Dual-State Emissive π-Extended Salicylaldehyde Fluorophores: Synthesis, Photophysical Properties and First-Principle Calculations

Article abstract of DOI:10.1002/ejoc.202100650

The search for simple, low-cost, versatile, easily accessible, stimuli-responsive, highly emissive molecular fluorophores emitting both in solution and in the solid-state has prompted us to investigate the optical properties of a series of synthetically accessible salicylaldehyde derivatives possessing a π-conjugated moiety at their 4-position. These dyes are mainly known as synthetic intermediates but can also display sizeable Excited-State Intramolecular Proton Transfer (ESIPT) fluorescence owing to the presence of a 6-membered H-bonded ring in their structure. The photophysical properties of these compounds have been studied in solution (multiple solvents) and in the solid-state, as doped in PMMA films, KBr pellets or as powders leading to the observation of a pronounced fluorosolvatochromism. Emission wavelengths in the range 400–654 nm, along with photoluminescence quantum yields up to 76 % were recorded. Modification of the spacer (ethynyl, vinyl or direct connection) involved the π-delocalization triggers major differences in terms of maximum emission wavelength and fluorescence quantum yields in the various media studied. All photophysical observations are rationalized by first-principle calculations.

Full text of DOI:10.1002/ejoc.202100650

European Journal of Organic Chemistry
Accepted Article
Accepted Article
Title: Dual-State Emissive π-Extended Salicylaldehyde Fluorophores:
Synthesis, Photophysical Properties and First-Principle
Calculations
Authors: Timothée Stoerkler, Denis Frath, Denis Jacquemin, Julien
Massue, and Gilles Ulrich
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To be cited as: Eur. J. Org. Chem. 10.1002/ejoc.202100650
Link to VoR: https://doi.org/10.1002/ejoc.202100650
01/2020

10.1002/ejoc.202100650
European Journal of Organic Chemistry
Dual-State Emissive π-Extended Salicylaldehyde Fluorophores:
Synthesis, Photophysical Properties and First-Principle
Calculations
Timothée Stoerkler,[a] Denis Frath,[a][b] Denis Jacquemin,*[c] Julien Massue*[a] and Gilles
Ulrich*[a]
[a] Mr. Timothée Stoerkler, Dr. Denis Frath, Dr. Julien Massue, Dr. Gilles Ulrich
Institut de Chimie et Procédés pour l’Energie, l’Environnement et la Santé (ICPEES), Equipe
Chimie Organique pour la Biologie, les Matériaux et l’Optique (COMBO), UMR CNRS
7515, Ecole Européenne de Chimie, Polymères et Matériaux (ECPM), 25 Rue Becquerel,
67087 Strasbourg Cedex 02, France
Emails: massue@unistra.fr, gulrich@unistra.fr
https://icpees.unistra.fr/en/welcome-to-icpees/
https://twitter.com/JulienMassue
[b] Dr. Denis Frath
Université de Lyon, ENS de Lyon, CNRS UMR 5182, Laboratoire de Chimie, 69342 Lyon,
France.
[c] Prof. Dr. Denis Jacquemin,
CEISAM/UMR CNRS 6230, Université de Nantes, Nantes, France
Emails: Denis.Jacquemin@univ-nantes.fr,
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10.1002/ejoc.202100650
European Journal of Organic Chemistry
Table of Contents
This article describes the synthesis and photophysical properties of a series of functionalized
salicylaldehydes derivatives which show strong fluorescence intensity in solution and in the
solid-state in multiple environments. These results are rationalized by ab initio calculations.
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10.1002/ejoc.202100650
European Journal of Organic Chemistry
Abstract
The search for simple, low-cost, versatile, easily accessible, stimuli-responsive, highly
emissive molecular fluorophores emitting both in solution and in the solid-state has prompted
us to investigate the optical properties of a series of synthetically accessible salicylaldehyde
derivatives possessing a π-conjugated moiety at their 4-position. These dyes are mainly
known as synthetic intermediates but can also display sizeable Excited-State Intramolecular
Proton Transfer (ESIPT) fluorescence owing to the presence of a 6-membered H-bonded ring
in their structure. The photophysical properties of these compounds have been studied in
solution (multiple solvents) and in the solid-state, as doped in PMMA films, KBr pellets or as
powders leading to the observation of a pronounced fluorosolvatochromism. Emission
wavelengths in the range 400-654 nm, along with photoluminescence quantum yields up to
76% were recorded. Modification of the spacer (ethynyl, vinyl or direct connection) involved
the π-delocalization triggers major differences in terms of maximum emission wavelength and
fluorescence quantum yields in the various media studied. All photophysical observations are
rationalized by first-principle calculations.
solution
solid
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10.1002/ejoc.202100650
European Journal of Organic Chemistry
Introduction
Functional fluorescent emitters based on a rationally (poly)substituted organic π-conjugated
scaffold have found applications in cutting-edge innovations in a wide range of fields owing
to their synthetic availability, modularity, absence of toxicity, non-invasiveness, low-cost, up
scalability, and industrial processability.1 Molecular fluorophores encompass several families
of well-known dyes including fluorescent heterocyclic architectures derived from cyanines,2
rhodamines,3 squaraines4 or boron complexes,5 just to name a few. Although highly emissive
in solution and presenting tunable fluorescence wavelengths, the majority of the dyes reported
so far are however heavily quenched in the solid-state due to detrimental aggregation
processes, generally called aggregation-caused quenching (ACQ) effects. In the same time,
aggregation-induced emission (AIE) dyes have been engineered with the view to compensate
for this drawback and deliver rotors-shaped emitters with strong fluorescence intensity at high
concentrations in solution or in confined matrixes like nanoparticles.6 Dual-state emitters
(DSE) have been developed more recently and originate from a rational design allowing the
observation of fluorescence emission both in solution and in the solid-state.7 DSE dyes have
been recently the focus of intense research since they appear to be able to fill the gap between
solution- and solid-state emitters (ACQ and AIE processes) and target a wider range of
applications. These innovative probes arise from a tenuous balance between molecular
rigidity, quasi-planarity and the introduction of appropriate solubilizing entities. There are no
established general rules to engineer DSE probes but a strong lack of molecular symmetry is
usually observed to entail DSE emission among these dyes. Recent examples including
imidazo[1,2-f]phenanthridine,8 2,5-dithienylpyrrole,9 2-(2’-hydroxyphenyl)benzimidazole,10
among others11 have opened up the possibility to interlink applications usually targeted by
traditional fluorophores, i.e., dyes emitting in solution and solid-state dyes, including AIE
probes. The search for original structures displaying DSE properties is strongly pursued by
heterocyclic chemists due to the imperious necessity to elaborate design rules as well as to
reach overall disruptive progress in the field of organic dyes.
Salicylaldehyde derivatives are synthetic intermediates widely reported in the literature as
precursors for the elaboration of Schiff bases,12 Salen-based probes13 and other heterocycles.14
They present with the advantage of being inexpensive commercially available reagents or
obtainable in a limited number of synthetic steps. Moreover, a recent example pointed out the
use of unsubstituted deprotonated salicylaldehyde as a photocatalyst relying on visible light.15
However, unlike acetophenones or benzophenones analogs,16 salicylaldehyde derivatives
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10.1002/ejoc.202100650
European Journal of Organic Chemistry
display reduced chelating ability toward metal or metalloid ions, unless an additional
chelating group is inserted in the vincinity.17 Nevertheless, some coordination complexes
involving salicylaldehyde derivatives have been described.18 Salicylaldehydes derivatives
typically display weak emission in solution due to the possibility of Excited-State
Intramolecular Proton Transfer (ESIPT) emission, owing to the presence of 6-membered H-
bonded ring.19 ESIPT is a photophysical process, consisting of a phototautomerism that can be
partially or fully frustrated depending on the position and nature of electronic substitution on
the molecular scaffold.20 During the course of previous studies, we serendipitously found that
substitution at the 4-position of selected salicyaldehyde derivatives could trigger strong
naked-eye fluorescence which prompted us to investigate more deeply the photophysical
properties of salicyaldehydes functionalized at the para position with respect to the formyl
group. The studies reported herein investigate the influence of the nature of the π-conjugated
spacer and the electronic substitution on the optical properties of the resulting dyes. Three
different spacers were considered: ethynyl, vinyl or a direct connection between
salicylaldehyde and various aryl groups (Figure 1).
Figure 1. 4-extended salicylaldehyde derivatives reported herein.
Synthesis
The synthesis of 4-extended salicylaldehydes 1a-d, 2a-b and 3a-b is described on Scheme 1.
Salicylaldehyde dyes 1a-d, 2a-b and 3a-b were obtained in one step from commercially
available 4-bromosalicylaldehyde, using classical Pd-catalyzed cross-coupling reactions,
Sonogashira coupling for 1a-d, Heck coupling for 2a-b and Suzuki-Miyaura coupling for 3a-
b. Compounds 1a-d, 2a-b and 3a-b were obtained as white to yellow powders or oil in 35-
97% yield, after purification on a column chromatography. All new compounds were
characterized by 1H and 13C NMR spectroscopy, as well as HR-MS spectrometry (see the SI).
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10.1002/ejoc.202100650
European Journal of Organic Chemistry
Scheme 1. Synthesis of 4-extended salicylaldehyde derivatives 1a-d, 2a-b and 3a-b
Experimental section
Materials and methods
All chemicals were received from commercial sources (Sigma Aldrich, Fluorochem) and used
without further purification. Tetrahydrofuran (THF) was distilled over metallic sodium (Na).
Triethylamine (NEt3) was distilled under argon over KOH. Thin layer chromatography (TLC)
was performed on silica gel coated with fluorescent indicator. Chromatographic purifications
were conducted using 40-63 μm silica gel.
13
1H NMR (500 MHz) and C NMR (126 MHz) spectra were recorded on a Bruker Advance
spectrometer with perdeuterated solvents containing residual protonated solvent signals as
internal references. Absorption spectra were recorded using a dual-beam grating Schimadzu
UV-3000 absorption spectrometer with a quartz cell of 1 cm of optical path length. The
steady-state fluorescence emission and excitation spectra were recorded by using a Horiba S2
Jobin Yvon Fluoromax 4. All fluorescence and excitation spectra were corrected. Solvents
used for spectroscopy were spectroscopic grade and were used as received.
The fluorescence quantum yields (Φexp) were measured in diluted solutions with an absorption
value below 0.1 at the excitation wavelength using the following equation:
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10.1002/ejoc.202100650
European Journal of Organic Chemistry
I is the integral of the corrected emission spectrum, OD is the optical density at the excitation
wavelength, and η is the refractive index of the medium. The reference system used was
Rhodamine 6G, Φ = 88% in ethanol (λexc = 488 nm).
Luminescence lifetimes were measured on a Horiba Scientific TCSPC system equipped with
a nanoLED 370. Lifetimes were deconvoluted with FS-900 software using a light-scattering
solution (LUDOX) for instrument response. The excitation source was a laser diode (λexc
=
320 nm).
N,N-dibutyl-3-iodoaniline,21 N,N-dibutyl-4-ethynylaniline22 and salicylaldehyde 2a23 were
synthesized according to reported procedures.
N,N-dibutyl-3-trimethysilylethynylaniline: To a solution of N,N-dibutyl-3-iodoaniline (1
mmol) in a mixture of THF/NEt3 (20 mL/5 mL) was added Pd(PPh3)2Cl2 (0.05 mmol). The
resulting mixture was degassed with Ar 30 minutes before ethynyltrimethylsilane (3 mmol)
and CuI (0.1 mmol) were added. The mixture which turned rapidly black was stirred
overnight at 60°C. The solution was then taken up in CH2Cl2, washed with water three times,
dried over MgSO4 and concentrated in vacuo. The crude residue was purified by silica gel
chromatography (pet. ether/ CH2Cl2, 8:2, v/v) to afford N,N-dibutyl-3-ethynylaniline as a
1
3
yellow oil. (0.68 mmol., 68%). H NMR (500 MHz, CDCl3) δ [ppm] = 7.11 (t, 1H, CH J =
10 Hz), 6.72-6.76 (m, CH, 3H), 6.61 (dd, 1H, CH, 3J = 10 Hz, 3J = 2.5 Hz), 3.25 (t, 4H, CH2,
3J = Hz), 1.53-157 (m, 4H, CH2), 1.34-1.38 (m, 4H, CH2), 0.97 (t, 6H, CH3, 3J = 10 HZ), 0.26
(s, 9H, CH3). 13C NMR (126 MHz, CDCl3) δ [ppm] = 147.9, 128.9, 123.5, 119.0, 114.8,
112.3, 106.4, 92.3, 50.6, 29.3, 20.3, 14.0, 0.0. ESI-HRMS: calcd for C19H32NSi: 302.2299
(M+H), found 302.2286 (M+H).
N,N-dibutyl-3-ethynylaniline: To a solution of N,N-dibutyl-3-trimethysilylethynylaniline (1
mmol) in a mixture of THF/MeOH (40 mL, 1:1, v/v) was added potassium fluoride (10 mmol)
in portions. The mixture was heated at 50°C for 5 hours, before it was then taken up in
CH2Cl2, washed with water three times, dried over MgSO4 and concentrated in vacuo, to
afford N,N-dibutyl-3-ethynylaniline, as a yellow oil. (0.92 mmol., 92%). 1H NMR (500 MHz,
CDCl3) δ [ppm] = 7.14 (t, 1H, CH, 3J = 10 Hz), 6.76-6.79 (m, CH, 3H), 6.64 (dd, 1H, CH, 3J
3
3
= 10 Hz, J = 2.5 Hz), 3.26 (t, 4H, CH2, J = Hz), 3.01 (s, 1H, CH), 1.54-157 (m, 4H, CH2),
1.33-1.40 (m, 4H, CH2), 0.97 (t, 6H, CH3, 3J = 10 HZ). 13C NMR (126 MHz, CDCl3) δ [ppm]
= 148.0, 129.2, 123.6, 119.0, 115.0, 112.6, 85.0, 75.7, 50.7, 29.4, 20.4, 14.1. ESI-HRMS:
calcd for C16H24N: 230.1903 (M+H), found 230.1893 (M+H).
General procedures for salicylaldehyde derivatives 1a-1d: To a solution of 4-
bromosalicylaldehyde (1 mmol) and N,N-substituted-3 or 4-ethynylaniline (1.2 mmol) in a
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10.1002/ejoc.202100650
European Journal of Organic Chemistry
mixture of THF/NEt3 (20 mL/5 mL) was added Pd(dppf)Cl2 (0.05 mmol). The resulting
mixture was degassed with argon 30 minutes before CuI (0.1 mmol) were added. The mixture
which turned rapidly black was stirred overnight at 60°C. The solution was then taken up in
CH2Cl2, washed with water three times, dried over MgSO4 and concentrated in vacuo. The
crude residue was purified by silica gel chromatography (pet. ether/ CH2Cl2) to afford
salicylaldehydes 1a-d as a white to yellow oil or powders.
1
salicylaldehyde 1a. yellow oil. (0.78 mmol., 78%). H NMR (500 MHz, CDCl3) δ [ppm] =
11.09 (s, 1H, OH), 9.87 (s, 1H, CHO), 7.50 (d, 1H, 3J = 10 Hz), 7.40 (dd, 2H, 3J = 8 Hz, 4J =
3
2.5 Hz), 7.09-7.13 (m, 2H), 6.60 (d, 2H, J = 10 Hz), 3.32 (t, 4H, CH2, 3J = 8 Hz), 1.57-1.64
3
(m, 4H, CH2), 1.34-1.43 (m, 4H, CH2), 0.99 (t, 6H, CH3, J = 8 Hz). 13C NMR (126 MHz,
CDCl3) δ [ppm] = 197.1, 162.2, 149.8, 134.7, 134.2, 133.8, 123.3, 120.8, 119.5, 112.3, 108.1,
97.3, 87.7, 51.2, 20.9, 14.3. ESI-HRMS: calcd for C23H28NO2: 350.2115 (M+H), found
350.2093 (M+H).
1
Salicylaldehyde 1b. yellow powder. (0.53 mmol., 53%). H NMR (500 MHz, CDCl3) δ
3
[ppm] = 11.05 (s, 1H, OH), 9.87 (s, 1H, CHO), 7.52 (d, 1H, J = 10 Hz), 7.14-7.19 (m, 3H),
3
6.77-6.82 (m, 2H), 6.66 (dd, 1H, 3J = 10 Hz, 4J = 4 Hz), 3.27 (t, 4H, CH2, J = 10 Hz), 1.55-
3
13
1.59 (m, 4H, CH2), 1.35-1.39 (m, 4H, CH2), 0.97 (t, 6H, CH3, J = 8 Hz). C NMR (126
MHz, CDCl3) δ [ppm] = 195.8, 161.4, 148.1, 133.5, 132.5, 129.3, 123.1, 122.8, 120.3, 120.0,
118.9, 114.6, 112.9, 95.6, 87.2, 50.7, 29.4, 20.4, 14.1. ESI-HRMS: calcd for C23H28NO2:
350.2115 (M+H), found 350.2096 (M+H).
Salicylaldehyde 1c. white powder. (0.68 mmol., 68%). 1H NMR (500 MHz, CDCl3) δ [ppm]
= 11.06 (s, 1H, OH), 9.88 (s, 1H, CHO), 7.49-7.52 (m, 3H), 7.11-7.14 (d, 2H, 3J = 8 Hz), 6.90
(d, 2H, 3J = 10 Hz), 3.84 (s, 3H, CH3). 13C NMR (126 MHz, CDCl3) δ [ppm] = 195.7, 161.4,
160.4, 133.6, 133.5, 132.5, 123.0, 120.1, 119.9, 114.4, 114.2, 94.4, 87.4, 55.4. ESI-HRMS:
calcd for C16H13O3: 253.0859 (M+H), found 253.0864 (M+H).
Salicylaldehyde 1d. white powder. (0.58 mmol., 58%). 1H NMR (500 MHz, CDCl3) δ [ppm]
= 11.05 (s, 1H, OH), 9.89 (s, 1H, CHO), 7.52-7.56 (m, 3H), 7.12-7.15 (m, 2H), 7.05-7.10 (m,
2H). 13C NMR (126 MHz, CDCl3) δ [ppm] = 195.8, 164.3, 161.8, 161.4, 133.9, 133.5, 131.8,
123.0, 120.3, 120.1, 116.0, 115.8, 92.8, 88.1. ESI-HRMS: calcd for C15H10FO2: 241.0659
(M+H), found 241.0646 (M+H).
Salicylaldehyde 2b. 4-bromosalicylaldehyde (1 mmol.) was dissolved in 15 mL of a mixture
of DMF/DIEA (2:1 v/v) before 1-ethenyl-4-methoxybenze (1.2 mmol.) and tris(2-
methylphenyl)-phosphine (0.7 mmol.) were added as a powder. Argon was bubbled through
the resulting solution before Pd(OAc)2 (0.05 mmol.) was added. The resulting mixture was
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10.1002/ejoc.202100650
European Journal of Organic Chemistry
stirred for 18h at 100°C, before it was cooled to room temperature, washed with a solution of
NH4Cl and extracted with CH2Cl2. Organic layers were dried over MgSO4 and the solvents
removed in vacuo. The crude residue was purified by silica gel chromatography to afford
salicylaldehyde 2b as a yellow powder (0.35 mmol., 35%).1H NMR (500 MHz, CDCl3) δ
3
[ppm] = 11.11 (s, 1H, OH), 9.84 (s, 1H, CHO), 7.48-7.52 (m, 3H), 7.22 (d, 1H, CH, J = 20
3
4
Hz), 7.14 (dd, 1H, CH, J = 10 Hz, J = 5 Hz), 7.07 (s, 1H, CH), 6.91-6.94 (m, 3H) 3.85 (s,
13
3H, CH3). C NMR (126 MHz, CDCl3) δ [ppm] = 195.5, 162.1, 160.2, 147.0, 134.0, 132.9,
129.1, 128.5, 125.0, 119.6, 118.0, 114.5, 114.3, 55.4. ESI-HRMS calcd for C16H15O3:
255.1016 (M+H), found 255.1024 (M+H).
General procedures for salicylaldehyde derivatives 3a and 3b: To a solution of 4-
bromosalicylaldehyde (1 mmol) in a mixture of dioxane/H2O (20 mL/5 mL) was added 4-
methoxyphenylboronic acid or 4-N,N-diethylphenylboronic acid (1.2 mmol) and potassium
carbonate (3 mmol). The resulting mixture was degassed with argon 30 minutes before
Pd(PPh3)Cl2 (0.05 mmol) was added. The mixture was stirred overnight at 60°C. The solution
was then taken up in CH2Cl2, washed with water three times, dried over MgSO4 and
concentrated in vacuo. The crude residue was purified by silica gel chromatography (pet.
ether/ CH2Cl2) to afford salicylaldehydes 3a and 3b as white to yellow powders.
1
Salicylaldehyde 3a. yellow powder. (0.77 mmol., 77%). H NMR (500 MHz, CDCl3) δ
3
[ppm] = 11.16 (s, 1H, OH), 9.84 (s, 1H, CHO), 7.56 (d, 2H, J = 10 Hz), 7.52 (d, 1H, 3J = 10
Hz), 7.23 (dd, 1H, 3J = 10 Hz, 4J = 2.5 Hz), 7.17 (s, 1H, CH), 6.74 (d, 2H, 3J = 10 Hz), 3.42 (q,
4H, CH2, 3J = 10 Hz), 1.21 (t, 6H, CH3, 3J = 10 Hz). 13C NMR (126 MHz, CDCl3) δ [ppm] =
195.5, 162.2, 150.0, 148.4, 134.0, 128.4, 125.2, 118.5, 117.5, 113.5, 111.7, 44.5, 12.7. ESI-
HRMS calcd for C17H20NO2: 270.1489 (M+H), found 270.1499 (M+H).
Salicylaldehyde 3b. white powder. (0.97 mmol., 97%). 1H NMR (500 MHz, CDCl3) δ [ppm]
= 11.13 (s, 1H, OH), 9.90 (s, 1H, CHO), 7.58-7.60 (m, 3H), 7.23 (dd, 1H, 3J = 10 Hz, 4J = 2.5
Hz), 7.18 (s, 1H), 7.00 (d, 2H, 3J = 10 Hz), 3.87 (s, 3H, CH3). 13C NMR (126 MHz, CDCl3) δ
[ppm] = 195.9, 162.0, 160.5, 149.5, 134.1, 131.7, 128.6, 119.2, 118.4, 115.1, 114.5, 55.4.
ESI-HRMS calcd for C14H13O3: 229.0859 (M+H), found 229.0867 (M+H).
Photophysical properties in solution
The photophysical properties of salicylaldehyde derivatives 1a-1d, 2a-b and 3a-b in solution
are compiled in Table 1. All dyes were studied in toluene but dyes 1a, 1b, 2a and 3a, which
demonstrated fluorosolvatochromic features, were consequently studied in several solvents.
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10.1002/ejoc.202100650
European Journal of Organic Chemistry
The absorption and emission of all dyes in toluene are presented on Figure 2, whereas those of
salicylaldehydes 1a, 2a and 3a in a wide range of solvents are presented on Figures 3a-3c
respectively.
Table 1. Photophysical data for salicylaldehyde dyes 1a-1d, 2a-b and 3a-b recorded in
aerated solutions at 25°C.
λabs
(nm)
ε
λem
(nm)
ΔS
τ
[c]
Kr[c]
Knr
[a]
Dye
Solvent
Φ F
(M-1.cm-1)
(cm-1)[b]
(ns)
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1b
1b
1b
1b
1b
1c
1d
2a
2a
2a
2a
2a
2a
2a
2a
2a
2a
2a
2b
3a
3a
3a
3a
3a
3a
3a
3a
3a
3a
3b
412
407
398
401
411
351
400
400
401
400
393
388
389
394
348
346
320
416
425
415
425
436
331
425
427
426
425
430
367
383
392
383
389
396
332
391
387
390
385
321
33000
31000
42300
33000
38000
11600
30800
27000
25500
30400
25100
26200
25400
28300
34100
26800
30300
31000
26000
15200
31000
25800
20600
30000
28600
26400
27000
26200
35700
50400
30100
31800
18200
32200
7800
424
490
508
555
605
690
0.53
0.68
0.60
0.11
0.02
1.3 4.08
3.1 2.19
3.4 1.76
2.5 0.44
3.62
1.03
1.18
3.56
CH[d]
toluene
Et2O
4200
5400
6900
7800
THF
0.6 0.33 16.30
0.4 0.25 24.80
CH2Cl2
CH2Cl2/H+
Acetone
CH3CN
DMSO
DMF
CH[d]
toluene
THF
CH2Cl2
CH2Cl2/H+
toluene
toluene
CH[d]
toluene
Et2O
THF
CH2Cl2
CH2Cl2/H+
Acetone
CH3CN
EtOH
DMSO
DMF
toluene
CH[d]
toluene
Et2O
THF
CH2Cl2
CH2Cl2/H+
Acetone
EtOH
DMSO
DMF
533
9700
0.01
[e]
[e]
[e]
[e]
[e]
[e]
[e]
[e]
[e]
[e]
[e]
[e]
442/594
587
445
506
410/605
654
2300
8000
3000
6000
9200
10100
10000
5300
12500
3980
3800
4900
5600
6100
6300
6900
7600
7900
6900
0.01
0.02
0.12
0.29
0.06
0.02
0.01
0.01
0.01
0.04
0.10
0.19
0.29
0.64
0.04
0.44
0.47
0.09
0.22
0.5 0.20 19,80
0.4 0,50 24,50
2.3 0.53
10.0 0.29
4.1 0.15
1.7 0.12
0.8 0.13 12.40
0.5 0.20 19.80
0.4 0.25 24.80
0.4 1.00 24.00
0.8 1.25 11.30
0.8 2.38 10.10
1.6 1.81
2.2 2.91
0.4 1.00 24.00
2.4 1.83
2.0 2.35
0.5 1.80 18.20
3.83
0.71
2.29
5.76
533
424/532
411/533
423
508
522
558
593
418/532
600
631
4.44
1.64
2.33
2.65
643
602
617
[e]
2.4 0.92
2.4 2.92
3.25
1.25
[e]
7100
[e]
0.70
[e]
[e]
[e]
410/568
459
464
1700
3700
4600
5900
0.24
0.48
0.38
0.76
0.2 12.0
1.3 3.69
2.1 1.81
2.4 3.17
2.3 2.43
38.0
4.00
2.95
1.00
504
532
6500
0.56
1.91
[e]
[e]
[e]
[e]
[e]
[e]
69800
27200
26800
27800
29800
548
546
533
520
400/527
7300
7500
6900
6700
6200
0.09
0.03
0.58
0.27
0.01
0.5 1.80 18.20
0.2 1.50 48.50
3.0 1.93
2.9 0.93
1.40
2.52
0.2 0.10 49.50
toluene
[a] Relative quantum yield determined in solution using Rhodamine 6G as a reference (λexc = 488 nm, Φ = 0.88 in ethanol), [b]
[c]
[d]
Stokes shift, kr (108 s-1) and knr (108 s-1) were calculated using: kr = ΦF/σ, knr = (1- ΦF)/σ where σ is the lifetime,
Cyclohexane [e] Non fluorescent.
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10.1002/ejoc.202100650
European Journal of Organic Chemistry
In toluene, salicylaldehydes 1a-d, functionalized at the 4-position by ethynyl aryl groups with
electrodonating (NBu2, OMe) or withdrawing substituents (F) display maximum absorption
wavelengths (λabs) in the 320-407 nm range with ε of 26200-31000 M-1.cm-1. A pronounced
bathochromic shift is observed when the electronic density on the aryl side increases (λabs 1a
> 1b > 1c >1d). This feature is also observed for the salicylaldehyde dyes 2a-b and 3a-b
where the introduction of a dialkylamino moiety triggers the observation of a red-shifted
absorption band, regardless of the nature of the spacer (λabs = 425 and 367 nm for dyes 2a and
2b, respectively and λabs = 392 and 321 nm for dyes 3a and 3b). The molar absorption
coefficients remain, however in the same range (Figure 2a). Additional absorption bands are
observed below 300 nm which can be assigned to π-π* transitions of the various conjugated
moieties of the dyes. In toluene, for all salicylaldehydes dyes 1a-1d, 2a and 3a-b, a sizeable
emission band is observed after photoexcitation in the lowest energy band (λexc = 310-420
nm). A notable exception is 2b which is non-fluorescent. Luminescent lifetimes recorded are
in the nanosecond range, which is typically expected for purely organic fluorophores.
(a)
1a
1b
1c
1d
2a
2b
3a
3b
(b)
1a
1b
1c
1d
2a
3a
3b
Figure 2. (a) UV-Vis. and (b) Emission spectra of salicylaldehyde 1a (plain blue), 1b (dotted
blue), 1c (dashed blue), 1d (dotted dashed blue), 2a (plain red), 2b (dotted red), 3a (plain
green) and 3b (dotted green) in aerated solutions of toluene at room temperature
(concentrations around 10-6 M).
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In toluene, salicylaldehydes 1a-d, functionalized with an ethynyl-extended spacer at the 4
position display a single or a dual emission band in the range 424-533 nm. Two types of
photophysical behaviors can be clearly evidenced: 1a and 1b, bearing a meta- or para-
substituted N,N-dibutylamino group feature a single emission band (λem = 490 and 506 nm for
1a and 1b, respectively) whereas 1c and 1d, incorporating a para-substituted methoxy or
fluoro group show a dual emission profile (λem = 424/532 nm and 411/533 nm for 1c and 1d,
respectively). The intense emission band observed in the case of 1a and 1b can be ascribed to
the sole presence of the excited enol tautomer E*, as a result of a complete inhibition of the
ESIPT process. This phenomenon was previously observed on other π-extended ESIPT
emitters, rationalized by the occurrence of a S1-S0 transition delocalized over the conjugated
spacer leads to a strong intramolecular charge transfer (ICT) process.24 This assumption can
be further confirmed by the strong values of the calculated quantum yields in toluene (Φ =
0.68 and 0.29 for 1a and 1b, respectively), indicative of a lack of ESIPT process, and is also
consistent with theoretical modelling (vide infra). As for 1c and 1d are concerned, a dual
emission is observed in toluene with a second band being much more intense than the first
one (in the case of 1d, the intensity of E* is very weak). Moreover, a strong bathochromic
shift for the main emission band, as compared to that of 1a and 1b (λem =532 and 533 nm for
1c and 1d, respectively). This optical behavior clearly emphasizes a partial inhibition of
ESIPT, translating into the observation of a dual E*/Keto (K*) process. The K* tautomer
generated by ESIPT is predominant in the excited-state, explaining the very low quantum
yield (Φ = 0.01 for both 1c and 1d); a feature consistent with the detrimental molecular
motions induced by the proton transfer in the excited-state, leading to more efficient non-
radiative deactivations pathways. The electronic nature of the functional group present on the
core of the aryl group (NBu2 vs. OMe or F) clearly influences the pKa* of the phenol moiety
and subsequently the efficiency of the ESIPT process. Salicylaldehydes 2a-b and 3a-b seem
to follow the same trends. Indeed, dyes 2a and 3a, both functionalized by a strongly
electrodonating N,N-dialkylamino moiety at the para position of the aryl ring display intense
K* single emission bands at 508 and 459 nm, respectively. Meanwhile, dye 2b appears to be
non-emissive and dye 3b shows a faint dual E*/K* emission (λem =400/527 nm, Φ = 0.01 in
toluene).
Intrigued by the bright emission recorded for the salicylaldehyde derivatives functionalized by
an aromatic amine group, i.e. 1a-b, 2a and 3a, regardless of the nature of the spacer, we
decided to investigate their photophysical properties in a range of solvents with increasing
dielectric constants (Figures 3a-c). First, the maximum absorption wavelength for each dye
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10.1002/ejoc.202100650
European Journal of Organic Chemistry
does not significantly change upon increase of the dipole moment of the solvent (λabs = 398-
412 nm for 1a, λabs = 388-394 nm for 1b, λabs = 415-430 nm for 2a, λabs = 383-396 nm for 3a),
highlighting a moderately polarized ground state.
In the excited-state, however, sizeable differences can be evidenced between dyes 1a, 2a, and
3a, evidencing a pronounced influence of the nature of the π-conjugated spacer on the
emissive properties. Ethynyl-extended salicylaldehyde 1a displays characteristic features of
strong fluorosolvatochromism, i.e., the observation of a gradual bathochromic shift upon
increase of the dielectric constant of the solvent. In apolar medium, i.e., cyclohexane, toluene,
diethylether or THF, a strong single red-shifted emission band is observed (λem = 424-555
nm) with a high quantum yield (Φ = 0.11-0.68). However, in polar media (dichloromethane,
acetone, acetonitrile, DMSO and DMF), fluorescence appears to be significantly quenched
(λem = 587-605 nm, Φ = 0.01-0.02). Similar fluorosolvatochromism behaviors are observed
for dyes 2a and 3a with a weaker intensity. Indeed, in the case of 2a functionalized with a
vinyl extended spacer, a strong fluorescence intensity is found in most solvents studied with a
maximum emission wavelength spanning the entire visible spectrum (λem = 423-643 nm, Φ =
0.04-0.70). Unlike 1a, the fluorescence quantum yield of 2a seems to significantly increase
along with the dielectric constant of the solvent, from 0.04 in cyclohexane to 0.70 in DMF. As
reported in the literature on similar dyes whose excited-state possesses a charge transfer
character, solvent polarity tends to strongly interfere with the photoisomerization process of
the vinyl spacer, with a stronger photoisomerization efficiency in apolar media.25
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10.1002/ejoc.202100650
European Journal of Organic Chemistry
(a)
(b)
(c)
Figure 3. Absorption (left) and emission (right) of salicylaldehydes (a) 1a, (b) 2a and (c) 3a
in cyclohexane (purple), toluene (navy blue), diethyl ether (blue), THF (light blue),
dichloromethane (green), acetone (light green), acetonitrile (orange), DMSO (yellow), DMF
(pink) and ethanol (red).
Dye 3a, where the aryl group is directly functionalized onto the salicylaldehyde core similarly
displays a strong ICT character with a maximum emission wavelength spanning from 410 to
560 nm with quantum yields in the range 0.03-0.76.
Protonation studies were performed by bubbling HCl gas into a dichloromethane solution of
dyes 1a-b, 2a and 3b, bearing protonable N,N-dialkylamino groups. In all cases, the
protonation led to strongly blue-shifted absorption and emission bands, along with a strong
quenching of fluorescence intensity. This trend is particularly noticeable for dye 2a (λabs/ λem
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10.1002/ejoc.202100650
European Journal of Organic Chemistry
= 436/593 nm vs. 331/532 nm in neutral and protonated dichloromethane, respectively) with a
quantum yield dropping from 0.64 to 0.04.
Figure 4. Photographs under irradiation (λexc = 365 nm) of salicylaldehyde 1a (top), 2a
(middle) and 3a (bottom) in solution (from left to right: cyclohexane, toluene, diethylether,
THF, dichloromethane, protonated dichloromethane, acetone, acetonitrile, ethanol, DMSO
and DMF).
The photophysical behavior observed for all dyes substituted with N,N-dialkylamino groups
is characteristic of the presence of a strong ICT process in the excited-state where dipole
moments of the excited molecules align with those of the solvent to provide highly polarized
solvated excited species. To ascertain the charge transfer nature of the excited-state, Lippert-
Mataga curves were plotted for dyes 1a, 2a and 3a (Figure S2.40). The dipole moment
differences between the excited and ground states (Δµ) were calculated to reach 25.6, 21.7
and 22.3D for dyes 1a, 2a and 3a, respectively, which are in the highest range of values for
similar fluorosolvatochromic emitters, highlighting a highly polarized excited-state.26
Moreover these values are very close the calculated ones (vide infra). To illustrate the strong
impact of the solvent in the emission color and intensity, Figure 4 represents photographs of
dyes 1a, 2a and 3a in solution in different solvents under irradiation with a UV-bench lamp
(λexc = 365 nm).
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10.1002/ejoc.202100650
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Viscosity-induced fluorescence enhancement
The intensity of the emission of dye 3a, which features a direct connection between the two
aryl rings appears to be higher in polar solvents with higher polarities such as DMSO (λem
=
533 nm, Φ = 0.58). This prompted us to explore the viscosity-induced fluorescence
enhancement in different mixtures of EtOH/glycerol (1:0 to 1:9) (Figure 5). Viscosity
molecular probes which often feature donor-acceptor rotor-shaped dyes and display twisted
intramolecular charge transfer (TICT) can be advantageously applied in many sensing
applications.27
Figure 5. (top) Emission spectra of salicylaldehyde 3a in ethanol/glycerol mixtures from
EtOH (100%) to EtOH/glycerol (90/10). (bottom) Photographs of solutions of 3b in
ethanol/glycerol mixtures under irradiation from a UV bench lamp (λexc = 365 nm).
Upon increasing the solution viscosity, a strong fluorescence enhancement can be clearly
detected by the naked eye. The maximum emission gradually shifts from 546 to 575 nm with
an approximate 5-fold enhancement when going from pure ethanol to a 1:9 mixture with
glycerol. This viscosity effect, which remains mild as compared to reported viscosity
sensors,27 can be attributed to the restriction of rotation between the electron donating aryl
group and the salicylaldehyde on the one hand and the rotation between the dialkylamino
moiety and the aryl group on the other hand, in high viscosity media.
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Solid-state emission
Salicylaldehyde 1a-b, 2a and 3a show significant fluorescence emission by the naked eye as
amorphous powders or oils under irradiation from UV bench lamp (λexc = 365 nm). The
fluorescence emission was recorded, as embedded in potassium bromide (KBr) pellets, as
10% wt doped in poly(methylmethacrylate) (PMMA) films and as powders (Figure 6). The
photophysical data can be found on Table S1. As can be seen on Fig.6, very significant
differences on the maximum emission wavelengths can be observed, i.e. single bands when
the emission is recorded in PMMA films or KBr pellets and dual emission in the case of
powders. In PMMA films, the maximum emission wavelength is centered on 467-482 nm,
with the exception of dye 2a which shows a red-shifted emission at 528 nm, which is
consistent with the sole presence of an E* band.
Figure 6. Emission spectra in PMMA films (blue), KBr pellets (red) and as powders (green)
of salicylaldehydes (a) 1a, (b) 1b, (c) 2a and (d) 3a (λexc = 390 nm).
In KBr pellets, a single emission band is observed for all dyes, strongly red-shifted as
compared to that observed in PMMA films (λem = 521-579 nm) which can be tentatively
attributed to the ESIPT tautomer K*. In powders, however, a dual emission band is recorded
for all dyes (λem = 465-469 nm for the first band and λem = 521-579 nm for the second band).
These fluorescence studies in the solid-state unambiguously validate these dyes as DSE dyes
with environment-responsive optical properties.
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10.1002/ejoc.202100650
European Journal of Organic Chemistry
First-principle calculations
To obtain a complementary view on the nature of the emitting species of the synthesized
salicylaldehydes, theoretical calculations have been performed using an ab initio approach
detailed in the SI. During the calculations, the long alkyl chains have been replaced by Me
groups for the sake of computational efficiency. The main theoretical results are collected in
Table 2. First, for absorption, one can note that all theoretical values are slightly blue-shifted
(by ca. 20 nm) as compared to the experimental values, which is attributed to the neglect of
vibronic effects in the calculation. The trends are however, nicely reproduced, i.e. the dyes
yielding the most red-shifted absorption bands being salicylaldehydes 1a, 2a and 3a, as in the
measurements (Tables 1 and 2).
Table 2. Computed vertical absorption, enol and keto vertical emission wavelengths together
with the free energy difference between the enol and keto forms (GE*-K*) corresponding to
the relative stabilities of the two species, the predicted origin of the emission and the excited-
state dipole moments of the two forms. See the SI for details regarding the selected theoretical
approach.
GE*-K*
(eV)
 (E*)  (K*)
λabs λem (E*) λem (K*)
Predicted
emission
Dye
Solvent
(nm)
(nm)
(nm)
(D)
(D)
[b]
[b]
[b]
1a
1b
380
361
336
336
407
351
365
343
431
426
379
364
476
418
416
369
E*
E*
22.4
22.6
15.6
8.6
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
466
466
+0.28
-0.10
7.3
6.9
1c
Dual
K*
1d
466
[b]
-0.24
[b]
4.4
[b]
2a
E*
20.2
14.5
18.9
12.2
2b
459
479
469
+0.19
+0.28
-0.08
E*
7.2
13.1
6.9
3a
3b[a]
E*
Dual
[a]
[b]
Corresponding to the second state, the first transition is nearly dark for E*, see text. K* form unfound in the excited-
state.
The predicted features for the fluorescence are nicely in line with the experimental ones.
Indeed, it has been shown that the relative free energies of the E* and K* tautomers guide the
final response, the ESIPT barrier being typically low in such keto/enol ESIPT dyes.28 In more
details, for salicylaldehyde 1a, which is a strong ICT dye, theory predicts that the K* tautomer
does not exist (optimizations starting from this structure yield back E*). The emission is thus
originating solely from E*, which is consistent with the experimental findings (very large
quantum yield). Interestingly, both the computed Stokes shifts (3113 cm-1) and excited-state
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10.1002/ejoc.202100650
European Journal of Organic Chemistry
dipole (22.4 D corresponding to a =13.2 D) agree quite well with the experimental findings
(4200 cm-1 and 25.6 D, respectively).
In the meta-substituted salicylaldehyde 1b, a very swallow K* minimum could be located but
it appears much less stable than its E* counterpart, so there is no ESIPT driving force and only
E* fluorescence should be observed. This is also fitting the experimental photophysical
measurements characterized by a large emission quantum yield and a large, yet not huge
Stokes shift. If one decreases the strength of the donor group (1c), the two tautomers become
almost isoenergetic with a slight driving force of -0.10 eV for the ESIPT process. Such value
is typical of dual emitters,28 which fits indeed the experimental observation in toluene (Figure
2) and with the observed small quantum yield. Note that the computed E* emission of
salicylaldehyde 1a falls in between the E* and K* emissions of 1c according to theory, again
matching the experimental trends. In going to 1d, one now obtains a clearly more stabilized
K* structure which should be the sole emitting species. This is what is found experimentally
(although a very weak residual E* emission can be detected in Figure 2). We note that the enol
excited states in 1c and 1d are much less polar than in 1a and 1b, confirming that the ICT is
much larger in the two latter dyes.
In the vinyl series, salicylaldehyde 2a behaves similarly as 1a according to the calculation:
only the enol emission can be observed and the excited state of 2a is very polarized, though
slightly less than in 1a. This fits well the observations of a significant Stokes shift, non-
negligible quantum yields, and strong solvatofluorochromism. In contrast, 2b departs from the
behavior of the homologous 1c, according to theory. Indeed in 2b, theory foresees that the
formation of the K* structure would be uphill by +0.19 eV, i.e. a sole E* emission should be
observed. However, experimentally 2b is non-emissive, maybe due to isomerization around
the double bond, making comparisons impossible.
As for the directly bonded salicylaldehydes 3a-b, unsurprisingly, 3a behaves like 1a and 2a,
i.e. the keto form is too unstable to exist and a single fluorescence band stemming from the
enol, characterized by a strong ICT nature is predicted by the calculations; again perfectly
fitting the experimental observations. Salicylaldehyde 3b is more specific, i.e. the sole dye of
the series for which theory predicts that the lowest absorption transition is associated to a
small oscillator strength. The optimization of this state leads to a dark state, which should be
non-emissive due to a trifling Kr. If one starts the optimization process from the second
(brighter state), one obtains a typical dual emission from both tautomers as the ESIPT driving
force is moderate (-0.08 eV). Therefore, the photophysical properties of 3b can be explained
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10.1002/ejoc.202100650
European Journal of Organic Chemistry
by the fact that the absorption to the strongly dipole-allowed S2, is followed by a competition
between ESIPT, leading directly to the S1 of K* and a weak emission, or internal conversion
to the S1 of E* which relaxes non-radiatively (dark state quenching). In the experimental graph
of Figure 2b, there is a small blue-shifted emission band, likely coming from a (non-Kasha
but very limited) residual S2 of E* emission. Eventually, we note that the excited state dipoles
of the E* forms of dyes 1a, 1b, 2a, and 3a are very large, of the order of 20 D, indicating that
their photophysical signatures will be very sensitive to the environment (strong ICT),
consistent with the exceptionally large impact of the solid-state effects noted herein.
Conclusion
In conclusion, a series of original π-extended salicylaldehyde dyes functionalized by electron
donating or withdrawing groups and various spacers have been synthesized and characterized
spectroscopically in solution and in the solid-state. The nature of the spacer has a strong
influence on the nature of the excited-states, i.e. the quantitative, partial or absence of ESIPT
process, leading to strong ICT processes. All experimental results have been rationalized by
first-principle calculations. Owing to their strong environment-sensitive emission profile,
these dyes appear to be attractive candidates for sensing applications. Work along these lines
is currently in progress.
Acknowledgments
The authors thank the CNRS and the ANR for support in the framework of the GeDeMi grant.
D.J. thanks the CCIPL (Centre de Calcul Intensif des Pays de la Loire) installed in Nantes for
generous allocation of computational time. T.S acknowledges the ministère de l’enseignement
supérieur, de la recherche et de l’innovation for a PhD fellowship.
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