"Hot Exciton" Organic Fluorophores For Realizing High Temperature Thermometers And Heat-resistant OLEDs

In modern and intelligence industries,real-time and in situ spatial temperature sensing is essential.Luminescent temperature measurement technology based on heat-resistant organic fluorescence materials exhibits attractive abilities of fast response,non-invasive detection,high spatial resolution,resistance to electrical/magnetic fields,ease of fabrication,safety of remote handling and applicability of monitoring moving objects.However,at heating,the luminescence of organic materials is usually significantly quenched due to drastically facilitated nonradiative deactivation,leading to a prominent decrease in detection sensitivity at high temperatures.This impedes their applications for wide-range and high-temperature measurements.Organic light emitting diodes（OLEDs）play an important role for interactive and timely information display,due to their advantages in high color quality,low-cost fabrication methods and low power consumption.Heat-resistant organic electroluminescent devices are highly demanded for real-time high temperature detection in industrial production and daily life.However,to the best of our knowledge,there are no research publications focusing on exploration of“hot exciton”materials for high temperature sensing and heat-resistant OLEDs.In this paper,a series of pyrrole substituted triaryl phosphine oxides with high heat-resistance"hot exciton"materials（C1 and C2）were studied.By mixing with a yellow excited-state intramolecular proton transfer（ESIPT）emitter（T4AC）,high-temperature ratiometric sensing has been developed.The relative sensitivity S_r is higher than 1%K^-1 in the high temperature region from 393 K（120°C）to 470 K（275°C）,with the maximum S_r reaching 1.26%K^-1 at 430 K（157°C）.Through a series of optimization,we prepared efficient cyan OLEDs based on C1 and C2.Among them,the cyan OLED device based on C1 is particularly excellent.The maximum CE,PE and EQE reach 13.72 cd/A,11.74 lm/W and 6.23%respectively,and Commission Internationale d’Eclairage（CIE）are（0.151,0.400）.By heating from room temperature to 530 K（257°C）,the electroluminescence（EL）is significantly enhanced by nearly 12-fold.On this basis,heat-resistant white OLEDs（W1 and W2）based on C1/T4AC and C2/T4AC were prepared.W1 and W2 devices both exhibit dual emissions of the cyan and yellow luminescence peaked at 480 and 545 nm,respectively.W1 device displays cold white EL with CIE coordinates of（0.27,0.40）,and W2 presents a warm white EL with CIE coordinates of（0.33,0.50）.The maximum EQE,CE and PE of W1reaches 1.83%,4.99 cd·A^-1and 4.27 lm·W^-1,while those of W2 are 1.33%,3.38 cd·A^-1 and 2.89lm·W^-1,respectively.Similar to the blue OLEDs based on C1 and C2 emitter,the temperature dependent EL spectra of both the white emitting devices show high tolerance to heat as well.The EL intensity significantly enhanced with increase of temperature,exhibiting a 7.4（W1）and 2.0-fold（W2）enhancement by heating from 80（-193°C）to 500 K（227°C）,and a 4.7（W1）and 1.7-fold（W2）enhancement by heating from 300（27°C）to 500 K（227°C）,respectively.we studied the mechanism due to the good electroluminescent properties and temperature resistance.The results show that the high singlet exciton utilization（100%for C1 and 47%for C2）is achieved by effective high-lying reverse intersystem crossing（h RISC）.Through theoretical calculation,we determined that C1 and C2 have the characteristics of"hot exciton",which has a great application prospect in high temperature sensing and high efficiency heat-resistant OLED.