Chapter 14
Luminescent Crystal–Control
of Excited-State Intramolecular Proton
Transfer (ESIPT) Luminescence
Through Polymorphism
Toshiki Mutai
Abstract Crystal polymorphism has a significant effect on the luminescence of
materials. Using 2-(2
-hydroxyphenyl)imidazo[1,2-a]pyridine (HPIP) as the key
compound, crystal structure-dependent luminescence, also known as polymorphdependent luminescence (PDL), is described. HPIP shows excited-state intramolecular proton transfer (ESIPT) luminescence, which is quite efficient in the solid state,
yet very weak in solution; it is mainly ascribed to the suppression of efficient radiationless decay via the S 0 /S 1 –conical intersection when the molecular conformation
is fixed coplanar. The two polymorphic forms of the crystals display blue-green
and yellow luminescence and can be interconverted via heat transfer between the
two states. Three-color PDL (yellow, orange, and red) is observed in 6-cyano HPIP.
The relationship between crystal structure and luminescence color was rationally
explained by TD-DFT calculations using dimer models. It is remarkable, that this
dimer model is reasonably supported by the femtosecond time-resolved transient
absorption spectroscopy. These results will contribute to the understanding of structure–property relationships in solid-state luminescence at the molecular level and
would further the design of new polymorph-dependent luminescent materials.
Keywords Luminescent crystal · Polymorph-dependent luminescence · Quantum
chemical calculation · ESIPT · Fluorescence · Femtosecond time-resolved
transient absorption spectroscopy
14.1 Introduction
Organic solid-state luminescent materials have garnered considerable interest in
various areas of science including OLEDs [1, 2], sensors [3], solid lasers [4], nearinfrared light sources [5], and so on [6, 7]. In most cases, the electronic properties
of materials are controlled via synthetic modification of luminescent molecules.
T. Mutai (B)
Department of Materials and Environmental Science, Institute of Industrial Science, The
University of Tokyo, 4-6-1, Komaba, Meguro-ku, Tokyo 153-8505, Japan
e-mail: mutai@iis.u-tokyo.ac.jp
© Springer Nature Singapore Pte Ltd. 2020
M. Sakamoto and H. Uekusa (eds.), Advances in Organic Crystal Chemistry,
https://doi.org/10.1007/978-981-15-5085-0_14
271
Luminescent Crystal–Control
of Excited-State Intramolecular Proton
Transfer (ESIPT) Luminescence
Through Polymorphism
Toshiki Mutai
Abstract Crystal polymorphism has a significant effect on the luminescence of
materials. Using 2-(2
-hydroxyphenyl)imidazo[1,2-a]pyridine (HPIP) as the key
compound, crystal structure-dependent luminescence, also known as polymorphdependent luminescence (PDL), is described. HPIP shows excited-state intramolecular proton transfer (ESIPT) luminescence, which is quite efficient in the solid state,
yet very weak in solution; it is mainly ascribed to the suppression of efficient radiationless decay via the S 0 /S 1 –conical intersection when the molecular conformation
is fixed coplanar. The two polymorphic forms of the crystals display blue-green
and yellow luminescence and can be interconverted via heat transfer between the
two states. Three-color PDL (yellow, orange, and red) is observed in 6-cyano HPIP.
The relationship between crystal structure and luminescence color was rationally
explained by TD-DFT calculations using dimer models. It is remarkable, that this
dimer model is reasonably supported by the femtosecond time-resolved transient
absorption spectroscopy. These results will contribute to the understanding of structure–property relationships in solid-state luminescence at the molecular level and
would further the design of new polymorph-dependent luminescent materials.
Keywords Luminescent crystal · Polymorph-dependent luminescence · Quantum
chemical calculation · ESIPT · Fluorescence · Femtosecond time-resolved
transient absorption spectroscopy
14.1 Introduction
Organic solid-state luminescent materials have garnered considerable interest in
various areas of science including OLEDs [1, 2], sensors [3], solid lasers [4], nearinfrared light sources [5], and so on [6, 7]. In most cases, the electronic properties
of materials are controlled via synthetic modification of luminescent molecules.
T. Mutai (B)
Department of Materials and Environmental Science, Institute of Industrial Science, The
University of Tokyo, 4-6-1, Komaba, Meguro-ku, Tokyo 153-8505, Japan
e-mail: mutai@iis.u-tokyo.ac.jp
© Springer Nature Singapore Pte Ltd. 2020
M. Sakamoto and H. Uekusa (eds.), Advances in Organic Crystal Chemistry,
https://doi.org/10.1007/978-981-15-5085-0_14
271