Spectral characterization of selected stilbentriazine dyes - Structural trans-cis isomerisation

Article abstract of DOI:10.1016/j.molstruc.2008.01.054

The paper deals with the spectral investigations of new stilbentriazine dyes containing amino groups or alkoxy group attached to the main molecular triazine ring. Absorption and fluorescence spectra of the dyes in aqueous solutions were recorded. The existence of two spectral structures absorbing at 275 and 350 nm was shown and they are assigned to cis- and trans- structures of dyes, respectively. It was indicated that absorption and fluorescence features of the dyes are not significantly affected by a kind of substituent; exception is ST2 which differs in molecular structure and in the absorption and emission properties from the remaining dyes. The quantum yields of dye fluorescence and the yields of cis-trans energy transfer were also evaluated. The influence of temperature, solution pH and radiation exposure on photoproperties of the dye spectral forms were also examined. The large impact of the radiative exposure time on molecular conformation changes was observed, whereas temperature and environmental pH do not affect the dye spectral parameters. The equilibrium ratios of the trans- to cis- structures were estimated.

Full text of DOI:10.1016/j.molstruc.2008.01.054

Journal of Molecular Structure 887 (2008) 165–171
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Spectral characterization of selected stilbentriazine dyes – Structural trans–cis
isomerisation
a
b
b
a,
*
_
I. Hanyz , R.M. Ion , A. Nuta , D. Wróbel
a
´
Institute of Physics, Faculty of Technical Physics, Poznan University of Technology, Nieszawska 13A, 60-965 Poznan, Poland
b ICECHIM-Bucharest, Analytic Department, 202 Splaiul Independentei, Bucharest, Romania
a r t i c l e i n f o
a b s t r a c t
Article history:
The paper deals with the spectral investigations of new stilbentriazine dyes containing amino groups or
alkoxy group attached to the main molecular triazine ring. Absorption and fluorescence spectra of the
dyes in aqueous solutions were recorded. The existence of two spectral structures absorbing at 275
and 350 nm was shown and they are assigned to cis- and trans- structures of dyes, respectively. It was
indicated that absorption and fluorescence features of the dyes are not significantly affected by a kind
of substituent; exception is ST2 which differs in molecular structure and in the absorption and emission
properties from the remaining dyes. The quantum yields of dye fluorescence and the yields of cis–trans
energy transfer were also evaluated. The influence of temperature, solution pH and radiation exposure
on photoproperties of the dye spectral forms were also examined. The large impact of the radiative expo-
sure time on molecular conformation changes was observed, whereas temperature and environmental
pH do not affect the dye spectral parameters. The equilibrium ratios of the trans- to cis- structures were
estimated.
Received 15 September 2007
Received in revised form 22 January 2008
Accepted 22 January 2008
Available online 21 March 2008
Keywords:
Stilbentriazine dye
Absorption
Fluorescence
Trans–cis isomerisation
Cis–trans energy transfer
Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction
forms: cis and trans, owing to the ethylene bondings which favour
trans–cis transition [2,3,12]. The degree of isomerisation is mark-
Stilbentriazine dyes are very interesting groups of molecules
due to their particular photophysical and photochemical proper-
ties; they are known as optical fluorescence brighteners. Their
molecular structure is planar with conjugated bonds and electron
donating groups are attached to the main molecular core [1]. These
dyes absorb energy in the range of 200–400 nm and therefore they
practically do not exhibit colour, but they show strong violet–blue
emission (400–450 nm) [2–4]. Stilbentriazine dyes could find
application as blue-emitted species in optoelectronics due to their
good fluorescence properties and skillness in changing colours
upon illumination when needed. Also their advantage is high de-
gree of whitening what makes them very good agents in the paper
and textile industry and also in colouring of both natural and syn-
thetic fibers [2,5–9]. Optical brighteners have also been used
against beetle larvae because they enhance bacterial infection of
it [10].
edly influenced by a kind of substituents attached to the main
molecular scaffold.
Photophysics and photochemistry of stilbentriazine dyes are
still a subject of numerous papers [13–16]; there are some papers
on fluorescent brighteners containing different amino and alkoxy
groups in triazine ring [1,2,8]. However in this paper, we study
new stilbentriazine dyes differing one from each other with sub-
stituents (alkoxy, amino groups) linked to the main molecular core.
The following measurements have been done for the dyes in aque-
ous solutions: absorption, fluorescence and excitation emission.
The general aim of this paper is to follow an influence of the exter-
nal conditions (pH, temperature, time-of-light exposure) which
could lead to stereoisomerization of the stilbentriazine dyes and
physical processes occurring in the systems (energy transfer) are
also under consideration.
Photophysical and photochemical properties of some stilbentri-
azine dyes with various substituents attached to the main molecu-
lar core depend very strongly on environmental conditions. Some
of these dyes are very sensitive on pH surrounding and/or light
exposure [11,12]. Moreover, they can exist in two stereoisometric
2. Materials and methods
Four stilbentriazine dyes were investigated; their molecular
structures are shown in Fig. 1. The dyes differ one from another
with substituents linked to two triazine rings; the substituents
R1 and R2 in ST1, ST3 and ST4 are the amino groups and the dye
ST2 differs from the remaining dyes with one alkoxy group instead
of one amino group (full names of the dyes are included in Fig. 1
* Corresponding author. Tel.: +48 61 665 3177; fax: +48 61 665 3178.
E-mail address: wrobel@phys.put.poznan.pl (D. Wróbel).
0022-2860/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2008.01.054

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I. Hanyz et al. / Journal of Molecular Structure 887 (2008) 165–171
166
The three stages of synthesis ware carried out in the presence of
an acid binding (e.g., NaOH, KOH, NaHCO3, NaH2PO4, Na2HPO4,
etc.).
The resulted stilbentriazine compounds were isolated from the
reaction mixture by precipitation and a filter at 85 5 °C, dried at
85–90 °C, purified by toluene extraction and double precipitation
in a methanol–water–NaOH mixture, and physically–chemically
analysed as was described in the literature [12].
The dyes were dissolved in chloroform and after chloroform
evaporation they were dissolved in the buffer of pH: 6, 7 and 9.
The dyes were dissolved in aqueous solutions (water and a buffer).
Their concentrations in water (pH 5.6) and in buffered solutions
are 10ꢀ5 and 5 ꢁ 10ꢀ6 M, respectively. The pH of the buffered solu-
tions was reached with 1 mM Tris; (organic compound tris-
hydroxymethylaminomethane with the formula (HOCH2)3CNH2,
widely used as a compound of buffer solution) and 1% HCl.
The course of chemical procedure and quality of dye’s molecular
structures were followed elemental analysis and spectroscopically
by taking the UV spectra on a double beam spectrophotometer
Specord M4000 and the IR spectra on a FT-IR GX Perkin-Elmer
983 instrument in the range of 20–10,000 cmꢀ1. The results of ele-
mental analysis for the dye molecular structure and the IR analysis
are presented in Tables 1 and 2, respectively. Results of the ele-
mental analysis and identification of the IR bands constitute good
arguments for the attributed structure.
Fig. 1. Molecular structure of stilbentriazine dyes. ST1, 4,40-bis{400-phenylamino-
600-[(4-sulfophenyleneamino)-1,3,5-triazin-200-yl]amino}stilbene-2,20-disulfonic
acid; ST2, 4,40-bis{400-methoxy-600-[bis(2-hydroxyethylamino)-1,3,5-triazin-200-yl]-
amino}stilbene-2,20-disulfonic acid; ST3, 4,40-bis{400-amino-600-[(4-sulfophenyl-
eneamino)-1,3,5-triazin-200-yl]amino}stilbene-2,20-disulfonic acid; ST4, 4,40-bis{40-
-(2000 5000-disulfophenyleneamino)-600-[bis(2-hydroxyethylamino)-1,3,5-triazin-200-
,
yl]amino}stilbene-2,20-disulfonic acid.
caption). Thus we can expect to find different photofeatures of
these two dye groups.
The synthesis of stilbentriazine compounds, corresponding to
the general formula (Fig. 1), was effectuated in aqueous medium,
by the common techniques described in [17,18], e.g., substitution
of the three chlorine atoms from cyanuric chloride by aliphatic
alcohols, secondary amines, aliphatic or aromatic in equivalent
quantities, and an order imposed by the reactivity of reagents.
Thus, the synthesis of the analysed structures required the fol-
lowing steps:
Absorption spectra were recorded with a spectrophotometer
Carry 4000 in the region from 250 to 450 nm. Fluorescence proper-
ties of the dyes were monitored with a fluorometer Hitachi F4500
in the range of 350–600 nm at two excitation wavelengths: 275
and 350 nm.
The fluorescence quantum yield was estimated on the basis of
Eq. (1) [20]:
FS ð1 ꢀ 10ꢀA Þ nS
a) substitution of the first chlorine atom from cyanuric chloride
molecule by 1-aminobenzene-4-sulfonic acid (ST1 and ST3),
methanol (ST2) or 1-aminobenzene-2,5-disulfonic acid (ST4)
at pH = 6 0.5 and temperature 0–10 °C;
2
R
UF ¼ UR
;
ð1Þ
ð1 ꢀ 10ꢀA
Þ
2
S
FR
n
R
where
b) substitution of the second chlorine atom from cyanuric chlo-
ride molecule by 4,40-diaminostilbene-2,20-disulfonic acid at
10–45 °C and pH max. 6 0.5;
c) substitution of the third chloride atom from cyanuric chlo-
ride by aniline, ammonia or diethanolamine, at 90–95 °C
and pH = 8 0.5.
ꢂ UR – fluorescence quantum yield of a reference sample,
ꢂ FS – integrated area under the emission spectrum of a sample,
ꢂ FR – integrated area under the emission spectrum of a reference,
ꢂ AS – sample absorbance at the excitation wavelength of
fluorescence,
Table 1
The results of elemental analysis done for the studied dyes
Compound
M.W.
1052
794
Elemental analysis
C(%)
H(%)
Calc.
3.42
N(%)
Calc.
15.96
O(%)
Calc.
18.25
S(%)
Calc.
Exp.
Exp.
3.44
Exp.
Exp.
Calc.
12.16
Exp.
ST1
50.19
50.21
15.99
18.26
12.14
C44 H36N12O12S4
R1 = 4-SO3H-C6H4NH
R
2 = NHC6H5
ST2
45.34
42.76
41.81
45.32
42.74
41.82
4.78
2.89
3.83
4.77
2.90
3.85
17.63
18.7
17.64
18.72
14.64
24.18
21.38
30.66
24.19
21.39
30.65
8.06
14.25
16.72
8.05
14.25
16.73
C30 H38N10O12 S2
R1 = OCH3
R2 = N(C2H4OH)2
ST3
898
C32 H26N12O12 S4
R1 = 4-SO3H-C6H4NH
R
2 = NH2
ST4
1148
14.63
C40 H44N12O22 S6
R1 = 2,5-(SO3H)2-C6H3NH
R
2 = N(C2H4OH)2

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167
Table 2
The IR data performed in KBr for the studied dyes
Compound
IR bands in KBr (cmꢀ1
)
ST1
ST2
3442 (NH), 3012 (aromatic C@CAH), 2968(C@CAH), 2810 (CH2), 2650 (SO3H),1609 (C@C), 1521 (aromatic C@C), 1400 (C@C), 1610 (CAN), 1650 (NAH)
3454 (NH), 3281 (AOH), 3014 (aromatic C@CAH), 2968 (C@CAH), 2812 (CH2), 2655 (SO3H), 1609 (C@C), 1531 (aromatic C@C), 1400 (C@C), 1612
(CAN),1653 (NAH)
ST3
ST4
3444 (NH), 3014 (aromatic C@CAH), 2969 (C@CAH), 2812 (CH2), 2652 (SO3H), 1613 (C@C),1523 (aromatic C@C), 1403 (C@C), 1615 (CAN), 1655 (NAH)
3452 (NH), 3286 (AOH), 3012 (aromatic C@CAH), 2975(C@CAH), 2813 (CH2), 2652 (SO3H), 1609 (C@C), 1524 (aromatic C@C), 1404 (C@C), 1613 (CAN),
1653 (NAH)
where eA and eD are molar absorption coefficients of the acceptor
ꢂ AR – reference absorbance at the excitation wavelength of
fluorescence,
and the donor, respectively, FA and FD are fluorescence intensities
of the acceptor excited directly and sensitized by donor absorption,
respectively.
In the temperature experiments, temperature was changed
from 6 to 30 °C. For the experiments of light exposure a xenon
lamp of 150 W was used and temperature (20 °C) was kept con-
stant and controlled with a temperature controller.
ꢂ nS – refractive index of a sample,
ꢂ nR – refractive index of a reference.
Coumarin 120 in ethanol was used as a reference sample for
quantum yield evaluation. The refractive indices nS for the sample
in water and for a standard in ethanol were 1.33 and 1.36,
respectively.
On the basis of the absorption and fluorescence examinations
(excitation emission spectra) the energy transfer yield from a do-
nor to an acceptor can be evaluated. Efficiency of energy transfer
can be determined from the sensitization of acceptor fluorescence
or the quenching of donor fluorescence [21]. In our paper the yield
of excitation energy transfer, UET can be expressed taking into ac-
count fluorescence intensities of an acceptor in the presence and
in the absence of a donor:
3. Results and discussion
3.1. Basic spectral parameters of dyes in water
First recognization of the basic spectral features has been done
for the dyes in water (pH 5.6). The absorption spectra are pre-
sented in Fig. 2A. The dyes absorb in two radiative regions: 250–
300 and 320–400 nm. These bands are assigned to the cis and trans
isomeric structure of the dyes [1–3,7,8,22]. The absorption param-
eters (band locations, the band intensity ratios) are gathered in Ta-
ble 3. The rough analysis of the absorption spectra evidently shows
ꢀ
ꢁ
eA FA ꢀ FD
UET
¼
ꢀ 1
;
ð2Þ
eD
FA
Fig. 2. (A) Absorption spectra of stilbentriazine dyes in water (pH 5.6). (B and C) Fluorescence spectra of dyes at excitation wavelengths 350 and 275 nm, respectively. (D)
Excitation spectra at observation of 425 nm.

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168
Table 3
Absorption and fluorescence parameters of ST1, ST2, ST3 and ST4
Sample
Absorption kA
(nm)
A275/350
Fluorescence kF
(nm)
max
FWHM350 (F) (cmꢀ1
)
UF
max
ST1
ST2
ST3
ST4
277
–
274
272
347
347
346
347
1.44
–
0.96
1.31
432
428
431
429
3340
3449
3347
3737
0.10
0.15
0.16
0.19
kA
– Wavelength of absorption maximum, A275/350 – absorption band intensity ratio,
max
kFmax – Wavelength of fluorescence maximum, FWHM350 (F) – the fluorescence band half-width for the main band with a maximum at about 430 nm for kex = 350 nm (data
evaluated on the basis of gaussian analysis), UF – fluorescence quantum yield.
the differences in ST2 behaviour when compared to that of the
remaining dyes. The dyes ST1, ST3 and ST4 exist in two isomeric
forms – the cis and trans forms are characterized by the bands with
the maxima at nearly 275 and 350 nm, respectively [1–3,7,8,22].
The lack of the band in the maximum range of 250–300 nm in
ST2 indicates that the cis form does not exist in this dye. The vari-
ations in the dye’s absorption features are attributed to the differ-
ences in the molecular structure of the substituents attached to the
main molecular skeleton; the alkoxy group in ST2 causes that the
molecule is more resistant to isomerization and the cis form in
ST2 does not occur.
by the excitation spectra presented in Fig. 2D; two emission bands
are observed (at 275 and 350 nm) when the trans-dye is registered
(at 425 nm).
The fluorescence quantum yields of the dyes is rather low (they
range from 0.10 to 0.19) and they are a little bit lower than the
quantum yields of other stilbentriazine dyes [3,22]. The UF values
slightly depend on the dye molecular structure (Table 3). The loss
of fluorescence in the cis-dyes is explained as due to processes in
which excitation energy is needed for photoisomerization
[2,11,23,24] and ET as well as non-radiative processes.
Figs. 2A–D evidently confirm occurrence of energy transfer from
the cis form (donor) to the trans form (acceptor). Excitation of the
cis does not lead to its own fluorescence, however emission of the
trans form, when excitation is in the range of cis absorbance, takes
place as seen in Fig. 2C. It is seen that cis–trans energy transfer
gives emission of trans fluorescence. The efficiency of energy trans-
fer, UET can be roughly calculated from direct and sensitized accep-
tor fluorescence on the basis of the absorption spectra (Fig. 2A) and
excitation spectra (Fig. 2D). The data show that about 50% of cis
excitation is transferred to the trans form.
Moreover the weak trans-dye fluorescence properties (UF
< 0.20) indicate that a part of excitation could also be deactivated
in some nonradiative processes like for example: heat liberation,
steric effects and/or activation of other photochemical processes.
The isomerisation process is a subject of the following chapter.
In Figs. 2B and C, the fluorescence spectra are shown when mea-
sured at two excitation wavelengths: 350 and 275 nm. Fig. 2B con-
firms the intensive fluorescence of the trans form of the dyes
(absorbing at about 350 nm) characterized by the spectra with a
hump at about 415 and maximum at 430 nm which differ slightly
from the emission spectra observed in [3,15]. The source of ob-
served difference and an asymmetry of the fluorescence spectra to-
gether with the existence of splitted bands are rather unknown. It
could be existence of an another mesomer form in the samples;
however this explanation needs further experiments with tri-
azine–stilbene dyes substituted with various peripheral groups.
Similar emission features were also observed for other substituted
triazine–stilbene dyes (data not published). Moreover, the results
in Fig. 2C (excitation at 275 nm) evidently show that although in
this experiment the cis form is excited, the fluorescence signal is
emitted in the same spectral range as that in Fig. 2B. The results
in Figs. 2B and C could be explained in two aspects: (i) only one
spectral form of the dyes exists characterised with the absorption
bands at nearly 275 and 350 nm regions, (ii) two forms (cis and
trans) are present in the samples. The (i) is excluded for at least
two reasons: first, the fluorescence signals of the samples when ex-
cited at 275 nm are nearly twice lower than those observed for
350 nm excitation although the absorbance of the samples in the
range of 270–275 nm is twice intensive than that at 340–350 nm
region; second, in the emission excitation spectra (Fig. 2D) the
intensity ratios (at 350 nm bands with respect to the bands at
275 nm) are reversed in intensity when compared to those in the
absorption spectra. Thus, we have every reason to believe that
there are two spectral dye structures in our samples: the cis form
and the trans form (ii) characterized by absorption at 270–275
and 340–350 nm, respectively [12,13]. As seen the shapes of fluo-
rescence are very similar and independent of excitation wave-
length for all investigated dyes; the band positions and half
widths are gathered in Table 3. On the basis of Figs. 2B and C we
can draw at least three conclusions: (i) trans dye form is responsi-
ble for fluorescence; fluorescence spectra shapes are not depen-
dent on the excitation wavelength, (ii) energy transfer (ET) from
the cis to the trans form takes place; excitation at 275 nm gives
fluorescence of the trans-dyes (it concerns to ST1, ST3 and ST4),
(iii) the cis dyes do not fluoresce; whatever excitation is used
(275 or 350 nm), only fluorescence of the trans form is observed.
These conclusions are consistent with the results of other papers
which deal with cis and trans forms [3,13–16,23] and are confirmed
3.2. Influence of pH, temperature and light exposure on cis–trans
isomerisation
In this chapter, we present the results obtained for the dyes ST2
and ST3. The dye ST2 seems to be the most interesting molecules
because it is the only investigated dye among others which does
not have a cis form. The dyes ST1, ST3 and ST4 have very similar
properties and therefore we present the results of the dyes ST3
and ST2 as examples of two dye groups. Fig. 3A presents an influ-
ence of pH on the absorption features of the ST2 dissolved in water
(pH 5.6) and in the buffered solutions (pH 6, 7 and 9). The results
evidently indicate that pH of the media in the range from 5.6 to 9
does not alter the structure of the investigated dyes and their
absorption properties (or only slightly). A similar conclusion we
can draw from the temperature experiment; in the range from 6
to 30 °C no changes are observed in the dye spectral properties
(Fig. 3B). These results evidently indicate that the reaction of media
and heat treatment do not alter the structure of the investigated
dyes and no trans–cis transition takes place.
In the next stage of our investigation, we have examined the
alternation in absorption and fluorescence caused by light expo-
sure. The experiments were done for the samples illuminated with
the light intensity of the order of mW/cm2 to avoid secondary ef-
fects (like for example two-photon processes). In Figs. 4A–D, we
have shown the results obtained for the sample ST2 when dis-
solved in the buffered solution of pH 7 (in pH 5.6, 7 and 9 the
changes are very similar and results are not shown). Fig. 4A shows
variations in absorption behaviour in ST2 upon light in the time

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I. Hanyz et al. / Journal of Molecular Structure 887 (2008) 165–171
169
Fig. 3. (A) pH influence on the absorption properties (pH 5.6, 6, 7, 9). (B) Absorption spectra of ST2 at 6–30 °C.
Fig. 4. (A) Absorption spectra of ST2 (pH 7) as measured at different time light exposure. (B and C) Fluorescence spectra of dyes at excitation wavelengths 350 and 275 nm,
respectively. (D) Excitation spectra at observation of 425 nm.
duration from 0 to 60 min. We can evidently see the marked
diminishing in the band intensity at 340–350 nm and simulta-
neously increasing bands with a maximum at 270–275 nm. Modi-
fication of the absorption spectra upon light confirms the presence
of process of dye isomerisation from the trans to cis structure. In
this point it is worth to underline that the cis form in ST2 is created
as a result of light-induced changes in the molecular structure; un-
der the ‘‘dark” condition this form does not exist. The cis and trans
structures are in thermodynamic equilibrium what is evidently
confirmed by the isobestic point at about 305 nm. The fluorescence
spectra (Figs. 4B and C for kexc = 350 and 275 nm, respectively), and
emission excitation spectra (Fig. 4D, for kob = 425 nm) show
decreasing of the trans form in time with simultaneous increase
of the cis form. The shapes of the fluorescence spectra are not al-
tered under light exposure (both at 350 and 275 nm excitations)
and only band intensities diminish markedly of about 6 times in
60 min of illumination. This diminishing in the fluorescence signals
is explained as a result of trans–cis isomerisation. The decrease in
trans fluorescence (Fig. 4B) is coherent with the absorption de-
crease at 350 nm (Fig. 4A). However, the loss of fluorescence at
275 nm excitation with light exposure seems to be inconsistent
with 275 nm absorbance increasing. To explain this point one has
to have in mind following facts: (i) the cis form does not fluoresce,
(ii) energy transfer could take place, (iii) new spectral form can ar-

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I. Hanyz et al. / Journal of Molecular Structure 887 (2008) 165–171
170
rive. The (i) is obvious and does not need a comment. Energy trans-
fer (ii) is rather unlike, because the band at 275 nm in the excita-
tion emission spectra (Fig. 4D) is hardly seen. Moreover, the
fluorescence intensity at 275 nm excitation is about 10 times lower
than that when excited at 350 nm; this result differs from that pre-
sented in Figs. 2B and C where the fluorescence intensity ratio is
equal to about 2. The presence of a new spectral form (absorbing
at nearly 275 nm) cannot be excluded (iii) and it differs from the
cis forms in ST1, ST3 and ST4. This supposition could again be con-
firmed by the results in Fig. 4D; the band at 275 nm is not seen
(compare with Fig. 2D). This new dye form in ST2 arriving upon
light illumination does not transfer energy (or very weakly) to
the trans form.
excitation spectra (Fig. 5D) are rather similar to those in
Fig. 2D but they are different from those in Fig. 4D. Also the fluo-
rescence intensity ratios of the signals when excited at 350 nm
(Fig. 5B) and at 275 nm (Fig. 5C) are of the same order as found
in Fig. 2B and C. Thus these results confirm creation of the cis
form under illumination. The decreasing in the fluorescence
intensity in the range of 275 and 340 nm (Fig. 5D) and simulta-
neous increase in 275 nm absorption (Fig. 5A) indicate also the
existence of the new form of the dyes ST3 which arises upon
light exposure and absorbs at 275 nm. Thus, we can confirm
the presence of a new spectral forms arriving upon illumination
in ST3 (ST1, ST4) and ST2 but we are not able to judge their
sources in this paper.
As shown the ST1, ST3 and ST4 have two spectral forms (cis and
trans) under the no-light condition. The responses of these dyes on
the light exposure are different from those observed in ST2. In Figs.
5A–D, we present the results for ST3 as an example, because the re-
sults for this group of dyes (ST1, ST3 ST4) are very similar one to
each other.
The changes in the absorption properties of ST3 (ST1, ST4)
(Fig. 5A) upon light show simultaneous declining of the 350 nm
band and increasing of the 275 nm peak can be observed. This re-
sult indicates that the trans form is isomerised to the cis form. The
occurrence of the isosbestic point at 305 nm again confirms ther-
modynamic equilibrium of these two structures.
The reduction of the fluorescence signals (Fig. 5B) is observed;
it again confirms trans–cis transformation in ST3 (ST1 and ST4).
However, fluorescence at 275 nm excitation (Fig. 5C) seems to
be incoherent with the ST3 absorption changes in the range of
275 nm (Fig. 5A); absorption rises and fluorescence decreases in
time (in similar way like in ST2). However, the shapes of the
The content of the cis and trans isomers after light exposure was
estimated on the supposition that DCtrans = DCcis (where DCtrans and
DCcis are changes in amount of the conformers upon illumination).
On the basis of Eq. (3), [19]:
A0 ðA0 ꢀ ACÞ
T
C
Ctrans
¼
100%;
ð3Þ
ð4Þ
A0 AT ꢀ ACA0
C
T
Ccis ¼ 100% ꢀ Ctrans
AT and A0T are the absorbances of the trans form before and after illu-
mination, respectively, AC and A0C are the absorbances of the cis form
before and after illumination, respectively. The results are collected
in Table 4. As seen the values of cis and trans content are about 87–
91% and 9–13%, respectively in ST1, ST3, ST4. The exception is ST2;
in the dark it has only the trans form and the cis form appears under
the light conditions. The equilibrium ratios of the two spectral
forms slightly depend on a kind of substituents in the triazine ring.
Similar effect was observed for other triazine–stilbene fluorescent
dyes [18].
Fig. 5. (A) Absorption spectra of ST3 (pH 7) as measured at different time light exposure. (B and C) Fluorescence spectra of dyes at excitation wavelengths 350 and 275 nm,
respectively. (D) Excitation spectra at observation of 425 nm.

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171
Table 4
5. The amount of the cis and trans forms depends on light expo-
sure time and degree of isomerization transition from trans to
cis structure is high.
The content of the cis and trans isomers in water after light exposure (60 min)
Sample
kiso (nm)
eiso (104 cm2/mol)
Ccis (%)
Ctrans (%)
6. The dyes are resistant to pH and temperature.
ST1
ST2*
ST3
ST4
305
305
305
307
2.28
2.06
2.10
2.78
87
88
91
90
13
12
9
The features described above indicate that these dyes could be
widely applied as perfect optical brighteners.
10
kcis – Maximum absorption wavelength of the cis form. ktrans – Maximum absorption
wavelength of the trans form. kiso – Absorption wavelength isosbestic point. Ccis
Percentage content of the cis form after light exposure. Ctrans – Percentage content
of the trans form after light exposure. eiso – Molar absorption coefficients in the
isosbestic point. In the case of ST2, the asterisk means that before light exposure
Ctrans = 100%.
–
Acknowledgments
*
This paper was supported by Poznan University of Technology –
Grant BW 62-207. The authors thank M.Sc. Marzena Tykocka-Piłat
for her assistance in measurements.
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4. Conclusions
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1. Absorption and fluorescence features of the stilbentriazine dyes
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2. The dyes ST1, ST3 and ST4 are characterized with two isomeric
forms cis and trans coexisting together in the dark and upon
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light induces trans–cis isomerization and cis form is then also
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3. Process of energy transfer from the cis to trans form occurs in
ST1, ST3 and ST4 with high efficiency.
4. Upon illumination a new spectral form arrives in ST1, ST3, ST4
and ST2, but in ST2 no energy transfer to the trans form occurs.