Pyrene-Cored Starburst Oligofluorenes with Diphenylamine End

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Pyrene-Cored Starburst Oligofluorenes with Diphenylamine EndCappers: Design, Synthesis, Stabilized Optical Gain and Lasing Properties Hao Zhang, Ting-Ting Lu, Wen-Yong Lai, Xin-Wen Zhang, Meng-Ke Zhang, Peng Lv, Cheng-Fang Liu, and Wei Huang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b08721 • Publication Date (Web): 09 Nov 2017 Downloaded from http://pubs.acs.org on November 12, 2017

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The Journal of Physical Chemistry

Pyrene-Cored Starburst Oligofluorenes with Diphenylamine End-Cappers: Design, Synthesis, Stabilized Optical Gain and Lasing Properties Hao Zhang,†,§ Ting-Ting Lu,†,§ Wen-Yong Lai,*,† Xin-Wen Zhang,† Meng-Ke Zhang,† Peng Lv,† Cheng-Fang Liu,† Wei Huang†,‡ †

Key Laboratory for Organic Electronics and Information Displays (KLOEID) & Institute of Advanced Materials (IAM), Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM), Nanjing University of Posts & Telecommunications (NUPT), 9 Wenyuan Road, Nanjing 210023, China

‡

Shaanxi Institute of Flexible Electronics (SIFE), Northwestern Polytechnical University (NPU), 127 West Youyi Road, Xi'an 710072, Shaanxi, China

* E-mail: [email protected] §

H. Zhang and T.-T. Lu contributed equally to this work.

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Abstract: A family of starburst conjugated molecules composed of a pyrene core and diphenylamine end-cappers with various oligofluorene bridge lengths, named as P1F, P2F, and P3F, were designed, synthesized and characterized. The resulting materials exhibited good photoluminescence (PL) properties and excellent thermal stability with high degradation temperatures (Td, the temperature with 5% weight loss) over 400 oC. A combination of the thermal, photophysical, electrochemical, fluorescence transients, electroluminescence (EL), amplified spontaneous emission (ASE) and lasing measurements were carried out to reveal further the influence of the diphenylamine moieties as electron-donating end-cappers on their optoelectronic properties. It is interesting to find that the PL spectra are significantly blueshifted for P2F (~ 11-15 nm) and P3F (~ 14-22 nm) relative to those of P1F in both dilute solutions and films with an extension in the conjugation length of the oligofluorenes, which is quite different from the common phenomenon of red-shifted PL spectra for the general starburst molecules with increasing the conjugation length. It is supposed that the introduction of strong electron-donating diphenylamine units helps to construct donor-π-acceptor structures that enable intramolecular excitation energy transfer transition from the diphenylamine donor to the pyrene acceptor. The diphenylamine units also play a key role in raising the highest occupied molecular orbital (HOMO) energy levels of the molecules, which is beneficial for improving the charge injection and transport properties. Consequently, efficient EL properties with rather low turn-on voltages of 2.7-3.0 V among non-doped blue OLEDs have been achieved. Moreover, stabilized EL and ASE with high net gain coefficients (around 73.6-81.9 cm-1) and low waveguide loss (about 2.10-4.90 cm-1) have been demonstrated. One dimensional distributed feedback (1D DFB) lasers demonstrated low lasing threshold of 2.1 kW cm-2 (25 nJ pulse-1) at 475 nm for P2F. All the results confirm that the novel molecular design strategy on constructing diphenylamine-capped pyrene-centered starburst molecules can obviously improve the EL performance by fine modulating the HOMO energy levels, but did not largely sacrifice their optical gain properties. The high gain 2 - Environment ACS Paragon -Plus

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and low loss with excellent thermal and optical stability have rendered these donor-π-acceptor starburst conjugated molecules rather attractive as robust gain media for organic lasers.

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1. Introduction Organic semiconductors have gained intense interest because of their attractive optoelectronic properties and facile fabrication via low cost and large area manufacturing, which have been widely investigated in optoelectronic fields such as organic thin-film transistors (OTFTs),1,2 organic light-emitting diodes (OLEDs),3-6 organic photovoltaics (OPVs),7-9 and organic lasers.10-12 One of most challenging goals in this area is to develop robust organic gain media to realize electrically-pumped organic lasers. Although numerous organic emitters have been demonstrated in OLEDs with great promise, it should be noted that only a small fraction of them can achieve high optical gain and low thresholds that are necessary for organic lasing. Particularly, it still remains a worldwide open issue for organic gain media to simultaneously achieve high luminescence efficiency, strong optical gain, excellent thermal stability, and favorable charge mobility for attempting electrically-pumped organic lasing.13-15 So far, the most widely pursued materials for organic lasers are π-conjugated molecules and polymers. Among them, fluorene-based conjugated oligomers and polymers (PFs) are considered to be promising candidates as gain media due to their high photoluminescence quantum efficiencies (PLQYs), good thermal stability, and facile tunability of optoelectronic properties by chemical modification.16-19 However, a critical issue of this material system is their strong tendency to form aggregates/excimers, which results in reduced efficiency, poor spectral stability and inferior electrical properties. Efforts on incorporating sterically hindered groups onto PFs or constructing hyperbranched/cross-linked PFs have been explored to address the above issue.20-23 Particularly, arylamine units (such as carbazole, diphenylamine, triphenylamine, or triazole) have been incorporated onto PFs, which has been proven to be effective to modulate the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels to improve the charge injection and transport properties.24-28 The resulting materials have demonstrated promising optoelectronic properties with improved spectral stability and enhanced electrical properties as well as 4 - Environment ACS Paragon -Plus

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superior device efficiency. Despite significant advances, it is still far from ideal for attempting electrically-pumped organic lasers. The development of novel robust organic gain media with excellent optical, thermal, and electrical properties is highly desirable. Unravelling the structure-function relationships is a vital research task to shed light on rational design and development of high-performance optical gain materials. In our previous contributions, a novel series of monodisperse starburst π-conjugated macromolecules based on various core structures including triazatruxene,29-35 truxene,36-41 and pyrene,42-46 etc, have been developed. Such starbursts combine both the merits of polymers and small molecules with excellent chemical purity, superior reproducibility, good thermal and environmental stability, facile solution processibility, prominent optical gain and low lasing thresholds, which have rendered them rather attractive for optoelectronic applications including OLEDs and organic lasers. Among these, pyrene-centered starburst oligofluorenes have stood out as promising organic emitters, demonstrating not only outstanding optoelectronic properties for OLEDs, but also interesting low lasing thresholds and relatively high environmental and thermal stability.42-46 However, the electrical properties of the materials have not been much improved. Especially, the low HOMO energy levels of the materials have resulted in high turn-on voltages, and low luminance efficiency, which would also limit their applications for attempting electrically-pumped organic lasers. In this contribution, we present our further effort to explore robust gain media with enhanced electrical properties to address the challenges for organic lasers. A novel set of starburst emitters with 1,3,6,8-tetraphenylpyrene core, oligofluorene arms and diphenylamine end-cappers (P1F, P2F, and P3F) have been designed, synthesized, and characterized. Pyrene was selected as the core structure because of its high PL efficiency, long excited-state lifetimes, enhanced charge carrier mobility.47-53 Oligofluorenes with varying conjugation length were chosen as the arms owing to their high PLQYs, excellent thermal and optical stability.29-41,54-61 Strong electron-donating diphenylamine units were incorporated onto the 5 - Environment ACS Paragon -Plus


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starburst molecules as the end-cappers to raise the HOMO energy levels and to fine modulate the electrical properties of the materials.62-64 The influence of the molecular structures and the introduction of diphenylamine units on their thermal, photophysical, electrochemical, electroluminescence (EL) characteristics, amplified spontaneous emission (ASE), and lasing properties have been systematically investigated. Efficient EL properties with rather low turnon voltages of 2.7-3.0 V among non-doped blue OLEDs have been achieved. Moreover, stabilized EL and ASE with high net gain coefficients (around 73.6-81.9 cm-1) and low waveguide loss (about 2.10-4.90 cm-1) have been demonstrated. All the results confirmed that the novel molecular design strategy on constructing diphenylamine-capped pyrene-centered starburst molecules significantly improved the EL performance by fine modulating the HOMO energy levels, but did not largely sacrifice their optical gain properties. The high gain and low loss with excellent thermal and optical stability have rendered these donor-π-acceptor starburst conjugated molecules rather attractive as robust gain media for organic lasers. C6H13C6H13 Br

Br

+

NH

O O B O

C6H13C6H13

trans-1,2-diaminocyclohexane 1,4-dioxane t-BuOK CuI 12 h

N

Br

1

C6H13C6H13 N

n-BuLi THF

2a

C6H13C6H13 Br

Br

+

n = 1, 3a n = 2, 3b

Pd(PPh3)4 Toluene K2CO3 12 h

1

n = 1, 3a O

C6H13C6H13 N

O

Br

n

O B B O

C6H13C6H13

1,4-dioxane N

Pd(dppf)Cl2 KOAc 12 h

n = 2, 2b n = 3, 2c

n

N n

R C6H13 C6H13

Br

+ Br

O B n O

N

Br

4

n = 1, 3a n = 2, 3b n = 3, 3c

Pd(PPh3)4 MW 150 oC

O B

nO n = 2, 3b n = 3, 3c

N

Br

O B

nO

R

R

R

R

R

15-30 min R

R R = n-C6H13

N n

P1F : n=1 P2F : n=2 P3F : n=3

nN

Scheme 1. Synthetic routes toward P1F, P2F, and P3F.

2. Experimental Section The synthetic routes toward the target molecules P1F, P2F, and P3F are depicted in Scheme 1. A convergent core-creating approach was adopted. The stepwise preparation of the 6 - Environment ACS Paragon -Plus

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key intermediates of diphenylamine end-capped oligofluorene boronic pinacol esters 3a-c with varying chain lengths (n = 1, 2, and 3) commenced with the synthesis of 2a, which was obtained by the selective amination of compound 1 with diphenylamine by a mild Buchwald cross-coupling reaction. Subsequent borylation of 2a afforded 3a. Suzuki cross-coupling reaction of 1 with 3a gave 2b, and subsequent borylation using bis(pinacolato)diboron afforded 3b. Similar stepwise procedures were used to obtain the key intermediates 3c. Finally, the target compounds P1F-P3F were conveniently accomplished by a microwave-assisted multiple Suzuki cross-coupling methodology by reactions of 1,3,6,8-tetrabromopyrene with the corresponding oligofluorene boronic pinacol esters 3a-c using Pd(PPh3)4 as the catalyst.29,37 The synthetic procedures for all the intermediates and the final starburst compounds P1F-P3F are detailed in Supporting Information. The chemical purity and structures of all new key intermediates and targets were adequately verified by 1H NMR and 13

C NMR spectra, MALDI-TOF mass spectrometry (see Figures S1−S9 in Supporting

Information). The resulting starburst molecules P1F, P2F, and P3F show good solubility in most common solvents, such as dichloromethane, chloroform, tetrahydrofuran (THF), and toluene.

3. Results and Discussion

P1F TGA P2F TGA P3F TGA

100 80

Weight ( %)

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


60 40 20 0 0

100 200 300 400 500 600 700 Temperature (oC)

Figure 1. TGA thermograms of P1F, P2F, and P3F recorded at a heating rate of 10 °C min−1. 3.1 Thermal properties 7 - Environment ACS Paragon -Plus


Thermal properties of P1F, P2F, and P3F were determined by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) under a nitrogen stream. As shown in Figure 1, the resulting starbursts exhibited good thermal stability with 5% weight loss temperatures (Td) of 417, 410, and 401 oC for P1F, P2F, and P3F, respectively, which are about 10-40 oC higher than their corresponding counterparts without diphenylamine endcappers (P1-P3).44,45 As revealed by DSC curves, P1F, P2F and P3F are amorphous with high glass-transition temperature (Tg) of 130, 132, and 133 °C, respectively (Figure S10a), which are apparently much higher than those of their corresponding counterparts P1-P3 (Tg = 62-92°C).44,45 The results suggest that the end-capping strategy with introduction of diphenylamine units can significantly increase the rigidity of the starburst molecular structures, endowing the materials with much enhanced thermal and morphological stabilities, which are highly desirable and beneficial for practical applications in OLEDs and organic lasers.

0.6

0.8 0.6

0.4

0.4

0.2

0.2

0.0

0.0 300

400 500 Wavelength (nm)

600

P1F-UV P1F-PL P2F-UV P2F-PL P3F-UV P3F-PL

1.0 0.8 0.6

1.0 0.8 0.6

0.4

0.4

0.2

0.2

0.0

0.0 300


PL Intensity (norm.)

0.8

(b)

1.0

Absorbance (norm.)

P1F-UV P1F-PL P2F-UV P2F-PL P3F-UV P3F-PL

PL Intensity (norm.)

(a) 1.0 Absorbance (norm.)

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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600

Figure 2. Normalized UV-vis absorption and PL spectra of P1F, P2F and P3F in (a) THF solution (10-6 M) and (b) thin films. 3.2 Photophysical properties Figure 2 shows the normalized absorption and photoluminescence (PL) spectra of P1F, P2F and P3F in dilute THF solutions and thin-solid films. The corresponding photophysical data are summarized in Table 1. The absorption spectra of P1F-P3F in dilute solutions and in films exhibit a broad absorption band with vibrational features, resulting from a strong π-π* 8 - Environment ACS Paragon -Plus

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electron absorption band in the UV-visible region.44,45 In light of the absorption spectra in solutions, three distinctive peaks are observed for P1F at 308, 369, and 407 nm. P2F and P3F show two main absorption peaks at 310 and 375 nm for P2F, 311 and 379 nm for P3F, respectively. The maxima absorption peaks at around 369-379 nm mainly originate from the π-π* transition of the oligofluorene-diphenylamine arms, which is progressively red-shifted with an extension of the arm length from P1F to P3F. The absorption peak at around 407 nm for P1F may be assigned to the π-π* absorption due to the donor-acceptor electronic coupling that causes electron transfer between the diphenylamine and the pyrene core via the oligofluorene bridge.65-68 Such electronic coupling effects strongly depend on the distance between the donor and the acceptor, and become negligible with increasing the oligofluorene arm length for P2F and P3F. Moreover, the maxima absorption peaks at around 369-379 nm become dominant in the absorption spectra of P2F and P3F, which is in accord with the fact that the content of the oligofluorene-diphenylamine arms increases significantly with an extension of the oligofluorene arm length. These assignments match well with previous studies on 1,3,6,8-tetraphenylpyrene derivatives43-45 and diphenylamine dendrimers.64-68 The absorption characteristics of P1F-P3F in films are similar to those recorded from dilute solutions except slight red shifts (7–12 nm). PL spectra for all the starbursts are quite similar in dilute solutions (Figure 2a) and in thin films (Figure 2b). One main emission peaks were recorded in solutions at λem = 484 nm for P1F, 469 nm for P2F, and 462 nm for P3F, respectively. The thin films of the starbursts show emission peaks at 485 nm for P1F, 474 nm for P2F, and 471 nm for P3F, which slightly bathochromic shift 1-9 nm relative to their solution spectra, suggesting the absence of strong intermolecular interactions in films. This may be attributed to the bulky starburst architectures that help to effectively suppress the intermolecular aggregation. It is interesting to find that the PL spectra are obviously blue-shifted with an extension in the conjugation length of the oligofluorenes in both dilute solutions and films, i.e. ~11 nm blue shift for P2F relative to 9 - Environment ACS Paragon -Plus


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P1F, and ~14 nm blue shift for P3F relative to P1F in films. This is quite different from the common phenomenon of red-shifted PL spectra for the general starburst molecules with increasing the conjugation length.29-41,54-61 It is supposed that the introduction of strong electron-donating diphenylamine units helps to construct donor-π-acceptor structures that enable intramolecular electronic transfer transition from the diphenylamine donor to the pyrene acceptor. The oligofluorene arm length also plays a role that impacts the electronic coupling between the donor and acceptor units. The donor-acceptor electron transfer is obvious for P1F with one fluorene short arm length. For P2F and P3F, the electronic coupling effect between the donor and acceptor becomes negligible with increasing the arm length, while alternatively the conjugation effect dominates the emissive behaviors instead. Such trends are also evident as confirmed in the absorption spectra. The PLQYs of the starbursts in THF solution and thin film states were measured by an integrating sphere with λabs as the excitation wavelength. P1F, P2F, and P3F achieved high PLQYs of 71%, 95%, and 90% in solution, which are slightly higher than the films of 65%, 90%, and 88%, respectively. The PLQYs in films are close to those in their dilute solutions, suggesting that the bulky star-shaped architecture can effectively reduce the intermolecular interactions in the condensed states. The PL decay transients recorded from THF solutions and films are plotted as shown in Figure S11 (Supporting Information). Excited-state relaxation of these startbursts in THF solutions were recorded as a single exponential decay profile with estimated fluorescence lifetimes (τ) of 1.91 ns, 1.40 ns and 1.27 ns for P1F, P2F, and P3F, respectively. The films of the samples showed di-exponential decays with τ1 = 0.78 ns (82%), τ2 = 2.25 ns (18%) for P1F, τ1 = 0.88 ns (76%), τ2 = 2.14 ns (24%) for P2F, and τ1 = 0.88 ns (76%), τ2 = 2.18 ns (24%) for P3F, respectively. The long-lived lifetime (τ2) may be attributed to the exciton migration from the electron-donating diphenylamine cappers to the electron-accepting pyrene core. The corresponding data are summarized in Table 1.

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Table 1. Thermal and photophysical data of P1F–P3F. Tga)/ Tdb)

λabs (nm)

τ (ns)e

λem (nm)

ФPLf

Compd

(oC)

THFc)

Filmd)

THF

Film

THF

Film

THF

Film

P1F

130/417

308,369,407

315,378,416

484

485

1.91

0.78(82%)

0.71

0.65

0.95

0.90

0.90

0.88

2.25(18%) P2F

132/410

310,375

317,387

469

474

1.40

0.88(76%) 2.14(24%)

P3F

133/401

311,379

318,388

462

471

1.27

0.88(76%) 2.18(24%)

a) 6

Obtained from DSC measurements; b) Obtained from TGA measurements; c) Measured in 10-

M THF solution;

d)

Measured in thin films; e) The excitation wavelength is the maxima UV-

visible absorption wavelength of each sample; f) Fluorescence quantum yields in solid films, measured on the quartz plate using an integrating sphere. 200

P1F-ox

-10

100

P1F-red

-20

0

-30

-100

0

200

P2F-ox

-200

100

P2F-red

0

-400 50

-100

Current (uA)

0

Current (uA)

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


80

0

P3F-ox 40

-50

P3F-red

-100

0 -3.0 -2.5 -2.0 -1.5 -1.0 -0.5

0.0

0.4

0.8

1.2

Potential V vs. Ag/Ag+

Figure 3. Cyclic voltammograms (CV) of P1F, P2F and P3F. 3.3 Electrochemical properties The electrochemical properties of P1F, P2F and P3F were investigated by cyclic voltammetry (CV), which employed a standard three-electrode electrochemical cell in nonaqueous acetonitrile solution with 0.1 M tetra-n-butylammonium hexafluorophosphate (Bu4NPF6).69,70 The results are depicted in Figure 3 and the data are summarized in Table 2. The onset oxidation potentials (Eox) for P1F, P2F, and P3F were recorded as 0.48, 0.52, and 0.55 eV, respectively. According to the formula (EHOMO = -[Eox - E(Fc/Fc+) + 4.8)] eV, 11 - Environment ACS Paragon-Plus


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Eox(Fc/Fc+) = 0.14 eV),71 the HOMO energy levels of P1F, P2F, and P3F are calculated to be of -5.14, -5.18, and -5.21 eV, respectively. The values are 0.47, 0.47 and 0.50 eV higher as compared with their corresponding P1-P3 counterparts without diphenylamine end-cappers (5.61~ -5.71 eV)44,45 respectively. Meanwhile, the onset reduction pontentials of the starbursts were measured ranging from -2.52 eV to -2.17 eV, the corresponding LUMO energy levels of P1F, P2F, and P3F are estimated be of -2.49, -2.20 and -2.14 eV by the formula (ELUMO = [Ered + 4.66] eV), respectively. Based on the equation E

g

= Eox – Ered, the band gaps (Eg) of

P1F, P2F, and P3F were calculated to be 2.65, 2.98 and 3.07 eV, respectively. The trends are similar to those of their optical band gaps estimated from the edges of the longest absorption wavelength for the thin film samples as listed in Table 2. The results manifested that the introduction of diphenylamine end-cappers effectively raised the HOMO levels of the starbursts, which are able to fine modulate the electrical properties for improved carrier injection and transport properties. Table 2. The electrochemical data of P1F–P3F. Eox a)

Ered a)

EHOMO b)

ELUMO b)

Eg c)

Egopt d)

(eV)

(eV)

(eV)

(eV)

(eV)

(eV)

P1F

0.48

-2.17

-5.14

-2.49

2.65

2.52

P2F

0.52

-2.46

-5.18

-2.20

2.98

2.57

P3F

0.55

-2.52

-5.21

-2.14

3.07

2.63

Compd

a)

Onset oxidation and reduction potentials versus Ag/Ag+;

b)

Estimated from the onset

oxidation and reduction potentials by using EHOMO = -(Eox + 4.66) eV and ELUMO = -(Ered + 4.66) eV; c) Estimated using E

g

= Eox – Ered; d) Band gap estimated from the edge of the

longest wavelength absorption (in films). To better understand the electronic properties of the starbursts at the molecular level, density functional theory (DFT) calculations were performed using a suite of Gaussian 09 programs. The Becke’s three parameterized Lee–Yang–Parr exchange functional combined with 6-31G basis set (B3LYP/6-31G) was used to fully optimize the molecular geometry. From the predicted global minimum ground-state structure, time-dependent (TD) DFT


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calculations were carried out using B3LYP hybrid functional theory to predict the electronic properties. All the calculations were performed in vacuum conditions. The optimized molecular geometries and the orbital distributions of HOMO and LUMO energy levels of the targets are shown in Figure S12. According to DFT calculations, the LUMOs of P1F, P2F and P3F were mainly located on the pyrene units, whereas the HOMOs came across the whole molecules from the pyrene core via the oligofluorene arms to the diphenylamine endcappers in P1F and mainly distributed on the core and the end-cappers in P2F and P3F. It is evident that there is strong electronic coupling existing between the pyrene core and the diphenylamine end-cappers in P1F. It is also feasible that varying the oligofluorene arm length can greatly modulate the HOMOs energy levels without largely affecting their LUMOs energy levels. The results confirmed that the incorporation of electron-donating diphenylamine end-cappers could effectively modulate the electronic distributions and raise the HOMO energy levels, compared with their P1-P3 counterparts without diphenylamine end-cappers. The HOMO/LUMO energy levels of P1F, P2F and P3F were calculated to be of -4.65/-1.69 eV, -4.75/-1.66 eV, and -4.79/-1.65 eV, respectively. 3.4 Electroluminescence properties To investigate the EL properties of the resulting starbursts, solution-processed OLEDs with the configuration of ITO/PEDOT: PSS/Emissive layer (EML)/TPBI/LiF/Al (EML: P1F, device A; P2F, device B; P3F, device C) were fabricated. PEDOT: PSS and LiF acted as hole- and electron-injecting layers, respectively. The device results are shown in Figure 4 and the key device data are summarized in Table 3. The devices based on P1F, P2F, and P3F exhibited sky-blue emission with the central peaks at 484, 480 and 474 nm, respectively (Figure 4). The EL spectra were quite similar to those of their PL spectra in solutions and films, manifesting that the formation of excimer or exciplex was effectively depressed during the EL process. Impressively, the EL spectra of all the devices remained almost the same at varying driving voltages from 6 to 11 V with negligible variation of the CIE coordinate values 13 - Environment ACS Paragon-Plus


as shown in Figure 4, implying a remarkable voltage-independent EL emission. The impressive spectral stability may be probably attributed to the bulky diphenylamine-capped starburst architectures that contribute to greatly depressing the molecular stacking in films.

1.0 device A

11V 10V 9V 8V 7V 6V

0.6

1.0 0.8

0.4 0.2 0.0

0.6 0.4 0.2 0.0

500

600

400

500

Wave length (nm)

Wave length (nm)

0.6 0.4 0.2 0.0

10k

2

A B C

600

)

0.8

(d) 800 Current Density (mA/cm )

11V 10V 9V 8V 7V

1.0 device C

Normalized

600

1k

400

100

200

10

2

400

(c)

9V 8V 7V 6V 5V

device B

Luminance (cd/m

Normalized

0.8

(b)

Normalized

(a)

1

400

500

600

2

4

Wave length (nm)

6 8 Voltage (V)

10

(f) 4

(e) 6 5

A B C

3

4

EQE (%)

Current Efficiency (cd/A)

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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3 2

A B C

1 0 0

20 40 60 80 2 Current Density (mA/cm )

100

2 1 0 0

20 40 60 80 Current Density (mA/cm2)

100

Figure 4. Performance of the solution-processed OLEDs based on P1F, device A; P2F, device B; P3F, device C: EL spectra of device A (a), device B (b), and device C (c); (d) J–V– L characteristics, (e) LE-current density and (f) EQE-current density. Figure 4 shows the current density−voltage−brightness (J−V−L) characteristics and efficiency versus current density curves of the devices. Rather low turn-on voltages (the


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voltage with a brightness of 1 cd m-2) of 2.7, 2.8 and 3.0 V were recorded for the devices based on P1F, P2F and P3F, respectively, which are among the lowest values for non-doped blue OLEDs.42-45,54-60 This is mainly attributed to the high-lying HOMO levels of the starbursts, which match well with the work function of the adjacent PEDOT:PSS/ITO anode (~-5.2 eV) and thereby facilitate the hole injection and charge balance of the devices. Furthermore, excellent brightness and efficiencies have been achieved for these devices. Device A shows a maximum luminance (Lmax) of 45l6 cd m-2 at 9.0 V, a maximum luminance efficiency (LEmax) of 3.9 cd A-1, a maximum power efficiency (PEmax) of 1.8 lm W−1, and a maximum external quantum efficiency (EQEmax) of 1.52% (6.3 V, 164 mA cm-2, 1802 cd m-2). The performance of device B is much better with an Lmax of 8923 cd m-2 at 10.3 V, an LEmax of 4.1 cd A-1, a PEmax of 2.1 lm W-1, and an EQEmax of 2.27% (5.8 V, 43 mA cm-2, 1465 cd m2

), whereas the best EL performance is observed for device C with an Lmax of 12057 cd m-2 at

8.7 V, an LEmax of 5.7 cd A-1, a PEmax of 4.8 lm W-1, and an EQEmax of 3.36% (4.7 V, 11.7 mA cm-2, 995 cd m-2). The EL performances are much better compared to those recorded from their corresponding counterparts without diphenylamine end-cappers,44,45 suggesting that the diphenylamine end-capped strategy is effective to improve the electrical properties and the EL performance. Table 3. EL characteristics of solution-processed OLEDs based on P1F-P3F. Device

a

Compd

Von

Luminance a) -2

LE a) -1

PE a) -1

EQE a)

CIE(x, y)

(V)

(cd m )

(cd A )

(lm W )

(%)

at 9 V

A

P1F

2.7

4516

3.9

1.8

1.52

(0.18, 0.39)

B

P2F

2.8

8923

4.1

2.1

2.27

(0.17, 0.35)

C

P3F

3.0

12057

5.7

4.8

3.36

(0.16, 0.26)

Maxima values recorded from the devices. The remarkable EL properties in these devices could be attributed to the following facts:

(I) the diphenylamine end-cappers raise the HOMO energy levels of the starbursts; (II) the resulting high-lying HOMO levels match well with the work function of the adjacent PEDOT:PSS/ITO anode and thereby facilitate carrier injection and transport balance of the 15 - Environment ACS Paragon-Plus


devices; (III) the bulky starburst architectures effectively suppress the intermolecular aggregation that help to enhance the luminescence properties in PL and EL conditions; (IV) Especially, device C based on P3F achieving the best EL performance is probably ascribed to its large starburst molecular structures and the proper HOMO and LUMO energy levels. 3.5 ASE and lasing properties To evaluate the potential of the resulting starbursts for optical gain applications, ASE properties of P1F, P2F, and P3F were investigated. ASE characterizations are facile and appropriate techniques for comparing the lasing performance of different materials excluding effects of resonant cavity.72,73 The ASE measurements of P1F, P2F, and P3F in thin films were conducted by an Nd:YAG laser (a repetition rate of 10 Hz, a pulse duration of 12 ns) with the excitation laser source at their absorption maxima. The surface emission was collected and analyzed by means of a fiber-coupled spectrograph and charge-coupled device (CCD) detector.

5

2

6

ASE Threshold (µ J/cm )

P1F P2F P3F

4

2

0 1

2 4 8 16 32 2 Pump Energy Density (µJ/cm )

64

120 100 80 60 40

Normalized ASE intensity

(b) 140

(a)

Output Intensity (10 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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P1F 25 C 0 P1F 240 C 0 P2F 25 C 0 P2F 260 C 0 P3F 25 C 0 P3F 280 C

0.8 0.6 0.4 0.2 0.0

P1F P2F P3F

0

1.0

400

440

480

520

Wavelength (nm)

20 0 0

50 100 150 200 250 o Annealing Temperature ( C)

300

Figure 5. (a) Evolution of the ASE thresholds at room temperatures, (b) Evolution of the ASE thresholds slab waveguides as a function of annealing temperatures; Inset: normalized ASE spectra at room termperature (solid) and high temperature (hollow) in ambient atmosphere. Figure S13 shows the ASE spectra of P1F–P3F films in comparison with those of their UV−vis absorption, and PL spectra. As the pulse energy increased, a narrow ASE peak appeared and grew to dominate the emission spectra. When the film samples were pumped at


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high excitation densities, P1F, P2F, and P3F show ASE spectra peaks at λASE≈ 498 nm, 489 and 479 nm, respectively. Figure 5a shows edge emission output intensity as a function of the pump energy density for the starbursts. A threshold pulse energy (EthASE) for ASE emission is defined at which the full width at half-maximum (FWHM) of the emission spectra drops to half of its low intensity excitation PL value. The lowest EthASE for the starbursts in films were determined as 119 nJ pulse

-1

(6.6 µJ cm-2, peaked at 498 nm) for P1F, 42 nJ pulse

-1

(2.3 µJ

cm-2, peaked at 489 nm) for P2F, and 83 nJ pulse-1 (4.6 µJ cm-2, peaked at 479 nm) for P3F, respectively.

P1F 4

10

2

3

-1

Ep= 33.2 µJ/cm

10

2

10

Ep= 66.3 µJ/cm

2

Ep= 99.5 µJ/cm

2

g= 36.9±2 cm

-1

g= 43.4±2 cm

-1

g= 54.6±2 cm 2

-1

2

-1

Ep= 165.8 µJ/cm g= 66.5±2 cm 1

10

Emission Intensity (a.u.)

(b)

(a) Emission Intensity (a.u.)

P2F

4

10

0.15

0.20 0.25 Strip Length (cm)

(c)

2

g= 64.9± 2 cm

2

g= 86.2± 2 cm

2

g= 63.9± 2 cm

Ep= 11.5 µJ/cm

3

10

Ep= 23.1 µJ/cm Ep= 34.7 µJ/cm

2

10

0.10

-1 -1

g= 74.6± 2 cm

2

g= 81.9± 2 cm

Ep= 92.4 µJ/cm

0.30

-1

2

Ep= 57.8 µJ/cm

Ep= 265.2 µJ/cm g= 73.6±2 cm

Emission 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


-1


-1

0.25

P3F 4

10

g= 37.3±2 cm

2

g= 51.7±2 cm

2

g= 68.2±2 cm

Ep= 46.2 µJ/cm Ep= 69.3 µJ/cm

2

10

-1 -1 -1

2

-1

2

-1

Ep= 115.5 µJ/cm g= 70.2± 2 cm Ep= 184.8 µJ/cm g= 79.3± 2 cm

1

10

2

Ep= 23.1 µJ/cm

3

10

0.10


0.25

Figure 6. The dependence of ASE emission intensity on the stripe length at different pump intensities of P1F–P3F. For electrically pumped organic lasers, high thermal and optical stability is one of the most important requirements. From the TGA and DSC thermal analysis as discussed above, the resulting starburst compounds exhibited excellent thermal stability, enabling them reliable in operational stability at high temperatures. To further evaluate the photostability, evolution 17 - Environment ACS Paragon-Plus


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

of the ASE thresholds slab waveguides as a function of annealing temperatures were investigated. The film samples were annealed for 10 min at each annealing temperature in open ambient condition during the experiment. The excitation stripe was moved to a new location on the films for every characterization to exclude the influence of photodegradation. The results are depicted in Figure 6b. Remarkably, EthASE of all the starbursts kept nearly the same when annealing from room temperature to 180 °C (1.2-fold to pristine films). With increasing the annealing temperatures for P1F at 200 °C and for P2F and P3F at 225 °C, the EthASE of the films increased by 2-fold in comparison to those of their pristine films. When the temperature reached 240 °C, the EthASE increased by 10.3-fold for P1F and 5.1-fold for P3F, while it increased only 2.5-fold for P2F. Even with annealing temperature up to 280 °C, ASE could still be detectable for P2F. The ASE spectra remained almost the same with very small shifts within 4 nm even annealing up to 280 °C for P2F and 260 °C for P3F, while the similar phenomenon was obversed for P1F under 240 °C, as shown in Figure 6b (inset). The results suggested outstanding ASE stability under thermal conditions. It is believed that excellent ASE thermal stability will help to improve and enhance the optical gain stability for attempting electrically pumped organic lasers. The variable stripe length (VSL) technique was used to determine the optical gain coefficients.73 Figure 6 shows the dependence of ASE emission intensity on the stripe length at different pump intensities. The output intensity increased exponentially with the pump energy increased at stripe lengths of less than 2 mm, but then at longer stripe lengths gain saturation can be observed. To verify if the saturation was a result of the sample degradation, we repeated the measurements at the same sample spot, obtaining the same results. The net gain coefficient (g) was determined by fitting the stripe length dependence of the ASE intensity according to: I(λ, l) ~ 1/g(λ)(exp(g(λ)×l)-1).10 As shown in Figure 6, the solid lines represent fits to the experimental data prior to the onset of gain saturation. The net gain gradually increased with the increase of pump energy and the light traveling in the waveguide 18 - Environment ACS Paragon-Plus

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was intensely amplified. When gain saturation occurs, the gain coefficient decreased and light is not amplified further as the length of the stripe increased. On the basis of the results, the maxima net gain coefficients (g) are determined as g = 73.6±2 cm-1 (pump energy E = 265.2 µJ cm-2) for P1F, g = 81.9±2 cm-1 (pump energy E = 92.4 µJ cm-2) for P2F, and g = 79.3±2 cm-1 (pump energy E = 184.8 µJ cm-2) for P3F, respectively. These net gain coefficients are at the same levels in comparison with those of their corresponding counterparts without diphenylamine end-cappers (75-78 cm-1).45 4

10

Emission Intentisy (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


P1F

3

10

-1

α=4.90 cm

P2F

4

10

3

10

-1

α=2.10 cm

P3F 4

10

-1

α=2.19 cm 3

10

0.00

0.05

0.10

0.15

0.20

0.25

Position (cm) Figure 7. The intensity of light emission from the edge of the waveguides for P1F, P2F, and P3F vs. the distance between the pump stripe and the edge of the sample. To provide further insights into the optical gain properties of the resulting starbursts, the loss coefficients were measured. The length of the pump stripe remained constant, but the pump stripe was moved away from the edge of the sample. Assuming that the emission from the end of the pump stripe (I0) remained constant, the emission from the edge of the sample should decrease (as a result of waveguide losses) according to I (λ) = I0 (λ) exp (–α(λ) × l),73 where l is the length of the non-pumped region between the excitation redion and the sample edge and α is the loss coefficient.10 By fitting the ASE intensity data at different stripe position as a function of l utilizing the above equation, the curve of loss coefficient versus 19 - Environment ACS Paragon-Plus


stripe position can be obtained. As shown in Figure 7, the intensity of light emission from the edge of the waveguides for P1F, P2F, and P3F varies with changing the distance between the pump stripe and the edge of the sample. The output declines exponentially because of waveguide loss in the unpumped region. By curve fitting these data to an exponential, loss coefficients of (4.90 ± 0.1), (2.10 ± 0.1) and (2.19 ± 0.1) cm-1 were thus determined for P1F, P2F, and P3F, respectively. Waveguide losses are usually a result of intrinsic absorption and scattering at interfaces caused by surface roughness and any particles or grain boundaries potentially present in the films.28 According to the low loss coefficients, it is speculated that the resulting starburst samples exhibit excellent waveguide characteristics and film-forming properties. The high gain and low loss values make these starbursts rather attractive as robust gain media for organic lasers. (a) 8

(b)

6 4

475 nm 25 nJ/pulse

2 0

15.6 31.3 62.5 125.0 Pump Energy Density (nJ/pulse)

Normalized Intensity (a. u.)

3

Output Intensity(10 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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1.2

1 23

1.0 0.8 0.6

4

5

475 nm 477 nm 479 nm 484 nm 495 nm

0.4 0.2 0.0 470

480


510

Figure 8. (a) Output intensity of a P2F 1D DFB laser as the function of pump energy. Inset shows the schematic illustration of the P2F 1D DFB laser device. (b) Lasing spectra from P2F 1D DFB laser base on different gain layer thickness and different periods: (1) 150 nm with Λ= 280 nm for λ = 475 nm; (2) 180 nm with Λ= 280 nm for λ = 477 nm; (3) 200 nm with Λ= 280 nm for λ = 479 nm; (4) 150 nm with Λ= 290 nm for λ = 486 nm; (5) 150 nm with Λ= 300 nm for λ = 495 nm. P2F was selected to further explore the lasing properties due to its good film-forming properties and superior ASE performance. One-dimensional (1D) distributed-feedback (DFB)


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gratings were fabricated by nanoimprint lithography (NIL). The 2nd order gratings (50% fill factor and 50 nm depth) comprise arrays of lines with a periods of 280 nm, 290 nm, and 300 nm, respectively. P2F was spin-coated on top of etched silica 1D gratings to form laser devices. The performance was recorded under pulsed optical excitation (the pumped wavelength at 380 nm with Q-switched, 10 Hz repetition rate) at room temperature. Figure 8a shows the output intensity of a P2F 1D DFB laser as the function of pump energy. A abrupt change in the slope of the output intensity was detected for the P2F 1-D DFB lasers with increasing excitation fluence, which was determined as the threshold of the DFB lasers. Consequently, a low lasing threshold of 2.1 kW cm-2 (25 nJ pulse-1) at 475 nm with a grating period of 280 nm was recorded for P2F as the gain media in the DFB lasers. In addition, the surface-emission spectra based on the DFB resonators exhibited very narrow peaks with FWHM around 0.2 nm. As shown in Figure 8b, by modulating the grating periods from 280 to 300 nm, the lasing spectra for P2F were facilely tunable across a spectral window of 20 nm (from 475 nm to 495 nm).

4. Conclusions In conclusion, a novel set of starburst conjugated molecules composed of a pyrene core and diphenylamine end-cappers with various oligofluorene bridge lengths, named as P1F, P2F, and P3F, were designed, and synthesized. The resulting starbursts exhibited good PL properties and excellent thermal stability with high Td over 400 oC. A combination of the thermal, photophysical, electrochemical, fluorescence transients, EL, ASE and lasing measurements were carried out to reveal further the influence of the diphenylamine moieties as electron-donating end-cappers on their optoelectronic properties. It is interesting to find that the PL spectra are obviously blue-shifted with an extension in the conjugation length of the oligofluorenes in both dilute solutions and films, i.e. ~11 nm blue shifts for P2F relative to P1F, and ~14 nm blue shifts for P3F relative to P1F in films. It is supposed that the introduction of strong electron-donating diphenylamine units helps to construct donor-π21 - Environment ACS Paragon-Plus


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acceptor structures that enable intramolecular excitation energy transfer transition from the diphenylamine donor to the pyrene acceptor. The diphenylamine units also play a key role in raising the HOMO energy levels of the molecules, which is beneficial for improving the charge injection and transport properties. Consequently, efficient EL properties with rather low turn-on voltages of 2.7-3.0 V among non-doped blue OLEDs have been achieved. Moreover, stabilized EL and ASE with high net gain coefficients (around 73.6-81.9 cm-1) and low waveguide loss (about 2.10-4.90 cm-1) have been demonstrated. Particularly, P2F films showed excellent ASE performance with low EthASE of 41.8 nJ pulse -1 (2.3 µJ cm-2, peaked at 489 nm), high net gain coefficient (g = 81.9 cm-1), low waveguide loss (2.10 ± 0.1) cm-1. Impressively, the ASE emission still remained detectable even upon annealing up to 280 oC. 1D DFB lasers demonstrated low lasing threshold of 2.1 kW cm-2 (25 nJ pulse-1) at 475 nm for P2F. All the results confirm that the novel molecular design strategy on constructing diphenylamine-capped pyrene-centered starburst molecules can significantly improve the electrical properties and the EL performance by fine modulating the HOMO energy levels, but did not largely sacrifice their optical gain properties. The high gain and low loss with excellent thermal and optical stability have rendered these donor-π-acceptor starburst conjugated molecules rather attractive as robust gain media for organic lasers.

Supporting Information Synthetic routes and structure characterization (MALDI-TOF, 1H NMR, 13C NMR) of the key intermediates and the final targets; Fluorescence transients, density functional theory calculations, and ASE spectra of P1F–P3F films in comparison with those of UV−vis absorption and PL spectra (PDF). The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.xxxxxx.

Acknowledgements We acknowledge financial support from the National Key Basic Research Program of China (973 Program, 2014CB648300, 2017YFB0404500), the National Natural Science Foundation 22 - Environment ACS Paragon-Plus

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of China (21422402, 21674050, 20904024, 61136003), the Natural Science Foundation of Jiangsu Province (BK20130037, BK20140060, BM2012010), Program for Jiangsu SpeciallyAppointed Professors (RK030STP15001), Program for New Century Excellent Talents in University (NCET-13-0872), Specialized Research Fund for the Doctoral Program of Higher Education (20133223110008), the Synergetic Innovation Center for Organic Electronics and Information Displays, the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), the NUPT ‘‘1311 Project’’, the 333 Project (BRA2015374) and the Qing Lan Project of Jiangsu Province. References (1) Ponomarenko, S. A.; Tatarinova, E. A.; Muzafarov, A. M.; Kirchmeyer, S.; Brassat, L.; Mourran, A.; Moeller, M.; Setayesh, S.; de Leeuw, D. Star-Shaped Oligothiophenes for Solution-Processible Organic Electronics: Flexible Aliphatic Spacers Approach. Chem. Mater. 2006, 18, 4101-4108. (2) Hotta, S.; Yamao, T.; Bisri, S. Z.; Takenobu, T.; Iwasa, Y. Organic Single-Crystal LightEmitting Field-Effect Transistors. J. Mater. Chem. C 2014, 2, 965-980. (3) Xie, L.-H.; Yin, C.-R.; Lai, W.-Y.; Fan, Q.-L.; Huang, W. Polyfluorene-based Semiconductors Combined with Various Periodic Table Elements for Organic Electronics. Prog. Polym. Sci. 2012, 37, 1192–1264. (4) Pu, Y. J.; Nakata, G.; Satoh, F.; Sasabe, H.; Yokoyama, D.; Kido, J. Optimizing the Charge Balance of Fluorescent Organic Light-Emitting Devices to Achieve High External Quantum Efficiency beyond the Conventional Upper Limit. Adv. Mater. 2012, 24, 17651770. (5) Shirota, Y.; Kageyama, H. Charge Carrier Transporting Molecular Materials and Their Applications in Devices. Chem. Rev. 2007, 107, 953-1010. (6) Park, B.; Kwon, O. E.; Yun, S. H.; Jeon, H. G.; Huh, Y. H. Organic Semiconducting Layers Fabricated by Self-Metered Slot-Die Coating for Solution-Processable Organic 23 - Environment ACS Paragon-Plus


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TOC Graphic

n N

N n

R

R

N

R

R

R

R

R

R

R = n-C6H13 n

P1F : n = 1 P2F : n = 2 P3F : n = 3

n

N

Normalized 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


1.0

UV PL ASE Lasing

0.8 0.6 0.4 0.2 0.0 300

400 500 600 Wavelength (nm)


Pyrene-Cored Starburst Oligofluorenes with Diphenylamine End

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