Stable High-Energy Excited States Observed in a Conjugated

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C: Plasmonics; Optical, Magnetic, and Hybrid Materials

Stable High-Energy Excited States Observed in a Conjugated Molecule with Hindered Internal Conversion Processes Qinglin Jiang, Yuwei Xu, Xiaoming Liang, Cong Wang, Xiaohui Tang, Ya Li, Xu Qiu, Nan Zheng, Ruiyang Zhao, Duokai Zhao, Dehua Hu, and Yuguang Ma J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b12544 • Publication Date (Web): 19 Feb 2019 Downloaded from http://pubs.acs.org on February 26, 2019

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The Journal of Physical Chemistry

Stable High-Energy Excited States Observed in a Conjugated Molecule with Hindered Internal Conversion Processes Qinglin Jiang,a Yuwei Xu,a Xiaoming Liang,a Cong Wang,a Xiaohui Tang,a Ya Li,a Xu Qiu,a Nan Zheng,a Ruiyang Zhao,b Duokai Zhao,a Dehua Hu,*a and Yuguang Ma*a a

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 b

College of Chemical Engineering, Qingdao University of Science and Technology, Qingdao 266042,

P. R. China

ABSTRACT When the excited states of a molecule above its lowest excited state (Sn or Tn, n > 1) have a sufficient lifetime, photophysical and photochemical processes may occur directly in these high-energy levels, bypassing Kasha’s rule and significantly influencing the system properties. Investigation of the relationship between molecular structure and intramolecular electronic relaxation processes targets molecular design to achieve long-lifetime higher excited states. Here, we report stable high-energy excited states with lifetimes approaching several nanoseconds in our newly designed and synthesized compound PPI-AnCN. Through experimental and theoretical investigation, we reveal that both the large energy gap and poor electronic coupling between the high excited state and the S1 state are responsible for this stability. More importantly, we find that the emission from the higher exited states can be promoted when the lowest excited state is quenched. This study provides new insights into understanding longlifetime higher excited states and intramolecular electronic relaxation processes. INTRODUCTION 1

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Since an azulene-in-benzene solution was first shown to exhibit emission from an S2 state with a 1.04 ns lifetime,1, 2 long-lifetime higher excited states have attracted considerable attention from researchers interested in intramolecular electronic relaxation processes3-5 and potentially relevant applications.6-9 Later, other organic species, including thiones,10 pyrene, and polyenes,11 were discovered with higher excited states—including singlets and triplets—having lifetimes approaching the nanosecond range. Within this time scale, radiative decay,12-15 bi-molecular energy transfer16, 17 and photo-chemical reaction18, 19 seem to occur from these states. Thus, the applications of such long-lifetime higher excited states include many promising areas, such as sensors for biological systems,20, 21 energy transfer sensitizers22 and excitationwavelength-dependent photochemical reactions.23,

24

For example, Liptak and

Aprahamian et al. recently found that a series of BF2-hydrazone based dyes with emission from a higher energy excited singlet state can be used as efficient fluorescent molecular rotors; limiting the rotor rotation can suppress internal conversion to the dark S1 state and consequently enhance fluorescence.25 The most thoroughly studied examples of energy transfer from higher excited states are anthracene derivatives, which usually have a long-lifetime T2 state.16 This stable T2 state is a demonstrated triplet donor sometimes used to sensitize the rearrangement of rigid diene systems and as a “hot exciton” material for OLED applications through a fast T2 to S1 reverse intersystem crossing.26-28 Unlike photochemical reactions which require energy transfer from the higher excited states, some reactions can directly occur from them. It has been reported that acetophenone, benzaldehyde, and benzophenone show wavelengthdependent photochemistry in both the vapor and solid phases.29, 30 Upon irradiation into the S1 state, these compounds undergo efficient intersystem crossing to give T1 which did not react, but irradiation of these triplets (T1) with another intense light source gave radicals derived from the α-cleavage process. Therefore, it appears as though α-cleavage occurs from either a higher triplet or a vibrationally excited T1, produced by T-T absorption. Considering these important applications of long-lifetime higher excited states, new materials and more comprehensive research should be conducted to survey this specific subject. 2


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In this paper, we report the investigation of long-lifetime higher excited states in a newly designed compound PPI-AnCN. The molecule consists of 1,2-diphenyl-1Hphenanthro[9,10-d]imidazole (PPI) and anthracene-9-carbonitrile (AnCN) units. We experimentally observed anomalous light emission from a higher excited state (Sn) with lifetime approaching several nanoseconds in a low polarity solvent, accompanying the radiative decay of the S1 state. Through experimental and theoretical investigation, we attributed this phenomenon to the large energy gap and poor electronic coupling between the S1 and Sn states, which lowers the rate of internal conversion. More importantly, we found that quenching the emission of the S1 state can significantly increase emission from the S2 state. This study investigated emission from the higher excited state in a conjugated molecule in relationship to the mechanisms of intramolecular electronic relaxation. 1. EXPERIMENTAL SECTION 2.1 Synthesis All the reagents and solvents used for the synthesis and characterization were purchased from Aldrich and Acros companies and used without further purification. PPIB: PPIBr (3 g, 6.7 mmol), bis(pinacolato)diboron (3.39 g, 13.4 mmol, 2 equiv), KOAc (1.97 g, 20.0 mmol, 3 equiv), Pd(dppf)Cl2 (0.15g, 0.20 mmol, 0.03 equiv) and 100 ml dioxane were placed in a round-bottom flask. The mixture was heated at 85℃ under nitrogen for 48 hours. After cooling to the room temperature, the mixture was washed with 60 ml water for 3 times and extracted with dichloromethane. The organic solution was dried by Mg2SO4, and then evaporated the solvent. The residue was purified by column chromatography eluting with petroleum ether-dichloromethane mixtures to give a white solid (2.13g,Yield: 64 %).1H NMR (500 MHz, CDCl3) δ (ppm)： 8.86 (d, J = 7.0 Hz, 1H), 8.67 (d, J = 8.3 Hz, 1H), 8.73 (d, J = 8.3 Hz, 1H), 7.75 – 7.69 (m, 3H), 7.69 – 7.61 (m, 1H), 7.66 – 7.52 (m, 5H), 7.52 – 7.41 (m, 3H), 7.31 – 7.24 (m, 1H), 7.16 (dd, J = 8.3, 0.9 Hz, 1H), 1.30 (d, J = 35.5 Hz, 12H). MALDI-TOF-MS(mass m/z ): calcd for C33H29BN2O2,496.23; found, 497.49 [M+]. PPI-AnCN: PPIB (2.0 g, 4 mmol), 9,10-dibromoanthracene (2 g, 6 mmol, 1.5 3



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equiv), 15 ml aqueous of K2CO3 solution (2.0 M, 4.14 g), 15 ml methanol, and 30 ml toluene was placed in a 250 ml round-bottom flask. After the mixture was degassed under liquid nitrogen, Pd(PPh3)4(0.14 g, 0.12 mmol, 0.03 equiv) was added under nitrogen. The mixture was heated at 90 ℃for 36 hours under nitrogen. After cooling to room temperature, the mixture was washed with 40ml water for 3 times and extracted with dichloromethane. The organic solution was dried by anhydrous Mg2SO4 ,and then the solvent was evaporated. The residue was purified via column chromatography by using petroleum ether/dichloromethane (2:1,v/v) as eluent to give a lightgreen solid. (1.53g,Yield: 65 %).1H NMR (500 MHz, CDCl3) δ 8.85 (dd, J = 8.0, 1.1 Hz, 1H), 8.71 (d, J = 8.3 Hz, 1H), 8.65 (d, J = 8.4 Hz, 1H), 8.40 (d, J = 8.6 Hz, 2H), 7.80 – 7.73 (m, 2H), 7.68 (tt, J = 10.7, 5.3 Hz, 1H), 7.64 – 7.54 (m, 10H), 7.49 – 7.42 (m, 1H), 7.40 – 7.33 (m, 2H), 7.29 – 7.19 (m, 3H), 7.15 (dd, J = 8.3, 1.0 Hz, 1H). 13C NMR (125 MHz, CDCl3) δ 149.15, 141.89, 137.74, 136.77, 136.54, 132.00, 129.67, 129.65, 129.29, 129.06, 128.41, 128.39, 128.34, 128.15, 127.64, 127.40, 127.34, 126.61, 126.35, 126.14, 125.35, 125.31, 124.76, 124.48, 124.07, 123.15, 122.16, 121.98, 121.72, 119.88,

116.41,

104.80.

MALDI-TOF-MS(mass

m/z

):

calcd

for

C42H25N3,571.20;found, 572.191 [M+]. Elemental analysis calculated [%] for C42H25N3, C, 88.24, H, 4.41, N, 7.35, found: C, 89.98, H, 4.224, N, 7.45. 2.2 Characterization and measurements General information: The 1H NMR spectra and 13C NMR spectra were recorded on a Bruker AVANCE 500 spectrometer at 500 MHz and 125 MHz, respectively, using tetramethylsilane (TMS) as the internal standard and CDCl3 as solvent. The MALDITOF-MS mass spectra were measured using an AXIMA-CFRTM plus instrument. The Elemental analysis were measured with Vario EL cube (Elementar company) elemental analysis instrument. Photophysical measurements: UV-vis absorption spectra were recorded on a Shimadzu UV-3100 spectrophotometer. Fluorescence measurements were recorded on Shimadzu RF-5301PC. PL efficiencies in solvents and films were measured with Hamamatsu Quantaurus-QY C11347-11. Electrochemical characterization: Cyclic voltammetry (CV) was performed 4


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with a BAS 100W Bioanalytical Systems, using a glass carbon disk (Φ = 3 mm) as the working electrode, a platinum wire as the auxiliary electrode with a porous ceramic wick, Ag/Ag+ as the reference electrode, standardized for the redox couple ferricinium/ferrocene. All solutions were purged with a nitrogen stream for 10 minutes before measurement. The procedure was performed at room temperature and a nitrogen atmosphere was maintained over the solution during measurements. Thermal stability measurements: Thermal gravimetric analysis (TGA) was undertaken on a Perkin-Elmer thermal analysis system from 30 ℃ to 850 ℃ at a heating rate of 10 K/min and a nitrogen flow rate of 80 ml/min. Differential scanning calorimetry (DSC) analysis was carried out using a NETZSCH (DSC-204) instrument from 30 ℃ to 380 ℃ at a heating rate of 10 K/min while flushing with nitrogen. 2. RESULTS AND DISCUSSION

Br

i

N Br N

PPIBr

N

O

BrAnCN

N

O

ii

N

B N

CN

PPIB

CN PPI-AnCN

(i) bis(pinacolato)diborn, Pd(dppf)Cl2 , KOAc, dioxane, 85℃, 48h (ii) Pd(PPh3)4, toluene, methanol, 2M K2CO3 (2:1:1), 90℃, 36 h

Scheme 1. The synthetic route of PPI-AnCN. The synthetic route of the molecule PPI-AnCN is shown in Scheme 1. Firstly, PPIBr was synthesized by a one pot reaction between 9,10-Phenanthraquinone, phenylamine, ammonium acetate, and 4-Bromobenzaldehyde. Then, the corresponding boron ester (PPIB) was synthesized. Finally, the precursor PPIB was combined (Suzuki coupling) with the BrAnCN to get the target product PPI-AnCN. The final product was purified using silica gel column chromatography, preparative HPLC, and vacuum sublimation, in sequence. The chemical structure was fully confirmed by the 1H NMR, 13C

NMR, mass spectrometry, elemental analysis, and single crystal analysis.

5



Figure 1. The frontier molecular orbitals (HOMO and LUMO) of PPI-AnCN. The frontier molecular orbital distributions of the molecule calculated through density functional theory (DFT) using the B3LYP method with a basis of 6-31g(d) is shown in Figure 1. The highest occupied molecular orbital (HOMO) is delocalized on the PPI unit, while the lowest unoccupied molecular orbital (LUMO) is delocalized on the AnCN moiety. The HOMO and LUMO have no overlap and are almost totally separated. The molecular orbital distribution reveals that the electronic transition is more like a charge-transfer (CT) transition from HOMO to LUMO. (b)

(a) 1.0

0.8

0.8

0.6

0.6

0.4

0.4

0.2 0.0 300

360 380 400 420

400

500

600

0.2

Hexane Triethylamine Butylether

PL Intensity (a.u.)

In Toluene

PL Intensity (a.u.)

1.0

Absorbance (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.0 700

Wavelength (nm)

Ethylether Ethylacetate Dichloromethane Acetone ACN

400

500

600

700

800

900

Wavelength (nm)

Figure 2. (a) UV spectra and PL spectra of PPI-AnCN in toluene solution (10-5 M). Insert: The amplified PL spectra from 350 nm to 420nm. (b) Solvatochromic PL spectra. The ultraviolet-visible (UV-vis) and photoluminescence (PL) spectra of PPIAnCN in toluene (10-5 M) were recorded to evaluate photo-physical properties (Figure 2a). The absorption bands appear as a combination of independent absorption bands of PPI and AnCN units. The well-resolved vibronic fine structure absorption bands from 6


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350 to 425 nm are attributed to the AnCN, while bands from 300 to 350 nm belong to the PPI unit. The PL spectrum of PPI-AnCN in toluene is structureless (λmax = 463 nm) and exhibits significant solvatochromic effects (Figure 2b), indicating that PPI-AnCN has more CT character than local excited (LE) character, which is consistent with the theoretically calculated prediction. Table 1 shows detailed photophysical properties of the molecule. Table 1. Photophysical parameters and energy levels of the PPI-AnCN λabs for sola) [nm]

λabs for filmb) [nm]

λPL for sol a)/filmb) [nm]

ΦPL c) for sol a)/filmb)

HOMO/LOMOf) [eV]

326,368,392,413

322,367,400,419

463/515

31.4/42.6

-5.53/-2.18

a) Measured in toluene solution (1×10-5 M) at room temperature; b) Measured in neat film; c) PL quantum yield evaluated using an integrating sphere; d) Measured by cyclic voltammetry. Interestingly, we observed the high-excited state emission at wavelengths from 360 to 400 nm in solution. Although this emission is relatively weak, it is still more obvious in some solvents and shows solvent-dependent characteristics (Figure S2a). In particular, this high-energy emission becomes absolutely dominant in the triethylamine solution. The normalized spectra in different solvents show that the high-energy emission has the same shape (including peak wavelength and fine structure), indicating the emission is contributed by the same excited state. It should be noted that this anomalous emission cannot be observed in vacuum-evaporated films (Figure S2b), likely due to the accelerated radiationless deactivation of the higher excited states in the condensed phase. To understand the origin of this anomalous emission, the PL spectra of the PPI and AnCN were measured in different solvents. As shown in Figure S3a and Figure S3b, both PPI and AnCN exhibit LE state emissions with vibronic fine structure. However, the emission of PPI-AnCN shows an obvious solvation effect as the emission peak shifts from 447 nm in hexane to 599 nm in acetonitrile. It is notable that the emission 7



of AnCN was completely quenched in triethylamine solution. The reason of this quenching may be attributed to the formed exciplex between AnCN and triethylamine in excited state as some published papers reported.31, 32 When the AnCN is excited in triethylamine, a non-radiative exciplex is formed between the electron-deficient AnCN and the electron-donating triethylamine, quenching the fluorescence of AnCN. Figure 3 shows the normalized PL spectra of PPI-AnCN and PPI. We found that the emission of PPI-AnCN was identical to the emission of PPI in triethylamine solution. Therefore, the PPI unit of PPI-AnCN may be responsible for the anomalous emission. This assignment can be further confirmed by TD-DFT results (Figure 6), the S1 state of PPIAnCN is mainly distributed on the AnCN unit. In excited state, there may also form an exciplex between the S1 state of PPI-AnCN and triethylamine, quenching the S1 emission of PPI-AnCN.

PPI-AnCN PPI

1.0

PL Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.8

In Triethylamine

0.6 0.4 0.2 0.0

350

400

450

500

550

Wavelength (nm)

Figure 3. Normalized PL spectra of PPI-AnCN and PPI in triethylamine solution. In order to further understand the photophysical properties of the molecule, the excitation spectra of PPI-AnCN were measured in triethylamine and toluene solutions. We detected the first three λmax values (370 nm, 390 nm, and 410 nm), corresponding to the emission peaks of the higher excited state, and the last emission peak (λmax = 463 nm), originating from the S1 state. As shown in Figure 4a, the excitation spectra of PPI-AnCN measured (at 370 nm, 390 nm, and 410 nm) in triethylamine are the same 8


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and are similar to the absorption spectra of PPI unit in triethylamine (Figure 4b). These observations suggest that the anomalous emission originates from the PPI unit. In addition, we found that the excitation spectrum measured at 463 nm in toluene is similar to the absorption spectrum of PPI-AnCN, indicating that the emission of the S1 state originates not only from the AnCN but also from the PPI unit. This is consistent with the experimental result determined from the solvation effect spectra.

(b)

(a) in Triethylamine 370 nm 390 nm 410 nm

400

0 800

in Toulene 463 nm

400

0

300

350

400

450

In Triethylamine PPI-AnCN PPI AnCN

1.0

Absorbance (a.u.)

800

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


500

0.8 0.6 0.4 0.2 0.0 300

350

400

450

Wavelength (nm)

Wavelength (nm)

Figure 4. (a) The excitation spectra of PPI-AnCN in triethylamine and toluene solution. (b) The absorption spectra of PPI-AnCN, PPI, and AnCN in triethylamine solution. Transient decay curves were measured to study the dynamics of the excited states. As shown in Figure 5a, the PL decay at 370 nm, 390 nm, and 410 nm in triethylamine exhibit identical fluorescence lifetimes of ~ 2.18 ns, indicating that these emission peaks originate from the same excited state. In toluene, the fluorescence lifetime of PPIAnCN, measured at 463 nm, is 1.29 ns. The relatively short fluorescent lifetime suggests that the radiative transition rate of the S1 state is larger than that of the highexcited state. The PL quantum efficiencies of PPI-AnCN measured in triethylamine and toluene are 0.046 and 0.314, respectively. Therefore, the radiative rate ( k r ) and nonradiative rate ( k nr ) can be calculated from the fluorescence quantum efficiency ( f ) and fluorescence life (  ) based on the following equations.33 k r (S1 )  f ( S1 ) /  (S1 )

(1) 9



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k nr (S1 )  (1  f (S1 ) ) /  S1 

(2)

k r (S2 )  f ( S2 ) /  (S2 )

(3)

k nr (S2 )  (1  f (S2 ) ) /  S2 

(4)

The photophysical parameters of PPI-AnCN including τ, φ, k r , and k nr are summarized in Table 2. In triethylamine solution, k r (Sn ) of PPI-AnCN can compete with k nr (Sn ) , and the emission from the Sn state can be observed. The emission difference of Sn in triethylamine and toluene shows that the emission quenching of the S1 state by the triethylamine may enlarge the intensity of the Sn emission. Triethylamine is assumed to slow down the internal conversion process from the Sn state to the S1 state, facilitating the fluorescence emission of the Sn state. To verify this hypothesis, we measured the emission spectra of PPI-AnCN in toluene-triethylamine solutions with varying triethylamine fraction. As shown in Figure 5b, with increasing triethylamine concentration, the emission of the S1 state gradually reduces and disappears in the pure triethylamine solution. Transient decay spectra in the mixed solution are monitored at 370 nm and 463 nm, respectively. The transient decay spectrum at 370 nm exhibit single-exponential decay expect in the pure toluene solution (Figure 5c). However, the decay spectrum at 463 nm exhibits bi-exponential decay (except in the pure toluene solution) and the proportion of the slow part (corresponding to the emission of the Sn state) increases with triethylamine fraction (Figure 5d). The experimental results confirmed our hypothesis that triethylamine slows down the internal conversion from Sn to S1, resulting in increased Sn emission. The detailed photophysical parameters are shown in Table S1. Table 2. Photophysical properties of PPI-AnCN in triethylamine and toluene solution

solvents

triethylamine

PLQY

0.046

 S  1

 S

(ns)

(ns)

--

n



2.18

k r (S1 )

k nr (S1 ) k r (Sn )

k nr (Sn )

(108 S-1)

(108 S-1)

(108 S-1)

(108 S-1)

--

--

0.221

4.38

10


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toluene

0.314

1.29

--

2.43

5.32

--

--

Normally, the higher excited states would not exhibit a long lifetime and would directly relax to the lowest excited state due to the ultrafast internal conversion process. Theoretically, the internal conversion rate from the zero-vibrational level of the higher excited state (S2) to the lower excited state (S1) with m isoenergetic vibrational levels is given by the following expression:34 k ic   2 J 1

2

 m  20 1m

2

 C12F

(5)

In this equation, Ψ1 and Ψ2 are the electronic wave functions of the two states, respectively; χ20 and χ1m are the nuclear wave functions for the appropriate vibrational levels of the two states; J is the nuclear kinetic energy operator; C12 is the electronic coupling factor, and F is the Franck-Condon factor. Therefore, the internal conversion rate kic is mainly decided by electronic coupling and the Franck-Condon factor. As for most organic molecules, the origin of the long-lifetime higher excited states is the large energy gap between the excited states, resulting in a small Franck-Condon factor and reducing the internal conversion rate from the higher excited state to the lowest excited state.13 This allows the radiative rate to efficiently compete with the internal conversion rate. Previously, G. Eber et al. found that the energy gap of azulene between S2 and S1 state can be adjusted by proper substitution, and the emission behavior could be changed from the dominant S1→S0 fluorescence to dual fluorescence, even to dominant S2→S0 fluorescence.35 Indeed, according to Eq.5, the small electronic coupling factor between the excited states can also result in small kic and emission from high-excited state. In other words, both the energy gap of the excited states and the electronic coupling factor between these excited states determine whether the higher excited state can stably survive for a long time.

11



(b)

(a) 4

103

10

2

101

0

10

20

30

40

50

60

1.0

PL Intensity (a.u.)

370nm(In Triethylamine) 390nm(In Triethylamine) 410nm(In Triethylamine) 463nm(In Toluene) IRF

Time (ns)

(c) 104

102

100 80 60 40 20 0

0.8 0.6 0.4 0.2

400

(d)

in Tol: x%Triethylamine 100 80 @ 370nm 60 40 20 0

103

in Tol: x%Triethylamine

0.0

70

Counts (cps)

Counts (cps)

10

Counts (cps)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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500

600

700

Wavelength (nm)

104

in Tol: x%Triethylamine

103

@ 463nm

100 80 60 40 20 0

102

101

101

0

10

20

30

40

50

60

0

Time (ns)

10

20

30

40

Time (ns)

Figure 5. (a) Transient PL decay spectra of PPI-AnCN in triethylamine and toluene solutions. (b) The normalized PL spectra of PPI-AnCN in toluene and triethylamine mixed solution with different triethylamine fraction. Transient decay spectra monitored at 370 nm (c) and 463 nm (d) in the mixed solution. To further understand the origin of long-lifetime higher excited states of PPIAnCN, we analyzed the natural transition orbitals (NTOs) of the singlet states and the energy landscape on the basis of TD-DFT results with M06-2X/6-31G (d,p). The NTOs of PPI-AnCN are shown in Figure 6a. The S1 states are localized on the AnCN unit and exhibit LE properties. As for the S2 state, the holes are localized on the PPI unit while the particles are localized on the AnCN unit, exhibiting a CT transition from PPI to AnCN. The S3 states also show LE state properties, localized on the AnCN units. The much higher excited singlet states, S4 and S5, are both localized on PPI units and exhibit LE state character. The oscillator strength of the S1 state is 0.3212 and the oscillator strengths of the S2 and S3 states are very small, indicating that the S2 and S3 12


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states can hardly radiate. However, the oscillator strengths of the S4 and S5 states are quite large, especially that of the S4 state, relative to the S1 state. Although the S3 and S4 state are both LE state with larger orbital overlap, their transition-orbital characteristics are different. For S4 state, the orbital symmetry of the hole and particle is different. While the orbital symmetry of the S3 is highly similar (both and particle are symmetric about y axis), so the transition dipole moment is small and the vibrator strength is relatively small, according to the transition selection rules.36 In addition, there is poor electronic coupling between S1 and S4 because the S1 state mainly distributes on the AnCN unit while the S4 state mainly distributes on the PPI unit. Figure 6b shows the energy landscape for the singlet states of PPI-AnCN. The energy gap between the S1 and S4 for PPI-AnCN is very large (0.75 eV), predicting slower internal conversion process.37 Therefore, the S4 states with large oscillator strengths could directly radiate to the ground state. Considering the orbital electron distribution and oscillator strengths of the excited states, it is more reasonable to assume that the high-excited emission observed in solution originated from the S4 state, which has the same characteristic of the LE state from the PPI unit. These theoretical results are consistent with the experimental observation that the emission of higher excited states originates from the PPI unit. In triethylamine solution, the formed exciplex between the S1 state of PPI-AnCN and triethylamine may further lower the energy level of S1 state, increasing energy gap between the S1 state and S4 state. This increased energy gap will further decrease the IC rate of S4→S1, leading to an increased emission of S4 state.

13



(a)

(b) 4.5

Excitation energy (eV)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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4.2 3.9

S5 S4 S3 S2

S4: 4.12 eV S4-S1=0.75 eV

3.6 3.3

S1: 3.37 eV

S1

3.0

Figure 6. (a) Natural transition orbitals of PPI-AnCN. (b) The energy landscape for singlet states of PPI-AnCN. 3. CONCLUSIONS In summary, we have designed and synthesized the molecule PPI-AnCN capable of photo-emission from both the S4 and S1 excited states. This is the first report that the PPI unit, which is an excellent blue fluorescent building block, can also radiate from higher singlet excited states. The experimental results and theoretical calculations indicate that the larger ES4 S1 and molecular orbital separation decrease the internal conversion rate, allowing stabilization of the higher excited states with long-lifetime. The long lifetime combined with the large oscillator strength of higher excited states makes that fluorescence from these states becomes possible. Particularly, we observed that the enhanced emission from the higher excited state was only observed in triethylamine solution, indicating that the quenching of emissions from the S1 state by triethylamine reduces Kic between the higher excited state and S1 states and improves the emission from the higher excited state. This can be explained by the formed exciplex between the S1 state of PPI-AnCN and triethylamine in excited state may lower the energy level of S1 state, which will increase energy gap between the S1 state and S4 state and consequently decrease the Kic of S4→S1. AUTHOR INFORMATION 14


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Corresponding Author *E-mail: [email protected]; [email protected]. Phone: +86-20-22236311. Fax: +86-20-87110606. Qinglin Jiang, Yuwei Xu and Xiaoming Liang contributed equally. Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work was supported by the Natural Science Foundation of China (21334002, 51521002, 51403063), the Major Science and Technology Project of Guangdong Province (2015B090913002), and the Foundation of Guangzhou Science and Technology Project (201504010012). Supporting Information The Supporting Information is available free of charge and includes 1H NMR spectrua of PPI-AnCN in CDCl3, supplementary photophysical properties, electrochemical properties, thermal properties, and crystal data. REFERENCES (1) Beer, M.; Longuet ‐ Higgins, H. C., Anomalous Light Emission of Azulene. J. Chem. Phy. 1955, 23, 1390-1391. (2) Viswanath, G.; Kasha, M., Confirmation of the Anomalous Fluorescence of Azulene. J. Chem. Phy. 1956, 24, 574-577. (3) Daub, J.; Engl, R.; Kurzawa, J.; Miller, S. E.; Schneider, S.; Stockmann, A.; Wasielewski, M. R., Competition between Conformational Relaxation and Intramolecular Electron Transfer within Phenothiazine−Pyrene Dyads. J. Phys. Chem. A 2001, 105, 5655-5665. (4) Zhang, W.; Xu, Y.; Hanif, M.; Zhang, S.; Zhou, J.; Hu, D.; Xie, Z.; Ma, Y., Enhancing Fluorescence of Naphthalimide Derivatives by Suppressing the Intersystem Crossing. J. Phys. Chem. C 2017, 121, 23218-23223. (5) Hochstrasser, R. M., The Luminescence of Complex Molecules In Relation To The Internal Conversion Of Excitation Energy. Part I. Can. J. Chem. 1959, 37, 1367-1372. 15



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TOC Graphic

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Stable High-Energy Excited States Observed in a Conjugated

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