A novel hot exciton blue fluorophores and white organic light-emitting diodes with simplified configuration

The two efficient non-doped blue emitters with hybridized local and charge transfer state namely, NDBNPIN and DBTPIN have been synthesised and characterised. These materials are employed as a host for green and red phosphorescent OLEDs. The white device based on DBTPIN:Ir(MDQ)2(acac) (4%) exhibit maximum external quantum efficiency (ηex) −24.8%; current efficiency (ηc) −57.1 cdA−1; power efficiency (ηp) −64.8 lmW−1 with Commission Internationale de l’Eclairage (CIE:0.49, 0.40) than NDBNPIN:Ir(MDQ)2acac (4%) device [ηex − 23.1%; ηc −54.6 cd A−1; ηp− 60.0 lm W−1 with CIE (0.47, 0.42)].

(0.42 eV) and DBTPIN (0.57 eV) is because of same acceptor (phenanthrimidazole) group and the energy gap of DBTPIN is larger when compared to NDBNPIN. In both NDBNPIN and DBTPIN, very small ΔE ST ≈ 0 facilitates RISC (T 3 → S 1 ) with hot exciton due to HLCT state. Thus, DBTPIN show high η PL and high η S compared with NDBNPIN. The η EQE (external quantum efficiency) of device with DBTPIN is increased due to high % LE. The HOMO as well as LUMO of NDBNPIN and DBTPIN exhibit partial separation which enhanced holeand electron-transportation (bipolar nature) with electron/hole transfer integrals, NDBNPIN (0.24/0.47 eV) and DBTPIN (0.21/0.41 eV) and minimised the ΔE ST (Fig. 2).
The optical characteristics of NDBNPIN and DBTPIN were studied in solution as well as in film ( Fig. 1 &  Fig. S1). Absorption (λ abs ) around 276 and 378 nm is attributed to π − π* and CT transitions, respectively and strong absorption is due to CT from triphenylamine (donor) to acceptor (naphthonitrilephenanthrimidazole) 18 . The suppressed π-π* stacking in film induced red-shifted λ abs relative to solution 19 and the larger red shift supports the CT in twisted DBTPIN or NDBNPIN. From the onset absorption in film, optical E g (band gap) is calculated as NDBNPIN (2.98 eV) and DBTPIN (2.90 eV). The emitters NDBNPIN and DBTPIN show emission maxima at 440 and 428 nm, respectively (Fig. 1). As solvent polarity increased the emission spectra is red-shifted with broadened structure (Fig. S1) and exhibits larger variation in ground state dipole moment (μ g ) relative to excited state dipole moment (μ e ). By employing DFT and Lippert-Mataga plot (Fig. S1) μ g /μ e was calculated as NDBNPIN (9.02/27.8 D) and DBTPIN (8.11/26.1 D). Solvents with f ≥ 0.2, CT state is stabilised [20][21][22] (strong interaction between solvent field and CT state, LE remains unchanged) whereas solvents with f ≤ 0.1 LE state is stabilised. Transformation in the slope observed between butyl ether (f = 0.10) and ethyl acetate (f = 0.20) reveal that the emitters show HLCT emissive state i.e., intercrossed excited state of LE and CT [E CT = E LE ] ( Fig. S1 and Table 1). The λ emi of DBTPIN and NDBNPIN in film and ether is almost same due to HLCT emissive state.
The PES of NDBNPIN and DBTPIN reveal twisting of D-A linkage with 20-50° angle be the origin for intercross of CT and LE states. At 90° twist angle, frontier orbitals (HOMO and LUMO) on TPA and PPI are separated results in CT transition from HOMO (donor) → LUMO (acceptor). At 90°, twisted conformation of NDBNPIN and DBTPIN is less stable because of higher energy NDBNPIN (≈0.6 eV) and DBTPIN (≈0.04 eV) than at ≈40° (stable conformation) [20][21][22] . The HOMO and LUMO orbital map is displayed in Fig. 2. The high ɸ soln/film (quantum yield) of NDBNPIN (83/80%) and DBTPIN (92/90%) is due to co-emission from LE and CT which is essential for efficient blue OLEDs and the enhanced quantum yield is due to decreased non-radiative (k nr ) transition 23 ( Table 1 show that oscillator strength of these compounds in CHCl 3 is high relative to gaseous phase due to higher luminance of HLCT state in CHCl 3 (Fig. S1). To further investigate the excited state properties, transient PL decay was recorded using time-correlated single photon counting method. The single-exponential lifetime of 1.61 ns (NDBNPIN) and 1.20 ns (DBTPIN) indicates that the hybridization of LE and CT components into a single emissive HLCT state (Fig. S1) [24][25][26] . The lifetime measurements in nanosecond scale further confirmed that they are fluorescent materials 22 . The triplet energies (E T ) estimated as 2.62 eV (DBTPIN) and 2.74 eV (NDBNPIN) which are sufficient for exciting red as well as green phosphorescent emitters. Also small ΔE ST is sufficient for energy-transfer from host [27][28][29] triplet to green and red emitters. The charge transportation of DBTPIN and NDBNPIN, CBP:Ir(ppy) 3 , DBTPIN:Ir(ppy) 3 and NDBNPIN:Ir(ppy) 3 was examined by single carrier device fabrication (Fig. 3). Current-density difference of DBTPIN and NDBNPIN compared to CBP device reveal that these bipolar-materials transport electrons and holes effectively 30 .
The carrier-current decreases in control device CBP: Ir(ppy) 3 due to trapping at HOMO of Ir(ppy) 3 ( Fig. 3c (i)) whereas carrier current increased in DBTPIN: Ir(ppy) 3 or NDBNPIN:Ir(ppy) 3 devices due to direct carrierinjection into HOMO of Ir(ppy) 3 followed by hopping transport via DBTPIN/NDBNPIN sites ( Fig. 3c (ii)). Hole-current density of DBTPIN < NDBNPIN because cyanonaphthyl limits carrier (hole) injection/transportation significantly 12 . For devices with DBTPIN and NDBNPIN, similar electron/hole current charges were measured by high/low electric-field, respectively which shows that these materials are potential emissive candidate at low V for efficient OLEDs.
The fabricated blue device, ITO/NPB (60 nm)/DBTPIN or NDBNPIN (30 nm)/LiF (1 nm)/Al (100 nm) with HOMO-LUMO energies are depicted in Fig. 3. Generally, flat-decay was shown by TADF materials because of slow TADF process in conversion of triplet state exciton → singlet state. The single-exponential decay of DBTPIN and NDBNPIN reveal that the radiative exciton are short-lived without TADF contribution and also supports single emissive state (HLCT) (Fig. 1). Therefore, high η s of DBTPIN and NDBNPIN is not due to TTA or TADF process 31 . The similar PL and EL emission of DBTPIN and NDBNPIN shows that both PL and EL stems from same source with similar radiative route. The DBTPIN device exhibit superior performance (  (Fig. 4). Higher η ex harvested from doped devices relative to non-doped one ascribed to doping concentration which reduced the exciton concentration quenching and minimised the intermolecular CT leads to bathochromic shift 32 .
The green device DBTPIN (30 nm): 5 wt% Ir(ppy) 3 Fig. 5. A part of host singlet exciton transferred to singlet blue fluorophore (BS 1 ) exhibits deep-blue emission whereas another part of singlet exciton transferred to singlet green/red phosphorescent emitters (GS 1 /RS 1 ) and then delivered to triplet green and red phosphorescent emitters (GT 1 /RT 1 ) by intersystem-crossing process and shows red and green phosphorescent emission. Furthermore, host triplet exciton (HT 1 ) is transferred to GT 1 /RT 1 to enhance the exciton utilization (Fig. 6).

Conclusion
We have reported two deep blue emitting materials DBTPIN and NDBNPIN with dual charge transport properties and exhibit high EQE of 6.5% and 4.8% with CIE (0.14, 0.13). The triplet energies (E T ) estimated as 2.62 eV (DBTPIN) and 2.74 eV (NDBNPIN) are sufficient for the excitation of green and red phosphorescent dopants. Efficient green and red PhOLEDs with EQE of 23.0%/20.1% and 24.1%/21.3% have been harvested based on

1
H and 13 C NMR and mass spectra were recorded at 298 K on Bruker 400 MHz spectrometer and Agilent (LCMS VL SD), respectively. Absorption (solution and film) were recorded on Perkin-Elmer Lambda 35 and Lambda 35 spectrophotometer with integrated sphere (RSA-PE-20), respectively. PerkinElmer LS55 fluorescence spectrometer and fluorescence spectrometer Model-F7100 with integrating sphere was employed to analyse PL and absolute quantum yield, respectively. Thermogravimetric analysis (TGA) and differential scanning calorimetric (DSC) were recorded with PerkinElmer thermal analysis system and NETZSCH-DSC-204, respectively (10 °C min −1 ; N 2 flow rate of 100 mL min −1 ). Lifetime was estimated with time correlated single-photon counting (TCSPC) method on Horiba Fluorocube-01-NL lifetime system. Cyclic voltammetry was performed with potentiostate CHI 630 A electrochemical analyzer. The HOMO [E HOMO = −(E ox + 4.8 eV)] energies and LUMO [E LUMO = (E red − 4.8 eV)] energies were calculated using oxidation and reduction potentials, respectively.

Computational Details
The ground and excited state analysis were studied by using Gaussian 09 program 68 and multifunctional wavefunction analyzer (Multiwfn) 69 (Figs. S7-S14).