Spectral features of substituted 9-(phenoxycarbonyl)-acridines and their protonated and methylated cation derivatives

Article abstract of DOI:10.1016/j.saa.2007.10.045

The long-wavelength absorption of eight 9-(phenoxycarbonyl)-acridines and the 10-H-9-(phenoxycarbonyl)-acridinium and 10-methyl-9-(phenoxycarbonyl)-acridinium cations derived from them, substituted with an alkyl or trifluoroalkyl group at the benzene ring

Full text of DOI:10.1016/j.saa.2007.10.045

Available online at www.sciencedirect.com
Spectrochimica Acta Part A 70 (2008) 394–402
Spectral features of substituted 9-(phenoxycarbonyl)-acridines
and their protonated and methylated cation derivatives
Karol Krzyminskia,∗, Alexander D. Roshalb, Agnieszka Niziołeka
´
a University of Gdan´sk, Faculty of Chemistry, J. Sobieskiego 18, 80-952 Gdan´sk, Poland
b Kharkiv V.N. Karazin National University, Institute of Chemistry, Svoboda Square 4, 61 077 Kharkiv, Ukraine
Received 21 January 2007; received in revised form 29 October 2007; accepted 31 October 2007
Abstract
The long-wavelength absorption of eight 9-(phenoxycarbonyl)-acridines and the 10-H-9-(phenoxycarbonyl)-acridinium and 10-methyl-9-
(phenoxycarbonyl)-acridinium cations derived from them, substituted with an alkyl or trifluoroalkyl group at the benzene ring, occurs above
300 nm as the superposition of four bands. Three of these bands occupy comparable positions (expressed in nm) in all the compounds; the fourth
one, however, changes position, appearing in neutral molecules as a long-wavelength shoulder below 400 nm, but in cations as an almost separate
band above 400 nm. The weak fluorescence resulting from excitation within the long-wavelength absorption band is red-shifted relative to absorp-
tion, such that Stokes shifts are similar for both neutral molecules and cations. Stokes shifts tend to increase with the orientational polarisability
of a medium. Computations predict that long-wavelength electronic transitions are accompanied by structural changes in molecules. They also
indicate that such transitions are followed by roughly uniform electron density changes in whole molecules accompanied by small changes in their
dipole moments, which accounts for the weak absorption in the long-wavelength region. The predicted radiative and non-radiative deactivation
rate constants suggest the occurrence of efficient spin–orbital coupling in the molecules investigated, which is the cause of the relatively low
fluorescence quantum yields. Apart from the cognitive significance of these investigations, the results demonstrate that absorption of radiation by
10-methyl-9-(phenoxycarbonyl)-acridinium cations above 400 nm may influence their chemiluminescence output.
© 2007 Elsevier B.V. All rights reserved.
Keywords: 9-(Phenoxycarbonyl)-acridines; 10-H-9-(phenoxycarbonyl)-acridinium cations; 10-Methyl-9-(phenoxycarbonyl)-acridinium cations; Electronic absorp-
tion and emission spectroscopy; Features of excited states
1. Introduction
cancer [18,19], anti-amoebic [20], anti-inflammatory [21] or
immuno-modulating drugs [22].
Acridines have been a topic of interest for a long time owing
to their interesting features and numerous applications. Their
spectral properties have been much investigated, since these
compounds absorb radiation in the near-UV region [1–5] and
fluoresce [4–6] or phosphoresce [5,7–9] at the short-wave end of
the visible region. Since both absorption and emission depend
on the properties of the medium, acridines have been used to
monitor the properties of liquid phases [10–12]. Further, the
tautomeric phenomena taking place in some acridines have been
investigated by electronic absorption and emission spectroscopy
[13–15]. Lastly, numerous acridines exhibit biological activity:
some of them have been used as antibacterial [1,16,17], anti-
This publication focuses on derivatives of acridine-9-
carboxylic acid, i.e. derivatives of 9-(phenoxycarbonyl)-
acridines and their cationic forms protonated or methylated
at the endocyclic N atom. These latter entities are chemilu-
minogenic in that after reaction with H2O2 in alkaline media,
the product – electronically excited 10-methyl-9-acridinone
– emits light [23–26]. The efficiency of chemiluminescence
depends on the rate at which the original cations are con-
verted to the latter molecules and the phenyl-containing entities
are removed [27]. Substitution can affect the rate of this
conversion, and hence the efficiency of chemiluminescence;
several phenyl-substituted derivatives of 9-(phenoxycarbonyl)-
acridines were therefore synthesised in order to investigate
their spectral properties. The chemiluminescence of 10-alkyl-
9-(phenoxycarbonyl)-acridinium cations is affected by the
presence of nucleophiles or anionic species, an effect that
∗
Corresponding author. Tel.: +48 58 523 5467; fax: +48 58 523 5472.
E-mail address: karolk@chem.univ.gda.pl (K. Krzymin´ski).
1386-1425/$ – see front matter © 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2007.10.045

K. Krzymin´ski et al. / Spectrochimica Acta Part A 70 (2008) 394–402
395
has been used to determine their content [28,29]. Alkylated
9-(phenoxycarbonyl)-acridines also occur as luminogenic frag-
ments of chemiluminescent labels [25,30,31].
The aim of this investigation was to examine the spec-
tral features of the title compounds and to discover the
extent to which the absorption or emission of 10-methyl-9-
(phenoxycarbonyl)-acridinium cations can affect originating
from them chemiluminescence.
confirmed by X-ray structure analysis [33,34]. Elemental anal-
yses (EAGER 200 CHNS Analyser, Carlo Erba) and melting
points (uncorrected) of 9-(alkylphenoxycarbonyl)-acridines
(% found/calculated): 1a: C 79.79/80.25, H 4.44/4.38, N
4.25/4.68, m.p. = 463–465 K; 1b: C 80.68/80.49, H 4.81/4.82,
N 4.45/4.47, m.p. = 411–412 K; 1c: C 80.90/80.71, H 5.12/5.23,
N 4.33/4.28, m.p. = 392–394 K; 1d: C 80.85/80.92, H 5.52/5.61,
N 4.08/4.10, m.p. = 395–397 K; 1e: C 80.85/81.10; H 6.02/5.96,
N 3.86/3.94, m.p. = 462–463 K; 1f: C 80.61/80.49, H 4.84/4.82;
N 4.41/4.47, m.p. = 428–429 K; 1g: C 80.42/80.49, H 4.86/4.82,
N 4.32/4.47, m.p. = 439–441 K; 1h: C 68.57/68.67, H 3.37/3.29,
2. Materials and methods
1
N 3.77/3.81, m.p. = 408–409 K. The H and 13C NMR spectra
2.1. Compounds
of the compounds will be presented and discussed elsewhere
[35].
9-(Alkylphenoxycarbonyl)-acridines (1, Scheme 1) were
synthesised as described in detail elsewhere [23,32]. In
brief, 9-(chlorocarbonyl)-acridine (obtained by treating
acridine-9-carboxylic acid with thionyl chloride) and
stoichiometric amounts (3.6%, w/w each) of phenol or
alkyl-substituted phenols (2-methylphenol, 3-methylphenol,
4-methylphenol, 2-ethylphenol, 2-isopropylphenol, 2-tert-
butylphenol, 2-trifluoromethylphenol) were mixed in anhydrous
dichloromethane. The esterification was carried out in the pres-
ence of N,N-diethylethanamine (twofold molar excess) with
continuous stirring and in the presence of catalytic amounts of
N,N-dimethyl-4–pyridinamine (room temperature, 18–36 h).
The crude esters were purified chromatographically (SiO2
column; mobile phase: cyclohexane/ethyl acetate (3/2–2/1,
v/v). Yields were 60–80%. The identity of 1b and 1c was
10-H-acridinium-9-(alkylphenoxycarbonyl) cations (2a–2h)
appeared in situ in acetonitrile or dichloromethane when
the esters (1a–1h), at concentrations of 1–5 × 10−5 M, were
treated with dilute hydrochloric acid (final concentration of
HCl = 1 × 10−4 M).
10-Methyl-9-(alkylphenoxycarbonyl)-acridinium trifluoro-
methanesulphonates (containing cations 3a–3h) were obtained
by reaction of the above esters with a fivefold molar excess
of methyl trifluoromethanesulphonate, both dissolved in anhy-
drous dichloromethane (Ar atmosphere, room temperature,
3–3.5 h). The crude products were dissolved in ethanol and the
solutions filtered. An excess of diethyl ether was then added to
each solution (25/1, v/v) and yellow precipitates were filtered off
(yields 60–88%). The purity of the acridinium salts was checked
Scheme 1. The entities investigated with the numbering of atoms indicated. (1) 9-(phenoxycarbonyl)-acridines: (1a) 9-(phenoxycarbonyl)-acridine, (1b) 9-(2-
methylphenoxycarbonyl)-acridine, (1c) 9-(2-ethylphenoxycarbonyl)-acridine, (1d) 9-(2-isopropylphenoxycarbonyl)-acridine, (1e) 9-(2-tert-butylphenoxycarbonyl)-
acridine, (1f) 9-(3-methylphenoxycarbonyl)-acridine, (1g) 9-(4-methylphenoxycarbonyl)-acridine, (1h) 9-(2-trifluoromethylphenoxycarbonyl)-acridine; (2)
10-H-9-(phenoxycarbonyl)-acridinium cations: (2a) 10-H-9-(phenoxycarbonyl)-acridinium cation, (2b) 10-H-9-(2-methylphenoxycarbonyl)-acridinium cation, (2c)
10-H-9-(2-ethylphenoxycarbonyl)-acridinium cation, (2d) 10-H-9-(2-isopropylphenoxycarbonyl)-acridinium cation, (2e) 10-H-9-(2-tert-butylphenoxycarbonyl)-
acridinium cation, (2f) 10-H-9-(3-methylphenoxycarbonyl)-acridinium cation, (2g) 10-H-9-(4-methylphenoxycarbonyl)-acridinium cation, (2h) 10-H-9-(2-
trifluoromethylphenoxycarbonyl)-acridinium cation; (3) 10-methyl-9-(phenoxycarbonyl)-acridinium cations: (3a) 10-methyl-9-(phenoxycarbonyl)-acridinium
cation, (3b) 10-methyl-9-(2-methylphenoxycarbonyl)-acridinium cation, (3c) 10-methyl-9-(2-ethylphenoxycarbonyl)-acridinium cation, (3d) 10-methyl-9-(2-
isopropylphenoxycarbonyl)-acridinium cation, (3e) 10-methyl-9-(2-tert-butylphenoxycarbonyl)-acridinium cation, (3f) 10-methyl-9-(3-methylphenoxycarbonyl)-
acridinium cation, (3g) 10-methyl-9-(4-methylphenoxycarbonyl)-acridinium cation, (3h) 10-methyl-9-(2-trifluoromethylphenoxycarbonyl)-acridinium cation.

396
K. Krzymin´ski et al. / Spectrochimica Acta Part A 70 (2008) 394–402
by TLC (Silicagel 60; mobile phase: 0.1 M trifluoroacetic acid in
chloroform); in each case a single signal was obtained (fluores-
cence detection, mid-pressure mercury UV lamp). The structure
of compound 3b was confirmed by X-ray structure analysis [36]
and all compounds (3a–3h) by 1H and 13C NMR spectrometry
[35].
solvents used for the spectral investigations – acetonitrile,
dichloromethane, trichloromethane, acetone, butyl acetate and
toluene – were of spectral grade and were selected so as to cover
a broad range of orientational polarisability [39,40].
2.3. Calculations
2.2. Spectral measurements
The geometries of the entities (Scheme 1) in the vibrationally
relaxed ground (S0), singlet (Si) and triplet (Ti) electronic states
wereobtainedusingthesemi-empiricalAM1andAM1/CImeth-
ods [41] implemented in the MOPAC 2002 program [42]. The
wavelengths and oscillator strengths of electronic absorption
and emission transitions were calculated using the CNDO/S
method implemented in HyperChem 7 with an 8 × 8 CI singly
excited matrix [43]. The intersystem crossing rate constants
(kISC) were estimated employing the CNDO/S/SOI method [44]
with a 15 × 15 singly excited CI matrix. To obtain the matrix
Absorption spectra were recorded on a Hitachi U3210
UV–vis spectrophotometer. The experimental absorption spec-
tra were deconvoluted using Spectral Data Lab software [37].
Fluorescence spectra and quantum yields were measured with
a Hitachi F4010 spectrofluorimeter with a solution of qui-
nine sulphate in 0.05 M aq. sulphuric acid as the quantum
yield reference standard (ϕ = 0.56 [38]). The concentrations
of the compounds investigated were 1–5 × 10−5 M. All the
Table 1
Absorption (above 300 nm) and fluorescence characteristics of 9-(phenoxycarbonyl)-acridines (1a–1h), 10-H-9-(phenoxycarbonyl)-acridinium cations (2b–2h) and
10-methyl-9-(phenoxycarbonyl)-acridinium cations (3b–3h)
Medium Compound
(Scheme 1)
λabs
λfl
ꢀνSt
φ
Compound
(Scheme 1)
λabs
λfl
ꢀνSt
5700
4860
5200
5510
5480
5540
5660
5420
φ
Compound
(Scheme 1)
λabs λfl
ꢀνSt
φ
1a
1b
1c
1d
1e
1f
2a
2b
2c
2d
2e
2f
3a
3b
3c
3d
3e
3f
CH2Cl2
CH3CN
382
363
381
361
455
454
5600
5700
1.3
1.6
413
363
408
362
428
367
423
366
510
3720
5400
< 0.1
530
527
527
526
529
532
535
524
0.3
0.2
0.2
544
0.1
CH2Cl2
CH3CN
383
362
381
361
456
453
5480
5620
0.8
1.6
413
364
420
365
430
367
423
366
501
533
3300
5260
< 0.1
0.4
CH2Cl2
CH3CN
383
362
381
361
454
455
5580
5560
0.6
1.8
413
364
414
364
429
367
424
366
503
534
3400
4860
< 0.1
0.3
CH2Cl2
CH3CN
382
363
381
361
455
454
5660
5640
0.6
1.6
413
364
408
363
428
367
423
366
506
532
3580
4820
< 0.1
<0.1
0.1
0.2
0.2
4.6
0.4
CH2Cl2
CH3CN
386
365
382
363
455
455
5460
5540
0.7
1.3
412
365
410
365
431
369
426
368
502
538
3260
4880
0.2
0.5
CH2Cl2
CH3CN
382
362
380
361
454
454
5600
5680
0.8
1.4
413
363
411
363
430
368
423
366
523
512
4140
4320
< 0.1
< 0.1
1g
1h
2g
2h
3g
3h
CH2Cl2
CH3CN
382
362
381
361
453
451
5560
5640
0.2
0.8
413
363
411
362
428
367
423
366
520
513
4060
4120
< 0.1
< 0.1
CH2Cl2
CH3CN
382
363
382
362
460
458
5760
5640
1.8
2.2
415
365
408
364
432
370
419
364
527
525
4180
4820
6.6
12.1
λabs, λfl: positions of band maxima in absorption and fluorescence spectra (in nm); ꢀνSt: Stokes shift (in cm−1; ꢀνSt = 1/λexc − 1/λfl, where λexc: position of the
long-wavelength band maximum in a fluorescence excitation spectrum (in nm); φ: fluorescence quantum yield (in %).

K. Krzymin´ski et al. / Spectrochimica Acta Part A 70 (2008) 394–402
397
elements of the spin–orbital coupling operator, single-, double-
and triple-centred integrals were taken into account. The inter-
system crossing rate constants kISC (s−1) were calculated from
the equation [44]:
ꢀ
2
−(E −E i /5×103)
kISC = 3.95 × 1011 | < S1|H|Ti > | × e
S
T
ˆ
1
i
ꢀ
= 3.95 × 1011
β12iF1i
where | < S1|H|Ti > | and β1i are the matrix elements of the
spin–orbital coupling operator, ES is the energy of state S1
1
(in cm−1), ET represents the energies of all the triplet lev-
i
els below S1 (in cm−1), and Fli is the Franck-Condon factor,
which includes the overlap between states S1 and Ti for all of
the above-mentioned intersystem crossing transitions. Fluores-
cence quantum yields were predicted on the assumption that
intersystem crossing is the main pathway of radiationless deac-
tivation. Rateconstantsofradiativedeactivation(kf)werefurther
calculated by the CNDO/S/SOI method, starting from oscillator
strength values [44,45].
*
The types of electronic transitions (␲ → ␲ or n → ␲*)
involved were discovered by analysis of the components of
configuration interaction matrices and excitation localisation
numbers [46,47].
3. Results and discussion
3.1. Experimental absorption and fluorescence spectra
9-(Phenoxycarbonyl)-acridines (1), 10-H-9-(phenoxy-
carbonyl)-acridinium cations (2) and 10-methyl-9-(phenoxy-
carbonyl)-acridinium cations (3) exhibit long-wavelength
absorption and fluorescence (Fig. 1 and Table 1); bathochromi-
cally shifted relative to that of acridine itself [48–50]. The
long-wavelength absorption of these molecules occurs above
300 nm and is a superposition of four bands (Fig. 1). The posi-
tions of two of these bands and those of the fluorescence bands
are given in Table 1. The band at the long-wavelength absorption
shoulder is poorly resolved in the case of 9-(phenoxycarbonyl)-
acridines but quite well resolved for the cationic forms. The
decadic molar absorption coefficients of these bands lie between
0.8–5 × 104 M−1 cm−1, which is characteristic of weak absorp-
tion. All the fluorescence bands are typically red-shifted in such
a way that the Stokes shifts of the neutral and cationic entities
are similar. Substitution at the phenyl ester ring only weakly
influences the positions of the absorption and fluorescence
bands within a given group of entities (1, 2, or 3). The Stokes
shifts are similar for all the 9-(phenoxycarbonyl)-acridines.
The differences in the Stokes shifts are the most pronounced
in the case of 10-methyl-9-(phenoxycarbonyl)-acridinium
cations.
Fig. 1. Absorption (measured: solid line; deconvoluted: dashed line) and fluo-
rescence (dotted line) spectra of 1a, 2a and 3a in CH3CN (c = 1–5 × 10−5 M;
l = 1 cm) (upper graphs) together with positions of electronic absorption
(ꢁ1 = S0 → S1, ꢁ2 = S0 → S2, ꢁ3 = S0 → S3, ꢁ4 = S0 → S4) and fluorescence
(ꢁ’1 = S1 → S0) transitions predicted at the CNDO/S level of theory (lower
graphs).
however, discernible. In general, the absorption and fluores-
cence characteristics of neutral 9-(phenoxycarbonyl)-acridines
are only weakly affected by solvent properties (Table 1).
There are some similarities between the spectral characteristics
of 10-H–9-(phenoxycarbonyl)-acridinium and 10-methyl-9-
(phenoxycarbonyl)-acridinium cations, but the latter are the
most susceptible to changes in the medium’s properties. This
is reflected not only in the positions of the absorption and fluo-
rescence spectra, but also in the values of the Stokes shifts. In
order to gain a broader insight into this problem, the Stokes shifts
3.2. Influence of the medium on spectral characteristics
The spectral features of each entity depend individually
on the properties of the medium. Certain regularities are,

398
K. Krzymin´ski et al. / Spectrochimica Acta Part A 70 (2008) 394–402
3.3. Electronic transitions in the context of computational
data
The computationally predicted positions of four long-
wavelength electronic absorption (S0 → Si) transitions are
red-shifted relative to the maxima of the equivalent bands in the
absorption spectra (Fig. 1). S0 → S1 transitions are always of
the ␲ → ␲* type, whereas S0 → S2, S0 → S3 and S0 → S4 tran-
sitions are predominantly of the ␲ → ␲* type, but sometimes of
the n → ␲* type (Table 2); this is borne out by analysis of con-
figuration interaction matrices, and is, moreover, in accordance
what is known so far about acridine spectroscopy [5].
Computations predict rather poorly the position of fluores-
cence (S1 → S0) transitions, which are shifted bathochromically
in cationic species and hypsochromically in neutral molecules
relative to the band maxima in experimental spectra. The
computational predictions are generally corroborated by the
experimentally revealed spectral behaviour.
Fig. 2. Plot of the Stokes shifts (ꢀνSt = 1/λexc − 1/λfl, where λexc, position
of the long-wavelength band maximum in a fluorescence excitation spec-
trum) for 3a dissolved in CH2Cl2 (1), toluene (2), chloroform (3), butyl
acetate (4), acetone (5) and CH3CN (6) against the orientational polarisabil-
ity (ꢀf = (ε − 1)/(2ε + 1) − (n2 − 1)/(2n2 + 1)), where ε, dielectric constant and
n, refractive index.
Electronic transitions bring about structural changes in neu-
tral molecules and cationic entities that are reflected particularly
in the mutual arrangement of the acridine, carboxyl and phenyl
fragments (Table 3). Changes in the angles between the mean
planes of these fragments apparently do not correlate with their
structures or properties.
Electronic absorption and emission always accompany
charge distribution changes; this always occurs during radia-
tive transitions. Changes in relative Mulliken charges in selected
corresponding to the long-wavelength band of 3a were plotted
against orientational polarisability (Fig. 2). This figure shows a
certain trend: the higher the value of ꢀf, the greater the differ-
ences in electron density distribution in the ground and excited
states of this entity. The value corresponding to dichloromethane
does not follow this trend. There is no simple explanation of this
finding.
Table 2
Wavelengths (in nm) and oscillator strengths (in parentheses) of electronic absorption (S0 → Si) and fluorescence (S1 → S0) transitions, together with relevant
excitation localisation numbers at acridinic (LAcr), carboxylic (LCOO) and phenylic (LPh) sites (in %), predicted at the CNDO/S level for selected compounds
Compound
Absorption
Transition type
Fluorescence
401 (0.298)
LAcr
LCOO
LPh
1a
379(0.272)
359(0.030)
351(0.121)
291(0.012)
␲ → ␲*
n → ␲*
␲ → ␲*
␲ → ␲*
100
100
100
99
0
0
0
1
0
0
0
0
1e
1h
3a
3e
3h
378(0.285)
359(0.027)
358(0.119)
301(0.017)
␲ → ␲*
n → ␲*
␲ → ␲*
␲ → ␲*
␲ → ␲*
n → ␲*
␲ → ␲*
␲ → ␲*
␲ → ␲*
␲ → ␲*
␲ → ␲*
␲ → ␲*
␲ → ␲*
␲ → ␲*
␲ → ␲*
␲ → ␲*
401 (0.283)
400 (0.276)
593 (0.006)
599 (0.002)
490 (0.148)
100
100
100
100
0
0
0
0
0
0
0
0
379(0.285)
359(0.022)
350(0.121)
292(0.012)
100
100
100
100
0
0
0
0
0
0
0
0
437(0.172)
378(0.009)
375(0.003)
354(0.154)
100
50
50
0
50
50
0
0
100
100
0
100
434(0.138)
408(0.024)
382(0.015)
372(0.160)
100
50
50
0
50
50
0
0
100
100
0
100
432 (0.151)
392(0.229)
368(0.019)
354(0.223)
␲ → ␲*
␲ → ␲*
␲ → ␲*
␲ → ␲*
100
100
50
0
0
50
0
0
0
100
0
100

K. Krzymin´ski et al. / Spectrochimica Acta Part A 70 (2008) 394–402
399
Fig. 3. Changes in relative Mulliken atomic charges accompanying electronic excitation (the difference between the relative atomic charges in the relaxed excited
(S1; AM1/CI level of theory) and the relaxed ground (S0; AM1 level of theory) states). Empty circles indicate the decrease and filled circles the increase in electron
density at a given atom; the area of the circles is proportional to the absolute values of the relative Mulliken charge change at a given atom.
entities demonstrate that charge distribution changes during
S0 → S1 transitions take place in whole molecules (Fig. 3).
These changes are roughly uniform, which means that differ-
ences in dipole moments in states S1 and S0 are small in the
case of neutral molecules (Table 3). This explains the relatively
low absorption capacity of the compounds investigated.
Analysis of configuration interaction matrices provides exci-
tation localisation numbers [45,46], which can be regarded as
a measure of involvement of the various molecular sites in
electronic transitions. It was predicted which of the acridinic
(Acr), carboxylic (COO) or phenylic (Ph) sites in selected neu-
tral molecules and cationic species contribute to the first four
electronic transitions (Table 2). The excitation localisation num-
bers for S0 → S1, S0 → S2, S0 → S3 and S0 → S4 transitions in
neutral molecules indicate that predominantly acridine systems
should be involved in these transitions. The situation is similar in
the case of S0 → S1 and S0 → S4 transitions in cationic species.
In S0 → S2 and S0 → S3 transitions of cationic species all three
of these sites should be involved. These findings do not confirm
the predictions concerning electron distribution changes (Fig. 3),
which suggest that excitation involves whole molecules. This
is probable, since the measured and predicted positions of both

400
K. Krzymin´ski et al. / Spectrochimica Acta Part A 70 (2008) 394–402
Table 3
Physicochemical and structural characteristics for selected compounds predicted at the AM1 (S0 state) and AM1/CI (S1 state) levels of theory
Characteristic
Compound
1a
1b
1e
1h
μ(S1)–μ(S0)
Angle: A
Angle: B
0.3
−0.1
−0.2
−0.6
91 (81)
21 (5)
50 (92)
80 (81)
4 (5)
88 (73)
12 (35)
62 (73)
86 (76)
8 (7)
Angle: C
87 (65)
78 (58)
2a
2b
2e
2h
Angle: A
Angle: B
Angle: C
56 (79)
16 (11)
54 (54)
60 (85)
9 (5)
65 (67)
80 (86)
62 (91)
34 (31)
86 (81)
48 (42)
50 (38)
3a
3b
3e
3h
Angle: A
Angle: B
Angle: C
95 (92)
3 (5)
89 (93)
96 (92)
3 (5)
81 (92)
75 (88)
52 (94)
22 (30)
84 (68)
43 (84)
44 (40)
μ: Dipole moments in the S0 and S1 states (in Debyes); A: between the mean plane of the acridine nucleus (delineated by C1–C9, N10, C11–C14) and the plane
delineated by C15, O16, O17 in the S0 and S1 states (in parentheses); B: between the mean plane of the acridine nucleus and the mean plane of the benzene ring
(delineated by C18–C23) in the S0 and S1 states (in parentheses); C: between the mean plane of the benzene ring and plane delineated by C15, O16, O17 in the S0
and S1 states (in parentheses); the values of angles A, B and C (in degrees) were obtained with the HyperChem program [43] using AM1 and AM1/CI optimised
structures.
absorptionandemissiontransitionsarebathochromicallyshifted
relative to those of native acridine [48–50], which would point
to an extension of the conjugated system at the phenyl site via
the carboxyl group. On the other hand, the pattern of the long-
wavelength absorption spectra of the compounds investigated
is similar to that of the corresponding acridine [48–50]. This
would indicate that the latter fragment predominantly affects
the spectral behaviour of 9-(phenoxycarbonyl)-acridines and the
protonated and methylated forms derived from them.
(3h) exceeds 10%. This means that non-radiative deactivation
is the predominant process in the depopulation of electronically
excited molecules. This is indeed confirmed by the results of our
predictions (Table 4). They demonstrate that rates of intersys-
tem crossing exceed rates of fluorescence decay by roughly two
orders of magnitude. In such a situation, predicted fluorescence
quantum yields are no greater than a few per cent. Rapid inter-
system crossing is a consequence of the efficient spin–orbital
coupling noted earlier in some acridine derivatives [5,51].
3.4. Fluorescence efficiency
3.5. Spectral features in relation to chemiluminescence
phenomena
The experimentally determined fluorescence quantum yields
are generally low for both neutral molecules and cationic enti-
ties (Table 1); the fluorescence quantum yield of only one entity
Oxidation of 10-methyl-9-(phenoxycarbonyl)-acridinium
cations with H2O2 in alkaline media produces electronically
Table 4
The rate constants of radiative deactivation (kf), the intersystem crossing rate constants (kISC) and fluorescence quantum yields (φ) predicted at the CNDO/S/S0I
level of theory
Characteristic
Compound
1a
1b
1e
1h
kf (s−1
)
9.33 × 106
8.11 × 108
1.14
7.18 × 106
9.02 × 108
0.79
8.00 × 106
1.33 × 109
0.60
1.25 × 107
7.76 × 108
1.59
kISC (s−1
φ (%)
)
)
)
2a
2b
2e
2h
kf (s−1
kISC (s−1
φ (%)
)
5.69 × 105
2.02 × 109
0.03
6.60 × 105
1.17 × 109
0.05
3.13 × 105
3.68 × 109
0.01
9.27 × 107
2.61 × 109
3.43
3a
3b
3e
3h
kf (s−1
kISC (s−1
φ (%)
)
1.71 × 106
3.51 × 109
0.04
7.11 × 105
3.28 × 109
0.02
4.28 × 105
2.79 × 109
0.02
5.91 × 107
2.06 × 109
2.79
Fluorescence quantum yields were calculated on the assumption that intersystem crossing is the main pathway of non-radiative deactivation; φ = kf/(kf + kISC).

K. Krzymin´ski et al. / Spectrochimica Acta Part A 70 (2008) 394–402
401
excited molecules of 10-methyl-9-acridinone, which exhibit
broad emission centred around 450 nm [52]. Such emission
thus overlaps both the long-wavelength absorption and the
fluorescence of the original cations. If the thus excited 10-
methyl-9-acridinone molecules emit radiation, this can be
partially absorbed by native 10-methyl-9-(phenoxycarbonyl)-
cations and then emitted by them with a low quantum yield.
Such processes will lower chemiluminescence efficiency, which
is unwanted when one intends to investigate or apply this phe-
nomenon analytically. It is difficult to assess to what extent
the internal absorption and emission of the original acridinium
cationsactuallyaffectschemiluminescence, sinceitisnotknown
how long these entities remain in unchanged form following oxi-
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[27] and some fraction of the original cations must always be
present during the whole process. Internal absorption and emis-
sion should thus be taken into account as a factor determining the
chemiluminogenic ability of 10-methyl-9-(phenoxycarbonyl)-
acridinium cations.
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4. Concluding remarks
The results of these investigations indicate that substitution in
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characteristics within a given group of entities.
The lowest energy electronic transitions in all compounds
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The patterns of the long-wavelength absorption spectra of 9-
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Acknowledgements
The financial support of this work from the Polish State Com-
mittee for Scientific Research (KBN) under grants 4T09A 123
23 (contract no. 0674/T09/2002/23) and BW/8000-5-0232-3 is
gratefully acknowledged.
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