Influence of Processes of Photobleaching on Spectral Characteristics

of Organic Nonlinear Optical Co-crystal 26DAP4N

K. E. Zhevaikin

, I. Yu. Denisyuk, M. I. Fokina and V. E. Sitnikova

Saint-Petersburg National Research University of Information Technologies, Mechanics and Optics, Kronverksky Pr.,

Saint-Petersburg, Russia

Keywords: Molecular Co-crystals, Aminopyridine, Nonlinear Mediums, Photobleaching, Spectroscopy.

Abstract: In this paper, we used organic nonlinear optical materials based on 4-nitrophenol and 2,6-diaminopyridine

co-crystals. The goal was to study the influence of photobleaching processes on spectral characteristics of

mentioned co-crystals. We presented spectral characteristics of transmittance coefficients of material before

photobleaching and after its exposure to light for 4 weeks. We also noted the breakdown of hydrogen bonds

in a molecular complex of co-crystal under the influence of photobleaching. However, this breakdown does

not lead to disruption of the integrity of the molecules that are a part of an aminopyridine-nitrophenol.

1 INTRODUCTION

Studies in the field of nonlinear optics are

accompanied by constant search and implementation

of new nonlinear optical materials with high values of

nonlinear optical coefficients. In recent years in

particular interest are molecular and polymeric

nonlinear optical materials with high values of the

nonlinear optical coefficients caused by high

polarizability of molecules (Debrus, et al., 2002).

An example of such materials is prospective

organic nonlinear optical co-crystals based on the

aminopyridine series and optical chromophore 4-

nitrophenol (Srinivasan, et al., 2007). These co-

crystals possess noncentrosymmetric crystalline grid

which leads to origin of nonlinear optical effects. This

makes them interesting, first of all, for their potential

uses as radiation transformers – generation of optical

harmonicas and generation of THz radiation. Early

studies co-crystals of aminopyridine series showed

that high enough values of nonlinear optical

coefficients exist and that degradation of material

under action of both an intensive laser radiation, and

humidity (Pavlovetc, et al., 2016) (Krishnakumara, et

al., 2012) (Draguta, et al., 2013) is absent. Results

provided in operation (Zhevaikin, et al., 2018), show

existence of considerable anisotropy of refraction

indexes between crystallographic axes. That, in turn,

brings to studies, provided in operations (Tu, et al

2016 (Esaulkov, et al., 2018) which show prospects

of use of these materials in terahertz area of radiation.

However, despite having promising nonlinear

optical properties, these organic co-crystals have a

flaw. They are brittle and require high complexity of

machining. This therefore makes it necessary to

search for another method of their processing in order

to create elements on these co-crystals, as it is equally

important. One of prospective methods is the use of

degradation optical (and nonlinear optical) properties

of crystals under the influence of active radiation.

Structural changes created by directional radiation in

certain sections of a crystal are nondestructive to the

general integrity and allow to create miniature optical

elements with high conversion factors of radiation

(Mutter, et al., 2003). In this study we have

researched stability of spectral characteristics of co-

crystals of the aminopyridine series under the

influence of optical radiation. Research of how

radiation wavelength depends on intensity and time

of illumination for degraded co-crystals will show a

possibility of practical application of co-crystals. That

will undoubtedly allow to expand a range of applied

materials, and, therefore, an element basis of

photonics in general.

2 MATERIALS AND METHODS

In this paper we have studied organic nonlinear optical

co-crystals based on optical chromophores 4-nitrophe-

220

Zhevaikin, K., Denisyuk, I., Fokina, M. and Sitnikova, V.

Inﬂuence of Processes of Photobleaching on Spectral Characteristics of Organic Nonlinear Optical Co-crystal 26DAP4N.

DOI: 10.5220/0007404102200224

In Proceedings of the 7th International Conference on Photonics, Optics and Laser Technology (PHOTOPTICS 2019), pages 220-224

ISBN: 978-989-758-364-3

nol and 2,6-diaminopyridine. These co- crystals were

synthesized and grown using a crystallization method

of slow evaporation of solvent (Srinivasan, et al.,

2007).

The co-crystals are named in the following way:

2,6-diaminopyridine-4-nitrophenol (2,6DAP4N).

The structural formula of co-crystal is shown on

Fig. 1.

Figure 1: A structural formula of co-crystal 26DAP4N.

Studied LEDs have the following characteristics:

1) A red LED with wavelength of 625 nm, emissive

power of 5 W and luminous flux of 400 lm;

2) A green LED with wavelength of 525 nm, emissive

power of 5 W and luminous flux is 400 lm;

3) A UV-LED with wavelength of 405 nm, emissive

power of 1 W and luminous intensity is 150 mcd.

We have used spectrophotometer Shimadzu

UV-1800 to produce transmittance spectrums of

studied co-crystals. To conduct a spectroscopic study

in the IR area we have used an IR spectrometer

Tensor 37 Bruker. The spectrometer was set to the

range of 4000-600 cm

-1

with resolution of 2 cm

-1

and

with averaging on 32 ranges using frustrated total

internal reflection (FTIR).

In this study, we aimed to find out how

photobleaching processes affect spectral

characteristics of co-crystals transmittance

coefficients. To do so we carried out experiences in

the following fashion: we exposed 26DAP4N co-

crystals to light of three LEDs that radiated different

wavelengths. These wavelengths were red (625 nm),

green (525 nm) and ultraviolet (405 nm). The way we

have ensured the results of this study were consistent

was by measuring transmittance spectrums of co-

crystals before photobleaching and after, during the

span of 4 weeks.

3 RESULTS AND DISCUSSIONS

The following is the summary of findings of this

research. During the study, we have discovered that

under the influence of light exposure all studied co-

crystals had their transmittance spectrums lowered.

3.1 UV-Spectroscopy of Co-crystals

26DAP4N

Photobleaching process for 26DAP4N co-crystals

was done in a span of 4 weeks. We have used the

following wavelength – 525 nm, 625 nm and 405 nm.

Produced transmission spectrums are shown on Fig. 2

– Fig. 4.

Figure 2: Transmittance spectrums of 26DAP4N co-

crystals under exposure to light with wavelength of 525 nm.

1 – before photobleaching; 2 – photobleaching for 1 day; 3

– photobleaching for 1 week; 4 – photobleaching for 2

weeks; 5 – photobleaching for 3 weeks; 6 – photobleaching

for 4 weeks.

Figure 3: Transmission spectrums of 26DAP4N co-crystals

under exposure to light with wavelength of 625 nm, where:

1 – before photobleaching; 2 – photobleaching for 1 day; 3

– photobleaching for 1 week; 4 – photobleaching for 2

weeks; 5 – photobleaching for 3 weeks; 6 – photobleaching

for 4 weeks.

Figure 4: Transmission spectrums of 26DAP4N co-crystals

under exposure to light with wavelength of 405 nm, where:

1 – before photobleaching; 2 – photobleaching for 1 day; 3

– photobleaching for 1 week; 4 – photobleaching for 2

weeks; 5 – photobleaching for 3 weeks; 6 – photobleaching

for 4 weeks.

Inﬂuence of Processes of Photobleaching on Spectral Characteristics of Organic Nonlinear Optical Co-crystal 26DAP4N

221

Spectral characteristics of transmittance

coefficient demonstrate reduction of transmittance

spectrums after photobleaching on all influencing

wavelengths. On 625 nm (a red range) it decreased

sharply from 56% to 9% on lengths of waves of 485-

900 nm. On 525 nm (a green range) it fell from 60%

to 31% on lengths of waves of 490-900 nm. On 405

nm (UV-range) it dropped from 73% to 7% on lengths

of waves of 500-900 nm. Subsequently to long light

exposure spectral characteristics reach the threshold

values of photobleaching for this material, which is

clearly visible for relations 5 and 6 (Fig. 4).

3.2 IR Spectroscopy of Co-crystals

26DAP4N

We have researched structural changes in 26DAP4N

co-crystals under the influence of photobleaching

using the IR method of spectroscopy. During IR-

spectrometry we used nonphotobleached co-crystal

26DAP4N and photobleached co-crystal 26DAP4N

exposed to wavelength 405 nm (UV-LED). Produced

IR spectrums are shown on Fig. 5.

Figure 5: IR absorption spectrum of co-crystals 26DAP4N

when illumination on wavelength of 405 nm.

During photobleaching of co-crystals the change in

ranges is generally linked to the absorption bands

connected to nitrogroup 4-nitrobensole and an

aminogroup 2,6-diaminopyridine. Bending vibrations

of N-H in the С

-NH

group 2,6-diaminopyridine do

not change the state after being exposed to light.

Absorption bands of C-N bonds in the C

-NO

group

also do not change their state. However, in symmetric

and asymmetric stretching of N-O bonds in the C

group absorption bands shift as a result of light

exposure. This means that bonds containing

nitrogroup are involved in co-crystal formation and

that these bonds break after being exposed to light.

The most significant changes happen in composite

band of 1243 cm

-1

, which includes bending of C-N

bands in an aromatic ring, bending vibrations of C-N

bands in C

-NH

and C-O-H phenolic group. It

suggests that C-N bands in C

-NH

participate in

formation of co-crystal, forming a band between NH

and NO

groups, which breaks as a result of light

exposure. Therefore, during the process of light

exposure molecular complex of aminopyridine-

nitrophenol breaks down.

In addition, the contrast between exposed and not

exposed areas (Fig. 6) is seen. This demonstrates

structural changes in volume of co-crystal.

Figure 6: Micrograph of co-crystal 26DAP4N after

photobleaching.

3.3 Spectral Characteristics of

Solutions of Co-crystals 26DAP4N

We studied molecular integrity of aminopyridine-

nitrophenol complex. In order to carry out this

analysis we used photobleached and on

nonphotobleached co-crystals 26DAP4N. We took

exposed and not exposed to light co-crystals and

dissolved them in isopropanol. Then we used

spectrophotometer to study the spectrums of acquired

PHOTOPTICS 2019 - 7th International Conference on Photonics, Optics and Laser Technology

222

solutions. Characteristics of transmittance

coefficients of solutions are shown on Fig. 7. Also, on

Fig. 8 shown spectral characteristics of the

components that are a part of a molecular complex

aminopyridine-nitrophenol: 2,6 – aminopyridine

(26DAP); 4 nitrophenol (4N - optical chromophore).

Figure 7: Transmission spectrums of solutions of co-

crystals 26DAP4N: 1 – solution not lit co-crystal; 2 –

solution lit co-crystal.

Figure 8: Transmission spectrums of the solutions forming

a complex aminopyridine-nitrophenol: 1 – component

26DAP solution; 2 – component 4N solution (optical

chromophore).

Transmittance spectrums of exposed and not exposed

to light co-crystals demonstrate that during

photobleaching of a molecule 2,6-diaminopyridine

and 4-nitrophenol internal complex did not fail (the

difference in spectral characteristics of components

of co-crystals before and after exposure was

minimal).

4 CONCLUSIONS

We studied the influence of photobleaching processes

on spectral characteristics of organic nonlinear

optical co-crystals of an aminopyridine series (2,6-

diaminopyridine-4-nitrophenol). The main focus of

the study was to investigate the influence of active

light emission on mentioned co-crystals on the

following wavelengths: 625 nm (red range); 525 nm

(green range); 405 nm (UV range). We have noted

significant decrease in values of transmittance

coefficients for co-crystals 26DAP4N with spectral

characteristics reaching the threshold values of

photobleaching for this material.

During IR-spectroscopy we discovered that

hydrogen bonds in co-crystals breakdown under the

influence of photobleaching, which leads to structural

changes and violation of integrity of an

aminopyridine-nitrophenol molecular complex.

Additionally we analysed the integrity of the

molecules that are composing aminopyridine-

nitrophenol molecule complex and confirmed that

during light exposure the initial molecules do not

break.

The results of this research aim to study stability

of nonlinear properties of co-crystals that is necessary

for practical application of these co-crystals in

devices of nonlinear optics and photonics.

ACKNOWLEDGEMENTS

The reported study was funded by RFBR according

to the research project № 18-32-00643/18.

We thank Tatiana Lebedeva, technical writer for

assistance with translating and editing of the article.

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