Introduction

Fluorescent dyes attract attention in the last decades owing to their different optical microscopic applications such as OLEDs [1], anti-forgery markers. [2], diverse materials as sunscreens [3], dye-injection solar cells [4], phosphorescent lighting strips [5], selective probes [6,7,8], bio-labels, [9], fluorescent markers, [10] and solid-state pH indicators [11].

The solvents have different effects on geometrical structure of solute molecules and their spectroscopic behavior have been previously monitored [12,13,14,15,16,17,18,19,20,21,22,23]. In this respect, the Stokes shift, which is the difference (in wavelength or frequency units) between positions of the band maxima of the absorption and emission spectra of the same electronic transition, have also studied as a function of; 1) the dielectric effects, which is, from a molecular point of view, the dipolar interaction between the solute and the solvent molecules; 2) the dispersive interaction because of van der Waals forces which exist in solvent-solute system; 3) the specific interaction between the solvent and the solute molecules, e.g., hydrogen bonding, electron transfer,.. etc.; 4) the aggregation effect between solute molecules; this effect changes with changing solute concentrations and may be neglected at low solute concentration; 5) the electrochemical effects, which is the changes in the degree of dissociation of a solute in different solvents; this effect is studied by varying the pH of the medium; 6) the intermolecular resonance effects which is important in emission spectra study.

On the other hand, upon photo-excitation [18, 24,25,26], intermolecular hydrogen bonding significantly strengthened in the electronic excited state resulting in an important role in photochemistry, such as intermolecular charge transfer [18], fluorescence quenching [24], excited-state proton transfer [25] and tuning effects on photochemistry [26].

In continuation of our recent interest in designing and preparing new dyes [27,28,29], we report herein on the synthesis of new fluorescent probe and studying the effect of different solvents on its optical and photophysical properties.

Experimental

General

Melting points were determined in open glass capillaries with a Gallenkamp apparatus and were not corrected. The infrared spectra were recorded in potassium bromide disks on a PyeUnicam SP3–300 and Shimadzu FTIR 8101 PC infrared spectrophotometer. The 1H and 13C NMR spectra were determined on a Varian Mercury VX 300 NMR spectrometer using TMS as an internal standard and DMSO-d6 as a solvent. Mass spectra were measured on a GCMS-QP1000 EX spectrometer at 70 eV. Elemental analyses were carried out at the Microanalytical Center of Cairo University, Giza, Egypt. 7-Hydroxy-4-methylcoumarin (1) and 9,10-bis(bromomethyl)anthracene (2) were purchased from Sigma-Aldrich.

Synthesis of Novel 9,10-bis(coumarinyl) Anthracene Derivative

Synthesis of the K-Salt of (1)

A solution of each of compounds 1 (10 mmol) and KOH (1.14 g, 20 mmol) in ethanol (10 ml) was stirred at room temperature for 10 min. The solvent was removed in vacuo and the remaining solvent was triturated with dry ether, collected and dried. It was then used in the next steps without further purification.

Synthesis of 7,7′-((anthracene-9,10-diylbis(methylene))bis-(oxy))bis(4-methyl-2H-chromen-2-one) (BisCA) (3)

A solution of each of the potassium salt of 1 (20 mmol) and 9,10-bis(bromomethyl)anthracene (2) (10 mmol) in DMF (20 ml) was heated under reflux for 5 min during which time KBr was precipitated. The solvent was then removed in vacuo and the remaining material was washed with water (50 ml) and crystallized from DMF to give compound 3 as orange crystals.; yield: 81%; mp 242 °C; IR: υ max 1714 (C=O) cm−1, 1H NMR (DMSO) δ 2.39 (s, 6H, CH3), 5.41 (s, 4H, OCH2), 6.20–7.92 (m, 10H, ArHs, Coumarin H) ppm. MS: m/z 554 (M+); Anal. for C36H26O6 (554.17), Calcd: C, 77.97; H, 4.73. Found: C, 77.70; H, 5.10.

Specroscopic Analysis

(1 × 10–5 M) Solutions of the new probe 910-bis(coumarinyl) anthracene 3 and its precursors coumarin 1 and anthracene 2 were prepared by mixing 100 μL 1 × 10−3 M in DMF with other solvents to get concentration of 1 × 10−5 M of probes. All solution samples were contained in 1 cm optical-path quartz cells. Absorption and emission spectra were measured using Camspec M501 uv-vis spectrophotometer and PF-6300 spectrofluorometer, respectively.

Polarity Parameters of Solvents and Onsager Cavity Radius Calclations

Solvent polarities play a significant effect on ground and excited sates of fluorescent molecules. Different direct methods for the assessment of the excited state dipole moment such as fluorescence polarization, stark splitting of rotational levels and electric dichroism are known and accurate, but the most common method still based on the solvatochromism analysis of the absorption and fluorescence frequency maxima using either microscopic bulk polarity (\( {E}_T^N \)) or functionized polarities f (n,ε) [30, 31]. In the latter method suggested by Bakhshiev, the Stokes shift between the absorption and fluorescence frequencies maxima is pointed to the dipole moment change on excitation as presented in the following equations [30, 31].

$$ {\upsilon}_a-{\upsilon}_f={m}_1f\left(\varepsilon, n\right)+ const. $$
(1)
$$ {\upsilon}_a+{\upsilon}_f=-{m}_2\left[f\left(\varepsilon, n\right)+2g(n)\right]+ const. $$
(2)

Where; υaand υf are the peak absorption and fluorescence frequencies.

However;

$$ f\left(\varepsilon, n\right)=\frac{2{n}^2+1}{n^2+2}\left[\frac{\varepsilon -1}{\varepsilon +2}-\frac{n^2+1}{n^2+2}\right] $$
(3)

and

$$ g(n)=\frac{3}{2}\left[\frac{n^4-1}{{\left({n}^2+2\right)}^2}\right] $$
(4)

are the solvent polarity parameters as functions in dielectric permittivity (ε) and refractive index (n) according to Bakhshiev method which express the contributions of the orientation and dispersive-induction-polarization interactions, respectively.

Where;

$$ {m}_1=\frac{2{\left({\mu}_e-{\mu}_g\right)}^2}{hc{a}^3} $$
(5)

is the slope of Eq. (1). And

$$ {m}_2=\frac{2{\left({\mu}_e+{\mu}_g\right)}^2}{hc{a}^3} $$
(6)

is the slope of Eq. (2). Where h is Plank’s constant and c, the velocity of light in vacuum whereas μg and μe are the ground and excited state dipole moments, respectively.

Also, the dipole moment of ground state (μg) and excited state (μe) parameters can be determined from Eqs. (7) and (8) assuming that the symmetry of the probe molecule remains unchanged through electronic transition, and the ground and excited state dipole moments are parallel [32],

$$ {\mu}_g=\frac{\mid {m}_2-{m}_1\mid }{2}{\left[\frac{hc{a}^3}{2{m}_1}\right]}^{\frac{1}{2}} $$
(7)
$$ {\mu}_e=\frac{\mid {m}_2+{m}_1\mid }{2}{\left[\frac{hc{a}^3}{2{m}_1}\right]}^{\frac{1}{2}} $$
(8)

Since parameters m1 and m2 are linear functions of the solvent polarity parameters f(ε, n) and f(ε, n) + 2 g(n) and can be determined from the slopes of straight lines, the Onsager cavity radius (α) can also easily be assessed according to the theory [33,34,35] that assumed the molecule to be sphere in shape and its Onsager radius can be evaluated using the relation:

$$ \alpha ={\left(\raisebox{1ex}{$3M$}\!\left/ \!\raisebox{-1ex}{$4\pi \delta N$}\right.\right)}^{\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$3$}\right.} $$
(9)

M: is the molecular mass of molecule, δ is the density and N is the Avogadro’s number.

On the other hand, Reichardt [36] reported an empirical polarity scale which gave better results of solvatochromic shift of dipolar molecules that depends mainly on microscopic solvent polarity \( {E}_T^N \) rather than on traditionally bulk solvent polarity functions. Accordingly, the excited state dipole moment is determined using Eq. (10) [37].

$$ {\nu}_a-{\nu}_b=11307.6\left[{\left(\frac{\varDelta \mu}{\varDelta {\mu}_B}\right)}^2{\left(\frac{a_B}{a}\right)}^3\right]{E}_T^N+ const. $$
(10)

where ΔμB= 9D and aB= 6.2 Å represents the change in dipole moment on excitation and Onsager radius, respectively, for betaine dye as reference probe, while (∆μ) and (α) are the corresponding quantities for the target probe.

Photo-Physical Properties

Studying of photo-physical properties such as fluorescence quantum yields φf (that was obtained by applying a comparative method [38]), fluorescence lifetime, radiative decay and non-radiative decay in different environmental solvent have been the subject of various investigations because of the wealth of information on photochemistry of target probe for various applications.

For determination of fluorescence quantum yield (ϕf) using Rhodamine 6G (R6G) in MeOH solution of a quantum yield (ϕf) of 0.96 [39] as a reference, the emission spectra were monitored at different wavelengths having the same absorbance intensities of R6G and target probe. To determine the quantum yield of a compound relative to a standard material, a very dilution of dye was used to avoid self-reabsorption phenomena, the following relationship (11) was applied: [40].

$$ {\phi}_u={\phi}_s\times \frac{I_u}{I_s}\times \frac{A_s}{A_u}\times \frac{n_u^2}{n_s^2}. $$
(11)

Where ϕu, Iu and Au are the fluorescence quantum yields, the areas under emission curve and the absorbance, respectively, of the unknown probe. While, ϕs, Is, Asare those of the standard dye. ns and nu are the refractive indices of solvents used.

Other important sequent photo-physical property is the lifetime of excited state which provides significant information about intermolecular kinetics interaction including dimmer formation [41] and different types of energy transfer. Lifetime fluorescence value (τ) is a function of the quantum yield, emission and absorption spectra by means of Eq. (12) [42] as follows:

$$ \frac{1}{\tau }=2.88\times {10}^{-9}{n}^2{\phi}_f^{-1}\frac{\int F(v) dv}{\int {v}^{-3}F(v) dv}\int \frac{\varepsilon (v)}{v} dv $$
(12)

ν: wavenumber (cm−1), F(ν): intensity of the emitted fluorescence, n: refractive index. Derived from the emission and absorption spectra and the wavelengths (λ) which were converted into wavenumbers (cm−1), the integral values were assessed and consequently the calculated lifetime of fluorescence (τ) was determined.

Oscillator strength Values are calculated by Eq. (13) [43]:

(13)

Also, the transition dipole moment (μ12) from ground to excited state was calculated in different media by the Eq. (14); where f is related to the transition moment and the Einstein coefficient by the following expressions:

$$ {\upmu}_{12}^2=2.36\times {10}^{-51}\times \mathrm{f}\times \uplambda $$
(14)

A Debye (D) is the traditional non-SI unit of dipole moment. The conversion between D and SI units is 1Dp = 2.36 × 10−30 ( coulomb. meter).

Since an excited dye molecule can return to the ground state by a number of pathways, either intramolecular or intermolecular interactions, [44] the fluorescence quantum yield is directly related to the radiative (kr) and nonradiative (knr) rate constants of deactivation by the relationship (15) [44].

$$ {\phi}_f=\frac{k_r}{k_r+{k}_{nr}} $$
(15)

This parameter has major importance not only since it is a physical characteristic of a molecule in specific conditions but it is also ultimately involved in the calculation of quenching-rate constants, energy transfer, lasing ability and radiative or nonradiative rate constants, from which the whole photophysical behavior can be deduced.

However, the radiative (fluorescence) decay rate constant (kr) of a probe can be assessed from Eq. (16) based on Strickler-Berg equation, which depends on Einstein’s spontaneous emission rate and Planck’s black body radiation law [45].

$$ {k}_r=\frac{1}{\tau_0}=2.88\times {10}^{-9}{n}^2\frac{\int F\left(\tilde{\nu}\right) d\nu}{\int F\left(\tilde{\nu}\right){\tilde{\nu}}^{-3} d\nu}\int \frac{\varepsilon \left(\tilde{\nu}\right)}{\tilde{\nu}}d\tilde{\nu}, $$
(16)

Where \( F\left(\tilde{\nu}\right) \), \( \tilde{\nu} \), \( \varepsilon \left(\tilde{\nu}\right) \) are the fluorescence intensity, wavenumber and molar extinction coefficient at a certain wavenumber (\( \tilde{v} \)), respectively, while n is the refractive index of the medium.

Results and Discussion

Synthesis and Characterization of BisCA

The synthetic utility of 7-hydroxy-4-methylcoumarin (1) as building block for novel bis(coumarin) 3 was investigated as outlined in Scheme 1. Thus, the reaction of the potassium salt (obtained upon treatment of 1 with ethanolic KOH), with 9,10-bis(bromomethyl)anthracene (2) in boiling DMF afforded 7,7′-((anthracene-9,10-diylbis(methylene))bis(oxy))bis(4-methyl-2H-chromen-2-one) (3) (BisCA) in 81% yield.

Scheme 1
scheme 1

Synthesis of compound 3 (BisCA)

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Compound 3 (BisCA) was characterized by elemental analyses, as well as its spectral data which agree with the proposed structure. Thus, the IR spectrum of 3 exhibits an absorption band at υ 1714 cm−1 characteristic for the carbonyl group. Moreover, the absence of an absorption band corresponding to OH stretching frequency of the parent coumarin 1 in the IR spectrum of the alkylated product clearly confirmed the formation of BisCA 3. Moreover, the 1H NMR spectra in Fig. 1 of BisCA showed a singlet signal at δ 2.39 integrated for six protons attributed to the methyl protons together with a singlet signal at δ 5.42 integrated for four protons characteristic for the OCH2 protons. In addition, 1H NMR spectra displays a singlet signal at δ 6.19 integrated for two protons attributed to the coumarin protons. All other protons were seen at the expected chemical shifts and integral values. Mass analysis of compound 3 showed an intense molecular ion peak at m/z 554, in agreement with their respective molecular formulae.

Fig. 1
figure 1

1H NMR of BisCA

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Optical Properties of Bis(coumarinyl) Anthracene (BisCA)

As shown in Fig. 2 which represents the absorption spectra of three dyes in DMF as aprotic solvent. The absorption spectra of BisCA is presented by broad band ranged from 280 nm to 415 nm of two separated characterized profiles. BisCA absorption spectra has a combination profile spectra of coumarin dye, which is broad band spectrum at maximum wavelength 320 nm, and anthracene dye which characterized by three fine spectral peaks at 355, 374 and 396 nm. The large bulk structure of BisCA molecule and intramolecular charge transfer (ICT) in coumarin part may result in decreasing of ground state energy of anthracene in BisCA compared with parent 9,10-dibromro anthracene probe (10 nm blue shift was noticed). There is no shifting in coumarin part of BisCA compared with parent coumarin, which may be attributed to the charge transfer resonance in coumarin part with lone-pair of etheric oxygen atom as shown in Scheme 2.

Fig. 2
figure 2

Absorption spectra of 1 × 10−5 M three dyes in DMF

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Scheme 2
scheme 2

Intramolecular charge transfer (ICT) in coumarin part of compound 3 (BisCA)

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The effect of protic solvent such as ethanol on electronic absorption spectra of BisCA is presented in Fig. 3. This spectra show about 5 nm red-shift of absorption spectra of BisCA compared with the corresponding one in aprotic solvent (DMF) in both coumarin and anthracene parts. The shifting may be attributed to intermolecular hydrogen bond donar (HBD) character of ethanol. The maximum peak of absorption spectrum of coumarin dye exhibits a higher wavelength absorption compared with its respective peak in BisCA probe due to dual inter-and intramolecular H-bond interactions of coumarin dye with itself as well as with protic environment. On the other hand, only intermolecular H-bond interaction occur in BisCA molecule.

Fig. 3
figure 3

Absorption spectra of 1 × 10−5 M three dyes in ethanol

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Normalized emission spectra of three dyes in aprotic solvent (i.e. DMF as shown in Fig. 4 shows a red-shift in fine band peaks of anthracene part of BisCA probe compared with its precursor anthracene dye. This shifting in fluorescence spectra indicates the higher stability of excited state of BisCA in aprotic solvent. This lower electronic energy of excited state of BisCA compared with that of the parent anthracene probe (9,10-dibromomethylanthracene) may be attributed to lower electronegativity of bromine atom compared with electron-withdrawing effect of etheric oxygen atom in BisCA dye.

Fig. 4
figure 4

Normalized emission spectra of 1 × 10−5 M three dyes in DMF

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On the other hand, protic solvent effect in Fig. 5 shows different fluorescence behavior of all dyes. Firstly, coumarin dye shows extra two new peaks in longer wavelengths (at 442 & 483 nm) which attributed to intermolecular H-bond interaction with solvent molecules. Anthracene part in BisCA dye has 8-nm blue shift compared with that of parent anthracene dye. This may be attributed to the increase of the excited state energy as a result of increasing the steric geometry of BisCA molecule by H-bond formation of its coumarin moiety with the protic ethanol solvent. The effect of prtoic and aprotic solvents on the spectral data, Stokes shift, photo physical parameters of three dyes were assessed under the same conditions and tabulated in Table 1.

Fig. 5
figure 5

Normalized emission spectra of 1 × 10−5 M three dyes in ethanol

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Table 1 Optical and photo—physical properties of three dyes in different solvents
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Solvent Polarity and H-Bonding Effect on Bis(coumarinyl) Anthracene BisCA

Solvent polarity simply defined as the overall solvation capability of solvents, which in turn depends on the action of all possible, nonspecific and specific, intermolecular interactions between solute molecules and solvent molecules, excluding, however, those interactions leading to definite chemical alterations of the solute molecules (such as oxidation, reduction, protonation, chemical complex formation etc.) [46, 47].

As shown in Fig. 6, the absorption spectra of BisCA probe has different trend in its two chromophore parts. However, the highest absorption intensity of the coumarin chromophore part was recorded in ethanol, while in anthracene chromophore part, the highest absorption intensity was recorded in DMF with slight blue shift. This different behavior may be attributed to the higher polarity of coumarin compared with anthracene.

Fig. 6
figure 6

Absorption of BisCA in different solvents

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As we noticed in Fig. 7, the emission spectra of BisCA probe which excited by wavelength of 323 nm in different solvents has the highest intensity in protic solvent (ethanol). This may be explained in terms of the intramolecular hydrogen bond with ethanol. On the other hand, blue shift in anthracene part may referred to intra charge transfer (ICT) in both coumarin sides of BisCA through etheric oxygen atom leading to increase of the electronic energy of excited state in anthracene part. In toluene, the slight red- shift in the maximum wavelength position (at 410 nm and 430 nm) may attributed to its low solvent polarity. However, decreasing in intersystem crossing efficiency is due to an increase in the energy of n → π* triplet state resulting in an increase of radiative fluorescence efficiency. In DMF a decrease of fluorescence efficiency with small red shift of the anthracene part of BisCA was assessed.

Fig. 7
figure 7

Emission of BisCA in different solvents

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The effect of solvent polarity by either microscopic or functionally methods using six solvents (ethanol & acetic acid as protic polar, DMF & acetone as aprotic polar and toluene & xylene as non-polar solvents) was also investigated. Thus, the solvents effect on Stokes shifts was shown in Figs. 8a, b and 9. Also, their effects on some photo-physical parameters of probe were assessed and compiled in Tables 1 and 2. It is noticed that, Stokes shifts have the lowest values in case of protic polar solvents, while it has the highest value in non-polar solvents.

Fig. 8
figure 8

Stockes shifts of BisCA dye versus different solvent polarity function (a) f(ε,n) and (b) f(ε,n) + 2 g(n)

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Fig. 9
figure 9

Stockes shifts of BisCA dye versus the microscopic solvent polarity of different solvents (\( {E}_T^N \))

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Table 2 Dipole moments, slope (m) of BisCA dye by Bakhshiev and Reichardt methods
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On the other hand, BisCA dye has the highest fluorescence quantum yields (i.e. φf = 0.42 and 0.48 in ethanol and acetic acid, respectively) in protic polar solvents. While the lowest fluorescence quantum yield (i.e. φf = 0.27 in acetone) was observed in non-polar solvent.

Finally, according to Bakhshiev and Reichardt equations [30, 31, 36] the dipole moment of BisCA was assessed and tabulated in Table 2.

Conclusion

Molar absorptivity, oscillator strength, transition dipole moment, excited-state lifetime, fluorescence quantum yield, absorption as well as emission cross sections, rate of radiative and non-radiative decay for our new target probe BisCA and its fluorescent starting materials have been deduced in various organic solvents. The characterized Onsager cavity radiuses as well as the dipole moments of ground and excited state according to Bakhshiev and Reichardt methods were estimated.

Intermolecular H-bonding interaction between dye molecules and solvent molecules was found to have distinctive impact on optical and photophysical properties of the focused probe. This may occur as a result of different factors related to geometrical chemical structure of probe, strengthening H-bond formation and physical properties of surrounding media.