Guanidine Substitutions in Naphthyl Systems to Allow a Controlled

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C: Energy Conversion and Storage; Energy and Charge Transport

Guanidine Substitutions in Naphthyl Systems to Allow a Controlled Excited-State Intermolecular Proton Transfer: Tuning Photophysical Properties in Aqueous Solution Pedro J. Pacheco-Liñán, Jesús Fernández-Sainz, Ivan Bravo, Andrés Garzón-Ruiz, Carlos Alonso-Moreno, Fernando Carrillo-Hermosilla, Antonio Antiñolo, and Jose Albaladejo J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b02576 • Publication Date (Web): 16 Apr 2018 Downloaded from http://pubs.acs.org on April 16, 2018

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The Journal of Physical Chemistry

Guanidine Substitutions in Naphthyl Systems to Allow a Controlled Excited-State Intermolecular Proton Transfer: Tuning Photophysical Properties in Aqueous Solution

Pedro J. Pacheco-Liñán†; Jesús Fernández-Sainz†; Iván Bravo*,†; Andrés Garzón-Ruiz†; Carlos AlonsoMoreno‡; Fernando Carrillo-Hermosilla§; Antonio Antiñolo§ and José Albaladejo∥

†

Departamento de Química Física, Facultad de Farmacia, Universidad de Castilla-La Mancha,

Campus Universitario, 02071-Albacete, Spain. ‡

Centro de Innovación en Química Avanzada (ORFEO-CINQA). Departamento de Química

Inorgánica, Orgánica y Bioquímica, Facultad de Farmacia, Universidad de Castilla-La Mancha, Campus Universitario, 02071-Albacete, Spain. §

Centro de Innovación en Química Avanzada (ORFEO-CINQA). Departamento de Química

Inorgánica, Orgánica y Bioquímica, Facultad de Ciencias y Tecnologías Químicas, Universidad de Castilla-La Mancha, Campus Universitario, 13071-Ciudad Real, Spain. ∥ Departamento

de Química-Física, Facultad de Ciencias y Tecnologías Químicas, Universidad de

Castilla-La Mancha, Campus Universitario, 13071-Ciudad Real, Spain.

ABSTRACT: The excited-state intermolecular proton transfer proccess (ESPT) in aqueuos solution is achieved and controlled by the incorporation of guanidine groups in a fluorescent structure. The bisguanidine under investigation exhibits a dual fluorescence emission with a very high Stokes shifts in water, ≈ 86 (7890) and 210 (14500) nm (cm-1), and an excited-stated deprotonation coupled to an intramolecular charge transfer (ICT) process contributes to this emission. The study demostrates that the emission properties of the different protonation states are strongly dependent on the solvent environment, which also allows luminescene of the molecule to be tuned. The results of this work show

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the potential utility of guanidine substitution for the stabilization of ESPT-ICT processes in water and allows the subsequent logical design of new stimulus-responsive fluorophores.

INTRODUCTION Life itself is governed by excited-state proton transfer processes (ESPT) given that many mechanisms of biological energy transduction are associated with electron and proton transfer reactions. As a consequence, fluorophores that undergo these intriguing processes have been studied in great depth for bio-imaging in living cells, sensing of ions, proteins and DNA, artificial energy-related systems such as fuel cells, and other electrochemical devices.1–7 Despite the volume of research carried in this area, current goals are to achieve new stimulus-responsive fluorophores with larger Stokes shifts without self-reabsorption, easier population inversion, and ultra-fast dual emission processes. In this respect, organic molecules that undergo ESPT processes are extremely important because of their exceptional fluorescence properties.8–10 Solute-solvent ESPT processes are ultrafast reversible reactions in which a series of elementary steps occur on different time scales and the stabilization of reactants and products generally plays a key role. In this sense, water warrants particular attention since it is the fastest solvent to stabilize polar solutes.11–14 However, achieving control of an intermolecular ESPT process in aqueous solution to modulate the fluorescence of a molecule is not a simple task, and it is difficult to observe the emission of reactants and products during the ESPT process simultaneously in steady state.13–18 On the other hand, intramolecular ESPT systems have been widely described and fluorescent molecules with enol-keto phototautomerism are the most common systems.8,9,19,20 It is noteworthy that the ESPT rate of that photosystem is easily tuneable by changing the microenvironment: i.e., the substituents present on the donor and acceptor groups, and the surrounding medium. Similarly, the few studies found in the literature


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that concern intermolecular ESPT show that the extent of the process also depends on the nature and position of the substituents in the structure.1,2,18,19,21–23 To enhance the optical properties of fluorophores, ESPT processes can be coupled to an intramolecular charge transfer reaction (ICT), because of the possible “push-pull pair”.4,9,20,23–25 In this sense, numerous fluorophores with intramolecular ESPT-ICT have been tested to identify single molecules that exhibit white-light emission for laser applications. Once again, the extent of this coupled ICT process depends on the features of the electron-donor or -acceptor substituent, the position of the substituents in the molecular structure, and the nature of the solvent.4,10,26 It is worth noting that intermolecular ESPT-ICT processes are very rare and are practically unknown in aqueous solution. The presence of amino groups linked to aromatic systems or N atoms inserted within the aromatic units are widely used to improve the fluorescence properties by increasing the aromatic electronic density, suppressing the non-radiative relaxation processes and providing pH sensitivity.27,28 Guanidines are very strong bases that have very interesting features, such as the high stability of the cation in aqueous solution over a wide pH range, and the participation of electrostatic and hydrogen bonding interactions with anions and polar species. As a consequence of these properties, guanidine derivatives have been successfully used in a wide range of biophysical and industrial applications.27,29–35 From a chemical point of view, these features provide an ideal template for the rational design of new stimulus-responsive fluorophores based on ESPT-ICT systems that could be stabilized in aqueous solution. In this sense, we envisaged that the incorporation of guanidine groups in a fluorescent structure might be perfectly suited to induce ESPT-ICT processes, with structural diversity achieved through the wealth of guanylation reactions.35 We report here how the facile guanylation of commercially available amines is indeed a highly promising tool for the rapid discovery of new ESPT-ICT fluorophores in aqueous solution. The mono- and bis-guanidine



naphthalenes MG and BG (see Figure 1), respectively, were initially obtained and then their photophysical properties were studied by absorbance and steady-state fluorescence spectroscopy and in time-resolved fluorescence experiments. Finally, Density Functional Theory (DFT) calculations were carried out in order to gain a better understanding of the phenomena observed. ESPT-ICT phenomena lead to abnormally high Stokes shifts, and BG present excellent skills for its application as fluorescent probe with ratiometric response. To our knowledge, this is the first example of application of guanidine moiety both for binding the analyte (proton) and forming analytical signal (two-banded fluorescence emission).

EXPERIMENTAL SECTION Materials. Acetonitrile, tetrahydrofuran, ethanol, N,N’-diisopropylcarbodiimide, amines, ZnEt2 (1M solution in hexane) and trifluoroacetic acid were purchased from Sigma-Aldrich (Spain). Dichloromethane, toluene, hexane, NaOH and HCl were purchased from Labkem (Spain). The highest purity grade available was used. Synthesis and Characterization of MG and BG. Synthesis reactions were performed using standard Schlenk and glove-box techniques under an atmosphere of dry nitrogen. Solvents were purified by passage through a column of activated alumina (Innovative Tech.), degassed under nitrogen and stored over molecular sieves in the glove box prior to use. NMR spectra were recorded on a Varian FT−400 spectrometer using standard VARIAN-FT software. In a glovebox, 0.04 mL of a solution of ZnEt2 in hexane (1M) was added to a solution of the corresponding amine (2 mmol) in dry THF (20 mL) in a Schlenk tube. N,N’diisopropylcarbodiimide (2 mmol for MG or 4 mmol for BG) was then added to the above reaction mixture. The Schlenk tube was taken outside the glovebox, and the reaction was carried out at 50 °C for 3 h. The solution was concentrated under reduced pressure, hexane was added and the mixture was placed in a refrigerator at -30 °C for 16 h. After filtration the guanidine


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products were obtained as white microcrystalline solids in 95% yield and characterized by comparing their NMR spectra with the literature data (see Supporting Information).36,37 Spectral equipment and measurements. Solutions of MG and BG (10 µM) were prepared in different solvents from stock solutions (1 mM) of MB and BG in the same solvent. For experiments in water, stock solutions (1 mM) of MG and BG in ethanol were used. For the pH titration experiments, an aqueous solution (10 µM) was titrated by the successive addition of small volumes (in the order of microliters) of HCl and NaOH solutions of different concentrations (0.01–10 M) to an initial volume of 20 mL in order to minimize changes in the sample. Steady State Fluorescence (SSF) spectra were recorded and the excitation wavelengths were 293 and 295 nm for MG and BG, respectively. Solutions of MG and BG (10 µM) in acetonitrile were titrated with trifluoroacetic acid. The [TFA]: [MG or BG] ratios were from 0 to 2. Steady State Fluorescence (SSF) spectra were recorded and the excitation wavelengths were 305 and 310 nm for MG and BG, respectively. Fluorescence spectra of different concentrations of BG in acetonitrile were recorded when increasing quantities of BG were added from 0 to 100 µM (295 nm excitation wavelength). The UV-Vis absorption spectra of MG and BG were recorded at room temperature using a Cary 100 (Varian) spectrophotometer in a 10 mm quartz cuvette (Hellma Analytics) using a slit width of 0.4 nm and scan rate of 600 nm/min. Fluorescence spectra of the samples were recorded on an FLS920 (Edinburgh Instruments) spectrofluorometer equipped with a time correlated single photon counting (TCSPC) detector. A TLC 50 temperature-controlled cuvette holder (Quantum Northwest) was used for the measurements (temperature was controlled at 296 K). For steady-state fluorescence spectra (SSF), a Xe lamp of 450 W was used as the light source and the excitation and emission slits were both fixed at 1 nm. The step and dwell times were 1 nm and 0.1 s, respectively. A subnanosecond pulsed Light-Emitting Diode, EPLED-290 (Edinburgh Photonics), was employed as



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a light source at 291 nm to record the time-resolved fluorescence spectra (TRF). TRF emission was also collected at different emission wavelengths, as indicated in the manuscript. The fluorescence intensity decay, I(t), was fitted to the following multiexponential function using an iterative least squares fit method = ∑ exp −/

(1)

where αi and τi are the amplitude and lifetime for each ith term. The mean lifetime of the decay was then calculated as:

=

∑

(2)

∑

The quantum yields of the sample, , were measured in solutions in the respective solvent. Tryptophan ( = 0.12 ± 0.01) was used in water at 23 °C (λexc = 278 nm) as standard38,39 and applying the equation:40 =

!

#$"

"

#$!

(3)

where I is the integrated density and the subscripts x and s correspond to the sample and standard, respectively. OD is the optical density and the refractive index was the same for sample and standard solutions.41 The step, excitation and emission slits were all fixed at 1 nm, dwell at 0.2 s and measured OD values were always lower than 0.10. We used the kinetic approach proposed by Weller et al.

56

to estimate the ESPT rate

constant, kPT, from the intensity-band ratio of the steady-state fluorescence spectra of the specie formed after and before the proton transfer. For aqueous solution we used the following equation: %&' ≈

)*+, )*+,

∙

)*+,


(4)

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while for acetonitrile we used the following equation: %&' ≈

)*+ , )*

∙

)*+,

(5)

./0 , is the lifetime for the specie formed after the proton transfer (123 4 in both solvents). Fluorescence rate constants, kF, can be estimated using the following equation: %5 =

67 7

(6)

where, 5 and 5 are the quantum yield and the fluorescence lifetime of the specie, respectively. Computational methodology. All calculations were performed with the Gaussian09 (revision D.01) program package.42 The initial conformational analysis was carried out at the PBE0/6-31G* level of theory43,44 while the ground (S0) and first electronic excited (S1) state geometries were optimized at the PBE0/6-31+G** level.45,46 TD-PBE0 is particularly recommended to estimate the low energy electronic transitions in organic dyes47 and basis sets including diffuse functions are also recommended in energy calculations.48 Vertical electronic transitions were computed at the time-dependent TD-DFT level for the solvated molecule within the Polarizable Continuum Model (PCM) methodology.49,50 Fluorescence emission energy was calculated as ∆Eem = ES1GS1 – ES0GS1

(4)

where ES1(GS1) is the energy of the S1 state at its equilibrium geometry (in the state-specific solvation approach) and ES0(GS1) corresponds to the energy of the S0 state at the S1 state geometry and with the static solvation from the excited state.

RESULT AND DISCUSSION Synthesis and structural characterization of fluorophores. In this work a bis-guanidine, BG in Figure 1, was selected to promote potential intermolecular ESPT-ICT processes. The



monoguanidine analogue, MG in Figure 1, was obtained to carry out a comparative study. These guanidines have been described previously and were obtained by catalytic methods involving lanthanide complexes.36,37 In recent years we reported the use of cost-effective and commercially available ZnEt2 as a very effective catalyst for the addition of primary and secondary aromatic amines, including aromatic diamines, aliphatic, heterocyclic, and secondary cyclic amines, to carbodiimides under mild conditions.51–54 Thus, as previously reported, guanidines were obtained in a single step by this catalytic method from commercially available bis- and mono-amines. Details of the corresponding synthesis procedures and characterization are provided in the experimental section.

Figure 1. Chemical formula of the studied compounds Photopysical properties of the ground state: pH-dependence. Guanidines are strongly basic groups and the optical properties of the synthetized compounds are therefore strongly dependent on their protonation state. The acidic form of MG is denoted as MGH+ and the mono- and diprotonated states of BG are denoted as BGH+ and BGH22+ respectively. The lowest absorption bands of MG and BG have maxima at 283 nm and 294 nm in water, respectively, which could be easily attributed to a typical π→π* electronic transition. As expected, upon excitation MG showed one emission band located at 380 nm. However, BG exhibited two emission bands with maxima at 376 and 520 nm (Figure 2a). Interestingly, when the 376 and 520 nm emission bands were scanned, identical excitation spectra were obtained and these correspond to the same absorption spectrum. In the first instance, this fact suggests


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that both emission bands arise from the same ground state origin and, therefore, two excitedstate species were generated after electronic excitation.

Normalized fluorescence

a)

1.0

0.8

0.6

0.4

0.2 250

300

350

400

450

500

550

Wavelength (nm) 1.0

b)

Normalized intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.8

0.6

0.4

0.2 250

300

350

400

450

500

550

Wavelength (nm) Figure 2. a) Normalized emission (lines) and excitation (symbol) spectra of BG (blue) and MG (red) in water (pH = 7). Excitation spectra at 375 nm (dot), 386 nm (square) and 498 nm (triangle) emission wavelength; b) Normalized emission (lines) and excitation (symbol) spectra of BG in acetonitrile (CH3CN, blue) and when 2 eq. of trifluoroacetic acid (TFA)(red) were added. Excitation spectra at 370 nm (square), 418 nm (triangle) and 490 nm (dot) emission wavelength. Concentrations used were 10-5 M

The absorption and emission spectra for both compounds in aqueous solution at different pH values are shown in Figure 3. In the case of MG, a linear response with pH and the existence of an isosbestic point indicate the presence of two distinct species, the acidic (MGH+ and the basic (MG) forms with maxima at 283 nm and 297 nm, respectively (Figure 3a). The absorbance



changes with pH for BG show a decrease in the band with a maximum at 294 nm as the pH increases, and the appearance of a red-shifted broad band at around 315 nm that increases with pH (Figure 3a). In principle, similar behaviour to MG is expected and the acidic (BGH+) and basic (BG) forms might correspond to the main absorption bands at 294 and 315 nm, respectively. However, the co-existence of mono- and di-cationic species in bis-guanidine derivatives has been described33 and the dicationic species (BGH22+) cannot be ruled out. Therefore, in order to assess the origin of the two bands for BG, the absorption spectra were measured upon addition of increasing equivalents of trifluoracetic acid (TFA) in acetonitrile (Figure 4).

Figure 3. a) Absorption spectra of MG and BG at different pH values; b) Emission spectra of MG and BG at different pH values. Concentrations used were 10-5 M


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Figure 4. Absorption spectra of BG in acetonitrile on addition of different equivalents (eq) of trifluoroacetic acid (TFA). Concentrations used were 10-5 M In this experiment two isosbestic points were observed but only two clearly defined absorption bands, which may indicate the presence of three distinct species. The band at 335 nm gradually decreased and was red-shifted when TFA was added up to 0.6 equivalents, with an isosbestic point observed at 355 nm. This band suddenly became blue-shifted and when one equivalent of TFA was added a new band at 296 nm gradually increased and remained even when 20 equivalents of TFA were added. This finding suggests that these bands (296 and 335 nm) are due to the di-cationic and neutral species, respectively. In addition, it is reasonable to assume that the band observed at 296 nm is equivalent to the absorption band previously observed for acidic aqueous solutions (294 nm), and this is therefore assigned to BGH22+. Furthermore, the band related to BGH+ may appear at around 360 nm according to the isosbestic point located at 355 nm. These assignments are in agreement with the results of theoretical calculations, which are discussed below (for instance a value of 361 nm was calculated for BGH+). It seems that the diprotonated form (BGH22+) is predominant in water up to very basic pH values and this is consistent with the fact that only one pKa value could be determined.33,34 pKa values of 10.8 and 9.4 for MG and BG were obtained, respectively, (see pKa

determination

in

Supporting


Information).


Photophysical properties of the excited state: identification of the intermolecular ESPTICT process. The emission spectra of MG in aqueous solution are shown in Figure 3b. Under acidic conditions a single weak emission band centred at 380 nm was observed when the sample was excited at 293 nm (an intermediate wavelength between the maximum absorption bands of MG, 283 nm, and MGH+, 297 nm). This band slowly decreased on increasing the pH to the pKa value (10.8), after which an intense new band at 480 nm gradually appeared. At pH values above the pKa the decrease in intensity showed a linear response, and at pH = 12.2 the emission spectrum mainly contained the band with a maximum at 480 nm. This suggests that the emission band at 380 nm should be assigned to the excited state of the acidic form (MGH+*) and, therefore, the emission band at 480 nm can be assigned to the emission from the excited state neutral form (MG*). The large Stokes shift can be related to an efficient excited state ICT process from the guanidine group to the naphthalene moiety at basic pH. In contrast, protonation of the guanidine group may suppress the electronic transfer to the naphthalene ring exclude the ICT mechanism. As far as the bis-guanidine derivative is concerned, two emission bands centred at 376 and 520 nm were observed in aqueous solution at different pH (Figure 3b). The emission intensity of the band at 376 nm is very weak at high pH values (pH ≥ 10), but increases gradually as the pH decreases down to pH = 4. A significant change in the intensity was not observed in the 1–4 pH range. In contrast, the band at 520 nm shows a high emission intensity at basic pH and this gradually decreases upon acidification of the medium. However, the emission band at 520 nm did not disappear even at very acidic pH values. Hence, both emission bands coexist in an acidic medium, but only a single band is observed in the corresponding excitation spectra. This band, which has an excitation maximum at 290 nm, was assigned to BGH22+* in agreement with the wavelength of the maximum absorbance (294 nm) previously measured for BGH22+. Therefore, an intermolecular ESPT reaction from the excited species BGH22+* seems to occur and generate a new emissive excited state, i.e., BGH+*. Emission bands at 520 and 376 nm come from the


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excited state emission of mono-cationic (BGH+*) and di-cationic (BGH22+*) species, respectively, in accordance with the theoretical values calculated for both transitions (385 nm for BGH22+* and 509 nm for BGH+*, see below and Scheme 2). The basic BG form is not present in water within the pH range studied.

Scheme 1. Proposal for the excited state proton transfer coupled to an intermolecular charge transfer mechanism between the protonated and deprotonated forms of BG in water and acetonitrile.

Figure 5. Emission spectra of BG in acetonitrile when different numbers of equivalents of trifluoroacetic acid (TFA) were added. Concentrations used were 10-5 M



As in the case of MG*, these observations – along with the large Stokes shift and high intensity of the BGH+* emission band – suggest that the ESPT reaction can be coupled to an ICT process. The sustainability of the ICT process might be attributed to a “push-pull” effect from the deprotonated guanidine group to the aromatic system in BGH+* species (see Scheme 1).4,18 To confirm this hypothesis and identify the BG* emission band, fluorescence spectra of BG were measured in another low protic solvent, namely acetonitrile, while increasing equivalents of TFA were added (Figure 5). BG shows an intense emission band at 418 nm in acetonitrile with with a long tail in the red region. The red edge of the band increased dramatically when TFA was added, with the highest intensity reached when one equivalent of TFA was added (one proton per BG molecule) and with a maximum at 490 nm. On adding more than one equivalent of TFA, the emission spectrum contained a new band at 369 nm that reached its maximum intensity at two equivalents of TFA (two protons per BG molecule). Therefore, emission bands at 369, 418 and 490 nm in acetonitrile could be assigned to the species BGH22+*, BG*, and BGH+*, respectively. The emission spectra of MG in acetonitrile when different equivalents of TFA were added are shown in Figure S4 in the Supporting Information. These results agree well with the previous assignments of bands in aqueous solution (bands at 376 and 520 nm for BGH22+* and BGH+*, respectively). It seems that the ESPT process also exists in acetonitrile for bis-guanidine, but this process occurs between different species when compared to the process in water. In the previous absorption spectroscopy experiment, only the species BG was observed when TFA had not been added. Nevertheless, a long tail at the red edge of the emission band recorded under the same experimental conditions seems to indicate that the excited species BGH+* is being formed from BG* through an ESPT reaction, with a proton taken from the solvent molecules. As mentioned previously, the emission band arising from BGH+* (490 nm) grows upon the addition of TFA up to one equivalent. However, only an excitation band at 334 nm, with minimal changes, was observed under these experimental conditions (0–1 equivalents TFA), which could correspond


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to both BG and BGH+ forms since they have similar electronic excitation energies (see Figure 2b). When two equivalents of TFA were added only BGH22+ was detected in the absorption spectroscopy experiments and, accordingly, only an emission band, attributed to BGH22+* (369 nm), was observed in the fluorescence emission spectrum. This behaviour contrasts with the emission profile observed for the same compound in high pH aqueous solutions, where two emission bands arising from the same excited species were recorded. This observation suggests that, in acetonitrile, the ESPT process does not occur from BGH22+* to the solvent molecules, a situation in contrast to the behaviour observed in water, while the BG* + H+ BGH+* reaction does occur in both solvents. Furthermore, the fluorescence of the BG* band (418 nm) remains when 0–0.2 equivalents of TFA are added, while the BGH+* fluorescence band gradually increases. However, in more acidic solutions both emission bands changed with the amount of TFA added, which suggests that the ESPT process is inhibited (see Figure 5). An additional experiment in acetonitrile solution was performed to shed light on the ESPT process observed in this solvent. It was demonstrated that, in the excited state, the species BG* tends to convert into BGH+* by proton removal from the solvent molecules instead of from a BG molecule. At at low concentration BG exhibited one main emission band (418 nm) and a long tail in the red part of the spectrum, which is associated with BGH+* and the ESPT process. When the concentration of BG was increased, the emission band narrowed and was blue-shifted (see Figure S7). Thus, the relative light emission of the species BG* increases with respect to BGH+* and, as a consequence, proton removal from another BG molecule can be ruled out since the ESPT process is not favoured by a higher BG concentration. Solvent-dependent photophysical properties. The extent of the intermolecular ESPT strongly depends on the relative polarity and proton exchange ability of the solvent. Given the strong dependence of ESPT efficiency with changes in the environment, emission spectra were measured in different protic and non-protic solvents (see Figure 6 and Table 1). In a non-protic and apolar solvent, such as hexane, the spectra show only one emission band at 400 nm, which



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may correspond to the neutral form BG*, and hence an ESPT process was not observed. A single emission band at 460 nm was observed for a low polar aprotic solvent such as dichloromethane and this is attributed to the excited monoprotonated form BGH+*, which is formed through an ESPT reaction from BG* with a proton taken from a molecule solvent. In a similar way, two emission bands were observed in acetonitrile and tetrahydrofuran, which may correspond to BG*and BGH+* species depending on the position of the band. In a more polar and protic solvent, i.e., ethanol, only an emission band assigned to BGH+* was recorded (band

centred at 492 nm). Figure 6. Relative emission spectra of BG in different solvents, λexc = 337, 324, 328, 332, and 334 nm, for tetrahydrofuran (THF), hexane, ethanol, dichloromethane and acetonitrile, Guanidine

Solvent

λex (nm)

λem (nm)

ϕ

Lifetime decay (ns)

respectively. Concentrations used were 10-5 M Table 1. Photophysical properties of BG in different solvents at room temperature. Lifetime decay data for different forms (neutral, monoprotonated and diprotonated), λexc = 291 nm, (λem ) ACS Paragon Plus Environment

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MG*/BG*

MG

BG

@AB4 */CAB 4 *

CABDD4*

H2O (pH = 6.1)

284

388

0.003 ± 0.001

7.82 ± 0.25 (388)

H2O (pH =12.7)

297

484

0.005 ± 0.001

1.08 ± 0.01 (484)

CH3CN

315

443

0.005 ± 0.001

0.24 ± 0.01 (443)

H2O (pH = 2)

290

376/520

0.005 ± 0.001

1.71 ± 0.03 (498)

0.16 ± 0.01 (376)

H2O (pH = 6.1)

290

376/520

0.005 ± 0.001

1.79 ± 0.02 (498)

0.19 ± 0.02 (376)

H2O (pH = 11)

315

376/520

0.002 ± 0.001

1.69 ± 0.01 (498)

0.56 ± 0.07 (376)

EtOH

328

492

0.043 ± 0.003

0.75 ± 0.19 (492)

CH3CN

334

418/490

0.007 ± 0.001

CH2Cl2

332

460

0.014 ± 0.001

THF

337

472

0.022 ± 0.003

0.20±0.01 (415)

0.29 ± 0.04 (490)

0.60 ± 0.02 (460)

It is important to note that emission bands were red-shifted as the polarity of the solvent increased, but the emission band of the monoprotonated form was markedly displaced to the red on changing from dichloromethane to water (by more than 50 nm). These results indicate that polar solvents stabilize the excited state via a “push-pull” effect within the ICT excited-state process in both forms (BG* and BGH+*), but stabilization is significant in the BGH+* form. In this species only one guanidine group is protonated and the “push-pull” effect via ICT from the other guanidine group is enhanced to stabilize the excited state. The importance of the ICT process in the stabilization of BGH+* is also evidenced by the fluorescence quantum yield (ϕ). The photophysical properties of mono- and bis-guanidine derivatives in different solvents are summarized in Table 1. In general, the fluorescence quantum yields of a particular species decreased as the polarity of the solvent increased, and this is due to the interaction between excited-state and solvent molecules leading to a non-radiative decay.40 These interactions are generally pronounced in protic solvents because of the possibility of forming hydrogen bonds. For MG, the φ value observed in water at pH 12.7 is significantly higher than at pH 6.1 (0.005 vs. 0.003) and similar to that observed in acetonitrile (0.005). This finding clearly indicates that the MG* form is more fluorescent than MGH+*. This fact seems to be related to the stabilizing effect of the ICT observed for the excited species MG*, while protonation of the guanidine



group inhibits this process. In contrast, for BG an ICT effect associated with a large Stokes shift was observed for the excited monoprotonated species BGH+*. In this sense, φ increases with the polarity of the solvent (dichloromethane

Guanidine Substitutions in Naphthyl Systems to Allow a Controlled

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