Effect of Substituent Position on the Photophysical Properties of

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Effect of Substituent Position on the Photophysical Properties of Triphenylpyrrole Isomers Lichao Dong, Guojun Shang, Jianbing Shi, Junge Zhi, Bin Tong, and Yuping Dong J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 15 May 2017 Downloaded from http://pubs.acs.org on May 17, 2017

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The Journal of Physical Chemistry

Effect of Substituent Position on the Photophysical Properties of Triphenylpyrrole Isomers

Lichao Dong†, Guojun Shang†, Jianbing Shi†, JungeZhi‡, Bin Tong†*, Yuping Dong†*

† Beijing Key Laboratory of Construction Tailorable Advanced Functional Materials and Green Applications, School of Materials Science & Engineering, Beijing Institute of Technology, 5 South Zhongguancun Street, Beijing, 100081, China ‡ School of Chemistry, Beijing Institute of Technology, 5 South Zhongguancun Street, Beijing, 100081, China

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ABSTRACT

The charge distribution, molecular structure, and morphological packing significantly affect the photophysical properties of organic photoluminescent materials. In this work, two triphenylpyrrole isomers, 1,2,5- (TPP1) and 1,3,4- (TPP2), were firstly synthesized and characterized. Because of their different substituent positions, TPP1 possesses aggregationcaused emission quenching (ACQ) behavior while TPP2 exhibits aggregation-induced emission (AIE). Their different photoluminescent properties were systematically investigated by using UV-visible absorption spectroscopy, fluorescence spectroscopy, density functional theory (DFT) calculations, and single-crystal structure analysis. The results indicate that substituent position of the two phenyl groups predominately affects the charge distribution of the isomers and determines their molecular packing structures, which further cause the different restriction of intramolecular rotation (RIR) capabilities of phenyl rings, thus resulting in different luminescence properties of these two triphenylpyrrole isomers under different aggregate states.


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INTRODUCTION In recent years，luminescent materials have attracted growing research attention because of their various potential applications such as organic light-emitting diodes (OLEDs),1-4 chemical sensors,5-8 and biological detectors.9-13 However, the traditional organic luminophores exhibit high fluorescence efficiency in dilute solution but suffer from an aggregation-caused quenching (ACQ) effect in their condensed phase. The concentration quenching effect is caused by the π–π stacking aggregation or the formation of sandwich-shaped excimers and exciplexes14, 15 aided by the collisional interactions between the aromatic molecules in the excited and ground states. Therefore, it is highly desirable to develop new luminophore systems that can overcome this obstacle. To temper the ACQ effect, researchers have devoted considerable efforts to study the aggregation effect on the optical properties of organic dyes. In 2001, Tang’s group discovered the phenomenon of aggregation-induced emission (AIE) in a certain type of propeller-like molecules, such as tetraphenylethylene (TPE) and hexaphenylsilole.16-22 These luminogens are nonemissive in good solvents but become highly luminescent in the aggregate state. Later on, restriction of intramolecular rotation (RIR) in the aggregation has been identified as the leading cause for the AIE effect.23 In solution, the active intramolecular rotation of the phenyl rings consumes energy. In the aggregate state, such motion is, however, restricted, which blocks the nonradiative pathway and activates radiative decay. However, the effect of molecular structure and packing arrangement on the AIE processes have rarely been investigated, although the structure-property relationship is significant in terms of gaining new insights into AIE mechanisms and guiding further research efforts in the development of new AIE luminogens.24-29


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In the previous work, we designed and synthesized a series of aryl-substituted pyrroles without other substituents, e.g. 1,2,5-triphenylpyrrole, 1,2,3,5-tetraphenylpyrrole, 1,2,3,4,5pentaphenylpyrrole, and etc.30 Except for 1,2,3,4,5-pentaphenylpyrrole, all other compounds showed the ACQ property. Their single-crystal structures demonstrated that the molecular structure, conformational twisting, structural rigidification, and morphological packing play important roles in the photophysical processes of aryl-substituted pyrroles. However, only the influences of the type and number of substituent on the photophysical properties of photoluminescent materials have been studied in the previous studies. Since it is well known that substituent position is also an important factor for photophysical properties of isomers, it was concluded that the variation in electronic structure resulted in different photophysical properties of the isomers, such as electron affinity, ionization potential, electronic absorption, fluorescence spectra, and dipole moment. Furthermore, it has been demonstrated that the photophysical behaviors of AIEgens in the solid state could be significantly affected by the isomerization. To date, there, however, are only a few scattered reports the effect of isomerization on fluorescence behaviors of fluorophores in the solid state. The results indicated that the subtle variation in the substituent position of aryl units at AIEgens has a profound impact on their AIE properties, thus resulting in vast differences in their steric conformations, conjugation degrees, and electronic distributions.31-33 Herein, we designed and synthesized two triphenylpyrrole isomers according to the literature method, 1,2,5-triphenylpyrrole (TPP1) 30, 34 and 1,3,4-triphenylpyrrole (TPP2) 35-37 (Chart 1). Due to their different substituent positions of two phenyl rings, the photophysical properties of these two isomers are entirely different. Consequently, TPP1 is ACQ while TPP2 is AIE. Both the experimental and theoretical results demonstrate that the charge distribution, molecular structure,


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and morphological packing play important roles in the photophysical processes of triphenylpyrrole isomers, which enable us to better understand the influence of the different substituent position on the AIE phenomenon.

Chart 1. The structures of TPP1 and TPP2 RESULTS AND DISCUSSION Synthesis of TPP1and TPP2 According to the literature method, we synthesized TPP1 via the Schulte-Reisch reaction of 1,4-diphenylbuta-1,3-diyne with aniline. 30, 34 Meanwhile, we synthesized TPP2 from the reaction of 2-phenylacetaldehyde with aniline in the presence of AgOAc and NaOAc in anhydrous THF. 35-37

These reactions performed under ambient conditions afforded good yields of desired

products. The products were characterized using spectral and crystallographic methods, from which satisfactory analysis data corresponding to their expected molecular structures were obtained (Figure S1-S8, Supporting Information). Photophysical properties of TPP1 and TPP2 in solution The absorption and fluorescence spectra of TPP1 and TPP2 in pure THF are shown in Figure 1, which exhibited significant difference. The absorption peaks of TPP1 at 242 nm and TPP2 at 256 nm were attributed to π-π* transitions of phenyl rings. Meanwhile, the absorption peaks of TPP1 at 304 nm and TPP2 at 290 nm corresponded to π-π* transitions of pyrrolyl-phenyl conjugation structure. This result reveals that TPP1 possessing more conjugated structure


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displays a stronger absorption than TPP2. In the emission spectra of these two isomers, the emission intensity of TPP2 at 345 nm was significantly weaker than that of TPP1 at 385 nm due to the lower oscillator strength of TPP2 (Figure 1 and Table 1). This phenomenon has also been reported by other research groups.38- 41 Moreover, the fluorescence quantum yields (ΦF) of these two isomers in pure THF solution further confirm this result. TPP1 showed a higher fluorescence quantum yield (ΦF) of 24.8%, which is considerably higher than that of TPP2 (ΦF =1.37%). Therefore, it is concluded that the electronic structures of these two isomers have a significant influence on their photophysical behaviors.

2.5

TPP1/Abs TPP2/Abs

3500

2.0

TPP1/Em TPP2/Em

3000 2500

1.5

2000 1500

1.0

PL Intensity

4000

Absorbance

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1000 0.5 500 0

0.0 250

300

350

400

450

500

550

600

Wavelength (nm)

Figure 1. The UV-vis absorption spectra and PL emission spectra of TPP1 and TPP2 in pure THF. Solution concentration: 1x10-4 mol/L. Excitation wavelength: 310 nm (TPP1) and 280 nm (TPP2).


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Table1. Properties of triphenylpyrrole isomers

Compounds

Melting points / oC

Abs. in THF / nm

Em. In THF / nm

Abs. in solid / nm

Em. In solid / nm

ΦF in THF （%）

ΦF in solid （%）

Dipole moment /DB

TPP1

234.45

242,304

385

284,330

385

24.8

1.88

0.3356

TPP2

152.75

256,290

345

302

382

1.37

13.1

2.1985

The charge distribution in these two isomers varies with their different substituent positions. The dipole moment values of these two isomers are the theoretical values obtained by Gaussian calculation. The results indicate that the dipole moments of TPP1 and TPP2 was 0.3356 and 2.1985 DB, respectively, corresponding to a non-polar molecule and a polar molecule, respectively. To characterize the solvent effect of these two isomers, we measured the maximal wavelengths for absorbance and emission in solvents with varying polarity. For TPP2, the UVvis absorption spectra and the PL spectra significantly changed with increasing solvent polarity parameter (∆f) (Figure S9, Supporting Information). In contrast, the emissions of TPP1 displayed only a slightly bathochromic shift. The influence of ∆f on the Stokes shift (∆υ) was further explored by the Lippert–Mataga equation in the Supporting Information. From the plots of ∆υ versus ∆f (Table S1 and Figure S10, Supporting Information), it was found that the fitting lines for TPP1 were positive with a small slope of 195, showing a slight solvatochromism effect. On the other hand, TPP2 exhibited a large negative slope of -650, exhibiting more significant solvatochromism effect. It suggests that the excited state of TPP2 has a larger dipole moment than the ground state because of its substantial charge redistribution through the strong polarity. The results indicate that TPP2 is more favorable than TPP1 for the ICT process, which further confirms that TPP2 possesses a higher molecular polarity. Photophysical properties of TPP1 and TPP2 in aggregate state


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Given the similar structures of TPP1 and TPP2, we next investigated their photophysical properties in the aggregate state and in solid. The UV-vis absorption spectra and PL spectra of these two isomers were measured in water-THF mixtures with different fractions of water, and their final concentrations were kept constant at 1 x10-4 mol/L according to the previous methods. As can be seen from Figure 2A, the absorbance of TPP1 at 304 nm was almost constant when the water content in the aqueous THF mixture was

Effect of Substituent Position on the Photophysical Properties of

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