Crystal structure and novel solid-state fluorescence behavior of the model benzoxazine monomer: 3,4-Dihydro-3,6-dimethyl-1,3,2H-benzoxazine

Article abstract of DOI:10.1016/j.molstruc.2014.05.057

Solid-state fluorescent behavior of the model benzoxazine monomer, 3,4-dihydro-3,6-dimethyl-1,3,2H-benzoxazine denoted as MM, was firstly reported. Three characteristic fluorescence peaks were observed, i.e., (1) the ultraviolet peak at 317 nm with the excitation wavelength of 258 nm, (2) the blue emission peak at 450 nm with the excitation wavelength of 324 nm and (3) the other blue fluorescent peak at 454 nm with the excitation wavelength of 360 nm. It was proved that the benzoxazine core was responsible for the fluorescence of blue light due to the absence of the emission peak in the ring-opening benzoxazine dimer with the same moieties as the benzoxazine monomer MM. The single crystal X-ray structure of the compound MM recorded at 100 K showed that the irregular chair conformation of the ring-closure benzoxazine MM gave rise to the fluorescent property since all the members of the oxazine ring other than nitrogen atom arranged themselves at the plane of the benzene ring.

Full text of DOI:10.1016/j.molstruc.2014.05.057

Accepted Manuscript
Crystal Structure and Novel Solid-State Fluorescence Behavior of the Model
Benzoxazine Monomer: 3,4-dihydro-3,6-dimethyl-1,3,2H-benzoxazine
Worawat Wattanathana, Sutharat Nonthaglin, Chatchai Veranitisagul,
Nattamon Koonsaeng, Apirat Laobuthee
PII:
S0022-2860(14)00571-7
DOI:
http://dx.doi.org/10.1016/j.molstruc.2014.05.057
MOLSTR 20656
Reference:
To appear in:
Journal of Molecular Structure
Received Date:
Revised Date:
Accepted Date:
26 March 2014
21 May 2014
23 May 2014
Please cite this article as: W. Wattanathana, S. Nonthaglin, C. Veranitisagul, N. Koonsaeng, A. Laobuthee, Crystal
Structure and Novel Solid-State Fluorescence Behavior of the Model Benzoxazine Monomer: 3,4-dihydro-3,6-
dimethyl-1,3,2H-benzoxazine, Journal of Molecular Structure (2014), doi: http://dx.doi.org/10.1016/j.molstruc.
2
014.05.057
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Crystal Structure and Novel Solid-State Fluorescence Behavior of the Model
Benzoxazine Monomer: 3,4-dihydro-3,6-dimethyl-1,3,2H-benzoxazine
a
b
c
a
Worawat Wattanathana , Sutharat Nonthaglin , Chatchai Veranitisagul , Nattamon Koonsaeng ,
b,*
Apirat Laobuthee
a
Department of Chemistry, Faculty of Science, Kasetsart University, Ladyao, Chatuchak, Bangkok
1
0900, Thailand
b
Department of Materials Engineering, Faculty of Engineering, Kasetsart University, Ladyao,
Chatuchak, Bangkok 10900, Thailand
c
Department of Materials and Metallurgical Engineering, Faculty of Engineering, Rajamangala
University of Technology Thanyaburi, Pathumthani 12110, Thailand
Author to whom correspondence should be addressed; Email: fengapl@ku.ac.th
Abstract
Solid-state fluorescent behavior of the model benzoxazine monomer, 3,4-dihydro-3,6-dimethyl-
1,3,2H-benzoxazine denoted as MM, was firstly reported. Three characteristic fluorescence peaks
were observed, i.e., (1) the ultraviolet peak at 317 nm with the excitation wavelength of 258 nm, (2)
the blue emission peak at 450 nm with the excitation wavelength of 324 nm and (3) the other blue
fluorescent peak at 454 nm with the excitation wavelength of 360 nm. It was proved that the
benzoxazine core was responsible for the fluorescence of blue light due to the absence of the
emission peak in the ring-opening benzoxazine dimer with the same moieties as the benzoxazine
monomer MM. The single crystal X-ray structure of the compound MM recorded at 100 K showed
that the irregular chair conformation of the ring-closure benzoxazine MM gave rise to the
fluorescent property since all the members of the oxazine ring other than nitrogen atom arranged
themselves at the plane of the benzene ring.
Keywords: Benzoxazine monomer, X-ray crystallography, Fluorescent spectroscopy
Highlights
u
u
u
1
The crystal structure of the compound 3,4-dihydro-3,6-dimethyl-1,3,2H-benzoxazine (MM)
recorded at low temperature was reported.
The structure parameters of the compound MM and the parameters obtained from preceding
literatures were compared.
Solid-state fluorescence behavior of the compound MM was firstly investigated.
. Introduction
Benzoxazines are a sort of heterocyclic compounds which have the core structure of benzene
ring fused with oxazine ring which can readily synthesized by the Mannich reaction of mixing three
components, namely, primary amines, paraformaldehyde, and phenols [1-5]. Many possible
combinations of the precursors are available leading to the diversity of molecular design. The
desired properties of the benzoxazine monomers can be accomplished by adjusting their functional
and substitution groups. Moreover, the importance of these compounds is for producing the
corresponding polymers called polybenzoxazines. Like the properties of benzoxazine monomers, the

properties of polybenzoxazines can also be fabricated. Numerous syntheses and applications of
benzoxazine monomers and their polybenzoxazines with different functional groups were reported
[6-19].
However, there are only few serious works on spectroscopic aspects of benzoxazines. The
published works on these points covered only in the field of vibrational [20-21], nuclear magnetic
resonance [22] and ultraviolet-visible [23-24] spectroscopies. Apart from that, no work on the
fluorescent spectroscopy of the benzoxazine monomers has been reported up to now. Recently, our
group accidentally discovered the fluorescent emission of the crude products from synthesizing
benzoxazine monomers when exposed to ultraviolet radiation. To make it clear whether the
fluorescence is from the benzoxazine derivatives or not, the model benzoxazine monomer; 3,4-
dihydro-3,6-dimethyl-1,3,2H-benzoxazine (MM), is synthesized and crystallized to provide the pure
crystals that are suitable for studying both the fluorescent property and the single crystal X-ray
analysis. The purpose of this study is, therefore, to clarify the detail of solid-state fluorescence
behavior of the model benzoxazine MM and to identify the important part of the molecule MM that
contributes to its fluorescent property by using the information from its crystal structure.
2
. Experimental details
2
.1. Materials
Paraformaldehyde was purchased from Fluka while 40% aqueous solution of methylamine,
4
-methylphenol and sodium hydroxide were supplied by Merck. Solvents 1,4-dioxane and
dichloromethane were obtained from LAB-SCAN. All reagents and solvents were analytical grade and
used without further purification.
2
.2. Synthesis
The compound 3,4-dihydro-3,6-dimethyl-1,3,2H-benzoxazine (MM) was synthesized by the one
pot Mannich reaction of phenol, formaldehyde and amine with the molar ratio of 1:2:1 [25]. In this
work; however, the molar ratio of phenol, formaldehyde and amine was adjusted to 0.9:2.2:1.2 to
make sure that all the phenol was used up. Therefore, 10.8 g (330 mmol) of paraformaldehyde was
transferred to the round bottom flask with dioxane solvent (60 mL) then 15.5 mL (180 mmol) of
aqueous methylamine was added. The reaction mixture was stirred to allow iminium ions to form for
2
0 minutes. Next, 14.6 g (135 mmol) of 4-methylphenol was added to the solution containing
iminium and the unreacted paraformaldehyde. The reaction mixture was heated for 6 hours to
complete. At the end, the reaction mixture turned yellow and clear. Dichloromethane (70 mL) was
added to the yellow solution obtained and then the solution was extracted by using
3
0 mL of 3N NaOH for three times together with 30 mL of distilled water for three times to remove
phenolic, formaldehyde and amine impurities. After extraction, the pale yellow solution obtained
was then evaporated by rotary evaporator to remove dichloromethane and dioxane solvents. When
left at room temperature for a day, the solution crystallized to colorless crystals of the compound
MM.
2
.3. Characterization
Fourier-transform infrared (FT-IR) spectroscopy
The FT-IR spectrum of the prepared benzoxazine was recorded by Alpha FT-IR spectrometer
from Bruker using ATR mode. The one hundred FT-IR scans of the sample were taken in the range of
-
1
-1
4
000-550 cm at the resolution of o2 cm .

1
Proton nuclear magnetic resonance ( H-NMR) spectroscopy
The synthesized benzoxazine (about 10-20 mg) was dissolved in 1 mL of CDCl .
3
Tetramethylsilane (TMS) was dropped to the solution as an internal standard. NMR measurement
was carried out by Varian INOVA spectrometer with a proton frequency of 400 MHz.
FTIR: 3000-2800 (w, CH), 1498 (oxazine ring), 1226 (s, CN), 1117 (m, CO)
NMR: D
H
3
2.26 (3H, s, Ar-CH ), 2.60 (3H, s, N-CH
3
), 3.92 (2H, s, Ar-CH
2
-N), 4.76 (2H, s, O-CH -N),
2
6
.69 (H, d, Ar-H, J = 2.10 Hz), 6.77 (H, s, Ar-H), 6.93 (H, d, Ar-H, J
3
3
= 2.10 Hz)
2.4. Single crystal X-ray diffraction
The rod shaped colorless crystal of MM with the size of 0.15r0.16r0.58 mm was selected to
mount on the tip. The measuring temperature was set at 100 K (under the flow of liquid nitrogen)
and the measuring conditions employed were set to a scan of 0.5 degree with the measuring time of
20 seconds/frame. All the data were collected on a Bruker APEXII CCD diffractometer controlled by
APEX2 software and the cell refinement and data reduction were carried out by SAINT [26]. The
programs used for solving and refining structure were SHELXS97 and SHELXL97, respectively [27]. All
non-hydrogen atoms were solved anisotropically. All H atoms were located in different Fourier map
and restrained to their hosts. The ortep plot for the structure of MM was drawn by Ortep-3 program
[
28] and the crystal packing of MM was obtained from Mercury 3.3 program [29].
2
.5. Study on fluorescent property
The pure crystals of compound MM were ground to get a fine powder. About 0.1 g of the
powder of the compound MM was packed in the sample holder before the measurement. All the
fluorescent spectra were taken from Perkin-Elmer spectrofluorometer (LS 55 Luminescence
spectrometer).
3
. Results and discussion
.1. Single crystal X-ray diffraction
The crystallographic data (CCDC#969107) for this manuscript can be obtained from The
3
Cambridge Crystallographic Data Centre via http://www.ccdc.cam.ac.uk. The molecular structure
including atom numbering for the compound MM is illustrated in Figure 1 whereas the crystal data
and the structure refinement for the compound MM are shown in Table 1. The ortep plot of the
compound MM showed that, the C , C , C , C and O in the oxazine ring were closely oriented in the
5
6
7
8
1
plane which was in line with the benzene ring while the N atom of the oxazine was nearly
1
perpendicular to the oxazine ring containing the rest of atoms. The orientation of oxazine members
generated the irregular chair conformer containing ring strain. The irregular chair structure of
oxazine ring seem to be the characteristic of the benzoxazine compounds since this feature was
observed in all the chosen structures from the previously reported derivatives [30]. The crystal
packing of the compound MM shows 4 molecules of MM in a unit cell. Two kinds of the attractions,
namely C-H•••pi and C-H•••O interactions, were found in the crystal packing of the compound MM.
Each MM molecule formed two contacts for both C-H•••pi and C-H•••O interactions (Figure 2(a)). For
C-H•••pi contacts, the MM molecule used the hydrogen of the methyl group attached to the nitrogen
atom of the oxazine ring (H
Moveover, the MM molecule also used its C
of another molecule. For C-H•••O contacts, the MM molecules formed two C-H•••O contacts via the
methylene hydrogen (H ) and the oxygen atom (O ). The C-H•••pi and C-H•••O contacts in the crystal
9
) to contact with the carbon C
4
of the benzene ring of another molecule.
4
atom to accept the interaction with methyl hydrogen
7
1

packing of the compound MM are displayed in Figure 2(b) and (c). The contact lengths of C-H•••O
interactions (2.531 Å) were shorter than those of C-H•••pi contacts (2.880 Å) (Table 2). Figure 2(d)
illustrates that two C-H•••pi contacts linked the motifs within the unit cell while the C-H•••O contacts
connected to the motifs of the other unit cell.
Insert Table 1
Insert Figure 1
Insert Figure 2
It was noteworthy understanding the effects of substituents on both benzene and nitrogen
moieties on the structure parameters of the molecules especially the parameters that were belong
to the oxazine part of the molecules. To clarify this issue, the data comparison was needed to be
constructed using the results from many previously published works [30-35]. The structure
parameters obtained from various sources had the different ways of numbering. It might cause the
complication for comparing data. In this work, the methods of numbering for core structures of both
the monobenzoxazine and dibenzoxazine derivatives were established as depicted in Scheme 1
whereas the structures of the preceding benzoxazines were in Scheme 2. The selected structural
parameters related to the oxazine part were listed in Table 3. Considering the carbon-oxygen bond
lengths in the oxazine moieties of all the selected compounds, we found that the bond lengths of
oxygen to two of its adjacent carbon atoms were significantly different. The one that was from the
C (1.438-1.468 Å) was longer than the other one from C (1.362-1.383 Å). All the compounds
7
6
chosen were 1,3-benzoxazine derivatives where the oxygen of the oxazine heterocyclic part was
connected with the carbon of the benzene ring and bonded to another methylene carbon. The
2
3
benzene carbon bonded the oxazine oxygen by overlapping its sp orbital with the sp orbital of the
3
oxygen while the methylene carbon used the sp orbital to form bond. It was obvious that the bond
3
3
2
3
made by sp -sp orbitals was longer than the one from sp -sp orbitals. Moreover, when looked
more closely with the O -C bond of the selected compounds, the bond distance was sensitive to the
electron-withdrawing group on the benzene ring. Introduction of the electron-withdrawing group to
the benzene ring shortened the O -C bond distance, seeing that the O -C found in the compound 3
the compound containing two electronegative chlorine atoms) was shortest. The decrease in the
1
6
1
6
1
6
(
bond length was because the electron-withdrawing chlorines pulled the electron cloud out of the
benzene ring led the oxygen atom to contribute its lone pair electrons to the benzene ring. In other
word, the oxygen atom was forced to donate its electrons to the ring then the oxygen orbital
overlapped more strongly to the C orbital of the benzene ring and, therefore, formed a shorter
6
bond. Not only the carbon-oxygen bonds of the benzoxazines but the nitrogen-carbon bonds also
showed the trends that would be a characteristic of this group of compounds. The nitrogen atom of
the benzoxazines bonded with three carbon atoms from miscellaneous moieties, namely, (1) the
methylene carbon that bridged two hetero atoms nitrogen and oxygen (C
carbon which linked the hetero atom nitrogen with the benzene carbon (C ) and (3) the carbon from
variety moieties that were derived from the amine reagent used for synthesizing benzoxazines (C ).
The general trend observed for the carbon-nitrogen bonds were, although there were three
categories of carbon-nitrogen bonds (N -C , N -C and N -C ), the distances of these three kinds were
7
), (2) the methylene
8
9
1
7
1
8
1
9

drastically different. For the compound MM and all the compounds selected, the N
-C
1 7
bonds (1.419-
1
.432 Å) were remarkably shorter than the N -C bonds (1.463-1.478 Å). Unlike the difference in
1
8
distance of carbon-oxygen bonds, the different type of hybridization was not the reason since two
3
methylene carbons adopted the same type of hybridization (sp ) to bond to the orbitals of nitrogen
atom. The distances of the N
value of regular C-N bond (1.469 Å) [36]. The third type of nitrogen-carbon bond was N
1
-C
8
bond lied in range from 1.463 to 1.478 Å which were close to the
-C bond.
1
9
The distances of this bond kind depended on the hybridization of the C
groups with sp hybrid orbital bonded further than the aryl carbon with sp hybridization. From
9
atom. The carbon of alkyl
3
2
Table 3, the N
group in the reference compound (1) were 1.469 and 1.472, respectively, while the N
distances varied around 1.406-1.427 Å with the different aryl moieties. In summary the N
lengths contracted while the O -C expanded compared with standard data form the previous
literature [36]. The result showed that the C atom was the most susceptible atom to undergo
chemical reactions to relax the distortion of its bonds. The contraction of N -C bonds made them
harder to be broken but the elongation of the O -C bonds caused them more readily to be broken.
Thus, these characteristic structure parameters suggested that when the C atom underwent
chemical reactions, its bond with the oxygen O would be broken rather than the bond connected to
the nitrogen N . Many works where benzoxazine derivatives reacted by breaking the O -C bond
1
-Calkyl bond distances for methyl substituent in the compound MM and for benzyl
1
-Caryl bond
1
-C bond
7
1
7
7
1
7
1
7
7
1
1
1
7
such as reverse Mannich reaction [1], ring opening dimerization [25] and ring opening
polymerization [37] were in agreement with the discussed bond lengths.
Bond angles of the benzoxazine compounds were the other significant point to consider. Table
3
shows the bond angle values at the oxazine ring of many benzoxazines. Most of the bond angle
values were in a good agreement with the basic concept of hybridization of organic compounds. For
o
example, the bond angles C
angle of the sp -hybridized atom because C
6
-C
5
-C
8
and O
1
-C
6
-C
and C
5
were near the angle of 120 which was the regular
2
2
5
6
were sp -hybridized atoms. However, there were
some angles which had the large difference from theoretical values, i.e., O
1 7 1
-C -N , C
5 8
-C -N
1
and
C
6
-O
1
-C
7
. The centre atoms of the three kind of distorted angles were C
7
, C
8
and O
1
, respectively.
3
From the concept of hybridization, the atoms C
angles were likely to be near 109.5 . Nevertheless, the experimental values were deviated to about
7
, C
8
and O
1
adopt sp hybridization so their bond
o
o
o
o
o
1
1
13 for the O
19.93(13) for the C
-C -N
1 7 1
angle, from 109.7(4) to 112.24 for the C
-C -N
5 8 1
angle and from 113.2(3) to
o
6
-O
1
-C
7
angle. The distortion of the mentioned angles can be explained as the
and O which were the oxazine members to adjust themselves to form
attempt of the atoms C
7
, C
8
1
the irregular chair conformation. Forming the irregular chair conformation seemed to have the
smallest impact on the C -C -N angle since the O -C -N and C -O -C angles distorted more. The
larger deviation of the O -C -N and C -O -C angles indicated benzoxazines were inclined to undergo
ring-opening reaction to relieve their ring strain. From the bond length data mentioned above, the
most elongated bond was O -C bond and the most deviated bond angle was the angle with the O
centre, it can be concluded that the most susceptible part of the benzoxazine monomers was,
therefore, at the O and C atoms. Apart from that, the angle of C -O -C bond was strongly
dependent on the electronic effect of the substituent on the benzene ring. As the electron with
5
8
1
1
7
1
6
1
7
1
7
1
6
1
7
1
7
1
1
7
6
1
7
drawing group attached the benzene ring, the C
6
-O
1
-C
7
bond angles expanded. Using the
o
benzoxazine compound 3 as an example, its C
6
-O
-C
1 7
bond angle was 119.93 which was close to the
2
bond angle of the sp -hybridized centre since two electron-withdrawing chlorines pull the electron
out of the benzene ring connected to the O
the ring leading to more sp character and expansion of the C
1
atom. The O
1
atom contributed its lone pair electrons to
-O -C bond. This result was confirmed
2
6
1
7

by the shortening in the C
remarkable influence on the angle of the O
effect on the bonds with the oxazine nitrogen centre. The angles with the nitrogen centre (C
6
-O
1
bond. Even though the formation of the chair orientation has a
-C -N , C -C -N and C -O -C bonds, It had a smaller
-N -C
1
7
1
5
8
1
6
1
7
7
1
8
,
7 1
C -N -C
9
and C
8
-N
1 9
-C ) depended more largely on the substituent group on its atom.
Insert Scheme 1
Insert Scheme 2
Insert Table 2
3
.2. Fluorescent spectroscopy
Fluorescence behavior of the compound MM was studied by varying the excitation wavelength
in the range from 250 to 380 nm using an increment of 10 nm. The fluorescence spectra of the
compound MM were shown in Figures 3 and 4. As observed in Figure 3, when the compound MM is
excited using the excitation wavelength in the range from 250 to 280 nm, the emission peak of the
ultraviolet light at the maximum wavelength of 317 nm was observed. The intensity of the emission
peak increased as the excitation wavelength increased from 250 nm and then reached the maximum
at the excitation wavelength of 260 nm. For excitation wavelengths 280 nm, the intensities of the
peak at 317 nm gradually decreased as illustrated in the inset of Figure 3. The result revealed that
the most suitable excitation wavelength for the emission signal at 317 nm is around 260 nm. The
emission spectrum of the compound MM scanned at the fixed emission wavelength of 317 nm was
shown in Figure 4. Some excitation peaks were found in the excitation spectrum but the most
intense one is at the wavelength of 258 nm. This result was in agreement with the result from
increasing excitation wavelength by the step of 10 nm in Figure 3.
As the excitation wavelengths were escalated further, the emission intensities dropped
gradually. When the excitation wavelength was changed up to 310 nm, the energy of the incident
light was not enough to excite the electron in the molecule of the benzoxazine MM to its excitation
energy level that triggered the emission peak at 317 nm but it brought about new fluorescence
signal at the emission maximum of 450 nm (Figure 5a). The peak at 450 nm was responsible for the
emission of blue light of the compound MM when exposed to the incident light (Figure S1).
However, the excitation wavelength of 310 nm was not the most appropriate wavelength to
generate the fluorescence at 450 nm. The most suitable excitation wavelength for emission at 450
nm was 324 nm as seen from the increase in the intensities of the peak at 450 nm when the
excitation wavelength approached 324 nm (Figure 5b). Longer excitation wavelengths than 324 nm
not only lowered the intensity of the peak at 450 nm, but also slightly shifted the emission maximum
to 454 nm. The intensity of the peak at 454 rose again with the excitation of 340 nm until reached
the maximum at the excitation wavelength of 360 nm and then dropped again with those of longer
excitation wavelengths than 360 nm (Figure 5c). The emission spectrum of the compound MM
shown in Figure 6 also confirmed there were two excitation maxima present. The emission
wavelength of 324 nm was to yield the emission wavelength of 450 nm while the emission
wavelength of 360 nm was for the emission wavelength of 454 nm (Figure 6). From all the results,
the energy level diagram related to the fluorescence behavior of the compound MM could be
proposed as shown in Figure 7. In summary, three characteristic fluorescence peaks were observed,
i.e., (1) the ultraviolet peak at 317 nm which is from the excitation wavelength of 258 nm, (2) the

blue peak at 450 nm which is from the excitation wavelength of 324 nm and (3) the other blue peak
at 454 nm which is from the excitation wavelength of 360 nm.
Insert Figure 3
Insert Figure 4
Insert Figure 5
Insert Figure 6
Insert Figure 7
It was noteworthy recognizing that the emission of the blue light at both wavelengths of 450
nm and 454 nm was characteristic of the benzoxazine MM since these emission did not observed
from those of other compounds containing only benzene or phenol moiety, so that the formation of
the benzoxazine core was likely to be a major contributor of the fluorescence. When considered the
crystal structure of the compound MM, the special feature that the atoms C , C , C , C and O in the
5
6
7
8
1
oxazine ring were nearly oriented in the plane in line with the benzene ring was the evidence to
show the rigidity the benzoxazine core which was responsible for the fluorescence. To confirm
whether this structural character had the influence on the fluorescence or not, the corresponding
ring-opening benzoxazine dimer derivative of the compound MM named N,N-bis-(5-methyl-2-
hydroxybenzyl)methylamine, synthesized as reported elsewhere [25], was subjected to study
fluorescence property. The dimer derivative represented the compound with the similar group as
the compound MM but the oxazine ring was broken into methylene-aza-methylene bridge where
the oxazine members were no longer located in the plane of the benzene ring. It was found that
there were no fluorescent peaks at both 450 and 454 nm emitted in the spectrum of the
benzoxazine dimer. Consequently, it was confirmed that the fluorescent behavior was resulting from
the ring-closure structure of benzoxazine.
4
. Conclusions
Low-temperature X-ray crystallography was employed to study the structure of the benzoxazine
MM. The refined structure showed that the benzoxazine MM adopt the irregular chair conformation
with the ring strain due to the expansion of the C -O -C angle and the elongation of the C -O bond.
6
1
7
7
1
The distortion of bond lengths and angles was comparatively studied. The results showed that
substituents on both benzene rings and nitrogen atom played an important role in the distortion.
The electron withdrawing group on the benzene ring is responsible for the shortening of the C
6
-O
1
2
bond and the expansion of the angle of the C
6 1
-O -C
7
bond due to the higher sp character of the O
1
atom. The substituents on the nitrogen atom of the benzoxazine ring had a greater impact on the
bond angles; C -N -C , C -N -C and C -N -C , than the benzene substituents.
7
1
8
7
1
9
8
1
9
Solid-state fluorescence of benzoxazine monomer MM showed characteristic fluorescence
peaks at the wavelengths of 317, 450 and 454 nm. The excitation wavelengths for those of
fluorescence were 258, 324 and 360 nm, respectively. From the crystal structure, it was found that
the irregular chair conformation where all the members of the cyclic oxazine ring other than
nitrogen atom, namely, C , C , C , C
5 6 7 8
and O , were co-planar to the benzene ring let the benzoxazine
1

structure be a novel fluorophore. To ensure this point, the fluorescent property of the ring-opening
benzoxazine dimer containing the resemble moieties to the benzoxazine monomer MM but its
moieties were not located in the plane of the benzene ring was studied. As expected, no
fluorescence peaks at the 450 or 454 nm were observed so it was proved that the ring-closure
structure of the benzoxazine monomer was the significant contributor to the fluorescent property.
The fluorescent property of solution state in various solvents of the compound MM and the other
benzoxazine derivatives with different functional groups is currently preparing for publication soon.
5
. Acknowledgements
The authors would like to thank Kasetsart University Research and Development Institute
(KURDI) and Department of Chemistry, Faculty of Science, Kasetsart University for financial support.
6
. References
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[
[
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Figure captions
Figure 1 Molecular structure of the compound MM, with the atom-labeling scheme. The
displacement ellipsoids are drawn at the 50% probability level while the H atoms are shown as small
spheres of arbitrary radii
Figure 2 (a) Contacts observed for each molecule of the compound MM (b) Stereoview of C-H•••P
contacts in the crystal structure of the compound MM indicated as blue lines (c) Stereoview of
C-H•••O contacts in the crystal structure of the compound MM indicated as blue lines and
(d) Stereoview of the crystal structure of the compound MM
Figure 3 Fluorescent spectra of the compound MM recorded by using excitation wavelengths in the
range from 250-280 nm. Inset: The intensity of the emission peak at 317 nm as a function of
excitation wavelengths in the range from 250-280 nm
Figure 4 Fluorescent excitation and emission spectra of the compound MM for the emission peak at
3
17 nm
Figure 5 (a) Fluorescent spectra of the compound MM recorded by using excitation wavelengths in
the range from 250-380 nm (b) The intensity of the emission peak at 450 nm as a function of
excitation wavelengths in the range from 250-280 nm and (c) The intensity of the emission peak at
4
54 nm as a function of excitation wavelengths in the range from 250-280 nm
Figure 6 Fluorescent excitation and emission spectra of the compound MM for the emission peaks at
50 and 454 nm
4
Figure 7 Proposed energy level diagram of the compound MM showing its fluorescent behavior
Figure S1 Appearance of the blue light emitted from the compound MM when exposed to incident
light
Scheme captions
Scheme 1 Atomic labeling system for (a) monobenzoxazine derivatives and
(b) dibenzoxazine derivatives
Scheme 2 Structure of benzoxazine derivatives form the preceding works for comparing with the
compound MM

Highlights
u
u
u
The crystal structure of the compound 3,4-dihydro-3,6-dimethyl-1,3,2H-benzoxazine (MM)
recorded at low temperature was reported.
The structure parameters of the compound MM and the parameters obtained from preceding
literatures were compared.
Solid-state fluorescence behavior of the compound MM was firstly investigated.

Table captions
Table 1 Crystal data and structure refinement for the compound MM
Table 2 C-H•••P and C-H•••O contacts in the crystal structure of the compound MM
Table 3 Bond lengths and bond angles of the compound MM compared to the previously reported
benzoxazines

Table 1
Crystal data and structure refinement for the compound MM
Crystal data
Empirical formula
Relative molecular mass (M
Space group
Hall symbol
a (Å)
C H13NO
163.21
Orthorhombic, Pna2(1)
P 2c -2n
8.490(1)
17.164(2)
5.8981(6)
859.50(17)
4
10
r
)
b (Å)
c (Å)
3
V (Å )
Z
F(000)
Calculated density (D
352
1.261
-
3
x
, mg m )
Wavelength of Mo K
Reflections for cell parameter measurement
Q range for cell parameter measurement ( )
Absorption coefficient (M, mm )
Temperature (K)
A
radiation (L, Å)
0.71073
1512
2.37-29.05
0.08
100
o
-
1
Appearance of crystal
Crystal dimension
Data collection
Diffractometer
Radiation source
Rod, colorless
0.58r0.16r0.15 mm
Bruker APEX-II CCD
Fine-focus sealed tube
Graphite
Monochromator
Measurement method
Q range for data collection ( )
Index ranges
Reflections collected/unique
Completeness to theta = 29.05 (%)
Maximum and minimum transmission
Structure refinement
J and W scans
o
2.68-29.05
-11 a h a 10, -11 a k a 23, -8 a l a 4
3601/1705 [R(int) = 0.0229]
99.6
0.9879 and 0.9542
2
Refinement method
Full-matrix least-squares on F
Data/restraints/parameters
Goodness-of-fit on F
1705/1/161
1.067
2
Final R indices [I>2S(I)]
R1 = 0.0332, wR2 = 0.0822
R1 = 0.0365, wR2 = 0.0845
0.322 and -0.197
R indices (all data)
-
3
Largest diffraction peak and hole (e Å )

Table 2
Contact
Type of contact
Symmetry operations
x,y,z and 1/2+x,1/2-y,z
x,y,z and -1/2+x,1/2-y,z
x,y,z and 2-x,1-y,-1/2+z
x,y,z and 2-x,1-y,1/2+z
Length (Å)
2.880
C
C
C
9
4
7
-H
•••H
-H •••O
••• H -C
9
•••C
4
C-H•••P (donor)
9
- C
9
C-H•••P (acceptor)
C-H•••O (donor)
C-H•••O (acceptor)
2.880
2.531
2.531
7
1
O
1
7
7