Synthesis, Optical, Electrochemical, DFT Studies, NLO Properties and

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C: Plasmonics; Optical, Magnetic, and Hybrid Materials

Synthesis, Optical, Electrochemical, DFT Studies, NLO Properties and Ultrafast Excited State Dynamics of Carbazole Induced Phthalocyanine Derivatives Somdatta Bhattacharya, Chinmoy Biswas, Sai Santosh Kumar Raavi, Jonnadula Venkata Suman Krishna, Narra Vamsi Krishna, Lingamallu Giribabu, and Venugopal Rao Soma J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b01531 • Publication Date (Web): 09 Apr 2019 Downloaded from http://pubs.acs.org on April 9, 2019

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The Journal of Physical Chemistry

Synthesis, Optical, Electrochemical, DFT Studies, NLO Properties and Ultrafast Excited State Dynamics of Carbazole Induced Phthalocyanine Derivatives Somdatta Bhattacharya,1 Chinmoy Biswas,2 Sai Santosh Kumar Raavi,2 Jonnadula Venkata Suman Krishna,3,4 Narra Vamsi Krishna,3,4 Lingamallu Giribabu,3,4,* Venugopal Rao Soma1,# 1Advanced

Centre of Research in High Energy Materials (ACRHEM), University of Hyderabad,

Hyderabad 500046, Telangana, India 2

Ultrafast Photophysics and Photonics Laboratory, Department of Physics, Indian Institute of

Technology Hyderabad, Kandi 502285, Telangana 3Polymers

& Functional Materials Division, CSIR-Indian Institute of Chemical Technology,

Tarnaka, Hyderabad-500007, India. 4Academy

of Scientific and Innovative Research, CSIR-Indian Institute of Chemical

Technology, India. Authors for correspondence: * [email protected] and # [email protected]

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Abstract In this paper we present results from the detailed investigations on the synthesis, optical, emission, electrochemical, ultrafast nonlinear optical (NLO) properties along with the excited state dynamics of Zinc (II) 2,10,16,24 tetrakis (9-phenyl-9H-carbazol-2-yl) phthalocyanine (CBZPC1), Zinc (II) 2,10,16,24 tetrakis (4-(9H-carbazol-9-yl)phenyl) phthalocyanine (CBZPC2). Due to the presence of carbazole moieties the Soret band was found to be broadened. The emission studies performed using different solvents revealed the fluorescence yields in the range of 0.10–0.27 and the time-resolved fluorescence data revealed radiative lifetimes of, typically, few ns. Femtosecond transient absorption measurements indicated the formation of triplet states within the first ns of photo-excitation. From the cyclic volumetric studies, the oxidation and reduction processes were found to be ring centered. Spectral changes in the UV-visible absorption were recorded by means of spectro-electrochemical analysis at an applied potential. The DFT and TD-DFT analysis was employed using B3LYP hybrid functional theory and 6-31G(d,p) basis set in the Gaussian 09 package. The NLO properties of CBZPC1 and CBZPC2 were investigated using the Z-scan technique and femtosecond (fs) pulses with kHz and MHz repetition rates. Closed and open aperture Z-scan data were recorded at three different wavelengths of 600 nm, 700 nm, 800 nm and the NLO coefficients were extracted from both the data. Two-photon absorption (TPA) was the dominant mechanism observed in the open aperture Z-scan data. The real and imaginary parts of the (3) along with the two-photon absorption cross-sections were evaluated. Our NLO data and large 2PA coefficients and crosssections obtained indicate the potential of these compounds for applications in optical limiting and optical switching applications. IICT Manuscript Communication Number: IICT/Pubs./2019/123 2 ACS Paragon Plus Environment

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1. Introduction Nonlinear optics (NLO) is considered the most promising technology towards the evolution of several fields such as photonics, opto-electronics, and biomedicine.1-5 Till date, several NLO were developed to study their potential in applications of optical signal processing, telecommunications.6-7 Organic compounds such as dyes have been found to be particularly potential NLO candidates.8-10The strong NLO properties originate from the π delocalized networks, resulting in a major intensification in the triplet excited state lifetimes. The research in this direction has led to the progress of the novel π conjugated materials.11 Incidentally, the tetrapyrrolic pigments like phthalocyanines and its metallic derivatives have received the huge responsibility towards the NLO properties, due to its ability of holding more than seventy (70) non-metallic and metallic ions in its cavity, incredible thermal, chemical, optical stability. In addition, phthalocyanines possess ultrafast and large third-order nonlinearity which can further be developed from its derivatives through rational synthetic procedures.12-15 The metallic phthalocyanines (MPcs) can also exhibit interesting properties.The optical constants of MPc thin films provide information concerning microscopic characteristics of the material. It has also been shown that the third order electronic susceptibility (χelec) in MPc thin films depends on the experimental conditions such as the temperature of the annealing process as well as the type of central metallic atom in metallophthalocyanine molecular ring, planarity of the molecule and filled ‘d’ valence orbitals16,17 In order to further enhance the NLO properties of MPc thin films they were deposited in the form of nanocrystals (NCs) and nanoparticles.18 In NLO applications, optical limiting (OL) is the important phenomenon to overcome the high intensity radiation and permitting only the transmission of low intense irradiation at ambient conditions.19 Usually, the OL is influenced by several mechanisms such as reverse saturable absorption (RSA), nonlinear 3 ACS Paragon Plus Environment


refraction and reflection, two-photon absorption and it has been proven earlier through many studies that phthalocyanines respond quickly to OL processes.13,20 The macrocyclic nature of the phthalocyanine is favourable for functionalizing the various substituents at its peripheral or nonperipheral region to enhance the solubility. Habitually, phthalocyanines have an intense absorption band at 600-700 nm (Q band) region, and less intense absorption band in 300-400 nm region (Soret band), which arises due to the π-π* transitions. The molar extinction coefficients of Q bands are very high with typical values of 105 M-1cm-1 and, hence, phthalocyanines demonstrate potential for various optoelectronic applications.21-23 The optical properties of phthalocyanines, in general, are influenced by their solubility and aggregation. The planar nature of phthalocyanine macrocycles tends to aggregate even at M concentration and hinders several device applications. In this regard, several efforts have been made towards the design and development of the highly soluble and aggregation minimized phthalocyanines.24-30 Over the last several years our group was actively involved in designing quite a few novel phthalocyanines and/or similar molecules along with their metallic derivatives by introducing substituents either at peripheral or non-peripheral positions and investigated their third-order NLO properties and excited state dynamics.31-35 Previous reports and their detailed studies suggest that it is indispensable to introduce functional groups at peripheral positions of phthalocyanine to suitable for various optoelectronic applications. To achieve the strong NLO coefficients, we have designed and synthesized two carbazole substituted phthalocyanines in which the carbazole moiety tethered to phthalocyanine by using two different positions of the carbazole i.e., Carbazole ring substituted (9-phenyl-9H-carbazol-2-yl) (CBZPC1) and N-phenyl substituted (4-(9H-carbazol-9-yl) phenyl) (CBZPC2) phthalocyanines (see figure 1). It is well established through various studies reported in recent literature that carbazoles are rich in 4 ACS Paragon Plus Environment

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photochemistry and have been used in several optoelectronic applications.36 Incorporation of these carbazole moieties at the periphery of the phthalocyanine would influence the sterical and electronic properties of the macromolecule. Also, these phthalocyanines have the ability to be soluble in the polar aprotic solvents and shifts the redox processes towards the positive potentials. Herein, we report the structural, optical, and ultrafast NLO property studies of the CBZPC1and CBZPC2. In addition, we implemented several photophysical, electrochemical, spectroelectrochemical studies in different solvents at varying concentrations to understand the structure-property relationship. Finally, the femtosecond transient absorption technique was employed to understand the excited state dynamics in both the molecules.

N

N

N

N N

N N

N

N N Zn N N N N

CBZPC1

N

N N

Zn

N N

N

N N

N

CBZPC2

N

Figure 1. Molecular structures of CBZPC1 and CBZPC2

2. Experimental Section: 2.1. Materials (9-phenyl-9H-carbazol-2-yl) boronic acid, (4-(9H-carbazol-9-yl)phenyl) boronic acid, [Pd(pph3)4], 1-pentanol, HCl, Li metal were purchased from TCI and were used as it is. All the solvents viz., dichloromethane, tetrahydrofuran, dimethylsulfoxide, N,N-dimethylformamide, toluene, methanol, potassium carbonate, chloroform, n-haxane were purchased from SD Fine 5 ACS Paragon Plus Environment


chemicals limited, India and were dried before further use. Zinc Acetate was purchased from Qualigens Chemicals Ltd, India.

2.2 Methods and Instrumentation 2.2.1. Characterization: 1H NMR spectra were recorded in CDCl3 solutions on AVANCE 300 MHz spectrophotometer. Electron spray ionization-mass spectrometer recorded on The Waters 2996 Photodiode array detector. Matrix-assisted laser desorption ionization time-of-flight (MALDI–TOF) mass spectrometry performed on Shimadzu Biotech Axima Performance 2.9.3.20110624: Mode Reflectron-Hi Res, Power: 85using TMS as standard. FT-IR spectra (KBr pellets) were recorded on a Bruker. 2.2.2. Optical Studies: Shimadzu UV-3600, UV-Visible-NIR spectrophotometer was used to record the absorption spectra of studied compounds. Fluorolog-3 spectrofluorometer (Spex model, JobinYvon) was used to record the steady-state fluorescence spectra, for solutions with an optical density at the wavelength of excitation (ex) ~0.06. Zinc tert-butyl phthalocyanine (=0.37 in benzene) was used as the reference to calculate the fluorescence quantum yields () by integrating its fluorescence bands.37 Picosecond time-correlated single photon counting (TCSPC) setup (FluoroLog3-Triple Illuminator, IBH Horiba JobinYvon) was employed to record the fluorescence lifetime measurements my means of a ps light emitting diode laser (NanoLED, λex= 670 nm) as excitation source. The decay curves were recorded by monitoring the fluorescence emission maxima of the phthalocyanine macrocycle (λem= 700 nm). Photomultiplier tube (R928P, Hamamatsu) was employed as the detector. The lamp profile was recorded by placing a scattered (dilute solution of Ludox in water) in place of the sample. The width of the instrument function was limited by full width at half maximum (FWHM) of the excitation 6 ACS Paragon Plus Environment

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source, ~635 ps at 670 nm. Decay curves were analyzed by nonlinear least-squares iteration procedure using IBH DAS6 (version 2.3) decay analysis software. The quality of the fits was judged by the χ2 values and distribution of the residuals. 2.2.3. Electrochemical Studies: Electrochemical measurements were performed on a computer controlled potentiostat of CH instruments (model CHI 620C). The experiments were performed using 1 mM concentration solutions ofCBZPC1 and CBZPC2 in THF solvent at a scan rate of 100 mV/s using 0.1 M tetrabutyl ammonium perchlorate (TBAP) as the supporting electrolyte. The working electrode was glassy carbon, standard calomel electrode (SCE) was reference electrode and platinum wire was an auxiliary electrode. After a cyclic voltammogram (CV) had been recorded, ferrocene was added, and a second voltammogram was measured. The optical thin layer electrochemical studies were carried on Maya 2000 Ocean Optics software using DTMINI-2-GS, UV-VIS-NIR LIGHTSOURCE. 2.2.4. Theoretical calculations: By using the Gaussian 09 package on a personal computer all the theoretical analysis were performed.38 The optimized energy minimized structures of CBZPC1 and CBZPC2 were stable with global minimal energy. These studies were performed using the density functional theory (DFT) at B3LYP hybrid functional39 theory at 6-31G(d,p) basis set40 in the Gaussian program. The excited state properties like percentage of molecular contribution, oscillatory strength, and singlet transition energy in the tetrahydrofuran solvent were obtained by performing the time-dependent density functional theory (TD-DFT). The integral equation formalism polarizable continuum model (PCM)41,42 within the self-consistent reaction field (SCRF) theory was used in the TD-DFT calculations to describe with M06-2x functionand the solvation of the phthalocyanines in tetrahydrofuran. The software GaussSum



2.2.5 was employed to simulate the major portions of the absorption spectra and to interpret the nature of transitions.43-45 2.2.5. Ultrafast NLO and transient absorption studies: For all the NLO studies, the standard Z-scan technique was used.46 In the open aperture Z-scan configuration the nonlinear absorption (2) of the sample is measured while closed aperture Z-scan data provides the sign and magnitude of the nonlinear refractive index (n2). The Z-scan experiments were performed using (1) Ti:Sapphire laser pulses from an oscillator (Chameleon, M/s Coherent) with input pulses of duration ~150 fs, 80 MHz repetition rate and tunable in the 680-1080 nm spectral region47-49 and (2) Ti:Sapphire laser pulses from an amplifier (Libra, M/s Coherent) with input pulses of duration ~70 fs, repetition rate of 1 kHz and in the 600-800 nm spectral regime. An input beam diameter of ~2 mm combined with a plano-convex lens of focal length 100 mm for experiment (1) and 150 mm for experiment (2) was used to focus the beam onto the sample.

Figure 2. A schematic of the complete transient absorption experimental setup. M1-M7 are the mirrors used for guiding the pump beam (solid line path) and m3-m9 are the mirrors used for guiding the probe beam (dotted line path). BS represents the beam splitter.


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For experiment (1), the wavelength range studied were 700-900 nm while for experiment (2), the wavelength range studied were 600-800 nm which was tuned with the help of an optical parametric amplifier (TOPAS-C). Neutral density filters were utilized for attenuating the input pulse energy. A 1-mm glass cuvette was used for keeping the sample solutions. The thickness of the samples was less than the typical Rayleigh range in both the experiments (1) and (2). The pump-probe experiments [transient absorption spectroscopy (TAS)] were performed using TiSapphire (~70 fs, 1 kHz repetition rate) laser pulses and an input wavelength of 800 nm. The pump pulse wavelength was achieved with the help of optical parametric amplifier (TOPAS-C) while the probe pulse used was a white light continuum (WLC) in the spectral range of 440-820 nm. The schematic of the setup used is shown in figure 2. For CBZPC1 (in THF), the excitation/pump wavelength used was 650 nm while for CBZPC2 (in DCM), the excitation/pump wavelength used was 690 nm.

2.3. Synthesis 2.3.1. Synthesis of 4-(9-phenyl-9H-carbazol-2-yl) phthalonitrile (CBZPN1): The compound CBZPN1 was synthesised by Suzuki coupling reaction (figure 3). To (9-phenyl-9H-carbazol-2yl) boronic acid (800 mg, 2.7 mmol), 4-iodophthalonitrile(704 mg, 2.7 mmol), and [1,1′ Bis (diphenylphosphino) ferrocene] dichloro palladium (II) (110.16 mg, 0.13 5mmol) were dissolved in 30 ml of dry toluene and 10 ml of dry under nitrogen atmosphere. To this a solution of 2M Na2CO3 was added to the reaction mixture. After that again reaction mixture was purged with nitrogen for another 30 minutes and then it was stirred for 4 hrs at reflux temperature. After completion of the reaction, the reaction mixture was cooled to RT and was dried under reduced pressure followed by extracted in ethyl acetate and water. Column chromatography was done by using Hexane and ethyl acetate (3:1) solvent mixture as eluent with silica gel as stationery phase. 9 ACS Paragon Plus Environment


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Yield 61%. ESI-MS (m/z) = 370 (calculated mass = 369.42), FT-IR (KBr) νmax (cm-1)=306.23, 2923.03, 2853.28, 1726.26, 1625.93, 1595.40, 1485.68, 1438.54, 1337.12, 1220.16, 1182.90, 907.92, 814.81, 747.16, 698.03, 628.86; 1H NMR (CDCl3, 500 MHz, TMS) δ (ppm) = 8.26 (d, 1H) 8.18 (d, 1H) 8.04 (d, 1H) 7.97 (dd, 1H) 7.84 (d, 1H) 7.67 (t, 2H) 7.58 (dd, 2H) 7.54 (m, 2H) 7.48 (m, 2H) 7.41 (d, 1H) 7.34(dd, 1H).

OH

N

B OH

N

I

N

+

+ N

N Pd(dppf)Cl2 Aq.Na2CO3 THF, Toluene, Reflux, 4h

Pd(dppf)Cl2 Aq.Na2CO3 THF, Toluene, Reflux, 4h

N

N

N

N CBZPN1

N

CBZPN2 N Li metal, Pentanol, 150oC, 6h HCl, DMF, Zn(OAc)2

Li metal, Pentanol, 150oC, 6h HCl, DMF, Zn(OAc)2

N

N

N

N N

N N

N N

Zn

N N

N

N

N N

N

I

OH B OH

N N

Zn

N N

N

N N

N

CBZPC1

Figure 3. Synthetic route for CBZPC1 and CBZPC2.


CBZPC2

N

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2.3.2. Synthesis of 2(3), 9(10),16(17), 23(24) tetrakis (9-phenyl-9H-carbazol-2-yl) phthalocyanine Zinc (II) (CBZPC1): CBZPC1 was synthesized by adding CBZPN1 (300mg, 0.81mmol) in 10 mL of 1-pentanol with a catalytic amount of Li metal, and was stirred for about 5 hrs at 150oC. Later, the reaction mixture was cooled to RT and was stirred at RT for 1hr by adding 2 mL of HCl. Then the reaction mixture was extracted with DCM and washed with water. Then the solvent was removed by using a rotary evaporator. The compound was collected in 50 mL round-bottomed flask, to this 10 mL of DMF was added and stirred at 150oC by adding zinc acetate (742 mg, 4.05 mmol) for 3 hrs. The reaction mixture was cooled to RT followed by removal of solvent under reduced pressure. Later, the compound was extracted with DCM and water to remove excess zinc acetate and solvent was removed by rotary evaporator. The obtained residue was subjected to silica gel column chromatography using with hexane, followed by 1:1 ratio of hexane and DCM, finally with pure DCM to get the desired compound in 77% yield. MOLDI-TOF (m/z) = 1543 (calculated mass=1543.05). FT-IR (KBr) νmax (cm-1) = 3290.35, 2854.13, 2551.64, 1772.17, 1725.24, 1598.08, 1565.52, 1501.55, 1483.99, 1453.36, 1435.03, 1394.05, 1333.27, 1234.48, 1133.12, 1098.47, 1018.40, 962.15, 896.75, 864.13, 813.33, 769.95, 627.70, 591.23. 2.3.3. Synthesis of 4'-(9H-carbazol-9-yl)-[1,1'-biphenyl]-3,4-dicarbonitrile (CBZPN2):The compound CBZPN2 was synthesised by a similar procedure adopted for CBZPN1. The only difference is that we have taken (9-phenyl-9H-carbazol-2-yl)boronic acid instead of (9-phenyl9H-carbazol-2-yl) boronic acid. Yield is 64%. ESI-MS (m/z) = 370 (calculated mass = 369.1). FT-IR (KBr) νmax (cm-1) = 3061.12, 2923.02, 2853.10, 2232.14, 1726.53, 1598.42, 1449.84, 1335.01, 1222.22, 1016.24, 829.85, 772.11, 724.50; 1H-NMR (CDCl3, 500 MHz, TMS) δ (ppm) = 8.17 (d, 2H) 8.11 (d, 1H) 8.03 (dd, 1H) 7.94 (d, 1H) 7.80 (dd, 4H) 7.45 (m, 4H) 7.33 (t, 2H). 11 ACS Paragon Plus Environment


ESI-MS, 1H

NMR, FTIR spectra of CBZPN1, CBZPCN2 are provided in supporting

information file (Figures S1-S6 and S8) 2.3.4. Synthesis of 2(3), 9(10),16(17), 23(24) tetrakis (4-(9H-carbazol-9-yl)phenyl) phthalocyanine Zinc (II) (CBZPC2):CBZPC2 was synthesized by a similar procedure that adopted for the synthesis of CBZPC2. The only difference is that we have taken CBZPN2instead of CBZPN1. MALDI-TOF (m/z) =1543.76(calculated mass=1543.05). FT-IR (KBr) νmax (cm-1) = 2956.02, 2923.08, 2853.39, 1725.16, 1601.87, 1521.17, 1476.68, 1334.67, 1221.21, 1099.22, 1065.28, 820.52, 772.25, 224.53.

3. Results and Discussion 3.1. Synthesis and characterization The phthalonitrile 4-(9-phenyl-9H-carbazol-2-yl)phthalonitrile (CBZPN1), and 4'-(9H-carbazol9-yl)-[1,1'-biphenyl]-3,4-dicarbonitrile(CBZPN2) were synthesized by Suzuki coupling reaction. The two phthalocyanines CBZPC1,andCBZPC2 were engineered by cyclotetramerization of CBZPN1, CBZPN2 in 1-pentanol at reflux temperature under nitrogen atmosphere in the presence of Li-metal as a non-nucleophilic base catalyst. The crude compound was purified by column chromatography and Li metal was removed by treatment with HCl and followed by zinc metallation was achieved by treatment with Zn(OAc)2in DMF solvent. Finally pure phthalocyanines were obtained throughrecrystallization process. Both the target compounds and their intermediates were characterized by elemental analysis, Mass, FT-IR, 1H NMR, UV-Visible absorption and emission spectroscopies as well as electrochemical techniques. The elemental analysis data is presented in the experimental section and found to be satisfactory. The MALDITOF spectrum consisted of a molecular ion peak at 1543 for CBZPC1 and for CBZPC2 at 12 ACS Paragon Plus Environment

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1543.76 that is assigned to the presence of corresponding phthalocyanine (See supporting information figures S7 and S9). 3.2. UV-Visible Absorption Studies Figure 4 illustrates the absorption spectra of both CBZPC1 and CBZPC2 phthalocyanines in various solvents ranging from non-polar solvent toluene to polar solvent DMF. The absorption spectra illustrated two or three peaks in 300-400 nm region belonging to the Soret band of phthalocyanine, which arises due to deeper -levels to LUMO transition as well as carbazole absorption due to π-π* transitions. Whereas the 600-700 nm spectral region consists of relative intense Q bands, which arises due to HOMO to LUMO transition (-*) [Figures 4(a) and 4(b)]. These peaks belong to pure phthalocyanine since carbazole does not have any absorption in this region. The corresponding absorption maixma (λmax) and logerithemic of molar extinction coefficients (log ε) are presented in Table 1. It is well known from literature that the solubility and aggregation of phthalocyanine macrocycle is due to its planarity nature and it is evident in absorption spectra depicting a peak in the high energy side of Q bands i.e. at ~625 nm for  concentration.36 These problems can be avoided by tethered soluble organic chromophores such as carbazole group at peripheral positions phthalocyanine either direct link (CBZPC1) or N-phenyl substituted carbazole (CBZPC2). As expected, no evidence of aggregation in both phthalocyaines as demonstrated by the sharp unperturbed single Q band, typical metallo phthalocyanine with D4h symmetry. From the Figure 4 data it is evident that both phthalocyanines CBZPC1and CBZPC2 have a sharp singh peak at 694 nm and 690 nm, respectively in DCM solvent. We have also measured the absorption spectra of both phthalocyaines in non-coordinating solvents such as toluene and DCM as well as coordinating 13 ACS Paragon Plus Environment


solvents THF, DMF and DMSO, which will coordinate at axial position of a metal ion in central cavity of phthalocyaine macrocycle leading to further minimization of the aggregation. Further, we studied aggregation phenomena by increasing the concentration of phthalocyaine in THF solvent [Figure 5 presented here depicts the CBZPC2 data in THF while figure S11 (supporting information file) shows the CBZPC1 and CBZPC2 data in DCM]. From this we concluded that the phthalocyanines exhibited as monomeric form at different concentrations i.e., no new blueshifted band was observed due to aggregation and the appearance of Q band absorption maxima at 690 nm and 686 nm remainedintact as the concentration increased. Thus, the molar extinction coefficient almost constant and obeying the Beer-Lambert Law in the dilute concentrations. Similarly, we have carried out analogous studies in solvent DCM and it also follows Beer– Lambert’slaw and as the concentration increases absorption increases without change of ratio between Q band peak and the shoulder at 640 nm (Figure S11).


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Figure 4 . (a), (b) Absorption spectra of CBZPC1, CBZPC2 in different solvents, respectively.



(7)

1.2 1.0 0.8

0.3x10-5 0.5x10-5

(1)

(6)

Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(2)

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(3) (4)

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500

600

700

800

Wavelength (nm) Figure 5. Absorption spectral changes of CBZPC2 in THF at different concentrations: 0.3×10-5 (black, 1) 0.5×10-5 (red, 2) 0.8×10-5 (blue, 3) 1.0×10-5 (dark cyan, 4) 1.5×10-5 (magenta, 5) 2.0×10-5 M (dark yellow, 6) 2.5×10-5 M (navy blue,7).

3.3. Emission studies Qualitative evaluation of emission, including quantitative analysis of the fluorescence spectra and determination of the quantum yields (f) was performed for CBZPC1 and CBZPC2 so as to understand the effect of peripheral substitution on phthalocyanine macrocycle. Figures 6(a) and 6(b) illustrate the emission spectra of CBZPC1 and CBZPC2 in various solvents (toluene, DCM, THF, DMF and DMSO) by exciting at ~700 nm and the corresponding emission maxima as well as quantum yiels are presented in Table1. The steady state fluorscence spectra of CBZPC1 and CBZPC2 are similar, except for different emission maxima. From the data presented in Figures 6(a), 6(b) and Table 1, the emission spectra of CBZPC1 and CBZPC2 are consistent with both the Stokes rule and the rule of mirror symmetry between the absorption and 16 ACS Paragon Plus Environment

30

DCM DMF DMSO THF Toluene

(1)

(4)

(2)

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


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(5)

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(3)

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(4)

DCM DMF DMSO THF Toluene

10 (2)

5 0 660

680

700

720

740

Wavelength (nm) Figure 6. Steady state fluorescence spectra of (a) CBZPC1 (b) CBZPC2 in different solvents.


760


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Table 1 Absorption, emission data of CBZPC1&CBZPC2 in different solvents Sample

Solvent

λ

max

nm

Emission

(logɛ, −1

−1 a

λ

M cm ) CBZPC1

CBZPC2

max

nm

τ, ns (A%)

b

d

c

Фf

DCM

694(3.9)

720

0.24

DMF

693(4.2)

695

0.11

DMSO

697(3.9)

705

0.13

0.04 (38.8) 3.44 (61.1) 0.25(40.3) 4.54 (59.7) 8.30(100)

THF

690(4.1)

718

0.18

7.46(100)

Toluene

695(4.0)

720

0.26

7.85(100)

DCM

690(3.5)

698

0.22

0.13(100)

DMF

689(3.5)

691

0.12

DMSO

692(3.7)

697

0.10

THF

686(3.5)

696

0.20

0.07(20.2) 2.63(79.8) 1.27(18.3) 3.32(81.7) 7.46(100)

Toluene

692(3.8)

701

0.27

7.85(100)

Error limits: aλmax, ±1 nm, ε ± 10%. bError limits: λem, ±1 nm. cϕ, ± 0.01%. dError limits τ ≈ 10%.

fluorescence bands. It is clear from the Table 1 data that if the polarity of solvent increases, quantum yield reduces. This might be due to the increase of aggregation in polar solvents, which reduces the possibility of radiative deactivation i.e., fluorescence through dissipation of energy by the aggregates.34 All these aspects lead to the direct influence on the excited state life time of the phthalocyanines. The exited state life time of the CBZPC1 and CBZPC2 were measured in various solvents (e.g. toluene, DCM, THF, DMF and DMSO).

The data is illustrated in

FigureS13and corresponding singlet state lifetime data presented in Table 1. As in the case of steady state emission, the polarity of the solvents also affects the exited state lifetimes.


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3.4. Electrochemical Studies To study the redox behavior of the phthalocyanines, we have have employed electrochemical studies by employing cyclovoltametric studies. Typically, phthalocyanineis a 18 π-electron aromatic system, carries two negative charges it common oxidation state. Phthalocyanines have the capability of losing one or two electrons in its oxidation, gaining one to four electrons in its reduction. Under experimental conditions, present phthalocyanines (CBZPC1 and CBZPC2) undergo two one electron oxidations and two one electron reductions, all these processes are either quasi-reversible or reversible. The relative positions of the HOMO and LUMO levels can be shifted via changes in the electron density of the molecule brought about by electron donating or electron withdrawing substituents. To explorethe redox potentials of CBZPC1 and CBZPC2, cyclc voltammetry technique wasmeasured in THF by usingwith 0.1M tetrabutyal ammaonium perchlorate (TBAP) as supporting electrolyte and ferrocene as the external standard (Figure 7). The oxidation potentials for CBZPC1 and CBZPC2 found to be 1.01 V and 1.03 V and that of reduction potentials are found to be -1.03 V and -1.05 V, respectively. Generally phthalocyanines have the well-known electrochemical behaviour, based on that all the redox couples for CBZPC1 and CBZPC2 are assigned to the phthalocyanine ring. By employing the spectro electrochemical studies the nature of the redox couples can be confirmed.



Figure 7 . Cyclic voltammogram of CBZPC1 and CBZPC2 in THF using a glassy carbon electrode.

3.5. Spectroelectrochemical studies: During the redox reactions the spectral changes were confirmed by using the spectroelectrochemical studies for CBZPC1 and CBZPC2 phthalocyanines. The spectral changes of CBZPC1 at applied potential was depicted in Figures 8(a)-8(d). In CBZPC1, at a potential of -1.03 V [Figure8(c)] shows the spectral changes in Q band at 692 nm increases in intensity without appearance of any shoulder and appearance of new bands at 369 nm and 733 nm were observed. At same time the peak at 319 nm decreases in intensity. The changes in the sholder at 621 nm indicates disaggregation of the phthalocyanine macrocycle. While doing the reduction process, clear isosbestic points were observed at 314 nm, 20 ACS Paragon Plus Environment

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371 nm, 443 nm, 657 nm, 708 nm and 734 nm, from this one can deduce that these are formation of single product during the reduction process. All These changes are typical of a ring based oxidation and wereassigned to be [ZnIIPc2-]/[ZnIIPc3-]1-.50-52 The spectroscopic changes occurred when controlled potential switched to -1.35 V [Figure 8(d)]support the further reduction of the mono anionic species confirming the cyclo voltametric assignment of the couple to[ZnIIPc3-]1-/ [ZnIIPc4-]2-. While applying the reduction potentialfor phthalocyanine macrocycle there is decreasing in the B band and Q band intensities, with the formation of a new band at the 737 nm of characteristic reduction in ring. Figure 8(a), exemplified the spectral changes during the oxidation process at an applied potential of 1.01 V. Q band absorption at 691 nm reduces its intensity while both B band at 358 nm and sholder at 624 nm also reduces its intensity without formation of any new absorption bands. During this process, clear isosbestic points were observed at 364 nm, 621 nm, 722 nm. These changes in the absorption spectrum were assigned to oxidation of species from[ZnIIPc2-]1+ to [ZnIIPc1-]1+. Figure 8(b) shows the spectral changes during the oxidation process at a controlled potential of 1.75 V, which supports the further oxidation of monocationic species confirming the CV assesment of cople to

[ZnIIPc1-

]1+/[ZnIIPc0]2+. At this potential, the intensity of Q band absorption at 691 nm further reduced without shift, while new bands at 369 and sholder at 624 nm appeared with decreasing intensity without ant shift. Especially the sholder peak at 624 nm almost deminished. These are characteristics of ring based oxidation process. Clear isosbestic points were recorded at 307 nm, 411 nm,610 nm, 716 nm, 761 nm. Similar spectral changes were also observed inCBZPC2during the controlled applied potential at -1.10 , -1.56 V [reduction, Figures 8(g) and 8(h)] and 1.03, 1.55 V[oxidation, Figures 8(e) and 8(f)]. When the potential applied at -1.10 V [Figure 8(g)], the increase in the intensity of Q band at 686 nm without the shift in absorption 21 ACS Paragon Plus Environment


maxima and the B band at 317 nm also increased in its intensity. The band at 394 nm fomed with increse in the intensity. All these factors assgined to [ZnIIPc2-]/[ZnIIPc3-]1-. The process gives isosbestic point at 332 nm, 358 nm, 417 nm, 625 nm and 715 nm. When the applied potential changed to -1.56V [Figure 8(h)], both B band at 319 nm and Q band at 687 nm have slight increasing in the intensity. As in CBZPC1, CBZPC2 also supports the further reduction process by spectroscopic changes, which are easily assigned to reduction of the mono-anionic species, [ZnIIPc3-]1- to dianionic species, [ZnIIPc4-]2-. When the applied potential of 1.03 V, Figure 8(e), shows there is slight deecrease in the Q band at 686 nm while B band at 331 nm, sholder peak at 629 nm are also slightly reduced. The isosbestic points were observed to be at 344 nmand 671 nm. These changes in the absorption spectrum were assigned to redox couple [ZnIIPc2-]1+ to [ZnII Pc1-]1+ species. The second oxidation potential of 1.75 V [Figure 8(f)]causes the spectral changes in the UV-vis spectra, which supports the further oxidation of the monomeric species [ZnIIPc1]1+/ [ZnIIPc0]2+. The Q band was reduced and B band also reduced in its intensity, and that of sholder peak also almost vanished. The clear isosbestic points were recorded at the wavelengths of 305 nm, 485 nm, 569 nm and 785 nm.


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Figure 8 . In-situ UV-Visible spectro-electrochemical changes of CBZPC1(a) Eapp= 1.01 V (b) Eapp= 1.75 V (c) Eapp= -1.03 V (d) Eapp= -1.35 V; CBZPC2(e) Eapp = 1.03V (f) Eapp= 1.55 V (g) Eapp= -1.10 V (h) Eapp= -1.56 V.



2.5.6. Theoretical calculations

Theoretical calculations were performed to examine the structural, optical and redox properties of CBZPC1 and CBZPC2 using density functional theory (DFT) and time-dependent DFT with a functional basis set of the B3LYP/6-31G (d,p) level by Gaussian 09 package. The optimized phthalocyanine derivatives of CBZPC1 and CBZPC2 were showen in Figure S14, comprise the two different positions of the carbazole units in pthalocyanine. Figure 9a, 9b and Table S1 shows HOMO, LUMO, HOMO−LUMO gap energies and ground state dipole moment in Debye units. In case of CBZPC1 electron density distribution of HOMO and LUMO is occupied in phalocyanine ring at-2.675 and -4.764 and that of HOMO-1 (-5.41) and LUMO+1 (2.64) is occupied in pthalocyanine ring and carbazole moiety respectively, at HOMO-2 (-5.41) and LUMO+2 (-1.29) the electron cloud is present on pthalocyanine and carbazole unit respectively. In CBZPC2, the HOMO and LUMO was occupied by the pthalocyanine ring only at -5.08 and -2.99 respectively. But in both HOMO-1 (-5.40), HOMO-2 (-5.40) and LUMO+1(2.96), LUMO+2 (1.56) the elecron cloud is exist on pthalocyanine and carbazole moiety. However, the electron density distribution is ambiguous to distinguish the electron donor and acceptor, thus, trivial changes of charge generation in CBZPC1 and CBZPC2. The TD-DFT studies for CBZPC1 and CBZPC2 were employed at the B3LYP/6-31G(d,p) level at polarizable continuum model (PCM) in tetrahydrofuran as solvent with M06-2X function. These results are in good agreement with the experimental values (Figure S12). The calculated vertical excitation energies for singlet together with calculated oscillator strengths are listed in Table S2.


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Figure 9. (a), (b) Isodensity plots of FMOs and the energy values in eV by using the B3LYP method 6-31G (d,p) for CBZPC1 and CBZPC2 respectively.



2.5.7. Ultrafast Nonlinear Optical Studies To investigate the NLO properties of phthalocyanines, we have employed the Z-scan technique with fs pulses of (i) MHz repetition rate and (ii) kHz repetition rate. The Z-scan experiments were performed using: (i) Ti:Sapphire laser (Chameleon, Coherent) with input pulses of duration ~150 fs, repetition rate of 80 MHz and tunable in the 680-1080 nm spectral region and (ii) Ti:Sapphire laser (Libra, Coherent) with input pulses of duration ~100 fs, repetition rate of 1 kHz at 800 nm wavelength. An input beam diameter of ~2 mm combined with a Plano-convex lens of focal length 100 mm for experiment (i) and 150 mm for experiment (ii) was used to focus the beam onto the sample. For experiment (i), the wavelength range studied were 700-900 nm while for experiment (ii), the wavelength range studied were 600-800 nm which was tuned with the help of an optical parametric amplifier (TOPAS-C). Neutral density filters were utilized for attenuating the input pulse energy. A 1-mm glass cuvette was used for the sample solutions. The length of the sample was less than the typical Rayleigh range of 2.2-3.65 mm in both the experiments. To scan the sample in closed aperture configuration, a variable aperture was placed in front of the photo detector. The experiments were repeated multiple times and best data were averaged to retrieve the NLO coefficients. The input peak intensities used were in the range of 1.3–6.2×1011 W/cm2. The data was fitted using standard formulae46-48 and the details are provided in the supporting information (see page S10 of the supporting information for further details). Figures 10(a) to 10(f) illustrate the Z-scan data for CBZPC1obtained in the wavelengths ranging from 600 nm to 800 nm for ~70 fs, 1 kHz input pulses. The open aperture Z-scan data obtained for all the wavelengths illustrated in figures 10(a)-10(c) showed a strong decrease in the 26 ACS Paragon Plus Environment

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transmission which is indicative of reverse saturable absorption (RSA) type of behavior. The obtained experimental open aperture Z-scan data was best fitted for two-photon absorption (TPA) from 600-800 nm and the calculated TPA coefficients (α2) magnitudes were in the range of 0.1-0.5 10-12 cm/W as shown in Table 2. The closed aperture data illustrated in figures 10(d)(f) depicted a valley followed by a peak, which clearly suggests positive nonlinear refractive index (n2) or self-focussing behaviour with the magnitudes in the range of 0.2-0.4 10-16 cm2/W and all the values are summarized in Table 2. The NLO behaviour of the solvent THF in kHz regime at a wavelength of 700 nm revealed self-focussing behaviour as well which might contribute for the overall NLO behaviour in CBZPC1 [figures 11(a), (b)]. However, the magnitude of nonlinear coefficients α2, n2 due to the solvent THF were 0.03×10-11 cm/W & 0.02×10-16 cm2/W which were quite small to effect the overall magnitude of NLO coefficients of CBZPC1 (and are in agreement with the literature reported values).53,54 Since the input laser pulses duration was ~70 fs (with kHz repetition rate) the simultaneous absorption of 1+1 photons (with an intermediate stop for very brief time) results in an instantaneous type of TPA. Figures 12(a)-12(f) illustrate the Z-scan data for CBZPC1 obtained in the wavelengths ranging from 700 nm to 900 nm for 150 fs, 80 MHz input pulses. The data obtained for all the wavelengths show a strong decrease in the transmission which is indicative of RSA type of behaviour. The obtained experimental open aperture Z-scan data was again best fitted for 2PA from 700-900 nm and the calculated TPA coefficients with magnitudes in the range of 1.9-2.8 10-8 cm/W are summarized in Table 2. The closed aperture data presented in figures 12(d)-12(f) evidently shows a valley followed by a peak which clearly suggests positive n2 with the magnitudes in the range of 1.7-2.3 10-11 cm2/W and the values are summarized in Table 2. The NLO coefficients for both the phthalocyanines are summarized in Table 2 along with their 27 ACS Paragon Plus Environment


imaginary χ(3) values and 2PA cross sections. It is possible that the 2PA is of instantaneous type since the input pulse duration in this case is ~150 fs and the duration of electrons in the intermediate quantum levels can only be

Synthesis, Optical, Electrochemical, DFT Studies, NLO Properties and

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