Conformational Relaxation and Thermally Activated Delayed

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Conformational Relaxation and Thermally Activated Delayed Fluorescence in Anthraquinone-Based Intramolecular Charge-Transfer Compound Zhuoran Kuang, Guiying He, Hongwei Song, Xian Wang, Zhubin Hu, Haitao Sun, Yan Wan, Qianjin Guo, and Andong Xia J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b11411 • Publication Date (Web): 18 Jan 2018 Downloaded from http://pubs.acs.org on January 19, 2018

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The Journal of Physical Chemistry

Conformational Relaxation and Thermally Activated Delayed Fluorescence in Anthraquinone-Based Intramolecular Charge-Transfer Compound Zhuoran Kuang,a,b Guiying He,a,b Hongwei Song,a,b Xian Wang,a,b Zhubin Hu,c Haitao Sun,c Yan Wan,d Qianjin Guo* a,b, 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

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

c

State Key Laboratory of Precision Spectroscopy, School of Physics and Materials Science, East

China Normal University, Shanghai 200062, People’s Republic of China d

College of Chemistry, Beijing Normal University, Beijing 100875, People’s Republic of China

ACS Paragon Plus Environment

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ABSTRACT

A

novel

donor-π-acceptor-π-donor-type

(D-π-A-π-D-type)

chromophore,

2,6-bis[4-

(diphenylamino)phenyl]-9,10-anthraquinone (AQ(PhDPA)2), has been reported as an efficient red thermally activated delayed fluorescence (TADF) emitter. Molecular structure and conformation, which directly determine the nature of excited states of a TADF emitter, are critical for obtaining efficient reverse intersystem crossing (rISC) and TADF. In this article, a series of excited-state deactivation processes of AQ(PhDPA)2, from the optical excitation to fluorescence and TADF emitting, have been investigated by theoretical calculations and ultrafast transient absorption (TA) spectroscopy. Theoretical calculations and steady-state spectra suggest that the TADF emitter appears to have conformational twisting in the excited state. Both the relaxed S0 and S1 conformations have a small energy difference between the lowest singlet and triplet excited states (∆EST) in favor of rISC, whereas ∆EST increases at the relaxed T1 conformation. Ultrafast TA spectra reveal that the intramolecular charge transfer (ICT) state of AQ(PhDPA)2 emits efficient fluorescence after a solvation-stabilization process in nonpolar toluene, while the fluorescence from the solvation-induced conformational relaxed ICT state is quenched in polar tetrahydrofuran. Additionally, we further reveal that the suppression of the conformational relaxation in long-lived triplet states contributes to maintaining a small ∆EST, which is critical for efficient rISC and TADF. These results provide a guidance for understanding the relationship between TADF and conformational relaxation dynamics, as well as for designing and synthesizing advanced TADF emitters.


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INTRODUCTION

The improvement of luminescence efficiency is always the primary issue to organic lightemitting diodes (OLEDs) or electroluminescent devices.1-3 Using thermally activated spin-flip up-conversion from the lowest triplet (T1) to singlet (S1) excited states (E-type),4 OLEDs based on conventional organic emitters enable all of singlet and triplet excitons into luminescence via fluorescence and thermally activated delayed fluorescence (TADF),1,5,6 where a theoretical internal quantum efficiency of almost 100% depending on up-conversion mechanisms has been achieved.7 Due to the different spin multiplicities of S1 and T1 states, the spin-flip process, that is intersystem crossing (ISC), is generally forbidden. The mixing of S1 and T1 state functions makes the ISC possible through the spin-orbital coupling (SOC). One strategy to enhance the SOC is to introduce heavy metal atoms into organic compounds. The enhanced SOC effect promotes the coupling between T1 and S1/S0. This approach has been applied into Cu(I) complexes, and both phosphorescence and TADF were observed at ambient temperature.8,9 The other strategy is to control a small energy gap between S1 and T1 (∆EST), typically < 0.3 eV.6,10 The decreased ∆EST opens an additional channel from lower-energy T1 to higher-energy S1, i.e. reverse ISC (rISC), and it is critical for realizing TADF. A small ∆EST is attributed to an effective spatially separation of the HOMO and LUMO, thus, TADF is an ICT-based fluorescence process. TADF materials with various molecular designs that suitably connect the donor (D) and acceptor (A) units have been fabricated.6,11-13 Pretwisted bipolar D-A-D-type TADF molecules have been proved to achieve a small ∆EST as well as a large fluorescence rate (kF).13,14


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A number of researches focusing on the mechanism of efficient rISC have been done by using quantum chemical calculations and time-resolved spectroscopy.15-22 Particularly, Penfold and Monkman et al. proposed that rISC is a complex second-order spin-vibronic SOC process, where the triplet locally-excited (3LE) state acts as an intermediate state to assist the coupling between the triplet (3CT) and the singlet charge transfer state (1CT).17,18 The vibronic coupling between the lowest 3LE and 3CT states leads to the rapid formation of an equilibrium via reverse internal conversion, while the rISC occurs via the weak SOC between the lowest 3LE and 1CT states.20,21 Thus, rISC is a dynamical process that is crucially dependent on the energy gap and vibronic coupling between the lowest LE and CT states. Molecular chemical structure and conformation, which intrinsically determine the nature of excited electronic states of an ICT compound, are critical for obtaining efficient rISC. Herein, for a pretwisted D-A-type molecule, the steric hindrance of D-A dihedral angle can affect the energies of lowest excited states, especially CT states, altering the relaxation pathways for TADF and/or phosphorescence.10,14,19,23,24 Except for conformation effects, it is also widely studied how the local environments, especially solvents with different polarities, affect the emission behaviors of ICT molecules.22,25-33 After the optical excitation to ICT states, the solvent molecules rearrange around the excited chromophores and “consume” the excess amount of energy. This phenomenon is commonly named “solvation effect”.34,35 For symmetric D-A-Dtype molecules with high structural flexibility, excited-state ICT usually couples with the solvation process and conformational relaxation, especially in polar solvents.36-39 The solvent fluctuation also leads to dynamical symmetry-breaking charge transfer in the lowest excited state of a symmetric molecule, and stabilizes the chromophore into a polarized state.40 Furthermore, since TADF emitters are usually used in solid-state devices, such as OLED, solid state solvation


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from a small polar dopant embedded in amorphous organic thin films are reported recently.31-33 Analogous to solvent−solute interactions in liquids, dopants in matrices are able to rotationally reorient in ultrafast time scales following the light-induced change of electronic configurations, minimizing the system energy though solvation in solid state matrices is slightly slower than that in liquid. These studies on solvation are helpful to understand how the host-guest interaction determines fundamental excited-state properties of TADF emitters.16,31-33,41 Scheme 1. Model for the D-π-A-π-D-type molecule, and molecular structure of AQ(PhDPA)2.

The purpose of this article is to provide a deeper understanding of the relationship between the excited-state conformational relaxation and TADF of a D-π-A-π-D-type TADF chromophore in various surrounding environments. Scheme 1 shows the molecular structure of the D-π-A-π-Dtype chromophore, 2,6-bis[4-(diphenylamino)phenyl]-9,10-anthraquinone (AQ(PhDPA)2), which has been recently reported as an efficient red TADF emitter.13 Quantum chemical calculations are conducted to simulate ground- and excited-state geometries and predict the excited energies of lowest LE and CT states. By analyzing the femtosecond excited-state dynamics, we identified the ultrafast solvation-induced conformational relaxation processes in 1CT, which markedly affects the molecular fluorescence properties. Furthermore, through analyzing the nanosecond-


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to-microsecond excited-state dynamics of AQ(PhDPA)2, we conclude that suppressing the conformational relaxation in triplet states is of significance to preserve a small ∆EST in favor of facilitating the dynamical rISC and TADF.

MATERIALS AND METHODS

Materials AQ(PhDPA)2 were purchased from Xi’an Polymer Light Technology Corp. Cycloolefin polymer Zeonex E48R (Zeon corp., Japan) were used as polymer matrixes. Rhodamine 6G was purchased from Sigma-Aldrich. All solvents including toluene and tetrahydrofuran (THF) were analytic reagent grade and used as received. Quantum Chemical Calculations On the basis of density-functional theory (DFT) and time-dependent DFT (TD-DFT), quantum-chemical calculations were performed to obtain the optimized geometries and excitedstate electronic structures for AQ(PhDPA)2. The ground-state geometries were optimized at the B3LYP42/6-31G(d) level, and the singlet and triplet excited-state geometries were optimized using the optimally-tuned range-separated LC-BLYP43 functionals, in which the optimal ω values were obtained by the “gap-tuning” procedure.54,55 The range-separation parameter ω (in Bohr-1) stands for the reciprocal of the interelectronic distance at which the exchange term switches from DFT-like to HF-like. This optimal tuning method, in brief, is to minimize the energy difference between the HOMO energy (εH(N)) and first ionization potential (IP(N)) of the N-electron neutral system and the energy difference between the HOMO energy of the anionic


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system (εH(N + 1)) and the electron affinity (EA(N), equivalent to IP(N + 1)) of the neutral system as showed in the following equation: 1

J 2 = ∑ ε H ( N + i ) + IP ( N + i ) 

2

(2)

i =0

Hereafter, the optimally-tuned range-separated functional is referred to as LC-BLYP*, and the optimal ω value is adjusted to 0.1891 Bohr-1. The prediction of the vertical excitation energies of low-lying excited states were carried out at the TD-LC-BLYP*/6-31G(d) level. The polarized continuum model (PCM) was applied to take into account solvent effects44, and the TammDancoff approximation (TDA) were implemented to avoid the issue of triplet instability.45 All simulations were carried out for isolated molecules using the Gaussian 09 software package.46 To analyze the electron excitation properties, the electron–hole (e–h) distribution analysis of the lowest excited states was performed on the Multiwfn.47 Steady-State and Transient Spectral Measurements The steady-state UV/Vis absorption and photoluminescence spectra were recorded on a U3010 (Hitachi, Japan) spectrophotometer and an F-4600 (Hitachi, Japan) fluorescence spectrometer, respectively. The fluorescence relative quantum yield (Φf) was determined by using Rhodamine 6G (Φf = 0.94, in ethanol) as a reference. For oxygen-free measurements, solutions were bubbled with nitrogen for 10 min before measurements. Fluorescence lifetimes were measured on a time-correlated single photon counting (TCSPC) spectrometer (F900, Edinburgh Instruments), excited by a picosecond LED source (PLS-500, PicoQuant) at 450 nm with the FWHM of 400 ps. The delayed fluorescence kinetics and nanosecond TA measurements were performed by using a flash photolysis spectrometer (LP920,


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Edinburgh Instruments), excited by a third harmonics generation Nd:YAG laser (Quanta-Ray, Spectra-Physics) at 355 nm with the FWHM of 8 ns. The experimental apparatus of femtosecond TA measurements were described in detail elsewhere.25,48 Briefly. a pulse with 400 nm, 90 nJ produced by doubling a portion of the 800 nm pulse with a BBO (type I, 0.5mm thickness) crystal from regenerative amplified femtosecond laser (50 fs, 1 kHz, Coherent Legend Elite USA) acts as the pump beam (spot size at the sample is ca. 130 µm). A white light supercontinuum (420–780 nm) generated by a water cell acts as probe beam after an optical delay up to 1 ns. The thickness of flowing sample cell is 1 mm for TA measurements. For isotropic measurements, the included angle between the pump and probe beam polarization was set to the magic angle (54.7°). Both the femtosecond and nanosecond time-resolved differential absorbances ∆A ( t , λ ) were analyzed by singular value decomposition (SVD) and target analysis using R-package TIMP software with the graphical interface Glotaran.49,50 ∆A ( t , λ ) is a superposition of several principal spectral components ε i ( λ ) weighed by their concentrations ci ( t ) :49,51 n

∆A ( t , λ ) = ∑ ci ( t ) ε i ( λ ) .

(3)

i =1

Viscosity Measurements Viscosity measurements of toluene-Zeonex solutions were performed on a DV2T (Brookfield) rotational viscometer at room temperature.


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RESULTS AND DISCUSSIONS

Quantum Chemical Calculations Density functional theory (DFT) and time-dependent density functional theory (TD-DFT) are employed to reproduce the molecular geometries and electronic structures of AQ(PhDPA)2. For an ICT molecule, it is proved that the simulation of CT transition energies is significantly dependent on the Hartree-Fock percentage in exchange-correlation functional.52-54 Recently, the optimally tuned range-separated functional has been successfully developed to accurately estimate the excitation energies of CT and LE states.55,56 We then employed this optimally tuned range-separated functional approach in the following calculations.

Table 1. Computed dihedral angles (θ), bond lengths (l), vertical transition energies of S1 (EVT(S1)) , energy difference between S1 and T1 (∆EST) for AQ(PhDPA)2 in toluene at optimized S0, S1 and T1 geometries at the optimally-tuned LC-BLYP*/6-31G(d) level. optimized

EVT(S1)

∆EST

(eV)

(eV)

1.413/1.480/1.480/1.413

2.93

0.41

32/30/31/32

1.409/1.475/1.475/1.409

2.70

0.42

27/20/21/27

1.398/1.462/1.462/1.398

2.63

0.51

geometry

θ1/θ2/θ3/θ4 (deg)

l1/l2/l3/l4 (Å)

S0

36/33/35/35

S1 T1

The conformational changes from S0 to S1, and then to T1 geometries are described by the dihedral angles and bond lengths between the DPA-donor, Ph-bridge, and AQ-acceptor (details seen Table 1). The HOMO and LUMO, which are correlated with the steric hindrance of molecular geometry, are displayed to reveal the relationship between ∆EST and conformational relaxation. In the S0 state, the dihedral angles between these units are all about 35°, indicating


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weak conjugation between D and A units. The HOMO (Figure 1) is symmetrically localized on DPA-donors and Ph-bridges, while the LUMO is mainly distributed over the AQ-acceptor. The effective spatial separation between the HOMO and LUMO benefits to gain a small ∆EST (0.41 eV) to induce TADF. The pretwisted bipolar chromophore preserves a high geometrical symmetry.

Figure 1. HOMOs and LUMOs for AQ(PhDPA)2 in toluene at optimized S0, S1, and T1 geometries. From the optimized S0 to S1, and then to T1 geometries, the twisted structure exhibits a trend of conjugation enhancing. The dihedral angles of two sides decrease to about 20°, and bond lengths also shorten obviously. These conformational changes strengthen the D-A electronic coupling. Accordingly, the spatial overlap of the HOMO and LUMO enlarges slightly, resulting in the ∆EST increasing to 0.51 eV at T1 geometry as shown in Table 1. To analyze the electron excitation properties, the electron-hole (e-h) distribution analysis for lowest excited states at optimized S0, S1, and T1 geometries are visually displayed in Figures 2 and S2 (in the Supporting Information). Excitation energies and electron transition properties of these states are listed in Table S1 (in the Supporting Information). Herein, we focus on the e-h distribution at optimized S1 and T1 geometries, between which the rISC happens. Figure 2 illustrates that, at optimized S1 geometry, S0 → S2/S3/T3 transitions contain significant CT (π-π*) character, while S0 → S1/T1/T2 transitions show LE (n-π*/π-π*) character. For T1 geometry, S0


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→S1/S3/T1/T2 transitions show CT (π-π*) character, while S0 → S2/T3 transitions show LE (nπ*/π-π*) character.

Figure 2. The electron-hole distribution of lowest singlet and triplet states of AQ(PhDPA)2 at optimized (a) S1 and (b) T1 geometries in toluene. The green and blue represent the electron and hole distribution, respectively. Excitation energies of CT transitions is closely dependent on the electronic coupling between D and A units, which can be adjusted by the conformational change. However, excitation energies of LE transitions localized on the AQ-acceptor is not significantly affected by the conformational change.57 Thus, the energy levels of CT states fall down from S1 to T1 geometries, while LE states remain nearly unchanged. The rISC mechanism depends on the nonadiabatic coupling between 3CT and 3LE.17,18,20 Thus, upper 3LE states lying between the lowest 1CT and 3

CT states act as intermediators for promoting rISC. Due to the conformational change, the

lowered 3CT state enlarges the energy difference between 3CT and upper 3LE, leading to weaker


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vibronic coupling. Therefore, the conformational relaxation from S1 to T1 state is adverse to rISC and TADF. The results of theoretical simulations suggest that the excited-state conformational change of the bipolar AQ(PhDPA)2 emitter enlarges ∆EST and adjusts the energy levels of CT and LE states, which may have influence on rISC efficiency and TADF properties. The excited-state dynamics, as well as the conformational relaxation, can be controlled by solvent polarity.36 To illustrate the excited-state relaxation dynamics of AQ(PhDPA)2, especially regarding the prompt and delayed fluorescence, steady- and transient-state spectral measurements are conducted and discussed in detail as follows.

Steady-State Spectra The normalized absorption and photoluminescence spectra of AQ(PhDPA)2 in toluene solution, tetrahydrofuran (THF) solution, AQ(PhDPA)2-doped Zeonex film, and neat AQ(PhDPA)2 film at 298 K are shown in Figure 3. For AQ(PhDPA)2 in nonpolar toluene and polar THF, the ICT absorption bands show a much slight blue-shift along with the increase of solvent polarity, indicating a nearly net zero electric dipole moment at the ground state. By contrast, fluorescence bands of AQ(PhDPA)2 in toluene and THF show an obvious bathochromic shift from 605 nm to 720 nm. The solvent-dependent emission with an unstructured spectral profile typically originates from the relaxed 1CT state. For the bipolar symmetric D-A-D-type molecule, polar solvent environment results in a relaxed symmetry-broken state that is also referred to as solvation-induced conformational adjustment, which is more obvious in the excited state than in the ground state. Thus, emission spectra show obvious solvatochromic effect, while the absorption spectra shift slightly with increasing solvent polarity.36,40


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Figure 3. Absorption (solid line) and photoluminescence (dotted line) spectra of AQ(PhDPA)2 in different solutions and films. The concentration of solutions is 10−5 M. The mass fraction of the doped film is 2 wt %. The AQ(PhDPA)2-doped nonpolar Zeonex film (2 wt %) is prepared to provide a conformational-relaxation-restricted environment. A neat AQ(PhDPA)2 film without a polymer matrix is also prepared as a reference. Absorption curves of the doped and neat films have similar spectral profiles to those in solutions but a slight red-shift of the ICT band. These evidence indicate the solvent-induced conformational adjustment lowers the energy of the ground state. Considering the emission spectra, the doped film, which is efficient green emitting, has a smaller Stokes shift than that in toluene. The small Stokes shift implies a low energy loss in excited states attributed to the confinement of geometrical relaxation in the matrix. Differently, the red-emitting neat film experiences a large Stokes shift. Thus, compared to the doped film, the chromophores in a neat film are described as a molecular aggregation state, in which chromophores locate in the immediate vicinity and cannot be approximated as isolated molecules. Therefore, the diluted doped film is appropriate to present the conformational relaxationrestricted environment with a favorable molecular dispersity and minimal intermolecular interactions.


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Before studying the excited-state dynamics, the fundamental photophysical properties of AQ(PhDPA)2 in different environments are evaluated (Table 2). After bubbling oxygen, the observed delayed fluorescence in solutions completely vanishes. For AQ(PhDPA)2 in THF, we observed an extremely low fluorescence quantum yield (Φf), and no delayed fluorescence. In oxygen-free toluene, strong delayed fluorescence is observed. To investigate the relationship between conformational relaxation and TADF, we prepared AQ(PhDPA)2-Zeonex toluene solutions (Solution I: 1.9 cP; Solution II: 7.4 cP) to provide gradually varied ambient viscosity. The increased solvent viscosity results in an increase in the TADF quantum yield (ΦTADF), but barely influences the Φf. These results imply intramolecular conformational motions in excited states, which are closely related to the TADF properties. Time-resolved TA spectral measurements are expected to have a complete description of the dynamical solvent-stabilized processes and conformational relaxation dynamics.

Table 2. Photoluminescence characteristics of AQ(PhDPA)2 in solutions and doped film.

environment THF a toluene (0.59 cP)

a

Φf

ΦTADF

0.002

τprompt

τTADF

kr,S c 6

kISC c -1

8

kRISC c -1

5

-1

knr,T c

(ns)

(µs)

(×10 s )

(×10 s )

(×10 s )

(×104 s-1)

--

5.1

--

0.39

2.0

--

--

0.17

0.23

7.2

13.7

24

1.2

1.2

5.3

a

0.17

0.26

7.2

3.4

24

1.2

5.4

20

toluene-Zeonex II (7.4 cP) a

0.17

0.28

7.2

2.1

24

1.2

9.5

32

0.15

0.16

4.9

1.7

30

1.7

75

4.9

toluene-Zeonex I (1.9 cP)

doped film

b

a

Measured at a concentration of 10−5 M in solutions.

b

The mass fraction of the doped film is 2 wt %.

c Rate constant of radiative decay of singlets kr,S, kISC, kRISC, and nonradiative decay of triplets knr,T determined by the method described by Masui et al.58


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Femtosecond Time-Resolved Conformational Relaxation Dynamics

Figure 4. Evolution of TA spectra at different delay time for AQ(PhDPA)2 in (a) toluene and (b) THF excited at 400 nm. The broadband femtosecond time-resolved TA measurements were further carried out to have a better insight into the nature of the emissive ICT state for AQ(PhDPA)2 in nonpolar toluene and polar THF. As shown in Figure 4, the TA spectra in both solvents are composed of broad excited-state absorption (ESA) across the entire spectral range, overlapping with ground state bleach (GSB) around 440 nm and stimulated emission (SE) around 650 nm. Specifically, for AQ(PhDPA)2 in toluene (Figure 4a), broad ESA occurs instantly in the initial 0.7 ps, simultaneously accompanied by a rapid generation and spectral evolution of an SE band between 570 to 700 nm. The SE maximum shifts from 600 to 640 nm with an increase in magnitude as the delay times become longer. According to the results of DFT calculations and steady-state spectra, the 400 nm optical excitation directly excites the chromophore to the upper vibrational energy levels of the singlet ICT (1CT) state. Thus, the SE spectral evolution indicates the solvation stabilizing process of the excited chromophore. During the following 10 – 1000 ps,


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the ESA and SE bands decay slowly with a neglectable spectral shift, indicating a long-lived emissive state. In contrast to the measurement for AQ(PhDPA)2 in nonpolar toluene, different spectral features are observed in polar THF (Figure 4b). TA features in The initial 1 ps are similar to those in toluene, but their evolution is much faster. A broad SE band is recognized at 640 nm with a rapid bathochromic shift to 680 nm, which is corresponding to the solvation process in THF. Simultaneously with the spectral shift, an obvious integral intensity decay of the SE band is found within ca. 4 ps, and leaving a broad ESA band (500 – 750 nm) with a slow decay in magnitude. Since the SE and ESA originating from the same population in the excited state, such rapid quenching of the SE band results from a reduction of the oscillator strength for the solvation-coupled ICT state, in line with the very low fluorescence quantum yield in THF.59 To the end of 1 ns, a new broad ESA band develops around 450 nm with a low intensity. The markedly different spectral dynamics of AQ(PhDPA)2 in THF and toluene indicates that solvent effect plays a significant role in excited-state conformational relaxation and affect the nature of emissive 1CT state obviously.37 To further figure out the excited-state relaxation mechanism, target analysis was performed to extract species-associated difference spectra (SADS) and concentration kinetics. According to the singular value decomposition (SVD) analysis results and the known excited-state nature of AQ(PhDPA)2, four components are required for the adequate fitting based on a target evolution model as shown in Scheme 2. Time constants of corresponding excited-state processes estimated from the target analysis are also shown in Scheme 2. Kinetics at selected wavelengths are depicted in Figure S4 in the Supporting Information to show the fitting qualities.


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Scheme 2. Proposed relaxation models of AQ(PhDPA)2 in toluene and THF. Time constants of corresponding excited-state processes estimated from the target analysis are depicted.

Figure 5a,c shows SADSs and concentration dynamics for AQ(PhDPA)2 in toluene. The first SADS (blue-black line, Figure 5a) with a decay time constant of 1.5 ps appears to a broad ESA character. We ascribe the first SADS to the high vibrational levels of the 1CT state (FranckCondon state, FC state) populated by the ultrafast optical excitation. The second SADS (skyblue line, Figure 5a), which rises in 1.5 ps and decays in 5.2 ps, displays an evident SE band peaking around 620 nm. Besides, the third SADS (orange line, Figure 5a) exhibits a similar spectral profile with the second SADS but a bathochromic-shifted SE peak. Thus, the second and third SADSs stand for two relaxation stages of 1CT states.37 The time constant of 1.5 ps is attributed to the solvation-stabilized process that surrounding solvent molecules rearrange around the excited chromophores. Subsequently, the time constant of 5.2 ps is ascribed to the solvation-induced conformational

relaxation

processes,

more

specifically,

the

intramolecular

twisting

rearrangement of the flexible D-π-A-π-D structure.37,38 Therefore, the second and third SADSs correspond to the solvation-stabilized 1CT’ state and the conformational relaxed 1CT’’ state,


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respectively. The 1CT’’ state relaxes to S0 with a time constant of 3.5 ns. The fourth SADS (red line, Figure 5a) is attributed to the 3CT state, and the time constant of ISC is estimated to 2.4 ns. 3

LE states, which are not depicted in Scheme 2, act as intermediates for the effective population

on 3CT states.

Figure 5. SADSs from target analysis for AQ(PhDPA)2 in (a) toluene and (b) THF. Concentrations of transient species as a function of time in (c) toluene and (d) THF. For TA spectra in THF, a same proposed relaxation model is used in the target analysis, giving four SADSs (Figure 5b,d) corresponding to 1CT (FC excited), 1CT’ (solvation-stabilized), 1CT’’ (conformational relaxed), and 3CT states, respectively. Time constants of solvation-stabilization and conformational relaxation processes are 0.7 ps and 1.5 ps, respectively, which are much shorter than those in nonpolar toluene. Unlike the spectra in toluene, the SADS of the 1CT’’ (conformational relaxed) state in THF does not exhibit obvious SE character but a broad ESA


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band, indicating the formation of a nonemissive twisted conformer. Additionally, the solvationinduced conformational relaxation significantly enhances the nonradiative relaxation. The population transfer to triplet states for AQ(PhDPA)2 in polar THF is suppressed on account of the competition from the rapid charge recombination (S1→S0, 250 ps) because of the strong solvation in THF. Besides, the nonemissive relaxed 1CT state results in the inhibition of fluorescence and TADF even if the re-population via rISC exists. The species-associated target analysis approach, which bases on the state-to-state evolution assumption, only mimics the observed spectral dynamics. When considering a continuous evolution model (e.g., solvation), SADSs merely depict a limited number of intermediate species.60 The spectral shift of TA spectra provides a supplemental description of the formation of the solvation-coupled relaxed 1CT state. The time-correlated dynamical Stokes shift is extracted from the corresponding TA spectra by multi-peaks Gaussian fitting. The peak shift correlation function is expressed as34,49

C (t ) =

ν ( ∞ ) −ν ( t ) ν ( ∞ ) −ν ( 0 )

(4)

where ν(t) denotes the position of SE peak at a time delay t, and ν(0) and ν(∞) represents the initial and final peak positions, respectively. Here, the peak position at 400 fs is set as ν(0); ν(∞) is fixed by ν(100 ps) and ν(2 ps) in toluene and THF, respectively. The resulting correlation functions, single exponential fitting curves, and fitted time constants are shown in Figure 6. It is seen that the fitted time constant from the correlation functions is ca. 2.6 ps in toluene, much slower than that in THF (0.5 ps). The dynamics Stokes shift suggests that solvation is more significant and rapid in polar THF than that in nonpolar toluene. In combination with the target analysis and the proposed relaxation models, the solvation time constant in toluene (2.6 ps) ascribes the apparent evolution of the potential energy surface of the excited 1CT state, from the


19


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FC excitation to the solvation-stabilized 1CT’ (1.5 ps), and then to the conformational relaxed 1

CT’’ state (5.2 ps). The solvation time constant in THF (0.5 ps) corresponds to the formation

process of solvation-stabilized

1

CT’ state, which rapidly evolves to the nonemissive

conformational relaxed 1CT’’ state. The dynamical Stokes shift results, in addition to the target analysis, is helpful for us to accurately infer the excited-state relaxation paths of AQ(PhDPA)2 in toluene and THF. Overall, these results indicate that a prominent solvation effect acts on the 1CT state of AQ(PhDPA)2. The solvent-dependent excited-state deactivation mechanism for bipolar D-π-A-πD-type chromophore contains substantial intramolecular conformation effect, which is critical for obtaining a superior fluorescence performance.61 Nanosecond TA spectral measurements are further performed to unravel the subsequent conformational dynamics in triplet states.

Figure 6. Excited-state peak shift time correlation functions C(t), fitting curves, and fitted time constants for AQ(PhDPA)2 in toluene and THF.


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Nanosecond Time-Resolved Conformational Relaxation Dynamics

Figure 7. (a) Nanosecond TA spectra of AQ(PhDPA)2 in toluene following the excitation at 355 nm. The shaded area represents the steady-state absorption spectrum. (b) Target evolution model, SADSs and corresponding concentration dynamics obtained from the target analysis. Nanosecond TA spectral measurements have been carried out for AQ(PhDPA)2 in toluene and doped-film to get an insight into the triplet-state dynamics and analyze the relationship between excited-state conformational relaxation and TADF. Figure 7a shows nanosecond TA spectra obtained in toluene. In the case of excitation at 355 nm, the initial spectrum acquired at 0.1 µs exhibits a dominant GSB band between 300 to 400 nm and an unapparent GSB band centered at 460 nm. Besides, a broad ESA band exists between 400 to 800 nm, which is assigned to the T1→Tn absorption. It is clearly seen that the spectral profiles have a distinct evolution as the delay time growing. In ca. 50 µs, the GSB band around 350 nm and the broad ESA band decay simultaneously, giving rise to an additional ESA peak at 380 nm. Subsequently, the evolution of the spectral profile becomes gradually unapparent in 50 – 300 µs. The decay dynamics of the GSB band intrinsically reflects the population recovery from the triplet manifold to the ground state. Obviously, the GSB decay is not a simple exponential process. Considering a TADF emitter, the emission of delayed fluorescence, as a radiative transition in microseconds, plays a


21


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major role in deactivating triplet states. Besides, the kinetics of TADF is always found to have a multi-exponential lifetime, and that is closely related to the molecular conformational relaxation.13 The nanosecond TA spectra of AQ(PhDPA)2 in toluene are analyzed using target analysis based on a target evolution model, and two SADSs and corresponding concentration dynamics are extracted (shown in Figure 7b). The estimated time constants are listed in Table 3. The kinetics of the first transient species describes the initial GSB recovery and ESA decay in TA spectra. The second transient species with a very long lifetime remains considerable GSB and ESA features, but its spectral profile changes significantly (compared with the first SADS). Meanwhile, the lifetime of TADF for AQ(PhDPA)2 in toluene is ca. 13.7 µs, approaching to the lifetime of the first SADS. Therefore, the initial fast recovery of the ground state population is attributed to rISC and TADF. Accordingly, we ascribe the first SADS to chromophores in the lowest 3CT state which preserve rISC-suited geometries. The long-lived second component suggests that a certain proportion of conformers in the 3CT state have relaxed to a twisted “dark state” (3CTdark) with enlarged ∆EST and inefficient rISC, eventually deactivating to the ground state through slow nonradiative transitions. As discussed above, the conformational evolution from initial S0 to S1 and then to T1 geometries leads to an increase of ∆EST which may result in efficiency decrease of rISC. The pretwisted D-π-A-π-D-type molecular structure with a low steric hindrance indicates various intermediate conformers between relaxed S1 and T1 geometries during the triplet-state relaxation. Thus, we speculate that the efficiency of rISC for the AQ(PhDPA)2 chromophore at the triplet manifold in solution declines along with the conformational relaxation. Thus, a higher proportion of chromophores at 3CT cannot up-convert to 1CT, and at last deactivate to the ground state via


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nonradiative relaxation. However, when flexible molecules are operating in polymer matrixes or high-viscosity microenvironments, intramolecular twisting and conformational changes would be restricted.62 We conducted the nanosecond TA measurements on AQ(PhDPA)2-doped (2 wt %) Zeonex matrix and AQ(PhDPA)2-Zeonex toluene solutions to further analyze the relationship between conformational changes and TADF.

Figure 8. Nanosecond TA spectra of AQ(PhDPA)2 in (a) doped film, (b) toluene-Zeonex solution I (the viscosity is 1.9 cP), and (c) toluene-Zeonex solution II (the viscosity is 7.4 cP) following the excitation at 355 nm. The shaded areas represent the corresponding steady-state absorption spectra. The TA spectra of AQ(PhDPA)2 in the doped film are shown in Figure 8a. Unlike the spectra in toluene, the spectral lineshape for 1 – 500 µs stays unchanged due to almost identical decay dynamics at different wavelengths. In comparison with AQ(PhDPA)2 in toluene, spectra of the doped film do not exhibit the feature of long-lived 3CTdark with shifted ESA band. Besides, the TA signal in the doped film almost vanishes after 500 µs, indicating that vast majority of excited chromophores have deactivated to the ground state (as depicted in Figure 9 in comparison with that in toluene). Generally, for emitters in rigid matrixes, collisions with surrounding molecules, which promote the non-radiative decay from T1 to S0, are suppressed.13 If the deactivation mechanism of a chromophore does not change when going from solution to solid state, the


23


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restriction of intramolecular motion prevents internal twisting from dissipating the excited-state energy, resulting in a longer excited-state lifetime in solid film than in solution.16,62 Considering the fact that AQ(PhDPA)2 in the doped film has a shorter excited-state lifetime than that in toluene but a similar τTADF, we deduce that the twisting process of AQ(PhDPA)2 in the doped film is highly suppressed. As a result, the relaxed chromophores in triplet manifold could preserve their initial conformations closed to S0 and S1 geometries with a small ∆EST, which is in favor of rISC and TADF. AQ(PhDPA)2-Zeonex toluene solutions (Solution I: 1.9 cP; Solution II: 7.4 cP) are prepared and the TA spectra are shown in Figure 8b,c. Based on the same target evolution model (as shown in Figure 7b), estimated time constants of relaxation dynamics are listed in Table 3. Focusing on the stacking spectra with equal time intervals, fast GSB recoveries around 350 nm and spectral lineshape evolutions in microseconds are observed in both Solutions I and II, which have similar spectral behaviors to those in toluene. The TA spectra after 10 µs for both Solutions I and II exhibit an ESA band around 400 nm and GSB dips at 340 and 450 nm, and the spectral profiles are consistent with the SADS of 3CTdark for AQ(PhDPA)2 in toluene (Figure 7b). The difference is that the GSB recovery, as well as the TADF decay, is significantly faster in Solution II (7.4 cP) than in Solution I (1.9 cP) and the toluene solution (0.59 cP). In addition, the measured quantum yields of TADF of these three solutions increase along with the viscosity. These results prove that the conformational relaxation rate is strongly dependent on the ambient viscosity.62 The high-viscosity environments suppress the intramolecular motions and conformational changes, and then inducing higher krISC and shorter τTADF.

Table 3. Relaxation Dynamics Parameters for AQ(PhDPA)2 after nanosecond laser pulse excitation.


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a

environment

viscosity (cP)

τTADF (µs)

τ1 (µs) a

τ2 (µs) a

τ3 (µs) a

toluene

0.59±0.1

13.7±0.3

27.0±0.8

64±5

550±50

toluene-Zeonex I

1.9±0.2

3.4±0.2

6.4±0.5

21±2

500±50

toluene-Zeonex II

7.4±0.2

2.1±0.2

3.9±0.5

17±2

420±50

doped film

--

16.7±0.4

15.5±0.8

50±2

240±30

dynamics parameters in nanosecond TA spectra obtained from the target analysis.

Figure 9. ESA and emission kinetics of AQ(PhDPA)2 in toluene and doped film. The results of nanosecond TA measurements indicate that the conformational relaxation plays a crucial role in rISC and TADF processes. For AQ(PhDPA)2, the Ph-bridged D-π-A-π-D structure has a large degree of twisting flexibility. The twisting of linearly positioned Ph-bridge and DPA-donor causes conformational relaxations, leading to the change of the energy order of lowest CT and LE states, as well as the energy gap between them. And we deduce that suppressing excited state conformational changes in terms of molecular structure is of great importance for achieving efficient TADF. Therefore, we establish a design strategy that proper steric hindrances should be introduced into pretwisted D-A-D-type TADF emitters to confine the excited-state conformational change. Multiple near-orthogonal D moieties suitably linking to the A unit, or adjusting the position of substituents to restrict intramolecular motions, will benefit a better TADF performance.


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CONCLUSIONS

In summary, we have elucidated the nature of the ICT state and conformation-related excitedstate relaxation dynamics of an anthraquinone-based TADF emitter, AQ(PhDPA)2, by theoretical calculations and ultrafast spectroscopic approaches. As a pretwisted D-π-A-π-D-type molecule, conformers of relaxed S0 and S1 states both have a small ∆EST for inducing TADF, while the relaxed T1 geometry has an increased ∆EST that is adverse to rISC. Femtosecond TA spectroscopy further identifies ultrafast solvation and conformational relaxation process of the emissive 1CT state in solutions. It is found that the 1CT state undergoes a solvation-stabilization process and emits fluorescence and TADF in nonpolar toluene; whereas in polar THF, the 1CT state undergoes a rapid solvation-induced conformational relaxation (ca. 1.5 ps) and stabilizes to a nonemissive state. The nanosecond-to-microsecond excited-state dynamics of AQ(PhDPA)2 in toluene solution and polymer matrix also point out that the suppression of the conformational relaxation in triplet states is of significance to facilitate the dynamical rISC process.

ASSOCIATED CONTENT

Supporting information. Additional TD-DFT calculation results, emission kinetics, fitting qualities of TA dynamics, and simulated geometry data. These materials are available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION

Corresponding authors *Andong Xia


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E-mail: [email protected] Telephone: 86-010-62562865

Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS

This work was supported by NSFCs (Nos. 21673252, 21333012, and 21773252) and the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No.XDB12020200).

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