Photoinduced excited-state hydrogen bonding strengthening of hemiindigo for the drastically fluorescence quenching in protic solvent and water sensing in aprotic solvent

Journal Pre-proof Photoinduced excited-state hydrogen bonding strengthening of hemiindigo for the drastically fluorescence quenching in protic solvent and water sensing in aprotic solvent Xi Zhao, Songqiu Yang PII:

S0022-2313(19)31785-5

DOI:

https://doi.org/10.1016/j.jlumin.2019.116993

Reference:

LUMIN 116993

To appear in:

Journal of Luminescence

Received Date: 11 September 2019 Revised Date:

29 November 2019

Accepted Date: 22 December 2019

Please cite this article as: X. Zhao, S. Yang, Photoinduced excited-state hydrogen bonding strengthening of hemiindigo for the drastically fluorescence quenching in protic solvent and water sensing in aprotic solvent, Journal of Luminescence (2020), doi: https://doi.org/10.1016/ j.jlumin.2019.116993. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.

Photoinduced Excited-State Hydrogen Bonding Strengthening of Hemiindigo for the Drastically Fluorescence Quenching in Protic Solvent and Water Sensing in Aprotic Solvent Xi Zhao, †‡

Songqiu Yang,† *

†

State Key Laboratory of Molecular Reaction Dynamics, Dalian Institute of Chemical

Physics (DICP), Chinese Academy of Sciences, 457 Zhongshan Road, Dalian, Liaoning 116023, China ‡

University of Chinese Academy of Sciences, Beijing 10049, China

AUTHOR INFORMATION Corresponding Author *Songqiu Yang: [email protected]

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ABSTRACT Hydrogen bonding in excited state could regulate the photochemical and photophysical processes of organic molecules. In this work, the fluorescence and photoisomerization properties of Hemiindigo (HI) in dioxane and methanol have been studied. Its fluorescence is bright in dioxane but faint in methanol and the photoisomerization in methanol is distinctly inefficient when compared with that in dioxane. By using time-resolved spectroscopy technology and quantum chemical calculation, hydrogen bonding strengthening between solutes and solvents in excited state, which induces enormously enhanced efficiency of internal conversion of HI in excited state, is assigned to the reason of significant fluorescence quenching and inefficient photoisomerization of HI in methanol. Because of the drastically fluorescence quenching effect of HI, trace water sensing with a limit of detection of 10 ppm in dioxane suggests the potential of HI as a fluorescence probe to quantitate the water content in aprotic organic solvents.

TOC GRAPHICS

KEYWORDS Hydrogen bond, Fluorescence quenching, Internal conversion, Excited-state, Water sensor, photoisomerization

2 / 22

1. Introduction

Hydrogen bonding is one of the fundamental interactions in chemistry [1-3], physics, biology[4-6] and material science[7, 8] and has a significant influence on the photophysics and photochemistry of chromophores in the hydrogen-bonding surrounding. As an important site-specific interaction between hydrogen bond donor and acceptor in gas phase and solution phase, the photoinduced hydrogen bonding dynamics in excited state has been widely investigated through experimental and theoretical methods[9-14]. It is have been demonstrated that the hydrogen bond in excited state can strongly tune and regulate the deactivation processes of functional molecules and materials, such as chromophores, chemosensors, etc.[15-19].Therefore hydrogen bonding interaction in excited state may be crucial to their applications in biosensing, imaging, light-emitting, photocatalysis, photovoltaic, etc.

Hemiindigo (HI) consists of an indigo and a stilbene fragment connecting with a C=C double bond, shown in Scheme 1, which was first described in 1883 by Adolf von Baeyer[20] and with a thermal stable Z isomer[21]. The photoisomerization of a pyrrole-derivative of HI under ultraviolet light and visible light was reported[22]. Furthermore, the interaction of pyrrole-substituted HI with bovine serum album in aqueous media shows the potential biological applications of HI derivatives[23]. In recent researches, some derivatives of HI were designed as photoswitches which characterized with highly bistable photoswitching at the biooptical window and chiroptical changes upon visible-light irradiation[21, 24]. 3 / 22

In the present work, we explore the photodynamics of HI in protic and aprotic solvents by using femtosecond time-resolved transient absorption spectroscopy, time-correlated single photon counting (TCSPC) technique and quantum chemical calculation. The fluorescence of HI in dioxane is bright while the fluorescence in methanol is sufficiently quenched. The photoisomerization of HI in dioxane and methanol was explored and the results indicated that the change of photoisomerization rates was trivial when compared with the change of internal conversion rates of HI from dioxane to methanol and the photoisomerization quantum yield of HI in dioxane is twenty times larger than that in methanol. Thus, organic solvents induce difference in photoisomerization is unable to explain the fluorescence quenching of HI in methanol. Quantum chemical calculation was employed to reveal the reason for the fluorescence quenching of HI in methanol. Hydrogen bonding strengthening between solutes and solvents in excited state, which induces enormously enhanced efficiency of internal conversion of HI in excited state, is account for the faint fluorescence and inefficient photoisomerization of HI in methanol. Due to the significantly fluorescence quenching, the potential as a trace water sensing probe of HI was examined. A limit detection (LOD) of 10 ppm of trace water sensing in dioxane indicates the excellent ability for water sensing in organic solvents. More details could be found in the supporting information.

SCHEME 1. Photoisomerization between HI

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Z and HI

E

2. Experimental and theoretical sections

2.1. Materials and steady state spectroscopy

I was synthesized through a condensation reaction between commercially available indoxyl acetate and benzaldehyde[25]. Further nuclear magnetic resonance was carried out to confirm the structure of HI. UV-Vis absorption spectra were obtained on a PerkinElmer Lambda 35 double-beam spectrometer in a 10 mm quartz cuvette. The fluorescence spectra were recorded on a Horiba Jobin Yvon FluoroMax-4P spectrofluorometer. The fluorescence spectra wavelength dependence was corrected using water Raman signal. The fluorescence quantum yield of HI in dioxane and methanol were obtained by using coumarin 153 as a standard sample, whose fluorescence quantum yield was 0.544 under 450 nm excitation in ethanol[26].

2.2. Time-resolved spectroscopy set up

The experimental setup of the pump-probe femtosecond transient absorption spectroscopy(fsTAS) was report in previous work[27-29]. Briefly, 800 nm fundament pulses were obtained from Spitfire pro system (Spectra Physics) with 50 fs width duration (full

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width at half-maximum, FWHM). The white-light continuum probe pulses (360-700 nm) were generated by focusing 800 nm pulses (0.1mJ) into a linearly translated CaF2 crystal. The 800 nm pulses to generate white light probe pulses were delayed with respect to the pump pulses using a computer control translation stage (Newport) before generating white light. The pulses were linearly polarized at magic angle with respect to probe pulse to eliminate any possible polarization dependent phenomenon. Typical instrument response function (IRF) of 90 fs (FWHW) was estimated according to the measurements of the Kerr effect of n-hexane. The experiment was carried out at room temperature. The pump power was 0.2 mW at 450 nm. The absorption at 450 nm of HI in methanol was adjusted to about 0.5 OD in 1 mm cuvette. The fluorescence lifetime of HI in dioxane is nanosecond and beyond the fsTAS time window. Thus, TCSPC was used to measure the fluorescence lifetime under the excitation wavelength at 460 nm.

2.3. Theoretical methods

Density functional theory (DFT) and time-dependent (TD) DFT calculation were performed by using Gaussian 16 program. Equilibrium ground-state geometries were optimized by DFT with the M062X[30, 31] function and 6-31+g (d, p) basis set. Polarizable continuum model (PCM) was used to simulate the solvent environment. The stationary point was checked by frequency calculation. The complexes of the solute and two solvent molecules were placed in a cavity with the solvent reaction field (SCRF). The vertical excited energies with linear-response (LR) were calculated at the optimized structure using TD-DFT method. － / 22

3. Results and discussion

3.1. Steady-state spectra and fluorescence quenching

The extinction coefficients and emission spectra of HI in dioxane and methanol are shown in Fig. 1a. The absorption spectrum of HI peaked at 458 nm in dioxane and 469 nm in methanol. This bathochromic shift of absorption spectrum is probably due the change of the polarity of solvent. The emission spectrum of HI peaked at 505 nm in dioxane and 570 nm in methanol. The stronger bathochromic shift and the relative wider full width at half maxima of emission spectrum in methanol indicate the interplay of solvent with the solute in the excited state. The fluorescence of HI is bright in dioxane but dim in methanol. Quantitatively, the fluorescence quantum yields of HI are 0.56 and 0.0024 in dioxane and methanol respectively. In other words, the fluorescence of HI is significantly quenching in methanol. The fluorescence spectra of HI in dioxane/methanol mixtures are shown in Fig. 1b. It’s clear that the fluorescence of HI in dioxane is obviously quenching with small quantity of methanol. The fluorescence intensity of HI in different dioxane/methanol mixtures are shown in the inset. The peak of fluorescence spectra of HI in different dioxane/methanol mixtures are shown in Fig. S1. The change of fluorescence intensity and fluorescence peak with different amount methanol in dioxane indicates that general solvent, such as solvent polar effect, can not explain the remarkable difference of fluorescence of HI in dioxane and methanol.

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Fig. 1. Figure (a)The steady-state absorption and emission spectra of HI in methanol. (b) The fluorescence spectra of HI in dioxane with different amounts of methanol(vol%), (inset)The fluorescence peak counts of HI in dioxane with different amounts of methanol.

3.2. Photoisomerization has no contribution on the fluorescence quenching of HI in methanol

Fig. 2. Photoisomerization of HI in dioxane(a) and methanol(b) at 450 nm irradiation.

Photoisomerization around C=C double bond of HI derivatives is reported[21, 24, 32]. In order to determinate the relationship between photoisomerization and fluorescence quenching of HI in methanol. The photoisomerization experiments were scrutinized under radiation wavelength at 450 nm which is around the absorption peak in dioxane and methanol. 8 / 22

As shown in Fig. 2, the change of absorption spectra and the present of isosbestic points indicate the photoisomerization between Z isomer and E isomer and interconvert without side reactions or decomposition.[21] The slower change of absorption spectra of HI under irradiation

in

methanol

compared

with

that

in

dioxane

indicate

the

smaller

photoisomerization efficiency of HI in methanol. The photoisomerization quantum yields of HI from Z isomer to E isomer in dioxane and methanol are 0.068 and 0.0033 respectively (More details would be found in supporting information). The photoisomerization and fluorescence emission processes of HI in the excited state are competing with each other. If the photoisomerization quantum yields of HI in methanol is obviously higher than that in dioxane, the high-efficiency photoisomerization of HI in methanol maybe the reason for the dramatically fluorescence quenching of HI in methanol. In fact, the photoisomerization quantum yield of HI in methanol is smaller than that in dioxane. Therefore it is important to noticed that the relatively small photoisomerization quantum yield and fluorescence quantum yield of HI in methanol suggest that the photoisomerization efficiency changed of HI in methanol can not account for the fluorescence quenching of HI in methanol. Otherwise, the photoisomerization quantum yield should increase. The present of isosbestic points suggests the interconversion between Z isomers and E isomers, but the decrease of photoisomerization quantum yield of HI from dioxane to methanol implies another photophysical nonradiative transition should be response for the fluorescence quenching of HI in methanol.

3.3. Excited-state dynamics in dioxane and methanol

In order to explore the decay rate constants of HI in excited state in dioxane and 9 / 22

methanol. Time-resolved spectroscopy technology was employed to measure lifetime of excited state of HI in dioxane and methanol. Excited-state relaxation of HI in methanol ranging from picosecond to nanosecond timescale was recorded by fsFTA, shown in Fig. 3b. Since the decay of HI in dioxane is as long as tens of nanoseconds and beyond the measurement time window of the fsFTA, TCSPC was used to record the emission decay of HI in dioxane, shown in Fig. 3a.

Fig. 3. (a) Fluorescence decay of HI in dioxane record at 505 nm at 460 nm excitation measured by TCSPC technique. (b) Excited-state absorption decay of HI in MeOH at 373 nm.

Table 1. Summary of the excited-state lifetimes (τ), fluorescence quantum yield(ΦFluo), fluorescence

rate

constant

(kF),

photoisomerization

quantum

yield(Φiso(Z-E)),

photoisomerization rate constant (kiso) and rate constant of internal conversion (kic) of HI in dioxane and methanol.

Solvent

τ (ns)

ΦFluo

kF

Φiso(Z-E)

(10^9 s-1) 10 / 22

kiso

kIC

(10^9 s-1)

(10^9 s-1)

Dioxane

13.02

0.56

0.042

0.068

0.010

0.023

MeOH

0.170

0.0024

0.014

0.0033

0.039

5.83

As shown in Fig. 3a, the fluorescence emission decay of HI at 505 nm in dioxane is ranging about tens of nanoseconds. The lifetime of excited state is fitted to be 13 ns. This relatively long excited-state lifetime is consistent with the bright fluorescence of HI in dioxane. The excited-state lifetime of HI in methanol significantly decreased, as shown in Fig. 3b, the lifetime is fitted to be 170 ps. As shown in Fig. S4, the wavelength evolution spectra of HI in methanol is dominated by an excited-state absorption (ESA) band peak at 520 nm. The missing data of ESA band between 430 and 470 nm is due to the scattering of the excitation wavelength (450 nm). In the early delay stage, the ESA band undergoes a slight blue shift following 450 nm femtosecond pulse excitation. This indicates the excited-state relaxation driven by the intramolecular vibrational energy redistribution (IVR) and solvent relaxation[27-29].

As discussed in last section, the difference of photoisomerization between methanol and dioxane can not explain the fluorescence quenching of HI in methanol. Furthermore, isosbestic points present in the absorption spectrum experiment exclude the participating of other photochemical relaxation decay. Thus, internal conversion (IC), which is a photophysical nonradiative relaxation process, is taking into considered. Excited-state lifetimes, photoisomerization and fluorescence quantum yields and the resulting rate constants for response relaxation processes of HI in dioxane and methanol are summarized in 11 / 22

Table 1. As shown in Table 1, the fluorescence rate constants are derived from ΦFluo/τ, fluorescence rate constants are in the same order suggests the similar property of the fluorescence emission of HI in dioxane and methanol. Photoisomerization rate constant form Z isomers to E isomers in methanol is four times faster than that in dioxane. (The details about determination of photoisomerization rates constants are shown in supporting information.) The faster of photoisomerization but the smaller photoisomerization quantum yield of HI in methanol imply the important role of IC for the fluorescence quenching of HI in methanol. The photophysical nonradiative relaxation rate corresponding to the IC rate constant could be derived from (1-ΦFluo-2Φiso(Z-E))/τ, as shown in Table 1, the relaxation rate of IC of HI in methanol is about three hundred times faster than that in dioxane. This tremendously increasing of efficiency of IC is responsible for the fluorescence quenching of HI in methanol.

3.4. Hydrogen bonding strengthening in excited state

Considering the different properties of solvents used in the experiment, methanol as a protic solvent, the hydrogen bond effect between the solvent and solute should be taken into consideration. In order to further confirm the role of the hydrogen bond effect on the fluorescence quenching, the fluorescence spectra of HI in different protic solvents are shown in Fig. 4. It’s clear that the degree of fluorescence quenching of HI in these four protic solvents is accord with the hydrogen donor ability of these protic solvents[9]. It should be noted that the polarity of these solvents are different, the peaks of fluorescence spectra of HI in these four protic solvents are slightly bathochromic shift along with the increase of the 12 / 22

polarity of the solvents and the quenching degree of fluorescence of HI in these protic solvents are stronger along with the increase of the polarity of these four protic solvents, it seems the higher the polarity of solvent, the stronger of the fluorescence quenching of HI. However, according to Fig. 1b and Fig. S1, we can find out that with small quantity of methanol added in dioxane which has little effect on the polarity of solvent, the fluorescence intensity of HI decreased dramatically and the peak wavelength of the fluorescence spectra of HI changed obviously. Thus, the difference in polarity of solvents play little role in the fluorescence intensity variation of HI in these solvents. Fluorescence quenching is therefore associated with hydrogen bonding.

Fig. 4. Steady-state fluorescence spectra of HI(~10uM) in different alcoholic solvents.

In order to give a deep insight into the hydrogen bond effect on fluorescence quenching of HI in methanol, quantum chemical calculations were employed. When inspecting the 13 / 22

molecular structure of HI carefully, two sites were chosen as hydrogen bonding sites. Thus, two methanol molecules were placed in the first solvation shell, which were directly hydrogen-bonded with HI. One methanol molecule was placed around the nitrogen atom of HI to form N-H O hydrogen bond and the other was placed around carbonyl group of HI for the formation of the hydrogen bond O H O. The optimized structure of this hydrogen bonding complex in ground state is shown in Fig. S5a. The length of hydrogen bond between hydrogen and oxygen atoms is 1.904 Å and 1.869 Å for N－ －H…O and O H O hydrogen bonds, respectively. The Hydrogen bond bonding energy of these two hydrogen bonds is 52 kJ/mol. The electronic excitation energy and the corresponding oscillator strengths of the hydrogen-bonded complex as well as the HI monomer are show in Table 2.

Table 2. Electronical excitation energies(eV) and corresponding oscillator strengths of the hydrogen-bonded complex as well as the HI monomer.

S1 HI─(MeOH) 2

HI

S2

S3

S4

S5

2.944

3.676

4.169

4.714

4.762

(0.274)

(0.010)

(0.595)

(0.020)

(0.005)

3.128

3.559

4.307

4.790

4.843

(0.311)

(0.023)

(0.595)

(0.012)

(0.005)

The calculated absorption maxima of HI in methanol is located at S3 state, which is accord with the experimental absorption spectrum presented in Fig. 1. Furthermore, the electronic excitation energies of the hydrogen-bonded complex are correspondingly decreased compared to that of the HI monomer indicate the solute-solvent intermolecular 14 / 22

hydrogen bonding interactions. It should be noted that visible light region is focused, HI will be excited to S1 state under 450 nm excitation. For the excitation from S0-S1 state, the frontier molecular orbitals (MOs) of HI (Fig. S6) in methanol show a predominantly π-π* type transition from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO). The optimized structure of the hydrogen-bonded complex in S1 state is shown in Fig. S5b. The calculated hydrogen bond lengths and hydrogen bond bonding energies of HI hydrogen-bonded complex are listed in Table 3.

Table 3. Calculated hydrogen bond bonding energies Eb (kJ/mol) and corresponding lengths of hydrogen bonds of hydrogen-bonded complex in ground and first excited state.

Eb

LN─H···O

LO···H─O

S0

52

1.904

1.869

S1

83

1.804

1.783

As shown in Table 3, the hydrogen binding energies between HI and methanol increase from 52 kJ/mol in the ground state to 83 kJ/mol in the excited state indicates the hydrogen bond between HI and methanol is significantly strengthened in the excited state. Furthermore, 15 / 22

the corresponding hydrogen bond lengths between hydrogen and oxygen atoms in excited state decrease 0.1 and 0.083 Å for N H O and O H O hydrogen bonds, respectively. In addition, monitoring the spectral shifts of some characteristic vibrational modes including the hydrogen bonds is been demonstrated an effective method to understand the hydrogen-bonding dynamics.[33-35] The vibrational spectra of HI hydrogen-bonded complex in conjunct vibrational regions of the O-H stretching modes in ground and excited state are shown in Fig. 5. In the S0 state, the stretching vibrational frequencies of O 3719 and 3509 cm-1 for O H O and N H

H stretching are

O hydrogen bonds, where as they change to

3549 and 3297 cm-1 in the S1 state. The markedly bathochromic shift of 170 and 212 cm-1 for O H O and N

H

O hydrogen bonds in excited state are the result of strengthen of

excited-state intramolecular hydrogen bonds, which accords with the higher hydrogen bond bonding energy and shorter hydrogen bond length in the excited state. All the calculation results demonstrated the strengthen of hydrogen bond in the excited state.

1－ / 22

Fig. 5. Calculated vibrational absorption spectra of HI hydrogen-bonded complex in the spectral region of O-H stretching bands in S0 and S1 state.

As discussed above, the fluorescence of HI in methanol is significantly quenching. Hydrogen bonding is associated with the fluorescence quenching by the degree of fluorescence quenching of HI in different protic solvents. According to our quantum chemical calculation, the hydrogen bonding between HI and methanol is significantly strengthened in the excited-state. A schematic view of the excited-state relaxation of HI in free and hydrogen-bonded forms is shown in Fig. 6. The formation of intermolecular hydrogen bonds between HI and solvents result in the lower energy of hydrogen-bonded (HB) forms. Because of the strengthen of hydrogen bonding in excited state, the stabilization of the energy is stronger in excited state. Therefore, the energy gap between excited state and ground state is smaller and the efficiency of IC becomes higher. This hydrogen bonding strengthening in the excited state is accounting for the fluorescence quenching of HI in protic solvents.

Fig. 6. Schematic view of excited-state relaxation of HI in free and hydrogen-bonded 17 / 22

(HB) forms.

In addition to alcohols, water was expected to form hydrogen bond with HI and induce significant fluorescence quenching of HI. The detection and quantitation of water is important in chemical reaction and industrial application[36, 37]. Therefore, the potential of trace water sensing of HI in organic solvent is examined. The fluorescence intensities of HI at 505 nm in water/dioxane mixtures are presented in Fig. 7, a very low water concentration (0-0.4 wt.%) induce obvious fluorescence quenching. A limit of detection (LOD) of 10 ppm was determined (supporting information), which indicates the excellent ability of trace water sensing of HI in organic solvent.

Fig. 7. Fluorescence intensities of HI at 505 nm in dioxane with 0-0.4 wt.% water, fitted to linear model, (mean±SD(n=3)); HI:20 uM; excitation wavelength: 450 nm.

4. Conculsion

18 / 22

In conclusion, we investigated the photodynamics of HI in methanol and dioxane by time-resolved technical spectrum and quantum chemical calculation. The fluorescence of HI is significantly quenching in methanol. The little difference of fluorescence and photoisomerization rates of HI between dioxane and methanol excluding the possibility of photoisomerization quenches the fluorescence of HI in methanol. The enormously enhanced efficiency of IC was proposed to account for the fluorescence quenching and tiny photoisomerization quantum yield of HI in methanol. According to the quantum chemical calculation, the hydrogen binding strengthening between HI and methanol in the excited state resulting a more efficient IC of HI in the excited state. This hydrogen binding strengthening in excited state is responsible for the significantly fluorescence quenching and inefficient photoisomerization of HI in methanol. Furthermore, the quenching effect of HI was used to quantify the trace water in dioxane with a LOD of 10 ppm indicates the potential of HI as a fluorescence probe trace water sensing.

AUTHOR INFORMATION

Corresponding Author *Songqiu Yang: [email protected]

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT 19 / 22

This work was supported by the National Natural Science Foundation of China (No. 21873100).

Appendix A. Supporting Information

Supplementary data associated with this article can be found in the online version.

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Highlight: 1. The fluorescence of hemiindigo (HI) is bright in dioxane but faint in methanol. 2. Internal conversion is the main deactivation way of excited HI in protic solvent. 3. H-bond strengthening in excited state induces efficiency of internal conversion. 4. HI is a fluorescence probe to quantitate the water content in aprotic solvents.

Author statement Xi Zhao: Methodology, Validation, Investigation, Data Curation, Writing-Original Draft, Visualization Songqiu Yang: Conceptualization, Software, Formal analysis, Resources, Writing-Review & Editing, Supervision, Project Administration.

Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: