Photostimulated Spiropyran for Instantaneous Visualization of Thermal Field Distribution and Flow Pattern

Thermal Field Distribution and Flow Pattern

Article

Photostimulated Spiropyran for Instantaneous Visualization of

Yahui Chen, Yefei Liu, Sheng Lu, Shuang Ye, Hao Gu, Jian Qiang, Yourong Li,* and Xiaoqiang Chen*

Cite This: https://dx.doi.org/10.1021/jacs.0c09460

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sı Supporting Information

ABSTRACT: The study of thermocapillary convection has attracted

the attention of researchers due to the importance in both

fundamental and industrial aspects. To trace the state of ﬂows in

real time during thermocapillary convection, the development of

imaging methods and tools is essential. Here we use a benzothiazole

unit-bearing spiropyran (BS1-SP) as a photostimulated indicator to

visualize the details of instantaneous temperature distribution and

ﬂow pattern on the surface of volatile solvent simultaneously with a

high spatial and temporal resolution during convection. This work

provides insights into dynamic self-organization and thermo-

hydrodynamics taking place in evaporating systems, and a useful

tool to study these behaviors.

5−17

INTRODUCTION

polarity and conformation, and optical properties.

Because

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of the reversible isomerization and unique properties of the

isomers, spiropyran has been widely utilized as a molecular

switch in the construction of dynamic materials in a various

The thermocapillary convection is commonly involved in

various industrial processes such as crystal growth, droplet

evaporation, and microﬂuidics. The understanding of the

dynamic behaviors of ﬂows during thermocapillary convection

becomes very important in both fundamental and industrial

aspects. Especially, the dynamic behaviors of ﬂows driven by a

horizontal temperature gradient in ﬂuid layers with a free upper

surface have attracted the attention of the researchers for many

18−20

range of applications.

In the study described below, we

found that the UV-irritated isomerization of a spiropyran

derivate (BS1-SP) generated colored self-organization aggre-

gates, precisely located at the cold regions on the surface of

volatile solvents. On the basis of the discovery, we developed a

method to realize the instantaneous visualization of thermal ﬁeld

distribution and thermocapillary ﬂows during the evaporation of

solvents and explored the application of BS1-SP in imaging the

ﬂuid thermodynamics.

years. Many experiments have been designed and carried out in

liquid pools with various geometries to investigate the

relationships between the dynamic behaviors of ﬂows and

environmental parameters.

−11

Over the course of these

experiments, some measurement methods and tools are

developed to obtain information on the state of ﬂows. For

instance, infrared cameras, thermo-chromic liquid crystals, and

phosphorescence-based visualization techniques are common

optical tools used for two-dimensional temperature measure-

RESULTS AND DISCUSSION

■

Two spiropyrans BS1-SP and BS2-SP (Figure 1 A) were initially

prepared using the routes depicted in Scheme S1 of the

Supporting Information. The structures of these substances

were assigned by using NMR spectroscopy and high-resolution

mass spectrometry (Figures S10 −S30 ). The conversions of

BS1-SP to BS1-MC promoted by irradiation with UV-light are

shown in Figure 1B. We observed that UV light irradiation of

BS1-SP in dichloromethane (DCM) promotes the formation of

green color in association with the growth of a visible absorption

ment; the conjunction technology of particle image velocim-

etry and polystyrene ﬂuorescent particles are employed to trace

the patterns of the thermocapillary ﬂows. However, until now,

it has been rather challenging to observe the details of

instantaneous temperature distribution and ﬂow patterns

simultaneously with a high spatial and temporal resolution

during thermocapillary convection.

Received: September 2, 2020

Spiropyran derivates are a class of molecules that can transit

between a parent ring-closed isomer of spiropyran (SP) and a

ring-open isomer of merocyanine (MC) upon external stimuli

such as temperature, UV/vis light, acidity, environmental

polarity, and mechanical stress. The two isomers show vast

diﬀerences in physicochemical properties including molecular

XXXX American Chemical Society

Journal of the American Chemical Society

J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

surface of DCM (Figure 3 A). After the UV light was removed,

Article

Figure 1. Isomerization and optical properties of the BS1 system. (A)

Molecular structures of BS1-SP and BS2-SP. (B) Interconversion

between BS1-SP and BS1-MC. (C) Blue line: absorption spectrum of

BS1-SP in DCM. Red line: absorption spectrum of UV-irradiated

solution of BS1-MC. (D) Repeated interconversion between the two

isomers of BS1 molecules (absorption at 670 nm) in DCM solution

under UV light irradiation and Vis light condition, showing the good

fatigue resistance. Concentration: 20 μM.

Figure 2. (A) Worm-like pattern of BS1-MC formed on the surface of

volatile DCM after irradiation using UV light (left, green pattern); the

worm-like temperature distribution on the surface of volatile DCM

captured by an infrared camera (right, blue pattern). (B) Proposed

mechanism for thermal ﬁeld distribution imaging by BS1-MC. Initially,

the BS1-SP molecules distribute randomly on the surface of solvent;

the volatilization of DCM leads to the uneven temperature ﬁeld; the

BS1-SP molecules move to cold area due to the surface tension; upon

the irradiation of UV light, the generated green BS1-MC visualizes the

cold area.

band at 670 nm, indicating the formation of the zwitterionic

BS1-MC (Figure 1C). The emergence and disappearance of

green color can be reversibly controlled by exposure to UV light

or daylight (Figure 1D). In contrast, UV-light irradiation does

not promote observable isomerization of the ring-closed form of

the nonbenzothiazole containing spiropyran BS2-SP to the ring-

opened form BS2-MC (Figure S1 ). Thus, the presence of the

benzothiazole moiety increases the stability of merocyanine

zwitterion.

The photostimulated color transition of BS1-SP in DCM was

then investigated in detail. When DCM solution containing

BS1-SP in a disk was subjected to UV irradiation (365 nm), a

green worm-like pattern associated with BS1-MC generated on

the surface of the solvent (Figure 2A and Video S1).

Surprisingly, we found that the worm-like markings are identical

to the distribution of the thermal ﬁeld on the surface of DCM

observed with an infrared camera (Figure 2A). Obviously, the

green BS1-MC aggregates are located in the cold area of volatile

DCM. Moreover, in contrast to the infrared image, the BS1-

based system visualizes the thermal ﬁeld with a much higher

resolution. The similar phenomenon was also found on the

an agate mortar was used as the container for pattern formation

in DCM. Diﬀerent from the pattern formed in disk, upon UV

irradiation, a ﬂower-like green pattern was generated on the

surface of volatile chloroform (CHCl ), 1-bromopropane (1-

BP), and 1,2-dichloroethane (DCE), respectively (Figure S2 ).

We further speculated the mechanism of BS1-SP imaging the

cold area of volatile DCM. In an open solvent system with a free

upper surface, the evaporation of DCM leads to the uneven

temperature distribution on the surface of the solvent. Because

of the higher surface tension on the low temperature area, BS1-

SP molecules on the surface of the solvent are prone to aggregate

spontaneously at the low-temperature area for keeping the

balance between the surface tension at the high and low-

temperature areas. Upon the irradiation of UV light, the

concentrated BS1-SP turns to green BS1-MC on the cold

regions of the solvent surface (Figure 2B).

The patterns observed for BS1-SP aggregates are identical to

the distribution of a thermal ﬁeld, which is also related to the

convection inside a liquid. Thus, we anticipated that the shape of

the container might also impact the thermal ﬁeld distribution on

the surface of an evaporating solvent. To assess this possibility,

Figure 3. Flower-like pattern formation of BS1-MC driven by

convection. (A) In a mortar, BS1-SP (2 mM) in DCM is irradiated

using UV light for 1 s to form the green ﬂower-like pattern associated

with BS1-MC on the surface of DCM. The pattern and color fade when

the solution is exposed to visible light. And the ﬂower-like pattern

formed by UV light irradiation of BS1-SP in DCM and its

disappearance under visible light with a repeatable mode. (B) Two

streams (red and blue dashed cycles) that reveal the formation of BS1-

SP aggregates at the edge of the solvent, followed by translocation of the

aggregates to the center area driven by the convective ﬂows. (C)

Temperature distribution on the free DCM surface determined by the

numerical simulation. (D) Streamlines on the free surface (top) and a

meridian plane (bottom) for DCM liquid determined by the numerical

simulation.

J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society

Article

the green color and ﬂower-like pattern faded with time, in

conjunction with the conversion of BS1-MC to colorless BS1-

SP (Figure 3A and Video S2). The formation and disappearance

of the green ﬂower-like pattern can be reversibly controlled with

UV light “ON” and “OFF” (Figure 3A). By using the high-speed

camera, we observed that the green color associated with BS1-

MC formed at the wall of the mortar, and then streamed toward

the center of the solution, eventually forming the ﬂ ower-like

pattern on the surface of the solution (Figure 3B, Video S3). The

results suggest that the green ﬂower-like pattern formed by UV

irradiation of BS1-SP is a consequence of the patterned

distribution of self-aggregates of BS1 molecules on the surface

of DCM driven by the surface tension and convections.

Additionally, we also observed the formation of a ﬂower-like

pattern of BS1-SP solids at the bottom of the mortar after

evaporation of DCM without UV irradiation, indicating the

patterned aggregation of BS1 molecules on the surface of DCM

is irrelevant to UV irradiation (Figure S3).

We speculated that the thermal ﬁelds at the evaporating DCM

surface induce convective ﬂows via the Marangoni eﬀect, which

serves as the driving force for pattern formation. To support this

proposal and gain insights into the mechanism of pattern

formation, dynamics simulations were carried out by using the

ﬁnite volume method. The simulation results demonstrate that

the calculated temperature distribution on the surface of DCM is

similar to that observed experimentally by using an infrared

camera (Figure 3C and Figure S4). The nonuniform temper-

ature distribution is a result of evaporative cooling, where heat

loss caused by evaporation of DCM is supplemented rapidly

from solvent at the periphery of the container, leading to a higher

temperature than that of the central region. Because a lower

temperature of the solvent is associated with a higher surface

tension, a surface tension gradient is generated that drives liquid

ﬂow from the periphery to the central region (Figure 3D, top).

This is consistent with the observed direction that the BS1-MC

stream follows during the process of pattern formation (Figure

too weak to overcome the viscous resistance of DCM. As a

consequence, no ﬂower-like pattern formed at this temperature.

When the temperature rises to 4 °C, convection becomes strong

enough to support the formation of a ﬂower-like patterned

thermal ﬁeld (Figure S6 and Video S4). However, a further

increase in the temperature leads to a disordered BS1-MC

pattern because the intense convection leads to the disruption of

the ordered thermal ﬁeld on the surface of DCM (Figure S7 and

Video S5). These ﬁndings suggest that a moderate surface

evaporation rate is needed to create a stable pattern of BS1-MC

self-aggregates. Meanwhile, we found that the concentration of

BS1-SP from (0.1 mM to 10 mM) barely a ﬀ ected the thermal

distribution at the surface of DCM (Figure S8), demonstrating

that the visualization of convective ﬂows using BS1 method

reﬂects the intrinsic property of the solvent.

We further considered whether the patterned BS1-MC self-

aggregates can be tuned by changing the environmental

parameters and the solvent properties. When placing the cold

source on one side of the mortar, the patterned BS1-MC self-

aggregates showed an oﬀ-centered convergence point, which

was closer to the location of the cold source. The pattern was

restored to the symmetric shape after the removal of the cold

source (Figure 4A and Video S6). Besides, we investigated the

formation of ﬂower-like patterns by BS1-MC in diﬀerent

haloalkane solvents including 1-BP, CHCl , and DCE. We

observed that the ﬂower-like patterns generated in these solvents

contain diﬀerent numbers of petals (Figure 4B). Fewer petals

were found to associate with the solvent with a higher viscosity.

Since the temperature and the viscosity of the solvent are two

key parameters that govern the driving force and resistance to

the formation of the self-assembled patterns, we tried to describe

the relationship between the number of petals and the two

parameters of the above halogenated alkanes by establishing an

equation below,

N = A[−(δγ/δT)× ΔT/μ]

B and Video S3). The simulated streamlines on a meridian

where N is the number of petals, which is dimensionless; ∂γ/∂T

is the change rate of surface tension coeﬃcient with temper-

ature; ΔT denotes the temperature diﬀerence between the

periphery and the center; and μ is the dynamic viscosity of the

solvent. The values of ∂γ/∂T and μ for each solvent were

obtained from the Lange’s Handbook of Chemistry (Figure

4C). The plot of ln N versus ln (−(∂γ/∂T) × ΔT/μ) (Figure

4D) shows a linear relationship between ln N and ln (−(∂γ/∂T)

× ΔT/μ). These results also demonstrated that the distribution

of benzothiazole-bearing spiropyran molecules and ﬂow

patterns on the solvent surface can be tuned by changing the

local temperature and solvent properties.

plane suggest that the cooler solvent in the central region

descends to the mortar bottom and circle back to the liquid

periphery, creating a loop of convective ﬂow (Figure 3D,

bottom). It is important to point out that surface evaporation

causes the formation of a large negative vertical temperature

gradient, resulting in instability of the ﬂows near the periphery of

the free surface of the liquid. This phenomenon induces the

formation of a group of convective cells with an opposite

rotational direction near the periphery (Figure 3 D, bottom),

which spread toward the central region by the thermocapillary

ﬂow on the free surface. As expected, when the agate mortar was

covered by a transparent glass to suppress the evaporation, the

UV irradiation of BS1-SP DCM solution led to the formation of

a uniformly distributed green color at the surface of the solution

instead of generating a ﬂower-like pattern (Figure S5).

Therefore, it appears that the ﬂower-like pattern is a

consequence of Rayleigh-Benard-Marangoni instability. This is

the reason why the green BS1-MC self-organization region

overlaps with the area of low temperature, resulting in the

formation of the “ﬂower petals” proﬁle.

CONCLUSION

■

In summary, we developed a benzothiazole-bearing spiropyran

derivate BS1-SP that served as an excellent photoactivated

probe for visual observation of temperature distribution

occurring on the surfaces of evaporating organic solvents.

Furthermore, Rayleigh-Benard-Marangoni convective ﬂows on

the surface of evaporating DCM can be visualized by the self-

aggregation of BS1-MC, which is produced by UV-irradiation

induced ring-opening of BS1-SP. Also, we tried to manipulate

the patterned BS1-MC self-aggregates on the surface of solvents

by changing the local temperature and the properties of the

solvents. The observations made in this eﬀort give new insights

into the dynamic self-organization and hydrodynamics taking

place in evaporating systems. This work also provides a useful

Given that the formation of patterned self-aggregates of BS1-

MC on the surface of DCM is closely related to the convection

status of the solvent in the mortar, when the temperature is <0

C, the surface of DCM exhibits an evenly distributed thermal

ﬁeld (Figure S6 ). Convection does not take place at this

temperature because the driving force arising from buoyancy is

Journal of the American Chemical Society

J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Article

“

worm-like pattern” in CHCl , 1-BP, and DCE; formation

of ﬂower-like pattern after DCM evaporation completed;

ﬂower-like temperature ﬁeld distribution of DCM

collected by FILR infrared camera; ﬂower-like pattern

destroyed by preventing convection; ﬂower-like pattern

formed with increasing temperature; ﬂower-like pattern w

destroyed by increasing temperature; inﬂuence of BS1-SP

concentration on formation of ﬂow pattern; model

formulation and numerical method; characterization of

compounds (PDF)

Video of DCM solution containing BS1-SP in disk

subjected to UV irradiation (MP4)

Video of conversion of BS1-MC to colorless BS1-SP

(MP4)

Video of green color associated with BS1-MC formed at

wall of mortar, then streamed toward center of solution,

eventually forming ﬂower-like pattern on surface of

solution (MP4)

Video of convection strong enough to support formation

of ﬂower-like patterned thermal ﬁeld (MP4)

Video of further increase in temperature leading to

disordered BS1-MC pattern because intense convection

leads to disruption of ordered thermal ﬁeld on surface of

DCM (MP4)

Video of pattern restored to symmetric shape after

removal of cold source (MP4)

AUTHOR INFORMATION

Corresponding Authors

■

Xiaoqiang Chen − State Key Laboratory of Materials-Oriented

Chemical Engineering, College of Chemical Engineering, Jiangsu

National Synergetic Innovation Center for Advanced Materials

https://pubs.acs.org/doi/10.1021/jacs.0c09460.

SICAM), Nanjing Tech University, Nanjing 211816, China;

orcid.org/0000-0003-2493-2067; Email: chenxq@

njtech.edu.cn

Yourong Li − Key Laboratory of Low-grad Energy Utilization

Technologies and Systems of Ministry of Education, School of

Energy and Power Engineering, Chongqing University, Chongqing

00044, China; Email: liyourong@cqu.edu.cn

Authors

Figure 4. Eﬀects of local cooling and solvent properties on ﬂower-like

pattern formation. (A) Patterns imaged by BS1-SP (upper row) and

infrared camera images (lower row) of the surface of evaporating DCM.

From left to right: no dry ice around the mortar; dry ice on the left side

of mortar; dry ice on the right side of the mortar; dry ice removed. (B)

Flower-like patterns in diﬀerent solvents were imaged by the BS1-SP

system (upper row) and their infrared images (lower row). From left to

Yahui Chen − State Key Laboratory of Materials-Oriented

Chemical Engineering, College of Chemical Engineering, Jiangsu

National Synergetic Innovation Center for Advanced Materials

(

SICAM), Nanjing Tech University, Nanjing 211816, China

Yefei Liu − State Key Laboratory of Materials-Oriented Chemical

Engineering, College of Chemical Engineering, Jiangsu National

Synergetic Innovation Center for Advanced Materials (SICAM),

Nanjing Tech University, Nanjing 211816, China

Sheng Lu − State Key Laboratory of Materials-Oriented Chemical

Engineering, College of Chemical Engineering, Jiangsu National

Synergetic Innovation Center for Advanced Materials (SICAM),

Nanjing Tech University, Nanjing 211816, China; orcid.org/

right: DCM; 1-BP; CHCl ; DCE. (C) Physical parameters of solvents

and the corresponding numbers of petals. (D) Linear relationship

between ln N and ln (−(∂γ/∂T) × ΔT/μ).

tool for real-time imaging of temperature distribution and ﬂow

patterns on the surfaces of evaporating solvents.

000-0003-0481-8531

Shuang Ye − Key Laboratory of Low-grad Energy Utilization

Technologies and Systems of Ministry of Education, School of

Energy and Power Engineering, Chongqing University, Chongqing

ASSOCIATED CONTENT

sı Supporting Information

The Supporting Information is available free of charge at

■

00044, China

Hao Gu − State Key Laboratory of Materials-Oriented Chemical

Engineering, College of Chemical Engineering, Jiangsu National

Synergetic Innovation Center for Advanced Materials (SICAM),

Nanjing Tech University, Nanjing 211816, China

Chemicals and apparatus; synthesis routes of compounds

BS1-SP and BS2-SP; preparation of samples; absorption

spectrum of BS2-SP in DCM solution; formation of

J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society

(15) Remon, P.; Li, S. M.; Grotli, M.; Pischel, U.; Andreasson, J. An

Article

Jian Qiang − State Key Laboratory of Materials-Oriented

Chemical Engineering, College of Chemical Engineering, Jiangsu

National Synergetic Innovation Center for Advanced Materials

acido- and photochromic molecular device that mimics triode action.

Chem. Commun. 2016, 52, 4659−4662.

16) Xie, X.; Mistlberger, G.; Bakker, E. Reversible Photodynamic

(SICAM), Nanjing Tech University, Nanjing 211816, China

Chloride-Selective Sensor Based on Photochromic Spiropyran. J. Am.

Chem. Soc. 2012, 134, 16929−16932.

Complete contact information is available at:

https://pubs.acs.org/10.1021/jacs.0c09460

(17) Zhang, J.; Zou, Q.; Tian, H. Photochromic Materials: More Than

Meets The Eye. Adv. Mater. 2013, 25, 378−399.

(18) Khazi, M. I.; Jeong, W.; Kim, J.-M. Functional Materials and

Notes

Systems for Rewritable Paper. Adv. Mater. 2018, 30, 1705310−

705331.

19) Samanta, D.; Galaktionova, D.; Gemen, J.; Shimon, L. J. W.;

The authors declare no competing ﬁnancial interest.

(

ACKNOWLEDGMENTS

Diskin-Posner, Y.; Avram, L.; Kral, P.; Klajn, R. Reversible chromism of

spiropyran in the cavity of a flexible coordination cage. Nat. Commun.

■

This investigation was supported ﬁnancially by the National

Natural Science Foundation of China (21722605, 21978131,

1776022, 11532015), the National Key R&D Program of

China (2018YFA0902200), the Natural Science Foundation of

Jiangsu Province (SBK2020043221), the Six Talent Peaks

Project in Jiangsu Province (XCL-034) and the Project of

Priority Academic Program Development of Jiangsu Higher

Education Institutions (PAPD).

(

018, 9, 641−649.

20) Qi, Q.; Li, C.; Liu, X.; Jiang, S.; Xu, Z.; Lee, R.; Zhu, M.; Xu, B.;

Tian, W. Solid-State Photoinduced Luminescence Switch for Advanced

Anticounterfeiting and Super-Resolution Imaging Applications. J. Am.

Chem. Soc. 2017, 139, 16036−16039.

(21) Dean, J. A. Lange’s Handbook of Chemistry, 15th ed.; McGraw-

Hill: New York, 1999.

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