Upper Excited Triplet State-Mediated Intersystem Crossing for Anti

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C: Physical Processes in Nanomaterials and Nanostructures

Upper Excited Triplet State-Mediated Intersystem Crossing for AntiKasha's Fluorescence: Potential Application in Deep-Ultraviolet Sensing Xianfeng Qiao, Yulong Liu, Jingwen Yao, Xin He, Hui Liu, Ping Lu, and Dongge Ma J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b12403 • Publication Date (Web): 14 Feb 2019 Downloaded from http://pubs.acs.org on February 18, 2019

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The Journal of Physical Chemistry

Upper Excited Triplet State-Mediated Intersystem Crossing for Anti-Kasha's Fluorescence: Potential Application in Deep-Ultraviolet Sensing Xianfeng Qiao,*,† Yulong Liu,‡,§ Jingwen Yao,† Xin He,‡ Hui Liu,‡ Ping Lu,*,‡ Dongge Ma*,†

†

Center for Aggregation-Induced Emission, Institute of Polymer Optoelectronic Materials and

Devices, State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou, 510640, P. R. China. ‡State

Key Lab of Supramolecular Structure and Materials, Jilin University, 2699 Qianjin

Avenue, Changchun 130012, P. R. China §

Present address: Northeast Agricultural University, HarBin, 150036, P. R. China

Corresponding Author * E-mail: [email protected], [email protected], [email protected]

ACS Paragon Plus Environment

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ABSTRACT. Owing to the Kasha rule, only the lowest excited state S1 contributes to the photoemission or other photo-induced processes in general, causing a waste of photoenergy and the limitation of application scenarios. Anti-Kasha effect offers the possibility of utilizing high-energy excited states Sn to develop novel functions and applications. Here, an anti-Kasha fluorescence has been experimentally found in a pure organic molecule and investigated in detail to reveal the underlying mechanism. The experimental

evidences

of

excitation

mapping

spectrum

and

time-resolved

photoluminescence spectrum suggest that the anti-Kasha emission is governed by an upper excited triplet state Tn. Intersystem crossing from S5 to Tn can successfully compete with internal conversion, followed by revere intersystem crossing from Tn to S2 to form upper state emission. Further, utilizing this effect for deep-ultraviolet light sensor is discussed. These findings provide keen insights into manipulating the excited state evolution, while offering the possibility for utilizing the high energy excited states to explore novel functions and applications of organic molecules.


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1. INTRODUCTION Manipulating the excited state evolution is of importance to develop new applications for organic molecular materials.1 For most cases in condensed phase, the excitons in upper excited states rapidly relax to the lowest excited state S1 from which photoemission, photochemistry and photoelectric processes take place.2 These is well known as Kasha’ rule.3,4 The physical picture behind this rule is the energy gap law. This law predicts a faster internal conversion (IC) and vibrational relaxation (VR) of the upper states to S1 when these excited states are closed to each other. As a result, the photoemission from high-energy excited states is hardly observed except from S1. In this case, the emission from the lowest excited state will be independent on the excitation wavelength. However, when the gap between upper and lowest states are large enough or these states are orthonormal, radiative recombination rate of upper states competes with IC and VR processes, leading to a photoemission from upper excited states, an anomalous emission broke Kasha’ rule.5-8 Actually, massive exceptions have been experimentally observed,9 raising the terminology of anti-Kasha effect.10,11 Limited to Kasha’ rule, generally only the lowest excited state has been intensively concerned, photon energy is partly wasted


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through IC and VR processes. In this sense, anti-Kasha effect becomes more and more important, since it opens a new route of utilizing high-energy excited states to develop novel functions and applications.12-14,18

To obtain anti-Kasha emission, a universal principle is to make the radiative rate of upper electronic states dominant compared to the nonradiative rate.2 In general, the nonradiative processes of upper states consist of IC, VR, intersystem crossing (ISC) and dissociation. In pure organic molecules, IC and VR processes mainly contribute to the deactivation channel for upper states and ISC process is generally neglected. In this case, enlarging the gap ∆𝐸 between upper states and S1 can be an effective approach as in the typical examples of Azulene, porphyrin and their derivatives.19-25 Since IC process is highly dependent on the electronic coupling between two states, large gap leads to a weak coupling between these involved electronic states and thus an inefficient internal conversion. As a result, photoemission from high-energy states can fairly compete with IC process. Numerous experimental works on azulene derivatives showed that the fluorescence yield from the second excited state S2 increase almost exponentially with increasing energy gap between S1 and S2 states.26,27,28,29 In contrary, in case of small


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energy gap, the S2 state can be populated by thermal excitation of S1. The typical examples of thermal population mechanism are diphenylhexatriene and pyrene in solution with S2 fluorescence.30-32

Actually, the contribution of ISC to nonradiative decay of upper states might be undervalued in organic molecules.33,34 In crystal pentacene, the vibronically induced ISC for high-energy excited states was reported.35 It is found that in dibromoantracebe ISC can be facilitated in upper states, causing an excitation wavelength-dependent fluorescence quantum yield, while in metalorganic compounds with internal strong spinorbital coupling, ISC process can successfully compete with IC process in upper states.36,37 Therefore, in case of incomplete ISC, both of high-energy fluorescence from S1 and low-energy phosphorescence from T1 can be observed.38,39 More importantly, the ISC process in metalorganic is strongly dependent on the excitation wavelength and becomes faster at high-energy excitation. As a result, the ratio of fluorescence and phosphorescence emission exhibits a strong wavelength dependence. Chou et al. have systematically investigated the anti-Kasha effect caused by ISC mechanism in systems


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that simultaneously exhibit both fluorescence and phosphorescence.36,37,40 Besides the mechanisms discussed above, revealing the origin of anti-Kasha effects in new material systems are important to develop novel technologies base on this intriguing property.41-43

In this paper, a strong excitation wavelength-dependent fluorescence is observed in a pure organic material 2-(4,6-Dihydropyren-1-yl)-1-phenyl-1H-phenanthro [9,10-d] imidazole (PyPyI). It is found that the deep ultraviolet excitation of about 250 nm can excite a new dominant fluorescence emission peaked at 392 nm together with the normal weak emission centered at 468 nm. In contrast, the low-energy excitation can only result in the 468 nm fluorescence emission from S1. The time-resolved photoluminescence, excitation mapping spectra and theoretical calculation are used to investigate the underlying mechanisms. These results demonstrate that the intersystem crossing from high energy excited S5 to Tn can fairly compete with internal conversion, followed by the reverse intersystem crossing to S2. This new route for excited state evolution leads to the anti-Kasha fluorescence from S2 with long lifetime of about 109.7 microsecond. Finally,


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the possibility of implementing this anti-Kasha fluorescence as deep ultraviolet sensor is discussed.

2. EXPERIMENTAL SECTION

Materials and film. PyPyI was once purified by thermal gradient sublimation prior to experiments. The investigated film was obtained by spin-casting on quartz substrate with 1mM THF solution.

Theoretical calculation. The ground-state (S0) and S1 geometries were optimized at the B3LYP/6-31G(d, p) level, which is a common method to provide molecular geometries. The higher energy levels of both singlet and triplet states were calculated using TD-M062X/6-31G (d, p) method on the basis of the optimized configuration of S1.44 All the calculations were performed with the Gaussian 09 package (D. 01 version). Spectra characteristics. The optical absorption spectra were measured by UV-3600 spectrophotometer (Shimazu Instruments). The steady and transient optical spectra were performed by spectrometer FLS 980 (Edinburgh Instruments). The steady PL, excitation and emission mapping spectra were excited by standard xenon lamp. The transient and time-resolved PL measurements were excited by microsecond lamp (250 nm) and pulsed diode laser (375 nm). The low-temperature steady PL measurement was conducted using


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an Oxford Instrument Optistat DN-V. The temperatures were measured within ±0.1K by an Oxford Instrument ITC601 temperature controller.

3. RESULTS AND DISCUSSION Steady-state spectra. The steady state photoluminescence (PL) spectra of PyPyI film excited at 350 nm and 250 nm are shown in Figure 1a. Clearly, the emission peaks are strongly dependent on the excitation wavelength. When excited with 350 nm xenon lamp, only one emission band centered at 468 nm is observed. Increasing the excitation energy to 250 nm, an additional dominant emission peaked at 392 nm is detected together with the relative weak 468 nm emission. Similar phenomenon is also observed when doping PyPyI into PMMA matrix (Figure S1). Furthermore, the excitation spectra for 468 nm and 392 nm are displayed in Figure 1b as well as the absorption spectrum of the film. The excitation spectrum for 468 nm shares similar line-profile with the absorption spectrum ranging from 230 nm to 430 nm, implying that the photogenerated excitons in all upper excited states investigated could undergo fast IC process to S1. In contrast, the excitation spectrum for 392 nm has a dominant sharp peak around 250 nm that is completely


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different from the line-shape of absorption spectrum, suggesting that the high-energy excitation is mainly responsible for the short-wavelength emission. To obtain global view of the excitation wavelength-dependent PL spectra, the emission mapping studies were conducted and these results are displayed in Figure 1c. Two emission band centered at 468 and 392 nm are observed. Clearly, the excitation with wavelength ranging from 230 to 430 nm contributes to the long-wavelength emission band with peak of 468 nm. But only the excitation to high energy states around 250 nm can activate the short-wavelength emission band centered at 392 nm, consistent with the excitation spectrum. These are typical character of anti-Kasha effect that the photoemission properties are strongly dependent on the excitation energy.


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Figure 1 (a) PL spectra of PyPyI films excited at 250 nm (black line) and 350 nm (red line). (b) Normalized excitation spectra of PyPyI films recorded at 468 nm (solid black line) and 392 nm (solid red line), together with the normalized absorption spectrum (black dashed line). (c) Emission mapping in wavelength range of 340 to 650 nm for emission by excitation ranging from 230 to 370 nm with 2 nm steps. The color indicator represents the emission intensity.


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Time-resolved PL measurement. To reveal the dynamics of the anti-Kasha photoemission, transient PL decay curves for 468 and 392 nm emissions are recorded with 375 and 250 nm excitations and shown in Figure 2a and 2b, respectively. In Fig 2a, for the case of 375 nm excitation, the emission of 468 nm wavelength follows the fluorescent decay form with lifetime of 4.5 ns. As shown in Figure 2b, the excitation of 250 nm leads the two emission bands to completely different decay fashion. The emission at short-wavelength of 392 nm exhibits single exponential decay with persisted lifetime of 109.7 μs, suggesting that triplet states should participate in this anti-Kasha PL process. In contrast, the emission at long-wavelength of 468 nm upon 250 nm excitation decays exponentially with two regimes. Flowing the prompt fast decay, the slower exponential decay shares the same lifetime of 104.1 us with the 392 nm emission within acceptable error. The identical lifetime suggests that the 392 nm emission and the slower emission of 468 nm are from the homologous process. However, it is still hard to judge whether the slower decay comes from the long-wavelength emission band or from the tail of shortwavelength band by single PL decay curve, since the two emission bands overlap upon


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250 nm excitation as shown in Figure1. Essentially, time-resolved photoluminescence (TRPL) technology is valid to distinguish the two overlapping emissions.

Figure 2 (a) Transient PL decay curves for 468 nm emission excited by pulsed diode laser of 375 nm. (b) Transient PL decay curves for 468 and 392 nm emissions excited by microsecond lamp of 250 nm. (c) Time-resolved PL spectra of PyPyI films by 250 nm excitation with 2 nm emission resolution. (d) Time resolved PL spectra at series delayed


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times after excitation extracted from Figure 2c. Insert in Figure 2 (d) is the spectrum at delayed time of 100 us in double-log form.

The TRPL spectra upon 250 nm excitation are shown in Figure 2c and the TRPL spectrum by 375 nm excitation can be found in supplementary Figure S2. Figure 2c clearly shows that these two emission bands are distributed in different time scales as expected. For comparison, the PL spectra at various delayed times after excitation are extracted from Figure 2c and shown in Figure 2d. After excited, the two emission bands reach their maximum intensity at about 2 μs, where 2 μs is the pulse width of excitation source at 250 nm. The long-wavelength emission peaked at 468 nm is initially stronger than the short-wavelength emission centered at 392 nm. As the delayed time increases, the long-wavelength emission decreases faster and the short-wavelength emission decreases relative slower. At delayed time of 100 μs, the long-wavelength emission is still observed though it is much weaker as shown in the inset of Figure 2d. These observations offer final verdict that the slower decay of 370 nm emission is partly from the long-


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wavelength emission band. The analysis of TRPL results demonstrates that upon 250 nm excitation, the prompt and slower decay of long-wavelength emission are from two different physical processes, while the slower decay is homologous with the shortwavelength emission.

Figure 3 Theoretical calculation results. Natural transition orbitals (NTOs) of the first five singlet and triplet excited states for PyPyI. Si ant Ti represent the singlet and triplet excited states, respectively, where the subscript i indicates the number of the excited state with 1 standing for the lowest excited state. HLCT is short for hybrid local and charge transfer. LE represents the local excited state. HLCT and LE describe the characters of the excited states.


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Theoretical calculation. To gain deeper insights into the mechanism of the anti-Kasha emission, theoretical calculations were carried out. Natural transition orbitals (NTOs) of the first five singlet and triplet excited-states are displaced in Figure 3. The studied material PyPyI consists of two chromophores. S1, S5 and T3 are hybrid local and chargetransfer (HLCT) states occupying the whole molecules, the other states are all local excited (LE) states. Among these LE states, S2, T1, T4 and T5 are located at pyrene unit, while S3, S4 and T2 are mainly occupied on pyrene derivative unit. The different locations and properties of excited states might account for the complex photophysical processes. The calculated excited state energy can be found in supplementary Figure S3. HLCT state combinates the Frenkel and CT type characters, which is believed to facilitate the ISC process.33 As in acene system, the conversion from singlet to triplet states can be faster within femtosecond scale with the vibronically induced ISC.45-47 Further investigations found that the closed degeneracy between triplet and singlet states can compensate the spin-orbital coupling necessary for ISC process, leading to the competing between ISC and IC processes in DNA nucleobase cytosine.48 Here, the intersystem crossing from the high-energy HLCT state of S5 is considered to be comparable with the


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internal conversion to S1. The reported vibronically induced ISC effect47 and the HLCT characters should be favorable for the ISC process from S5 in this studied molecule.

Mechanisms for the anti-Kasha emission. Based on the above experimental evidences, a possible pathway responsible for this anti-Kasha emission is proposed in Figure 4. For the excitation states lower than S5, the photogenerated excited singlet states mainly undergo a fast IC process to S1, wherein the fluorescence occurs peaking at 468 nm with lifetime of nanosecond scale. In contrast, when excited to the states above S5, part of the photogenerated excitons cool down to S1 by IC process, forming the prompt fluorescence decay as observed in Figure 2b. Additionally, ISC from S5 to T5 or T4 becomes comparable with internal conversion to S1. Meanwhile, IC from Tn to T2, T1 and ground state S0 seems weak for the large energy gap. The experimental evidence for this assumption is the lack of phosphorescence from T2 and T1 in 80 K (See Figure S4). In this case, reverse ISC from Tn to S2 could be a competitive deactivation channel, since these two excited states are closed in energy and located on the same group. This route of S5-Tn-S2 for exciton evolution eventual leads to the anti-Kasha fluorescence from S2.


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This mechanism well interprets the microsecond lifetime of the high-energy fluorescence emission around 392 nm, since it is modulated by a high-energy triplet excited state. Moreover, the excitons of direct internal conversion from S5 to S1 lead to the prompt fast decay, and the internal conversion from S2 contributes to the slower decay. This suggested pathway for excitons evolution is well consistent with the TRPL analysis.

Figure 4 Excitons evolution route. Schematic potential key photophysical processes for anti-Kasha photoemission. Abs, IC, ISC and RISC represent the absorption, internal conversion, intersystem crossing and revers intersystem crossing, respectively. The absorption and emission processes are donated as solid arrow lines. The nonradiative processes are indicated as dashed arrow lines


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Our experimental observations reveal that anti-Kasha fluorescence is formed via ISC and reverse ISC processes mediated by a high-energy triplet excited state Tn. The key feature of this anti-Kasha effect is the excitation wavelength-dependent emission as shown in Figure 1c. An intensity ratio R392/468, defined as the relative intensity of high- and low-energy emission peaks, is extracted from Figure 1c and shown in Figure 5. The R392/468 ratio exhibits a strong dependence on the excitation energy as expected with the maximum value at 244 nm. Further increasing the excitation wavelength, the R392/468 monotonously decreases and closes to unit at 262 nm, suggesting that the two emission bands share the same intensity at 262 nm excitation. At excitation range longer that 270 nm, the R392/468 continuously reduces and finally approaches to zero, implying the intensity of short-wavelength emission becomes negligible. Obviously, the value of R392/468 can be an indicator for sensing deep-ultraviolet light. For digital optical sensor, two distinctly different states identified as on and off are necessary. Here, the values of R392/468 larger and smaller than 1 are assigned as on/off states to sense the deep-


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ultraviolet light. With the light illumination, dual-channel calibrated detector fixed at 392 and 468 nm outputs dual PL intensities for comparison. R392/468 large than 1 indicates that the excitation light must contain deep-ultraviolet light, while R392/468 smaller than 1 is strong indicator of lack of deep-ultraviolet excitation.

Figure 5 Excitation wavelength dependent PL spectra. Ratio of R392/468 as a function of excitation wavelength. The solid red line represents the value of 1. 4. CONCLUSIONS In summary, an anti-Kasha fluorescence in pure organic molecule PyPyI have been investigated by experimental measurement and theoretical calculations. A high excited triplet state modulated ISC and reverse ISC processes has been revealed to be


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responsible for the photoemission from upper states. The ability of emitting variable emission in response to excitation wavelength provides a convenient approach for deepultraviolet sensing. These results prove the possibility of utilizing high-energy excited states to explore novel functions and applications of organic molecules.

AUTHOR INFORMATION

Corresponding Author * E-mail: [email protected], [email protected], [email protected] Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT The authors gratefully acknowledge the National Natural Science Foundation of China (Grant No. 21788102, 51527804 and 61575195), for the support of this research.

ABBREVIATIONS IC, internal conversion; VR, vibrational relaxation; ISC, intersystem crossing; PyPyI, 2(4,6-Dihydropyren-1-yl)-1-phenyl-1H-phenanthro

[9,10-d]

imidazole;

PL,


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photoluminescence, TRPL, time-resolved photoluminescence; NTOs, Natural transition orbitals, HLCT, hybrid local and charge-transfer.

Supporting Information.

The Supporting Information is available free of charge on the ACS Publications website at DOI:

Experimental methods and measured pictures are shown in Supporting Information.

REFERENCES

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Upper Excited Triplet State-Mediated Intersystem Crossing for Anti

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