410
Y. Kubo
red (R), green (G), and blue (B). To avoid subjective color perception, the International Commission on Illumination (Commission Internationale de l’Éclairage,
CIE) coordinates in the chromaticity diagram (known as CIE 1931) have been used
as the colorimetric standard (Jones 1943). All colors are represented by two color
coordinates (x and y) in the system; accordingly, white light is defined as (0.33,
0.33). Thus, white light can refer to any color with coordinates in a region with
energy equal to that of white light. The diagram also indicates that white light can
be obtained by mixing two complementary colors for which the connection line of
their color coordinates crosses the white light region. The quality of white light is
evaluated by the color rendering index (CRI) and correlated color temperature (CCT)
(Xu et al. 2018). For organic materials, great effect has been devoted to prepare a
single molecule-based white-light emission system owing to the potent application to
white organic light-emitting devices (Lili and Michael 2014). However, there is still
a challenge to provide systems with “panchromatic emission” because of Kasha’s
rule (Turro 1991), which states that photon emission occurs in an appreciable yield
from the lowest excitation state. To overcome such difficulty, plausible dyes have
been designed to have at least two forms, such as an emissive tautomer, to entirely
cover the visible region. As a typical example, excited state intramolecular proton
transfer (ESIPT) has great promise to provide the desired white-light emitters (Liu
et al. 2017a; Sakai et al. 2016), whose molecules have a stable enol form in the
ground state, which is switchable to the corresponding keto form in the excited state
through intramolecular proton transfer (Zhao et al. 2012a). Such a unique photophysical feature leads to a large Stokes shift of the emission, providing a very useful
methodology to cover the longer wavelengths of the visible region. Aggregationinduced emission (AIE) is also a promising way toward this end (Mei et al. 2015).
A propeller-like organic compounds involving tetraphenylethylene (TPE) (La et al.
2018), hexaphenylsilole (Hong et al. 2009), and tetraphenylpyrazole (TPP) (Chen
et al. 2015) have been developed as AIE luminogens. Those compounds are nonemissive in solution but emit in the aggregation state, which is the exact opposite of
aggregation-caused quenching (ACQ) phenomena. Such a unique feature is responsible for the restriction of intramolecular rotation. In this context, Tang, et al. prepared
single molecules with dual fluorescent modes containing TPE and pyrene fluorophore
through covalent bond formation to realize WLE (Feng et al. 2018a). The structureproperty relationship was investigated to understand the origin of WLE. Thermally
activated delayed fluorescence (TADF) (Uoyama et al. 2012) is known as an effective
tool to obtain a high photoluminescence efficiency in the development of materials
for OLEDs. For the strategy, it should be considered that common TADF molecules
can easily aggregate through π–π interactions, resulting in exciton concentration
quenching. The drawback was overcome by Zheng, Chi, and co-workers who developed single organic material composed of carbazoly- and phenothiazinyl-substituted
benzophenone. The compound exhibited an aggregation-induced emission delayed
fluorescence (Xie et al. 2015).
Facile way to tune fluorescence involving WLE is to control the Förester resonance
energy transfer (FRET) (van der Meer 2013) from donor dye to acceptor one in
Y. Kubo
red (R), green (G), and blue (B). To avoid subjective color perception, the International Commission on Illumination (Commission Internationale de l’Éclairage,
CIE) coordinates in the chromaticity diagram (known as CIE 1931) have been used
as the colorimetric standard (Jones 1943). All colors are represented by two color
coordinates (x and y) in the system; accordingly, white light is defined as (0.33,
0.33). Thus, white light can refer to any color with coordinates in a region with
energy equal to that of white light. The diagram also indicates that white light can
be obtained by mixing two complementary colors for which the connection line of
their color coordinates crosses the white light region. The quality of white light is
evaluated by the color rendering index (CRI) and correlated color temperature (CCT)
(Xu et al. 2018). For organic materials, great effect has been devoted to prepare a
single molecule-based white-light emission system owing to the potent application to
white organic light-emitting devices (Lili and Michael 2014). However, there is still
a challenge to provide systems with “panchromatic emission” because of Kasha’s
rule (Turro 1991), which states that photon emission occurs in an appreciable yield
from the lowest excitation state. To overcome such difficulty, plausible dyes have
been designed to have at least two forms, such as an emissive tautomer, to entirely
cover the visible region. As a typical example, excited state intramolecular proton
transfer (ESIPT) has great promise to provide the desired white-light emitters (Liu
et al. 2017a; Sakai et al. 2016), whose molecules have a stable enol form in the
ground state, which is switchable to the corresponding keto form in the excited state
through intramolecular proton transfer (Zhao et al. 2012a). Such a unique photophysical feature leads to a large Stokes shift of the emission, providing a very useful
methodology to cover the longer wavelengths of the visible region. Aggregationinduced emission (AIE) is also a promising way toward this end (Mei et al. 2015).
A propeller-like organic compounds involving tetraphenylethylene (TPE) (La et al.
2018), hexaphenylsilole (Hong et al. 2009), and tetraphenylpyrazole (TPP) (Chen
et al. 2015) have been developed as AIE luminogens. Those compounds are nonemissive in solution but emit in the aggregation state, which is the exact opposite of
aggregation-caused quenching (ACQ) phenomena. Such a unique feature is responsible for the restriction of intramolecular rotation. In this context, Tang, et al. prepared
single molecules with dual fluorescent modes containing TPE and pyrene fluorophore
through covalent bond formation to realize WLE (Feng et al. 2018a). The structureproperty relationship was investigated to understand the origin of WLE. Thermally
activated delayed fluorescence (TADF) (Uoyama et al. 2012) is known as an effective
tool to obtain a high photoluminescence efficiency in the development of materials
for OLEDs. For the strategy, it should be considered that common TADF molecules
can easily aggregate through π–π interactions, resulting in exciton concentration
quenching. The drawback was overcome by Zheng, Chi, and co-workers who developed single organic material composed of carbazoly- and phenothiazinyl-substituted
benzophenone. The compound exhibited an aggregation-induced emission delayed
fluorescence (Xie et al. 2015).
Facile way to tune fluorescence involving WLE is to control the Förester resonance
energy transfer (FRET) (van der Meer 2013) from donor dye to acceptor one in