Syntheses and studies of electron/energy transfer of ...

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Syntheses and studies of electron/energy transfer of new dyads based on an unsymmetrical perylene diimide incorporating chelating 1,10-phenanthroline and its corresponding square-planar complexes with dichloroplatinum(II) and dichloropalladium(II)† Sebile Işık Büyükekşi, a Ahmet Karatay, b Nursel Acar, Ayhan Elmali b and Abdurrahman Şengül *a

c

Betül Küçüköz,

b

Perylene diimides (PDIs) are among the most versatile and functional dyes for supramolecular structures displaying characteristic high absorptions and photo-luminescence properties as the prerequisite for optoelectronic thin film devices. Despite intense investigations into these semi-conducting and electroactive materials, details of their electronic structure are still under examination. In particular, non-planar twisted PDIs as an electron acceptor is a promising model system for efficient charge generation and transport processes. Therefore, a new dyad, an unsymmetrical PDI, N‘-(2-ethylhexyl)-N‘-(1,10-phenanthroline)-1,6,7,12-tetrakis-(4-methoxyphenoxy)-3,4,9,10-tetracarboxylic acid diimide (1) and its corresponding dichloroplatinum(II) and dichloropalladium(II) complexes as new dyads, [(Cl2)M(II)-(1)] where, M(II) = Pt(II) (2) and Pd(II) (3), were prepared. These dyads were fully characterized by FT-IR, 1D-NMR (1H-NMR and 13C-NMR), 2D-NMR (1H–1H COSY, 1H–13C HSQC, 1H–13C HMBC), MALDI TOF mass and UV-Vis spectroscopy. Electronic structure calculations have been employed based on Time-Dependent Density Functional Theory (TDDFT) calculations for the geometry-optimized electronic ground state structures in the gas phase and in dichloromethane (DCM). Current results indicate that 2 and 3 have similar HOMO–LUMO energy gaps which are smaller than 1. The energy and charge transfer processes with moleReceived 24th March 2018, Accepted 26th April 2018 DOI: 10.1039/c8dt01135d rsc.li/dalton

cular structures are crucial for the design of future functional dyads based on donor and acceptor moieties for hybrid optoelectronic devices. Charge transfer mechanisms were also investigated with linear absorption, fluorescence and ultrafast transient absorption spectra for the newly synthesized compounds in DCM. The observed ultrafast intramolecular charge transfer from donor units on the PDI-2 compound is related to fluorescence quenching and faster singlet decay on transient measurements.

1.

a

Department of Chemistry, Faculty of Arts and Sciences, Bülent Ecevit University, 67100 Zonguldak, Turkey. E-mail: [email protected], [email protected]; Fax: +90 372 257 4181; Tel: +90 372 291 1126 b Department of Engineering Physics, Faculty of Engineering, Ankara University, 06100 Beşevler, Ankara, Turkey c Department of Chemistry, Faculty of Science, Ege University, 35100 Bornova, Izmir, Turkey † Electronic supplementary information (ESI) available: Detailed experimental procedures, characterization of the PDI-3, FT-IR, 1D-NMR (1H-NMR and 13 C-NMR), 2D-NMR (1H–1H COSY, 1H–13C HSQC, 1H–13C HMBC), MALDI TOF mass and absorption and emission spectra), tables of calculated electronic transitions related to molecular orbitals of 1, 2 and 3. See DOI: 10.1039/c8dt01135d

7422 | Dalton Trans., 2018, 47, 7422–7430

Introduction

Among the organic semiconductors, PDIs were applied in many electronic and photonic devices i.e. PVs and DSSCs1–9 owing to their high photochemical and thermal stability, high absorption coefficient for visible light, high fluorescence quantum yield, propensity to self-assemble, and strong electron-accepting character.10–12 Energy and charge transfer processes play a key-role in optoelectronic devices and have been extensively studied in a variety of systems.13–17 A detailed understanding of the structure–property relationship still requires design of novel functional molecular or supramolecular systems presenting highly efficient charge and/or energy transfers suitable for organic optoelectronic applications. For this purpose, a variety of model molecular systems based on

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dyads and triads containing donor and acceptor units have been developed to explore and identify the underlying key processes that govern energy- and charge-transfer mechanisms in organic conjugated materials.18–21 As an electron acceptor, PDIs are the potential alternatives to fullerene due to their good electron mobility, flexibility of functionalizing the molecule to control its electronic structure and tuning its energy levels, strong absorptions in the whole range of the UV-Vis region, and also their high photo-stability.22 We have recently reported on metal ion-driven welldefined PDI-based metallosupramolecular systems consisting of imidazole-functionalized PDI and redox-active squareplanar terpyridylplatinum(II) and also palladium(II) units.23 By considering the previous works, the Pt(II)-terpyridyl functionalization of the PDI unit caused a decrease of the short circuit current while the Pd(II)-terpyridyl functionalization caused an increase of the short circuit current and photovoltaic conversion efficiency up to 6.82%. It is generally considered that the short circuit current in a bulk heterojunction device is a combination of the dissociation of photo-generated excitons into free charge carriers at the donor–acceptor interface.23 The improvement in the photocurrent for device (the Pd-triad) was attributed to the efficiencies of exciton dissociation at the P3HT/PDI interfaces and charge collection at the electrodes. Later we have reported on 1,10-phenanthroline modified twisted PDI and its corresponding platinum(II) and palladium(II) complexes as triads and the device performance of the twisted PDIs based triads, whereby disrupting the planarity of the PDI helped to increase the device efficiencies up to 4.48%, depending on the presence of the metal ion.24 These studies have shown that the attachment of the square-planar Pt(II) and Pd(II)-complex units to the twisted PDI core strongly influenced the performance of the photovoltaic devices. In this respect, the relatively high performance (>2.75%) of the twisted PDI based BHJ device was reported by others.25 The relatively increased efficiency was reportedly attributed to an improved morphology and optimal electronic donor–acceptor (D–A) interface. Carlos et al.26 have reported that structurally rigid 1,10-phenanthroline ( phen) as a strong field ligand coordination increases the stability of the complex that hampers its photodissociation. They have also reported that the dyad as [Ru(II)( phen)2( phen-PDI-phen)] photosensitizes the formation of singlet-oxygen by triplet–triplet energy transfer. These before mentioned studies have stimulated us to synthesize an unsymmetrical and twisted 1,10-phenanthroline-modified PDI (1) carrying four bulky methoxyphenoxy groups at the bay positions thereby disrupting the planarity of the PDI to cause an increase of the short circuit density and device efficiency. These bulky groups at bay positions usually lead to increase in the solubility and decrease in the photoluminescence quenching in the solid state as observed for [Ir( ppy)2-phen-PDI (dyad)]+ (ref. 27) and other related cases,23,28–30 in addition to a pronounced effect on the optical spectra as well. As reported by Sastre-Santos et al.27 this dyad exhibited highly efficient electroluminescence (EL) in the deep-red region for light-

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emitting electrochemical cell (LEC) application. Moreover, the chelating phen ligand owing to its low lying π* antibonding orbitals can readily form complexes with the reducing metal cations hence generating a low energy metal-to-ligand charge transfer (MLCT) electronic state.31,32 The previous studies on the well-known ruthenium complex have shown that charge separation within the sensitizers is an important characteristic for efficient charge transfer within the system. Spatial separation of the positive charge left on the dye and the injected electrons after the MLCT result in decrease of the rate of recombination between the injected electrons and the oxidized dye molecules.33,34 Hence, the objective of the present work is to synthesize thermally and chemically stable neutral mononuclear metal complexes with increased molar extinction coefficient.35 Although both platinum(II) and palladium(II) ions adopt the same geometry, they have different binding strengths and reversibility and also different optoelectronic properties, important in the PV applications.36 Carlos et al.26 reported that unlike the Pd(II)-dyad, the Pt(II)-dyad showed a strong spin–orbit coupling that resulted in a populated longlived triplet state of PDI.26 Here, we study the photophysical properties of an unsymmetrical 1,10-phenanthroline modified PDI (1) as a free ligand dyad and its corresponding dichloro-Pt(II) and dichloro-Pd(II) complexes as 2 and 3, respectively, in DCM solution. The chemical structures of these compounds are shown in Scheme 1. In particular, PDIs have been incorporated as energy- or electron-acceptors in various conjugated dyads and triads with attractive photovoltaic properties.23,24 In contrast to these systems already reported in the literature, the molecules investigated herein were specifically designed as precursors of photoactive materials with an unsymmetrical manner of the donor and acceptor moieties. Donor–acceptor structures were often used to obtain fluorescence resonance energy transfer (FRET) or through bond energy transfer (TBET) in the molecular structures.37–42 The photophysical properties of the dyad molecules in solution were investigated with steady-state

Scheme 1 Synthetic Pd(DMSO)2Cl2, DCM.

pathway

of

2

and

3.

i.

Pt(DMSO)2Cl2/

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absorption, fluorescence measurements and ultrafast transient absorption spectroscopy as well as DFT calculations. Intramolecular charge transfer from the phen donor unit to the perylene acceptor unit was observed. The results show an intricate interplay between photo-induced intramolecular charge transfer processes that depend not only on the conformational effects allowed by the chelating phen ligand but also on the presence of the metal ion. While such a donor–acceptor supramolecular architecture is of strong interest for optoelectronic applications, the present study in solution provides useful information that will be needed to assess the structure– property relationship in this class of promising organic materials. Obviously, this study is also very relevant to future works devoted to the design and the development of novel functional D–A molecular or supramolecular systems for organic optoelectronic applications.

2. Results and discussion 2.1.

Synthesis and characterization

The synthesis of the precursor PDI N′-(2-ethylhexyl)-1,6,7,12tetrakis-(4-methoxyphenoxy)perylene-9,10-dicarboxylic imide3,4-dicarboxylic anhydride (PDI-3) was carried out according to the reported method as described in the ESI† and it was characterized by FT-IR (Fig. S1†), MALDI TOF mass (Fig. S2†), 1 H NMR (Fig. S3†) and 13C DEPT NMR spectroscopy (Fig. S4†). As shown in Scheme 1, the synthesis of 1 from PDI-3 was adapted from the previously reported method by us.23 The compounds 2 43 and 3 43 were synthesized from 1 by treatment with Pt(DMSO)2Cl2 and Pd(DMSO)2Cl2, respectively. The newly synthesized PDIs 1, 2 and 3 were fully characterized by FT-IR, MALDI-TOF mass, UV-Vis spectrophotometry, and 1D-NMR and 2D-NMR spectroscopy. The chemical shifts for proton and carbon were assigned with the combined information from 1H NMR, 1H–1H COSY, 13C DEPT, 1H–13C HSQC, 1 H–13C HMBC spectra. The numbering of atoms for NMR is indicated in Scheme 1. The IR (ATR) spectrum of 1 (Fig. S5†) clearly indicates the completion of the imidization by the disappearance of the anhydride peaks at 1765–1735 cm−1 observed in that of compound PDI-3. The FT-IR spectrum of 2 (Fig. S6†) and 3 (Fig. S7†) were also consistent with the proposed structures. The mass spectra of the PDIs were obtained using the MALDI-TOF technique for 1, 2 and 3. The molecular ion peaks observed at m/z 1169.2 as [M + H]+ for 1 (Fig. S8†), 1457.5 as [M − H + Na]+ for 2 (Fig. S9†) and 1369.2 as [M + Na]+ for 3 (Fig. S10†), respectively, confirm the integrity and the stoichiometry of the compounds. The 1H NMR spectra of 1 (Fig. S11†), 2 (Fig. S12†) and 3 (Fig. S13†) were consistent with the proposed structures. The PDI core protons and the methyl groups at bay positions were differentiated as two singlets HP1, HP1′ and HP4, HP4′ for 1, 2 and 3 because of loss of symmetry at the imide position. Upon coordination to PtII- and PdII-metal centers, the PDI core hydrogens showed slight deshielding relative to those of the

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corresponding 1. As expected, protons in the PtII complex (2) show more deshielding than those of the PdII complex (3) for PDIs.23,24 The correlation of H–H coupling was given by the 1 H–1H COSY NMR spectra of 1 (Fig. S14†), 2 (Fig. S15†) and 3 (Fig. S16†). The observed cross peaks indicate the following correlations for 1, 2, 3: H2 ↔ H3, H9 ↔ H8, H4 ↔ H3, H7 ↔ H8 for the phen ring and HH1 ↔ HH2, HH3,H5,H6,H7 ↔ HH4,H7 for 2-ethylhexyl alkyl protons. Identification of the chemical shifts and coupling constants of the protons is summarized in Table S1.† The 13C DEPT NMR spectra of 1 (Fig. S17†), 2 (Fig. S18†) and 3 (Fig. S19†) are also well consistent with the proposed structures. Their combination with the 1H–13C HSQC spectra of 1 (Fig. S20†), 2 (Fig. S21†), and 3 (Fig. 22†) and the 1H–13C HMBC spectra of 1 (Fig. S23†), 2 (Fig. S24†), and 3 (Fig. S25†) carbon chemical shifts are readily identified and the results are given in Table S2.† The C–H coupling of 2-ethylhexyl and phen groups are clearly indicated by 1H–13C HSQC spectra. The C8 and C3 carbons of 2 and 3 have the same chemical shift. The chemical shifts of Ca, Cb, and Cc and Ca′, Cb′, and Cc′ for PDIs are verified by the HMBC spectra of the compounds. The aggregation behaviours of 1 (Fig. S26†), 2 (Fig. S27†) and 3 (Fig. S28†) were also investigated at different concentrations in DMSO. As the concentration increases, the intensity of the absorption also increases. The Beer–Lambert law was obeyed for the entire compound in concentrations ranging from 1.2 × 10−5 to 2 × 10−6 M. The UV-Vis spectra of PDIs in DMSO show characteristic absorptions between 400 and 650 nm and three absorption maxima appeared. The absorption spectra of 2 and 3 are almost identical with the free ligand 1 with only slight red-shifting of the absorption maxima (∼2 nm). The fluorescence emission spectra of 1, 2 and 3 were determined in DMSO (1.2 × 10−5 M) as depicted in Fig. S29.† All compounds exhibited emission at ambient temperature when the excitation wavelength was chosen at 550 nm. The PDIs have the same emission maxima at 617.5 nm. The UV-Vis absorption and emission of the investigated compounds were also studied in DCM (Fig. S30 and S31†). The absorption spectra show similar characteristics for 1, 2 and 3. Fluorescence emissions were compared in Fig. S31† to understand the intramolecular energy transfer process better. Fluorescence intensity decreases after the formation of (Pt and Pd) metal complexes. The fluorescence quenching for 2 and 3 might be related with the increasing of the intramolecular charge transfer. Femtosecond transient absorption spectroscopy measurements and DFT calculations also support the intramolecular charge transfer approach.

2.2

Computational results

The investigated molecules (Fig. 1 and Fig. S32†) were optimized using B3LYP/6-31G(d,p) and LANL2DZ (for metals) in DCM and the results are summarized in Table S3.† Table S3† shows dipole moments (µ, Debye), sum of electronic energies and zero point correction energies, complexation energies

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Fig. 1 Ground-state optimized minimum energy structures of 1, 2 and 3 at B3LYP/6-31G(d,p) and LANL2DZ (for metals) in DCM (top and side views).

(ΔEC) and the selected torsion angles of the molecules in DCM. The dipole moments are high because of the unsymmetric molecular structures and the presence of the electronegative atoms. Compounds of 2 and 3 which are metal complexes of 1 are very stable. It is observed that the Pt complex (2) was formed more easily than the Pd complex (3) by approximately 10 kcal mol−1. The planar structure of the perylene ring is distorted due to the up and down orientation of the phenyl rings (Fig. S32 and Table S3,† C1C2C3C4: 28°). The phenyl rings were deviated from the perylene ring by 121° in both directions. The torsion angles of these groups are approximately 58°. In 3, the torsion angles for upward orientation of the phenyl rings in the direction from perylene ring to phenyl (COCC) are −31° and −30°, whereas the corresponding downward dihedral angles for the phenyl rings (COCC) are −31° and −28°. The C5N1C6C7 torsion angle (ca. 90°) indicates that the phen moiety is perpendicular to the perylene ring. In 2 and 3, the torsions have slightly changed (89° and 92°). The bands that arise from vibrations involving the related functional groups are shown in Fig. S33.† Coordination of MCl2 units makes a minimal contribution to the IR spectrum of 1. Frequencies at 324 and 327 cm−1 are assigned to Pt–Cl symmetrical and antisymmetrical Pt–N stretchings, respectively, in the spectrum of 2. These modes appear at 345 and 338 cm−1 for 3. Unfortunately, it is very difficult to observe these bands experimentally as their magnitudes are very small. Electronic properties of these compounds were calculated at the B3LYP/6-31G(d,p) and LANL2DZ (for metal) level in the ground state using TDDFT. The energies for the frontier highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) are depicted in Fig. 1 for all investigated systems in DCM. The HOMO–LUMO energy gap decreases in the order of 1 > 2 = 3.

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Tables S4, S5 and S6† show S0 → S20 electronic transitions (λabs), oscillator strengths ( f ), transition dipole moments (μtr), excitation characters, and molecular orbitals with percentage contributions of 1, 2 and 3, respectively. Electronic transitions are named according to the transition types. A number is added as a suffix when more than two transitions of the same type were observed. Molecular orbitals corresponding to those transitions for 1, 2 and 3 are given in Fig. S34, S35 and S36,† respectively. While the HOMO of 1 is a π-bonding orbital located on the phenyl groups of the molecule, the LUMO is a π*-antibonding orbital located on the perylene moiety. The first peak in the electronic spectrum of 1 (Table S4†) at 661 nm arises from a singlet transition from HOMO to LUMO. Therefore, it belongs to an intramolecular charge transfer (ICT1) between the phenyl and the perylene groups. The second peak located at 640 nm corresponds to the excitation involving HOMO−2 to LUMO and has the same charge transfer character as in the S1 transition. The HOMO−1 is distributed over the phenyl rings and the perylene ring. As a result, the S3 transition includes local excited PDI (LE) in addition to intramolecular charge transfer (ICT1). The maximum peak corresponding to the S5 transition (543 nm) also carries both ICT and LE characters. Another observation is the intramolecular charge transfer (ICT2) from HOMO−5 to LUMO with π–π* character for the transitions from S0 to S6–S9–S15 (447 nm, 394 nm, 382 nm) directed from the phen towards the perylene core. These transitions require more energy in comparison with ICT1. Coordination of the metal ions leads to a decrease in ΔEHL and a corresponding shift of all peaks to longer wavelengths (Tables S5 and S6†). The S0–S1 transition of 2 (679 nm) and 3 (678.5 nm) has intramolecular charge transfer (ICT1) between HOMO and LUMO. The second peak (653 nm) of 2 and 3 has also ICT1 character between HOMO−2 and LUMO. The maximum wavelength (563 nm) has intramolecular charge transfer from the phenyl rings to the perylene ring (ICT1) and locally excited perylene diimide. There is a ligand–metal charge transfer from the phenyl units to Pt (LMCT1) at 453 nm in 2. This transition has also intramolecular charge transfer from the phenyl rings to the phen (ICT5). Although there are similar transitions for 3, compound 2 has more LMCT character and requires slightly less energy for LMCT compared to 3. Calculated and experimental UV-Vis absorption spectra of 2 in DCM are compared in Fig. 2. The spectra at longer wavelength are in the same region; however, there is a 20 nm red shift in the experimental spectra. Thus, it may be concluded that calculated and the experimental spectra are in good agreement. Total electron densities based on electrostatic potential surface for ground state geometries of the studied molecules are obtained in DCM (Fig. 3). It was displayed using side view molecular geometries for a better visualization. Molecular Electrostatic Potential (MEP) was used to investigate the charge distribution. The molecular electrostatic potential surface color scheme depends on the electron density: red, high electron density, partial negative charge; blue, highly electron deficient, partial positive charge; light blue, slightly elec-

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static potentials of the molecules did not change significantly. The charge density is localized on the chloride substituents in 2 and 3. On the contrary, the charges are mostly on N and O atoms in 1 (a more homogeneous distribution). Generally, the charge distribution increases with substituent change (1 < 2 < 3). It is known that Mulliken charges are not very reliable so additional NBO values are calculated (Table S7†). The NBO charges are in agreement with the Mulliken charges. 2.3 Fig. 2 Calculated (in gas phase) and experimental (in DCM) UV-Vis absorption spectra of 2 in DCM.

Fig. 3

Femtosecond transient absorption spectroscopy

In order to investigate intramolecular electron transfer in 1, 2 and 3, ultrafast transient absorption measurements were performed at 590 nm as an excitation wavelength. It is expected that ultrafast spectroscopy studies confirm intramolecular charge transfer from the phen unit with Pt (II) and Pd(II) and phenyl rings to the perylene moiety. All investigated compounds exhibit similar characteristics on transient absorption spectra. Upon excitation at 590 nm, ground state bleaching signals occur at 590 nm, 550 nm and 460 nm wavelengths for 1, 2 and 3 (Fig. 4, Fig. S37 and S38†). These bleaching signals are attributed to the absorption bands on the steady-state absorption spectrum of the perylene moiety. There is an extra bleach band for all compounds on the transient spectra around 660 nm, which has an ultrafast lifetime, and this is attributed to stimulated emission.44 There are positive signals above 680 nm and around 430 nm in the ultrafast transient absorption spectra of these samples. These excited state absorptions correspond to the S1–Sn transition of the compounds since they occur simultaneously with the pump excitation. The absorption band above the 680 nm region can be related to the presence of a PDI radical anion absorption band according to the literature.45–47 The broadening positive absorption band and fast increasing absorption suggest the formation of a radical anion. The decay curves of ground state bleach signals for 1, PDI-1 and PDI-2 samples are compared with each other (Fig. 5) as an

Total electron density surfaces for 1, 2 and 3 in the gas phase.

tron deficient; yellow, low electron density; green, neutral. Gross orbital population based on Mulliken charges was used to obtain the electron density. It was observed that the electro-

7426 | Dalton Trans., 2018, 47, 7422–7430

Fig. 4 Ultrafast transient absorption spectra of 1 with different time delays at excited 590 nm femtosecond pulsed laser.

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stronger charge transfer in 2 and 3 compared to 1. The ground state bleach signal decreases fast around 3 ps; then it increases a little from 3 ps to 6 ps for 2 and 3, respectively, as presented in Fig. 6. This behaviour might be explained with electron oscillation between singlet energy levels of the PDI (∼590 nm) and MLCT state (∼540 nm) due to the Pt (II) and Pd(II) atoms.

3. Experimental 3.1.

Fig. 5 Comparison of decay curves of compounds 1, PDI-1 and PDI-2 at the singlet state of the perylene bleach wavelength.

indication of the intramolecular charge transfer between donor units i.e. phen and phenyl rings and acceptor unit i.e. perylene moiety. As seen in Fig. 5, the lifetime of the excited state decreases while binding of the phenyl ring and the phen unit to the perylene moiety. The decreasing of excited state lifetime proves that intramolecular charge transfer occurs from phenyl rings and the phen unit to the perylene moiety. The charge transfers also explain why the fluorescence intensity of 1 decreases compared with PDI-1 and PDI-2. The ground state bleach decay curves of 1, 2 and 3 are shown in Fig. 6 upon excitation using a femtosecond laser at 590 nm. In Fig. 6, the decay rates of 2 and 3 are faster than 1 because of the effect of Pt and Pd atoms on the intramolecular charge transfer. In addition to that occurrence of the MLCT also affected the decreasing lifetime for 2 and 3. DFT calculations indicate that charge distribution is not homogeneous for 2 and 3 complexes. The nonhomogeneous charge distribution leads to a

All chemicals were purchased from commercial sources, and used without further purification. 1,6,7,12-Tetrachloroperylene-3,4-9,10-tetracarboxylic acid dianhydride and 1,10phenanthroline were purchased from Aldrich. The starting materials 5-amino-1,10-phenanthroline,48,49 Pt(DMSO)2Cl2 and Pd(DMSO)2Cl2 50 were synthesized by the previously reported methods. The synthesis and characterization of N,N′-di(2-ethylhexyl)-1,6,7,12-tetrachloroperylene-3,4,9,10-tetracarboxylic acid diimide (PDI-1) and N,N′-di(2-ethylhexyl)-1,6,7,12-tetrakis-(4methoxyphenoxy)perylene-3,4,9,10 tetracarboxylic acid diimide (PDI-2) were previously reported by us.23 N′-(2-Ethylhexyl)1,6,7,12-tetrakis-(4-methoxyphenoxy)perylene-9,10-dicarboxylicimide-3,4-dicarboxylic anhydride (PDI-3) was synthesized by the previously reported methods.51 Column chromatography was performed on silica gel 60. 3.2.

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Equipment

Melting points were recorded on a Bushi Melting Points B-540 apparatus. FT-IR (ATR) spectra were recorded with a PerkinElmer FT-IR Spectra 100 spectrophotometer at room temperature. Mass spectra were obtained using a Voyager-DE matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometer and a SCIEX 4000 QTRAP LC-MS/MS mass spectrometer. The 1H NMR, 13C NMR DEPT, 1 H–1H COSY, 1H–13C HSQC and 1H–13C HMBC spectra were measured in DMSO-d6 with an Agilent 600 MHz Premium Compact NMR spectrometer at room temperature. Chemical shifts are reported in ppm ( parts per million) with reference to TMS. Coupling constants J are given in Hertz (Hz). The UV-Vis spectral measurements were carried out with a CARY 100 Bio UV-Visible spectrophotometer and fluorescence spectra were measured on a PerkinElmer LS55 fluorescence spectrometer using 1 cm path length cuvettes at room temperature. 3.3.

Fig. 6 Comparison of decay curves of compounds 1, 2 and 3 at 600 nm probe wavelength with excitation at 590 nm.

Materials

Synthesis

3.3.1. N′-(2-Ethylhexyl)-N′-(1,10-phenanthroline)-1,6,7,12-tetrakis-(4-methoxyphenoxy)-3,4,9,10 tetracarboxylic acid diimide (1). 5-Amino-1,10-phenanthroline (0.05 g, 0.26 mmol) and N′-(2ethylhexyl)-1,6,7,12-tetrakis-(4-methoxyphenoxy)perylene-9,10dicarboxylic imide-3,4-dicarboxylic anhydride (PDI-3) (0.10 g, 0.10 mmol) were dissolved in a mixture of 2.0 ml pyridine and 2.0 g imidazole and stirred for 48 h at 120 °C under an argon atmosphere. After cooling to room temperature the reaction mixture was added dropwise into 15 ml of 15% aqueous HCl solution. The precipitate was collected and washed with 50 ml

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water. The crude product was purified by column chromatography on silica gel using DCM/EtOH (100/20) as eluent. The product (0.07 g, 60%) was obtained as a purple solid. M.p.: 381 °C. FT-IR spectrum [(ATR)/cm−1]: 3057–1998 (Ar–CH), 2954–2835 (CH), 1698 (CvO imide), 1659 (CvO imide), 1586 (CvC), 1497, 1463, 1439, 1408, 1339, 1311, 1283, 1246, 1192 (C–O–C), 1178 (C–O–C), 1101, 1083, 1030, 1008, 939, 875, 826, 801, 755, 740, 705, 675, 659. MALDI-TOF (m/z): C72H56N4O12, calculated: 1169.26; found: 1169.2 [M]+. 1H NMR (600 MHz, d6-DMSO) δ ppm: δ = 9.16 (d, J = 3.89, 1H, H2), 9.10 (d, J = 4.07, 1H, H9), 8.49 (m, 1H, H7), 8.47 (m, 1H, H4), 8.09 (s, 1H, H6), 7.85 (s, 2H, HP1), 7.84 (s, 2H, HP1′), 7.80 (dd, J = 4.08, 1H, H3), 7.64 (dd, J = 4.11, 1H, H8), 7.00 (m, 8H, HP2, HP2′), 6.95 (d, 8H, HP3, HP3′), 3.87 (m, 2H, HH1), 3.76 (s, 6H, HP4), 3.69 (s, 6H, HP4′), 1.75 (m, 1H, HH2), 1.21 (m, 8H, HH3, HH5, HH6, HH7), 0.80 (m, 6H, HH4, HH8). 13C NMR (DEPT, 600 MHz, d6-DMSO) δ ppm: (C) 163.564, 163.342, 156.875, 156.798, 156.714, 156.415, 148.369, 148.323, 146.300, 145.924, 132.813, 131.410, 126.667, 123.410, 122.966, 119.571, 119.012 (CH) 151.334, 150.698, 136.912, 132.437, 127.916, 124.192, 124.000, 121.908, 121.770, 118.153, 117.992, 115.793, 115.755, 37.653 (CH2) 43.998, 30.588, 28.573, 23.937, 22.825 (CH3) 55.945, 55.907, 14.358, 10.871. UV-Vis (DMSO) λmax nm (log ε): 588 (4.71), 546 (4.48), 453 (4.19), 267 (5.11). 3.3.2. [(1)-PtCl2](2). Pt(DMSO)2Cl2 (0.01 g, 0.02 mmol) was dissolved in 4.0 mL DCM; then 1 (0.02 g, 0.016 mmol) was added and stirred for 6 h at 55 °C under an argon atmosphere. Then the reaction mixture washed with 100 ml water. The product (0.02 g, 95%) was obtained as a purple solid. M.p.: >400 °C. FT-IR spectrum [(ATR)/cm−1]: 3074–3001 (Ar–CH), 2954–2850 (CH), 1698 (CvO imide), 1659 (CvO imide), 1584 (CvC), 1497, 1462, 1431, 1409, 1388, 1339, 1314, 1286, 1246, 1193 (C–O–C), 1178 (C–O–C), 1101, 1029, 1008, 936, 877, 828, 801, 800, 754, 718, 679, 666. MALDI-TOF (m/z): C72H56Cl2N4O12Pt, calculated: 1435.2; found: 1457.5 [M − H + Na]+. 1H NMR (600 MHz, d6-DMSO) δ ppm: δ = 9.75 (d, J = 5.53, 1H, H2), 9.73 (d, J = 5.52, 1H, H9), 9.13 (d, J = 8.53, 1H, H7), 9.05 (d, J = 8.17, 1H, H4), 8.48 (s, 1H, H6), 8.19 (dd, J = 5.59, 1H, H3), 8.07 (dd, J = 5.55, 1H, H8), 7.86 (s, 2H, HP1), 7.84 (s, 2H, HP1′), 7.01 (m, 8H, HP2, HP2′), 6.95 (d, 8H, HP3, HP3′), 3.89 (m, 2H, HH1), 3.76 (s, 6H, HP4), 3.70 (s, 6H, HP4′), 1.75 (m, 1H, HH2), 1.21 (m, 8H, HH3, HH5, HH6, HH7), 0.80 (m, 6H, HH4, HH8). 13C NMR (DEPT, 600 MHz, d6-DMSO) δ ppm: (C) 163.580, 163.304, 156.882, 156.798, 156.308, 148.346, 148.277, 148.238, 147.917, 132.966, 132.782, 129.671, 129.158, 123.303, 123.019, 120.031, 119.786, 118.897 (CH) 150.369, 150.124, 140.169, 136.330, 129.096, 127.073, 121.901, 121.694, 118.176, 117.969, 115.770, 37.638 (CH2) 43.983, 30.580, 28.565, 23.929, 22.818 (CH3) 55.953, 55.922, 14.350, 10.863. UV-Vis (DMSO) λmax nm (log ε): 590 (4.64), 547 (4.41), 454 (4.12), 280 (5.09). 3.3.3. [(1)-PdCl2](3). Pd(DMSO)2Cl2 (0.01 g, 0.03 mmol) was dissolved in DCM (4.0 mL); then 1 (0.02 g, 0.016 mmol) was added and stirred for 6 h at 55 °C under an argon atmosphere. Then the reaction mixture was washed with 100 ml water. The product (0.02 g, 97%) was obtained as a purple solid. M.p.: 285 °C (decompose). FT-IR spectrum [(ATR)/cm−1]: 3069–2999 (Ar–CH), 2953–2851 (CH), 1698 (CvO imide), 1659 (CvO

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imide), 1585 (CvC), 1497, 1463, 1438, 1408, 1388, 1339, 1285, 1245, 1191 (C–O–C), 1177 (C–O–C), 1101, 1030, 1007, 940, 929, 876, 827, 801, 754, 721, 678, 662. MALDI-TOF (m/z): C72H56Cl2N4O12Pd, calculated: 1346.5; found: 1369.2 [M + Na]+. 1H NMR (600 MHz, d6-DMSO) δ ppm: δ = 9.39 (d, J = 5.27, 1H, H2), 9.36 (d, J = 5.13, 1H, H9), 9.05 (d, J = 8.47, 1H, H7), 8.96 (d, J = 8.15, 1H, H4), 8.44 (s, 1H, H6), 8.14 (dd, J = 5.52, 1H, H3), 8.02 (dd, J = 5.39, 1H, H8), 7.85 (s, 2H, HP1), 7.82 (s, 2H, HP1′), 6.98 (m, 8H, HP2, HP2′), 6.93 (d, 8H, HP3, HP3′), 3.84 (m, 2H, HH1), 3.75 (s, 6H, HP4), 3.69 (s, 6H, HP4′), 1.74 (m, 1H, HH2), 1.20 (m, 8H, HH3, HH5, HH6, HH7), 0.79 (m, 6H, HH4, HH8). 13C NMR (DEPT, 600 MHz, d6-DMSO) δ ppm: (C) 163.564, 163.304, 156.882, 156.798, 156.300, 148.346, 148.277, 147.457, 147.104, 132.974, 132.782, 129.433, 128.843, 123.311, 123.027, 120.031, 119.770, 118.889 (CH) 151.587, 151.265, 140.698, 136.821, 129.020, 126.759, 121.901, 121.694, 118.169, 117.969, 115.770, 37.638 (CH2) 43.983, 30.580, 28.565, 23.929, 22.818 (CH3) 55.953, 55.914, 14.350, 10.863. UV-Vis (DMSO) λmax nm (log ε): 590 (4.70), 547 (4.47), 454 (4.18), 280 (4.98). 3.4.

Computational details

Gaussian09 52, Gaussview5.0 53 and Spartan08 54 were used in the calculations. Conformational analyses were performed with Spartan 08 to obtain initial structures. Density Functional Theory (DFT) was used to optimize ground state geometries.55 Calculations were done with the hybrid B3LYP functional (Becke’s three-parameter nonlocal exchange functional,56,57 Lee–Yang–Parr’s correlation function58) for DFT and TDDFT with the LANL2DZ basis set. All the structures were optimized in the gas phase. All of the optimized geometries have positive frequencies, which verify that they correspond to minima. The most stable conformer for each molecule was determined and used in calculations. For every investigated system, 20 lowest singlet excited states were calculated. Ground state geometries were used to obtain molecular orbital energies and the UV-Vis spectra. The electrostatic potential values are calculated using the SCF density to determine the total electron density surface of studied molecules in the gas phase. 3.5.

Ultrafast transient absorption spectroscopy

The ultrafast transient absorption spectroscopy measurements were performed using a Ti:Sapphire laser amplifier-optical parametric amplifier system (Spectra Physics, Spitfire Pro XP, TOPAS) and a commercial setup of an ultrafast transient absorption spectrometer (Ultrafast Systems, Helios). Pulse duration was measured as 100 fs inside the commercial setup. Wavelengths of the pump beam were chosen according to the steady-state absorption spectra of the studied compounds. The white light continuum generated with a sapphire crystal was used as a probe beam.

Conclusions In conclusion, we synthesized new unsymmetrical 1,10-phenanthroline-appended perylene diimide and its metal com-

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plexes with Pt (II) and Pd(II) to investigate intramolecular charge transfer process in these dyads. The photophysical properties of these compounds were studied with steady-state absorption, fluorescence emission and femtosecond transient absorption spectroscopy, as well as DFT calculation. According to the reference samples of PDI-1 and PDI-2, ultrafast intramolecular charge transfers were confirmed by transient absorption spectroscopy. The donor units on PDI-2 provide intramolecular charge transfer that can be clearly seen as fluorescence quenching and faster singlet decay on transient measurements. Formation of the metal complexes with Pt (II) and Pd(II) metal ions also led to strong intramolecular charge transfer and metal–ligand charge transfer from the donor/metal unit to the acceptor unit. This has been proven with observation of fluorescence quenching and transient measurements of the metal complexes as a faster singlet decay. TDDFT calculations show that bay-substituted PDI is highly distorted and deviated from planarity. The lowest bandgap in 2 makes it the best candidate for photosensitive material design among all studied molecules.

Conflicts of interest There are no conflicts to declare.

Acknowledgements The authors are grateful to the Turkish Scientific and Technical Research Council for support of this project under the grant [TÜBİTAK-Project No: 214Z090]. We also acknowledge the computer time on FenCluster provided by Ege University Faculty of Science and TUBITAK-ULAKBIM Truba Resources.

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