Synthesis and Photophysical Properties of Conjugated and


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Synthesis and Photophysical Properties of Conjugated and Non-Conjugated Phthalocyanine-Perylenediimide Systems Jorge Follana-Berná, Damla Inan, Vicente M. Blas-Ferrando, Natalie Gorczak, Javier Ortiz, Félix Manjón, Fernando Fernández-Lázaro, Ferdinand C. Grozema, and Angela Sastre-Santos J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b07160 • Publication Date (Web): 04 Oct 2016 Downloaded from http://pubs.acs.org on October 7, 2016

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Synthesis and Photophysical Properties of Conjugated and Non-Conjugated Phthalocyanine-Perylenediimide Systems Jorge Follana-Berná§, Damla Inan†, Vicente M. Blas-Ferrando§, Natalie Gorczak†, Javier Ortiz,§ Félix Manjón, § Fernando Fernández-Lázaro§, Ferdinand C. Grozema†*, and Ángela Sastre-Santos§* §

División de Química Orgánica, Instituto de Bioingeniería, Universidad Miguel Hernández, Elche 03202, Spain Department of Chemical Engineering, Delft University of Technology, 2628 BL Delft, The Netherlands Supporting Information Placeholder †

ABSTRACT: The synthesis and characterization of different conjugated phthalocyanine-perylenemonoimidebenzimidazole [ZnPcPBIm(OR)4] and non-conjugated phthalocyanine-perylenediimide [ZnPc-PDI(OR)4] dyads are carried out. UV-vis, 1H-NMR and electrochemistry measurements reveal the interaction between perylene and phthalocyanine moieties in the ground state in the conjugated hybrid and the lack of interaction in the non-conjugated one. Ultrafast transient absorption measurements show that a state with substantial charge transfer character is formed in both compounds, but the rates for the formation and recombination from this state are much are much faster for the conjugated compound.

INTRODUCTION Nowadays, the synthesis of new donor-acceptor systems as molecular materials is becoming an interesting issue due to their outstanding optoelectronic properties,1,2 useful for optical data storage, as electronic component and as light harvesting systems.3 Like artificial photosynthetic analogues they can simulate the photosynthetic reaction center obtaining long-lived chargeseparated states as in the natural photosynthesis process.4 Normally, after photoexcitation of the acceptor subunit an excited state is obtained which can be deactivated through some processes, such as electron transfer process to generate a charge-separated state, potentially usable in electronic devices.5-7 As donor moieties are used phthalocyanines (Pcs), aromatic macrocycles with 18 π-electron with photochemical and thermal stability and intense absorption in the red-near IR region.8-13 Moreover Pc properties can be modified by introduction of metal ions in the central cavity and substituents at α- and/or β-positions. Perylenediimides (PDIs) are used as electron acceptor units because of their chemically, thermally and photochemically stability.14-20 Mostly their acceptor properties can be modified through linked to different electron-withdrawing groups at the bay positions and their solubility through imide position. In addition, they show absorbance in the visible area, about 580 nm, just in a region complementary to that of phthalocyanine absorption. Both units can be connected by a bridge which can help with the photoinduced electron-separation processes and there are some cases of phthalocyanine-PDI systems that obtained a long-lived charge-separated states.21-23 In a previous work, it was synthesized a triad made of two phthalocyanine subunits and a perylenediimide through an alcoxy bridge, achieving a lifetime in the range of picoseconds.23 Recently, adding an electron-donating tertoctylphenoxy group in six peripheral positions of phthalocyanine and two electron-withdrawing p-tolylsulfonyl moieties at

the PDI bay positions it was achieved to stabilize the chargeseparated state in the range of microseconds.24 On the other hand, it was observed how affected the coupling between bisimidazole group in the Pc and PDI, showing an increase of lifetime.25 More recently, a protonated fused bisimidazoleZnPc has been described presenting interesting properties as pH sensor.26 As a continuation of our work, we want to study in a more exhaustive way the influence on the conjugation between Pc and PDI subunits in the energy or/and electronic transference processes. To carry out our goal, two new dyads, ZnPcPBIm(OR)4 1 and ZnPc-PDI(OR)4 2 (Chart 1), have been synthesized. The first one, dyad 1 presents a fully conjugated bridge between Pc and PDI moieties through a benzimidazole ring, and the second one, dyad 2, in which there is no connection between both subunits to be the Pc ring directly bonded to the imide group of the PDI.

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Chart 1. Chemical structures of ZnPc-PBIm(OR)4 1 and ZnPc-PDI(OR)4 2 dyads.

Scheme 2. Synthesis of ZnPc-PBIm(OR)4 1 and ZnPcPDI(OR)4 2 from ZnPc 3 and PMAMI 4.

EXPERIMENTAL SECTION All solvents were used as received from Sigma-Aldrich. H NMR spectra were recorded at 25ºC using a Bruker AC300 spectrometer and it was referenced to tetramethylsilane. The solvent for spectroscopic studies were of spectroscopic grade and used as received. UV-vis spectra were measured with a Helios Gamma spectrophotometer. High resolution mass spectra were obtained from a Bruker Microflex LRF20 matrixassisted laser desorption/ionization time of flight (MALDITOF) using dithranol as matrix. IR spectra were measured with Nicolet Impact 400D spectrophotometer. 2,3,9,10,16,17-hexaquis-[p-(tert-octyl)phenoxy]-22,23diaminophthalocyaninate zinc (II), ZnPc 3,25 and N-(2’ethylhexyl)-1,6,7,12-tetrakis-[4’(1’’,1’’,3’’,3’’tetramethylbutyl)phenoxy]perylene-9,10-dicarboximide-3,4dicarboxyanhydride, PMAMI 4, were synthesized according to the processes described in the literature.27 Synthesis of Dyad ZnPc-PBIm(OR)4 1 and Dyad ZnPcPDI(OR)4 2. 50 mg of ZnPc 3, 17 µL (0.3 mmol) of acetic acid and 40 mg of PMAMI 4 were dissolved in 1.5 mL of NMP and heated to 85ºC under argon atmosphere during 18 h. The crude was diluted with CHCl3 and washed with HCl, NH4Cl (aq) and H2O. The organic layer was dried with MgSO4, concentrated in vacuum and purified by column chromatography (CHCl3:acetone/99:1), affording 15.4 mg (18 %) of ZnPc-PBIm(OR)4 1 as a green solid and 21.9 mg (26%) of ZnPc-PDI(OR)4 2 as a blue solid. Data for dyad 1: 1 H-NMR (300 MHz, THF-d8, 25ºC): δ= 10.38 (s, 1H; Pc), 9.68 (s, 1H; Pc), 9.17 (s, 1H; Pc), 8.97 (s, 4H; Pc), 8.65 (s, 1H; Pc), 8.53 (s, 1H; H-PBIm), 8.13 (s, 1H, H-PBIm), 8.09 (s, 2H, H-PBIm), 7.68-7.41 (m, 20H, phenol), 7.20-6.94 ppm (m, 20H, phenol), 1.94-1.83 (m, 31H), 1.49-1.42 (m, 66H); 0.940.89 (m, 90H). UV-vis (CHCl3): λmax/nm (log Ԑ): 356 (4.74), 627 (4.57), 692 (4.95), 731 (4.64). MS: HR-MALDI-TOF (dithranol): m/z: for C204H239N11O13Zn calcd, 3314.766; found 3314.777. νmax (KBr)/cm-1: 2955, 1728, 1699, 1659, 1506, 1453, 1401, 1272, 1218, 1173, 1092, 1029, 890, 825. Data for dyad 2: 1H-NMR (300 MHz, THF-d8, 25ºC): δ= 9.02 (s, 1H; Pc), 8.97-8.94 (m, 4H; Pc), 8.84 (s, 1H; Pc), 8.82 (s, 1H; Pc), 8.66 (s, 1H; Pc), 8.22 (s, 1H; H-PDI), 8.15 (s, 2H; H-PDI), 8.09 (s, 1H; H-PDI), 7.47-7.36 (m, 20H; phenol), 7-15-6.98 (m, 20H; phenol), 5.64 (t, 2H; NH2-Pc) 1.94-1.82 (m, 31H), 1.43-1.29 (m, 66H), 0.88-0.80 (m, 90H). UV-vis (CHCl3): λmax/nm (log Ԑ): 356 (4.92), 593 (4.61), 619 (4.72), 686 (5.32). MS: HR-MALDI-TOF (dithranol): m/z: for C204H227N11O14Zn calcd, 3132.777; found 3132.793. νmax (KBr)/cm-1: 3440, 1

3379, 2955, 1667, 1703, 1589, 1505, 1453, 1404, 1272, 1216, 1174, 1091, 1030, 1030, 1015, 891. Electrochemical Measurements. Cyclic voltammetry was measured in a conventional three-electrode cell using a µAUTOLAB type III potentiostat/galvanostat at 298K, over benzonitrile and deaerated sample solutions, containing 0.10 M tetrabutylammonium hexafluorophosphate (TBAPF6) as supporting electrolyte. A platinum working electrode, Ag/AgNO3 reference electrode and a platinum wire counter electrode were employed. Ferrocene/ferrocenium couple was used as an internal standard for all measurements. Transient Absorption Spectroscopy. A tunable Yb:KGW laser system is used for pump-probe transient absorption measurements. The systems consists of a Yb:KGW laser (1028nm) which operates at 5 kHz and optical parametric amplifier (ORPHEUS-PO15F5HNP1, Light Conversion). Generation of white light probe pulse is achieved by focusing laser light in a sapphire crystal. The photoinduced charges in the absorption were measured using a transient absorption spectrometer (HELIOS, Ultrafast Systems), with a probing range of 490-910nm. Samples were measured in spectroscopic grade toluene and measured in 2 mm quartz cuvettes. The optical densities of solutions in the cuvette were kept below 0.2 at the maximum absorption wavelength to prevent aggregation and they were stirred with a magnetic stirrer during the experiment.

RESULTS AND DISCUSSION The synthesis of dyads 1 and 2 were accomplished by condensation between ZnPc 3 and PMAMI 4 in a mixture of dry NMP/acetic acid and subsequent chromatographic isolation as shown in Scheme 2. The ZnPc-PBIm(OR)4 dyad was obtained in 18% yield and the ZnPc-PDI(OR)4 dyad in 26% yield in the same reaction. PMAMI 4 presenting four tertoctylphenoxy groups in the bay positions was used as starting material because of its high dihedral angle between the naphthalene units (330) prevents the PDI aggregation and increase the solubility in organic solvents. Also, tert-octylphenoxy groups were chosen for the non-peripheral position of the phthalocyanine due to the donor character of this group, conferring also high solubility and high volume that avoid π−π staking and as a consequence avoiding aggregation. Worth to mention is that both dyads are specially designed to be only one isomer. All these factors, will allow a better study of their characterization by spectroscopic techniques. Figure 1 shows the very well resolved 1H-NMR of ZnPcPBIm(OR)4 and ZnPc-PDI(OR)4 using THF-d8 as solvent. The conjugated character between Pc and PDI in dyad 1 is unambiguously demonstrated by the appearance of two broad singlets integrating as 1H quite de-shielded at 10.40 and 9.68 ppm which could be assigned to Ha and Hb protons respectively, due to the influence of the fused imidazole ring. On the other hand, Ha and Hb protons in ZnPc-PDI(OR)4 appears at higher champ, at 9.00 and 8.66 ppm as a consequence of a non-conjugated connection between Pc and PDI. Moreover, all the other aromatic protons of the green dyad 1 and the blue dyad 2 are perfectly identified in the spectrum. All the assignment is carried out by a comparative study with the perylene monoimide benzimidazole reference compound 5 and the diamino ZnPc 3. Infrared spectroscopy also demonstrated the existence of the amine group in ZnPc-PDI(OR)4 by

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the existence of the unsymmetrical bands of the NH2 group at 3440 and 3379 cm-1 (See Figure S3).

Dyad 1

Dyad 2

Figure 2. UV-vis absorption spectra of ZnPc-PBIm(OR)4 1, ZnPc-PDI(OR)4 2, PBIm 5 and ZnPc 3 measured in CHCl3.

The UV-vis absorption spectra of both dyads were carried out in different polar and apolar solvents (THF, CHCl3 and hexane). For both dyads, almost no variation in the bands was observed going from THF (green line), a polar coordinate solvent with the central Zn atom, to CHCl3 (yellow line), a non-coordinate solvent. However, it is remarkable the broad UV-vis spectrum with lower molar extinction coefficients in hexane (black line), due to π-π aggregation effects. a)

Figure 1. 1H-NMR spectra of ZnPc-PBIm(OR)4 1, ZnPcPDI(OR)4 2, PBIm 5 and ZnPc 3 in THF-d8 as solvent.

UV-vis spectra of the dyads and the reference compounds in CHCl3 as solvent are represented in Figure 2 and their data are included in Table 1. Both dyads showed different colors in a 28 µM solution in chloroform because of its conjugation. The Q band absorption maxima in both dyads are located at 690 nm. It is remarkable the influence of conjugation in ZnPcPBIm(OR)4 1 between the Pc and the PDI units due to the new band located at 731 nm. (green line). The presence near infrared bands has been previously reported in fully conjugated phthalocyanine-benzoimidazole systems.25 Furthermore, the absorption spectrum of the perylene monoimide benzimidazole (PBIm 5) reference shows two bands at 572 and 615 nm with that are red shifted to a just one band at 593 nm in the fully conjugated phthalocyanine, which also proves the strong coupling between both subunits as previously described.25 However, this is not shows for the ZnPc-PDI(OR)4 2 because of it shows two absorption bands around 590 and 620 nm, similar to reference perylene. Moreover, fluorescence was totally quenched in both dyads, which could be attributed to electron transfer processes from the ZnPc to the PDI moiety. Compound ZnPc-PBIm(OR)4 1 ZnPc-PDI(OR)4 2 PBIm 5 ZnPc 3

356 (5.51) 355 (8.29) 455 (2.22) 355 (7.62)

λabs [nm] (ε x 104 M-1cm-1) 627 692 (3.69) (8.88) 593 619 (4.07) (5.28) 572 615 (4.60) (7.29) 617 679 (2.62) (9.25)

Table 1. Optical properties measured in CHCl3.

731 (4.33) 686 (20.90)

693 (9.07)

b)

Figure 3. UV-vis spectra in THF (green line), CHCl3 (yellow line) and hexane (black line) of a) ZnPc-PBIm(OR)4 1 and b) ZnPc-PDI(OR)4 2.

The electrochemical characterization was performed using cyclic voltammetry in dry PhCN as solvent containing 0.1 M TBAPF6 as supporting electrolyte (Figure 4). The nonconjugate dyad, ZnPc-PDI(OR)4, shows two oxidation potentials at 0.17 and 0.52 V (vs Fc/Fc+) assigned to the Pc moiety, and two reduction peaks at -1.13 and -1.38 V (vs Fc/Fc+),

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assigned to the perylene moiety. The first reduction peak is 50 mV positively shifted in comparison with the PBIm 5 as consequence of the influence of the Pc ring. In contrast, ZnPcPBIm(OR)4 dyad, shows two oxidation potentials at 0.23 and 0.55 V assigned to the Pc subunit, and two reduction potentials at -1.08 and -1.27 V (vs Fc/Fc+) assigned to the perylene subunit. As it can be seen, the conjugated dyad is reduced easier than the non-conjugated one which corroborated the existence of molecular interactions between two subunits because of its conjugation. Regarding with the oxidation potentials, it is not well understood that the conjugated dyad is easier oxidized than the non-conjugated dyad. 0.17

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ties as shown in the Supporting Information (Figures S7 and S8).

0.52

-1.13 -1.38

a)

0.45 -0.005

-1.44 -1.60

b)

0.23

-1.08

Figure 5. Transient absorption of ZnPc-PBIm(OR)4 1 in toluene at 680nm (left) and in benzonitrile at 690nm(right) and their kinetics at 620 nm (acceptor bleach), 690 nm (normalized donor bleach) and 850 nm (photo-induced absorption).

0.55

-1.27

c)

0.68 -1.18 -1.37

d)

-2,0

-1,5

-1,0

-0,5

+

0,0

0,5

1,0

E (V vs Fc/Fc ) -1

Figure 4. Cyclic voltammograms (100 mV s ) of deaerated PhCN solutions of a) ZnPc-PDI(OR)4 2, b) ZnPc 3, c) ZnPcPBIm(OR)4 1 and d) PBIm 5 containing TBAPF6 (0.1 M) at 298K.

Photophysical Studies. For both dyads we have performed ultrafast transient absorption measurements to study the dependence of the excited state dynamics on the level of conjugation between the perylene and phthalocyanine moiety. In Figure 5 the transient absorption data for dyad 1 dissolved in toluene and benzonitrile is summarized. In both solvents, the photo-induced absorption spectrum is dominated by a bleach signal between 600 and 800 nm that roughly resembles the ground state absorption spectrum of dyad 1. Additionally, at wavelengths longer than 800 nm a broad induced absorption signal is observed. In this range, beyond 800 nm, there are typically absorption features due to both the excited state and radical anions of perylenediimides. However, the shape of the spectrum of dyad 1 does not resemble either the excited state of anion of the isolated PDI. This leaves some uncertainty about the nature of the final excited state. In both solvents, the formation of the excited state species occurs with a rise time of roughly 0.2 ps which is roughly the time resolution of the transient absorption setup used. The decay is shown to depend strongly on the polarity of the solvent used. The decay in benzonitrile (~10 ps) is more than an order of magnitude faster than in toluene (~150 ps). This suggests that the excited state has substantial charge transfer character, even if it is not a fully charge separated state. This indicates that the excited state species that is observed is directly formed on excitation, or at least well within the time resolution of the experiment. This is consistent with time-dependent density functional theory calculations that show that several peaks in the spectrum correspond to excited states that contain contributions from both the perylenediimide and the phthalocyanine moie-

Figure 6. Transient absorption of ZnPc-PDI(OR)4 2 in toluene at 680nm (left) and in benzonitrile at 690nm(right) and their kinetics at 620 nm (acceptor bleach), 690 nm (normalized donor bleach) and 850 nm (photo-induced absorption).

For dyad 2 the transition absorption data upon excitation of the phthalocyanines at 680-690 nm is summarized in Figure 6. As in dyad 1, the photoinduced spectrum shows clear characteristics of the bleach of the ground state between 550 and 750 nm, but superimposed on that are induced absorption features. These occur in the range where the radical cation of phthalocyanines show several absorption features. At wavelengths longer than 750 nm two clear maxima are observed that occur in the range where the anion and excited state of the perylenediimide absorb. In toluene, this long wavelength band has a lifetime of 4.14 ps, much slower than the time resolution of the equipment. In benzonitrile the formation of the long wavelength bands is faster, 2.86 ps, but the rise is still clearly observable. As evident in Figure 6, the increase of the bleach signal at 620 nm, corresponding to the ground state absorption of the perylenediimide, rises with the same time constant as observed for the induced absorption between 750 and 900 nm. In contrast, the bleach of the ground state absorption of the phthalocyanine at 690 nm rises instantaneous during the excitation pulse, as expected since the initial excitation is on this chromophore. The decay of the observed induced absorption

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and bleach signals occurs with a lifetime of 809 ps in toluene which is much longer than in dyad 1. In benzonitrile the decay is twenty times faster (42.2 ps) but still considerably slower than for dyad 1 in the same solvent.

CONCLUSION In summary, we present the synthesis of different conjugated phthalocyanine-perylenemonoimidebenzimidazole, ZnPcand non-conjugated phthalocyaninePBIm(OR)4, perylenediimide, ZnPc-PDI(OR)4 dyads as well as their characterization. From comparing the results for both dyads we conclude that for dyad 2 the formation of a charge transfer state is clearly observed. Both the formation and decay are strongly dependent on the polarity of the solvent. For dyad 1 the induced absorption spectrum is formed within the time resolution of the experiment and the shape of the perylenediimide anion is clearly different from that of the isolated chromophore. The change in the shape of the spectrum can be cause by the different structure in the dyad where the perylene is fused into the conjugated structure of the phthalocyanine. It is also possible that an excited state with only partial charge transfer character is formed, leading to a distorted spectrum. In any case, also for dyad 1 the strong dependence of the kinetics on solvent polarity show that the should be substantial CT character in the species that is formed.

ASSOCIATED CONTENT Supporting Information (SI) available: Characterization spectra of new compounds.

AUTHOR INFORMATION Corresponding Author [email protected] Phone: +34 966658408 Fax: +34 966658351

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

ACKNOWLEDGMENT This research was financially supported by the Spanish Ministry of Economy and Competitiveness (Mineco) of Spain (CTQ201106789/BQU and CTQ2014-55798-R), Generalitat Valenciana (PROMETEO 2012/010 and ISIC/2012/008) and European Research Council under grant nr. 648433.

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Induced Charge Transfer Reactions in Perylene Imide Dye Sensitised Solar Cells. J. Mater. Chem. 2008, 18, 5802-5808.

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