Introduction

In a series of papers [1–10], it has been shown that methyl p-dimethylaminobenzoate (I) and its ortho derivatives (–OCH3 (II) and –OH (III)) exhibit dual fluorescence in polar solvents, i.e., the normal (locally excited (LE)) fluorescence S 1 (LE) → S 0 (LE) and the twisted intramolecular charge transfer fluorescence S 1 (ICT) → S 0 (ICT). The two emission modes originate from independent electronic transitions of planar and non-planar spatial conformers, in which the plane of the electron donor substituent (–NMe2) is parallel or perpendicular to the plane of benzene skeleton. In polar solvents, the initially populated LE state reacts further to produce a stable twisted intramolecular charge transfer (TICT) state, which gives rise to a second (long-wavelength) fluorescence band. This conformation change facilitates an electron transfer process from the donor (–NMe2) to the methyl benzoate skeleton (charge accepting moiety). The appearance of two or three radiative modes in the fluorescence spectrum, associated with a definite isomeric or tautomeric form of the excited molecule, has been confirmed by picosecond transient absorption measurements [3] and steady-state absorption and fluorescence studies [1, 2, 4–9] in solvents of different polarity.

The isomeric or tautomeric spatial form of the parent molecule possesses a higher electric dipole moment and emits fluorescence quanta of lower energy. Its photophysical parameters, e.g., transition energy E 00, fluorescence rate constant k F = Q F /τ F (fluorescence quantum yield Q F and lifetime τ F ) differ significantly from that of the parent molecule [2, 4, 8, 9]. The efficiencies of the photoinduced intramolecular reactions i.e., the intramolecular charge transfer S 1 (LE) → S 1 (ICT) or the intramolecular proton transfer S 1 (LE) → S 1 (IPT) are determined by the dipole-dipole solute-solvent interactions, as well as they can be influenced by external factors such as a short range anisotropic interaction (hydrogen bonding) with one or more solvent molecules causing changes in the absorption and fluorescence spectra (changes in fluorescence yield, the shape of fluorescence emission spectra, position of fluorescence maxima) [10]. Ghosh and co-workers [11] have shown that a chemical reaction of a H+ ion with the –NMe2 substitutent changes the C–N bond length and as a consequence makes torsional motions to be more complex. As a result of that the dialkylamino group forms a specific complex, which severely polarises one end of the molecule.

Since our earlier papers [7, 8] concern spectroscopic studies of methyl p-dimethylaminobenzoate and its ortho derivatives, it is very interesting to study the influence of the H+ ion concentration on the steady-state absorption and fluorescence spectra of methyl p-dimethylaminobenzoate (I) and its o-methoxy (II), o-hydroxy (III) derivatives. The chemical structures of compounds of interest are given in Scheme 1. The results of spectroscopic studies were compared with results of our quantum-chemical calculations.

Scheme 1
scheme 1

Structures of methyl p-dimethylaminobenzoate and its o-methoxy and o-hydroxy derivatives (first column) and their monoprotonated (second column) and diprotonated (third column) forms

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Methods

Steady-state absorption and fluorescence measurements at room temperature were carried out using a computer-controlled Shimadzu UV-2401 PC Spectrophotometer and Shimadzu RF-5301 PC spectrofluorometer with a 3 nm bandwidth in excitation and 5 nm bandwidth in emission. Fluorescence measurements were carried out using the triangular cuvette with frontal excitation to minimize the effect of reabsorption. The emission was observed perpendicular to the direction of the exciting beam. Thus the errors due to fluorescence reabsorption were reduced in a way that mathematical corrections were superfluous. The luminescence spectra have been corrected for the spectral response of the photomultiplier Hamamatsu R-928. In the conversion between wavelength and wavenumber representation of the absorption and emission spectra the reweighing intensities factor has been taken into account [12].

The solvents tetrahydrofuran (THF) and hydrochloric acid (HCl) were purchased from Aldrich Chemical Co as highest grade commercially available. Additionally, solvents were checked in both steady-state and time-resolved fluorescence apparatuses for lack of fluorescent impurities in the wavelength ranges of interest. The concentration of molecules in the solutions was about 10−4 M.

All calculations concerning absorption spectra of neutral and protonated molecules (see Scheme 1) were performed using the Gaussian 09 code [13]. The first problem that we addressed was identification of the most stable conformers. We used the Systematic Screening of Conformers (SSC) software [14, 15] to generate a library of conformers for each molecule. For the first four molecules (I, IA-B, and II) all rotational degrees of freedom were considered at this stage. Next, we recognized that the contribution from methyl groups to the overall stability of molecules is relatively small and can be decoupled from contributions associated with other rotational degrees of freedom. Thus for the remaining five molecules (IIA-B, III, IIIA-B) the rotational degrees of freedom of methyl groups were not considered when creating initial libraries of conformers with SSC. The stability of each conformer was initially determined in the course of geometry optimization with the B3LYP exchange-correlation functional [16, 17] and the 6-31G* basis set [18], including geometrical relaxation of all methyl groups. A typical library created with SSC contained a few hundreds of conformers and the B3LYP/6-31G* optimization led to a few tens of distinct local minima, see Table 1. For these local minima all meaningful conformers derived from rotations of methyl groups were considered. At the second stage, the structures of the most stable conformers of each molecule were further refined at the B3LYP/6-311G** level [19]. The number n of conformers included at the second stage of search was selected in this way that the (n +  1)-th conformer was less stable than the n-th conformer by at least 2 kcal/mol. The details of our two-step procedure are summarized in Table 1. For example, for the diprotonated methyl o-hydroxy p-dimethylaminobenzoate (IIIB) the SSC software created 218 initial conformers and the B3LYP/6-31G* prescreening step led to 30 distinct conformers. The two most stable conformers spanned a narrow range of 0.2 kcal/mol and the third conformer was less stable by 7.6 kcal/mol. Hence, only the first two conformers were considered at the second stage of optimization. The effect of solvation with THF was studied by performing additional optimizations within the polarized continuum model (ε = 7.58) [20] at the B3LYP/6-311G** level. Excited singlet electronic states for the most stable conformers were calculated using the time-dependent B3LYP/6-311G** method [21], with the effect of THF included by using the PCM model.

Table 1 Number of optimized structures and local minima for each step of conformation searches
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Results and Discussion

Absorption Spectra and Their Analysis on the Base of the Quantum-chemical Calculations

Figure 1(a, b, c) shows the absorption spectra of I, II and III in neat THF and its hydrochloric acid solutions at different HCl concentrations (from 0.098 to 6.065 M) keeping a constant solute concentration of 10−4 M. The absorption spectra, as it is seen in Fig. 1a, b and c, possess two bands: the long-wavelength band in the region of 260–340 nm and the short-wavelength band in the 210–260 nm region. In our paper the bands are signed as C and B, respectively. In order to clarify the folded structure noted in the absorption spectrum of the molecules I, II and III, in our previous study we performed quantum-chemical calculations on these systems in vapour-phase [8]. From these calculations, it is evident that the long-wavelength absorption band in vapour-phase consists of two overlapping components (see Fig. 1 and Table 2). It has been shown [7, 8] that the calculated S 0 → S 1 transition in the studied compounds is described by the same linear combination of HOMO and LUMO one electron π-orbital transitions (only with different composition) as the S 0 (1 A) → S 1 (1 L b ) band of benzene and MB (methyl benzoate). Additionally, the strong peak located at 211 nm (47,400 cm−1) for II and at 215 nm (46,500 cm−1) for III (see Fig. 1b and c) corresponds to the allowed transitions of benzene (S 0 (1 A) → S 1 (1 B b ), signed by us as A band) at ν ≅ 54,000 cm−1.

Fig. 1
figure 1

Absorption spectra of I (a), II (b) and III (c) as a function of the HCl concentration in THF; 0-neat THF; 1–0.1 M; 2–0.2 M; 3–0.29 M; 4–0.49 M; 5–0.98 M; 6–1.97 M; 7–2.79 M; 8–3.6 M; 9–4.43 M and 10–6.065 M. The bars under the absorption curves give the theoretical electronic state energy values and corresponding oscillator strengths of neutral molecules in vapour-phase (taken from [8]) and in THF, and mono- and dicationic forms of the parent molecule in THF

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Table 2 Theoretical values of singlet excited states, E i , and corresponding oscillator strengths, f i , of molecules under study
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Here we would like to note that for all studied molecules in vapour-phase the longer wavelength transition possesses weaker intensity than that of the shorter wavelength (see Fig. 1 and Table 2). Its relative intensities \( \left( {I_{{{S_2}}}^{{abs}}/I_{{{S_1}}}^{{abs}}} \right) \) depend on the molecules and equal 37 (for I), 8 (for II) and 9 for (III). In order to explore the role of solute-solvent interactions, all calculations concerning absorption spectra of neutral and protonated molecules were performed in THF. It is important to note here that results of quantum-chemical calculations suggest that in THF the very weak long-wavelength transition is overlapped by the red-shifted shorter wavelength transition peak. As a consequence in polar solvents long-wavelength absorption spectrum of molecules under study possesses only one transition (see Fig. 1 and Table 2). Despite the fact that the theoretical calculations suggest only one transition in the long-wavelength absorption spectrum in THF solution the long-wavelength band consists of two transitions pictured by Gaussian profiles [7, 8], possessing maxima at 304.9 nm and 291.3 nm for I, 309.9 nm and 288.2 nm for II and 318.7 nm and 299.3 nm for III, respectively. It is important to note here that these two transitions undergo an energy-level reversal of the charge transfer and normal electronic transitions upon the environment change from vapour phase to the solvents [8, 22–24]. This phenomenon (energy-level reversal) has also been observed in the absorption spectrum of p-aminosalicylic acid and methyl p-dimethylaminobenzoate by the change of solvent from non-polar to polar one [22–24]. It must be noted that performed calculations using the Gaussian 09 code within the polarized continuum model of the solvent used do not confirm this phenomenon although the obtained absorption spectra show its existence.

As can be seen in Figs. 1 and 2, the absorption and the fluorescence spectra of molecules under study undergo very complex changes when adding HCl to the THF solution. The absorption spectra of each compound in acidic THF solutions form isobestic points, i.e., for molecule I only one is located at ν = 37,500 cm−1 (266.7 nm), for molecules II and III two points are located at ν = 42,300 cm−1 (236.4 nm) and 45,200 cm−1 (221.2 nm), and at 41,200 cm−1 (242.7 nm) and 45,000 cm−1 (222.2 nm), respectively. Noted changes of the absorption spectra suggest that the solute molecules undergo prototropic reactions in the electronic ground, S 0, and excited, S 1, states.

Fig. 2
figure 2

Normalized fluorescence spectra of I (panels I (a) and I (a’)), II (panels II (b) and II (b’)) and III (panels III (c1), III (c1’), III (c2) and III (c2’)) excited at different wavelengths in neat THF and for different concentrations of HCl. The concentration of HCl is: 0–0 M; 1–0.1 M; 2–0.2 M; 3–0.29 M; 4–0.49 M; 5–0.98 M; 6–1.97 M; 7–2.79 M; 8–3.6 M; 9–4.43 M and 10–6.065 M. Arrows indicate the fluorescence intensity changes caused by an increasing HCl concentration

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As can be seen in Fig. 1, upon increasing the HCl concentration the intensity of the C absorption band decreases. In the case of molecule I this behaviour is accompanied by the intensity increase of the B band. On the other hand, for molecules II and III new bands at ν ≅ 44,000 cm−1 (227.3 nm) (molecule II) and 42,500 cm−1 (235.3 nm) (molecule III) are being created (see Fig. 1). For these molecules a significant increase of the A band intensity, and a decrease of theirs full-width at half maximum, Δν1/2, is also noticed.

In order to make the understanding the prototropic reactions appearing in the molecules under study more clear, Table 3 assembles the long-wavelength absorption and LE and ICT fluorescence bands maxima (obtained using a linear combination of Gaussian profiles, it is not shown here) of molecules in acidic solutions of THF. Analysing the assembled spectra in Fig. 1 and data collected in Table 3, we can state that:

  1. i.

    Addition of HCl acid to the neat solution generates a H+ reaction with the non-bonding electrons of the –NMe2 or –COOMe (–OAc) substituents of the molecules under study. The H+ ion forms a chemical bond with the n-electrons of N atom of the dimethylamino substituent or with the –OAc group. In consequence, new cationic functional groups –NH+Me2 and COH+OMe are formed. The –NH+Me2 group does not conjugate with the benzene skeleton whereas the COH+OMe substituent decreases the electron accepting abilities of MB (methyl benzoate) moiety.

  2. ii.

    The positions of the absorption maxima, \( \lambda_{{SW}}^G \) and \( \lambda_{{LW}}^G \) (obtained using the Peak Fit program to decompose the long-wavelength C band into a linear combination of Gaussian functions), do not change when adding HCl to the THF solution. The noticed differences in the \( \lambda_{{SW}}^G \) and \( \lambda_{{LW}}^G \) for different HCl concentrations (see Table 3) lie in the error limit of their experimental and computer fitting program determination.

Table 3 The long-wavelength absorption and LE and ICT fluorescence bands maxima (obtained using a linear combination of Gaussian profiles) (in nm/cm−1) of molecules I, II and III in acidic solutions of THF
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To test whether the changes in the absorption spectra originate from the mono- and/or dications of the parent molecules, a series of quantum-chemical calculations were carried out. As mentioned earlier, Fig. 1 presents the absorption spectra of molecules under study as a function of the HCl concentration in THF. Additionally in the lower panels of Fig. 1, the bars under the absorption curves give the theoretical electronic state energy values and corresponding oscillator strengths of the neutral molecules in vapour-phase (taken from [8]) and in THF, and the mono- and dicationic forms of the parent molecule. Table 2 assembles the spectroscopic data (energy values of the S i = 1,… states and the associated oscillator strengths) concerning the low-lying states (up to 50,000 cm−1) for the neutral molecules and their mono- and dicationic forms. Analyzing the data assembled in Table 2 in connection with the graphical presentation in Fig. 1, it follows that:

  1. i.

    The absorption spectra of molecules under study in hydrochloric acid solutions consist of electronic transitions of three different luminescent centers, absorbing at different wavelengths (i.e., neutral molecule and its mono- and dicationic forms—their formation are represented by the reactions (1), (1a) and (1b), see Chapter 3.4). Generally, the participation of protonated forms of studied molecules in absorption spectrum is higher for high concentrations of HCl. The absorption of these components contributes more at the short-wavelength at the absorption spectrum for I, whereas in the case of II and III it contributes in the long-wavelengths part, also (see Fig. 1 and Table 2).

  2. ii.

    The intensity of the C absorption band decreases upon increasing the HCl concentration. The long-wavelength absorption band in the presence of HCl at its maximum concentration used consists of two overlapping components (mono- and dicationic forms) for II and III, whereas for I, it corresponds to the S 0 →S 1 transition of the dicationic form of the parent molecule (see Fig. 1 and Table 2).

  3. iii.

    For molecule I, the intensity of the B absorption band increases as the HCl concentration increases. This behaviour is understandable in terms of the S 0 →S 1 transition of the monocationic form of molecule I. It is important to note here that the new bands at ν ≈ 40,000 cm−1 (for I), ν ≈ 44,000 cm−1 (for II), and ν ≈ 42,500 cm−1 (for III) correspond to the S 0 →S 2 transition of the dicationic form of I and of the monocationic forms of II and III, respectively.

Steady State LE and ICT Fluorescence Spectra Analysis

As it was shown in the previous section (see also Fig. 1), in acidic solutions a part of neutral solute molecules is excluded from the fluorescence emission as a result of the prototropic reaction. Figure 3 shows exemplary fluorescence spectra of III in neat THF and its hydrochloric acid solutions at different HCl concentrations. It is seen that the intensity of both (LE and TICT) fluorescence bands decreases with increasing concentration of HCl. To obtain more information about the HCl concentration effect on the both emission modes of the fluorescence spectrum of molecules under study the analyzed fluorescence spectra are normalized in a way that each of the areas under the fluorescence curves were normalized to unity. In this case, each fluorescence spectrum corresponds to the emission from the same number of emitting solute molecules under study. In the studied concentration range of HCl, the fluorescence spectra consist of two partially overlapping bands, i.e., the short-wavelength (LE) band at about 350 nm (28,571 cm−1) and the long-wavelength (ICT) band at about 450 nm (22,222 cm−1). Figure 2 shows that in a neat THF solution the ICT fluorescence band of I, which is assigned to the emission from the non-planar (TICT) form, dominates. In the 0–1.79 M concentration range of HCl the ratio of non-planar (TICT) to planar (LE) emitting forms (for λexc = 286 nm) changes from 81 vs. 19 to 63 vs. 37%, respectively, whereas for λexc = 311 nm the ratio shows very week dependence on the HCl concentration. In the case of molecule II in the neat THF solution (see Fig. 2), the number of both emitting forms is almost equal (52 vs. 48%). The concentration change of HCl from 0 to 0.49 M (the reaction of H+ ion binding to the N or O atom of the –NMe2 or –OAc substituent, respectively) causes the intensity change of both bands giving the final ratio 11 vs. 89% (for λexc = 286 nm).

Fig. 3
figure 3

Fluorescence spectra of III in neat THF and its hydrochloric acid solutions at different HCl concentrations. Insert—Stern-Volmer plots of the fluorescence quenching of III in THF using HCl as a quencher

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In the case of molecule III, (see Fig. 2), the changes of the fluorescence spectra are more complex, i.e., the fluorescence spectrum of III in a mixed solution leads to two isosbestic points, in distinction to one isosbestic point for I and II. The presence of these points indicates a formation of well defined 1:1 or 1:2 H+ complexes in the emitting ensemble of molecules. The above observation is supported by respective changes in the location of the LE and ICT emission peaks maxima. In the present paper, the fluorescence behaviour of the molecules under study has been studied as a function of the excitation wavelength (i.e. the short-wavelength (λexc = 286 nm for all studied molecules) and the long-wavelength excitation ((λexc = 311 nm for I, II and 319 nm for III)). It is noteworthy that for higher concentration of HCl, when excitation is carried out at the long-wavelength part of the absorption spectrum mainly neutral form of molecule I is excited, whereas in the case of molecules II and III the mono- and dicationic are excited also (see Fig. 1).

Table 3 assembles the absorption (decomposed into two Gaussian profiles) and emission peak maxima of the neutral and cationic forms of the molecules under study. A scrupulous analysis of the emission spectra determined in neat and mixed solutions (see Fig. 2 and Table 3) allows to state that:

  1. i.

    When the concentration of HCl increases from 0 to the maximum concentration used and the excitation wavelength equals 286 nm the LE fluorescence band of I and II is shifted to the shorter wavelength by about 900 cm−1, whereas the ICT band is shifted to longer wavelength by about 550 cm−1 (I) and 310 cm−1 (II). Upon increasing the concentration of HCl, the intensity of the LE band increases. This behaviour is accompanied by a decrease of the ICT fluorescence band.

  2. ii.

    The fluorescence spectra of I and II recorded by excitation at λexc = 311 nm show the same general dependences as observed for the short-wavelength excitation (λexc = 286 nm).

  3. iii.

    The fluorescence spectra of III will be discussed for two regions of HCl concentrations: 0–0.98 M and 1.97–6.06 M (see Fig. 2). In the first region, the emission spectra of III excited at 286 nm constitute two distinct isosbestic points at 417 nm (23,981 cm−1) and 337 nm (29,674 cm−1). The intensities of both bands change in an opposite direction in comparison to these of molecules I and II, i.e., the intensity of the ICT band increases whereas the LE band decreases by increasing the HCl concentration. Additionally, a new band located at λmax ≅ 328 nm (30,488 cm−1) appears—for a low HCl concentration as a distinct tail and for higher HCl concentrations as a distinct band (see Fig. 2). It is important to note that for molecule III (λexc = 319 nm) dissolved in acidic solutions in the studied concentration range of HCl the LE band position does not change whereas the ICT band is hypsochromically shifted by about 400 cm−1, in comparison to the λmax (ICT) value of III in the neat THF solution. The above findings point that the fluorescence of the two spatial forms (LE and TICT) of studied molecules are quenched by the H+ ions differently. The next sections of this paper are devoted to the above phenomenon.

Static Studies of the H+ Reaction in the S 0 and S 1 States

At it was shown earlier the steady-state absorption and emission spectra of the molecules under study in acidic THF solutions undergo complex changes, i.e., the intensity of the long-wavelength absorption band decreases and simultaneously in the UV region a new band is created. Similar changes take place in their fluorescence spectra. The isobestic points noted in the absorption and fluorescence spectra suggest a formation of complexes in the S 0 and S 1 (LE) states. We suppose that between the studied molecules and the H+ ion the following reactions can take place: in the S 0 state

$$ {\hbox{MBp}} - {\hbox{NM}}{{\hbox{e}}_{{2}}} + {{\hbox{H}}^{ + }}{\xrightarrow{{K_g^{\prime }}}}{\hbox{MBp}} - {\hbox{N}}{{\hbox{H}}^{ + }}{\hbox{M}}{{\hbox{e}}_{{2}}} $$
(1)
$$ {\xrightarrow{{K_g^{{\prime \prime }}}}}{\hbox{MB}} \cdots {{\hbox{H}}^{ + }}{\hbox{p}} - {\hbox{NM}}{{\hbox{e}}_{{2}}} $$
(1a)
$$ {\xrightarrow{{K_g^{{\prime \prime \prime }}}}}{\hbox{MB}} \cdots {{\hbox{H}}^{ + }}{\hbox{p}} - {\hbox{N}}{{\hbox{H}}^{ + }}{\hbox{M}}{{\hbox{e}}_{{2}}} $$
(1b)

and in the S 1 state

$$ {\hbox{MBp}} - {\hbox{NMe}}_2^{*} + {{\hbox{H}}^{ + }}{\xrightarrow{{K_e^{\prime }}}}{\hbox{MBp}} - {\hbox{N}}{{\hbox{H}}^{ + }}{\hbox{Me}}_2^{*} \to {\hbox{MBp}} - {\hbox{N}}{{\hbox{H}}^{ + }}{\hbox{M}}{{\hbox{e}}_{{2}}} + {{\hbox{Q}}_{{1}}} $$
(2)
$$ {\text{MBp}} - {\text{NH}}^{ + } {\text{Me}}_{{\text{2}}} + h\nu ^{\prime }_{{LE}} $$
(2’)
$$ {\xrightarrow{{K_e^{{\prime \prime }}}}}\quad {\hbox{MB}} \cdots {{\hbox{H}}^{ + }}{\hbox{p}} - {\hbox{NM}}{{\hbox{e}}_{{2}}}^{ * } \to {\hbox{MB}} \cdots {{\hbox{H}}^{ + }}{\hbox{p}} - {\hbox{NM}}{{\hbox{e}}_{{2}}} + {{\hbox{Q}}_{{2}}} $$
(2a)
$$ \to {\hbox{MB}} \cdots {{\hbox{H}}^{ + }}{\hbox{p}} - {\hbox{NM}}{{\hbox{e}}_{{2}}} + h\nu_{{ICT}}^{\prime } $$
(2’a)
$$ \to {\text{MB}} \cdot \cdot \cdot {\text{H}}^{ + } {\text{p}} - {\text{NMe}}_{{\text{2}}} + h\nu ^{\prime }_{{IE}} $$
(2”a)
$$ {\xrightarrow{{K_e^{{\prime \prime \prime }}}}}\quad {\hbox{MB}} \cdots {{\hbox{H}}^{ + }}{\hbox{p}} - {\hbox{N}}{{\hbox{H}}^{ + }}{\hbox{NMe}}_2^{*} \to {\hbox{MB}} \cdots {{\hbox{H}}^{ + }}{\hbox{p}} - {\hbox{N}}{{\hbox{H}}^{ + }}{\hbox{M}}{{\hbox{e}}_{{2}}} + {{\hbox{Q}}_{{3}}} $$
(2b)
$$ \to {\hbox{MB}} \cdots {{\hbox{H}}^{ + }}{\hbox{p}} - {\hbox{N}}{{\hbox{H}}^{ + }}{\hbox{M}}{{\hbox{e}}_{{2}}} + h\nu_{{LE}}^{{\prime \prime \prime }} $$
(2’b)

where Q1, Q2 and Q3 present vibrational energies converted into heat. Naturally, the different luminescent centers (i.e., neutral molecule and its mono- and dicationic forms) absorb at different wavelengths (see Fig. 1). Also, it is important to note that light absorption can promote different luminescent centers (products of reactions (1), (1a), and (1b)).

In the first group of reactions, Eqs. 1, 1b, 2, 2’, 2b and 2’b, the electron donating substituent (–NMe2) of methyl benzoate (MB) is replaced by the –NH+Me2 group, in which the H+ ion forms an ionic bond with the non-bonding n-electrons of N. The new formed substituent loses the electron donating ability and does not conjugate with the MB skeleton. This mono-cation possesses (as shown in Fig. 1 and Table 2) an absorption spectrum appearing in a different spectral region. Its spectrum is similar to the absorption spectrum of MB and its ortho derivatives [25]. The formed complex, described by reactions (1a) and (2a), (2’a), (2”a), and (2’b) possesses a chemical bond with the –OAc group, i.e.,—CO⋅⋅⋅H+OMe, which does not influence the conjugation between the benzene skeleton and the acceptor moiety of these molecules. However, it must be added that the bond between benzene and the –CO⋅⋅⋅H+OMe group is weakened causing a blue shift of the long-wavelength Gaussian profile of the absorption band (see Fig. 1 and Table 3). This complex emits LE fluorescence quanta, the energies of which differ from those in the neat THF solution by 950 cm−1, 860 cm−1 and 2,160 cm−1 for I, II and III, respectively. The efficiency of these reactions depends on the reaction equilibrium constant for each channel. The equilibrium constants for the formation of 1:1 complex in the S 0 state and in the S 1 state can be determined from the steady-state spectroscopic measurements using the expression proposed by Benesi and Hildebrand for absorption and fluorescence data [26, 27]:

$$ \frac{1}{{A - {A_0}}} = \frac{1}{{{K_g}\left( {A - {A_0}} \right)}}\frac{1}{{\left[ {{H^{ + }}} \right]}} + \frac{1}{{A - {A_0}}}, $$
(3)
$$ \frac{1}{{I - {I_0}}} = \frac{1}{{{K_e}\left( {I - {I_0}} \right)}}\frac{1}{{\left[ {{H^{ + }}} \right]}} + \frac{1}{{I - {I_0}}} $$
(4)

where A 0, A are the absorbances and I 0, I are fluorescence intensities of the neat and acidic solution and [H+] is the ion concentration. The available absorption and emission spectra do not allow determining the reaction equilibrium constant for each separate reaction channel. Thus the K g and K e values are resultant equilibrium constants of reactions in the ground, S 0, and excited, S 1, states, respectively.

Figure 4a and b illustrated the double reversal dependence of 1/(A−A 0 ) vs. [H+]−1. For each molecule, A 0 and A are the areas under the absorption profile of the long-wavelength band \( \int {A\left( {\widetilde{\nu }} \right)d\widetilde{\nu }} \) of the neat and acidic solutions of THF. As it was shown in Fig. 4 the linearity of the plots (Eq. 3) is satisfactory. The correlation coefficient being larger than 0.94 indicates a good linearity. Using the Benesi-Hildebrand equation (Eq. 3) the K g data are determined and their values are assembled in Table 4.

Fig. 4
figure 4

The Benesi-Hildebrand dependence (1/(A−A0) vs. [H+]−1) for I, II and III in acidic solutions of THF. c1, c2—low and high concentration range, respectively

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Table 4 The equilibrium constants of the H+ complex formation in the ground, K g , and excited state, K e , determined using Benesi-Hildebrand procedure
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Figure 5 illustrated exemplary straight lines resulting from the linear regression of the LE and ICT fluorescence data fitted to Eq. 4. The correlation coefficient being larger than 0.92 indicates good linearity. Table 4 collects the \( K_e^{\rm{LE}} \) and \( K_e^{\rm{ICT}} \) data determined using the Benesi-Hildebrand procedure. As it was mentioned above the \( K_e^{\rm{LE}} \) and \( K_e^{\rm{ICT}} \) values are resultant equilibrium constants of reactions (2’), (2”a), (2’b) and (2’a) initiated by exciting the fluorophore at \( \lambda_{{SW}}^G \) and \( \lambda_{{LW}}^G \), respectively. Analysing the K g and the \( K_e^{\rm{LE}} \), \( K_e^{\rm{ICT}} \) data, it follows that the equilibrium constants of protopropic reactions in the S 0 and S 1 states differ for each molecule under study. The obtained data fulfil the inequalities \( {K_g} > K_e^{\rm{LE}} \) and \( {K_g} > K_e^{\rm{ICT}} \). Also, their values fulfil the following dependencies K g (I) > K g (II) > K g (III) and K e (I) > K e (II) > K e (III), what points that the protopropic reaction of molecules under study is most efficient for molecule I. From Eqs. 2, 2a, and 2b, it follows that these reactions are exothermic, the formed complexes are radiationless and supports fluorescence quenching phenomenon, whereas their emission (see Eqs. 2’, 2”a, 2’b and 2’a) supplies quanta at wavelengths that are shorter than those observed for the LE fluorescence band (if the fluorophore is excited at λ SW (see Table 3)) or quanta at wavelengths that are longer than those observed for the ICT fluorescence band of the neat solutions (see Figs. 1 and 2).

Fig. 5
figure 5

The Benesi-Hildebrand dependence (\( {1}/\left( {I_{{LE}}^{{{H^{ + }}}} - {I_{{LE}}}} \right),\;{1}/\left( {I_{{ICT}}^{{{H^{ + }}}} - {I_{{ICT}}}} \right) \) vs. [H+]−1) for I—panel (a) and III—panel (b) in acidic solutions of THF

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Analysing the K g and K e data collected in Table 4 in connection with the spectra pictured in Figs. 1, 2 and 3, we can state that:

  1. i.

    The molecules I, II, and III show different ability to react with the H+ ion. The affinities to H+ also differ between the S 0, S 1 (LE), and S 1 (FC) states.

  2. ii.

    The changes in the absorbances as well as in the LE and ICT fluorecscence band intensities are well described by the respective Benesi-Hildebrand equations.

  3. iii.

    The \( K_e^{\rm{LE}} \) and \( K_e^{\rm{ICT}} \) values depend on the excitation wavelength.

Analysing the LE and ICT fluorescence intensity changes, it is evident (see Figs. 1, 2 and 3), that the prototropic reactions possess different yields for each molecule. As it was mentioned earlier, Fig. 2 shows the variation of the normalized emission spectra (fluorescence spectrum corresponds to the emission of the same number of emitting molecules) of molecules under study. From Fig. 2, it is evident that in the case of molecules I and II, upon increasing the H+ concentration the intensity of the ICT fluorescence band decreases. This behaviour is accompanied by an increase of the LE emission band. Above changes show an apposite direction for molecule III. It is important to note that these fluorescence intensity changes are small in comparison to those noted for I and II.

In is interesting to note that the intensity changes of fluorescence spectra described by the ratio I ICT /I LE or by the coefficient of the ICT fluorescence band contribution \( r = {I_{{ICT}}}/\left( {{I_{{ICT}}} + {I_{{LE}}}} \right) \) show a linear dependence on the H+ concentration (see Fig. 6). The correlation coefficient being larger than 0.94 indicates a good linearity. As can be seen in Fig. 6 the fluorescence intensity ratio I LE /I ICT as well as r data are very sensitive to H+ concentration for I and II. These findings predestine molecules I and II to be used as acid fluorescence probes in a region of 0–2.5 M. For III these intensity changes are small, are in the error limit of their determination.

Fig. 6
figure 6

Fluorescence intensity ratio I ICT /I LE vs. [H+] of I, II and III (panel a and b) and the ICT fluorescence band contribution coefficient, \( r = {I_{{ICT}}}/\left( {{I_{{LE}}} + {I_{{ICT}}}} \right) \) vs. [H+] (panel c) of I and II determined for two excitation wavelengths

Full size image

Fluorescence Quenching Caused by Photoinduced Reaction of H+

Figure 3 shows the fluorescence spectra of III obtained in THF, in the presence of the HCl at its various concentrations as an example. The spectra have been obtained at fixed solute concentration (c = 10−4 M) and for the HCl, which concentration changes from 0 to 1.97 M. As can be seen in Fig. 3, the increase of HCl concentration results in a strong decrease of the short- (LE) and long-wavelength (TICT) fluorescence bands. It is important to note here that the intensity of both fluorescence modes decreases with increasing concentration of HCl for the all studied molecules (dependence for I and II is not shown here).

It is well known that for ideal bimolecular quenching process, the Stern-Volmer equations can be applied [28, 29]:

$$ I_F^0/{I_F} = 1 + {k_q}\tau_F^0\left[ Q \right] = 1 + {K_{{SV}}}\left[ Q \right], $$
(5)

and

$$ \tau_F^0/{\tau_F} = 1 + {k_q}\tau_F^0[Q] = 1 + {K_{{SV}}}\left[ Q \right], $$
(6)

where k q is the bimolecular quenching constant, \( I_F^0 \) and \( \tau_F^0 \) are the fluorescence intensity and lifetime in the absence of quencher. I F and τ F are the equivalent parameters in presence of the quencher at a concentration [Q]. If only dynamic quenching is operative without any static quenching, the K SV values obtained from Eqs. 2 and 3 should be identical [28, 29].

The insert in Fig. 3 shows straight line from the plot of the I 0 /I versus HCl concentration only at low quencher concentrations. Similar behavior is observed for I and II. Taking this into account, it can be concluded that the fluorescence quenching of the molecule under study in acidic THF solutions is very complex and a simple bimolecular process of the Stern-Volmer type is not sufficient to describe the noted changes in the fluorescence intensities.

Concluding the above findings, it must be stated that in a hydrochloric acid solution the studied molecules exist as a mixture of neutral, mono-, and dicationic forms. The reaction of H+ with the neutral excited molecules under study creates emissive and/or non-emissive complexes. They arise in a reaction of H+ with the –OAc or/and –NMe2 substituents creating mono- or/and dications of the parent molecule. Naturally, the neutral, mono-, and dicationic forms of the molecules under study possess own characteristic absorption and fluorescence spectra. Their characteristics need further detailed study using time-resolved measurements.