Solvent effects on the photophysics of 3-(benzoxazol-2-yl)-7-(N,N-diethylamino)chromen-2-one

Abstract

The photophysics of 3-(benzoxazol-2-yl)-7-(N,N-diethylamino)chromen-2-one was studied in different solvents and in SDS micelles. This compound presents characteristics which include an S₀ → S₁¹(π,π*) transition with a ¹(n,π*) perturbative component, due to the electronic coupling between the diethylamino group and the coumarin ring, considerable solvatochromism, dual fluorescence and high fluorescence quantum yields in almost all solvents studied. The electronic structure of the S₁ and S₂ excited states permits vibronic coupling between them, making configurational changes of the S₂ excited state possible, leading to the formation of an S₂(TICT) state. Analysis of the TCSPC data indicates an equilibrium between the S₂(TICT) and S₁(LE) states in favour of the former. In protic solvents, the hydrogen bonding between the solvent and the diethylamino moiety results in the formation of an S₂(HICT) state, making internal conversion an important deactivation process. Quantum mechanical calculations for the isolated molecule show that the diethylamino group in the S₂(TICT) state is twisted at least 56° from the plane of the coumarin ring, with partial electronic decoupling between –NEt₂ and the coumarin ring. This twisting angle must be positively influenced by solute–solvent interactions. Φ_ST is found to be small, but not negligible. However, Φ_Δ can be considered negligible, an indication that T₁ is a short-lived state. Based on the experimental data and theoretical calculations, the most probable sequence for the first excited states, including the TICT state, is T₁(n,π*) < S₂(TICT) < S₁(π,π*) ≈ S₂(n,π*).

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1 Introduction

Coumarins have been extensively studied due to their practical applications,^1–6 which include uses as biological and chemical sensors, fluorescent probes and laser dyes.^7–10 Additionally, coumarins containing an electron-releasing group in the 7-position and a heterocyclic electron-acceptor residue in the 3-position are recognised as fluorescent dyes suitable for application to synthetic fibres.¹¹ In sensor applications, fluorescence is frequently the observable phenomenon of choice. Sensors have been used for remote sensing of many different kinds of phenomena, all related to their solvatochromic properties.

Electron-donor groups in the 4- and 7-positions of the coumarin ring cause bathochromic and bathofluoric shifts, which are become larger as the electron-donor behaviour of the substituents becomes more pronounced.^4,12,13 The combination of electron-withdrawing groups in the 3-position and electron-donor groups in the 7-position intensifies electron delocalisation, with favourable implications for the fluorescence properties of these molecules. Alkylaminocoumarins, in particular, show solvatochromic behaviour. However, interactions with the solvent and the formation of hydrogen bonds involving the amino group tend to reduce the fluorescence quantum yield. As the interaction becomes more intense, internal conversion (IC) increasingly becomes the deactivation route for the excited state due to the increase in vibronic coupling.

In the present work, the photophysics and spectral properties of 3-(benzoxazol-2-yl)-7-(N,N-diethylamino)chromen-2-one (Scheme 1) were investigated in different solvents. The E_T(30) solvatochromic scale was used to quantify the observed changes. Experimental and theoretical evidence on the coexistence of S₁(LE), S₂(TICT) and S₂(HICT) states is also presented.

Scheme 1 Structure of 3-(benzoxazol-2-yl)-7-(N,N-diethylamino)chromen-2-one (I).

2 Experimental

3-(Benzoxazol-2-yl)-7-(N,N-diethylamino)chromen-2-one (I) was synthesised by Dr A. M. F. Oliveira-Campos and co-workers.¹⁴

All solvents were of spectroscopic grade and were used without further purification. The concentration of I in the solutions was around 10⁻⁶ mol L⁻¹. Solutions containing sodium dodecyl sulfate (SDS; 30 mmol L⁻¹) were prepared in phosphate buffer solution (PBS; pH 7.4).

UV-Vis absorption and emission spectra were recorded using a Hach 4000U spectrophotometer and a Hitachi F-4500 spectrofluorimeter, respectively, equipped with accessories for low temperature measurements. The fluorescence spectra were obtained, using the right-angle configuration, by exciting the samples at the wavelength of maximum absorption. The fluorescence quantum yields were estimated from the corrected fluorescence spectra, using methodology proposed by Eaton.¹⁵ The fluorescence standard was 9,10-diphenylanthracene in cyclohexane (Φ_F = 0.90 at 293 K). The solutions were prepared in such a way as to keep the absorbance below 0.100 at the excitation wavelength in order to avoid light reabsorption effects. Low temperature measurements were carried out at 77 K (liquid nitrogen) using methylcyclohexane as the solvent. Solutions were deoxygenated by bubbling argon through them prior to making the measurements.

Fluorescence decays were measured by the single-photon counting technique using an Edinburgh CD-900 spectrometer operating with a hydrogen-filled nanosecond flash lamp at 40 kHz pulse frequency. All lifetime values reported reflect data taken with pulses between 3000 and 5000 counts in the maximum channel and 0.900 < χ² < 1.100.

The quantum yield of singlet oxygen generation (Φ_Δ) was measured in chloroform, based on methodology proposed by Schmidt et al.,¹⁶ using solutions with an absorbance of 0.300 at 355 nm. The measurement was carried out with an Edinburgh LP 900 time-resolved system, using 355 nm laser pulses (5 ns) from a Continuum Surelite II Nd:YAG laser (Q-switched delay 200 µs). The laser power was varied from 0 to 8 mJ. A North Coast EO-817 detector was used for the detection of singlet oxygen phosphorescence at 1270 nm. The compound 1-perinaphthenone (ϕ_Δ = 0.97 at 293 K)¹⁶ was used as the standard in these measurements.

Quantum mechanical calculations were performed for the isolated molecule in the ground and excited states using the Gaussian 98W suite of programs,¹⁷ after optimisation of the ground-state geometry using the PM3 semi-empirical method (UHF calculation, gradient 0.1000 kcal Å⁻¹ mol⁻¹, Polak–Ribiere optimisation algorithm).¹⁸ The ground-state geometry was refined using a density functional theory (DFT) method (B3LYP) with a 6-31G* basis function. The Berny analytical gradient was used in this optimisation. The requested convergence limit on the RMS density matrix was 1 × 10⁻⁸ and the threshold values for the maximum force and the maximum displacement were 0.000450 and 0.001800 a.u., respectively. The ab initio RCIS method at the 3-21G** level was used to optimise the geometry of the S₁, S₂ and S₂(ICT) states. The geometry of the S₂(ICT) state was initially obtained by evaluating the changes in the state energy of the S₂ state on varying the dihedral angle C(31)–N(27)–C(20)–C(15) and the distance N(27)–C(20) (single point calculations, RCIS, 3-21G basis function) to obtain the minimum energy structure, which was then optimised (Fig. 1). The state energies of I were evaluated using PM3-CI (4 occupied MOs and 4 unoccupied MOs, 4900 configurations) for the isolated molecule.¹⁸ The calculations were conducted using the optimised ground-state structure.

Fig. 1 Optimised structure (DFT, B3LYP/6-31G*) of the S₀ state of I, with atom numbering scheme.

3 Results and discussion

3.1 Absorption spectra

Table 1 presents spectroscopic and photophysical data related to the compound under study, in different media. The absorption maximum corresponding to the S₀ → S₁ transition shows high molar absorptivity, between 40 000 and 54 000 L mol⁻¹ cm⁻¹, which is also observed for analogous compounds^19,20 and is attributed to a ¹(π,π*) transition. In fact, the relationship between the frequency, in wavenumbers, of the absorption maximum and the solvent polarity, expressed in terms of the E_T(30) scale (Fig. 2), is typical of a ¹(π,π*) transition.^21–23

Table 1 Spectral characteristics of I in different media

Solvent	λ abs ^a/nm	λ em ^b/nm	Δ[small nu, Greek, macron]^b/cm^−1	log ε	ϕ F	τ 1 ^c/ns	E T(30)/kcal mol^−1	k nr/kF
a λ abs = λexc. b The first value corresponds to the LE state and the second to the TICT state. c Mean values. d Value from ref. 19.
Cyclohexane	423	473, 490	2499, 3233	4.76	0.66	—	30.9	—
Methylcyclohexane	426	478, 490	2553, 3066	4.64	0.64	2.23	—	0.562
n-Hexane	421	474, 491	2656, 3386	4.74	0.83	2.37	31.0	0.205
n-Decane	423	473, 491	2499, 3274	4.68	0.95	2.26	31.0	0.053
n-Decane–nujol (2 ∶ 8)	423	458, 489	1807, 3191	—	—	2.36	—	—
Nujol	425	458, 489	1695, 3079	—	—	—	—	—
Carbon tetrachloride	428	474, 493	2267, 3080	4.66	0.84	2.28	32.4	0.190
Toluene	431	476, 498	2194, 3122	4.62	0.79	2.14	33.9	0.266
1,4-Dioxane	428	476, 495	2356, 3162	4.64	0.79	2.26	36.0	0.266
Tetrahydrofuran	432	482	2401	4.66	0.89	—	37.4	—
Ethyl acetate	430	480, 495	2423, 3054	4.62	0.81	—	38.1	—
Diisopropylamine	426	474, 495	2377, 3272	4.72	0.52	—	—	—
Chloroform	440	478, 498	1806, 2647	4.71	0.90	2.28	39.1	0.111
Dichloromethane	439	482	2032	4.78	0.86	—	40.7	—
Acetone	434	487	2507	4.66	0.76	—	42.2	—
N,N-Dimethylformamide	441	493	2392	4.67	0.45	1.93	43.1	1.222
Dimethyl sulfoxide	444	497	2402	4.66	0.60	1.76	45.1	0.667
Acetonitrile	438	490	2423	4.66	0.80	1.87	45.6	0.250
2-Butanol	438	487	2297	4.65	0.78	2.37	47.1	0.282
2-Propanol	438	488	2339	4.68	0.76	2.33	48.4	0.316
Ethanol	441	489	2226	4.70	0.57	1.95	51.9	0.754
Ethylene glycol	448	497	2200	4.69	0.46	1.75	53.8	1.174
Methanol	442	490	2216	4.70	0.42	1.43	55.4	1.381
SDS micelles/PBS	463	495	1396	4.83	0.33	1.16	61.3^d	2.030
Water	452	497	2003	4.61	0.08	0.66	63.1	11.11

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Fig. 2 Frequency of absorption maximum ([small nu, Greek, macron]) for I (S₀ → S₁) vs. solvent E_T(30): (a) n-hexane; (b) carbon tetrachloride; (c) toluene; (d) tetrahydrofuran; (e) 1,4-dioxane; (f) chloroform; (g) dichloromethane; (h) acetone; (i) N,N-dimethylformamide; (j) dimethyl sulfoxide; (k) acetonitrile; (l) 2-butanol; (m) 2-propanol; (n) ethanol; (o) methanol; (p) water.

A bathochromic shift occurs as the solvent polarity increases. This trend is approximately the same for protic and aprotic solvents, being the result of a diminishing energy gap between the ground and excited states due to the increasing strength of the polar interactions between I and the solvent.²¹

As a result of the anticipated electronic coupling between the diethylamino group and the coumarin ring,^24,25 it seems likely that the S₀ → S₁ transition possesses a ¹(n,π*) perturbative component, similar to that proposed for the deprotonated form of the analogue containing a hydroxyl group in the 7-position.²⁰ This could be evidence that S₂ is a ¹(n,π*) state and that the energy gap between S₁ and S₂ is sufficiently small to make mixing of the states viable. Theoretical estimates show that the absorption band related to the S₀ → S₂ transition [maximum at 333 nm; oscillator strength f = 0.07, typical of a ¹(n,π*) transition] must be overlapped by the band corresponding to the S₀ → S₁ transition.²⁶ Despite the fact that these estimates refer to an isolated molecule, the calculated absorption maximum corresponding to the S₀ → S₁ transition (431 nm, f = 1.08)²⁶ shows agreement with the experimental values observed for aprotic (around 430 nm) and protic (440 nm) solvents. The estimate of ΔE(S₂,S₁) based on these two theoretical absorption maxima (0.8 kJ mol⁻¹) suggests that these two states must be very close.

Fig. 3 shows the absorption spectrum of I in cyclohexane and methanol. Seixas de Melo et al., from studies on the photophysics of coumarins,²⁷ concluded that, for this class of compounds, the S₁ state is typically ¹(n,π*) and that the potential energy surfaces corresponding to S₁ and S₂ states are close. Thus, substitutions or changes in the solvent polarity may reduce the energy gap between these states, promoting the mixing or the inversion of states, which can result in an S₁(π,π*) state.

Fig. 3 UV-Vis absorption spectra of I in cyclohexane (a) and methanol (b).

For this compound, the nature of the S₀ → S₁ transition can be attributed to the combination of the electronic effects caused by the diethylamino and benzoxazole groups on the coumarin ring. A comparison of the geometrical data related to the S₀ and S₁ states shows that the bond lengths along the long axis of the molecule are shortened for S₁ (Table 2), indicating an increase in the electronic conjugation over the coumarin ring in the excited state.

Table 2 Optimised bond lengths (Å) for the S₀ and S₁ states of I

Bond	S0	S1
R(1–6)	1.457	1.391
R(2–4)	1.418	1.383
R(3–5)	1.406	1.402
R(9–13)	1.389	1.374
R(10–14)	1.370	1.378
R(11–15)	1.381	1.359
R(12–16)	1.378	1.357
R(13–18)	1.399	1.392
R(14–19)	1.386	1.368
R(18–23)	1.394	1.378
R(19–24)	1.397	1.388
R(20–27)	1.376	1.358

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In particular, the shortening of the C(20)–N(27) bond shows the electronic coupling between the diethylamino group and the coumarin π system with electronic excitation, confirming the participation of a ¹(n,π*) perturbative component in the S₁ state. The bond and dihedral angles indicate that the diethylamino group nitrogen has sp² character. For S₀ and S₁, this group is slightly out of the plane of the coumarin ring, being more planar in the S₁ state than in the S₀ (Table 3).

Table 3 Bond and dihedral angles (°) for the diethylamino group of I in the S₀ and S₁ states

State	Bond angle		Dihedral angle
	C(30)–N(27)–C(20)	C(31)–N(27)–C(20)	C(30)–N(27)–C(20)–C(15)	C(31)–N(27)–C(20)–C(15)
S0	122.118	121.736	−173.595	+5.727
S1	122.443	121.997	184.995	+4.716

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This lower dihedral angle must imply an increase in the state energy of S₁, due to the steric effects, and can be seen as one of the causes of the small energy gap between the S₁ and S₂ states. In addition, the vibronic coupling between S₁ and S₂, favoured by the interactions with the solvent, creates the conditions for configurational changes of the S₂ state, which leads to the formation of an S₂(TICT) state.

An estimate of the geometrical structure of I in the S₂(TICT) state reveals that the dihedral angle C(31)–N(27)–C(20)–C(15) (Fig. 1) must be twisted at least 56° from the plane of the coumarin ring. The expected C(30)–N(27)–C(20) and C(31)–N(27)–C(20) bond angles for this state are 121.826 and 119.761°, respectively, indicating that, despite the torsion, the sp² character of the nitrogen is not lost at all. This suggests only a partial electronic decoupling between the diethylamino group and the coumarin ring for the isolated molecule.²⁶ However, in principle, interactions between the S₂(TICT) state and the solvent might increase the electronic decoupling, with a consequent increase in the twisting angle.

3.2 Fluorescence and photophysics

Fig. 4 and Table 1 show that I exhibits considerable solvatochromism. As solvent polarity increases, the fluorescence maximum wavelength undergoes a bathofluoric shift, estimated from the Stokes' shift, which is more intense than the bathochromic shift observed for the absorption spectra. Plotting solvent E_T(30) against Stokes’ shift gives two distinct graphs, one for non-protic and one for protic solvents, with good linear correlations (R < −0.97). Since the E_T(30) scale is essentially a polarity scale based on the static dielectric constant, f(D), with the addition of a protic hydrogen-bonding effect, this behaviour is expected.^21,28,29 The plots reveal compound I to be more sensitive to polarity changes in aprotic solvents than in protic ones. However, specific interactions induced by protic solvents seems to induce an efficient vibronic coupling with the excited states of I, reducing the fluorescence quantum yield and increasing the rate of internal conversion. Fig. 5 shows the exponential growth in the k_nr/k_F ratio with increasing solvent polarity. The trend is more evident for protic solvents.

Fig. 4 Stokes' shift for Ivs. solvent E_T(30): (a) n-hexane; (b) n-decane; (c) carbon tetrachloride; (d) toluene; (e) 1,4-dioxane; (f) chloroform; (g) N,N-dimethylformamide; (h) dimethyl sulfoxide; (i) acetonitrile; (j) 2-butanol; (k) 2-propanol; (l) ethanol; (m) ethylene glycol; (n) methanol; (o) SDS micelles/PBS.

Fig. 5 k _nr/k_F ratio for Ivs. solvent E_T(30): (a) n-decane; (b) n-hexane; (c) carbon tetrachloride; (d) toluene; (e) 1,4-dioxane; (f) chloroform; (g) N,N-dimethylformamide; (h) dimethyl sulfoxide; (i) acetonitrile; (j) 2-butanol; (k) 2-propanol; (l) ethanol; (m) ethylene glycol; (n) methanol; (o) SDS micelles/PBS; (p) water.

In contrast to the situation with 4-dimethylaminobenzonitrile, I exhibits two fluorescence bands in aprotic solvents, independent of their polarity (Table 1 and Fig. 6), attributed to the dynamic equilibrium between the S₁(LE) and S₂(TICT) states.^{21,24,25,30–32}

S₁(LE) ⇄ S₂(TICT)

Fig. 6 Normalised fluorescence spectra of I in n-decane (a), diisopropylamine (b) and methanol (c).

In protic solvents, a red-shifted band, attributed to the S₂(TICT) state, is more evident. The rate constants (Table 4) show that formation of the S₂(TICT) state is favoured as solvent polarity increases. If the polar interactions are due to protic solvents, mainly alcohols capable of hydrogen bonding with the amino group, only one emission band is observed. This band, red-shifted toward the S₁(LE) state, is due to the emission of the S₂(TICT) state. Despite this, for all media, analysis of the TCSPC data (Table 4) indicates that the equilibrium between these species is favourable to the formation of the S₂(TICT) state.

Table 4 Kinetic data for I in different media

Solvent	τ 1/ns		τ 2/ns		χ ^2		k TICT/s^−1	k LE/s^−1
n-Decane	2.197 ± 0.035	0.976	5.266 ± 1.605	0.024	1.108	40.67	4.44 × 10^8	1.09 × 10^7
n-Hexane	2.356 ± 0.031	0.976	5.013 ± 0.945	0.024	1.126	40.67	4.16 × 10^8	1.02 × 10^7
1,4-Dioxane	2.280 ± 0.014	0.986	28.74 ± 9.78	0.014	1.157	70.43	4.33 × 10^8	6.15 × 10^6
Chloroform	2.277 ± 0.021	0.976	5.944 ± 1.590	0.024	1.132	40.67	4.38 × 10^8	1.05 × 10^7
N,N-Dimethylformamide	1.917 ± 0.009	0.991	22.83 ± 10.02	0.009	1.036	—	—	—
Dimethyl sulfoxide	1.734 ± 0.009	0.994	11.29 ± 3.27	0.006	1.138	—	—	—
2-Propanol	2.334 ± 0.006	1.000	—	—	0.965	—	—	—
Ethanol	2.037 ± 0.006	1.000	—	—	1.095	—	—	—
Ethylene glycol	1.859 ± 0.006	1.000	—	—	1.091	—	—	—
Methanol	1.449 ± 0.005	1.000	—	—	1.089	—	—	—
SDS micelles/PBS	1.203 ± 0.006	0.994	15.41 ± 2.87	0.006	1.099	—	—	—

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TCSPC measurements show a biexponential profile for the fluorescence decay of I in aprotic solvents,

and monoexponential for protic solvents, mainly alcohols. This profile is also observed in N,N-dimethylformamide, dimethyl sulfoxide and SDS micelles. The preexponential factor predominates, the participation of the second term being less than 3% in the best cases. For the majority of the protic solvents, is very small, and the fluorescence decay was considered, in principle, as almost completely due to the deactivation of the S₂(TICT) state.

Since and ,²⁸ the rate constants for the forward (k_TICT) and backward (k_LE) ICT reactions can be estimated. The calculated constants (Table 4) confirm the favourability of populating the S₂(TICT) state, even in aprotic solvents. The preference for populating the S₂(TICT) state is pertinent if we consider that the activation energy, E_a, for the formation of this state is lower than that for the reverse reaction.

Considering the expected role of the solvent polarity on the formation of these states, the inequality E_a(TICT) < E_a(LE) is perfectly plausible. A rough estimate of the energy of the S₂(TICT) state can be made by considering the energy difference between the emission maxima attributed to the S₁ and S₂(TICT) states. Such an estimate indicates that E{S₂(TICT)} is at least 11 kJ mol⁻¹ lower than S₁, very close the value estimated by TD-DFT calculations.²⁶ Since low temperature measurements have given a value of 260 kJ mol⁻¹ for E(S₁),³³ the expected value for E{S₂(TICT)} is 249 kJ mol⁻¹.

The formation of a HICT state in equilibrium with the S₂(TICT) state, as proposed by Kwok et al.,³⁴ justifies this. This state involves hydrogen bonding between the diethylamino group and the solvent. Since the S₁(LE), S₂(TICT) and S₂(HICT) states must coexist, the fluorescence of I in protic solvents should be triple and not dual. However, considering that the S₂(HICT) state must decay preferentially by internal conversion due to the increase in the vibronic coupling between the excited molecule and the solvent, its direct observation by fluorescence spectroscopy could well be very difficult. In spite of this, an obvious reduction in the Φ_F values can be observed for alcohols more susceptible to forming hydrogen bonds with the diethylamino group (methanol, ethanol and ethylene glycol), and for some other protic solvents (water, N,N-dimethylformamide and dimethyl sulfoxide) and SDS micelles, proportional to their polarities (Table 1 and Fig. 5). In recent work, Kwok et al. used time-resolved IR spectroscopy to demonstrate the existence of a hydrogen-bonded excited state, designated HICT, in equilibrium with the S₂(TICT) state for 4-dimethylaminobenzonitrile in methanol.³⁴

In water–dioxane mixtures, fluorescence suppression is observed, principally if the proton concentration in the medium increases.³³ Estimating the fluorescence quenching constant due to the interaction between coumarin and water in these mixtures (Fig. 7) gives a value of 3.09 × 10⁶ L mol⁻¹ s⁻¹, typical of charge-transfer processes,^21,22 indicating that the formation of the S₂(HICT) state from S₂(TICT) is a very favourable process in protic solvents.

Fig. 7 Stern–Volmer plot for the fluorescence quenching of I by water, in water–dioxane mixtures. K_SV = 2.56 × 10⁻² L mol⁻¹.

On the other hand, the polar interactions between the solvent and other parts of the molecule are not determining for the induction of routes capable of competing with the solvent–diethylamino group interactions for excited-state deactivation, as shown by the fluorescence data obtained for 3-(benzoxazol-2-yl)chromen-2-one.¹⁹

3.3 Excited states

The evaluation (PM3 CI) of the energy gap between S₁ and T₁ states (ca. 81 kJ mol⁻¹) shows that the population of the triplet state should, in principle, be unfavourable. However, although no detectable phosphorescence could be observed in measurements at low temperature, analysis of the Φ_F values at 77 and 298 K in methylcyclohexane shows that Φ_ST is not negligible. Since Φ_F = 0.90 at 77 K, whereas it is 0.64 at 298 K, and Φ_IC ≈ 0.26, then Φ_ST = 0.10.

Since Φ_ST is not that small and the singlet oxygen quantum efficiency, measured in chloroform, is very small (Φ_Δ = 0.06), it is possible to state that the T₁ and S₁ orbital symmetries must be different.^22,23 Thus, as S₁ is a ¹(π,π*) state, T₁ must be a short-lived ¹(n,π*) state. This is reasonable considering that ¹(π,π*) → ³(n,π*) intersystem crossing is a spin–orbit favoured process,^22,23 and that this compensates for the considerable energy gap between T₁ and S₁. On the other hand, it is known that T₁(n,π*) ketones are not usually good singlet oxygen sensitisers due to their short lifetimes.³⁵ Therefore, considering the above-mentioned results, the occurrence of intersystem crossing from the electronically excited aminocoumarins cannot be generalised as being negligible, as proposed by Priyadarshini et al.³⁶

4 Conclusions

The S₀ → S₁ transition of I is ¹(π,π*) with a ¹(n,π*) perturbative component, due to the electronic coupling between the nitrogen of the diethylamino group and the coumarin ring, combined with the electron-withdrawing action of the benzoxazole in the 3-position. The ¹(n,π*) component must be related to an S₂ state very close to S₁. The small energy gap between these two states (ca. 0.8 kJ mol⁻¹) makes vibronic interactions between them feasible, enabling configurational changes in the S₂ state, which leads to an S₂(TICT) state. The formation of this state causes partial electronic decoupling between the diethylamino group and the rest of the molecule, which can be increased by specific interactions with the solvent. These interactions, mainly hydrogen bonding involving the diethylamino group, result in an S₂(HICT) state in equilibrium with the S₂(TICT) state. In this case, internal conversion is favoured as the excited-state deactivation pathway due to the increase in vibronic coupling. The twist angle for the rotation of the diethylamino group relative to the coumarin plane was estimated as 56° for an isolated molecule in the S₂(TICT) state.

Compound I is fluorescent, exhibiting dual fluorescence, usually with high fluorescence quantum yields and considerable solvatochromism. The dual fluorescence is more evident in aprotic solvents. In protic solvents, only the band attributed to the emission from the S₂(TICT) state, red-shifted towards the S₁(LE) emission, is observed. TCSPC data show that the formation of the S₂(TICT) state is favoured in both kinds of solvents. An estimate of the energy difference between these states gave a value around 11 kJ mol⁻¹, a result which is supported by quantum chemical calculations.²⁶ Furthermore, E{S₁(LE)} ≫ E(T₁). Considering that Φ_ST is not negligible (Φ_ST = 0.10), and based on the small Φ_Δ value in chloroform, T₁ must be a ³(n,π*) state.

The behaviour observed for compound I in different kinds of solvents makes this compound suitable for application as a fluorescent probe or laser dye; it exhibits the requisite lasing characteristics (i.e. high fluorescence yield, low intersystem crossing and low tendency to aggregate), making useful as an active medium for a tunable laser for a wide range of solvents.

A more detailed theoretical investigation of the state energies and the relationship to solvent polarity, aimed at obtaining detailed information on the solvent effects on ΔE(S₁,S₂) and, consequently, on the formation of the S₂(TICT) and S₂(HICT) states, is under way.

Acknowledgements

The authors acknowledge the FAPEMIG, FAPESP and CAPES Foundations and the CNPq for research grants and Fellowships. The authors are grateful to Professors Ira M. Brinn and David E. Nicodem (LERT/IQ/UFRJ, Brazil) for access to time-resolved measurement facilities. Thanks are also due to Dr Ana Maria F. Oliveira-Campos (Univ. Minho, Portugal), who kindly furnished the coumarin derivative studied herein.

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