A new type of triphenylamine based coumarin–rhodamine hybrid compound: synthesis, photophysical properties, viscosity sensitivity and energy transfer

Abstract

A series of novel core modified triphenylamine coumarin–rhodamine systems (compounds MCMR, MCDR and DCMR) was designed and synthesized by incorporating a coumarin moiety on one and a rhodamine moiety on the other phenyl ring of the triphenylamine molecular skeleton. The resulting dyes possessed the individual advantages of coumarin and rhodamine derivatives to show high fluorescence in their spirocyclic and open form respectively. Positive solvatochromism observed in the spirocyclic form is well supported by a linear (Lippert–Mataga and Mac-Rae) and a multi-linear regression analysis (Kamlet–Taft and Catalan parameters) suggesting that for all three compounds solvent polarizability and solvent dipolarity are the main factors responsible for their slightly red shifted absorption spectra, and highly red shifted emission spectra. The viscosity studies are indicative of the induced fluorescence enhancement of open form rhodamine derivatives over the viscosity insensitive rigid lactone ring of spirocyclic form of rhodamine derivatives. In the present studies compound MCMR in its open form when excited at 440 nm shows through space energy transfer from coumarin donor to rhodamine acceptor. This design strategy is straightforward and adaptable to various deep red dyes by simply modifying the core of different fluorophores, thereby generating a through space energy transfer systems.

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Introduction

Many hi-tech applications such as cell imaging are highly dependent on the photophysical and photochemical properties of fluorescent dyes. Among the commonly used fluorescent dyes, rhodamine, coumarin, BODIPY and cyanine derivatives are highly favoured in bioimaging studies because of their excellent photophysical properties, such as high molar extinction coefficients, excellent fluorescence quantum yields, and great photo stability. Rhodamine derivatives are widely used as fluorescence standards,^1,2 laser dyes,^3,4 molecular switches,⁵ in fluorescent labelling of biomolecules,⁶ and bio imaging in living cells.^7,8 There can be equilibrium between spirocyclic and open-ring forms in these derivatives. The spirocyclic forms of these dyes are colourless and non-fluorescent due to the small π-conjugated systems, while the open-ring forms show intensive spectroscopic signals in the absorption and fluorescence spectra by generating the large π-conjugated systems. Thus, they are looked up on as building blocks and as efficient fluorescence turn-on sensors for heavy metal ions such as lead,⁹ mercury^10–13 copper,^14,15 iron,^16–19 etc.

Despite their huge array of function they suffer from limitation of very small Stokes shifts (typically less than 25 nm), which can lead to self-quenching and fluorescence detection errors arising from background auto fluorescence. Especially rhodamine dyes are known to absorb or emit below 600 nm which again limits their use in biological applications. The emission spectra of some of these fluorescent dye based sensors are also found to be susceptible to be perturbed by various environmental factors, such as pH, temperature, solvent polarity, viscosity and so on.²⁰ To overcome these limitations, new design strategies for the development of deep red fluorescent dye with great photo stability and large Stokes shift are targeted through (i) extension of the conjugation system (ii) introduction of donor and acceptor groups into the chromophore, and (iii) by core modification of the fluorophores molecular skeleton. Recently many energy transfer dyes based on fluorescence resonance energy transfer (FRET)^21–24 and through bond energy transfer (TBET)^25–29 have been constructed to get the large pseudo-Stokes shifts.^30–34 The FRET process occurs only when there is a substantial overlap between the donor emission spectrum and the acceptor absorption spectrum. However, the complexity of synthesis, chemical instability and difficulties to incorporate all the desirable photophysical properties in a single organic dye has necessitated the concept of hybridization of two different dyes with distinctive photophysical properties into a new hybrid dye with improved photophysical properties. For example Excited State Intramolecular Proton Transfer (ESIPT)-BODIPY,^35–37 ESIPT-coumarin,^38,39 BODIPY–coumarin,^40–42 BODIPY–rhodamine^43,44 and coumarin–rhodamine⁴⁵ dyes were synthesized successfully and found to show superior photophysical properties to their precursors.

In this context we constructed three novel triphenylamine based coumarin–rhodamine hybrid dyes namely mono-coumarin mono-rhodamine (MCMR), mono-coumarin di-rhodamine (MCDR) and di-coumarin mono-rhodamine (DCMR). Further the photophysical, viscosity sensitivity and energy transfer properties in both spirocyclic as well as in open form are studied.

Overall the newly synthesized hybrid dyes showed red shifted absorption and emission spectra with comparative high Stokes shits. Unlike other rhodamine derivatives, in the present studies, presence of coumarin moiety on the other side of triphenylamine core resulted in high fluorescence for their spirocyclic form. All three compounds in their open form, show spectral overlap between the energy donor coumarin and energy acceptor rhodamine. The emission peak in the deep red region at 640 nm in chloroform, toluene and hexane solvent is assigned as energy transfer peaks. The above findings and synthesis of novel triphenylamine based rhodamine–coumarin hybrid dyes can serve as a platform for the construction of viscosity induced emission enhancement molecular rotors for their biological applications (Fig. 1 and 2).

Fig. 1 Structure of the synthesized compounds MCMR, MCDR and DCMR for their spirocyclic and open form.

Fig. 2 Photographs of dyes MCMR, MCDR and DCMR for their spirocyclic and open form in chloroform solvent.

Result and discussion

Synthetic strategy

All three derivatives were synthesized starting from meta-substituted triphenylamine di-hydroxyl derivative (3,3′-(phenylazanediyl)diphenol) (compound-1) and tri-hydroxyl derivative (3,3′,3′′-nitrilotriphenol) (compound 4), obtained by the demethylation of respective methoxy derivatives as per the reported literature.^46,47 The compounds 1 and 4 were converted into their mono formylated derivatives compound 2 and compound 5 respectively by using only one equivalent of Vilsmeier hack adduct and controlled reaction temperature (i.e. 50–60 °C). Similar strategy was tried to synthesize compound 9 the di-formylated derivative, starting from compound 4 However, even after using excess POCl₃ and higher reaction temperature trace amount of pure product was isolated. Hence we performed Vilsmeier Haack formylation reaction directly on the triphenylamine-trimethoxy derivatives of (compound 7) and the tri-methoxy bis-aldehyde derivative (compound 8) was isolated in good yield. The de-methylation reaction of compound 8 was carried out using BBr₃ as de-methylating reagent, which was converted to its bis-coumarin derivative by reacting with intermediate A. The respective hydroxyl coumarin derivatives were converted to their rhodamine derivatives (MCMR and MCDR) by treating them with intermediate B in TFA. In the case of DCMR, we used methane sulphonic acid as reaction media rather than TFA for the improved yield (Schemes 1 to 3).

Scheme 1 Synthetic route for coumarin–rhodamine hybrid MCMR (spirocyclic and open form).

Scheme 2 Synthetic route for coumarin–rhodamine hybrid MCDR (spirocyclic and open form).

Scheme 3 Synthetic route for coumarin–rhodamine hybrid DCMR (spirocyclic and open form).

Materials and methods

All the required chemicals and solvents were purchased from S D Fine Chemical Limited, Mumbai, India and used without further purification. ¹H (500 MHz) and ¹³C (125 MHz) NMR spectra were recorded on a Varian Cary Eclipse Australia using Tetramethylsilane (TMS) as an internal standard. Melting points were measured on standard melting point apparatus from Sunder Industrial Product Mumbai and are uncorrected. The absorption and emission spectra of the compounds were recorded using freshly prepared solutions at the concentration of 1 × 10⁻⁶ mol L⁻¹ on a PerkinElmer UV-visible spectrophotometer Lambda 25 and Varian Cary Eclipse fluorescence spectrophotometer respectively. Fluorescence quantum yields were determined by using coumarin 6 (Φ = 0.94 in chloroform) as reference standard for spirocyclic form while Rhodamine 101 (Φ = 0.94 in ethanol) as a reference standard for open form rhodamine derivatives using the comparative method.

Photophysical properties

Due to the presence of coumarin moiety on the other side of the triphenylamine phenyl ring, these rhodamine derivatives are highly fluorescent in their spirocyclic form. Hence to get the insight of this fascinating result, detailed photophysical properties of the spirocyclic and open form in all solvents were required for comparison. Normalized absorption and emission spectra of all three dyes in their spirocyclic form in chloroform solvent are represented in Fig. 3. The spirocyclic forms of both compound DCMR show distinct red shifted absorption with λ_max at 481 nm and compound MCMR shows red shift in its emission spectra at 529 nm. In their open form, compound MCDR shows red shifted absorption as well as emission λ_max as represented in Fig. 4. Detailed photophysical parameters of all three dyes are presented in Table 1. In their open form both molar extinction coefficients as well as full width half maximum (fwhm) values are higher as compared to the respective spirocyclic form. The oscillator strength and transition dipole moment values are calculated by utilizing the well-known expression.⁴⁸ High values of oscillator strength (f) and transition dipole moments (μ_ge) for all three compounds are obtained for both the open and spirocyclic form. Further with the increasing solvent polarity both the values of oscillator strength (f) as well as transition dipole moment (μ_ge) increase in both spirocyclic as well as open form of these compounds (Tables S1–S6†). Compound MCMR shows higher value of Stokes shift (62 nm) for its open form in chloroform solvent as compared to MCDR and DCMR derivatives (40 and 33 nm respectively) (Table 1).

Fig. 3 Normalized absorption and emission spectra of MCMR, MCDR and DCMR (spirocyclic form) in chloroform solvent (10⁻⁵ M concentrations) at room temperature (λ_ex: 450, 448 and 481 nm for MCMR, MCDR and DCMR respectively).

Fig. 4 Normalized absorption and emission spectra of MCMR, MCDR and DCMR (open form) in chloroform solvent (10⁻⁵ M concentrations) at room temperature (λ_ex: 577, 600 and 588 nm for MCMR, MCDR and DCMR respectively).

Table 1 Photophysical parameters of MCMR, MCDR and DCMR in their spirocyclic and open form in chloroform solvent

Compound	λabs (nm)	εmax × 10^4 (M^−1 cm^−1)	fwhm (nm)	λems (nm)	Stokes shift		ΦF	f	μge (debye)
					(nm)	(cm^−1)
Spirocyclic form
MCMR	450	6.01	70	529	79	3319	0.71	1.07	10.11
MCDR	448	7.16	71	526	78	3310	0.66	1.27	10.99
DCMR	481	8.68	97	520	39	1559	0.64	1.76	13.43
Open form
MCMR	577	8.09	74	639	62	1682	0.26	0.95	10.83
MCDR	600	10.1	96	640	40	1042	0.24	1.35	13.14
DCMR	588	5.96	109	621	33	904	0.21	0.8	9.99

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Effect of solvent on absorption and emission spectra

With increase polarity of the solvents the values for emissions λ_max and Stoke shifts increase for all three derivatives in both spirocyclic as well as in open form (Tables S1, S3 and S5†). Fig. S1, S5 and S10† represents absorption spectra of compound MCMR, MCDR and DCMR respectively in spirocyclic form and as we go from non-polar to polar solvents slight red shift absorption λ_max is observed. The emission spectra of the compounds in the spirocyclic form (Fig. S2, S6 and S11†), with the increasing solvent polarity showing intense red shifted emission spectra with slight quenching of fluorescence (Fig. S3, S4, S8, S9, S13 and S14†). However for the open form of all the compounds do not much shift in absorption as well as emission spectra with increase in solvent polarity. In ethanol the spirocyclic form of the compound are easily converted to their opened form which is evident from the peak for both forms in the absorption spectra. Compound MCMR and MCDR mostly preferred to remain in their open form in ethanol, while compound DCMR in DCM solvent.

Solvatochromism and Lippert–Mataga analysis

Polarity of organic solvent highly influences the excited state of donor-π-acceptor compounds as the excited state is stabilized in highly polar solvents through dipole–dipole, hydrogen bonding, and solvation interactions, thereby leading to red shift in emission spectra. In the present study the absorption and emission spectra of all three dyes are recorded in solvents of different polarities for both the spirocyclic and open form. In the spirocyclic form we observe slight red shift in its absorption spectra while intense red shift in the emission spectra with increasing polarity of solvent. However, in the open form for all the compounds no significant shift in the absorption as well as emission spectra is evident. Among all three derivatives compound MCMR shows highest shift in absorption (15 nm) as well as emission spectra (70 nm) in spirocyclic form with increasing solvent polarity (Fig. 5, S7 and S12†). In this context the Lippert–Mataga function gives more insight on the role of solvent parameters such as dielectric constant and refractive index for the observed red shift emission spectra, with linear correlation between the orientation polarizability (i.e. Lippert–Mataga function) and the Stokes shift. As is evident from the linear graph plot of Stokes shift as a function of orientation polarizability (Lippert–Mataga function) the dipole moment and refractive index of solvents are collectively responsible for the observed red shift in emission spectra (Fig. 6). Further Mac-Rae function an improved version of Lippert–Mataga function takes into account the solute polarizability for dye molecules with the observed solvatochromism. A graph plot of Stokes shift vs. Mac-Rae function gave a linear fit suggesting that the observed red shift in emission spectra to be arising from dipoles created by the dye molecule (Fig. S15†).

Fig. 5 Normalized emission spectra of MCMR in all solvents (spirocyclic form).

Fig. 6 Lippert-Mataga solvent polarity plots of compound MCMR, MCDR and DCMR in all solvents (spirocyclic form).

As we observed quenching of fluorescence with the red shift in emission λ_max with increasing solvent polarity twisted intramolecular charge transfer (TICT) is predicted in these compounds and the molecule may get twisted in its excited state for the better charge transfer from donor nitrogen atom of triphenylamine to the acceptor coumarin moiety. The normalized emission spectra of MCMR, MCDR and DCMR in their spirocyclic form are represented in Fig. 5, S7 and S12,† where they showed 70 nm, 57 nm and 41 nm of red shift in emission from non-polar polar solvent for MCMR, MCDR and DCMR respectively.

Multilinear regression analysis of observed shift in absorption and emission spectra using Kamlet–Taft and Catalan parameters

The solvent polarity effects on the shift in emission spectra are satisfactorily established through single parameter scale such as Lippert–Mataga, Mac-Rae, E^T_N (30). However, these scales are inappropriate to explain precisely which solvent factor like solvent acidity, basicity, dipolarity or polarizability is responsible for the observed shift in absorption and emission spectra. Initially Kamlet and Taft^49,50 proposed one expression as represented in eqn (1)

y = y₀ + a_αα + b_ββ + c_π*π* (1)

where, ‘y’ is the solvent affected physiochemical property (e.g. absorption maxima ([small upsilon, Greek, macron]_abs), emission maxima ([small upsilon, Greek, macron]_emi), Stokes shift (Δ[small upsilon, Greek, macron]), etc.). ‘y₀’ is the studied physiochemical property in the gas phase. a, b, c and d are adjustable coefficients which reflect the dependence of ‘y’ to the various solvent parameters such as α (effect of acidity of solvent), β (effect of basicity of solvent) and π* (collective effect of solvent polarity/dipolarity and polarizability).

Later on Catalan^51–53 proposed another expression as represented in eqn (2)

y = y₀ + a_SASA + b_SBSB + c_SPSP + d_SdPSdP (2)

where, Catalan utilized same solvent parameters i.e. solvent acidity (SA) and solvent basicity (SB) which already employed by Kamlet and Taft, but he splitted the last parameter into two separate parameters i.e. solvent polarizability (SP) and solvent dipolarity (SdP) to know among these two factors which is mainly responsible for the observed shift in absorption or emission spectra. In the present study both approaches to find out the factor responsible for the shift in absorption and emission spectra are implemented. We utilized the reported Kamlet–Taft⁵⁴ and Catalan⁵⁵ parameters.

Scrutinising the absorption maxima ([small upsilon, Greek, macron]_abs), emission maxima ([small upsilon, Greek, macron]_emi) and Stokes shift (Δ[small upsilon, Greek, macron]) of compound MCMR as represented in Table 2, it indicates that all three correlation coefficient of absorption maxima ([small upsilon, Greek, macron]_abs), emission maxima ([small upsilon, Greek, macron]_emi) and Stokes shift (Δ[small upsilon, Greek, macron]) are higher for Catalan parameters (0.97, 0.92 and 0.90) as compared to the Kamlet–Taft parameters (0.76, 0.72 and 0.71) respectively. In the absorption spectra of MCMR as slight red shift is observed experimentally, the acidity (a_α or a_SA) and basicity (b_β or b_SB) factors are ruled out by both Kamlet–Taft and Catalan method as they shows positive values. Among solvent polarity and polarizability due to the high estimated coefficient and minimum standard error, solvent polarizability predominantly appears as the main factor responsible for the slight red shift in absorption spectra. This is further confirmed by plotting the absorption values against each individual factor, where solvent polarizability factor shows higher correlation coefficient (0.46) than solvent dipolarity factor (0.15). For the emission spectra of MCMR positive values were obtained for solvent acidity and basicity factor by Catalan method and hence they can be again ruled out from the competition. Again between solvent dipolarity and polarizability higher value of estimated coefficient and minimum standard error is observed for solvent dipolarity factor, which is also confirmed by the individual plots of emission values against each individual factor, where solvent dipolarity shows higher correlation coefficient (0.95) as compared to the solvent polarizability factor (0.08). Hence solvent dipolarity can be considered as the key factor responsible for the high red shift in emission spectra of MCMR. Similarly slight red shift in emission is observed for this compound with increase in solvent polarity, and solvent polarizability can be considered as the main factor responsible for the observed shift in emission as all the other factor show positive values.

Table 2 Estimated coefficients (y₀, a, b, c, d), their standard errors and correlation coefficients (r) for the multi-linear analysis of ([small upsilon, Greek, macron]_abs), ([small upsilon, Greek, macron]_emi) and (Δ[small upsilon, Greek, macron]) of MCMR as a function of Kamlet–Taft (2) and Catalan (3) solvent scales. Where, α or SA for solvent acidity, β or SB for solvent basicity, π* for collective parameter of solvent dipolarity and polarizability in the case of Kamlet–Taft equation, SdP and SP for solvent dipolarity and polarizability respectively in the case of Catalan equation

Kamlet–Taft	y0 × 10^3	aα	bβ	cπ*	r
[small upsilon, Greek, macron]abs	22.96 ± 0.12	0.15 ± 0.23	0.67 ± 0.26	−1.12 ± 0.23	0.76
[small upsilon, Greek, macron]emi	20.55 ± 0.37	−0.62 ± 0.67	−0.28 ± 0.77	−2.45 ± 0.68	0.72
Δ[small upsilon, Greek, macron]	2.42 ± 0.32	0.77 ± 0.58	0.94 ± 0.66	1.34 ± 0.59	0.71

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Catalan	y0 × 10^3	aSA	bSB	cSdP	dSP	r
[small upsilon, Greek, macron]abs	24.15 ± 0.17	0.72 ± 0.17	0.61 ± 0.09	−0.73 ± 0.06	−1.94 ± 0.24	0.97
[small upsilon, Greek, macron]emi	20.85 ± 0.81	0.05 ± 0.81	0.19 ± 0.46	−2.50 ± 0.30	−0.63 ± 1.14	0.92
Δ[small upsilon, Greek, macron]	3.30 ± 0.78	0.66 ± 0.78	0.42 ± 0.44	1.76 ± 0.29	−1.32 ± 1.10	0.90

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Similar to compound MCMR, multilinear regression analysis for compound MCDR and DCMR were also performed (Tables S7 and S8†), where it is found that again solvent polarizability play key role for the slight red shift in the absorption spectra of both these compounds and solvent dipolarity is emerged out as the main factor responsible for the highly red shifted emission spectra of these compounds. Furthermore, as in the case of MCMR, solvent polarizability is the main factor responsible for the red shift in Stokes shift with increase in polarity of solvents. We observed very good linear correlation between the predicted and experimental absorption and emission wave numbers for all three derivatives as represented in Fig. 7. Hence it can be concluded that for all three compounds in their spirocyclic form, solvent polarizability is the key factor responsible for the slight red shift in absorption spectra, while solvent dipolarity is the main factor responsible for the highly red shifted emission spectra with increase in solvent polarity.

Fig. 7 Correlation in experimental and predicted absorption and emission wave numbers (spirocyclic form) for MCMR, MCDR and DCMR by Catalan method.

Ring opening mechanism of coumarin–rhodamine compounds with the addition of TFA

In general, all rhodamine derivatives are interconvertible into their open or spirocyclic form in the presence of acid or base respectively. Hence we studied their spirocyclic to open form conversion with the increasing percentage of trifluoroacetic acid (TFA) in its spirocyclic form. Fig. 8 represents the absorption spectra of MCDR compound with the increasing percentage TFA in 10 μM solution of compound in toluene. It is clearly visible that with the increasing percentage of TFA the absorbance of spirocyclic form decreases with simultaneous increasing absorbance of its open form. An isobestic point for this inter-conversion is obtained at 488 nm. Similarly Fig. 9 represents emission spectra of MCDR for its spirocyclic as well as open form. The high fluorescence of the molecules in the spirocyclic form is attributed to the presence of coumarin moiety on the other side of triphenylamine. Hence comparatively higher emission intensity for their spirocyclic form is observed. Addition of TFA from 5 to 90 μL to the 10 μM toluene solution of compound caused diminishing for the emission peak of spirocyclic form simultaneously forming a new peak at 642 nm in the deep red region for its open form. An isosbestic point at 610 nm is obtained for this inter-conversion of spirocyclic form to open form in acidic medium. Fig. S16 and S17 in the ESI† represent the absorption and emission spectra of compound MCMR with the increasing amount of TFA in its spirocyclic form.

Fig. 8 Absorption spectra of MCDR with the increased percentage of TFA (5 to 70 μL) in toluene (10⁻⁵ M concentrations) at room temperature.

Fig. 9 Emission spectra of MCDR with the increased percentage of TFA (5 to 70 μL) in toluene (10⁻⁵ M concentrations) at room temperature.

Viscosity sensitivity in spirocyclic and open form

Fluorescent molecular rotors (FMR) are the compounds which form twisted intramolecular charge transfer state (TICT) upon photo excitation due to the molecular rotation between donor and acceptor, which is also called as dark state due to its non-emissive nature. Initially various chemical changes in the structure of organic dye were reported to avoid these TICT states. The rigidification of chemical structure was enviable to avoid intramolecular rotation of the compound around single bond between donor and acceptor. Recently physical processes such as viscous solvents are used to hinder the rotation of molecule around single bond, to block the TICT state and regain the fluorescence intensity by increasing the population of locally excited state.^56–58 Due to the rotating phenyl groups triphenylamine based styryl dyes are reported to show viscosity induced emission enhancement.⁵⁹ In this work with the coumarin and rhodamine moieties are attached on the freely rotating phenyl rings of triphenylamine, making it mandatory for the viscosity sensitive studies. Viscosity sensitive emissions of MCMR and MCDR with increasing percentage of PEG in ethanol for both spirocyclic as well as open form are studied. Interestingly for both MCMR and MCDR compound we observed very good viscosity induced emission enhancement in their open form (Fig. 11 and 12) and quenching of fluorescence in spirocyclic form (Fig. S18†). In the spirocyclic form both the compounds MCMR and MCDR showed quenching of fluorescence with the increased percentage of PEG in ethanol due to the increased polarity of solvent mixture and no fluorescence enhancement was observed (Fig. S18†). Recently BODIPY dyes are utilized as effective viscosity sensors by hindering the free rotation of phenyl ring present at meso position in viscous environment.^60–63 In our case, as represented in Fig. 10, the only change in the spirocyclic and open form is the availability of the free rotating phenyl ring attached to the xanthene core of open form of rhodamine derivative. Hence it can be predicted that the hindering of freely rotating phenyl ring at xanthene core of open form of rhodamine dyes in viscous environment is mainly responsible for the viscosity induced fluorescence enhancement. As two rhodamine moieties are present in MCDR structure, comparatively higher fluorescence enhancement (12.84 fold) is observed for this compound than the single rhodamine moiety of MCMR (10.15 fold). We also observed 47 nm red shifted emission λ_max for compound MCMR from 0 to 100% of PEG in ethanol. This viscosity induced emission enhancement for both MCMR and MCDR above 600 nm they also can be used as molecular rotors for their biological applications also.

Fig. 10 Spirocyclic (viscosity insensitive) and open form (viscosity sensitive) structures of MCMR.

Fig. 11 (a) Emission enhancement spectra of MCMR in different ratio of PEG (0 to 100%) in ethanol (10⁻⁵ M concentrations) at room temperature. (b) Plot of maximum emission intensity of MCMR versus percentage of PEG (0 to 100%) in ethanol.

Fig. 12 (a) Emission enhancement spectra of MCDR in different ratio of PEG (0 to 100%) in ethanol (10⁻⁵ M concentrations) at room temperature. (b) Plot of maximum emission intensity of MCDR versus percentage of PEG (0 to 100%) in ethanol.

Energy transfer through space (FRET) observed in MCMR compound

Due to their spectroscopic feasibility, coumarin–rhodamine couple has been already employed as through bond as well as through space energy transfer dyes where energy is particularly absorbed through coumarin moiety and it is emitted through the open form of rhodamine moiety.⁶⁴ Different coumarin–rhodamine based and FRET induced ratiometric fluorescent probes have been developed for the selective detection of metal ions such as copper,⁶⁵ HOCl,^66,67 H₂O₂–NO,⁶⁸ where specifically piperazine unit is used as spacer between coumarin donor and rhodamine acceptor moieties, which is replaced by the N-phenyl unit in our case. Not only through space but the coumarin–rhodamine couple also utilized as through bond energy transfer cassettes^25,33 and were also employed for the selective detection of mercury ion through desulfurization reaction.^28,69 For any through space energy transfer (FRET) to occur spectral overlap between the emission spectrum of energy donor and absorption spectrum of energy acceptor compound is an essential condition. In our case we observed very good spectral overlap between the energy donor coumarin and energy acceptor rhodamine for all three compounds in their open form as represented in Fig. 13.

Fig. 13 Spectral overlap observed between coumarin emission (left side) and rhodamine absorption (right side) for MCMR, MCDR and DCMR in chloroform solvent.

Hence to check the possible energy transfer experimentally we excited compound MCMR in all solvents in its open form at 440 nm which is the absorption λ_max of coumarin moiety and observed emission peak at visible region (511 nm) as well as in the deep red region (640 nm) in different solvent (Fig. 14). A emission peak observed at 640 nm in the deep red region in chloroform, toluene and hexane solvents can be assigned as energy transfer peaks, while the peak observed at 511 nm in dioxane and EtOAc solvents can be as normal emission peak of coumarin moiety. In DMF and ethanol solvents both the peaks are observed simultaneously, hence these peaks can be assigned for the partial energy transfer from the coumarin donor towards the rhodamine acceptor. Interestingly though we excited compound MCDR and DCMR at around 440 nm, we observed only normal emission peaks in the visible region and no energy transfer peak in the deep red region is observed, which may be due to the improper orientation of the energy donor coumarin moiety and energy acceptor rhodamine moiety of these derivatives.

Fig. 14 Emission spectra of compound MCMR in its open form in different solvent when excited at 440 nm.

Conclusion

In conclusion, novel triphenylamine based coumarin–rhodamine hybrid compounds were synthesized. Since both the spirocyclic and open form show high fluorescence, a detail photophysical studies in different organic solvents for both spirocyclic as well as open forms were performed. Positive solvatochromism observed in their spirocyclic form is supported by linear (Lippert–Mataga and Mac-Rae function) as well as multi-linear regression analysis (Kamlet–Taft and Catalan method). Thus it can be concluded that solvent polarizability is responsible for the slightly red shifted absorption spectra and solvent dipolarity is responsible for the highly red shifted emission spectra for all three compounds. Due to the hindered rotation of freely rotating phenyl ring attached to the xanthene core, with the increased percentage of PEG in ethanol viscosity sensitivity for the open form of rhodamine derivatives is observed, unlike the viscosity insensitive rigid lactone ring of spirocyclic form of rhodamine derivatives. Excitation at 440 nm for compound MCMR in its open form exclusively shows energy transfer from the coumarin donor to the rhodamine acceptor in various solvents.

Experimental section

General procedure for the synthesis of compound 2, 5 and 8

POCl₃ was added drop-wise into ice-cooled N,N-dimethyl formamide at 0 °C under nitrogen atmosphere and the mixture was stirred at room temperature for an hour. The Vilsmeier adduct formed was again cooled to 0 °C, the reactant was dissolved in DMF and added drop-wise to the reaction mixture and it was stirred at 50 °C for 2 h. On cooling water was added to the reaction mixture, the solid precipitated out was filtered and dried well as pure product.

2-Hydroxy-4-((3-hydroxyphenyl) (phenyl)amino)benzaldehyde (2). POCl₃ (0.50 mL, 5.41 mmol) was added to N,N-dimethyl formamide (1.4 mL, 18.03 mmol). Compound 2 (1 g, 3.61 mmol) dissolved in DMF (5 mL) was added to the Vilsmeier adduct. Yield = 0.9 g, (82%); melting point = 148–152 °C. ¹H NMR (500 MHz, DMSO-d₆) δ 10.69 (s, 1H), 9.87 (s, 1H), 9.56 (s, 1H), 7.45 (d, J = 9.0 Hz, 1H), 7.39 (t, J = 7.5 Hz, 2H), 7.15–7.22 (m, 4H), 6.58–6.61 (m, 2H), 6.54 (t, J = 2 Hz, 1H), 6.28 (dd, J = 9.0 and 2.0 Hz, 1H), 6.18 (d, J = 2.5 Hz, 1H). ¹³C NMR (126 MHz, DMSO-d₆) δ 190.2, 162.6, 158.9, 154.8, 146.6, 145.6, 131.8, 131.0, 130.4, 127.3, 126.1, 117.7, 115.6, 114.0, 113.4, 110.5, 104.4. Elemental analysis calcd (%); Mol. formula: C₁₉H₁₅NO₃ (C: 74.74, H: 4.95, N: 4.59, O: 15.72; found: C: 74.71, H: 4.87, N: 4.59, O: 15.73).

4-(Bis(3-hydroxyphenyl)amino)-2-hydroxybenzaldehyde (5). POCl₃ (1.67 mL, 17.89 mmol) was added to N,N-dimethyl formamide (4.63 mL, 59.66 mmol). Compound 2 (3.5 g, 11.93 mmol) dissolved in DMF (15 mL) was added to the Vilsmeier adduct. Yield = 2.5 g, (64%); melting point = 194–196 °C. ¹H NMR (500 MHz, DMSO-d₆) δ 9.84 (s, 1H), 7.42 (d, J = 8.5 Hz, 1H), 7.17 (t, J = 8 Hz, 2H), 6.57–6.61 (m, 4H), 6.52 (t, J = 2 Hz, 2H), 6.28 (dd, J = 9 and 2.5 Hz), 6.18 (t, J = 2 Hz, 1H). ¹³C NMR (126 MHz, DMSO-d₆) δ 190.1, 162.7, 159.0, 154.7, 146.7, 131.6, 130.9, 117.6, 115.6, 113.8, 113.3, 110.7, 104.7. Elemental analysis calcd (%); Mol. formula: C₁₉H₁₅NO₄ (C: 71.02, H: 4.71, N: 4.36, O: 19.92; found: C: 71.01, H: 4.69, N: 4.37, O: 19.89).

4,4′-((3-Methoxyphenyl)azanediyl)bis(2-methoxybenzaldehyde) (8). Compound 7 (5 g, 14.91 mmol) was reacted with POCl₃ (4.18 mL, 44.72 mmol) and DMF (5.79 mL, 74.53 mmol) by following the Vilsmeier Haack formylation reaction condition. Yield = 3.7 g, (63%); melting point = 118–122 °C. ¹H NMR (500 MHz, CDCl₃) δ 10.30 (s, 2H), 7.73 (d, J = 8.5 Hz, 2H), 7.25–7.30 (m, 1H), 6.79 (dd, J = 8.5 and 2.5 Hz, 1H), 6.75 (dd, J = 7.5 and 1.5 Hz, 1H), 6.68–6.70 (m, 3H), 6.63 (d, J = 2.0 Hz, 2H), 3.76 (s, 3H), 3.75 (s, 6H). ¹³C NMR (126 MHz, CDCl₃) δ 188.1, 162.8, 160.8, 153.2, 146.4, 130.5, 129.8, 120.3, 119.3, 115.5, 112.8, 111.2, 105.3, 55.6, 55.4. Elemental analysis calcd (%); Mol. formula: C₂₃H₂₁NO₅ (C: 70.58, H: 5.41, N: 3.58, O: 20.44; found: C: 70.61, H: 5.37, N: 3.49, O: 20.45).

4,4′-((3-Hydroxyphenyl)azanediyl)bis(2-hydroxybenzaldehyde) (9). Compound 8 (0.3 g, 0.76 mmol) was dissolved in dry dichloromethane (10 mL) and cooled to −78 °C under nitrogen atmosphere. A solution of boron tribromide (0.29 mL, 3.06 mmol) in dry dichloromethane (5 mL) was added drop-wise to the reaction mixture at −78 °C. The temperature was increased slowly up to 20 °C and stirred at room temperature for next 6 h. Reaction mixture was quenched with the addition of ice-water and extracted with dichloromethane (3 × 20 mL). Organic layer was dried over anhydrous Na₂SO₄ and concentrated on rotavapour to get the crude product, which was further purified on column chromatography (100–200 mesh silica gel) using 3% MeOH in chloroform as eluting system to get the pure product.

Yield = 0.18 g, (67%); melting point = 139–142 °C. ¹H NMR (500 MHz, CDCl₃) δ 10.72 (s, 2H), 10.07 (s, 2H), 9.68 (s, 1H), 7.57–7.59 (d, J = 8.5 Hz, 2H), 7.24 (t, J = 8.5 Hz, 1H), 6.70 (dd, J = 8.0 and 2.0 Hz, 1H), 6.58 (dd, J = 8.5 and 2.0 Hz, 2H), 6.53–6.55 (m, 3H). ¹³C NMR (126 MHz, CDCl₃) δ 189.92, 162.32, 159.23, 153.10, 146.18, 131.38, 130.98, 118.20, 114.60, 114.24, 109.87. Elemental analysis calcd (%); Mol. formula: C₂₀H₁₅NO₅ (C: 68.76, H: 4.33, N: 4.01, O: 22.90; found: C: 68.74, H: 4.33, N: 3.98, O: 22.91).

General procedure for the synthesis of compound 3, 6 and 10

Hydroxyl aldehyde compound 2 (1 eq.) and intermediate 1 (1.1 eq.) was dissolved in absolute ethanol (20 mL). Piperidine (1 eq.) was added to the reaction mixture and refluxed for 4–5 h. After completion the solid precipitated out was filtered out as iminocoumarin intermediate which was further hydrolyzed by dissolving it in ethanol and 15% HCl mixture (1 : 1) and stirred at 70–75 °C for 12 h. The solid precipitated out was filtered, washed with ethanol and dried well to get the red colored pure product.

3-(Benzo[d]thiazol-2-yl)-7-((3-hydroxyphenyl)(phenyl)amino)-2H-chromen-2-one (3). Compound 2 (0.8 g, 2.62 mmol) was reacted with intermediate 1 (0.54 g, 3.14 mmol) in ethanol (20 mL). Yield = 0.92 g, (76%); melting point = >300 °C. ¹H NMR (500 MHz, DMSO-d₆) δ 9.67 (s, 1H), 9.06 (s, 1H), 8.11 (d, J = 7.5 Hz, 1H), 7.99 (d, J = 8.0 Hz, 1H), 7.81 (d, J = 9 Hz, 1H), 7.51 (t, J = 7.0 Hz, 1H), 7.39–7.43 (m, 3H), 7.18–7.28 (m, 4H), 6.78 (dd, J = 9.0 and 2.0 Hz, 1H), 6.65–6.67 (m, 2H), 6.59 (t, J = 2.0 Hz, 1H), 6.56 (d, J = 2.0 Hz, 1H). ¹³C NMR (126 MHz, DMSO-d₆) δ 160.8, 160.1, 159.2, 155.7, 152.9, 152.4, 146.3, 145.3, 142.4, 136.1, 131.6, 131.2, 130.5, 127.1, 126.9, 126.5, 125.3, 122.5, 117.4, 116.2, 114.6, 113.8, 113.7, 112.1, 103.7. Elemental analysis calcd (%); Mol. formula: C₂₈H₁₈N₂O₃S (C: 72.71, H: 3.92, N: 6.06, O: 10.38, S: 6.93; found: C: 72.69, H: 3.85, N: 6.04, O: 10.34, S: 6.94).

3-(Benzo[d]thiazol-2-yl)-7-(bis(3-hydroxyphenyl)amino)-2H-chromen-2-one (6). Compound 5 (0.5 g, 1.55 mmol) was reacted with intermediate 1 (0.32 g, 1.86 mmol) in ethanol (20 mL). Yield = 0.55 g, (74%); melting point = 290–295 °C. ¹H NMR (500 MHz, DMSO-d₆) δ 9.69 (s, 2H), 9.02 (s, 1H), 8.10 (d, J = 7.5 Hz, 1H), 7.97 (d, J = 8.0 Hz, 1H), 7.77 (d, J = 9.0 Hz, 1H), 7.49 (t, J = 8.0 Hz, 1H), 7.39 (t, J = 8.0 Hz, 1H), 7.20–7.30 (m, 3H), 6.78 (dd, J = 9.0 and 2.0 Hz, 1H), 6.56–6.67 (m, 7H). ¹³C NMR (126 MHz, DMSO-d₆) δ 160.8, 160.1, 159.4, 159.1, 155.6, 152.8, 152.4, 146.3, 142.3, 136.1, 131.5, 131.1, 126.9, 125.3, 122.5, 117.4, 116.5, 114.5, 113.7, 113.7, 112.1, 103.8. Elemental analysis calcd (%); Mol. formula: C₂₈H₁₈N₂O₄S (C: 70.28, H: 3.79, N: 5.85, O: 13.37, S: 6.70; found: C: 70.26, H: 3.72, N: 5.86, O: 13.31, S: 6.72).

7,7′-((3-Hydroxyphenyl)azanediyl)bis(3-(benzo[d]thiazol-2-yl)-2H-chromen-2-one) (10). Compound 9 (0.1 g, 0.28 mmol) was reacted with intermediate 1 (0.13 g, 0.74 mmol) in ethanol (5 mL). Yield = 0.08 g, (42%); melting point = >300 °C. ¹H NMR (500 MHz, DMSO-d₆) δ 9.79 (s, 1H), 9.16 (s, 2H), 8.14 (d, J = 8.0 Hz, 2H), 8.04 (d, J = 8.0 Hz, 2H), 7.99 (d, J = 8.5 Hz, 2H), 7.54 (t, J = 8.0 Hz, 2H), 7.44 (t, J = 8.0 Hz, 2H), 7.32 (t, J = 8.0 Hz, 1H), 7.12 (dd, J = 8.5 and 2.0 Hz, 2H), 7.05–7.06 (m, 2H), 6.79 (d, J = 8.0 Hz, 1H), 6.73 (d, J = 8.0 Hz, 1H), 6.67 (t, J = 2.0 Hz, 1H). ¹³C NMR (126 MHz, DMSO-d₆) δ 160.5, 159.8, 159.4, 155.2, 152.4, 150.8, 145.8, 142.1, 136.2, 131.8, 127.1, 125.6, 122.7, 122.6, 120.2, 118.2, 117.1, 114.9, 114.8, 114.4, 109.2. Elemental analysis calcd (%); Mol. formula: C₃₈H₂₁N₃O₅S₂ (C: 68.76, H: 3.19, N: 6.33, O: 12.05, S: 9.66; found: C: 68.77, H: 3.11, N: 6.28, O: 12.07, S: 9.67).

General procedure for the synthesis of MCMR, MCDR and DCMR

Compound 3, 6 or 10 (1 eq.) and intermediate 2 (1.1 eq.) were dissolved in trifluoroacetic acid (10 mL) and stirred at 95 °C in a sealed tube for 12 h. After completion, the reaction mixture was poured in ice-water mixture and neutralized by saturated solution of NaHCO₃. The solid precipitated out was filtered, dried well and purified on column chromatography (100–200 silica gel) using 1–5% MeOH in chloroform as eluting system to get the pure products.

MCMR. Compound 3 (0.3 g, 0.64 mmol) was reacted with intermediate 2 (0.29 g, 0.71 mmol) in trifluoroacetic acid (10 mL). Yield = 0.36 g, (75%); melting point = >300 °C. ¹H NMR (500 MHz, CDCl₃) δ 8.96 (s, 1H), 8.03 (dd, J = 7.5 and 2 Hz, 2H), 7.95 (d, J = 7.5 Hz, 1H), 7.72 (t, J = 7 Hz, 1H), 7.64 (t, J = 7.5 Hz, 1H), 7.49–7.52 (m, 2H), 7.38–7.42 (m, 3H), 7.22–7.30 (m, 4H), 7.03 (d, J = 2.0 Hz, 1H), 7.01 (dd, J = 8.5 and 2.0 Hz, 1H), 6.95 (d, J = 2.0 Hz, 1H), 6.83 (dd, J = 8.5 and 2.0 Hz, 1H), 6.73 (d, J = 8.5 Hz, 1H), 6.60 (d, J = 8.5 Hz, 1H), 6.38–6.42 (m, 2H), 3.33–3.37 (q, 4H), 1.15–1.18 (t, J = 7.0 Hz, 6H). ¹³C NMR (126 MHz, CDCl₃) δ 169.4, 160.6, 160.2, 155.5, 152.7, 152.7, 152.5, 152.1, 147.4, 145.1, 141.2, 136.5, 134.8, 130.1, 130.1, 129.6, 129.3, 128.9, 127.3, 127.1, 126.4, 126.3, 124.9, 124.9, 124.1, 122.5, 121.6, 120.1, 117.8, 116.2, 116.1, 113.1, 113.1, 106.8, 97.5, 44.5, 12.4. Elemental analysis calcd (%); Mol. formula: C₄₆H₃₃N₃O₅S (C: 74.68, H: 4.50, N: 5.68, O: 10.81, S: 4.33; found: C: 74.63, H: 4.44, N: 5.73, O: 10.77, S: 4.35).

MCDR. Compound 6 (0.2 g, 0.42 mmol) was reacted with intermediate 2 (0.43 g, 1.04 mmol) in trifluoroacetic acid (10 mL). Yield = 0.24 g, (56%); melting point = >300 °C. ¹H NMR (500 MHz, DMSO-d₆) δ 9.99 (s, 1H), 9.38 (s, 1H), 9.34 (s, 1H), 8.38 (dd, J = 8 and 2.5 Hz, 1H), 8.24–8.28 (m, 2H), 8.17 (d, J = 9.0 Hz, 1H), 8.12 (d, J = 9.0 Hz, 1H), 8.03–8.07 (m, 2H), 7.95–7.98 (m, 2H), 7.76–7.80 (m, 1H), 7.66–7.69 (m, 1H), 7.60–7.62 (m, 2H), 7.51 (t, J = 8.5 Hz, 1H), 7.27–7.34 (m, 2H), 7.15–7.21 (m, 3H), 6.88 (m, 1H), 6.68–6.74 (m, 6H), 3.56 (q, 8H), 1.30 (t, J = 7.0 Hz, 12H). ¹³C NMR (126 MHz, DMSO-d₆) δ 169.4, 160.9, 160.8, 160.2, 160.1, 159.5, 155.7, 155.6, 152.5, 152.2, 151.5, 149.9, 147.9, 147.4, 146.4, 142.5, 136.4, 132.1, 131.6, 130.8, 130.2, 130.1, 129.6, 126.9, 125.3, 124.9, 122.9, 121.5, 121.1, 118.1, 116.8, 116.2, 115.8, 114.5, 113.9, 113.5, 113.1, 109.4, 106.3, 104.9, 97.6, 83.7, 44.5, 12.9. Elemental analysis calcd (%); Mol. formula: C₆₄H₄₈N₄O₈S (C: 74.40, H: 4.68, N: 5.42, O: 12.39, S: 3.10; found: C: 74.38, H: 4.65, N: 5.32, O: 12.44, S: 3.12).

DCMR. Compound 10 (0.06 g, 0.09 mmol) was reacted with intermediate 2 (0.08 g, 0.27 mmol) in methane sulphonic acid (10 mL) and the reaction mixture was stirred at 75 °C for 16 h. Yield = 0.03 g, (41%); melting point = >300 °C. ¹H NMR (500 MHz, DMSO-d₆) δ 9.13 (S, 2H), 8.11 (t, J = 7.0 Hz, 2H), 7.96–8.01 (m, 5H), 7.77 (d, J = 7.5 Hz, 1H), 7.68 (d, J = 7.5 Hz, 1H), 7.49 (d, J = 7.0 Hz, 2H), 7.34–7.42 (m, 3H), 7.11–7.15 (m, 5H), 6.94 (t, J = 7.0 Hz, 1H), 6.79 (t, J = 7.5 Hz, 1H), 6.42–6.45 (m, 2H), 6.37 (s, 1H), 3.38 (q, 4H), 1.02 (t, J = 7.0 Hz, 6H). ¹³C NMR (126 MHz, DMSO-d₆) δ 169.1, 160.3, 159.8, 155.2, 152.6, 152.4, 152.4, 152.4, 150.4, 149.7, 146.8, 141.9, 136.2, 136.1, 132.1, 130.6, 130.2, 129.1, 127.1, 126.6, 125.7, 125.1, 124.7, 122.8, 122.6, 121.8, 117.5, 117.4, 115.6, 110.3, 104.7, 83.4, 44.2, 12.7. Elemental analysis calcd (%); Mol. formula: C₅₆H₃₆N₄O₇S₂ (C: 71.47, H: 3.86, N: 5.95, O: 11.90, S: 6.81; found: C: 71.45, H: 3.81, N: 5.92, O: 11.91, S: 6.82).

Acknowledgements

Author SSK is thankful for University Grants Commission (UGC), India as well as Technical Education Quality Improvement Program (TEQIP) for a Research Fellowship. We are also thankful to the Sophisticated Analytical Instrument Facility (SAIF), IIT, Mumbai, India for the recording of mass and HR-MS spectra.

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Footnote

† Electronic supplementary information (ESI) available: Absorption–emission spectra in different solvents and ¹H and ¹³C NMR spectra of all intermediates and final rhodamine dyes were included. Tables containing photophysical parameters in different solvents in their spirocyclic and open form as well as Kamlet–Taft and Catalan parameters are provided. See DOI: 10.1039/c6ra24485h

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