Persistent room temperature blue phosphorescence from racemic crystals of 1,1-diphenylmethanol derivatives

Article abstract of DOI:10.1016/j.jphotochem.2020.113043

Among the six alkyldiphenylmethanol derivatives assessed in this study, five showed persistent room temperature phosphorescence (pRTP), whereas the original diphenylmethanol (DPhHOH) did not. pRTP is a unique luminescence phenomenon that appears only in t

Full text of DOI:10.1016/j.jphotochem.2020.113043

Journal Pre-proof
Persistent room temperature blue phosphorescence from racemic
crystals of 1,1-diphenylmethanol derivatives
Masaru Yamada, Kaname Ishigaki, Tatsuo Taniguchi, Takashi
Karatsu
PII:
S1010-6030(20)30840-6
DOI:
https://doi.org/10.1016/j.jphotochem.2020.113043
JPC 113043
Reference:
To appear in:
Journal of Photochemistry & Photobiology, A: Chemistry
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Revised Date:
Accepted Date:
6 August 2020
11 November 2020
11 November 2020
Please cite this article as: Yamada M, Ishigaki K, Taniguchi T, Karatsu T, Persistent room
temperature blue phosphorescence from racemic crystals of 1,1-diphenylmethanol
derivatives, Journal of Photochemistry and amp; Photobiology, A: Chemistry (2020),
doi: https://doi.org/10.1016/j.jphotochem.2020.113043
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Persistent room temperature blue phosphorescence from racemic crystals of 1,1-
diphenylmethanol derivatives
Masaru Yamadaa, Kaname Ishigakia, Tatsuo Taniguchia, and Takashi Karatsu*a,b
a Department of Applied Chemistry and Biotechnology, Graduate School of Engineering, Chiba University,
b Molecular Chirality Research Center, Chiba University,
1-33 Yayoi-cho, Inage-ku, Chiba 263-8522, Japan, E-mail: karatsu@faculty.chiba-u.jp.
Graphical Abstract
Solution
Crystal
Highlights

1,1-Diphenylmethanol derivatives are one of the simplest small molecules showed persistent room
temperature phosphorescence (pRTP) in their solid state.

Themaxs of pRTP are around 500 nm that is 0.75 eV smaller than those in Glassy matrices at 77K.



1,1-Diphenylmethanol derivatives are achiral molecules but existing as a pair of conformationally chiral
racemate in the crystal.
The interaction of racemic pair must be important factor to confine excitation energy in the crystal.
ABSTRACT
Among the six alkyldiphenylmethanol derivatives assessed in this study, five showed persistent room
temperature phosphorescence (pRTP), whereas the original diphenylmethanol (DPhHOH) did not. pRTP is a
unique luminescence phenomenon that appears only in the crystalline state and is different from the
luminescence produced by dispersed molecules, such as those in a glassy solvent matrix at 77 K or a
polymeric matrix. The data also suggest that pRTP appears in conjunction with a specific crystal structure.
The present work demonstrates that various crystal structures show pRTP regardless of the presence or
absence of weak hydrogen bonds. However, nonradiative deactivation is suppressed by both intermolecular
interactions and steric regulation in the crystal, such that the emission quantum yield is dependent on the
crystal structure. In addition, although the molecular structure in the crystalline state necessary for the
appearance of pRTP was not fully elucidated, intermolecular interactions between face-to-face and face-to-
edge benzene rings appear to play a major role. Density functional theory calculations indicate that
intersystem crossing is promoted in the crystalline state because the number of paths that allow this
intersystem crossing is greater in a structure having weak interactions between molecules as compared to a
dispersed state. DPhHOH did not show pRTP and is considered to have a structure based on the column-like
stacking of multiple molecules. In contrast, the crystals that exhibited pRTP had pseudo racemic conformation
in their crystals even though each molecule was achiral. In these racemic crystals, the interactions between
pairs of molecules were important to the generation of pRTP.
Keywords: Persistent room temperature phosphorescence; 1,1-Diphenylmethanol derivatives; Crystal
structure; Racemic pair; Triplet energy delocalization

1. Introduction
Recently, there have been reports concerning the solid state phosphorescence of simple organic
molecules at ambient temperature and atmospheric pressure. This phenomenon is referred to as
persistent room temperature phosphorescence (pRTP) or afterglow.
Because phosphorescence is originally a forbidden transition, it has a longer emission lifetime than
fluorescence. Specifically, the fluorescence emission lifetime is generally 10-11 to 10-6 s [1], while the
phosphorescence lifetime is 10-4 to 101 s. In addition, phosphorescence is characterized by a lower
energy emission maximum. The associated emission energy is typically lower than that observed for
typical phosphorescence in a glassy solvent matrix at 77 K.
In the solid state, molecular motion is fixed and knr might become small [2], but it is important that
kp is also small in order to achieve an ultralong lifetime. However, obtaining high luminous efficiency
requires that knr is less than kp. The emission quantum yield,
the formula p isc × kp × p where isc is the quantum yield of the intersystem crossing (ISC).
Achieving a high p therefore requires a molecular design that raises the value of isc
p, of the T1 state can be expressed by
=




.
Phosphorescence is an extremely useful means of exciting organic Electro-Luminescence. It has
been reported that organometallic complexes containing platinum group atoms emit phosphorescence
with very high efficiency [2–11]. This has led to research into metal-free organic light-emitting
materials for thermally activated delayed fluorescence.
In 2010, Tang and his collaborators demonstrated that benzophenone derivatives, which are not
phosphorescent in solution, generate phosphorescence in the crystalline state at room temperature in
air [12]. Benzophenone has a high

isc value of 1.0, and is known to generate an excited triplet state
p. However, a p value of 15.9% was obtained in the
with a high yield but also exhibits a negligible


crystalline state at room temperature. Tang et al. proposed that this effect was induced by the
suppression of nonradiative deactivation due to intramolecular rotation and vibration following
crystallization. Therefore, this phenomenon is referred to as crystallization-induced phosphorescence
(CIP).
Huang and his collaborators later reported that 9-(4,6-diphenyl-1,3,5-triazin-2-yl)-9H-carbazole
shows pRTP [13]. Single crystal X-ray structural analysis of this compound determined that H-
aggregates were formed in the solid and that this structure further stabilized the T1 state by forming
energy traps, thus promoting pRTP [14]. Time dependent density functional theory (TD-DFT)
calculations performed for the monomer and dimer based on the crystal structure demonstrated that
the number of ISC paths (that is, S1 → Tn) was increased in the dimer relative to the monomer.
However, efficiency of each process is unclear. Such studies prompted additional research into pRTP.

Recently, authors reported that important insight into the impurities caused persistent room temperature
phosphorescence from the crystals of carbazole derivatives [15]. This is also interesting from the crystal
doping point of view. Interestingly, the first report regarding pRTP was published by Clapp in 1939,
who found that the exposure of tetraphenylmethane or tetraphenylsilane crystals to light produced
pRTP [16]. Much later, in 1978, Morantz et al. showed that pRTP was also obtainable from carbazole,
dibenzthiophene, dibenzofuran and triphenylene crystals [17]. Following Morantz’s work, there were
few publications concerning the generation of pRTP by pure organic compounds, and the associated
luminescence mechanism was not studied in detail. However, more recently, there have been
assessments of many different organic molecules that exhibit this phenomenon.
Among the various molecules that have been found to show RTP in the solid state, several organic
compounds are able to exhibit phosphorescence with a long emission lifetime of several seconds that
can be visually observed [18–21]. As noted, this process is referred to as pRTP, and often generates a
different wavelength from the more typical phosphorescence exhibited in glassy matrices at 77 K.
Various mechanisms have been proposed for this emission, including π-π interactions that induce
rigidity in crystals, association via hydrogen bonds and other phenomena that promote ISC.
Li and collaborators demonstrated that the molecular packing structure of a compound affects the
appearance of pRTP. Specifically, pRTP was observed in work with a crystalline derivative [22], and
then 4-(10H-phenothiazin-10-yl)benzonitrile, which produced three different crystalline polymorphs
and showed that variations in the packing structure affected the pRTP characteristics [23]. Analyses of
the interactions between the molecules in the three crystal structures indicated that the crystal phase
forming the H-aggregate showed low luminous efficiency. The crystalline regions of these
compounds evidently had the highest
p values and so the molecular structure, the intermolecular
interactions in the crystal and the molecular stereostructure were all related to the pRTP
characteristics.
Li and co-workers established the importance of hydrogen bonding between molecules based on
work with a compound without aromatic rings [24]. Kim et al. examined the pRTP behaviour of 4-
bromo-2,5-bis(hexyloxy)benzaldehyde), which has aligned molecules in the crystalline state. Only
weak fluorescence was obtained from this compound in solution, but the incorporation of the heavy
bromine atom worked efficiently between the molecules in the crystal [25]. Since then, efforts have
been made for pRTP that does not utilize the heavy atom effect [26
–28].
Another useful approach to obtaining pRTP utilizes charge separation and recombination [29
–
32].
Yuasa and co-workers proposed that the ISC mechanism involved hyperfine coupling in radical ion
pairs formed from molecules in the crystalline state [29]. In the case that a singlet radical ion pair is

generated, the ISC to the triplet excited radical ion pair occurs because the acceptor side radical and
the donor side radical are situated in different magnetic field environments. Because there is a large
distance between these radicals, the degree of overlap between orbitals is minimal and the transition to
other levels (such as S0) is slow. This generation of a triplet excited state is also involved in various
other useful physical processes such as those occurring in organic light emitting diodes [33, 34]. The
concept of fixing the molecular structure has also been applied to the development of inorganic-
organic hybrid systems [35–40], bio-inspired system [41], emission color tuning [42–44], and
mechanoluminescence [45].
As noted, pRTP has been actively studied in recent years and various luminescence mechanisms
have been proposed. However, the reason for the long lifetimes and some parts of the luminescence
mechanisms remain unclear, and there is little agreement in this area. Therefore, the design of
molecules that display pRTP is still difficult, and further research is needed to predict which
molecules will exhibit this phenomenon. In the present study, we determined that 1,1-
diphenylmethanol derivatives (Fig. 1) show pRTP emission lifetimes of several hundred milliseconds
and investigated the associated emission mechanism [46]. 1,1-Diphenylmethanol derivatives are one
of the simplest small molecules showed persistent room temperature phosphorescence (pRTP) in their
solid state. These pRTP are around 500 nm that is 0.75 eV smaller than those in Glassy matrices at
77K. 1,1-Diphenylmethanol derivatives are achiral molecules but existing as a pair of
conformationally chiral racemate in the crystal. Interaction of the racemic pair must be important
factor to confine excitation energy in the crystal as similarly reported recently [47-49]. The
delocalization of singlet excitation energy into multiple molecules in a crystal has been proposed by a
couple of research groups in recent years [50, 51], and is considered to be an important process even
in the long-lived triplet excited state proposed in this study.
<Fig. 1. The 1,1-diphenylmethanol derivatives examined in the present study.>
2. Experimental
2.1. Syntheses of DPhEtOH, DPhPrOH and DPhcHexOH

DPhEtOH (1,1-diphenylpropanol), DPhPrOH (1,1-diphenylbutanol), and DPhc-HexOH
(cyclohexyldiphenylmethanol) were obtained using Grignard reactions. The initial reagent was
prepared from magnesium and bromobenzene in dry tetrahydrofuran (THF) and then reacted with 6.8-
15.3 mmol (0.7 equivalents) of propiophenone, 1-phenylbutan-1-one or
cyclohexyl(phenyl)methanone. The products were separated using an open column (with an ethyl
acetate:hexane mixture as the eluent) and purified by recrystallization. The products were obtained as
white solids with yields between 62 and 72%.
DPhEtOH; 1H NMR (400 MHz, CDCl3):
 (ppm)=7.40–7.43 (m, 4H), 7.28–7.33 (m, 4H), 7.19–7.24
(m, 2H), 2.32 (q, J=7.22 Hz, 2H), 2.07 (s, 1H), 0.88 (t, J=7.22 Hz, 3H) (Fig. S1). mp 95 °C (range of
less than 1 °C). Elemental analysis (%): calculated for C15H16O: C, 84.87; H, 7.60; N, 0; found: C,
84.81; H, 7.62; N, 0.10.
DPhPrOH; 1H NMR (400 MHz, CDCl3):
 (ppm)=7.39–7.42 (m, 4H), 7.28–7.32 (m, 4H), 7.19–7.23
(m, 2H), 2.24–2.27 (m, 2H), 2.08 (s, 1H), 1.25–1.33 (m, 2H), 0.93 (t, J=7.28 Hz) (Fig. S2). mp 37 °C
(range of less than 1 °C). Elemental analysis (%): calculated for C16H18O: C, 84.91; H, 8.02; N, 0;
found: C, 85.04; H, 8.05; N, 0.17
DPhc-HexOH; 1H NMR (400 MHz, CDCl3):
 (ppm)=7.46–7.49 (m, 4H), 7.25–7.29 (m, 4H), 7.13–
7.18 (m, 2H), 2.40–2.47 (m, 1H), 2.06 (s, 1H), 0.90–1.77 (m, 10H) (Fig. S3). mp 68 °C (range of less
than 1 °C). Elemental analysis (%): calculated for C20H18O2N: C, 85.67; H, 8.32; N, 0; found: C,
85.61; H, 8.32; N, 0.19.
2.2. Synthesis of DPhMeOMe (1-methoxy-1,1-diphenylethane)
In a 50 ml three necked flask, 1,1-diphenylethanol (402 mg; 2.03 mmol), sodium hydride (208 mg; 2.60
mmol) and 5 ml dry THF were combined under dry nitrogen. This mixture was stirred for 30 min with cooling
by ice, after which iodomethane (620 mg; 4.36 mmol) was added and the mixture was allowed to warm to
room temperature and stirred for 19 h. The crude product was separated using an open column (with ethyl
acetate:hexane = 1:12 as the eluent) and purified by recrystallization. The product was a white solid with a
yield of 207 mg (48%).
1H NMR (400 MHz, CDCl3):  ppm = 7.33 – 7.36 (m, 4H), 7.28 – 7.32 (m, 4H), 7.20 – 7.24 (m, 4H), 3.16 (s,
3H), 1.86 (s, 3H), (Fig. S4). mp 35 °C (range of less than 1 °C.) Elemental analysis (%): calculated for
C15H16O: C, 84.87; H, 7.60; N, 0; found: C, 84.71; H, 7.60; N, 0.09.
2.3. Purification of compounds

DPhc-PrOH was purchased from the Tokyo Chemical Industry Co., Ltd. and treated with charcoal to
remove pale yellowish impurities. Each compound was purified by recrystallization from hexane at room
temperature (for DPhHOH, DPhMeOH, DPhEtOH, and DPhc-PrOH), from hexane at -20 °C (for
DPhPrOH and DPhc-HexOH) or from methanol at -20 °C (for DPhMeOMe).
2.4. Instrumentation and measurement
Samples were analysed by 1H nuclear magnetic resonance (NMR; Bruker AVANCE III-400M, 400
MHz) and also assessed by CHN elemental analysis (Perkin Elmer PE2400II). UV-vis absorption and
emission spectra were acquired with JASCO V-570 and JASCO FP-8300 spectrophotometers, respectively.
Absolute emission quantum yields were determined using a JASCO FP-8300 instrument with an
integrated sphere unit for solutions and a Hamamatsu photonics C11347-01 apparatus with an integrated
sphere unit for solid samples. Since the wavelength intensity of the light source of the device and the
wavelength sensitivity of the detector can be corrected only at wavelengths longer than 250 nm, considering
the band path of the excitation light, the excitation wavelength could only be performed at 260 nm. Therefore,
in the measurement of solid-state emission, the fluorescence is very close to the excitation light and it is
difficult to separate this incident light. Therefore, the quantum yield of phosphorescent light appearing at a
long wavelength was firstly determined by 260 nm excitation. Then the quantum yields of fluorescence were
estimated by the spectral area of fluorescence and phosphorescence spectra obtained by 250 nm excitation. At
this time, it was assumed that the emission spectra obtained by excitation at 260 nm and 250 nm are identical.
Emission lifetime measurements were performed with a HORIBA NAES-550 or IBH 5000U-CS
instrument (for single photon counting) or a JASCO FP-8300 instrument. Single crystal X-ray structural
analyses were carried out using a Bruker AXS SMART APEX II.
3. Results and discussion
3.1. Emission characteristics in the crystalline state
Irradiation with a UV lamp (ex = 254 nm) produced blue emission from crystals of DPhMeOH,
DPhEtOH, DPhPrOH, DPhc-PrOH, DPhc-HexOH and DPhMeOMe that remained visible for several
seconds after the irradiation was stopped, indicating that pRTP was possibly generated (Fig. 2). However, in
the case of the DPhHOH, visible light emission and pRTP could not be observed visually.

<Fig. 2. Photographic images of crystals under white light and emissions under UV
irradiation at ex = 254 nm and after irradiation.>
To assess the solid-state emission characteristics of these compounds in more detail, emission spectra were
acquired at room temperature (298 K). In Fig. 3, the black solid line is the emission spectrum obtained in the
steady state with the excitation at ex = 250 nm, while the red solid line is the time-resolved emission
spectrum 50 ms after the excitation light was cut off. The DPhMeOH, DPhEtOH, DPhPrOH, DPhc-PrOH,
DPhc-HexOH and DPhMeOMe all generated pRTP that was visible to the naked eye. This emission was
characterized by time-resolved emission spectra having a similar vibrational structure around 444-508 nm. In
addition, similar narrow band emission was observed at approximately 310 nm for these same compounds,
and might be attributed to delayed fluorescence caused by triplet-triplet annihilation (TTA) [52], and further
study must be required. In the case of the DPhHOH, which did not exhibit pRTP, only a broad emission
spectrum having no vibrational structure was obtained, consistent with the emission spectrum in a solution
state (Fig. S5).
<Fig. 3. The emission spectra of (a) DPhMeOH, (b) DPhEtOH, (c) DPhPrOH, (d) DPhc-
PrOH, (e) DPhc-HexOH and (f) DPhMeOMe. Black solid lines: steady state emission
spectra, red solid lines; time resolved spectra (delay time: 50 ms, gate width: 25 ms, ex =
250 nm).>
Table 1 summarizes the emission characteristics of the diphenylmethanol derivative in the solid state. It
has been reported that aromatic compounds such as naphthalene have isc values of approximately 1-f when
the energy difference between S1 and S0 (S1 → S0) is at least 2.6 eV [29]. In the present study, this condition
was satisfied, and so the values of kp and knr + kq [Q] presented in this table were calculated based on the isc
= 1-f relationship. It is known that this assumption applies well to condensed polycyclic aromatic
compounds, but it is uncertain whether this assumption is fit to this case because isc is not experimentally
determined for this series of compounds. Here, bimolecular quenching processes by molecular oxygen and
inevitable impurities are summarized as a rate of kq [Q].
<Table 1 Absorption and emission characteristics of diphenylmethanol derivatives in
ethanol and in the solid state.>

The fluorescence quantum yield (f) in the solid state was determined to be higher than that in ethanol
(Table 1). In general, two contrasting luminescence-related phenomena are known to occur in the solution
state. One is aggregation-caused quenching (ACQ), in which aggregates are formed at a high concentration
and the emission quantum yield is decreased [53]. However, nonradiative deactivation is also suppressed by
aggregation, such that the emission quantum yield is increased in an effect known as aggregation-induced
emission (AIE) [54]. Because f was found to be increased when the compounds were in the solid state in this
study, it is considered that a phenomenon like AIE occurred. Since the structural change of the molecule is
suppressed in the crystal and the transition probability from S1 to S0 maybe increased by good HOMO-LUMO
overlap. In addition, f and isc may increase by the decrease of process of non-radiative deactivation.
Focusing on the solid-state phosphorescence characteristics, the p of DPhMeOH was very high at
14.9%, while the values for the other diphenylmethanol derivatives showing pRTP were between 3.8 and
9.1%. The p values were determined to be in the range of 515-766 ms, which are extremely long
luminescence lifetimes for organic compounds at room temperature (Fig. S6). The kp calculated for the
compounds for which pRTP was observed were on the order of 10-1 to 10-2 s-1, while knr + kq[Q] was on the
order of 102 s-1. In prior work, an iridium complex exhibiting RTP was found to have knr + kq[Q] and kp values
as high as 105 s-1 [55]. Therefore, the present p values were large and the p values were small. In our study,
low knr + kq[Q] values were obtained in conjunction with very small kq, and pRTP with long p values were
observed at room temperature in ambient air.
3.2. Luminescence in ethanol
The UV-Vis absorption and emission spectra of the diphenylethanol derivatives showing pRTP in ethanol
solutions (at 0.3 mM) are presented in Fig. S7. Each spectrum was found to have absorption maxima in the
ranges of 211-215 nm and 258-259 nm. Based on the molar extinction coefficient for each molecule (Table 1),
these absorption maxima are consistent with * transitions at the benzene sites, and the less intense
absorption band can be attributed to a symmetric forbidden transition [56].
A broad emission band without a vibrational structure was observed over the range of 282-286 nm for all
compounds. In addition, the luminescence properties in solution were almost the same for all the derivatives,
including DPhHOH, which did not show pRTP (Fig. S5).
3.3. Luminescence in an ethanol glassy matrix
The emission spectra of the diphenylmethanol derivatives in a glassy matrix were acquired in ethanol (at
0.3 mM) at 77 K (Fig. 4). At room temperature, only an emission maximum in the UV region was obtained, as
also discussed in the preceding section, while a new blue emission was obtained in the glassy matrix. At 77 K,
nonradiative deactivation was effectively suppressed by limiting the thermal motion of the molecules [13].
The emission characteristics in these glassy matrix states were almost the same for all derivatives including
DPhHOH (Fig. S5).

<Fig. 4. The solution state emission spectra of (a) DPhMeOH, (b) DPhEtOH, (c)
DPhPrOH, (d) DPhc-PrOH, (e) DPhc-HexOH and (f) DPhMeOMe (in ethanol, 0.3 M)
as acquired at room temperature and 77 K (ex = 250 nm).>
In Fig. 5, the luminescence spectra of DPhMeOH in the solution state and the solid state are plotted
against the energy values (in eV). Here, S1 and T1 are the energy levels associated with the fluorescence and
phosphorescence maxima in the solution and glassy matrix and S1* and T1* are the energy levels calculated
from the fluorescence and phosphorescence peaks in the time-resolved emission spectrum obtained from the
solid state.
<Fig. 5. Emission spectra and energy diagrams of DPhMeOH in an ethanol matrix at 77 K
and in the crystalline state at ambient temperature.>
In the solid state, light emission occurs based on transitions from the S1* and T1* states. The
energy difference between S1 and S1*, E (S1S1*), is 0.40 eV while that between T1 and T1*, E
(T1T1*), is 0.75 eV. These values indicate that the triplet state was more stabilized than the singlet
state. This trend was found to be common to all the compounds that showed pRTP in this study, and
all showed equivalent emission behaviour. These findings suggest that the excited state was stabilized
by the common structure involved in the examined molecules. That is, phosphorescence generated
from the stable T1* state was observed as pRTP. Further detailed investigation is required to
understand energy aspect of pRTP. In addition, nonradiative transition obeys to energy gap low. In
this sense, stabilization of T1* state may affect on knr. However, these compounds may have less
susceptible small knr.
The time-resolved emission spectra acquired from these compounds in the solid state were similar,
although the steady-state emission behaviour was different for each compound. In the case of each of
DPhEtOH, DPhc-PrOH and DPhc-HexOH, strong fluorescence resulting from S1 (like the state in solution)
was observed, whereas DPhMeOH also exhibited significant fluorescence but from the S1* state (stabilized
state). The DPhPrOH and DPhMeOMe showed both type of intense fluorescence from S1 and the S1* state.
Therefore, in the solid state, light emission from single molecules was observed just as in the solution state,
and so photophysical processes from the S1, S1*, T1 and T1* states occurred. There were differences between
the compounds, and no emission from T1 was observed in the case of phosphorescence, indicating that
emission from T1* occurred preferentially. The melting points of DPhPrOH and DPhMeOMe, for which
fluorescence from S1 and S1* was observed, were 37 and 35 °C, respectively, and so were lower than those of

the other diphenylmethanol derivatives (DPhHOH: 67 °C, DPhMeOH: 81 °C, DPhEtOH: 95 °C, DPhc-
PrOH: 86 °C, DPhc-HexOH: 68 °C)
This suggests that weak intermolecular interactions in the solid state were related to the appearance of
fluorescence.
3.4. Luminescent properties in PMMA thin films
Kim et al. reported that pRTP was observed upon dispersing a phosphorescent material in a poly(methyl
methacrylate) (PMMA) matrix, which provided a rigid environment that suppressed radiationless deactivation
[57]. We also employed a PMMA matrix in conjunction with the diphenylmethanol derivatives (Fig. S8). Each
of these compounds generated a fluorescence spectrum having an emission maximum similar to that seen in
the solution state, and no phosphorescence was observed in spite of the use of a more rigid environment than
in the ethanol solutions. In the case of fluorene derivative, energy transfer from the compound to the PMMA
was observed [58]. However, in this case no further detailed experiment was performed. This finding suggests
that a rigid environment alone was insufficient to promote pRTP, and that the specific structure in the
crystalline state may be a major factor related to inducing this type of emission.
3.5. Single crystal X-ray structural analysis
In order to obtain detailed information regarding the molecular packing structures, we attempted to
prepare single crystals of each compound by a solvent evaporation method [59]. Single crystals of
DPhMeOH, DPhEtOH, DPhc-PrOH and DPhc-HexOH were obtained by allowing saturated hexane
solutions of each compound to stand in the dark at room temperature. Single crystals of DPhHOH were
obtained in the same manner from a saturated nitromethane solution. The resulting crystallographic data are
summarized in Fig. 6 and Table S1 [60].
<Fig. 6. Single crystal structure packing diagrams for (a) DPhMeOH, (b) DPhEtOH, (c)
DPhc-PrOH, (d) DPhc-HexOH and (e) DPhHOH. Yellow or orange molecules in (a), (d)
and (e) are in the adjacent crystal lattice.>
All the compounds that showed pRTP had similar structures (two benzene rings and a hydroxy group), and
the luminescent behaviour of the crystals showed a similar pRTP emission that was clearly different from that
obtained in the PMMA thin film state or solution state. These data suggest that pRTP resulted from a common
interaction in the crystal structures of these compounds. The structures of those achiral molecules in the
crystals are taking pseudo racemic conformations, which is a pair of mirror images (Fig. S9).
The effect of intermolecular interactions can be discussed by assessing the single crystal structures of
DPhMeOH, DPhEtOH, DPhc-PrOH, DPhc-HexOH and DPhHOH, all of which gave single crystals. It can

be assumed that hydrogen bonds or  interactions were present between pairs of molecules in these crystals
regardless of the unit cell or Z value, with the exception of DPhHOH, which showed a columnar stacking
structure. The intermolecular interactions in the crystal structures (excluding the face-to-face  interactions)
and the associated distances (in Å) are shown in Fig. 7. Here, values in red indicate that the distances between
atoms are shorter than the sum of the van der Waals radii, while values in parentheses for the edge-to-face
interactions represent the angles between the plane of the benzene ring and the line connecting the hydrogen
with the centre of the ring (Fig. S10a). In previous studies, compounds that expressed pRTP were found to
form hydrogen bonds in the crystal [13]. However, even though all the present compounds except
DPhMeOMe had a hydroxy group, only DPhMeOH, and DPhHOH incorporated a hydroxy group
positioned so as to form a hydrogen bond. Since the distance between the oxygen atoms in the two hydroxy
groups on adjacent molecules was greater than 0.27 Å, which equates to a typical hydrogen bonding distance
[61], it appears that weak hydrogen bonds were present. In addition, DPhMeOMe (in which the hydroxy
group is replaced by a methoxy group) also showed pRTP, meaning that hydrogen bonding was not essential
for pRTP to be generated by the compounds of this study. Although there was no strong interaction common
to all these molecules, such as hydrogen bonding, there were various weak interactions in each crystal
structure, as a result of intermolecular forces and steric restrictions, that suppressed nonradiative deactivation
in the crystalline state.
<Fig. 7. Coupled A, B, C, and D intermolecular interactions in the crystals of (a)
DPhMeOH, (b) DPhEtOH, (c) DPhc-PrOH, (d) DPhc-HexOH and (e) DPhHOH.
Colors in parentheses correspond to the molecular color in the packing diagram in Fig. 6.>
We next considered  interactions, which were the only common interactions among these
diphenylmethanol derivatives. These  interactions were based on a stacking structure resulting from edge-
to-face interactions in the case that the benzene rings were arranged in a T shape or face-to-face interactions
where the benzene rings were arranged in parallel [62]. Because there have been many reports that pRTP is
caused by face-to-face C interactions [14, 22], the crystal substructures having the strongest face-to-
face  interaction forces are presented in Figs. 8 and 9. Here,  is the angle between the planes of the two
benzene rings and d is the distance between the centres of gravity of the two benzene rings (in Å). With regard
to the values in parentheses, the first is the shortest distance between the carbon atoms of the two facing
benzene rings while the second is the distance between the centre of gravity of the benzene ring and the
intersection of perpendicular lines drawn on the planes of the benzene rings facing each other (Fig. S10b). If
both values are small, this indicates a larger degree of overlap of the two benzene rings and thus a stronger
interaction.

<Fig. 8. Coupled C, B, A and B intermolecular interactions in crystalline (a) DPhMeOH,
(b) DPhEtOH, (c) DPhc-PrOH and (d) DPhc-HexOH, respectively.>
<Fig. 9. Coupled B (lower), C (upper left) and D (upper right) intermolecular interactions
in DPhHOH.>
It has been established that phenylboronic acid derivatives exhibiting pRTP show long-range
face-to-face interactions even when the distance between the centres of gravity of the two
benzene rings is in the range of 5.0-6.0 Å [63]. In the present work, the distance between the centres
of gravity was in the same range, and the two benzene rings were arranged almost in parallel.
Assuming that the strength of the face-to-face  interactions depends on the distance between the
centres of gravity, the strength should decrease in the order of DPhHOH
>
DPhMeOH
> DPhEtOH
>
DPhc-PrOH DPhc-HexOH. Although there have been reports that the
>

p and p values

associated with pRTP are proportional to the strength of the face-to-face  interactions [22], no
correlation was found in this work. In addition, although the DPhHOH did not show pRTP, it had the
strongest interactions. Therefore, we assessed another common phenomenon: edge-to-face 
interactions. Assuming that the strength of these interactions was dependent on the distance between
the nearest hydrogen atom and the benzene ring, the order should decrease in the order of DPhc-
PrOH
>
DPhEtOH
>
DPhHOH
>
DPhc-HexOH
>
DPhMeOH. However, there was no evident
p andp are determined by knr,
correlation between the luminescent properties and this order. The

which in turn is suppressed by the restricted molecular motion that results from intermolecular
interactions. However, it is interesting to note that the pRTP emission appeared at almost the same
wavelength for each compound, although the interactions between molecules were not consistent
despite the similar structures of these molecules.
The present crystals composed of achiral molecules had a fixed conformation and their space groups
were either chiral (DPhMeOH) or racemic (with the exception of DPhMeOH). Examining pairs of molecules
that may interact from a fixed conformation established that either racemic or chiral pairs were obtained (Fig.
7). Racemic pairs such as those produced by DPhMeOH (Coupled A, B and C), DPhEtOH (Coupled A and
B) and DPhc-PrOH (Coupled A and B) made greater contributions to the stabilization of the crystal
compared to pairs having the same chirality, based on both CH/ and  interactions. These pairs are
believed to have functioned as cages that trapped and confined the excitation energy. Compact packing of
racemic crystals is known as the Wallach’s rule [64–66].
The crystal structure of DPhHOH, which had a similar structure to those of the other diphenylmethanol
derivatives but did not generate pRTP, indicates the presence of  interactions and strong intermolecular
forces. In addition, a continuous column of Coupled C and Coupled D molecules can be seen along the c-axis
in Fig. 10 (The molecules in each column, shown in blue and green, have the same chirality and adjacent

columns are composed of different enantiomers). This packing structure resulted in the strongest face-to-face
 interactions among the diphenylmethanol derivatives assessed in this work. In addition, there was a
continuous packing structure both between pairs of molecules and between greater numbers of molecules
stacked on top of each other. It is therefore probable that pRTP was not observed from this compound due to
ACQ [28, 67]. This column like structure composed of strong packing by the same enantiomer may cause
delocalization of triplet excitation energy or migration of triplet exciton. However, excitation energy was
confined in racemic pairs of such as DPhMeOH as similar to the concept of multimer proposed to the singlet
excited state [50, 51]. Based on the above discussion,  interactions are evidently an important factor in
the appearance of pRTP, but further investigation is needed to clearly determine the structural characteristics
that cause pRTP. We already started experiment along this direction. In particular, the possibility of
preventing ACQ should be examined.
<Fig. 10. Aggregate formation by hydrogen bonding in DPhHOH.>
3.6. DFT calculations
Using the crystal structures obtained by single crystal X-ray structural analyses, TD-DFT calculations
were performed for both the ground and excited states of single molecules and pairs of molecules [68]. The
M062X functional and 6-31G (d) basis function were used [14]. The calculation results are summarized in Fig.
11, Fig. S11-S15 (MO contour of the coupled molecules) and Table S2-S16. It is generally believed that two
factors are necessary for efficient S1 → Tn ISC. The first is that the energy gap between S1 and Tn must be
within 0.3 eV, the second is that S1 and Tn must have the same symmetry components capable of enabling ISC
[14,23].
<Fig. 11. Tn states related to ISC in (a) DPhMeOH, (b) DPhEtOH, (c) DPhc-PrOH and
(d) DPhc-HexOH.>
The Tn shown in red in Fig. 11 are those that promote ISC based on the DFT calculations. The data for
the four compounds that showed pRTP indicate that the number of Tn capable of enabling ISC was greater for
pairs of molecules than for single molecules. As such, a crystal structure existed for each that promoted ISC to
a greater extent than in a dispersed state (such as in solution) even efficiency of each process was unclear. In
future, calculation by using spin orbit coupling matrix element (SOCME) between S1 and Tn [69]. Further
discussion about ISC in the crystalline state will be done after accumulating more data in future.
4. Conclusions

In this study, we found that pRTP was exhibited by DPhMeOH
DPhc-HexOH DPhPrOH and DPhMeOMe in their crystal states. This is a specific type of
luminescence that is observed only in the crystal state and is different from the luminescence
, DPhEtOH, DPhc-PrOH,
,
generated in a state in which the molecules are dispersed, such as in a solution, glassy matrix or
PMMA. It has been suggested that this is caused by a unique structure formed in the crystal.
These crystal structures generated pRTP regardless of the presence or absence of weak hydrogen
bonds, and nonradiative deactivation of the molecules was suppressed by intermolecular interactions
and steric regulation in the crystals. As such, the emission quantum yield was improved. Those
crystalline materials that expressed pRTP had racemic space groups, and the interaction between
racemic pairs played a major role. Computational may propose that ISC was promoted in the
crystalline state because the number of transition states promoting ISC was greater than in the
dispersed state even efficiency of each process was unclear. Therefore, this is considered to be one of
the factors that caused pRTP to appear.
The molecules studied in the crystalline state were found to possess newly stabilized S1* and T1*
states, and emissions from S1, S1* and T1* states were found at ambient temperature. It is likely that
observed decreases in emission energy were caused by interactions between racemic pairs of
molecules.
Authorship statement
All authors contributed in the following manner:
Conception and design of study, analysis and/or interpretation of data. Revising the manuscript critically
for important intellectual content.
M. Y. contributed in the acquisition of data and analysis, T. I. contributed in the acquisition and analysis of
X-ray single crystallography.
Declaration of Competing Interest
We declare no conflict of interest.
Acknowledgments
This work was supported by a KAKENHI Grant-in-Aid for Scientific Research (no. 26288111) from the
Ministry of Education, Culture, Sports, Science and Technology (MEXT) and also received funding from the
Strategic Priority Research Promotion Program (Chiral Materials Science) at Chiba University. This work was
also performed under the Five-star Alliance Research Program within NJRC Materials & Devices. The
authors thank Professors Masami Sakamoto and Hyuma Masu and Dr. Naohiro Uemura for providing support
during the single crystal X-ray structural analyses. The authors also acknowledge the Center for Analytical

Instrumentation for performing absolute emission quantum yield measurements, single photon counting, 1H
NMR spectroscopy and MS analyses.
Appendix A. Supplementary data Supplementary material related to this article can be found, in the online
version, at doi:https://doi.org/10.1016/j.jphotochem.2020.

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Table 1
Absorption and emission characteristics of diphenylmethanol derivatives in ethanol and in the solid statea).
In ethanol
Solid state
f
Absorption max / nm
 / M-1cm-1)
f
f
p (77K)

f
p
p
T
kp
knr+kq[Q]
/ s-1
0.24
1.6
/ nm
282
283
283
282
286
282
282
/ %
3.1
3.3
3.0
6.0
2.5
4.4
3.9
/ nm
/ nm
/ %
/ nm
/ %
8.3
6.0
3.8
8.6
7.6
9.1
N.D.
/ ms
552
515
589
648
654
766
N.D.
/ s-1
1.6
DPhMeOH
DPhEtOH
213 (8990),
214 (9230),
213 (9100),
213 (8850),
211 (9160),
215 (9280),
213 (9230),
258 (360)
350, 370, 384
349, 371, 383
349, 370, 383
350, 370, 384
349, 384
310
90.4
61.1
46.4
62.2
75.5
67.7
55.4
444, 475, 503
448, 478, 504
448, 479, 507
450, 478, 504
449, 480, 508
449, 480, 508
N.D.
259 (400)
259 (383)
258 (350)
259 (373)
258 (453)
259 (417)
285
0.30
0.12
0.35
0.47
0.37
−
DPhPrOH
284, 309
291
1.6
DPhc-PrOH
DPhc-HexOH
DPhMeOMe
DPhHOH
1.2
285
1.1
351, 372, 385
349, 369, 384
285, 308
283
0.94
−
a) For data analysis, relation of f is assumed, and also following equations were used, kp

isc = 1 −

p / (isc T) and knr + kq[Q] = (1 / T) − kp.
×


Fig. 1. The 1,1-diphenylmethanol derivatives examined in the present

Fig. 2. Photographic images of crystals under white light and emissions under UV irradiation at ex = 254
nm and after irradiation.

Fig. 3. The emission spectra of (a) DPhMeOH, (b) DPhEtOH, (c) DPhPrOH, (d) DPhc-PrOH, (e) DPhc-
HexOH and (f) DPhMeOMe. Black solid lines: steady state emission spectra, red solid lines; time resolved
spectra (delay time: 50 ms, gate width: 25 ms, ex = 250 nm).

Fig. 4. The solution state emission spectra of (a) DPhMeOH, (b) DPhEtOH, (c) DPhPrOH, (d) DPhc-
PrOH, (e) DPhc-HexOH and (f) DPhMeOMe (in ethanol, 0.3 M) as acquired at room temperature and
77 K (ex = 250 nm).

Fig. 5. Emission spectra and energy diagrams of DPhMeOH in an ethanol matrix at 77 K and in the
crystalline state at ambient temperature.

(a)
(b)
(c)
(d)
(e)
Fig. 6. Single crystal structure packing diagrams for (a) DPhMeOH, (b) DPhEtOH, (c) DPhc-PrOH, (d)
DPhc-HexOH and (e) DPhHOH. Yellow or orange molecules in (a), (d) and (e) are in the adjacent crystal
lattice.