NIR Dual Luminescence from an Extended Porphyrin. Spectroscopy

Article pubs.acs.org/JPCA

NIR Dual Luminescence from an Extended Porphyrin. Spectroscopy, Photophysics and Theory Christophe Gourlaouen,† Chantal Daniel,†,* Fabien Durola,‡ Julien Frey,‡ Valérie Heitz,‡ Jean-Pierre Sauvage,‡ Barbara Ventura,§ and Lucia Flamigni§,* †

Laboratoire de Chimie Quantique, Institut de Chimie de Strasbourg UMR-7177 CNRS-UdS, 1 Rue Blaise Pascal BP 296/R8, F-67008 Strasbourg cedex, France ‡ Laboratoire de Chimie Organo-Minérale, Institut de Chimie de Strasbourg UMR-7177 CNRS-UdS, 4 Rue Blaise Pascal, F-67000 Strasbourg, France § Istituto ISOF-CNR, via P. Gobetti 101, 40129 Bologna, Italy S Supporting Information *

ABSTRACT: Spectroscopic and photophysical properties of an extended Zn porphyrin with fused bis(tetraazaanthracene) arms including a 2,9-diphenyl1,10-phenanthroline incorporated in a polyether macrocycle are investigated in solvents of different polarity pointing to the presence of two emitting singlet excited states. The absorption and emission features are identified and ascribed, on the basis of solvent polarity dependence, to a π−π* and to a charge transfer (CT) state, respectively. Whereas the intraligand π−π* transition is assigned to the intense absorption observed at 442−455 nm, the CT states contribute to the bands at 521−525 nm and 472−481 nm. The theoretical analysis of the absorption spectrum confirms the presence of two strong bands centered at 536 and 437 nm corresponding to CT and π−π* states, respectively. Weak CT transitions are calculated at 657 and 486 nm. Two emission maxima are observed in toluene at 724 nm from a 1π−π* state and at 800 nm from a 1CT state, respectively. 1CT bands shift bathochromically by increasing the solvent polarity whereas the energy of the 1π−π band is less affected. Likewise, the emission yield and lifetime associated with the low energy 1CT band are strongly affected by solvent polarity. This is rationalized by a 1π−π* → 1CT internal conversion driven by solvent polarity, this process being competitive with the 1π−π* to ground state deactivation channel. Time resolved absorption spectra indicate the presence of two triplet states, a short-lived one (nanoseconds range) and a longer lived one (hundreds of microsecond range) ascribed to a 3π−π* and a 3CT, respectively. For them, a conversion mechanism similar to that of the singlet excited states is suggested.

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elsewhere.22 1 has been successfully used as an aromatic plate incorporated in a cyclic [4]rotaxane acting as switchable molecular receptor.22−24 We have recently communicated on the observation of a dual fluorescence from this metalated porphyrin.25 Dual fluorescence is a relatively common phenomenon and it has been observed before in nonsymmetrical metal-free porphyrins in solutions due to a well-known NH tautomerization, yielding isomers characterized by different spectroscopic features.26−29 Several cases of observation of dual fluorescence due to the emission from the upper (S2) and the lower (S1) excited state of porphyrins have also been reported, as well as dual (or even higher order) fluorescence for porphyrins in microheterogeneous solutions, involving encapsulation/complexation. However, to the best of our knowledge, no detailed study of dual

INTRODUCTION

Extended porphyrins are the subject of intense research activity in several fields due to their appealing linear and nonlinear optical properties.1,2 The extended NIR absorption and emission, typical of π expanded porphyrins, in addition to the interesting two-photon absorption (2PA) cross section associated with some of these structures, offers new opportunities for optical materials to be used in light energy conversion or in photodynamic therapy.3−12 In particular, their use in dye-sensitized solar cells (DSSC), has improved the efficiency of light conversion.4 In spite of their high potential as optical materials, photophysical properties of extended porphyrins have been studied only in a limited number of cases.6,13−21 The synthesis of the porphyrin object of the present study, Zn−porphyrin 1, constituted by a central tetraphenyl Zn− porphyrin core extended with two tetraazaanthracene units fused on the back of a 2,9-diphenyl-1,10-phenanthroline incorporated in a macrocycle (Figure 1) has been reported © 2014 American Chemical Society

Received: April 4, 2014 Revised: April 25, 2014 Published: April 25, 2014 3616

dx.doi.org/10.1021/jp5033383 | J. Phys. Chem. A 2014, 118, 3616−3624

The Journal of Physical Chemistry A

Article

Figure 1. Schematic structure of Zn−porphyrin 1.

Scheme 1. Model Molecule 2

resembles the aromatic arms, is on the order of 15 000 M−1 cm−1 in the region 440−460 nm. Quite obviously, the complex pattern of bands displayed by the absorption spectrum of 1, is not the result of a simple superposition of the Zn−porphyrin features and of that of the aromatic arms, but a strong coupling exists between the components of the structure and the extended delocalization imparts new properties to the molecular structure, as formerly reported for porphyrins corefused with large aromatic units.5−12,19−21,32−37 Given the poor solubility of 1 in toluene, the occurrence of association phenomena should be taken into account. In order to verify this, a dilution experiment was performed over 2 orders of magnitude, and the results are reported in Figure 2b. The molar absorption coefficients and spectral shape are not affected by the concentration of 1, excluding thus any effect of association on the spectroscopic features. The spectroscopic properties of 1 are affected by solvent polarity, as can be noticed in Figure 3, where the normalized absorption spectra of 1 in TOL, tetrachloromethane (CCl4), tetrahydrofuran (THF) and dichloromethane (DCM) are shown. The absolute values of molar absorption coefficients for each solvent are also reported in the figure. The absorption spectral features are significantly solvent dependent. They display in more polar THF and DCM, compared to TOL and CCl4, the following features: (i) an increase of the 450 nm with respect to 530 nm molar absorption coefficient ratio; (ii) a strong enhancement in the 430−550 nm absorption; (iii) a broadening and modest enhancement in the 600−900 nm region. It can be stressed that different ground state conformers can be excluded on the basis of the rigidity of 1 and the presence of impurities was excluded by characterization techniques that ensured the high purity of the sample.22

fluorescence corresponding to a single ground state structure and originating from low energy, equilibrated excited states of different nature, has been reported for metalated porphyrin structures in homogeneous solutions. In this report, we expand the spectroscopic and photophysical studies and present the results of a theoretical study that confirm the preliminary assignment of the absorption spectrum.

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RESULTS AND DISCUSSION Information on the structural properties of 1 can be found in the original article dedicated to the related cyclic [4] rotaxanes.22 The measured Zn−N bond lengths of the porphyrin entity in 1 range from 2.034 to 2.183 Å whereas the corresponding theoretical bond distances (2.012−2.128 Å) calculated in the model molecule 2 (Scheme 1) are in good agreement with experimental data. Several conformations have been explored but the lowest one corresponds to the D2h symmetry. Consequently the subsequent TD-DFT calculations have been performed under this symmetry constraint. The absorption spectrum of 1 in toluene (TOL) extends up to 900 nm (Figure 2a) with a molar absorption coefficient of the order of 105 M−1 cm−1 in the range 450−550 nm and of ca. 104 M−1 cm−1 in the 600−800 nm region. These values are well lower than those of the component Zn porphyrin and of a related mono tetraazaanthracenic fused Zn (II) porphyrin ZnB (Figure 2a). Furthermore, the spectrum of 1 is markedly bathochromically shifted in comparison with that of the parent monosubstituted structures (Figure 2a).13−18 On the other hand, the reported absorption of compound 9,11,20,22tetraazatetrapyridopentacene, TATPP,30,31 which somehow 3617



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Figure 4. Theoretical TD-DFT (B3LYP) absorption spectrum of the model complex 2 in a vacuum (Gaussian fwhm = 20).

symmetry (S2, Table 1). Despite a slight blue shift of the lowest band, the overall shape of the theoretical spectrum (Figure 4) of the model complex 2 is in good agreement with the experimental absorption characteristics of 1 (Figure 3). The lowest states S1 and S2 contribute to the experimental shoulder observed between 600 and 800 nm. Whereas the first intense band (S2) at 657 nm corresponds to an excitation from the porphyrin ring to the pyrazino quinoxaline, the next intense peak (S3) calculated at 536 nm is described mainly by an excitation from the lateral phenoxy groups to the pyrazino quinoxaline with a contribution of an intra porphyrin π−π* excitation. The change in electronic densities when going from the electronic ground state to the excited states is depicted in Figure 5 for the most intense S2, S3, S4, and S5 singlet states of 2. The band calculated at 536 nm (S3) is attributed to the intense peak observed at 541 nm (Figure 3). Similarly to the experimental spectrum of 1 the theoretical spectrum of 2 is characterized by intense absorption between 550 and 450 nm with two intense bands calculated at 486 nm (S4) and 437 nm (S5). The band at 486 nm corresponding to an excitation from the lateral phenoxy groups to the pyrazino quinoxaline is assigned to the shoulder observed at 487 nm and the second peak at 437 nm reproduces the intraporphyrin π−π* band observed at 449 nm (Figure 3). The absorption wavelengths of the corresponding neutral porphyrin free base are reported in Table 1 for comparison (values in parentheses). The lowest energy transitons are blueshifted but can be assigned to the electronic densities depicted in Figure 5. The S3 state calculated at 536 nm in the Zn metalated porphyrin 1 and corresponding to a mixing between the localized porphyrin ππ* excitation and the excitation from the lateral phenoxy groups to the pyrazino quinoxaline (Figure 5b) is splitted into two peaks in the porphyrin free base, calculated at 525 and 500 nm. Indeed, the blue shift of the porphyrin ππ* excitation in the free base does not allow the state mixing observed in complex 1 and leads to these distinguishable peaks not observed in the experimental spectrum depicted in Figure 3. Both the structured and unstructured parts of the experimental spectrum can be assigned with confidence to Zn metalated porphyrin 1 that differs significantly from the one of the porphyrin free base. The most distinguishable important features are (i) the single band centered at 536 nm assigned to S3 instead of two peaks in

Figure 2. (a) Absorption spectrum in TOL of 1 (thin black line), ZnB (gray line) and ZnP (thick black line). (b) Molar absorption coefficient of 1 calculated from solutions spanning 2 orders of magnitude.

Figure 3. Absorption spectra of 1 normalized at 527 nm in CCl4 (open circles), TOL (dash), THF (black line) and DCM (gray line).

The theoretical absorption spectrum of 2 calculated in vacuum is depicted in Figure 4 and absorption wavelengths are reported in Table 1 and Table S1 (Supporting Information). The theoretical spectrum based on TD-DFT calculations using PBE functional is represented in Figure S1. Because of a better overall agreement between TD-DFT (B3LYP) theoretical spectrum of model complex 2 and the experimental one of 1 the spectrum depicted in Figure S1 is not discussed. The theoretical spectrum starts at about 780 nm with a very weak shoulder composed of a low-lying 1B2u state (S1, Table 1), the most intense peaks starting at 657 nm and belonging to B3u 3618



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Table 1. Theoretical Absorption Wavelengths (in nm) to the Low-Lying 1B1u, 1B2u, and 1B3u States of the Model Complex 2 and Corresponding Oscillator Strengths f (Only the Lowest S1 State and the States with Oscillator Strengths Greater Than 0.1 Are Reported) stte

symmetry

one-electron excitationa

absorption wavelengthb

f

S1 S2 S3 S4 S5 S6 S7

B2u B3u B3u B3u B3u B2u B3u

πp → π*pq πp → π*pq πphen → π*pq/πp → π*p πphen → π*pq πp → π*p πp → π*p/πp → π*pq/πpq → π*pq πphen → π*p

784 (680) 657 (620) 536 (525, 500) 486 (460) 437 (437) 422 401

0.031 0.275 3.014 0.297 2.938 0.193 0.378

Key: πp, porphyrin ring localized; πpq, pyrazino quinoxaline localized; πphen, phenoxy groups localized. bAbsorption wavelengths calculated for the corresponding porphyrin free base are given in parentheses. a

Figure 5. Change in electronic densities when going from the electronic ground state to the excited state: (a) S2 state calculated at 657 nm; (b) S3 state calculated at 536 nm; (c) S4 state calculated at 486 nm; (d) S5 state calculated at 437 nm. Color code: in red, decrease of electronic density; in green, increase of electronic density.

the porphyrin free base and (ii) the lowest energy absorption starting at 780 nm assigned to S1 that is blue-shifted to 680 nm in the porphyrin free base. Obviously, this does not exclude the presence of porphyrin free base in the solution but confirms the assignment of the experimental spectrum to the Zn metalated porphyrin. The fluorescence spectra of 1 in the various solvents are displayed in Figure 6. They extend in the NIR up to 1200 nm and the luminescence quantum yield, of the order of 10−2 in apolar CCl4 and TOL, drops by 2 orders of magnitude in more polar THF and DCM (Table 2). By inspection of Figure 6, two different types of bands are identified: one at 724 nm, whose position is independent of solvent polarity, and another set of bathochromically shifted bands (around 800 and 900 nm in apolar solvents) whose energy shifts to lower values upon increasing the polarity of the solvent. In DCM the 724 nm band disappears and only a weak, broad and structureless emission with maximum at 890 nm is left (Table 2). We will refer to the two different types of bands as “high energy” (HE) and “low energy” (LE) bands.

Figure 6. Corrected emission spectra of optically matched solutions of 1 upon excitation at 450 nm in CCl4 (open circles), TOL (dash), THF (black line), and DCM (gray line). The spectra in THF and DCM are multiplied by 15 and 10 respectively.

Excitation spectra collected on the different emission bands in the less polar solvents, where the dual emission features are 3619



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In order to get a further insight on the nature of the absorption and emission bands, we performed an experiment aiming at engaging the phenantroline-polyether macrocycle in a metal complexation. This should alter electronic distribution at the extremities of the extended arms and it is therefore expected to affect transitions involving electron density changes at the coordination site. We therefore designed an experiment where a Zn2+ salt was added to a DCM solution of 1. The results obtained for addition of up to 10 equiv are represented in Figure 8. The experiments indicate a decrease and a slight red shift of the band localized at 526 nm and an increase of the shoulder localized at 472 nm in the absorption spectrum, whereas the main band at 449 nm is apparently unaltered. The results agree rather well with the following theoretical attributions: the band at 449 nm to a π−π* transition (S5), the shoulder at 472 nm and the band at 526 nm to CT transitions from the lateral phenoxy groups to the tetraazaanthracene units (S4 and S3, respectively). Only the transitions involving the peripheral units are expected to be modified by the complexation of the phenoxy groups with Zn2+. As far as luminescence is concerned, addition of a Zn2+ salt reduces the luminescence of the detected LE band to zero (Figure 8b). This behavior leads to ascribe the LE emission to a CT singlet excited state involving the peripheral groups. It would be tempting to make a direct correlation with the absorption band at 536 nm, in agreement with the excitation spectra scenario previously discussed, but the nature of the emission that originates from a relaxed state, does not allow such a direct assignment. Any attempt to perform a similar experiment on a low polarity solvent, which being characterized by an higher emission yield might give more reliable indication on both LE and HE emission bands, failed because of solubility problems of both the Zn2+ salt and/or 1. Time resolved luminescence studies were performed with a fast streak camera allowing picosecond resolution and with a spectral sensitivity extending up to 850 nm. In Figure 9, the streak camera images registered in the experiments are displayed; the time profiles in the lower part of the figure show both the 724 and the 800 nm bands in the various solvents. They clearly display different lifetimes from each other, with longer lifetimes for the HE bands. Furthermore, one can notice that the polarity of the solvent affects the lifetime of both bands, this is in fact reduced in passing from the lower polarity to the higher polarity solvents, however this effect is

Table 2. Luminescence parameters of 1 in CCl4, TOL, THF and DCM. The dielectric constant ε is reported ε

λmax (nm)a

ϕflb

τ (ns)c

Ed (eV)

CCl4

2.24

1.7 × 10−2

TOL

2.38

THF

7.58

DCM

8.93

724 786; 881; 982 724 795; 888; 997 (sh) 726 819; − − 890

4.2 2.6 4.0 1.9 0.93 0.15 − 0.15

1.71 1.58 1.71 1.56 1.71 1.51 − 1.40

9.8 × 10−3 1.1 × 10−4 1.1 × 10−4

a

From corrected emission spectra. bFluorescence quantum yields, excitation at 450 nm. cExcitation at 465 or 532 nm. dFrom emission maxima.

better resolved, reveal that different transitions are responsible for the observed luminescence: the emission band at 724 nm arises from absorption mainly in the 400−450 nm region whereas the absorption transition at 527 nm is primarily responsible for the LE bands (Figure 7). The dual behavior is

Figure 7. Arbitrarily scaled corrected excitation spectra of 1 collected at 720 and 800 nm in TOL. The arbitrarily scaled absorption spectrum is also reported (black line).

clearly observed on the spectral distribution upon excitation of 1 at different wavelengths in TOL both at room temperature and in rigid solvent at 77 K. Excitation at 520 nm leads to a spectrum where the emission band at 724 nm is significantly reduced with respect to the same spectrum recorded upon excitation at 450 nm (Figure S2).

Figure 8. Absorption (a) and emission (b) spectra of a DCM solution of 1 (1 × 10−6 M) after titration with ZnSO4·7H2O up to 10 equiv. Excitation was on an isosbestic point at 447 nm. 3620



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Figure 9. Streak camera images of the luminescence of 1 in CCl4, TOL, and THF. In the lower frames are reported the corresponding time profiles collected at the specified wavelength with the exponential fitting as gray lines.

much more important for the LE band than for the HE band (Table 2 and Figure 9). The lifetime values of the HE band are 4.2, 4.0, and 0.93 ns in CCl4, TOL and THF respectively, whereas those of the LE band are 2.6, 1.9, 0.15, and 0.15 ns in CCl4, TOL, THF, and DCM, respectively. No HE band was detected in DCM (see Figure 6). The observed behavior can be explained in terms of two singlet excited states. We assign a π−π* nature to the HE band on the basis of the fact that the energy, and to some extent also the lifetime (see below for an explanation) of the HE band is not affected by the polarity of the solvent. On the other hand, the extreme sensitivity of both the energy and the lifetime on the solvent polarity for LE bands, indicates a state with a large charge transfer (CT) character, with a large dipole moment change upon excitation. Furthermore, on the basis of the previously discussed excitation spectra and titration experiments, the main associated absorption bands are identified around 450 nm with the π−π* and at 470 and 520−530 nm with the CT transitions, respectively. The separation between the bands is obviously not so well-defined and a certain degree of overlap is present. On the basis of the excitation spectra collected at 720 and 800 nm (Figure 7) one can observe that both CT and π−π* components contribute to the absorption in the 600−900 nm region. The experimental interpretation is confirmed by theory that put in evidence a mixed character of the S3 state calculated at 536 nm involving CT from the arms as well as porphyrin centered excitations. Accordingly, the absorbing singlet state calculated at 437 nm (S5) is purely localized on the porphyrin ring (Figure 5) in agreement with the experimental features. The behavior of the π−π* emission band whose lifetime is still affected by polarity, contrary to expectations, can be rationalized by considering the scheme of Figure 10. The difference between the HE (π−π*) and LE (CT) excited state energy levels increases with the polarity of the solvent, which

Figure 10. Schematic energy level illustration and reactivity of the lowest singlet and triplet excited states of 1 in solvents of different polarity.

more and more stabilizes the latter. Whereas in an apolar solvent the driving force for conversion of the HE to LE state is low, and therefore the reaction is slow, when the LE (CT) state is stabilized in polar solvents, the driving force for conversion of HE to LE increases and the reaction competes more effectively with the ground state deactivation and decreases the lifetime of the HE state. The limit is in the DCM case, when the driving force for the π−π* → CT is so high that the conversion is very fast, the HE state decays completely via the LE state and no emission is detect from HE state. On the contrary, the effect of polarity on the lifetime of the LE (CT) state is expected and can be rationalized by the energy gap low: decreasing the energy of a state increases the internal conversion rate to the ground state. A former report on the photophysical properties of a simpler Zn(II) monotetraazaanthracenic porphyrin (ZnB) has provided 3621



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emissive properties of 1 as deduced from the experiments described above.

experimental and theoretical evidence for the presence of an emitting state CT in nature.13 In agreement with the present case the solvent effect on this luminescence was remarkable not only for bathochromic shift but also for the reduction of the emission quantum yield and lifetime.13 To shed more light on the peculiar photophysics of 1, laser flash photolysis experiments where performed in two air-free solvents, TOL and DCM, chosen as cases for lower and higher polarity, respectively. In air free solvents a long-lived species, τ = 220 μs in TOL and 105 μs in DCM, was observed. In air equilibrated solutions the species was quenched to a lifetime of 350 ns in TOL and 480 ns in DCM. This allows to derive a reaction rate with oxygen, kox, of the order of 1−1.5 × 109 M−1 s−1. The spectrum is characterized by a weak absorption below 450 nm, between 600 and 700 nm and in the NIR above 800 nm and by strong bleaching features in the region 450−530 nm (Figure 11). In addition, in TOL solutions a second species is

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CONCLUSION The spectroscopy and photophysics of a Zn−porphyrin extended with two tetraazaanthracenic arms containing two 2,9-diphenyl-1,10-phenanthrolines included in a polyether macrocycle have been examined in details. Zn−porphyrin 1 exhibits a complex absorption spectrum extending in the NIR and displaying solvent dependence. The experimental absorption spectrum of 1 has been assigned on the basis of TD-DFT calculations performed for a model system 2. The agreement between the theoretical and experimental spectra is fine. It has been shown that CT transitions involving units of the arms play a role in the emissive properties. The photophysical characterization of porphyrin 1 has provided evidence for the existence of two nonequilibrated singlet excited states emitting in the NIR range. They are close in energy but characterized by significantly different dipole moments. On the basis of the experimental results, the high energy one (724 nm) is identified as a 1π−π* and the low energy one (800−890 nm) as a 1 CT state, respectively. The character of the relevant absorbing states deduced from theory confirms this analysis. The collected data are rationalized on the basis of a deactivation of the 1π−π* through the 1CT state, a process favored in polar solvents. Two different triplets have also been identified, ascribed to a 3π−π* and a 3CT state, for which a deactivation scheme similar to that of the singlet manifold is suggested.

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EXPERIMENTAL SECTION

Photophysics. Solvents used for photophysical determinations were spectroscopic grade (C. Erba). Absorption spectra were recorded in diluted solutions by using a Perkin−Elmer Lambda 950 UV/vis/NIR spectrophotometer. The determination of molar absorption coefficients in the different solvents has been performed by different methods and at various concentrations, due to the low solubility of 1 in apolar solvents. In TOL the solubility was very low, of the order of 5 × 10−6 M. Emission spectra were measured on sample solutions with A ≤ 0.1 at the excitation wavelength in right-angle mode by using a NIR FLS920 spectrofluorimeter (Edinburgh) equipped with an Hamamatsu R5509−72 supercooled photomultiplier tube at 193 K and a TM300 emission monochromator with a grating blazed at 1000 nm. Corrected spectra were employed throughout this work, unless otherwise specified, by applying a correction curve of the wavelength dependent phototube response to the raw data. Luminescence quantum yields in solution were evaluated by comparing the wavelength integrated densities (I) with reference to [Os(ttpy)2]2+ (ϕr = 0.021, O2-free butyronitrile, ttpy is tolylterpyridine),38 by using the following equations:39 ϕem = ϕr (Arn2I/nr2IrA) in which r stands for reference, and A and n are the absorbance values at the employed excitation wavelength and refractive index of the solvent, respectively. The 1 cm optical quartz cuvettes were used for measurements at room temperature, and quartz capillary tubes immersed in liquid nitrogen in a homemade quartz Dewar were used for measurements in frozen media at 77 K. Absorption and emission titrations of 1 (1 × 10−6 M) in DCM with ZnSO4. 7H2O have been performed by adding small aliquots of a MeOH solution of the salt to the solution of 1, with a final added amount not exceeding 2% in volume.

Figure 11. Transient absorption spectra of 1 in DCM (triangles) and in TOL (full circles) detected 40 μs after the laser pulse. The spectrum registered in TOL 60 ns after the pulse (open circles) is also shown, ΔA is divided by 5. λexc= 532 nm, A = 0.79, and energy = 3.0 mJ/pulse.

detectable on a faster time-scale. This species, with a lifetime of 87 ns in air purged solutions, presents a broad and almost featureless absorption all over the spectral range, except for the bleaching bands in the region 450−530 (Figure 11). The lifetime is reduced in air equilibrated solutions to 50 ns, which allows to derive a reaction rate with oxygen kox= 5 × 109 M−1 s−1. Both the slow and the fast species can be identified as triplets, on the basis of their reactivity with oxygen. They can be identified as the lowest triplets corresponding to the 1π−π* and to the 1CT excited states discussed above. In agreement with the absence of the “fast” triplet in polar DCM, we identify the fast species as a 3π−π*state and the slow as a 3CT state. The lifetime of this 3π−π* is surprisingly short for a triplet and we propose that it is the results of a conversion toward the lower 3 CT state, likewise the mechanism proposed for the corresponding singlet (Figure 10). A theoretical analysis of the photophysics should be based on the calculation of the absorption spectrum taking into account the solvent effects. Moreover, a more realistic model of the system should be able to reflect the nuclear relaxation effects in the S3, S4 and S5 singlet excited states calculated at 536, 486, and 437 nm, respectively, especially in the CT states from the peripheral units, namely S3 and S4 (Figure 5). This is beyond the scope of the present study. However, the calculations confirm the character of the transitions responsible for the 3622


The Journal of Physical Chemistry A Luminescence lifetimes were determined by an apparatus based on a Nd:YAG laser (Continuum PY62−10) with excitation at 532 nm, 35 ps pulse duration, 1.7 mJ/pulse, and a Streak Camera (Hamamatsu C1587 equipped with M1952). More details can be found elsewhere.40,41 The overall time resolution of the system after deconvolution procedure is 5 ps.42 Luminescence lifetimes in the nanosecond range were also measured with higher accuracy by using an IBH 5000F timecorrelated single-photon counting device, by using a pulsed NanoLED excitation source at λ = 465 nm. Analysis of the luminescence decay profiles against time was accomplished with the Decay Analysis Software DAS6 provided by the manufacturer. Laser flash photolysis in the nanosecond range was performed by a Nd:YAG (JK) laser (18 ns pulse duration, 532 nm, 3.0 mJ/pulse). More details on the transient absorption apparatus can be found elsewhere.43 Estimated errors are 2 nm on band maxima and 20% on quantum yields and 10% on the lifetimes. Computational Details. The synthesized Zn−porphyrin 1 has been modeled by the molecule 2 depicted below (Scheme 1) where the poly(ethylene glycol) (PEG) loops have been replaced by −OCH3 groups. The structure of 2 has been fully optimized at the DFT/B3LYP level of theory.44−46 Both out-ofplane and planar D2h geometries were investigated. The theoretical absorption spectrum of the model system 2 has been computed by means of TD-DFT47 using B3LYP functional with all-electrons double-ζ quality basis sets. The emission properties of 1 have not been investigated theoretically for two main reasons: (i) the size of the real system; (ii) the limitation of the model system. Indeed in order to interpret the experimental luminescence spectra it is necessary to investigate the change of structure when going from the electronic ground state to the low-lying singlet and triplet states. This type of calculations is feasible nowadays for transition metal complexes with ∼100 atoms. Moreover the flexibility of the system investigated here will complicate the theoretical study and whereas the model molecule 2 is acceptable for investigating electronic ground state characteristics and the absorption spectrum it could be too crude for the calculation of luminescence properties, especially the fluorescence. The calculations have been performed with ADF-2010 quantum chemistry software48 and the electronic transitions have been analyzed with the Dgrid module.49

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ACKNOWLEDGMENTS

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REFERENCES

Funding from Italian CNR (Project PM.P04.010 “MACOL”), Eurocores project “Solarfueltandem” and PRIN 2010CX2TLM are acknowledged. The quantum chemical calculations have been performed thanks to the computer facilities of the High Performance Computing regional centre of University of Strasbourg and on the computer nodes of the LCQS, Strasbourg, France. The COST action Perspect-H2O is acknowledged. We also thank the Agence Nationale de la Recherche (ANR No. 07-BLAN-0174, MolPress) for financial support as well as the French Ministry of Education for fellowships to J.F. and F.D.

(1) Tsuda, A.; Osuka, A. Fully Conjugated Porphyrin Tapes with Electronic Absorption Bands that Reach into Infrared. Science 2001, 293, 79−82. (2) Letwak, J. P.; Gryko, D. T. Synthesis of π-extended Porphyrins via Intramolecular Oxidative Coupling. Chem. Commun. 2012, 48, 10069−10086. (3) Li, L.-L.; Diau, E. W. -G. Porphyrin-Sensitized Solar Cells. Chem. Soc. Rev. 2013, 42, 291−304. (4) Imahori, H.; Iijima, H.; Hayashi, H.; Toude, Y.; Umeyama, T.; Matano, Y.; Ito, S. Bisquinoxaline-Fused Porphyrins for Dye-Sensitized Solar Cells. ChemSusChem 2011, 4, 797−805. (5) Anderson, H. L. Building Molecular Wires from the Colours of Life: Conjugated Porphyrin Oligomers. Chem. Commun. 1999, 2323− 2330. (6) Drobizhev, M.; Stepanenko, Y.; Dzenis, Y.; Karotki, A.; Rebane, A.; Taylor, P. N.; Anderson, H. L. Extremely Strong Near-IR TwoPhoton Absorption in Conjugated Porphyrin Dimers: Quantitative Description with Three-Essential-States Model. J. Phys. Chem. B 2005, 109, 7223−7236. (7) Collins, H. A.; Khurana, M.; Moriyama, E. H.; Mariampillai, A.; Dahlstedt, E.; Balazi, M.; Kuimova, M. K.; Drobizhev, M.; Yang, V. X. D.; Phillips, D.; et al. Blood-Vessel Closure using Photosensitizers Engineered for Two-Photon Excitation. Nat. Photonics 2008, 2, 420− 424. (8) Pawlicki, M.; Collins, H. A.; Denning, R. G.; Anderson, H. L. Two-Photon Absorption and the Design of Two-Photon Dyes. Angew. Chem., Int. Ed. 2009, 48, 3244−3266. (9) Tsuda, A.; Furuta, H.; Osuka, A. Completely Fused Diporphyrins and Triporphyrin. Angew. Chem., Int. Ed. 2000, 39, 2549−2552. (10) Kim, K. S.; Lim, J. M.; Osuka, A.; Kim, D. Various Strategies for Highly-Efficient Two-Photon Absorption in Porphyrin Arrays. J. Photochem. Photobiol. C 2008, 9, 13−28. (11) Nakamura, Y.; Jang, S. Y.; Tanaka, T.; Aratani, N.; Lim, J. M.; Kim, K. S.; Kim, D.; Osuka. Two-Dimensionally Extended Porphyrin Tapes: Synthesis and Shape-Dependent Two-Photon Absorption Properties A. Chem.Eur. J. 2008, 14, 8279−8289. (12) Ikeda, T.; Aratani, N.; Osuka, A. Synthesis of Extremely πExtended Porphyrin Tapes from Hybrid meso-meso Linked Porphyrin Arrays: An Approach Towards the Conjugation Length. Chem.Asian J. 2009, 4, 1248−1256. (13) Flamigni, L.; Marconi, G.; Johnston, M. R. Bis-Porphyrinic Clamp for Photo- and Electro-Active Guests: a Spectroscopic and Photophysical Study. Phys. Chem. Chem. Phys. 2001, 3, 4488−4494. (14) Crossley, M. J.; Sintic, P. J.; Hutchison, J. A.; Ghiggino, K. P. Chemical Models for Aspects of the Photosynthetic Reaction Centre: Synthesis and Photophysical Properties of Tris- and TetrakisPorphyrins that Resemble the Arrangement of Chromophores in the Natural System. Org. Biomol. Chem. 2005, 3, 852−865. (15) Hutchison, J. A.; Bell, T. D. M.; Ganguly, T.; Ghiggino, K. P.; Langford, S. J.; Lokan, N. R.; Paddon-Row, M. N. Photoinduced Electron Transfer Dynamics in Porphyrin Donor Dyads. J. Photochem. Photobiol., A 2008, 197, 220−225.

ASSOCIATED CONTENT

S Supporting Information *

Transition energies, DFT/B3LYP optimized structure of 2, and corrected emission spectra of 1. This material is available free of charge via the Internet at http://pubs.acs.org

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AUTHOR INFORMATION

Corresponding Authors

*(C.D.) Telephone: +33 036 885 1314. E-mail: c.daniel@ unistra.fr. *(L.F.) Telephone: +39 051 639 9812. E-mail: lucia.flamigni@ isof.cnr.it. Notes

The authors declare no competing financial interest. 3623



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(16) Sendt, K.; Johnston, L. A.; Hough, W. A.; Crossley, M. J.; Hush, N. S.; Reimers, J. R. Switchable Electronic Coupling in Model Oligoporphyrin Molecular Wires Examined through the Measurement and Assignment of Electronic Absorption Spectra. J. Am. Chem. Soc. 2002, 124, 9299−9309. (17) Yeow, E. K. L.; Sintic, P. J.; Cabral, N. M.; Reek, J. N. H.; Crossley, M. J.; Ghiggino, K. P. Photoinduced Energy and Electron Transfer in Bis-Porphyrins with Quinoxaline Tröger’s Base and Biquinoxalinyl Spacers. Phys. Chem. Chem. Phys. 2000, 2, 4281−4291. (18) Roberts, D. A.; Fückel, B.; Clady, R. G. C. R.; Cheng, Y. Y.; Crossley, M. J. Synthesis and Ultrafast Excited-State Dynamics of Zinc and Palladium Triply Fused Diporphyrins. J. Phys. Chem. A 2012, 116, 7898−7905. (19) Cho, H. S.; Jeong, D. H.; Cho, S.; Kim, D.; Matsuzaki, Y.; Tanaka, K.; Tsuda, A.; Osuka, A. Photophysical Properties of Porphyrin Tapes. J. Am. Chem. Soc. 2002, 124, 14642−14654. (20) Kim, P.; Ikeda, T.; Lim, J. M.; Park, J.; Lim, M.; Aratani, N.; Osuka, A.; Kim, D. Excited-State Energy Relaxation Dynamics of Triply Linked Zn(II) Porphyrin Arrays. Chem. Commun. 2011, 47, 4433−4435. (21) Heo, J. H.; Ikeda, T.; Lim, J. M.; Aratani, N.; Osuka, A.; Kim, D. Molecular-Shape-Dependent Photophysical Properties of meso-β Doubly Linked Zn(II) Porphyrin Arrays and Their Indene-Fused Analogues. J. Phys. Chem. B 2010, 114, 14528−14536. (22) Collin, J. P.; Durola, F.; Frey, J.; Heitz, V.; Reviriego, F.; Sauvage, J.-P.; Trolez, Y.; Rissanen, K. Templated Synthesis of Cyclic [4] Rotaxanes Consisting of Two Stiff Rods Threaded through Two Bis-macrocycles with a Large and Rigid Central Plate as Spacer. J. Am. Chem. Soc. 2010, 132, 6840−6850. (23) Collin, J. P.; Durola, F.; Heitz, V.; Reviriego, F.; Sauvage, J.-P.; Trolez, Y. A Cyclic [4] rotaxane that Behaves as a Switchable Molecular Receptor: Formation of a Rigid Scaffold from a Collapsed Structure by Complexation with Copper (I) Ions. Angew. Chem., Int. Ed. 2010, 49, 10172−10175. (24) Ventura, B.; Flamigni, L.; Collin, J.-P.; Durola, F.; Heitz, V.; Reviriego, F.; Sauvage, J.-P.; Trolez, Y. NIR Emission of Cyclic [4]Rotaxanes Containing π-Extended Porphyrin Chromophores. Phys. Chem. Chem. Phys. 2012, 14, 10589−10594. (25) Ventura, B.; Durola, F.; Frey, J.; Heitz, V.; Sauvage, J. -P.; Flamigni, L. Near-Infrared Dual Luminescence From an Extended Zinc Porphyrin. Chem. Commun. 2012, 48, 1021−1023. (26) Ermilov, E. A.; Buge, B.; Jasinski, S.; Jux, N.; Roder, B. Spectroscopic Study of NH-Tautomerism in Novel CycloketoTetraphenylPorphyrins. J. Chem. Phys. 2009, 130, 134509−8. (27) Lash, T. D.; Chandrasekar, P.; Osuma, A. T.; Chaney, S. T.; Spence, J. D. Porphyrins with Exocyclic Rings. 13. Synthesis and Spectroscopic Characterization of Highly Modified Porphyrin Chromophores with Fused Acenaphthylene and Benzothiadiazole Rings. J. Org. Chem. 1998, 63, 8455−8469. (28) Mack, J.; Nakamura, J.; Okujima, T.; Yamada, H.; Uno, H.; Kobayashi, N. MCD Spectroscopy and TD-DFT Calculations of LowSymmetry Acenaphthoporphyrins with Dual Fluorescence. J. Porphyr. Phthalocyanines 2013, 17, 996−1007. (29) Uttamlal, M.; Holmes-Smith, A. S. The Excitation Wavelength Dependent Fluorescence of Porphyrins. Chem. Phys. Lett. 2008, 454, 223−228. (30) Chiorboli, C.; Fracasso, S.; Ravaglia, M.; Scandola, F.; Campagna, S.; Wouters, K. L.; Konduri, R.; MacDonnell, F. M. Primary Photoinduced Processes in Bimetallic Dyads with Extended Aromatic Bridges. Tetraazatetrapyridopentacene Complexes of Ruthenium(II) and Osmium(II). Inorg. Chem. 2005, 44, 8368−8378. (31) Guo, W.; Obare, S. O. Tuning the reduction of 9,11,20,22tetraaza-tetrapyridopentacene (TATPP). Tetrahedron Lett. 2008, 49, 4933−4936. (32) Davis, N. K. S.; Thompson, A. L.; Anderson, H. L. BisAnthracene Fused Porphyrins: Synthesis, Crystal Structure, and NearIR Absorption. Org. Lett. 2010, 12, 2124−2127.

(33) Davis, N. K. S.; Pawlicki, M.; Anderson, H. L. Expanding the Porphyrin π-System by Fusion with Anthracene. Org. Lett. 2008, 10, 3945−3947. (34) Jiao, C. J.; Zu, N. N.; Huang, K. W.; Wang, P.; Wu, J. S. Perylene Anhydride Fused Porphyrins as Near-Infrared Sensitizers for DyeSensitized Solar Cells. Org. Lett. 2011, 13, 3652−3655. (35) Xu, H. J.; Mack, J.; Descalzo, A. B.; Shen, Z.; Kobayashi, N.; You, X. Z.; Rurack, K. Meso-Aryl Phenanthroporphyrins: Synthesis and Spectroscopic Properties. Chem.Eur. J. 2011, 17, 8965−8983. (36) Akhigbe, J.; Zeller, M.; Brükner, C. Quinoline-Annulated Porphyrins. Org. Lett. 2011, 13, 1322−1325. (37) Lash, T. D.; Young, A. M.; Rasmussen, J. M.; Ferrence, G. M. Naphthiporphyrins. J. Org. Chem. 2011, 76, 5636−5651. (38) Sauvage, J.-P.; Collin, J.-P.; Chambron, J.-C.; Guillerez, S.; Coudret, C.; Balzani, V.; Barigelletti, F.; De Cola, L.; Flamigni, L. Ruthenium(II) and Osmium(II) Bis(terpyridine) Complexes in Covalently-Linked Multicomponent Systems: Synthesis, Electrochemical Behavior, Absorption Spectra, and Photochemical and Photophysical Properties. Chem. Rev. 1994, 94, 993−1019. (39) Demas, J. N.; Crosby, G. A. Measurement of Photoluminescence Quantum Yields. Review. J. Phys. Chem. 1971, 75, 991−1024. (40) Flamigni, L.; Talarico, A. M.; Serroni, S.; Puntoriero, F.; Gunter, M. J.; Johnston, M. R.; Jeynes, T. P. Photoinduced Electron Transfer between the Interlocked Components of Porphyrin Catenanes: Effect of the Presence of Nonequivalent Reduction Sites on the Charge Recombination Rate. Chem.Eur. J. 2003, 9, 2649−2659. (41) Flamigni, L.; Ventura, B.; Oliva, A. I.; Ballester, P. Energy Migration in a Self-Assembled Nonameric Porphyrinic Molecular Box. Chem.Eur. J. 2008, 14, 4214−4224. (42) Tasior, M.; Gryko, D. T.; Pielachinska, D. J.; Zanelli, A.; Flamigni, L. Trans-A2B-corroles Bearing a Coumarin Moiety - From Synthesis to Photophysics. Chem.Asian J. 2010, 5, 130−140. (43) Flamigni, L.; Marconi, G.; Dixon, I. M.; Collin, J.-P.; Sauvage, J.P. Switching of Electron- to Energy-Transfer by Selective Excitation of Different Chromophores in Arrays Based on Porphyrins and a Polypyridyl Iridium Complex. J. Phys. Chem. B 2002, 106, 6663−6671. (44) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865−68. (45) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1997, 78, 1396−1396. (46) Becke, A. D. Density-Functional Thermochemistry. III. The Role of Exact Exchange. J. Chem. Phys. 1993, 98, 5648−52. (47) Casida, M. E.; Wesolowski, T. A. Generalization of the Kohn− Sham Equations with Constrained Electron Density Formalism and its Time Dependent Response Theory Formulation. Int. J. Quantum Chem. 2004, 96, 577−588. (48) te Velde, G.; Bickelhaupt, F. M.; van Gisbergen, S. J. A.; Fonseca Guerra, C.; Baerends, E. J.; Snijders, J. G.; Ziegler, T. Chemistry with ADF. J. Comput. Chem. 2001, 22, 931−967. (49) Kohout, M. DGrid, version 4.5, Radebeul, 2009.

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NOTE ADDED AFTER ASAP PUBLICATION This article was published ASAP on May 7, 2014, without all of the corrections. The corrected article was published ASAP on May 9, 2014.

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NIR Dual Luminescence from an Extended Porphyrin. Spectroscopy

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