Experimental and theoretical insights into molecular and solid-state properties of isomeric Bis(salicylaldehydes)

Article abstract of DOI:10.1021/acs.jpca.9b07360

A series of five bis(salicylaldehydes), including four isomeric compounds based on a benzene scaffold and a closely related naphthalene derivative, were investigated in order to elucidate the impact of resonance effects and intramolecular hydrogen bonds (HBs) on the macroscopic properties of these systems. Density functional theory (DFT) computations revealed important differences between isomers on the molecular level, which was reflected in different charge distributions, aromatic C-C bond orders, and aromaticity characters. The consequences of these features were evidenced by the UV-vis absorption spectra: for 1,3-diformyl-4,6-dihydroxybenzene (2), the longest wavelength absorption band is observed at 285 nm, while its isomers 1,4-diformyl-2,5-dihydroxybenzene (1), 1,4-diformyl-2,3-dihydroxybenzene (3), and 1,2-diformyl-3,6-dihydroxybenzene (4) are characterized by absorption in the visible range (379-407 nm). The specificity of 2 results from simultaneous lowering and elevation of HOMO and LUMO energy levels, respectively. We have found that the HOMO/LUMO energy variations follow trends observed in isomeric dihydroxybenzenes (HOMO) and phthalaldehydes (LUMO), and these effects operate separately to some extent. Furthermore, theoretical calculations indicate that the UV-vis spectral properties of bis(salicylaldehydes) are directly transferable to the corresponding bis(salicylaldimines) and their boron complexes. Finally, the influence of structural and molecular stabilization effects was analyzed by means of X-ray structural analysis and periodic DFT computations.

Full text of DOI:10.1021/acs.jpca.9b07360

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A: Spectroscopy, Molecular Structure, and Quantum Chemistry
Experimental and Theoretical Insights into Molecular and
Solid-State Properties of Isomeric Bis(salicylaldehydes)
Krzysztof Durka, Bartosz Górski, Krzysztof B#ocki, Mateusz Urban,
Krzysztof Wozniak, Micha# Barbasiewicz, and Sergiusz Luli#ski
J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.9b07360 • Publication Date (Web): 13 Sep 2019
Downloaded from pubs.acs.org on September 14, 2019
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Experimental and Theoretical Insights into Molecular
and Solid-State Properties of Isomeric
Bis(salicylaldehydes)
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Krzysztof Durka,a* Bartosz Górski,b Krzysztof Błocki,b Mateusz Urban,a Krzysztof Woźniak,c
Michał Barbasiewicz,b Sergiusz Lulińskia
a) Faculty of Chemistry, Warsaw University of Technology, Noakowskiego 3, 00-664
Warsaw, Poland
b) Faculty of Chemistry, University of Warsaw, Pasteura 1, 02-093 Warsaw, Poland
c) University of Warsaw, Biological and Chemical Research Centre, Żwirki i Wigury 101,
02-089 Warsaw, Poland
ABSTRACT: A series of five bis(salicylaldehydes) including four isomeric compounds based on
a benzene scaffold and a closely related naphthalene derivative were investigated in order to
elucidate the impact of resonance effects and intramolecular hydrogen bonds (HBs) on
macroscopic properties of these systems. Density functional theory (DFT) computations revealed
important differences between isomers on the molecular level, which reflected in different charge
distribution, aromatic C–C bond orders and aromaticity characters. Consequences of these features
were evidenced by the UV-Vis absorption spectra: 1,3-diformyl-4,6-dihydroxybenzene 2 longest
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wavelength absorption band is observed at 285 nm, while its isomers 1,4-diformyl-2,5-
dihydroxybenzene 1, 1,4-diformyl-2,3-dihydroxybenzene 3, and 1,2-diformyl-3,6-
dihydroxybenzene 4 are characterized by absorption in the visible range (379-407 nm). The
specificity of 2 results from simultaneous lowering and elevation of HOMO and LUMO energy
levels, respectively. We have found that the HOMO/LUMO energy variations follow trends
observed in isomeric dihydroxybenzenes (HOMO) and phthalaldehydes (LUMO), and these
effects operate separately to some extent. Furthermore, theoretical calculations indicate that UV-
Vis spectral properties of bis(salicylaldehydes) are directly transferable to corresponding
bis(salicylaldimines) and their boron complexes. Finally, the influence of structural and molecular
stabilization effects was analyzed by means of X-ray structural analysis and periodic DFT
computations.
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Introduction
Comparative studies on a series of similar model systems provide an opportunity to get insight
into substantial structural relations and interplay between molecular conformation, electron
density distribution, solid-state structure and macroscopic properties of organic compounds. For
those reasons, polymorphs, isomeric and isostructural compounds are the subject of many
comparative analyses.1–6 Such systems usually differ in one particular molecular or structural
feature, which decreases the number of possible contributors influencing their micro- and
macroscopic properties. Nevertheless, the structure-property relationships are usually not
straightforward, because they depend on many, usually closely-related aspects originating from
both molecular (molecular mass, electron distribution) and environment effects. It is thus
reasonable to inspect relatively simple systems, which possess one distinct molecular feature that
can be finely tuned among studied series (strength of hydrogen bonds, steric hindrance of
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substituent, etc.). Following these general considerations, we have decided to focus our attention
on hydrogen-bonded isomeric systems possessing two pairs of ortho-located hydrogen bonds (HB)
donors (hydroxyl groups) and acceptors (formyl groups), namely bis(salicylaldehydes) - the
difunctional analogues of salicylaldehyde.
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Due to the presence of two reactive groups, salicyladehydes are highly versatile synthons for the
preparation of various organic molecules. Examples include Wittig olefination, oxidation, and
preparation of Schiff bases such as multidentate salen ligands which are extensively used in
coordination chemistry.7 A number of related N-salicylideneanilines (SANS) exhibit interesting
optical properties and can be used in synthesis of luminescent molecules.8–12 Due to reversible
proton transfer involving keto-enamine/enol-imine tautomerization, selected systems exhibit
thermochromic and photochromic behavior.13–16 Salicylaldehydes have also attracted a
considerable interest from the structural and theoretical point of view. The presence of an
intramolecular resonance-assisted HB is a key factor responsible for their specific physicochemical
and spectroscopic features including ESIPT (excited-state induced proton transfer)
phenomenon.17,18 Approaches to evaluation of aromaticity (or more precisely - quasi-aromaticity)
of a six-membered H-bonded ring comprising hydroxyl and formyl groups have also been
reported.19–23 As shown by Gilli, Krygowski, Solà, Simon, Palusiak, Houjou and others, in such
systems intramolecular HBs gain additional stabilization due to the resonance effect of π-electrons
(Chart 1).20,24–26 This is accompanied by elongation of the C=O bond and shortening of the C–OH
bond. Furthermore, it results in shortening of the O…O distance and elongation of the OH bond,
1
which can be monitored experimentally by H NMR or IR spectroscopy27 and quantified by
theoretical calculations. Thus, the term “resonance-assisted hydrogen bond” (RAHB) coined by
Gilli et al. in 198928 is commonly used to describe the H-bonding situation in various -ketoenol
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and other related systems, although (as pointed by Sanz et. al.) in some particular cases it may also
result from the constraints imposed by the σ skeleton.29–31 Palusiak demonstrated that RAHB has
strong impact on mesomeric effects operating in aromatic hydrocarbons and is sensitive to
substituent effects.32
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Chart 1. Resonance assisted hydrogen bonding in salicylaldehydes.
More recently, much attention has been paid to bis(salicylaldehydes), due to their utilization as
building units for the construction of bi- and multifunctional materials for catalysis and
optoelectronics.33–45 They are also extensively used for the construction of thermotropic mesogens
exhibiting a rich variety of liquid crystal (LC) phases.46–50 Furthermore, they were successfully
incorporated into the imine-based porous Covalent Organic Frameworks (COFs) and studied for
their potential catalytic and optoelectronic applications.51–58
Despite the importance of bis(salicylaldehydes), their molecular and structural features have not
been thoroughly studied and compared yet. The Cambridge structural database (CSD) search gives
279 hits of salicylaldehyde structures including several systems bearing two pairs of
salicylaldehyde units. However, they are either based on the naphthalene backbone or
salicyladehyde units located in different parts of the molecule, i.e., comprising two different
aromatic rings. Naphthalene-based bis(salicyl) aldehydes have already been studied by Houjou et.
al.20 In the latter examples the mutual influence of CHO/OH pairs is rather limited. Surprisingly,
there are no reports on crystal structures of simple bis(salicylaldehydes) based on benzene ring,
namely dihydroxydiformylbenzenes. This has prompted us to examine molecular and structural
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features of four isomeric bis(salicylaldehydes) 1-4 (Chart 2). To elucidate the effect of the direct
π-conjugation, the series was supplemented by symmetrical bis(salicylaldehyde) based on
naphthalene backbone, namely 1,8-dihydroxy-2,7-diformylnaphthalene (5), parent
salicylaldehyde (A), and 1-hydroxy-2-naphthaldehyde (B). At this point it is noticeable that
compound 5 is isomeric to previously studied by Houjou et. al. naphthalene bis(salicylaldehydes)
including 2,7-dihydroxy-1,8-diformyl-, 2,6-dihydroxy-3,7-diformyl- and 2,6-dihydroxy-1,7-
diformyl- derivatives.20 It is expected that studied series exhibit number of useful features: (a) all
molecules are rigid and can be treated as topological quasi-analogues of aromatic systems, namely
anthracene (1-2), phenanthrene (3-4) or chrysene (5) aromatics, (b) in all cases formation of
strongly favored RAHB is expected, which partially saturates HB donor and reduces the number
of possible intermolecular hydrogen interactions in the solid state, (c) the molecules are very
similar one to another, that facilitates the rationalization of their differences in macroscopic
properties (such as UV-Vis spectra) using relatively simple experimental and theoretical tools.
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Chart 2. Bis(salicyladehydes) discussed in this study.
Experimental Section
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Synthesis
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General Information. Starting dimethoxyphthalaldehydes and 2,3-dihydroxy-1,4-
diformylbenzene were synthesized according to the literature methods.59 Reactions and
manipulations involving moisture-sensitive reagents were carried out under an argon atmosphere.
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1H, and C NMR spectra were recorded on Agilent NMR 400 MHz spectrometer. H and C
chemical shifts were referenced to TMS using known chemical shifts of solvent residual peaks.
Synthesis of 1.
A
100 ml round-bottom flask was charged with 2,5-
dimethoxyterephthalaldehyde59 (0.873 g; 4.5 mmol) and argonated. Then, anhydrous DCM (40
ml) was added, and the resulted solution was cooled with ice-water bath. Then BBr3 (11.3 ml; 11.3
mmol; 1M solution in hexane) was added. After 5 min cooling bath was removed and stirring was
continued at r.t. for 3 h. Then water (50 ml) was added, and the mixture was extracted with DCM
(5×100 ml). Combined organic phases were washed with water (100 ml), brine (100 ml), and dried
over MgSO4. The mixture was filtered through a pad of silica gel at Schott filter, and evaporated
to obtain 1 (0.70 g; 4.2 mmol; 93%) as yellow crystalline solid, mp. 245 °C (dec.); (Lit: 240 °C60)
1H NMR (400 MHz, DMSO-d6) δ 10.30 (s br, 2H), 10.295 (s, 2H), 7.22 (s, 2H). C NMR
13
(100 MHz, DMSO-d6) δ 190.2, 152.8, 127.7, 115.1.
Synthesis of 2. Dialdehyde 2 was prepared from 4,6-dimethoxyisophthalaldehyde59 by
demethylation with AlCl3 in nitrobenzene (120 C, 1 h), according to the procedure described by
Widiger et. al.61 Crude product was crystallized from ethanol, mp. 186.5-187.5 °C (Lit. 191 °C)60).
1H NMR (200 MHz, CDCl3) δ 11.77 (s, 2H), 9.78 (s, 2H), 7.81 (s, 1H), 6.45 (s, 1H). 13C NMR
(50 MHz, CDCl3) δ 193.9, 168.5, 142.5, 115.6, 104.5.
Synthesis of 3. Sample 3 was available from our previous project reported by us in lit.59
Multistep synthesis of 4
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Alkylation of 2,3-dicyanohydroquinone. A 100 ml round-bottomed flask was charged with
2,3-dicyanohydroquinone (5.545 g; 34.63 mmol) and flushed with argon. Then, under argon
atmosphere K2CO3 (19.15 g; 138.6 mmol), DMF (40 ml), and butyl iodide (15.7 ml; 138.5 mmol)
were added. The flask was covered with aluminum foil to protect from light, tightly closed with
cap, and mixture was vigorously stirred at 90 °C for 20 h. Then, the mixture was cooled to r.t.,
poured into aqueous solution of NH4Cl (100 ml; 10 %), and concd aqueous solution of HCl (10
ml; ca. 37%) was added. The resulted mixture was extracted with DCM (3×150 ml), combined
organic phases were washed with aqueous solution of LiCl (150 ml; 1M), water (150 ml), brine
(150 ml), and dried over anhydrous MgSO4. The mixture was filtered, evaporated and dried under
vacuum to obtain dinitrile 8 as a cream-colored solid (8.175 g; 30.0 mmol; 87%), mp. 187.5-189.5
C (Lit. 193 C62,63). 1H NMR (400 MHz, CDCl3) δ 7.14 (s, 2H), 4.03 (t, J=6.4 Hz, 4H), 1.86-1.70
(m, 4H), 1.57-1.41 (m, 4H), 0.95 (t, J=7.4 Hz, 6H). 13C NMR (100 MHz, CDCl3) δ 155.1, 118.5,
113.1, 105.1, 69.9, 30.9, 19.0, 13.7.
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Hydrolysis and reduction of 8. A 250 ml round-bottomed flask was charged with 8 (8.175 g;
30.0 mmol) and aqueous solution of KOH (30 g KOH + 30 g H2O). The flask was fitted with reflux
condenser, and refluxed (bath temp.=170 C) over 6 h. The mixture was cooled with ice-water
bath, and aqueous solution of H2SO4 (ca. 100 ml; 20%) was added slowly, until the liquid become
acidic (tested with universal indicator). Then water (150 ml) was added, and the mixture was
extracted with DCM (3150 ml). Combined organic phases were washed with water (150 ml),
brine (150 ml), and dried over anhydrous MgSO4. The mixture was filtered, evaporated and dried
under vacuum to obtain a yellowish solid, which was used directly for the next step. The solid
contained two components, most likely diacid and anhydride, and their relative content varied
depending on conditions of the aqueous workup.
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A 250 ml round-bottomed flask was charged with yellowish solid from the previous step, and
argonated. Then, anhydrous Et2O (60 ml) was added, the suspension was cooled with ice-water
bath, and a solution of LiAlH4 (60 ml; 60 mmol; 1.0 M in Et2O) was added dropwise with stirring
over 15 min. Then, the flask was fitted with reflux condenser and heated to 40 C under argon for
3 d. Then, ice-water bath was applied, water (100 ml) was added, and most of the solvent was
evaporated. An aqueous solution of HCl (150 ml; 3:1 concd HCl:H2O) was added, and the resulted
mixture was extracted with DCM (3150 ml). Combined organic phases were washed with
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aqueous solution of NaOH (3150 ml; 5%), brine (150 ml), and dried over anhydrous MgSO4. The
mixture was filtered, evaporated and separated with column chromatography ( 9cm  6cm; ca.
200 ml of silica gel; cyclohexane:ethyl acetate 10:11:1) to obtain diol 9 (6.87 g; 24.33 mmol;
81% over two steps) as a cream-brownish powder, mp. 69.5-74 C. 1H NMR (400 MHz, CDCl3)
δ 6.78 (s, 2H), 4.81 (s, 4H), 3.92 (t, J=6.4 Hz, 4H), 2.76 (s, 2H), 1.82-1.67 (m, 4H), 1.55-1.39 (m,
4H), 0.95 (t, J=7.4 Hz, 6H). 13C NMR (100 MHz, CDCl3) δ 151.3, 129.6, 112.3, 68.8, 56.7, 31.4,
19.4, 13.8. MS (EI, m/z, relative intensity): 282 (M+•, 52), 264 (3), 208 (42), 179 (7), 166 (6), 152
(54), 134 (100), 123 (18), 106 (14), 95 (11). Elem anal, calcd for C16H26O4: C, 68.06; H, 9.28;
found: C, 67.84; H, 9.37.
Swern oxidation-dealkylation of diol 9. A 100 ml Schlenk flask was argonated, charged with
oxalyl chloride (2.79 ml; 33.0 mmol) and anhydrous DCM (30 ml), and the mixture was cooled to
-78 ºC. Then, a solution of DMSO (4.69 ml; 66.0 mmol) in DCM (12 ml) was added dropwise
over 5 min. The mixture was stirred for 5 minutes, and a solution of diol 9 (4.25 g; 15.1 mmol) in
DCM (18 ml) was added dropwise over 10 min. After 90 minutes triethylamine (20.88 ml; 150
mmol) was added dropwise over 10 min, and after next 5 min cooling bath was removed, and the
mixture was stirred at r.t. for 1 h. Then, the mixture was poured into water (150 ml), extracted with
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DCM (3150 ml), and combined organic phases were washed with water (150 ml), brine (150 ml),
and dried with anhydrous Na2SO4. The mixture was filtered through a Schott filter, evaporated and
used for the next step.
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Warning! The intermediate tends to easily decompose. We recommend to cleave the etheral
function as soon as possible, once the oxidation is finished.
A 100 ml Schlenk flask was argonated, charged with dialdehyde from the previous step and
anhydrous DCM (30 ml), and cooled in ice-water bath. Then, BBr3 (30.0 ml; 30.0 mmol; 1.0 M
solution in hexanes) was added dropwise with stirring over 5 min. After next 5 min cooling bath
was removed, and stirring was continued for 40 h at r.t. Then, the mixture was poured into aqueous
solution of NH4Cl (150 ml; 10%), and extracted with DCM (3150 ml). Combined organic phases
were washed with water (150 ml), brine (150 ml), and dried over anhydrous MgSO4. The mixture
was filtered through a pad of silica gel at Schott filter, evaporated and separated with column
chromatography ( 6cm  22cm; ca. 400 ml of silica gel; cyclohexane:DCM 8:10:1) to obtain
aldehyde 4 (0.843 g; 5.07 mmol; 34% over two steps) as a yellow-orange powder, mp. 153.0-154.5
C. 1H NMR (400 MHz, CDCl3) δ 11.86 (s, 2H), 10.73 (s, 2H), 7.27 (s, 2H). 13C NMR (100 MHz,
CDCl3) δ 191.3, 156.9, 129.4, 115.5. MS (EI, m/z, relative intensity): 166 (M+•, 100), 137 (89),
120 (14), 109 (19), 92 (32), 81 (24), 63 (13), 53 (31). Elem anal, calcd for C8H6O4: C, 57.84; H,
3.64; found: C, 57.97; H, 3.74.
The aldehyde 4 is a stable compound and can be crystallized by slow evaporation of concentrated
chloroform solution from open glass vial on air.
Structural analysis
X-ray structural measurements and refinement details. The single crystals of samples 1-5 were
obtained by slow evaporation of CHCl3 solutions. All except 4 were measured at 100 K on
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SuperNova diffractometer equipped with Atlas detector (Cu-Kα radiation, λ = 1.54184 Å). Crystal
structure of 4 was exceptionally measured at 250 K due to the crystal instability presumably
resulted from phase transition at lower temperature. Data reduction and analysis were carried out
with the CrysAlisPro program.64 All structures were solved by direct methods using SHELXS-
9765 and refined using SHELXL-2014.66 All non hydrogen atoms were refined anisotropically.
The position of OH hydrogen atoms were refined freely, whereas their ADPs were constrained
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O
with UisoH = 1.5∙Ueq . All hydrogen atoms were visible on the difference-density maps in the
proximity of hydroxyl oxygen atom, whereas no density was detected close to the carbonyl atom
pointing that hydrogen atom resides on the hydroxyl oxygen atom (Figure S2-S6, SI). All
important crystallographic data including measurement, reduction, structure solution and
refinement details are placed in Table 1. Crystallographic Information Files (CIFs) have been
deposited with the Cambridge Crystallographic Data Centre as supplementary publications no.:
1938693 (1), 1938694 (2), 1938695 (3), 1938696 (4), 1938697 (5).
Table 1. Selected crystal data, data collection and refinement parameters for 1-5.
5
1
2
3
4
Formula
C8H6O4
166.13
C8H6O4
166.13
C8H6O4
166.13
100
C8H6O4
166.13
250
C12H8O4
216.18
Molecular mass, Mr / a.u.
Temperature, T / K
Crystal system
Space group
a / Å
100
100
100
Monoclinic
P21/n
triclinic
P-1
orthorhombic monoclinic
monoclinic
P21/n
P21 21 21
4.7435(1)
12.9275(2)
22.1367(4)
90
P21
3.7157(2)
5.5117(2)
16.3825(6)
90
7.4756(4)
8.2973(5)
11.9343(7)
78.758(5)
3.8070(1)
7.3986(2)
12.5470(4)
90
15.5165(18)
3.7812(1)
16.046(3)
90
b / Å
c / Å
α / °
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β / °
93.399(4)
90
76.136(5)
84.789(5)
704.14(7)
1.567
90
95.445(3)
90
102.91(2)
90
γ / °
90
Volume, V / Å3
dcalc / gcm−3
334.92(3)
1.647
172
1357.46(4)
1.626
351.81(2)
1.568
917.7(2)
1.565
F(000)
344
688
172
448
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Absorption coefficient, μ / mm−1
1.158
1.102
1.143
1.102
1.003
No. of measured, independent reflections 2878 / 700
8615 / 2409
1.28
7438 / 2369
2.49
3284 / 1433
1.78
9403 / 1937
2.15
Rint / %
2.08
GooF
1.036
1.050
1.050
1.059
1.058
R[F] / wR[F2] (I> 2σ(I)) / %
Max. and min. residual density / eÅ−3
3.29 / 9.11
-0.155 /0.287
3.20 / 8.85
2.83 / 7.38
3.39 / 9.40
4.36 / 11.87
-0.238 / 0.181 -0.230 / 0.161 -0.144/ 0.136 -0.263 / 0.282
CSD search. The Cambridge Crystallographic Database was searched using the CONQUEST67
program (version 1.21). The search procedure included all compounds containing the
salicylaldehyde fragment excluding ionic compounds and polymers (279 entries). Within this
group 243 salicylaldehydes possess intramolecular O–H…O HB. Histogram representing O…O
distance distribution is presented in SI (Figure S1).
Theoretical calculations
Single-molecule computation. Ab initio calculations were performed using Gaussian16
programme package.68 In the first step the molecules were optimised in their ground states using
M06-2X (DFT)69 method with aug-cc-pVTZ71 basis set. The geometries of N-Me and N-Ph Schiff
bases along with their corresponding boron complexes were optimised at the same level of theory.
After geometry optimization, the vibrational frequencies were calculated and the results showed
that optimized structures are stable geometric structures (no imaginary frequencies). Natural Bond
Orbital (NBO) analysis was used to calculate Wiberg bond indexes and atomic charges. The O–H
distance constrained optimisation energy scan was performed at the same level of theory. The O–
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H distance was constrained on each step, while the rest of geometrical parameters were fully
optimised (Figure S7). In the next step, the absorption process was simulated using TD-DFT
methods (at PBE071 / 6-311++G(d,p)72). To take into account the conditions of absorption
measurements (dilute DCM solution), calculations were carried out in the presence of the solvent
(DCM) with the polarizable continuum model (PCM) using the polarizable conductor calculation
model (CPCM).73
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Aromaticity indicators. Nucleus-independent chemical shift (NICS) were calculated for points
located at the centre of the aromatic rings (NICS(0)) and the points 1Å above and below the centre
of the ring. The latter values were averaged to yield NICS(1).74 NICS are defined as the negative
value of the absolute shielding computed at certain point. the NICS(0) values may contain
important spurious contributions from the in-plane tensor components and are affected by the σ-
bond density, so it was postulated that NICS(1) values better reflects aromaticity patterns. The
harmonic oscillator model of aromaticity (HOMA) index was calculated according to the
definition proposed by Kruszewski and Krygowski:75
푛
훼
퐻푂푀퐴 = 1 − ∑(푅opt − 푅i)2
푛
푖=1
where n corresponds to the number of bonds within the analysed ring, Ropt being a optimal bond
length (1.396A from our calculations for benzene) and 훼 was set to 257.7.
Topological analyses of electron density. The topological analyses of the calculated electron
densities of all studied systems was prepared for the ground state optimised geometries at MP276/6-
311++G(d,p)72 level of theory. It was accomplished in terms of QTAIM approach77 and was
carried out using AIM2000 programme.78 In the framework of this approach critical points (CPs)
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together with the bond paths (BPs) were found. The bond critical points (BCPs) were analysed for
intramolecular O-H…O HBs. The energy of the intramolecular HB interactions were estimated on
the basis of empirical equations proposed by Rusinska-Roszak79–81 and further developed by
Afonin.82 The equations correlates the energy of intramolecular HBs with H…O distance (Ed
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O-H…O
= f(dH…O)) or are based on electron density value at BCP (EρO-H…O = f(ρ(BCP))):
EdO-H…O = -15.27dH…O +34.02
EρO-H…O = 216.08 ρ(BCP) – 1.71
Periodic calculations. All energy computations within the CRYSTAL09 program package were
performed at the DFT(B3LYP) level of theory with POB triple-zeta valence + polarization basis
set (TZVP).83 Grimme dispersion correction and correction for the basis set superposition error
was applied.84 Ghost atoms were selected up to 15 Å distance from the studied molecule in a crystal
lattice, and were used for the basis set superposition error estimation. The evaluation of Coulomb
and exchange series was controlled by five thresholds, set arbitrary to values of 10−7, 10−7, 10−7,
10−7, 10−25. The condition for the SCF convergence was set to 10−7 on the energy difference
between two subsequent cycles. Shrinking factor was equal to 4, which refers to 30-36 k-points
(depending on space group symmetry) in the irreducible Brillouin zone in the case of the studied
systems, and assures the full convergence of the total energy. The cohesive energy (Ecoh) was
calculated as described below:
1
Ecoh
=
Ebulk − Emol
Z
where Ebulk is the total energy of a system (calculated per unit cell) and Emol is the energy of a
molecule extracted from the bulk . Z stands for the number of molecules in the unit cell.85
UV-Vis and fluorescence spectra
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The UV-Vis absorption spectra were recorded using a Hitachi U-2300II spectrometer. The
emission spectra of solutions were recorded using a Edinburgh FS5 spectrofluorometer, equipped
with an extended range photomultiplier detector (PMT-EXT). The measurements were performed
at room temperature, according to published procedures.87,88 Suprasil quartz cuvettes (10.00 mm)
were used. 1.5 nm slits were used for absorption and 1.0 nm slits were used for emission spectra.
To eliminate any background emission, spectrum of pure solvent was subtracted from the samples’
spectra. qy was determined in diluted solution (c = 1·10-5 mol·dm-3) by comparison with known
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standard – coumarin 153 in ethanol (c = 4·10-6 mol·dm-3 qy = 0.544).88 Sample was excited at 418
,
nm. Quantum yield was calculated using following equation:
퐹 1 − 10−퐴 푛x2
st
x
qyx = qyst ∙
∙
∙
x
푛s2t
퐹
1 − 10−퐴
st
where F is the relative integrated photon flux of sample (x) and standard (st), A is the absorbance
at the excitation wavelength, n is the refractive index of used solvents. Photon fluxes (F) were
calculated by integration of corrected spectra (Ic)
(
)
)
퐼 휆em
퐹 = ∫ 퐼c 푑휆em = ∫
푑휆em
(
푠 휆em
Results and Discussion
Synthesis
1,4-Diformyl-2,5-dihydroxybenzene (1) and 1,4-diformyl-2,3-dihydroxybenzene (3) were
prepared by double metalation of 1,4- and 1,2-dimethoxybenzenes with n-BuLi, followed by
reaction with DMF, and subsequent removal of the methyl groups with BBr3.59 The same strategy
applied to 1,3-dimethoxybenzene is known to metalate mainly at 2-position,89,90 thus in the first
step the arene was brominated to obtain 1,3-dibromo-4,6-dimethoxybenzene, and then double
halogen-metal exchange at specific positions gave dialdehyde 2.59
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In turn, synthesis of 2,3-diformyl-1,4-dihydroxybenzene (4) was not previously described in the
literature, although numerous derivatives of 4, bearing additional substituents present at positions
5 and 6, are known.91 In contrast to syntheses of the remaining isomers 1-3, preparation of 4 would
require a double metalation of 1,4-dimethoxybenzene at adjacent positions 2 and 3, that is
disfavored due to the Coulombic forces.92 Our initial approach assumed introduction of the formyl
group to 2,5-dimethoxybenzaldehyde, using methodology developed by Comins.93,94 Accordingly,
the formyl group was transformed to the lithium salt of N,N,N'-trimethylethylenediamine, giving
an adduct, which then was treated with excess of n-BuLi, and quenched with DMF. Chelating part
of the diamine is known as ortho-directing group in metalations, and was utilized for
functionalization of m-anisaldehyde at the position 2.94 However, in our hands dimethoxy analogue
6 underwent metalation only at positions 3 and 4 (directed by the OMe groups), instead of the
expected internal position 6 (Scheme 1, below).95
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Scheme 1. Attempt at metalation-formylation of 2,5-dimethoxybenzaldehyde (6) according to
Comins methodology.93,94
Therefore we changed the synthetic plan and used commercially available
2,3-dicyanohydroquinone as a substrate. Butylation of the phenolic oxygen atoms,96 followed by
hydrolysis of the nitrile groups, and reduction with LiAlH4 gave diol 9 in 70% overall yield. Then,
Swern oxidation, followed by cleavage of the ethereal bonds gave desired dialdehyde 4, whereas
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attempts at oxidation of 9 with PCC led exclusively to lactone 10, isolated in 76% yield (Scheme
2).
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Scheme 2. Successful synthesis of 2,3-diformyl-1,4-dihydroxybenzene (4).
Interestingly, the 2,3-diformyl-1,4-dihydroxybenzene (4) appeared to be stable, and few mm size
crystals were grown by slow evaporation of its chloroform solution in open glass vial under air.
Finally, 2,7-diformyl-1,8-dihydroxynaphthalene (5) was prepared according to a 4-step procedure
involving
double
bromine/lithium
exchange
in
1,5-bis(methoxymethoxy)-2,6-
dibromonaphthalene followed by quenching with DMF and hydroxyl groups deprotection.35
Molecular properties of 1-5
Single crystals of bis(salicylaldehydes) were obtained by slow evaporation of the corresponding
acetone solutions and investigated by X-ray diffraction. Molecular structures are presented in
Figure 1. Molecules 1, 2 and 5, laying on the inversion center, crystallize in centrosymmetric
space group (P21/n or P-1), whereas 3 and 4 which are located on the two-fold axis, crystallize in
non-centrosymmetric space groups P212121 and P21, respectively. All molecules are essentially
planar reflecting effective conjugation between substituents and the benzene ring. Their metric
features are in the range expected for such aromatic derivatives. In case of compounds 2, 3 and 5,
two symmetrically independent molecules (abbreviated 2a/2b, 3a/3b, 5a/5b) were observed. In
case of 2 and 5, their geometries differ only marginally. Unexpectedly, molecule 3b features only
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one intramolecular RAHB. The second formyl group is oriented anti with respect to the adjacent
OH substituent which in turn is engaged in an intramolecular HB with the neighbouring OH group
(dO…O = 2.727(2) Å, dH…O = 2.37(3) Å).
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Figure 1. X-ray molecular structures of 1-5 with the labelling scheme (only asymmetric part).
Displacement ellipsoids are drawn at the 50% probability level. Note that 4 was exceptionally
measured at 250 K, remaining structures were measured at 100 K.
Hydrogen atoms from the OH group were refined freely only constraining their isotropic
displacement parameters with respect to oxygen atoms. Fourier difference density maps show that
they reside exclusively on the hydroxyl site (Figures S2-S6, Supporting Information). Theoretical
calculations show that proton transfer is rather not favoured (Figure S7, SI), nevertheless,
significant O…O bond shortenings (dO…O = 2.588-2.709 Å) with respect to intermolecular HBs
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(dO…O = 2.7-3.0 Å) originate from RAHB, as observed by Gilli and others for many
salicylaldehydes and conjugated β-diketo enols.23-24,28 A closer inspection of geometries of
intramolecular OH…O=C bridges (Table 2) reveals significant differences between 1-5. The
weakest OH…O=C bridge was found for 1 (2.709(1) Å), whereas the strongest one is observed
for 4 (2.588(3) Å) and 5 (2.591(2) Å). The CSD analysis revealed that those distances are typical
for other salicylaldehydes (Figure S1, SI). However, the strength of the intramolecular HB in
crystal structure is influenced to some extent by the intermolecular interactions with neighbours.
Furthermore, structure 4 was exceptionally measured at 250 K as the crystal collapsed below this
temperature presumably due to phase transition. Thus, to quantitively compare the intramolecular
HBs in studied systems, the use of geometries derived from single-molecule DFT (M06-2X/aug-
cc-pVTZ) optimizations seemed to be more reliable. Indeed, in the case of 1, significant shortening
of the O…O distance (to 2.664 Å) is observed and it is comparable to the corresponding O…O
distances in salicylaldehyde (A) and structure 3. We have found that the strength of RAHB is
significantly influenced by mutual resonance interactions of electron-withdrawing formyl and
electron-donating hydroxyl groups. In compound 1, the formyl groups are in the para position to
each other, which decreases the H-bond acceptor ability. Simultaneously, the hydroxyl groups are
also in the para position one to another which results in their reduced acidity consistent with
weaker H-bond donor properties. Similarly, in 3 the formyl groups are in the para position,
whereas hydroxyl groups are in ortho positions. Thus, in both cases cumulation of resonance
effects results in relative weakening of the RAHB. In contrast, in 2 the cooperation of mesomeric
effects is observed resulting in enhancement of proton acceptor character of the CHO groups and
acidity of the OH groups. In case of 4, the location of hydroxyl and formyl groups causes the
opposite resonance effects somehow puzzling the observed significant shortening of the O…O
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distance in this system (dO…O = 2.570 Å). However, it can be rationalized by the steric repulsion
of ortho positioned formyl groups (dH…H = 2.035 Å), which is also reflected by the deformation of
C2–C1–O1(OH) and C1–C2–C7(CHO) bond angles. The influence of mesomeric effect is also
clearly visible for naphthalene system 5. Although naphthalene monoaldehyde (B) is characterized
by shorter O…O distance with respect to salicylaldehyde A, introduction of hydroxyl groups at
the position 5 reduces the acidic character of both groups leading to elongation of O…O distance
in intramolecular HBs.
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The O–H…O bond distances correlate with the electron density values at O–H…O bond critical
point (ρ(BCP)) calculated at MP2/6-311++G(d,p) level of theory (Table 2, Figure S8, SI).
Consequently, in 1 the ρ(BCP) equals to 0.231 eÅ-3 and is lower than for A, then it increases
following the order 3 < 2 < 5 to reach 0.295 eÅ-3 for 4. Similar trend is observed for the Laplacian
(Figure S9, SI). These observations are consistent with well described continuous relations of both
functions (ρ(BCP), ∇2ρ(BCP)) on bond path length.97 On the basis of calculated O…O distance
and electron density values we have estimated the energy of the intramolecular HB using empirical
equations proposed by Afonin.73 According to these calculations the energies of single
intramolecular HB in 1-5 oscillate between 24-33 kJmol-1. It should be noted that in the case of 4,
the high value of electron density partially results from the H…H repulsion between formyl
groups. Thus the strength of RAHB is overestimated. The removal of hydrogen atoms followed by
the optimisation in triplet state (4’)98 provided more reasonable values of electron density and
energy of RAHB. According to these computations the electron density drops from 0.295 eÅ-3 to
only 0.223 eÅ-3 after release of steric repulsion.
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Table 2. Characterization of intramolecular H-bonding interactions for the salicylaldehyde motifs
in 1-5 derived from the experimental single crystal XRD data (Exp) and theoretical calculations
(Theor). Topological parameters derived from QTAIM theory at O–H…O Bond Critical Points
(BCPs) together with estimated interaction energies. d - distance; α – angle, ρ - electron density;
∇2ρ - Laplacian; EdO-H…O , EρO-H…O - interaction energy derived from empirical equations proposed
by Afonin.
Exp
Theor
Theor
Theor
Theor
∇2ρ(BCP)
/ eÅ-5
2.87
dO…O
dO…O
dO…H
dO-H
αO–H…O
ρ(BCP)
/ eÅ-3
0.248
0.284
Ed
Eρ
O–H…O
O–H…O
/ Å
/ Å
/ Å
/ Å
/ °
/ kJ·mol−1 / kJ·mol−1
22
A
2.623
-
2.645
2.586
1.785
1.725
0.978
0.981
144.91
144.35
25.4
29.0
26.1
30.9
23
24
B
3.02
25
26
27
28
29
1
2.709(1)
2.664
1.817
1.758
0.973
0.981
143.64
146.06
0.231
0.264
2.87
2.88
23.4
27.0
23.8
28.2
a
30
2
2.651(1), 2.656(1), 2.630
2.652(1), 2.652(1)
31
32
a
33
3
2.641(2), 2.599(2), 2.644
2.615(2)
1.791
0.977
144.00
0.246
2.92
25.0
25.7
34
35
36
37
38
4
2.606(3), 2.588(3)
2.570
2.679
2.622
1.707
1.833
1.762
0.980
0.979
0.982
144.75
142.79
144.16
0.295
0.223
0.261
3.09
2.45
2.92
30.1
21.9
26.8
32.4
23.3
27.8
39
4’
-
40
a
41
5
2.613(2), 2.591(2)
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a two independent molecules in the unit cell
Theoretical calculations confirmed that for all studied compounds, conformations featuring
RAHB are the most stable (Table 3). However, in case of 3, the energy of a conformer consistent
with the structure 3b (showing the contact between formyl hydrogen and oxygen atom of the anti-
oriented OH group) is only 6.6 kJmol-1 higher. This small difference reflects the stabilization
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provided by the intramolecular H-bond between neighbouring hydroxyl groups. It should also be
5
6
noted here that the RAHB in the X-ray molecular structure 3b is relatively stronger (dO…O
=
7
8
9
2.615(2) Å vs dO…O = 2.727(2) Å). It can be explained by the effect of adjacent hydroxyl group
forming another H-bond with the OH group participating in RAHB. As a result, acidity character
of the latter OH group is increased, which, in turn, enhances its H-bond donor character. In
addition, comparison of total energies of the most preferred conformations (i.e., possessing two
RAHB motifs each) of 1-4 shows that compound 2 is much more stable (by ca. 35-48 kJmol-1)
than its isomers. It seems that this effect is due to the resonance interactions of formyl and hydroxyl
groups which are in the para positions with respect to each other. Thus, an extra-stabilization
would be provided by the charge transfer which occurs preferably in the p-quinoid-type structures
comprising electron-donating and electron-withdrawing substituents.
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Table 3. Comparison of conformational energies related to the most stable form within each
system calculated at M06-2X/aug-cc-pVDZ level of theory. Conformation of one CHO/OH pair
varied, while conformation of second pair was set to OH…OCH. Additionally, the most stable
conformations are compared between isomers. All values are given in kJmol-1.
Overall
comparison
between all
isomers
A
1
0.0
0.0
31.6
29.5
38.7
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0.0
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Observed resonance effects have naturally a strong impact on the C–C bond lengths in the
aromatic ring. Figure 2 presents the C–C bond lengths together with calculated Wiberg bond
indexes derived from NBO analysis (M06-2X/ aug-cc-pVTZ). In salicylaldehyde A aromatic C–
C bond linking the OH and CHO groups is longer with respect to other C–C bonds reflecting
contribution of the keto-enol resonance structure. Similar relation is observed for molecules 1 and
2. This resembles bond relations in anthracene molecule, where endocyclic C–C bonds are
elongated (1.438 Å) with respect to other C–C bonds.99 Calculated bond indexes clearly show that
deformation of aromatic ring geometry is more pronounced in the case of 3 and 4. The C–C bonds
between carbon atoms bearing OH and CHO groups (C1–C2 and C3–C4) become slightly shorter
which is accompanied by elongation of neighboured C–C bonds. This is followed by strong
localization of π-electron density at C5–C6 bond with bond indexes of 1.531 (3) and 1.552 (4).
Interestingly, this resembles the bond situation of the inner ring in phenanthrene,101 where C9–
C10 and C4a–C4b bond distances (i.e., the opposite bonds from inner phenantrene ring bisecting
the two-fold axes) are 1.373 Å and 1.448 Å, respectively. Similar conclusions can be also drawn
from the comparison of naphthalene bis(salicylaldehyde) 5 and chrysene molecule.102 The C1–C2
and C3–C4 bond lengths in 5 equal to 1.386 Å and 1.360 Å, respectively. In chrysene the analogous
bonds distances are 1.417 Å and 1.356 Å, showing strong localization of the electron density in
the latter bond. Summing up, bis(salicylaldehydes) exhibit some molecular features of the PAH
molecules, thus by this sense it is reasonable to regard them as PAHs quasi-aromatic topological
analogues.23
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Figure 2. Comparison of bond lengths and Wiberg bond indexes (given in italic) for 1-5 and
referential salicylaldehydes (A, B) (geometries optimized at B3LYP/6-311+G(d,p)). Red and
yellow colors indicate bond shortening and elongation, respectively.
The changes in bond lengths are quite informative in connection with a geometrical criterion of
local aromaticity. In order to compare aromatic characters of the studied systems, we have
employed two popular aromaticity indicators: harmonic oscillator model of aromaticity (HOMA)75
and nucleus-independent chemical shift (NICS)74 The latter parameter was derived from
calculations of the magnetic shielding (taken with a negative value) at the centroid of the aromatic
ring (NICS(0)) and 1 Å above the centroid (NICS(1)). Calculations were performed at M06-
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2X/aug-cc-pVTZ level of theory. In general, rings with large negative NICS values are considered
aromatic and the aromaticity increases with the decrease (toward more negative values) of the
NICS parameters. Calculated values show that the substituent effects have strong impact on the
aromaticity character of the molecules (Table 4). For all studied bis(salicylaldehydes) loss of
aromaticity was reflected by lower values of HOMA parameter. This is especially visible for
compounds 2 and 4 owing to important contribution of p-quinoid-type resonance structure in the
first example, and to the ring deformation due to formyl-formyl repulsion in the second case. Thus,
the HOMA index drops from 0.965 for 1 to 0.847 for 4. Furthermore, obtained results suggest
interference between HOMA and energy of intramolecular HB. Strengthening of the RAHB
induces decrease of the HOMA index (Figure S10, SI). Qualitatively similar conclusions can be
derived from the analysis of NICS values. First of all, the reduction of aromaticity in
salicylaldehyde A is accompanied by the increase of NICS(0) and NICS(1) values with respect to
benzene. In case of 1 and 3, the aromatic ring current becomes stronger which can be ascribed to
positive mesomeric effects of para (1) and ortho (3) positioned hydroxyl groups. In case of 4, the
effect becomes weaker, probably resulting from above discussed deformations of aromatic ring
reflected by small HOMA values, while for molecule 2, both NICS values significantly increase
confirming augmented contributions of CHO/OH pseudorings in π-electron delocalization.
Regarding the naphthalene derivatives, HOMA and NICS indicators suggest that the
introduction of one CHO/OH pair (structure B) hardly affects the aromatic character of
salicylaldehyde ring, but aromaticity increases in unsubstituted one. The addition of second
CHO/OH pair realized in structure 5, averages the HOMA indexes of both rings as compared with
monohydroxyaldehyde B. On the other hand, NICS parameter suggest that aromatic character of
5 is weaker with respect to monoaldehyde and naphthalene. We have also calculated the magnetic
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shielding constants in the points located 1 Å above central hydrogen-bonded quasi-aromatic rings.
For benzene-based bis(salicylaldehydes) obtained NICS(1) values negatively approach to zero. In
contrast, in B and 5 they adopt slightly positive values.
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Table 4. Aromaticity indicators. HOMA parameters are calculated from X-Ray experimental and
optimized geometries. NICS(0) and NICS(1) are based on optimized geometries. NICS(1)RAHB
relate to hydrogen bonded ring (for 1-5 calculated as average value).
HOMAExp
1
HOMATheor NICS(0)
NICS(1)
-11.31
-9.63
NICS(1)RAHB
Benzene
1
-9.82
-
A
1
2
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4
0.971
0.935
0.967
0.965
-9.18
-0.07
-0.19
-0.16
-0.20
-0.36
-
-10.50
-7.92
-9.90
0.896 / 0.900a 0.895
0.932 / 0.940a 0.913
-8.04
-10.22
-9.49
-9.60
0.850
0.847
0.833
-9.12
Naphthalene 0.858b
-10.10
-11.68
B
5
-
0.834/0.882c -9.42/-11.24c -9.51/-11.27c 0.10
0.832/0.836a
0.856
-8.86
-9.66
0.09
b
aTwo independent molecules in the unit cell; geometry taken from lit.102 cSalicyl/distant
aromatic rings.
Comparison of atomic charges (computed by the NBO method) shows that molecule 2
experiences highly anisotropic distribution of the electron density in the aromatic ring (Figure 3).
Except for 2, in all systems carbon atoms bearing hydroxyl group are positively charged of about
+0.34 e and magnitude of negative charge on the remaining carbon atoms oscillates near -0.2 e. In
contrast, in 2 the cooperative positive mesomeric effects of hydroxyl groups induce high negative
charge on carbon atom located at ortho position to both groups (-0.31 e). Simultaneously, formyl
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groups withdraw the electron density from ortho- and para- positions increasing charge on C–OH
carbon atoms to +0.42 e and carbon atom located in between these two groups, which become less
negative (-0.07 e).
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Figure 3. NBO atomic charges for aromatic carbon atoms together with dipole moment vector and
its magnitude. Positive and negative charges are depicted in green and blue colors, respectively.
Intensity of the color corresponds to the charge magnitude.
The variations in charge distribution should also affect the molecular orbitals levels and, in
consequence, UV-Vis spectral properties. According to our suppositions, there is a distinct difference
between UV-VIS absorption spectrum of compound 2 and the spectra of remaining isomers as in the
first case there is no absorption in the visible range (Figure 4, Table 5). As a result, 2 is colorless,
whereas other compounds have an intense yellow colour. The spectrum of 2 shows three distinct bands
in the UV range at 256, 288 and 325 nm. The longest wavelength absorption bands (λmax) for 1, 3, 4
are ranging from 400-423 nm with quite high molar absorption coefficients ( = 3500-8500 M-1 cm-1).
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TD-DFT calculations of absorption spectra (PBE0/ 6-311++G(d,p)) show that these bands are
attributed to π- π* HOMO-LUMO transition.
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Among studied series, compound 5 is characterized by the longest wavelength of absorption (439
nm) and the highest molar absorption coefficients ( = 9700 M-1 cm-1). Interestingly, we have found
that UV-Vis spectra of previously studied by us Schiff base organoboron complexes, namely
bis(boranils),34 qualitatively match to spectrum of parent bis(salicylaldehyde) 5. Furthermore, 5 is
strongly luminescent, emitting light with maximum at 506 nm and quantum yield of 0.60 in DCM. At
this point it is noticeable, that naphthalene-based bis(boranils) are characterized by exceptionally high
quantum yields (up to 83%), that presumably results from molecular features (i.e. substitution pattern,
structural stiffness) of parent bis(salicylaldehyde) 5.
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Regarding the most intriguing system 2, the longest absorption band results from the transition
between HOMO and LUMO levels with some contribution from HOMO-1 to LUMO states, however,
the calculations show that HOMO level is significantly lowered, while LUMO level is elevated with
respect to remaining molecules (Figure 5). This results in the increase of band gap and observed
hypsochromic shift of the absorption band. We have found that the differences in HOMO and LUMO
levels are attributed to mesomeric effects of hydroxyl and formyl groups, respectively, and those effects
seem to operate independently to some extent. To support this thesis, we have performed computations
for para, meta and ortho isomers of dihydroxybenzene, diformylbenzene and hydroxybenzaldehydes
(9 structures in total). Results of these computations are presented on Figure S12 in SI. Comparing the
energy levels in a series of isomeric dihydroxybenzenes, the lowering of the HOMO orbital is clearly
visible in m-dihydroxybenzene. In turn, the LUMO level is elevated in isophthalaldehyde (m-
diformylbenzene). It is also noticeable that the energy gap decreases in series of p-, m-, o-
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hydroxybenzaldehyde, and further decreases in bis(salicylaldehydes) following the order 3 > 4 > 5 to
reach 3.33 eV for compound 1.
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To address the question whether the structure-property relation in bis(salicylaldehydes) is transferable
to other systems such as Schiff bases, we have extended our research by studying corresponding N-Ph
and N-Me diimines, and their boron BF2 complexes. According to our computations (M06-2X/aug-cc-
pVT)), all spectroscopic trends are preserved in those systems (Figures S13-16, SI). Furthermore,
aromatic character of bis(salicylaldimines) and their BF2 complexes monitored by HOMA and NICS
values follows the same trend as observed for their aldehyde precursors (Figure 6, Table S1, Figures
S17-S22, SI). It is thus supposed that the observed trends would be preserved in other systems to some
extent, although one have to keep in mind that the introduction of strong electron-donating or electron-
withdrawing substituent may shuffle the orbital scheme.
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Figure 4. UV-Vis absorption spectra of 1-5 in DCM supplied by insert of normalized spectra (left)
and overlay of emission and absorption spectra of 5 in DCM (right).
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Table 5. Optical properties of compounds 1-5.
5
6
7
Experimental
TD-DFT
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9
Compound solvent
λmax/nm ε /M-1 cm-1 λabs/nm
F
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2
DCM
DCM
278
423
256
288
325
284
400
263
408
266
304
439
31000
7000
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409
235
259
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250
384
247
389
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400
0.402
0.109
0.456
0.267
0.109
0.268
0.168
0.189
0.127
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0.173
65000
14000
5000
3
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5
DCM
DCM
DCM
27000
3500
13500
8500
103300
13100
9700
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