Solvent Polarity Dependent Excited State Dynamics of 2

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Solvent Polarity Dependent Excited State Dynamics of 2’-Hydroxychalcone Derivatives Hongwei Song, Zhuoran Kuang, Xian Wang, Yuanyuan Guo, Qianjin Guo, Hongyu Zhang, and Andong Xia J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b03133 • Publication Date (Web): 19 Jun 2018 Downloaded from http://pubs.acs.org on June 21, 2018

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The Journal of Physical Chemistry

Solvent Polarity Dependent Excited State Dynamics of 2’-Hydroxychalcone Derivatives Hongwei Song,a,b Zhuoran Kuang,a,b Xian Wang,a,b Yuanyuan Guo,a,b Qianjin Guo,a, Hongyu Zhang,*c Andong Xia,*a,b a

Beijing National Laboratory for Molecular Sciences (BNLMS), Key Laboratory of

Photochemistry, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, People’s Republic of China b

c

University of Chinese Academy of Sciences, Beijing 100049, People’s Republic of China

State Key Laboratory of Supramolecular Structure and Materials College of Chemistry, Jilin

University Qianjin Street, Changchun, People’s Republic of China

ACS Paragon Plus Environment

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ABSTRACT

The excited-state properties of 4-dimethylamino-methoxychalcones (DEAMC) and its derivative 4-dimethylamino-hydroxychalcones (DEAHC) were investigated in various solvents with different polarities by using steady-state and femtosecond transient absorption spectroscopy combined with quantum chemical calculations. It is found that their photophysical parameters such as fluorescence quantum yields, lifetimes and excited-state relaxation path strongly depend on the solvent polarity. Quantum-chemical calculations elucidate that the geometry of DEAMC in the ground state is slightly torsional whereas DEAHC adopts a near planar conformation stabilized by O-H···O chelated hydrogen bonds. Steady state spectra show that DEAHC is weak fluorescent in all solvents due to non-radiative relaxation in the excited enol and keto states, whereas, the fluorescence quantum yield of DEAMC increases with the increasing of solvent polarities, and the emission yield is as large as 0.16 in acetonitrile. Femtosecond and nanosecond transient absorption spectra further prove that in nonpolar solvent the deactivation of S1 in DEAMC is strongly governed by efficient formation of triplet states; whereas in polar solvent, stronger solvation induced energetically stabilization of ICT state, limiting the intersystem crossing to triplet state. The stabilization of ICT state not only leads to a higher fluorescence quantum yield for DEAMC but also restricts intramolecular twisting process in the enol form of DEAHC, facilitating efficient excited-state intramolecular proton-transfer reaction. These results clearly illustrate the dominant role of excited state solvation in modulating the emission behavior and deactivation mechanisms of fluorophores.


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INTRODUCTION

Over the last decades, efficient near-infrared (NIR) organic luminescent materials are of fundamental interest because of their large penetration for molecular probes in bioimaging, as well as multiple applications in organic light-emitting diodes (OLEDs), optical communication, etc.1-5 However, according to the energy gap law,6-8 the molecular syntheses of efficient NIRemitting luminescent materials is intrinsically difficult because their fluorescence quantum yield tends to decrease as the emission wavelength shifts to NIR region. The most common strategy that has been taken to develop NIR organic emitters is to employ a large π-conjugated framework, but this coplanar molecular structure can also cause fluorescence quenching due to attractive dipole–dipole interactions or effective intermolecular π-stacking.9-13 Some studies illustrated that introduction of bulky space group may be favored for the prevention of π-π stacking, which can slightly restore the fluorescence.14 On the other hand, there are also reports highlighted the use of thermally activated delayed fluorescence feature with hybridized local and charge-transfer state mechanism for NIR emitters.15-17 Besides these types of NIR emitters, derivatives of chalcone, whose fluorescence have a remarkable Stokes shift upon attaching electron-donor and acceptor as well as hydroxyl functional group, can also provide a good chance to develop versatile NIR emitters.18-20 Among these chalcone derivatives, 4-dimethylamino-methoxychalcones (DEAMC) and 4dimethylamino-hydroxychalcones (DEAHC, see Scheme 1) have exhibited promising performance in organic NIR-fluorescent dyes.21-24 With the presence of an electron donor dimethylamino group and electron acceptor carbonyl group in the molecular skeleton, DEAMC and DEAHC undergo extensive intramolecular charge transfer (ICT) upon photoexcitation in


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their S1 state. Furthermore, for DEAHC the hydroxyphenyl can induce excited-state intramolecular proton-transfer (ESIPT) reaction, resulting in complicated proton-charge coupled excited state relaxation processes. Previous comprehensive theoretical investigations on DEAHC proposed that in solution it undergoes the intramolecular twisting motion around the hydroxyphenyl ring, leading to the formation of non-fluorescent twisted intramolecular chargetransfer (TICT) state,22,24 though a near planar conformation stabilized by the O-H···O chelated hydrogen bond in DEAHC can also facilitate efficiently ESIPT as a competition relaxation channel for observed fluorescence in the NIR region. Therefore, both charge transfer and proton transfer are strongly coupled with structure relaxation upon excitation, which modulate the excited-state relaxation dynamic. Meanwhile, ICT, ESIPT and intramolecular structural rearrangement are seriously sensitive to their surrounding dielectric polarization of environments such as aqueous-based biological systems, different polar solvents and polymer matrices, which will play a very important role in modifying the coupled reaction dynamics.25-32 Generally, for most D-π-A molecules upon photoexcitation an ICT instantaneously occurs, the dipole moment of D-π-A molecules often has a large change in the excited state relative to its ground state.33 Then, the solute–solvent interaction and the electronic polarization of the surrounding solvent molecules will reorientate their spatial arrangements to reach an equilibrium situation.34 The process is typically called solvation where the solvent polarity, the changes of dipole moments between the ground and excited state are major factors for such solvation effect. Normally, in the case of solvents with large polarity, drastic solvation in the excited state can open a non-radiative channel resulting in significantly decreased fluorescence yield.35-38 In the other case, the enhanced fluorescence yield of many D-π-A molecules such as coumarin-151, PTZ-4, and NIAD-4, c-DMAC had been observed before with an increase in polarity of the


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aprotic solvents, where the increased solvent polarity can stabilize the S1 state, leading to the formation of rigid ICT state and suppress many non-radiative decay process, such as intramolecular conformational twisting, intersystem crossing (ISC) process.39-42 Earlier studies have shown that DEAHC has no or weak green fluorescence in solution but is strongly emissive around 700 nm when in bulk crystals; DEAMC is dark in both crystalline form and nonpolar solution but emits brightly in polar solution.21,24 To explore their crucial microscopic dynamic mechanism, in this work, solvent-dependent excited state dynamics of both DEAMC and DEAHC are extensively investigated by using steady state spectroscopy, femtosecond and nanosecond transient absorption spectroscopy. We found that dramatic solvation effect induces stabilization of ICT state, restricting efficient formation of triplet states, enhancing the fluorescence yield of DEAMC.41,43,44 Moreover, the stronger solvation process in polar solvents can further facilitate efficient ESIPT reaction in DEAHC. Scheme 1. Molecular structure involved in this work. DEAMC: R1=CH3; DEAHC: R1=H;


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MATERIALS AND METHODS

Materials The chalcone derivatives (DEAMC and DEAHC) were synthesized and purified as previously described.21,23 For the spectroscopic measurements, purified samples were dissolved in three solvents: nonpolar cyclohexane (CHX), medium polar tetrahydrofuran (THF), and strong polar acetonitrile (ACN). Steady State and Femtosecond Transient Absorption Measurements Steady-state absorption and fluorescence spectra were recorded by an UV−Vis spectrometer (model U-3010, Hitachi) and a fluorescence spectrometer (F-4600, Hitachi) at ambient temperature, respectively. Fluorescence quantum yields of samples in different solvents were estimated using 9,10-diphenylanthracene in CHX as the reference (fluorescence quantum yield, ΦF= 0.955).45 Fluorescence lifetimes were measured using time-correlated single-photon counting (TCSPC) spectrometer (F900, Edinburgh Instrument). The samples in all solvents were excited at 380 nm and instrument response function (IRF) of TCSPC instrument was about 200 ps. The excited state properties of ESIPT, ICT and intramolecular structure rearrangement within DEAMC and DEAHC were investigated by using a home-made femtosecond transient absorption setup as described previously,46,47 which is introduced in detail in S1 section (see Supporting Information). The femtosecond transient absorption data were analyzed by using the Glotaran software based on the fitting package TIMP.48,49 Nanosecond transient absorption spectra were recorded by an LP920 laser flash photolysis spectrometer (Edinburgh Instruments Ltd), combined with a Nd:YAG laser (Surelite II, Continuum Inc.). Samples were excited by a


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355 nm laser pulse (1 Hz, FWHM ~ 8 ns). The concentration for all the samples was adjusted to an absorbance of ~ 0.3 OD at 355 nm in a 1 cm path length quartz cuvette. Quantum Chemical Calculation Theoretical calculations were performed using the Gaussian 09 software package.50 The ground state molecular geometries of DEAMC and DEAHC were optimized by density functional theory (DFT) at the B3LYP/6-31G(d,p) level. Based on their optimized ground state structures, the charge different densities (CDDs) were calculated by time-dependent DFT (TDDFT) at the same level.

RESULTS AND DISCUSSIONS

Steady-State Spectroscopy Figure 1 shows the normalized steady-state absorption and fluorescence spectra of DEAMC, DEAHC in CHX, THF and ACN, and the corresponding spectral parameters are listed in Table 1. The absorption spectra of both molecules exhibit an absorption band around 400 nm, which undergoes significant red shift (10 – 20 nm) with the increasing of solvent polarity. Such a bathochromic shift is originated from an ICT transitions due to a strong electron donating ability of dimethylamino group. It can be seen that in DEAMC there is also a high energy peak around 330 nm in three solvents, corresponding to the π – π* transition of the anisole moiety.51-53 It is important to point out that in the same solvent the absorption peak around 400 nm in DEAMC has a blue shift as large as 25 – 32 nm relative to DEAHC. In view of the absence of intramolecular hydrogen bonds (IMHBs), the relative blue shift ICT absorption can be rationalized by the decreased degree of π-conjugated framework of DEAMC compared with


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DEAHC. In other words, the strong IMHBs increase the structural rigidity, leading to the enhancement of π electron delocalization in the ground state of DEAHC.

Figure 1. Normalized steady-state absorption (solid line) and fluorescence (dash-dot line) spectra of DEAMC (a) and DEAHC (b) in CHX, THF, ACN at 298K. There is no any fluorescence observed for both DEAMC and DEAHC in CHX. As shown in Table 1, the emission maxima, fluorescence quantum yields and lifetimes strongly depend on solvent polarities. For example, upon optical excitation at 380 nm, nearly resonant with the ICT transition, DEAMC is fluorescent in medium polar THF and strong polar ACN with single peak at 503, 536 nm, respectively, but non-fluorescent in nonpolar CHX. The remarkable bathochromic shift in polar solvent indicates that strong solvent effect occurs in the excited ICT state. Specifically, for DEAMC, fluorescence quantum yields are increased


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significantly in strong polar solvents relative to that in nonpolar solvent. Fluorescence decays are monoexponential at any monitored wavelengths of the emission band. The lifetimes are substantially longer in stronger polar solvents as shown in Table 1 and Figure S1. Such solvent polarity dependence of DEAMC agrees well with recent reported fluorescent probes coumarin 151 and NIAD-4, c-DMAC,39,41,42 where a higher Φ and a longer τ in more polar solvent may be expected from stabilization of ICT state by dipolar interaction with solvents molecules, suppressing non-radiative torsion or efficient ISC process, which will be after-discussed in detail. In DEAHC, a hydroxy group replaces the methoxy group in DEAMC, the fluorescence spectra are relatively complicated due to the additional ESIPT reaction. For example, in THF, DEAHC shows dual emission, consisting of a short and a long wavelength bands around 510, 610 nm, respectively (see Figure 1b). In addition, the picosecond time-correlated single photon counting (TCSPC) measurement clearly resolve their population decays. As shown in Figure S1 b and Table S1, the biexponential decay of fluorescence profiles of DEAHC in THF at 510 and 610 nm suggests that the consecutive populations of two different kinds of excited states with lifetimes of < 200 ps and 1.2 ns, respectively. Similar fitting results are obtained for DEAHC in ACN, consisting of two decay components with different amplitudes in various monitored emission wavelengths as compared with that of DEAMC. We deconvolute the fluorescence spectra of DEAHC in ACN with two Gaussian components in order to estimate its emission peak positions (see Figure S2). It is found that the short wavelength emission band is solventpolarity dependence and has a 30 nm bathochromic shift relative to that in THF, however, the long wavelength band is decoupled from the solvent polarity effect and remains at 610 nm. This observation indicates that the peak in blue side is attributed to normal ICT emission and emission peak around 610 nm belongs to proton-transfer tautomer emission.26,54,55 Because of weak


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solvation, low fluorescence quantum yields and short fluorescence lifetime of both DEAMC and DEAHC in nonpolar solvents must result from the occurrence of rapid non-radiative process in the excited state.22,24,42 Table 1. Solvent parameters, spectral properties, fluorescence quantum yields (Φ) and lifetimes (τ) of DEAMC and DEAHC in different solvents.

DEAMC

a

∆fa

λabs (nm)

CHX

0.006

326,379

THF

0.211

ACN

0.305

DEAHC Φb

τ (ns)

-

0

-

404,422

-

0

-

330,400

503

0.10

0.34

432

510,610

0.001

Solvent Polarity Dependent Excited State Dynamics of 2

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