information, Figure S6a and Table S2). The enlarged TPA cross-section value of protonated forms qualitatively indicates that as the structural planarity is enhanced by protonation, π-electrons are delocalized more effectively, resulting in larger TPA cross-section values. However, more detailed analyses are needed for the conspicuous enlargement of TPA cross-section values of protonated forms. On the basis of the second-order perturbation theory, TPA cross-section, σ(2) can be derived from a simple three-level approximation as shown in eq 123
σ (νp) ) (2)
Aν2p
|µi0 | 2 |µfi | 2 2 (νi0 - νp)2 + Γi0
g(2νp)
(1)
where A is a combination of the universal constants, refractive index of the medium, and a factor describing the mutual orientation of transition dipole moments (µi0 and µfi). The normalized Lorentzian line shape function for the two-photon transition, g(2νp) can be expressed as shown in eq 2
g(2νp) )
Γf0 1 π (2ν - ν )2 + Γ2 p f0 f0
(2)
5800 J. Phys. Chem. B, Vol. 113, No. 17, 2009 where νp is the incident photon frequency, νmn and Γmn are the frequency and homogeneous line width of the m f n transition (m, n ) ground (0), intermediate (i), and final (f) states). In eq 1, if the incident photon frequency (νp) is tuned close to the energy of intermediate state and/or the transition dipole moment (µi0) becomes larger, the TPA cross-section values increase.24 When the TPA occurs near the Soret-like band regions, the Q-like bands of expanded porphyrins play a significant role because they can act as an intermediate state in the TPA process.25 One can find that the Q-like bands of the lowest energy in [30]heptaphyrin 1 and [38]nonaphyrin 2 become stronger by addition of acid showing hypsochromic shifts (Figure 3). Therefore, we can conclude that the strong Q-like bands bring about the enlarged transition dipole moment (µi0) in eq 1 and provide the enhanced TPA cross-section values of 1p and 2p. The enlargement of TPA values is governed by electronic structure of molecule as well as structural planarity. Geometry Optimization and NICS Values. Because the NICS values are nowadays widely accepted as a quantitative measure of aromaticity in π-conjugated molecules, even in expanded porphyrins,6,26 we have calculated the NICS(0) values of [30]heptaphyrins and [38]nonaphyrins to confirm whether the aromaticity is affected by molecular planarity or not in a quantitative manner. In π-conjugated cyclic molecules, the magnitude of magnetic shielding or deshielding can be computed as a NICS value at the unweighted geometric center of molecule and the negative value indicates the aromatic character. At first, according to the definition of NICS value, we have optimized the molecular geometries by DFT calculations at the B3LYP level with 6-31G* basis set. While the X-ray crystal structures were used for the geometry optimization in the cases of 1p and 2, the expected structures based on NMR spectroscopy were used for the geometry optimization of 1 and 2p due to the absence of X-ray crystal structures. In order to examine the reliability of the optimized structures of 1 and 2p, their 1H NMR spectra and oscillator strengths were also calculated. As a result, the optimized geometry of [30]heptaphyrin 1 and protonated [38]nonaphyrin 2p exhibit twisted and rather planar structures, respectively, as expected by 1H NMR analysis (Figures 6b and 7b). Particularly, the optimized structure of 2p has five pyrroles pointing inward and four pyrroles pointing outward, which is consistent with 1H1H-COSY spectroscopy (Supporting Information, Figure S7). Since the calculated chemical shifts of inner-/ outer-NH and β-CH protons in these optimized geometries revealed a linear correlation with the experimental chemical shifts (Figures 8 and Supporting Information, S8), we believe that the real molecular structures are not far from these optimized structures even though the optimization procedures have not started from X-ray crystal structures. In addition, the optimized structures of protonated [30]heptaphyrin 1p and neutral [38]nonaphyrin 2 are coincident with the planar and figure-eight structures elucidated by X-ray crystal analysis (Figures 6b and 7b). With these optimized structures, we performed TD-DFT calculations to evaluate the oscillator strengths of each sample (Figures 6a and 7a). The calculated transition frequencies show a correlation with the experimental results, especially for the narrow distribution of oscillator strengths, increased intensity in lower energy Soret-like band, and decreased intensity in higher energy Soret-like band in protonated forms, which also supports the reliability of the optimized structures. After geometry optimization, we have calculated NICS(0) values at the central position of the unweighted geometric mean
Shin et al.
Figure 6. Calculated (a) oscillator strength and (b) NICS(0) values (in ppm) of neutral (top in panel a, left in panel b) and protonated (bottom in panel a, right in panel b) forms of [30]heptaphyrin 1 based on the optimized structures. To show clearly, meso-pentafluorophenyl rings are substituted for hydrogen atoms after the optimization procedures.
Figure 7. Calculated (a) oscillator strength and (b) NICS(0) values (in ppm) of neutral (top in panel a, left in panel b) and protonated (bottom in panel a, right in panel b) forms of [38]nonaphyrin 2 based on the optimized structures. To show clearly, meso-pentafluorophenyl rings are substituted for hydrogen atoms after the optimization procedures.
in macrocyclic molecules as well as at the additional positions (Figures 6b and 7b). Although [30]heptaphyrin 1 and [38]nonaphyrin 2 carries [4n + 2] π-electrons in π-conjugation pathway, the NICS(0) values are in the range of small negative values (-3.1 ∼ -7.8 ppm, -8.7 ppm in the cavity center of macrocycle for 1 and -0.4 ∼ -7.1 ppm, -8.7 ppm in the cavity center of macrocycle for 2) indicating the weak aromaticity due to their highly distorted structures. Upon protonation, the efficient ring current is induced by their planar structures and the NICS(0) values in protonated forms become negatively larger as -25.5 ∼ -26.7 ppm (-14.3 ppm in the cavity center of macrocycle) for 1p and -18.8 ∼ -21.9 ppm (-11.5 ppm in the cavity center
meso-Aryl Substituted Expanded Porphyrins
J. Phys. Chem. B, Vol. 113, No. 17, 2009 5801
Figure 8. (a) Experimental (top) and calculated (bottom) 1H NMR chemical shifts of protonated [38]nonaphyrin 2p and linear correlations between them of (b) β-CH protons and (c) NH protons. Experimental 1H NMR spectrum was measured by adding 2 equiv of MSA to [38]nonaphyrin 2 in THF-d8. * indicates solvent peaks. The calculated 1H NMR chemical shifts were obtained by GIAO method at the B3LYP/6-311G** level based on geometry optimized structure. Solid lines indicate the best linear fit.
of macrocycle) for 2p. Even though the NICS values at the molecular center increase by a small magnitude, that is enough to verify the enhancement of ring current because the values at the central positions include the effect of difference in distance from center to π-conjugation pathways; the central positions in both 1 and 2 are located near the molecular frame due to twisted conformations compared to their planar protonated forms 1p and 2p. In line with this, the significantly increased NICS values at meso-positions in which the distances between probed positions and molecular frame are not variable depending on the conformational change clearly show the enhancement of ring current. As expected by 1H NMR spectroscopies, the NICS values calculated at various positions of neutral and protonated forms of 1 and 2 are in a good agreement with that the aromaticity increases through molecular planarization by protonation, otherwise they exhibit the weak aromatic character due to their distorted structures in neutral state. Summary All observed photophysical properties of meso-hexakis(pentafluorophenyl) [30]heptaphyrin (1.1.1.1.1.1.0) 1 and mesohexakis(pentafluorophenyl) [38]nonaphyrin (1.1.0.1.1.0.1.1.0) 2 clearly represent that the molecular conformations are changed
from twisted to planar structures by protonation. Upon protonation, the photophysical properties are changed remarkably and all experimental results are in accord with the enhanced rigidity and planarity; for example, relatively sharp absorption bands, decreased Stokes-shift, longer excited-state lifetimes and much larger TPA values of protonated forms in 1 and 2 compared with their distorted neutral counterparts. Finally, the enhanced molecular planarity influences the aromaticity and gives rise to large negative NICS values. Our study demonstrates that the distorted molecular conformations of expanded porphyrins, which are unavoidable especially as the number of pyrrole rings increases, can be controlled by protonation and anion binding leading to the planar structures to increase the aromaticity determined by Hu¨ckel’s [4n + 2] rule. This study will give us further insight into understanding of the relationships between molecular planarity, photophysical properties, and aromaticity. Acknowledgment. The work at Yonsei University was financially supported by the Star Faculty and World Class University (WCU) Programs (2008-8-1955) of the Ministry of Education, Science, and Technology and the AFSOR/AOARD Grant (FA4869-08-1-4097). The work at Kyoto University was supported by Grant-in Aids for Scientific Research (Nos. 19205006 and 18655013) from MEXT. J.-Y.S., J.M.L., Z.S.Y.,
5802 J. Phys. Chem. B, Vol. 113, No. 17, 2009 K.S.K., and M.-C.Y. acknowledge the fellowship of the BK 21 program from the Ministry of Education, Science, and Technology of Korea. S.H. and S.S. acknowledge the Research Fellowships of the JSPS for Young Scientists. The quantum calculations were performed by using the supercomputing resource of the Korea Institute of Science and Technology Information (KISTI). Supporting Information Available: 1H NMR spectra, photophysical data of 1 and 1p treated with TFA in chloroform, triplet excited-state decay profiles, open-aperture femtosecond Z-scan traces, optimized structure of 2p, and correlation plot of chemical shifts. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (a) Sessler, J. L.; Davis, J. M. Acc. Chem. Res. 2001, 34, 989. (b) Srinivasan, A.; Furuta, H. Acc. Chem. Res. 2005, 38, 10. (c) Chandrashekar, T. K.; Venkatraman, S. Acc. Chem. Res. 2003, 36, 676. (d) Misra, R.; Chandrashekar, T. K. Acc. Chem. Res. 2008, 41, 265. (2) (a) Jasat, A.; Dolphin, D. Chem. ReV. 1997, 97, 2267. (b) Sessler, J. L.; Camiolo, S.; Gale, P. A. Coord. Chem. ReV. 2003, 240, 17. (ba) Lash, T. D. Angew. Chem., Int. Ed. 2000, 39, 1763. (c) Sessler, J.; Seidel, D. Angew. Chem., Int. Ed. 2003, 42, 5134. (d) Furuta, H.; Maeda, H.; Osuka, A. Chem. Commun. 2002, 1795. (3) (a) Maiya, B. G.; Harriman, A.; Sessler, J. L.; Hemmi, G.; Murai, T.; Mallouk, T. E. J. Phys. Chem. 1989, 93, 8111. (b) Bonnet, R. Chem. Soc. ReV. 1995, 24, 19. (4) (a) Sessler, J. L.; Mody, T. D.; Hemmi, G. W.; Lynch, V.; Young, S. W.; Miller, R. A. J. Am. Chem. Soc. 1993, 115, 10368. (b) Caravan, P.; Ellison, J. J.; McMurry, T. J.; Lauffer, R. B. Chem. ReV. 1999, 99, 2293. (5) (a) Rath, H.; Sankar, J.; PrabhuRaja, V.; Chandrashekar, T. K.; Nag, A.; Goswami, D. J. Am. Chem. Soc. 2005, 127, 11608. (b) Ahn, T. K.; Kwon, J. H.; Kim, D. Y.; Cho, D. W.; Jeong, D. H.; Kim, S. K.; Suzuki, M.; Shimizu, S.; Osuka, A.; Kim, D. J. Am. Chem. Soc. 2005, 127, 12856. (6) (a) Yoon, Z. S.; Kwon, J. H.; Yoon, M.-C.; Koh, M. K.; Noh, S. B.; Sessler, J. L.; Lee, J. T.; Seidel, D.; Aguilar, A.; Shimizu, S.; Suzuki, M.; Osuka, A.; Kim, D. J. Am. Chem. Soc. 2006, 128, 14128. (b) Tanaka, Y.; Saito, S.; Mori, S.; Aratani, N.; Shinokubo, H.; Shibata, N.; Higuchi, Y.; Yoon, Z. S.; Kim, K. S.; Noh, S. B.; Park, J. K.; Kim, D.; Osuka, A. Angew. Chem., Int. Ed. 2008, 47, 681. (c) Park, J. K.; Yoon, Z. S.; Yoon, M.-C.; Kim, K. S.; Mori, S.; Shin, J.-Y.; Osuka, A.; Kim, D. J. Am. Chem. Soc. 2008, 130, 1824. (d) Lim, J. M.; Yoon, Z. S.; Shin, J.-Y.; Kim, K. S.; Yoon, M.-C.; Kim, D. Chem. Commun. 2009, 261. (7) (a) Bucher, C.; Seidel, D.; Lynch, V.; Sessler, J. L. Chem. Commun. 2002, 328. (b) Werner, A.; Michels, M.; Zander, L.; Lex, J.; Vogel, E. Angew. Chem., Int. Ed. 1999, 38, 3650. (c) Sprutta, N.; Latos-Graz˙yn´ski, L. Chem.sEur. J. 2001, 7, 5099. (d) Shin, J.-Y.; Fruta, H.; Yoza, K.; Igarashi, S.; Osuka, A. J. Am. Chem. Soc. 2001, 123, 7190. (e) Setsune, J.-i.; Katakami, Y.; Iizuna, N. J. Am. Chem. Soc. 1999, 121, 8957. (8) (a) Seidel, D.; Lynch, V.; Sessler, J. L. Angew. Chem., Int. Ed. 2002, 41, 1422. (b) Shimizu, S.; Taniguchi, R.; Osuka, A. Angew. Chem., Int. Ed. 2005, 44, 2225. (9) (a) Shimizu, S.; Shin, J.-Y.; Furuta, H.; Ismael, R.; Osuka, A. Angew. Chem., Int. Ed. 2003, 42, 78. (b) Shimizu, S.; Aratani, N.; Osuka, A. Chem.sEur. J. 2006, 12, 4909. (10) Arand, V. G.; Saito, S.; Shimizu, S.; Osuka, A. Angew. Chem., Int. Ed. 2005, 44, 7244.
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