Synthesis and Photophysical Properties of Fluorescein Esters as Potential Organic Semiconductor Materials

Article abstract of DOI:10.1007/s10895-021-02789-y

Fluorescein (1), a known fluorescent tracer in microscopy with high photophysical properties, was esterified to have fluorescein ethyl ester (2) and O-ethyl-fluorescein ethyl ester (3) in excellent yields. All of them were investigated for the photophysic

Full text of DOI:10.1007/s10895-021-02789-y

Journal of Fluorescence
https://doi.org/10.1007/s10895-021-02789-y
ORIGINAL ARTICLE
Synthesis and Photophysical Properties of Fluorescein Esters
as Potential Organic Semiconductor Materials
Gamal A. E. Mostafa1,2 · Abu Syed Mahajumi3 · Haitham AlRabiah1 · Adnan A. Kadi1 · Yang Lu4 ·
A. F. M. Motiur Rahman1
Received: 8 February 2021 / Accepted: 13 July 2021
© The Author(s), under exclusive licence to Springer Science+Business Media, LLC, part of Springer Nature 2021
Abstract
Fluorescein (1), a known fuorescent tracer in microscopy with high photophysical properties, was esterifed to have fuo-
rescein ethyl ester (2) and O-ethyl-fuorescein ethyl ester (3) in excellent yields. All of them were investigated for the pho-
tophysical and electrochemical properties as potential organic semiconductor materials. Absorptions and emission spectra
were taken in various solvents, compound 2 showed emission maxima at λmax =545 and compound 3 showed λmax =550 nm.
Optical band gap energy (Eg) was calculated for 1–3 and the values were found in between 2.34 – 2.39 eV. Possibility of
shifting emission maxima was studied in various pH (5–9) bufers, and fnally the thermal stability was examined using dif-
ferential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). Increasing of conjugation system of 2 and 3
were studied by HOMO and LUMO distributions of 1–3. Experimental results showed that compounds 2 and 3 have excellent
photophysical and electrochemical properties hence can be used as excellent organic semiconductor materials.
Keywords Fluorescein · Fluorescein ester · Fluorescence · Optical band gap · Organic semiconductor
Introduction
made from organic compounds, got enormous achievements
attributed to their comparatively low energy consumption,
high efciency and thermal stability. Their realistic color
tunability can be further improved through chemical struc-
tural modifcation of those organic compounds. Organic
molecules could also be used to produce thin, light, transpar-
ent, fexible, and efcient devices [2]. For example, OLED
screens are mainly used in digital devices such as advanced
television systems, desktop and laptop computer monitors,
portable small sized systems such as smartphones, media
players, digital cameras, and portable gaming consoles
etc. However, some important characteristics should be
considered before using an organic molecule in an organic
semiconductor device. Figure 1 shows the electron distri-
bution on the highest occupied molecular orbital (HOMO)
and lowest unoccupied molecular orbital (LUMO) level of
organic semiconductor [3], the energy diference between
HOMO and LUMO is defned as energy gap. The energy gap
determines the minimum energy that can be absorbed by the
molecules. The ionization potential is defned as the mini-
mum energy required when taking an electron from HOMO
to the vacuum, whereas electronic afnity is defned as the
minimum energy released when supplying an electron from
the vacuum level to LUMO. This energy gap is also known
Photo- and electroactive organic compounds, bearing high
π-conjugate structures and/or fused aromatic system, have
gained great attraction in most scientifc, academic, and
industrial societies due to their excellent potential applica-
tion in full-color-fat-panel displays and solid-state light
source [1]. In recent years, organic semiconductor devices
(e.g. organic solar cells, OSCs; organic feld efects transis-
tors, OFETs; and organic light-emitting diodes, OLEDs etc.)
* A. F. M. Motiur Rahman
afmrahman@ksu.edu.sa
1
Department of Pharmaceutical Chemistry, College
of Pharmacy, King Saud University, Riyadh 11451,
Saudi Arabia
2
Micro-Analytical Laboratory, Department of Applied
Organic Chemistry, National Research Center, Dokki 12622,
Cairo, Egypt
3
Department of Electronic and Electrical Engineering,
Southwest Jiaotong University (SWJTU), University of Leeds
UK Joint School, XIPU Campus, Chengdu, China
4
Institute of Drug Discovery Technology, Ningbo University,
Ningbo, China
Vol.:(0123456789)
1 3

Journal of Fluorescence
Due to the high photophysical properties of compound
1 (λabs = 494 and λemission ~ 512 nm in water), it has been
functionalized with an isothiocyanate (-N = C = S) group
(FITC, Chart 1) and the formed product has been studied
for their physicochemical, photophysical properties before
its application in biological implications as FITC-conjugates
[8–22]. Compound 1 has also been functionalized on the top
ring with a carboxylic acid group (FCA) [23] for the prepa-
ration of peptide/glycosides conjugates [24–29]. Hydroxyl
groups of 1 have also been functionalized (FCA-OR) [30]
for various purposes [31–42], such as artifcial photosyn-
thesis and solar energy conversion [33]. A detailed search
gives us the information that fuorescein derivatives were
derived from cyclic form (Fig. 2, cyclic form) rather than
open form of 1 (Fig. 2, open form) and have been studied
for the detection of membrane-bound DNA [43], detection
of damaged nucleic acids [44], measurement of mammalian
phosphoinositide-specifc phospholipase C activity [45] and
proteolytic activities [46].
Fig. 1 Electron distribution on the HOMO and LUMO level of
Organic semiconductor [3]
Early this year, Shumin Feng and co-workers reported
that alkyl protected fuorescein enhanced its fuorescence
properties [47]. In addition to this article, a review on 2017,
done by Elisabete Oliveria and co-others after they had
reviewed 265 articles, concluded that derivatization of fuo-
rescein (hydroxyl protection, esterifcation, hydrazone for-
mation as well as metal complexes formation) also enhanced
the fuorescence properties a lot [48]. Since fewer conju-
gated π-bonds give lower λmax (towards the UV) while more
conjugated π-bonds give higher λmax (towards the visible),
we assumed that, it might be due to the free acid group of
1, which might have been responsible for cyclic formation
of 1. It should be noted that if cyclic formation occurs in
1, the conjugated π-bonds became less, resulting in lower
λmax, and if it stays in open form, the conjugated π-bonds
get more, resulting in higher λmax. Based on this hypothesis,
we therefore, designed and synthesized fuorescein ethyl
ester (2) and ethyl protected fuorescein ethyl ester (3) and
compared the photophysical properties with 1, and distri-
butions of HOMO and LUMO of 1–3 were studied to get
more theoretical information, for further uses as potential
organic semiconductor materials candidates. A resonance
structure and conjugated π-bonds in those compounds (1–3)
are shown below (Scheme 1).
as optical band gap energy (Eg). If Eg is larger than 4 eV
for a molecule, it acts as an insulator, and if it is lower than
3 eV, it acts as semiconductor, therefore, Eg plays a vital role
in selecting an organic molecule as organic semiconductor
material [4].
Fluorescein (1) is a synthetic organic molecule with open/
cyclic form (Fig. 2), it is powder with dark orange/red in
color, soluble in water as well as in alcohol. It was frst syn-
thesized in 1871 [5]. It is commonly used as a fuorescent
tracer in microscopy, a type of dye laser, and in forensics
and serology to detect latent blood stains [6]. Compound
1 has a pKa of 6.4, and its ionization equilibrium leads to
pH-dependent absorption and emission over the range of 5
to 9. In addition, the fuorescence lifetimes of the protonated
and deprotonated forms of compound 1 are approximately
3 and 4 ns, which allows for pH determination through non-
intensity-based measurements. The lifetimes can be recov-
ered using time-correlated single photon counting or phase-
modulation fuorimeter. Compound 1 has an isosbestic point
(equal absorption for all pH values) at 460 nm [7].
O
Experimental Section
COOH
O
General
HO
O
O
HO
O
OH
Chemicals and solvents were commercial reagent grade
and were used without further purifcations. Melting points
were determined on a Barnstead electrothermal digital
melting point apparatus model IA9100, BIBBY scientifc
Fluorescein
(Open form)
Fluorescein
1
(Cyclic form)
1
Fig. 2 Structures of fuorescein
1 3

Journal of Fluorescence
NCS
O
O
COOH
HOOC
HOOC
HO
O
O
R
O
R
HO
O
O
O
O
O
OH
FCA
Fluorescein carboxilic acid
FITC
Fluorescein isothiocyanate (
)
Fluorescein carboxilic acid (
)
FCA-OR
(
, R = alkyl, aryl, amino
acids, etc.)
O
H
N
O
H
N
N
N
N
R
O
P
O
O
O
O
O
N
HO
O
OH
tBu
O
O
O
tBu
FAM phosphoramides
Fluorescein hydrazone (FH)
Chart 1 Structure of some functionalized fuorescein
Scheme 1 Resonance structures and conjugated π-bonds in 1–3
1 3

Journal of Fluorescence
limited, Stone, Stafordshire, ST15 0SA, UK. IR spectra
were recorded on a Jasco FT/IR-6600 spectrometer, Japan.
NMR spectra were taken using an Agilent Technologies
400 MHz premium shielded NMR spectrometry, CA, USA.
Electrospray ionization (ESI) mass spectrometry (MS)
experiments were performed using an Agilent 6320 ion
trap mass spectrometer ftted with an electrospray ioniza-
tion (ESI) ion source (Agilent Technologies, Palo Alto, CA,
USA) with direct injection. UV–vis spectra were measured
using a Genesys G10S UV–Vis spectrophotometer (Thermo
scientifc, CA, USA). For the compounds studied, the meas-
urements were performed using various solvent and quartz
cells with a path length of 10.00 mm. The UV–vis spectra
in solution were measured over the range of 200–700 nm.
Based on each spectrum, the optical band gap was calculated
according to Equation (i), which represents the optical band
gap expressed in eV, and λae denotes the absorption edge
wavelength expressed in nm [4, 49–51]. Fluorescence spec-
tra were recorded on JASCO spectrofuorometer FP-8200
(Japan). Thermogravimetric analysis (TGA) spectra were
recorded on a Perkin Elmer model Pyris 1 (Perkin Elmer
Life and Analytical Sciences, Shelton, CT, USA). Diferen-
tial scanning calorimetry (DSC) spectra were recorded on a
Perkin Elmer model 4000 (PerkinElmer, Inc. Waltham, MA,
USA). The geometrical, distribution of HOMO and LUMO,
energy levels were performed using the Gaussian 16 pro-
gram package using Becke’s three-parameter hybrid func-
tional with Lee–Yang–Parr correlation functions (B3LYP)
and the 6-31G(d) atomic basis set.
Solvent was evaporated under vacuum, ethyl acetate was
added and subsequently washed with water, 5% NaHCO3,
brine and dried over anhydrous Na2SO4, fltered, and the
solvent was evaporated to obtain compound 2 (3.35 g, 93%)
as white powder. Mp. 241–242 °C [52–54].
Synthesis of Ethoxyfluorescein Ethyl Ester (3)
Fluorescein (1, 30.00 g, 90.000 mmol) was suspended in
DMF (30 mL) in a 500 mL round bottom fask equipped
with magnetic stirrer. K2CO3 (2.2 equiv.) was added to the
reaction mixture followed by the addition of ethyl iodide
(2.2 equiv. g~16 mL, d=1.94). The reaction mixture was
stirred at room temperature until the starting materials were
fully consumed (approximately for 24 h). The reaction was
diluted with 500 mL water and the pale-yellow color solid
precipitation was collected by fltration followed by washing
it with excess amount of water (100×3), aforded analytical
pure 3 (26.00 g, 82%). Rf =0.5 (Ethyl acetate: n-hexane=3:
7). Mp. 151–152 °C [54].
Results and Discussion
Esterifcation of fuorescein (1) was carried out in the pres-
ence of catalytic amount of concentrate sulfuric acid in etha-
nol to aford fuorescein ethyl ester (2) in an excellent yield
(93%). O-Ethyl fuorescein ethyl ester (3) was also prepared
from fuorescein (1) with ethyl iodide in the presence of
potassium carbonate (K2CO3) in DMF at room temperature
for 24 h in very good yield (82%) (Scheme 2).
Eg(eV) = h × f = h × c∕ꢀa.e ≈ 1240∕ꢀa.e(nm)
(1)
Photophysical properties of synthesized fuorescein esters
were evaluated to see the possibility of using these materials
as organic semiconductor materials (i.e. OLEDs/OSCs). Ini-
tially, absorbance and fuorescence spectra were taken under
various solvents in constant concentration (2.77×10–5 M for
absorbance; 6.94× 10–7 M for fuorescence) at room tem-
perature to see which solvent would give higher absorbance
and emission. Figure 3A, C and E show the absorption
spectra of compounds 1, 2 and 3 in ethanol, methanol, water,
Synthesis
Synthesis of Fluorescein Ethyl Ester (2)
Fluorescein (1, 3.32 g, 0.01 mol) was dissolved in 50 mL of
absolute ethanol, a catalytic amount of concentrate sulfuric
acid was added and the mixture was refuxed for overnight.
Scheme 2 Synthesis of fuorescein esters 2 and 3
1 3

Journal of Fluorescence
7000
6000
5000
4000
3000
2000
1000
B
0.8
Ethanol
A
Methanol
0.7
0.6
Water
Tetrahedrofuran
Chloroform
Acetonitrile
Ethylacetate
Acetone
CO2H
0.5
HO
O
O
1
0.4
0.3
0.2
0.1
0
0.0
500
550
600
650
350
400
450
500
550
5000
Wavelength (nm)
Wavelength (nm)
D
C
0.3
4000
3000
2000
1000
0
CO2Et
HO
0.2
O
O
2
0.1
0.0
350
400
450
500
550
500
550
600
650
Wavelength (nm)
Wavelength (nm)
3500
3000
2500
2000
1500
1000
500
0.7
F
E
0.6
0.5
CO2Et
EtO
O
O
0.4
0.3
0.2
0.1
0.0
3
0
500
550
600
650
350
400
450
500
550
Wavelength (nm)
Wavelength (nm)
Fig. 3 Absorbance and emission spectra of compounds 1–3 in diferent solvents with λmax (nm), A absorbance of compound 1, B emission of
compound 1, C absorbance of compound 2, D emission of compound 2, E absorbance of compound 3, F emission of compound 3
1 3

Journal of Fluorescence
Table 1 Photophysical properties of compounds 1–3 in respective solvents
Solvent
1
2
3
λ
abs (nm)
λ
FL (nm)
ε×104
λabs (nm)
λ
FL (nm)
ε×104
λabs (nm)
in
λ
FL (nm)
ε×104
in
in
(L mol−1 cm−1
)
in
in
(L mol−1 cm−1
)
in
(L mol−1 cm−1
)
3.10×10–5
M
7.53×10–7
M
2.77×10–5
M
6.94×10–7
M
2.57×10−5
M
6.44×10−7
M
Ethanol
Methanol
Water
THF
CHCl3
ACN
500
480
486
510
462
509
460
509
523
512
510
530
520
540
520
540
1.8
0.6
1.7
0.04
0.02
2.5
0.02
0.3
469, 508
464, 501
465, 496
460, 520
462, 490
477, 517
460, 490
481, 519
527
501
516
545
515
541
534
545
0.8
0.6
0.8
0.4
0.7
0.9
0.3
1.1
460, 490
457, 487
457, 480
460, 490
460, 490
456, 489
457, 487
455, 485
518, 547
515
515
522, 550
520, 550
524, 546
520 548
521, 549
2.4
2.3
2.3
2.1
1.9
1.7
1.7
1.9
EtOAc
Acetone
tetrahydrofuran, chloroform, acetonitrile, ethyl acetate and
acetone, respectively. Figure 3B, D and F display/dem-
onstrate the fuorescence spectra of compounds 1, 2 and 3
in the same solvents.
gave significant broad peaks at 480–600 nm because of
the π* → n / π* → π transitions. Photophysical proper-
ties of 1–3 in different solvents are illustrated in Table 1
in details. Molar absorptivity was calculated for 1–3 in
the solvent evaluated. As shown in Table 1, compound 3
displayed a higher molar absorptivity than compound 2,
which displayed a higher molar absorptivity than com-
pound 1 in most cases.
We found that polarity of the solvents playeds a cru-
cial role in taking absorption/fluorescence spectra for
compound 1–3. Since compound 1 possesses a free acid
group, it was not properly dissolved in all the solvents
studied and therefore it didn’t show excellent absorption.
But in case of compounds 2 and 3, absorption/emission
spectra were perfect as expected. Bathochromic shift
was observed in absorption/emission spectra depend-
ing on change in polarity of the solvent. According to
electronic transition, electrons are promoted from their
ground state to an excited state when an atom or mol-
ecule absorbs energy. In a molecule, the atoms can rotate
and vibrate with respect to each other. These vibrations
and rotations also have discrete energy levels, which can
be considered as being packed on top of each electronic
level. The electronic transitions involved in the ultravio-
let and visible regions of these compounds [55, 56]. As
shown in the Fig. 3A, C and E, all three compounds
gave n → π* / π → π* transitions. Emission spectra of 1–3
Photographs of 1–3 were taken in standard methanolic
solution under naked eye (Fig. 4A), 254 nm (Fig. 4B)
and 365 nm (Fig. 4C), respectively. Compounds 1–3 hav-
ing high luminescence in naked eyes. The photographs
clearly indicate that compounds 1–3 having photophysical
properties.
Emission spectra of compounds 1–3 at diferent pH
(5–9) in THF were also taken to observe the stability of
emission maxima and to clarify the correlation among the
fuorescence behavior of 1–3 (Fig. 5). It was found that
the emission maxima of 1 is lower in all the bufer solu-
tions than in normal THF solutions. Emission maxima was
obtained at 530 nm for normal THF solution (Fig. 5A)
but shifted 4–7 nm (523 – 526 nm) for diferent pH bufer
solutions (Fig. 5B). In case of compound 2 (545 nm)
Fig. 4 Luminescence properties of compounds 1–3 in MeOH: A under normal light, B under a 254 nm light, C under a 365 nm light
1 3

Journal of Fluorescence
B
A
THF (Normal); 530 nm
pH=9; 525 nm
pH=8; 523 nm
pH=7; 526 nm
pH=6; 523 nm
pH=5; 523 nm
2500
2000
1500
1000
500
4000
3000
2000
1000
CO2H
HO
O
O
1
0
0
525
550
575
600
525
550
575
600
3000
C
THF (Normal); 545 nm
CO2Et
D
pH=9; 531 nm
pH=8; 532 nm
pH=7; 534 nm
pH=6; 532 nm
pH=5; 534 nm
2500
2000
1500
1000
500
300
200
100
0
HO
O
O
2
0
525
525
550
575
600
550
575
600
500
400
300
200
100
0
E
THF (Normal); 522, 550 nm
F
2000
pH=9; 532 nm
pH=8; 533 nm
pH=7; 534 nm
pH=6; 518 nm
pH=5; 534 nm
CO2Et
1500
1000
500
EtO
O
O
3
0
500
525
550
575
600
475
500
525
550
575
600
Wavelength (nm)
Fig. 5 Comparison of emission spectra of compounds 1–3 in THF (normal) vs THF (pH 5–9). A Emission of 1 in THF, B Emission of 1 in THF
in various pH, C Emission of 2 in THF, D Emission of 2 in THF in various pH, E Emission of 3 in THF, B Emission of 3 in THF in various pH
1 3

Journal of Fluorescence
25
20
15
10
5
25
20
15
10
5
B
A
1
2
3
(E/eV=2.36)
1
2
3
(498 nm)
(508 nm)
(458 nm)
(E/eV=2.34)
(E/eV=2.37-2.39)
0
0
300
4.0
3.5
3.0
2.5
2.0
350
400
450
500
550
Fig. 6 UV–Vis spectra recorded in ethanol, at room temperature for compounds 1–3. A Molar absorptivity vs wavelengths maxima of 1–3 in
ethanol, B Molar absorptivity vs optical band gaps of 1–3 in ethanol
(Fig. 5C), it has also shifted 14–11 nm, becoming 531-
534 nm (Fig. 5D). Fluorescence spectra were obtained
with little diferences for compound 3 in varying pH buf-
ers. It should be noted that compound 3 showed 522 nm
and a shoulder at 550 nm (Fig. 5E), but in case of dif-
ferent pH bufers, normalized wavelength at 532–534 nm
was observed (Fig. 5F). Interestingly, all three compounds
gave maximum emission at pH = 7. It can be concluded
that pH has no/negligible efect on emission maxima of
compounds 1–3.
materials, thus compounds 1, 2 and 3 are suitable for
organic semiconductors materials.
Differential scanning calorimetry (DSC) was taken
under nitrogen atmosphere for compounds 1–3 to study
their thermodynamic stability (Fig. 7). As shown in
Fig. 6, from the DSC curve, compound 1 has a glass tran-
sition (Tg) peak at 77 ℃ without exhibiting other phase
transitions, whereas compound 2 gave a glass transition
peak at 102 ℃ and underwent crystallization (Tc) at 151
℃. On the other hand, compound 3 gave all three phases
transition with glass transition (Tg) peak at 53 ℃, crys-
tallization (Tc) peak at 140 ℃ and melting (Tm) peak at
153 ℃. From the data analysis of DSC, it turneds out
that, compound 2 has more thermodynamic stability than
compounds 1 and 3.
Molar absorptivity (ε), wavelengths at the maxima
(λmax.), absorption edge wavelengths (λedge) and opti-
cal band gaps (Eoptical band gap eV) were calculated using
UV–Vis spectra for compounds 1–3 in ethanol, and
were depicted in Fig. 6. The values are presented in the
Table 2.
Thermogravimetric analysis (TGA) was also taken under
nitrogen atmosphere for compounds 1–3 to study their ther-
modynamic stability (Fig. 8). It can be seen from the TGA
curve in Fig. 7 that the compounds 1–3 have thermal decom-
position temperature (5% weight loss temperature) at 70,
144 and 245 ℃, respectively. From the DSC and TGA it can
be concluded that compounds 2 and 3 have more thermal
stability than 1, thus can better serve as excellent candidates
of organic semiconductor materials which are necessary for
light emitting devices.
As expected, the UV–Vis spectra of 1–3 showed peak
maxima at 498 nm (optical band gap ~ 2.36 eV), 508 nm
(2.34 eV), and 458 & 488 nm (2.37 – 2.39 eV), respec-
tively (Fig. 6 & Table 2). The obtained optical band gap
values are consistent with standard organic semiconductor
Table 2 Values of the molar absorptivites and wavelengths at the
maxima, absorption edge wavelengths and experimental optical band
gaps for compounds 1–3
Molecular modeling is an iterative process of analyzing
and predicting compound with desired properties. In order
to have a better understanding of the molecular features,
fuorescein derivatives were optimized and calculated by
Gaussian 16 program package. The stability of compound
1 is important for this scenario because we are focusing on
the development of stable fuorescein compounds. Hence,
Bond Dissociation Energy (BDE), a crucial indicator to
evaluate the stability of the compound, has been computed.
Entry ε×103
λmax (nm) λedge (nm) Eoptical band gap eV
L mol−1 cm−1
1
2
3
18.2
7.57
23.9
18.7
498
508
458
488
527
531
2.36
2.34
521
2.37
517
2.39
1 3

Journal of Fluorescence
Fig. 7 Diferential scanning
calorimetry (DSC) of com-
pounds 1–3
-2
0
Tc=140oC
Tg=102oC
Tg=77oC
Tg=53oC
1
2
3
2
Tm=153oC
4
6
8
10
12
Tc=151oC
50
100
150
200
250
300
Temperature (oC)
The calculated BDE value of “C-O” bond in the lactone of
cyclic 1 is 52.06 kcal/mol, which is much lower than the
common “C-O” bond (109.3 kcal/mol), means the cyclic
state is readily to be formed [57]. Meanwhile, the energy of
singlet (S1) and triplet (T1) states of cyclic 1 are calculated
to be 94.24 kcal/mol, 86.96 kcal/mol respectively, all of
which are higher than the value of “C-O” BDE, indicating
that the cyclic 1 is also not stable and can easily undergo
Fig. 8 Thermogravimetric
analysis (TGA) of compounds
1–3
110
100
90
80
70
60
50
40
30
20
1
2
3
: (5% weight loss at 70 oC)
: (5% weight loss at 144 oC)
: (5% weight loss at 245 oC)
50
100
150
200
250
300
350
Temperature (oC)
1 3

Journal of Fluorescence
A
E (S1) 94.24 kcal/mol
E (T1) 82.96 kcal/mol
BDE (C-O)=Δ
f
H
0
(A·+B·) -Δ
fH0
(AB)
52.06 kcal/mol
B
open 1-LUMO
open 1-HOMO
open 1
cyclic 1-LUMO
cyclic 1
cyclic 1-HOMO
Fig. 9 A Stability analysis of cyclic and open state compound 1, B Optimized structure and distribution of HOMO and LUMO of cyclic and
open state compound 1
photodegradation to form the open 1 as illustrated in
Fig. 9A. Additionally, distributions of HOMO and LUMO
of 1 also apparently show that open form gets more con-
jugated systems because the HOMO and LUMO of open
1 are located mainly on the xanthene system while cyclic
1 are separated located on xanthene and isobenzofuran
systems, respectively, as depicted in Fig. 9B, which is con-
sistent with optical properties discussed above. Therefore,
on the one hand, it is necessary to prevent the cyclic ring
formation by protecting carboxyl group with inert substitu-
ents, while on the other hand, steric hindrance also has to
be taken into consideration that to avoid the carboxylate to
1 3

Journal of Fluorescence
Table 3 Values of the calculated data of compound 1–3
a.u., and the value of BDE is 79.11 kcal/mol, even close to
the phenol BDE [58]. All of the data indicate that protec-
tion of phenol group is also important. Hence, we used
large steric hindrance group, ethyl groups, to stabilize and
protect the active compound 1.
Entry
HOMO (eV)
LUMO (eV)
S1 (eV)
T1 (eV)
1open
1cyclic
2
-5.53
-6.03
-5.52
-5.81
-2.36
-1.29
-2.34
-2.76
2.65
4.09
2.66
2.53
1.58
3.60
1.59
1.50
Likewise, HOMO and LUMO distributions of compound
2 and 3 are all dispersed on the xanthene rings, which are
signifcant proofs of the increasing conjugated system as
summarized in Table 3. All of the data we calculated were
well associated with the experimental ones, which are direct
evidences to verify our ideas, illustrating our molecular
design strategy is on the right way.
3
be attacked by nucleophile. Additionally, the acidic phe-
nol group on xanthene system is another unstable factor
because it is easy to lose proton under base conditions.
Calculated Mulliken charge value of the proton is 0.416
Optimized structure and distribution of HOMO and
LUMO of compounds 2 and 3 is depicted in Fig. 10.
Fig. 10 Optimized structure and distribution of HOMO and LUMO of compounds 2 and 3
1 3

Journal of Fluorescence
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