Irradiation Specified Conformational Change in a Small Organic

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Irradiation Specified Conformational Change in Small Organic Compound and Its Effect on Electrical Properties Shibashis Halder, Arka Dey, Joaquin Ortega-Castro, Antonio Frontera, Partha Pratim Ray, and Partha Roy J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b10081 • Publication Date (Web): 21 Oct 2016 Downloaded from http://pubs.acs.org on October 26, 2016

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The Journal of Physical Chemistry

Irradiation Specified Conformational Change in Small Organic Compound and Its Effect on Electrical Properties Shibashis Halder,† Arka Dey,‡ Joaquín Ortega-Castro,╪ Antonio Frontera,*,╪ Partha Pratim Ray,*,‡ and Partha Roy*,†

†

Department of Chemistry Jadavpur University Kolkata 700 032 India Tel: +91-3324572970 Fax: +91-3324146414 Email: [email protected] (PR)

‡

Department of Physics Jadavpur University Kolkata, 700 032, India Tel: +91-9475237259 Fax: +91-3324138917 E-mail: [email protected] (PPR)

╪

Departament de Química, Universitat de les IllesBalears, Crta. deValldemossa km 7.5, 07122 Palma de Mallorca, Baleares, Spain E-mail: [email protected] (AF)

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Abstract Simple and small organic compound bis((quinolin-4-yl)methylene)benzene-1,4-diamine, (BQD), has been synthesized by one step Schiff-base condensation reaction and it has been characterized by elemental analysis, some standard spectroscopic methods and X-ray single crystal diffraction technique. It shows unique property of photosensitivity with about 10 times increase in electrical conduction under irradiation of visible light of specific wavelength (700 nm), in comparison to dark conditions. However, UV light (350 nm) or visible light of different wavelength (500 nm or 600 nm) cannot cause an enhancement in its electrical conduction. The electric current measurement of BQD exhibits its response ability towards visible light but not to UV illumination when current is measured several times under a constant bias voltage by putting light on and off with successive repetitions. Theoretical calculations indicate slight conformational change in C=N bond and dihedral angle leading to the increase in planarity in the molecule caused under visible light of specific wavelength (700 nm) is responsible for its photosensitivity and electrical conductivity. A new molecule (Me2-BQD) with two methyl groups in the central phenyl ring shows lower conductivity in comparison to BQD under visible (700 nm), UV (350 nm) light and dark conditions.


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1. Introduction In recent years scientists have been concentrating on the development of organic semiconducting materials. Nowadays organic semiconductors and their various derivatives gain more interest than conventional inorganic semiconductor because of their impending applications in markets e.g. OLED, solid state lighting, devices of organic compounds for the generation of solar energy, diodes made of organic compounds as identification tags of radio-frequency and many more.1-6 The reasons behind choosing them over inorganic materials are that (i) the organic compounds can easily be synthesized with easy tuning of their structure with the expectation of their property regulation and (ii) device fabrications with them are reasonably simple compare to conventional inorganic materials. However, there are reports on organic semiconducting materials which can function as Schottky barrier diode.7-9 In general, metalsemiconductor contacts with rectifying property are termed as Schottky barrier diodes.10 But conjugated polymers based on organic optoelectronic devices have been employed so far for such studies and applications because of their proficient optoelectronic properties and easy processing.11-13 Probably there is no study on the analysis of wavelength dependent conformational change in small organic compound and its effect on electrical properties. Our recent paper9 describes optical and electrical properties of a Schiff-base compound, 1,4-bis-(quinolin-6-yliminomethyl)benzene (BQB) with rarely observed Schottky barrier diode character. I–V measurement of ITO/BQB/Al configuration shows the Schottky behavior with a moderate rectification ratio value. However, it is not sensitive to the change in wavelength of the incident wavelength. We report here a new organic compound, bis((quinolin-4-yl)methylene)benzene-1,4diamine, (BQD) based electronic device that behaves like a Schottky barrier diode. We have measured and analyzed the current(I)-voltage(V) characteristics of indium tin oxide (ITO)/BQD/aluminium (Al) sandwiched device under the exposure of varying wavelength of light. It has been revealed that the rectification ratio of BQD is a function of wavelength of light. When the device fabricated with BQD on ITO coated glass has been irradiated with light the I-V curve indicates retention of its diode character but the rectification ratio changed appreciably with the change of the wavelength of light. Another new compound, 2,5-dimethyl-N1,N4-bis((quinolin-4-yl)methylene)benzene-1,43 ACS Paragon Plus Environment


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diamine (Me2-BQD), has been prepared to check the effect of the presence of bulkier group on the conductivity under diverse illuminations.

2. Experimental Section Materials and physical methods Quinoline-4-carboxaldehyde and 1,4-diaminobenzene were bought from Sigma Aldrich and used as received. Solvents and other chemicals were purchased from different commercial sources and the chemicals were used without any purification. Determination of amounts of some elements i.e. C, H and N was performed on Perkin Elmer 2400C elemental analyzer. Fourier Transform IR spectra of the samples were recorded on Perkin Elmer make Spectrum Two spectrometer by ATR method. UV-visible spectra were obtained on Shimadzu 2401 PC UV-Vis spectrophotometer. Powder X-ray diffraction (PXRD) patterns of the samples were recorded on a Bruker D-8 Advance instrument operated at 40 kV and 40 mA using Cu-Kα (λ = 1.5406 Å) radiation. Thermal stability of BQD was determined by thermogravimetric analysis performed on Mettler Toledo TGA 850 thermal analyzer with flow rate of 50 cc/min, temperature range of 25–500°C and heating rate of 2°C/min. Preparation of bis((quinolin-4-yl)methylene)benzene-1,4-diamine (BQD) Quinoline-4-carboxaldehyde (3.14 g, 20 mmol) was dissolved in methanol (20 mL). Then, 1,4-diaminobenzene (1.08 g, 10 mmol) dissolved in methanol (20 mL) was added drop by drop to the above mentioned solution of aldehyde and the mixture was stirred for 30 min. Then, it was refluxed for six hour and the mixture was allowed to cool to ambient condition. A yellow color precipitate appeared. It was then filtered and dried in air. Bock yellow crystals appropriate for its structure determination by X-ray single crystal diffraction were grown upon recrystallization from toluene within one day. Yield: 2.6 g (72%). Anal. Calc. (%) for C26H18N4: C, 80.83; H, 4.66; N, 14.51. Found: C, 80.91; H, 4.57; N, 14.63. 1H NMR (300 MHz, C6D6; δ): 8.98 (d, J = 4.6 Hz, 2H, Ar), 8.75 (s, 1H, imine), 8.52 (d, J = 8.0 Hz, 1H, Ar), 7.59 (d, J = 4.3 Hz, 1H, Ar), 7.51 (t, J1 = 8.4 Hz, J2 = 6.9 Hz, 1H, Ar), 7.43 (t, J1 = 7.0 Hz, J2 = 7.0 Hz, 1H, Ar), 7.35 (s, 2H, Ar);

13

C NMR (300 MHz, CDCl3, δ ppm,): 157.2, 150.2, 149.8, 136.8, 130.2, 129.8,

127.9, 125.8, 124, 122.3, 121.3, 115.5; ESI-MS+ (m/z) (BQD + H+): Calculated 387.16; Found 387.19. 4 ACS Paragon Plus Environment

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Preparation of 2,5-dimethyl-N1,N4-bis((quinolin-4-yl)methylene)benzene-1,4-diamine (Me2BQD) Me2-BQD was synthesized following similar procedure as mentioned above. Typically, quinoline-4-carboxaldehyde (3.14 g, 20 mmol) was dissolved in 20 mL of methanol. Then, to the solution, 20 mL of methanolic solution of 2,5-dimethyl-1,4-diaminobenzene (1.36 g, 10 mmol) was added dropwise and slowly. The mixture was stirred for 30 min. Then, it was refluxed for 6 h and cooled to room temperature. A bright yellow color precipitate appeared. It was then filtered and dried in air. The bright crystalline products were obtained when it was recrystallized from toluene. Yield: 3.1 g (74%). Anal. Calc. (%) for C28H22N4: C, 81.16; H, 5.31; N, 13.52. Found: C, 81.18; H, 5.29; N, 13.53. 1H NMR (300 MHz, CDCl3; δ ppm, TMS): 9,09 (s, 1H, imine), 9.089 (d, J = 4.4 Hz, 1H, Ar), 9.03 (d, J = 8.25 Hz, 1H, Ar), 8.23 (d, J = 8.43 Hz, 1H, Ar), 7.97 (d, J = 4.23 Hz, 1H, Ar), 7.82 (t, J1 = 7.02 Hz, J2 = 7.71 Hz, 1H, Ar), 7.71 (t, J1 = 7.38 Hz, J2 = 7.11 Hz, 1H, Ar), 7.04 (s, 1H, Ar), 2.51 (s, 3H, methyl). 13C NMR (300 MHz, CDCl3, δ ppm): 156.3, 150.3, 149.2, 144.9, 138.8, 131.6, 130.3, 129.6, 127.8, 125.8, 124.2, 121.7, 119.4, 17.8. ESI-MS+ (m/z) (BQD + H+): Calculated 414.18; Found 414.19. Crystallographic Data Collection and Refinement One selected yellow color block single crystal of BQD was mounted on the tip of a glass fiber by commercial glue. Single crystal data collection was performed at 273 K using a Bruker APEX II diffractometer, having a normal focus, using graphite monochromated Mo−Kα radiation of wavelength, λ = 0.71073Å. Data integration was performed on a SAINT program.14 Absorption correction was done with SADABS. The structure of the compound was solved on SHELXS 9715 by employing Patterson method and then with successive Fourier and difference Fourier synthesis. Full matrix least-squares refinements were done on F2 by SHELXL-9716 followed by anisotropic displacement parameters for all atoms except hydrogen. All of the H atoms were set geometrically by HFIX command and placed in perfect positions. Calculations were performed by the use of SHELXL 97, SHELXS 97, PLATON v1.15,17 ORTEP-3v2,18 and WinGX system Ver-1.80.19 Different parameters for the compound are submitted in Supplementary Info as CIF format. Selected bond lengths and bond angles are listed in Table S1.



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Selected data collection and structure refinement parameters are given in Table S2. CCDC 1478835 includes supplementary crystallographic data of BQD. Device fabrication Schottky device was fabricated upon indium tin oxide (ITO) coated glass substrate. To clean ITO coated glass substrate we used isopropanol, acetone and de-ionized water and an ultrasonication system followed by a vacuum chamber for drying. To develop the thin film of active layer on ITO coated glass substrate, firstly a well dispersed solution of complex BQD was prepared in DMF solution by sonication for 30 minutes in an ultrasonication unit. After obtaining the desired solution, thin film was deposited on to the ITO coated substrate with the help of SCU 2700 spin coating unit. By controlling the spinning rate and time of the unit, the thicknesses of the active layer was prepared as 1.0 µm measured by DEKTAK surface profiler. Before depositing the aluminum electrode as front contact, the as-deposited thin film was dried inside a vacuum oven at 100 °C. The aluminum electrodes were deposited on to the film through by a Vacuum Coating Unit 12A4D of HINDHIVAC under pressure 10-6 Torr. The effective area of the film was maintained by the shadow mask as 7.065x10-2 cm-2. For electrical characterization, the current-voltage (I-V) characteristic was measured with the help of a Keithley 2400 source meter by two-probe technique. The I-V characteristics of the fabricated Schottky Diode were measured in dark and under illumination of various light sources of specific wavelengths. All the preparation and measurements were performed at room temperature and under ambient conditions. Computational details The monoclinic BQD crystal structure (cell formulae of C26 H18 N4) was optimized with the density functional theory method using the CASTEP program code of Accelrys, Inc.20 BQD with cell formulae of C26 H18 N4. It was relaxed with the experimental unit cell parameters fixes (a=9.28Å b=8.79 Å and c=12.27 Å α=γ=90o β=104.5o). The calculations were performed within the Generalized-Gradient Approximation (GGA) and the Perdew-Burke-Ernzerhof (PBE) formulation for the exchange-correlation functional. 21,22 Nom conserving pseudopotentials were use in this work. A plane-wave basis set with 500-eV cutoff was applied. The k-mesh points over the Brillouin zone were generated with parameters 1×1×1 the Monkhorst-Pack-scheme. The 6 ACS Paragon Plus Environment

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energy tolerance for self-consistent field (SCF) convergence was 2×10-6 eV/atom for all calculations. The long-range dispersion correction has been included in the calculations with the Grimme’s scheme.23 Band structures were calculated along the k-vector of the first Brillouin zone of the crystal and Total and Partial density of states (TDOS and PDOS, respectively) were plotted with respect to the Fermi level, for comparison purposes. The optical properties including dielectric function, refractive index and optical conductivity of BQD crystal are calculated. Optical properties are averaged over all polarization directions, thereby imitating an experiment on a polycrystalline sample. A smearing of 0,2 eV was employed. Electronic excitation energies and geometry optimized of first four excited stated of the organic crystal were calculated with time-dependent density functional theory (TD-DFT) implemented in CASTEP. The implementation in CASTEP follows the work of Hutter,24 which takes a linear response approach to computing excitation energies directly. For this study choose the optimization of BQD crystal like starting point.

3. Results and Discussion BQD or Me2-BQD has been synthesized by condensation reaction between 2 eqv. of quinoline-4-carboxaldehyde and 1 eqv. of p-phenylenediamine (or 1 eqv. of 2,5-dimethyl-1,4diaminobenzene) in methanol (Scheme S1). Single crystals of BQD have been obtained by recrystallization of the compound in toluene. Unfortunately, we could not get single crystal of Me2-BQD suitable for X-ray diffraction analysis after several attempts. BQD is crystallized in P 21/c space group. An ORTEP of BQD is shown in Figure 1. The asymmetric unit contains a quinoline moiety, an imine group and half of a six member aromatic ring. X-ray single crystal analysis confirms the formation of the Schiff-base compound where one eqv. of amine moiety condenses with two eqv. of aldehyde derivative. A close inspection of the crystal structure of BQD reveals that it is not planar molecule rather two quinoline rings reside in the same plane. The central phenyl ring resides in a different plane from that of quinoline groups with a torsion angle of 147.49º. It is necessary to examine thermal stability of BQD as when we prepare thin coating on the ITO glass surface, it is dried at 120°C. Thus, thermogravimetric analysis has been carried out under nitrogen flow with the powdered sample upto 500°C [Figure S9]. There is no significant



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loss in mass at least upto 120 °C which confirms the stability of the material. TGA study suggests that BQD retains its stability during thin film preparation.

Figure 1. ORTEP diagram of BQD with 50% ellipsoidal probability; Symmetry code: (a) -x, -y, 1-z. To ensure whether the compound is stable up to 120°C, PXRD has also been performed with the samples before and after heating it at 120°C and thereafter these patterns have been compared with the PXRD pattern obtained from single crystal X-ray diffraction analysis [Figure S10]. It has been observed that the peak positions remain very much similar in all of them. To further examine the thermal stability of BQD during coating it on ITO surface, IR spectra of BQD were obtained on sample before (a) and after (b) heating it to 120°C [Figure S11]. Similar IR spectral pattern once again confirms the thermal stability of the compound. The optical characterization of BQD has been performed in a UV-visible spectrophotometer in the range of 200–1000 nm. Absorption spectrum of BQD thin film is given in Figure S12. Optical band gap of the thin film has been determined from fundamental absorption. This is corresponding to the excitation of electron from valence band to conduction band. The optical band gap energy (Eg) of the as synthesized BQD compound has been evaluated using Tauc’s equation:25,26 હ‫ܐ‬ν ൌ ‫ۯ‬ሺ‫ܐ‬ν െ ۳܏ ሻ‫ܖ‬


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where ‘α’ is the absorption coefficient, ‘Eg’ is the band gap, ‘h’ is Planck’s constant, ‘ν’ is the frequency of light and ‘n’ is the nature of transition dependent constant (n = ½ and 2 for direct and indirect allowed transitions).25,26 ‘A’ is a constant and it is dependent on temperature, photo energy and phonon energy. Its value is taken as 1 for ideal case. With the help of Tauc’s equation the type of the absorption transition of BQD thin film was determined as indirect transitions (see Supporting Information, p. S13). Hence the value of ‘n’ in the above equation has been considered as n = 2.25,26 By extrapolating the linear portion of the plot (αhν)0.5 vs. hν to α = 0 absorption [Figure S12], the value of optical indirect bandgap of BQD has been evaluated as 2.02 eV.

Figure 2: I-V plot of BQD under different illumination conditions. As the synthesized material exhibits an absorption peak in the visible range with band gap energy in semiconducting region, we have executed the electrical characterization of BQD compound at room temperature. To observe the electric behavior of the synthesized material, a sandwich structured device of Al/BQD/ITO configuration has been fabricated. The electrical conductivity of the fabricated film has been determined in dark condition and under illumination conditions. In this work to study the photo-responsivity of the material, we have used different



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light sources of distinct wavelength. We have used a light source of wavelengths 350 nm in the UV region and 500, 600, 700 nm in the visible region. The conductivity of the material in dark condition has been measured as 4.716 × 10-3 S cm-1 whereas, the conductivity has been found to be 4.379 × 10-3 S cm-1, 4.24 × 10-3 S cm-1, 4.07 × 10-3 S cm-1 and 1.179 × 10-2 S cm-1 for the light source of wavelengths 350, 500, 600 and 700 nm, respectively by maintaining the same experimental conditions. Light intensities were set at 40 mW/cm2 at the point of measurement with calibrated photodiodes for all the experiments under illumination. These results show that the conductivity of the material increases almost ten times under the light source of wavelength 700 nm compared to the dark condition. But no significant change in conductivity has been observed when illuminated with the light source of wavelengths 350, 500 and 600 nm. Figure 2 shows the I-V characteristic curves in different illumination conditions. As BQD exhibits responsivity only in light source of specific wavelength 700 nm rather than others, hence we completed our rest of the experiment with only two different light sources of wavelength 350 nm (UV light) and 700 nm (visible light). We have also measured the electric current of the compound several times at constant bias voltage + 2.0 V by repeating light on and off. Figure 3 presents the results in the form of array. The photosensitivity of the material has been measured in the presence of 350 and 700 nm light. At + 2.0 V the photosensitivity of the material has been computed as 2.5 and 1.01 by using light source of wavelength 700 nm and 350 nm, respectively. Photosensitivity under exposure of 350 nm light is measured as 1.01 which indicates that there has no significant difference in photo and dark current. On the other hand under the 700 nm light illumination, the photosensitivity is 2.5 which clearly establishes that there is a substantial difference between photo and dark current. From these results it can be easily concluded that the sample does not show any kind of photosensitivity under exposure of 350 nm light whereas it shows definite photosensitivity under 700 nm light exposure. The responsitivity in the specific wavelength of a device may occur due to the filter effect of the active layer. To cross check the occurrence of any kind of filter effect, the same experimental procedure was conducted with the devices of different thickness. In this regard we prepared two different devices having 0.8 and 0.6 µm thick active layers. In this extended studies we have used two different sources of light (700 and 350 nm). The I-V characteristics for both the devices having 0.8 and 0.6 µm thickness (Figure S14) shows same kind of responsitivity as seen in device of 1 µm thickness. The photosensitivity doesn’t change but the absolute value of current 10 ACS Paragon Plus Environment

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(in dark and under illumination) changes due to the change in thickness. Thus, we can nullify the occurrence of filter effect due to change in thickness.

Figure 3: Electric current, at bias voltage 2.0 V for the on and off conditions in visible light. Obtained experimental result (Figure 2) indicates the improvement in conductivity as well as photosensitivity in our device under the exposure of 700 nm light compared to 350, 500 and 600 nm light. In order to justify this process, a theoretical study with DFT and TD-DFT has been conducted. The experimental crystal lattice has been chosen like started point optimized the atomic position. Afterwards, crystal structure analysis has been done by standard band theory and total/partial density of states calculation which indicates that BQD is an indirect semiconductor with a band gap value of 1.754 eV (Figure 4). Usually, bandgaps calculated by DFT are lower than that are determined by experiments,27 in this case, bandgap from DFT is in good agreement with the experimenatlly obtained value (2.02 eV). The obtained experimental bandgap (Eg= 2.02 eV) demonstrates that the material belongs to semiconductor family which is also confirmed from the DOS calculation as shown in Figure 5. DOS analysis suggests a high contribution from carbon and nitrogen to the top of valence bands, with a p-character for both PDOS involved in delocalization. The carbon p-component is the main one of the conduction bands in the crystal. 11 ACS Paragon Plus Environment


Figure 4: Electronic band structure of ground state an of BQD crystal. Points of high symmetry in the first Brillouin zone are labelled as follows: Z = (0,0,0.5), G = (0,0,0), Y = (0,0.5,0), A = (0.5,0.5,0), B = (-0.5,0,0), D = (-0.5,0,0.5), E = (-0.5,0.5,0.5), and C = (0,0.5,0.5).

50

N(S)

45 40 Density of states

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N(P) C(S)

35

C(P)

30 25

Total

20 15 10 5 0 -12

-10

-8

-6

-4 -2 Energy (eV)

0

2

4

Figure 5: Calculated the total (blue line) and partial DOS of carbon atoms (solid lines) and nitrogen atoms (dashed lines) of BQD crystal. 12 ACS Paragon Plus Environment

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Theoretically, it is possible to know the frequency dependence of an incident photon in a material with dielectric function ε (ω). This function relates the interaction of photons with electrons. The optical properties calculated for the BQD crystal structure are presented in Figure 6-8 for dielectric function (ε (ω)), refractive index (n/k (ω)) and optical conductivity (σ (ω)), respectively. The real and imaginary parts of the complex dielectric constant as function of the photon energy of BQD crystal are shown in Figure 6. This function provides information regarding the absorption properties of the crystal. The peaks can be assigned to electronic transitions from the top of the valence band to the low-energy in the conduction band. It can be clearly observed the peaks of the imaginary part of dielectric function at 2.0, 3.3 and 4.9 eV. The first peak to 2.0 eV is mainly originated from the 2p valence band to the 2p conduction band of C and N transition (π-π* transitions). This also explains the origin of the peak structures in the refractive index and extinction coefficient spectra shown in Figure 7. From the real part of refractive index n (ω), a value of 1.76 eV is calculated which is in good accordance with the band calculation (Figure 4). Figure 8 shows the optical conductivity as a function of the photon energy. Semiconductor optical conductivity is the change in conductivity caused by illumination, either an increase or a decrease. Our semiconductor does not present photoconductivity below the bandgap energy of the compound. The calculated optical conductivity also coincides with the approximate value of the experimental bandgap (~2.0 eV). In addition to the first peak, it is also found a second maximum at 3.2 eV and a third one at 4.8 eV.

8 6 Re/Im (ω)

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4 2 0 0 -2

2

4

6

8

10

Photon energy (eV)



Figure 6: Calculated a) real (blue line) and b) imaginary part (red line) of the complex dielectric function of BQD crystal

n (ω) and k(ω)

3 2 1 0 0

2

4

-1

6

8

10

Photon energy (eV)

Figure 7: Calculated refractive index (blue line) and extinction coefficient (red line) of BQD crystal

2.5 σ (ω) (1/fs)

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1.5 0.5 -0.5 0 -1.5

2

4

6

8

10

Photon energy (eV)

Figure 8: Calculated real (blue line) and imaginary (red line) of optical conductivity of BQD crystal

The calculation of optical properties shows that only some wavelengths obtain an optical response. As first approximation, it is possible to relate the optical conductivity of the material with electrical conductivity measured experimentally, verifying whether any change in its 14 ACS Paragon Plus Environment

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electronic configuration occurs when the material absorbs a photon. By using the TDDFT, it is possible to know the crystal electronic excited states and check if these changes affect the properties of the ground state. The 1st, 2nd, 3rd and 4th electronic excitation energies along with the characteristic geometric differences between these excited states and the ground state of the BQD crystal are shown in Figure 9. The first excited state corresponds to HOMO-LUMO electronic transitions from the valence band to the conduction band or near levels. The principal changes into the BQD molecules into the crystal are found in the -C=N- bond distance and the dihedral angles -C=NCar-Car (Scheme 1, angle between the ‘a’ and ‘b’ planes). As can be seen, a decrease of 3.7o in this dihedral angles and a slight increase of -C=N- bond length in the 1st excited state produce a significant decrease in the energy needed for an allowed electronic transition (1.501 eV). If the population of these states with more flat geometry was large enough an increase in conductivity could be detected. 29 It is well known that light is an attractive external stimulus because of its easy access and fast response time.28 So these results could substantiate with the change of molecular conformation by exposure to light of specific wavelength. Although other factors may influence the conductivity increase that have not been taken into account theoretically, such as the interface metal/ BQD electrode, electrode work function, carrier density, charge transfer states, etc. Our theoretical study demonstrates that the planarity of the molecule increases when it is illuminated with specific visible light (700 nm). To examine the effect of conformational changes behind the photoresponsitivity another compound has been synthesized with methyl substitution at the ortho positions of the six membered aromatic ring and the newly synthezied compound is designated as Me2-BQD (Scheme S1). We assume that the presence of bulkier group, like methyl group, will effect on the planarity of the molecule and hence its conductivity should be different. In this regard the conductivity of Me2-BQD was measured under illumination with visible light of specific wavelength (700 nm), UV light (350 nm) and in dark condition. The conductivity was found to be 4.48 × 10-3 S cm-1, 2.26 × 10-3 S cm-1 and 1.73 × 10-3 S cm-1, respectively (Figure S15). Hence it can be concluded that Me2-BQD shows photosensitivity under illumination of visible light of specific wavelength (700 nm) compared to that under UV light or in dark, but the extent of photosensitivity is much lower than that of BQD. Presence of methyl groups probably decreases



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planarity of the compound and hence its conducvity under various conditions. These analyses are quite relevant and give a good explanation for our earlier experimental observations.

Figure 9: Geometric and energetic comparison between the ground state and the four first excited stated in the BQD Crystal. Changes into the BQD molecules into the crystal are found in the -C=N- bond distance and the dihedral angles -C=N-Car-Car (a and b planes).

4. Conclusions In summary, we have been able to synthesize and characterize a new simple organic compound, bis((quinolin-4-yl)methylene)benzene-1,4-diamine, (BQD). Electrical conductivity of BQD in Al/BQD/ITO configuration shows that electrical conductivity increases significantly under illumination of 700 nm light whereas there is almost no change in its conductivity under illumination of 350, 500 and 600 nm light indicating its photosensitivity under visible light exposure. Theoretical studies support these observations. Slight change in azomethine bond length and dihedral angle in BQD molecule reduces the energy needed for the electronic transition. This conformational change of molecule achieves more planarity along with the significant change in conductivity, when it is illuminated not with light of arbitrary wavelength 16 ACS Paragon Plus Environment

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but with light of specific wavelength (700 nm). A derivative of BQD with two methyl groups in the central benzene ring reduces its conductivity under different illumination conditions. Thus, simple and easy to synthesize organic compounds may have its potential application in the field of organic semiconducting industries. This study could open a new window of research field with promising applications in optoelectronic devices. Acknowledgments PR thanks DST, New Delhi for financial supports. SH thanks CSIR, New Delhi for fellowship. AF thanks MINECO of Spain (projects CTQ2014-57393-C2-1-P and CONSOLIDER INGENIO CSD2010-00065, FEDER funds) for funding. Authors acknowledge DST Special Grant to the Department of Chemistry, Jadavpur University in the International Year of Chemistry 2011. Supporting Information Scheme for synthesis; 1H and

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C NMR spectra; ESI mass spectral data; TGA; Powder

XRD; FT-IR and absorption spectra; Table for Selected bond lengths and bond angles; Table for Crystal data for BQD. References 1

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Table of Content

A

small

organic

compound,

bis((quinolin-4-yl)methylene)benzene-1,4-diamine,

shows

distinctive property of a diode that results in ~10 times enhancement in electrical conduction only under visible light illumination in comparson to dark, but not under UV exposure or dark.


Irradiation Specified Conformational Change in a Small Organic

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