Access to Triplet Excited State in Core-Twisted Perylenediimide - The

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Access to Triplet Excited State in Core-Twisted Perylenediimide Kalaivanan Nagarajan, Ajith R. Mallia, V. Sivaranjana Reddy, and Mahesh Hariharan* School of Chemistry, Indian Institute of Science Education and Research Thiruvananthapuram, CET Campus, Sreekaryam, Thiruvananthapuram, Kerala 695016, India S Supporting Information *

ABSTRACT: Solvent-free crystal structure of N,N-bis(propylacetyl)-1,6,7,12-tetrabromoperylene-3,4:9,10-bis(dicarboximide), PDI-Br4, obtained by X-ray diffraction reveals the core-twisted perylene motif having π−π stacks at an interplanar separation of 3.7 Å. Slip-stacked arrangement of PDI units in PDI-Br4 arises due to the presence of bulky bromine atoms. Femtosecond pump−probe measurements of monomeric PDI-Br4 in toluene reveal ultrafast intersystem crossing (τISC < 110 fs) when excited at 400 nm. Triplet quantum yield (ΦT) of 19 ± 1% and 105 ± 5% for PDI-Br4 in toluene and vaporannealed polycrystalline 60 nm thick film respectively are estimated from nanosecond transient absorption measurements. Quantum chemical calculations show that the combined effects of heavy atom and core-twist in PDI-Br4 can activate the intersystem crossing by altering the singlet−triplet energy gap. Enhanced quantum yield accounts for the singlet fission mediated generation of triplet excited state in the PDI-Br4 thin film.

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on the rate of SF is reported in tetracene polymorphs.20,38 Two pathways for SF where formation of polar transition state followed by triplet generation and a direct single step mechanism have been reported.39 Wasielewski and co-workers reported significant triplet formation in cofacial/slip-stacked PDI dimers40,41 through polar transition state and polycrystalline thin films of slip-stacked PDI through single step mechanism.35 State-of-the-art theory supports that slip-stacked co−facial arrangement of molecular pairs is a prerequisite for singlet fission.42,43 Access to triplet excited states in monomeric/aggregate/ single crystalline PDI-based chromophores continues to possess immense interest.44 Our ongoing interest toward the twisted chromophoric structures,45−48 for promising excited state properties, encouraged us to investigate the influence of coretwist in generating the triplet excited state in monomeric and crystalline PDI. Here we report the first example of the heavy atom and core-twist induced triplet formation in monomeric PDI-Br4. Interestingly, slip-stacked arrangement of PDI-Br4 exhibits combination of spin−orbit (SO) coupling and SF mediated triplet generation in polycrystalline thin film.

INTRODUCTION Perylenediimide (PDI) chromophore received immense attention in materials and biological applications due to remarkable photo, thermal, and chemical stability.1,2 Despite being well-explored for photofunctional applications,3−11 PDIs lack essential features such as solid-state fluorescence and access to triplet state. Reported methods to access triplet state of PDI makes use of bimolecular triplet sensitization,12 incorporation of sulfur13 and heavy metals such as Ir,14 Pt,15 and Ru16 that promote intersystem crossing (ISC). Unprecedented triplet state of perylenediimide was observed in unsymmetrically substituted PDIs.17 Though the triplet excited state is achieved in monomeric form, most of the organic molecules undergo triplet−triplet annihilation and lose their triplet excited state properties upon formation of assembly.18 Retention/generation of the triplet excited state in assembled/ solid state is still a challenging task in organic materials.18 Generation of triplet excited state in assembled chromophoric systems through singlet exciton fission (SF) process is an emerging topic of interest. SF is a spin allowed process whereby a singlet excited chromophore is energetically downconverted into two triplet excitons.18 SF mediated formation of two triplet excitons per photon can, in principle, increase the Shockley-Queisser limit for power conversion efficiency from 32% to 44% in solar cells.19−21 Till date, SF has been observed in crystalline anthracene,22 tetracene,23 pentacene,24 1,3diphenylisobenzofuran,25 rubrene,26 bipentacene,27 carotenoid,28 hexacene,29 TIPS-pentacene,30,31 zeaxanthin,32 terrylene,33 thiophene polymer,34 and perylenediimide35 having triplet quantum yield ranging from 1 to 200%. SF was also observed in concentrated solution of TIPS-pentacene36 and very dilute solution of bipentacene37 with quantitative triplet yield. Recently, crystal packing and crystallite size dependence © 2016 American Chemical Society

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EXPERIMENTAL METHODS

Synthesis. The synthetic details and characterization of the reported molecules PDI-Br0−4 are presented in the Supporting Information. Crystal structure data of the derivatives PDI-Br2−4 are tabulated in Table S1, Supporting Information. Electrochemistry. Cyclic voltammetry was performed on 1 mM solutions of PDI-Br0−4 in dry CH2Cl2 with 100 mM Received: January 23, 2016 Revised: April 2, 2016 Published: April 7, 2016 8443

DOI: 10.1021/acs.jpcc.6b00755 J. Phys. Chem. C 2016, 120, 8443−8450

Article

The Journal of Physical Chemistry C

photolysis studies. Triplet quantum yields upon direct photoexcitation (532 nm) were determined using [Ru(bpy)]Cl2 in methanol as standard with nonsaturating laser intensities. Equal volume of 0.2 mM solution of β-carotene was added to optically matched solutions of reference and the sample (see Supporting Information for details). Relative triplet quantum yield of PDI-Br4 in toluene solution on excitation at 355 nm was calculated from the following equation,

tetrabutylammonium hexafluorophosphate (TBHFAP) as the supporting electrolyte. All measurements were done using a platinum disc working electrode, a platinum wire counter electrode, and a Ag/AgCl reference electrode. All redox potentials were referenced to ferrocene as an external standard. Thin Film Characterization. Film samples PDI-Br0−4 were spin-coated from a 10 mg per mL solution of chloroform at 1000 rpm on quartz plate. Annealed samples were placed in a glass Petri dish above a reservoir of CH2Cl2 that had been allowed to equilibrate for 15 min prior to annealing. The samples were removed, dried under reduced pressure and stored in the nitrogen atmosphere. Film thickness was measured by profilometry using Veeco Dektak 150 surface profiler. Powder X-ray diffraction (PXRD) measurements on the annealed films were performed using Emperean, PANalytical powder diffractometer with reference irradiation of Cu Kα = 1.540 Å. Steady-state Raman spectra of PDI-Br4 polycrystalline thin film were recorded using a HR800 LabRAM confocal Raman spectrometer, operating at 20 mW laser power using a Peltier cooled (−74 °C) CCD detector. Raman spectra were collected in a quartz cuvette using a He−Ne laser source having an excitation wavelength of 633 nm and with an acquisition time of 30 s using a 50x objective. Optically matched neat and PMMA blend films of PDI-Br4 having an absorbance of 0.03 at excitation wavelength (λex = 355 nm) were prepared by spin coating the chloroform solution of PDIBr4 and chloroform solution of PMMA+PDI-Br4 mixture, respectively. Steady State Absorption and Emission Spectroscopy. Absorption spectra were recorded in Shimadzu UV-3600 UV− vis-NIR while emission (fluorescence/phosphorescence) and excitation spectra were performed in Horiba Jobin Yvon Fluorolog spectrometer. All spectroscopic experiments were performed using standard quartz cuvettes of path length 1 cm for solution in dried and distilled solvents. The solution state fluorescence quantum yields were determined by using optically matched solutions. Fluorescein dissolved in ethanol (Φf = 79%) was used as the standard for quantum yield measurements. Film measurements were carried out directly on the films, which were placed at a 20° angle to the excitation beam. Fluorescence decay measurements were carried out in an IBH picosecond single photon counting system. The excitation laser source used was 439 nm with a pulse width of less than 100 ps. The fluorescence decay profiles were deconvoluted using IBH data station software version 2.1 and fitted, minimizing the χ2 values of the fit to 1 ± 0.05. Steady state resonance Raman spectroscopy of PDI-Br4 in toluene solution was performed with the method described earlier. Femtosecond Transient Absorption Spectroscopy. Complete details of the fTA experiments are reported earlier.45,47 PDI-Br0−4 in toluene solution and polycrystalline thin film samples were excited at 400 nm with 4 mJ per pulse at 1 kHz. Solution samples were dissolved in toluene to an optical density between 0.5 and 0.7 at the excitation wavelength in a 0.4 mm path length quartz cuvette. Nanosecond Transient Absorption Spectroscopy. Laser flash photolysis experiments of the argon purged solutions were carried out in an Applied Photophysics Model LKS-60 laser kinetic spectrometer using the second and third harmonic (355 and 532 nm, pulse duration ≈ 7 ns) of a Quanta Ray INDI-40−10 series pulsed Nd:YAG laser. Triplet states of the PDI-Br0−4 in toluene were confirmed using the measurement of oxygen purged solutions through nanosecond flash

φT(355nm) = φT(532nm) ×

ΔA355nm A × 532nm ΔA532nm A355nm

(1)

Triplet Quantum Yield Measurement in Thin Film. Method employed by Wasielewski and co-workers35 to quantify quantum yield of triplet excited state in polycrystalline thin film of PDI-Br4 is briefly described here. For quantification of SF, 60 ± 5 nm polycrystalline thick film of PDI-Br4 was used (see Supporting Information for details). The number density (Nd) of molecules in the polycrystalline thin film of PDI-Br4 evaluated from X-ray structure is 12.6 × 1020 cm−3 (see Supporting Information for details). The excitation density ξ in the film is calculated from the following equation, ξ=

EλK (1 − 10−A) la

(2) 15 −1

−1

where, E = 2.00 ± 0.1 mJ, K = 5.034 × 10 J nm , λ = 532 nm, A = 0.195 ± 0.004 at 532 nm, l = (0.60 ± 0.05) × 10−5 cm, and a = 0.785 cm2 (see Supporting Information for details). Using these parameters, we have found the excitation density to be ξ = (4.11 ± 0.05) × 1020 cm−3. Percentage of molecules initially excited = ξ/Nd = (4.11 ± 0.05) × 1020/12.6 × 1020 = (32.56 ± 1) %. Since the absorbance at 550 nm is 0.240 ± 0.004, the expected initial ΔA at 550 nm = -(0.3256 ± 0.01) x (0.240 ± 0.004) = −0.0782 ± 0.002, that arises exclusively due to ground state bleaching and no T-T absorbance in the nTA measurement. In order to estimate the observed ground state bleach, A(S0), A(T1), and A(T1) − A(S0) spectra were generated from the nTA and ground state absorption spectra as per the report35 (Figure S15, Supporting Information). From the generated transient spectra, observed ground state bleach is found to be 0.0616 ± 0.004. Hence the calculated SF triplet quantum yield = (0.0616 ± 0.004)/(0.0782 ± 0.002) = 79 ± 5%. On the basis of eq 1, the relative triplet quantum yield in polycrystalline thin film of PDI-Br4 is estimated to be 105 ± 5% (see Supporting Information for details). Computational Details. Ground-state optimized structure and harmonic oscillator frequencies were computed using density functional theory (DFT) at the Becke’s three parameter functional 49 in combination with the Lee−Yang−Parr correlation functional50 (B3LYP) and 6-31+G(d,p) basis set. Vertical excitation energies and oscillator strengths were calculated employing time dependent DFT (TD-DFT) at the B3LYP/6-311+G(d,p) level of theory. Vertical excitation energy and oscillator strength for the slip-stacked dimer were calculated from TD-DFT at the B3LYP/LANL2dz level of theory. All computations were performed with the Gaussian 09 program suite.51

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RESULTS AND DISCUSSION Synthesis and Single Crystal X-ray Structural and Electrochemical Analyses. Brominated perylenediimide derivatives PDI-Br1−4 (Figure 1a) were synthesized and 8444

DOI: 10.1021/acs.jpcc.6b00755 J. Phys. Chem. C 2016, 120, 8443−8450

Article

The Journal of Physical Chemistry C

distance and the longitudinal shift between the adjacent PDI units in the crystalline state. Orbital overlap between adjacent perylenediimide units in PDI is estimated from X-ray crystal structure to be 46%.2 Percentage overlap between the vicinal PDI π−surfaces in crystalline state is found to be 22.1% for PDI-Br2, 16.8% for PDI-Br3 and 9.5% for PDI-Br4 (Figure 1e-g, Figure S2a-b, Supporting Information). Quantum theory of atoms in molecules analysis suggests the presence of CO···π58,59 (Figure S2d, Supporting Information) interaction at a distance of 3.10 Å between the neighboring PDI units in crystalline PDIBr4. PDI-Br3 displays (Figure S2c, Supporting Information) C− H···π60,61 interaction at a distance of 2.76 Å along with the C− Br···π62 interaction at a distance of 3.48 Å. Cyclic voltammetric measurement of PDI in DCM (Figure S3, Supporting Information) shows two reversible reduction peaks (−0.547 and −0.728 V) with reference to Ag/AgCl electrode, characteristic of PDI, as reported.63 On successive increase in number of electron releasing bromine atoms per PDI unit, reversible reduction potential (Table S2, Supporting Information) is significantly decreased to −0.319 and −0.528 V for PDI-Br4. Decrease in the reduction potential of PDI-Br4 when compared to the PDI could arise from the presence of additional bromine atoms in PDI-Br4. Steady State Photophysical Properties in Solution. UV−vis absorption spectra of PDI-Br0−2 in toluene (Figure 2a,

Figure 1. (a) Structure of the model derivative PDI (also referred hereafter as PDI-Br0) and brominated derivatives PDI-Br1−4; (b−d) side view of dimers in PDI-Br2, PDI-Br3 and PDI-Br4, respectively; (e− g) corresponding top view.

characterized as per the reported procedure (Scheme S1 and Figure S1, Supporting Information).52 Except for PDI-Br3, synthesis of PDI-Br1−2/PDI-Br4 has been reported.53−56 Würthner and co-workers have explored the extent of coretwist upon halogenation of PDI at bay-positions.57 X-ray structure of crystalline solvated PDI-Br457 and crystalline solvent-free PDI-Br253 were well-documented, however, crystalline solvent-free PDI-Br4 was not reported earlier. Slow evaporation from different ratios of dichloromethane/hexane mixture offered fluorescent crystals of PDI-Br2−4 in triclinic space group P-1 (Table S1, Supporting Information). π−π interactions between the neighboring PDI units appear unimpeded due to solvation in crystalline PDI-Br3 (vide infra). The present study focuses on the impact of core-twist in the photophysical properties of PDI-Br2−4 in monomeric (solution) vs crystalline state. The dihedral angle between the two identical halves (long axis) is found to be 2.4° and 39° in PDI-Br2 and PDI-Br4, respectively. PDI-Br3 exhibits dihedral angle of 27.9° and 37.8° between the two nonidentical halves due to asymmetry, suggesting varying degrees of core-twisting nature of PDI on successive bromination at the bay region. In the solvent-free single crystals of PDI-Br2, face to face π-surfaces are found at a distance of 3.51 Å along with the transverse shift of 2.93 Å and longitudinal shift of 1.0 Å (Figure 1b). Adjacent π−π surfaces are found at the distance of 3.54 Å along with the transverse shift of 2.39 Å and longitudinal shift of 4.23 Å in PDI-Br3 crystals (Figure 1c). Solvent free single crystals of PDI-Br4 exhibit nearest π surfaces at the distance of 3.66 Å along with the transverse shift of 2.40 Å and longitudinal shift of 5.64 Å (Figure 1d). On increasing the number of bromine atoms at the bay region, perylenediimide exhibits a gradual increase in the

Figure 2. (a) Absorption and (b) emission spectra (excitation wavelength: 480 nm) of PDI-Br0−4 in toluene; (c) fTA spectra of PDIBr4 in toluene (excitation wavelength: 400 nm); (d) corresponding SVD analysis.

Table S3, Supporting Information) exhibit three distinct bands corresponding to S0 → S1 transition, oriented along the longitudinal axis.64 Red-shifted absorption maxima with the increase of bromine atoms is fully consistent with the decreasing trend of S0 → S1 transition energy evaluated at time dependent density functional theory (TD-DFT) method (Table S4 and Figure S4, Supporting Information). Whereas S0 → S2 transition is symmetry forbidden in planar PDIs (i.e., PDI-Br0−2), significant absorbance at 425−440 nm in PDIBr3−4 arises due to core-twist, consistent with earlier reports.64,65 Oscillator strength calculated from TD-DFT method for S0 → S2 transition shows gradual increase from 8445

DOI: 10.1021/acs.jpcc.6b00755 J. Phys. Chem. C 2016, 120, 8443−8450

Article

The Journal of Physical Chemistry C

Br0−4 in toluene were carried out upon excitation of the samples with 7 ns, 355 and 532 nm laser. Photoexcited PDI shows weak signal corresponding to triplet excited state as a consequence of poor ISC (ΦT ∼ 0.3%).12 Substitution of one/two bromine atoms at the bay region of PDI resulted in slight increase of ISC (ΦT ≤ 1%), estimated using triplet−triplet energy transfer method upon photoexcitation at 532 nm.12 Ultrafast inherent fluorescence vs slower ISC could be attributed to the lack of heavy atom effect in PDI-Br1−2 (Figure S7a, Supporting Information), which is in good agreement with reported perylene based chromophoric systems.67,68 Upon excitation at 532 nm, PDI-Br3−4 in toluene exhibit strong absorption bands at 370 and 575 nm in addition to ground state depletion at 440, 490, and 530 nm (Figure 3, and

PDI (0.00) to PDI-Br4 (0.11), in agreement with our experimental data (Table S4, Supporting Information). Steady state fluorescence spectrum (Figure 2b) of PDI in toluene shows vibronic bands at 536, 577, and 627 nm as reported.12 On increasing the number of bromine atoms per PDI unit, fluorescence quantum yield (Table 1) is reduced Table 1. Photophysical Properties of PDI-Br0‑4 molecule PDI PDI-Br PDI-Br2 PDI-Br3 PDI-Br4

εa, L mol−1cm−1 71000 60088 51587 39801 26743

Φf,b % 97 97 94 85 64

ΦT,c % e