Colorless Copolyimide Films Exhibiting Large Stokes-Shifted Photoluminescence Applicable for Spectral Conversion

Abstract

A series of semi-aliphatic polyimide (PI) copolymers (CoPIs) were prepared through the copolymerization of two dianhydrides, 1-hydroxy pyromellitic dianhydride (PHDA) and 4,4′-oxydiphthalic anhydride (ODPA), with 4,4′-diaminodicyclohexylmethane, in which the PHDA molar ratio was controlled at less than 5% to suppress aggregate formation in a solid state. Upon increasing the molar ratio of the PHDA unit, the fluorescence color of the CoPI films continuously changed from pink to yellow through an orange color owing to the enhanced emission from the anionic form of the PHDA unit, and the efficiency of the energy transfer from the locally excited S1 state to the anionic state simultaneously increased. Moreover, the CoPI film with a PHDA content of 3 mol % that formed on a silica substrate was colorless and transparent and showed bright orange fluorescence. However, this CoPI film formed on a soda-lime-silica (soda) glass substrate showed a pale-yellow color as well as yellowish fluorescence originating from the anionic form owing to the high basicity of the soda glass. To suppress the absorption and emission from the anions, a small amount of sulfuric acid was doped into the CoPI film, and a colorless and transparent film exhibiting large Stokes-shifted orange fluorescence was successfully obtained on the soda glass substrate. The wavelength-converting spectrum of the CoPI demonstrated that UV irradiation in solar light was efficiently absorbed, and its energy was converted into visible light between 500 and 700 nm with a quantum efficiency of 20%. These CoPI films exhibiting large Stokes-shifted ESIPT fluorescence on both silica and the soda glass substrates are promising materials for solar spectral conversion applications.

Synthesis and Characterization of White-Light Luminescent End-Capped Polyimides Based on FRET and Excited State Intramolecular Proton Transfer

“…of H 2 SO 4 with variable r values, along with photographs under white light and UV irradiation. When a small amount of non-volatile H 2 SO 4 was added to the PAA (H 2 SO 4 -doped ODDC-TfA), the fluorescence at 440 nm was effectively suppressed [21], indicating that this fluorescence is attributable to the deprotonated anionic form, as shown in Figure S7. The E FRET of the H 2 SO 4 -doped ODDC-TfA increased by 9% with an increase in r from 1.98 to 7.92, which is higher than that of ODDC-TfA.…”

“…In our previous studies [21,45], we reported that ESIPT molecules are sensitive to the surrounding environment and may induce dissociation of the hydrogen atom in the amide group, generating an anionic form. The formation of anions in organic solvents was effectively suppressed with the addition of a small amount of TFA.…”

“…However, since it has been reported that PI films with acid dianhydrides, such as PMDA and BPDA, emit fluorescence in the visible region, the improvement of the fluorescence property of PI is desired [16][17][18][19]. To suppress the CT and LE(n−π*) transitions and enhance the LE(π−π*) transitions between the S 0 and S 1 states, we have developed a series of highly fluorescent PIs with high Φ values by combining dianhydrides with high electron-accepting ability, whose HOMO-LUMO transitions are LE(π−π*), and alicyclic diamines with low electrondonating ability [15,[20][21][22][23][24][25][26][27][28][29]. With other methods, highly fluorescent fully aromatic PIs have been developed by introducing aggregation-induced emission (AIE)-active bulky moieties, such as modified triarylamine [30,31] or tetraphenylethylene structures [32], in their main or side chains.…”

“…Hence, white emission, which requires long-wavelength fluorescence with a very large ν, is challenging to achieve. To overcome this problem, the uses of excited-state intramolecular proton transfer (ESIPT) and intersystem crossing (ISC) followed by phosphorescence emission are advantageous [21,28,33]. In addition, molecules having a donor-acceptor structure exhibit large Stokes-shifted fluorescence owing to the effective spatial separation of HOMO and LUMO [34].…”

Enhancement of Ultraviolet Light Resistance of Colorless and Transparent Semi-Alicyclic Polyimide Nanocomposite Films via the Incorporation of Hindered Amine Light Stabilizers for Potential Applications in Flexible Optoelectronics

“…Colorless and transparent polyimide (CPI) films have been well known for the excellent combined thermal and optical properties for many years in advanced optical and optoelectronic applications [ 1 , 2 , 3 ]. In many practical applications, CPI films have to be 1qconfronted with the long-term ultraviolet-visible (UV) light exposure, such as the fluorinated CPI thermal control films for the space aircrafts have to endure the large doses of ultraviolet (UV) and vacuum ultraviolet (VUV) irradiation during the servicing life, and the CPI substrates for flexible photovoltaic solar cells will be irradiated by sunlight in the whole life.…”