Different mechanisms of ultrafast excited state deactivation of coumarin 500 in dioxane and methanol solvents: experimental and theoretical study

Abstract

Solute–solvent intermolecular photoinduced intramolecular charge transfer (ICT) and twisted intramolecular charge transfer (TICT) states are proposed to account for the unusual properties of coumarin 500 (C500) in 1,4-dioxane (Diox) and methanol (MeOH) solvents. Our femtosecond transient absorption experiment on C500 shows that in Diox, there exists a single exponential component with a time constant of τ₁ ∼ 1.4 ps, however in MeOH two exponential components with lifetimes of τ₁ ∼ 0.5 ps and τ₂ ∼ 8.0 ps are observed. DFT and TDDFT methods were used to optimize the geometries of complexes C500–Diox and C500–(MeOH)₃ in the ground and excited states, respectively. They show that the rapid decay time could be due to ICT and ICT → TICT could be responsible for the slow decay time. Strengthening of the hydrogen bond N–H⋯O–H and the weakening of the hydrogen bond N⋯H–O in the excited state of the C500–(MeOH)₃ complex could facilitate the process of ICT from the 7-NHEt group to the CF₃ group and induce the formation of the TICT state in hydrogen bonding with MeOH. Together, the experimental and theoretical results reveal that C500 exhibits unusual deactivation mechanisms in Diox and MeOH solvents.

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1. Introduction

The coumarin dyes, generally known as the derivatives of 1,2-benzopyrone, were an important class of well-known laser dyes for the blue-green region.^1–7 The 7-aminocoumarin dyes were special among the coumarin dyes, due to their excellent laser activity and great applicability in studying many processes of physicochemistry.^1–7 The 7-aminocoumarin dyes often had very strong fluorescence with their fluorescence quantum yields approaching unity.^1–10 In these compounds the deexcitation of fluorescence occurred to some extent via internal conversion (IC) and negligible intersystem crossing (ISC).^8–10 During photoexcitation, most 7-aminocoumarin dyes showed large changes in dipole moment when going from ground state to their electronic excited states.^1–10 All 7-aminocoumarin dyes exhibited large solvatochromic shifts in their absorbance and fluorescence spectra.^8–17 The remarkable solvatochromic properties of these dyes can be attributed to intramolecular charge transfer (ICT) from the electron donor amino group to the electron acceptor carbonyl group.^18,19

Coumarin 500 (C500, Fig. 1A) was a typical 7-aminocoumarin dye that was widely used as a fluorescence probe doped into varied systems and has been researched using many steady-state and transient-state spectroscopic methods.^20–38 A few years back, Nad et al. studied the photophysical properties of C500 in a series of solvents, from nonpolar solvents to polar solvents and alcoholic solvents. They measured the optical absorption and fluorescence spectra of C500 in different solvents and the results indicated that C500 exhibited unusual spectral behaviors in nonpolar solvents. Importantly, the Stokes shifts and fluorescence lifetimes for C500 were found to be unusually high. Also the fluorescence quantum yields and radiative decay rate constants for C500 were seen to be unusually low in alcoholic solvents, which indicated that the solute–solvent hydrogen bonding had a strong influence on the photophysical properties of C500.³⁹ Recently, the rotational reorientation dynamics of C500 in methanol, dimethyl formamide, and dimethyl sulfoxide were investigated using femtosecond time-resolved stimulated emission pumping fluorescence depletion spectroscopy using a 400 nm pump pulse and a 490 nm probe pulse. Zhou et al. had proposed that the rapid anisotropy decays may be associated with solute–solvent intermolecular hydrogen bonding.⁴⁰ In fact, Zhao et al. demonstrated that the electronically excited state early time hydrogen bonding dynamics of coumarin 102 were closely related to many important phenomena such as internal conversion (IC), intermolecular charge transfer and fluorescence quenching in hydrogen bonding solvents.^41–48 However, the influence of solute–solvent intermolecular hydrogen bonding on the unusual behavior of C500 has not been investigated so far. In order to obtain complete photochemical information, it was necessary to explore the spectral properties of the broadband transient absorption of C500 in a hydrogen bonding solvent system.

Fig. 1 Molecular structures of (A) C500 and (B) Diox.

C500 may accept hydrogen bonds at the nitrogen lone pair and the carbonyl group from hydrogen bond donating solvents, and C500 may also establish such bonds with hydrogen bond accepting solvents from the H-atom on the amino group, as shown in Fig. 2.³¹ 1,4-Dioxane (Diox, Fig. 1B) and methanol (MeOH) have a similar polarity, but their ability to form different types of hydrogen bonds is different; MeOH has hydrogen bond acceptor and hydrogen bond donor properties, while Diox only has the hydrogen bond acceptor property.³¹ In this work, we have presented a femtosecond transient absorption study on the excited state dynamics of C500 in Diox and MeOH solvents. In addition, we have also employed quantum chemical calculations to simulate the two different environments. The results from the above experiments and calculations have been used to aid understanding of the different ultrafast excited state deactivation mechanisms of C500 in two different hydrogen bonding systems.

Fig. 2 Three positions to form hydrogen bonds in C500.

2. Methods

2.1. Experimental methods

Ultrapure grade C500 was purchased from Exciton (USA) and used without further purification. The Diox and MeOH used were spectrum pure quality reagents. The concentration of C500 in Diox and MeOH was 50 μM.

The steady-state absorption and fluorescence spectra were measured using a Cary 500 UV-Vis-NIR Spectrophotometer and a Cary Eclipse Fluorescence Spectrophotometer (Varian), respectively. The ultrafast dynamics of C500 in Diox and MeOH were measured using the transient absorption technique. The details of the experimental setup have been described elsewhere.⁴⁹ Briefly, the system consisted of a fundamental laser using a Ti:sapphire pulse pumped oscillation and a regenerative amplifier (Spectral Physics) both operating at 1 kHz repetition rate. The output wavelength of the system was 800 nm where the full width at half maximum (FWHM) was 90 fs and the output power was approximately 600 mW. The fundamental laser was separated into two beams in the ratio 9 : 1.

The more intense beam was used for generating the second harmonic (λ_exc = 400 nm) of the fundamental laser using a 0.5 mm BBO (β-BaB₂O₄, Fujian Castech Crystals Inc. China), and was applied as the pump laser for exciting the samples. The energy per pump pulse at the sample was about 2 μJ. The other beam passed through a controlled delay line and then focused onto a sapphire plate to generate a sub-picosecond white-light supercontinuum, which served as the probe laser. Two laser beams were incident on the sample in a 0.5 cm quartz cuvette at a small angle (θ ≤ 5°). The fused quartz sample cell was placed in the beam path where the beam diameter was 2 mm. The sample cuvette was stirred using a magnetic stirrer bar during the data acquisition. The FWHM of the correlation function of the two beams was 153 fs and the intrinsic temporal resolution was 7 fs. The intrinsic resolution of the fiber optics coupled multichannel spectrometer with CMOS sensor was 1.5 nm.

The transient absorption spectra were measured at room temperature between 420–650 nm and were corrected for group velocity dispersion of the white light continuum.

2.2. Computational methods

The ground state geometric structures and energies of both isolated monomers and solute–solvent hydrogen-bonded complexes were calculated using the density functional theory (DFT) method with the generalized gradient approximation (GGA) for exchange correlation potential BP86. Vertical singlet-state excitation energy calculations and excited state geometry optimizations were performed using the time-dependent density functional (TDDFT) method and the same exchange correlation potential that was used for the ground state calculations.^50–61 The resolution of the identity (RI) approximation was also used to improve the efficiency without sacrificing the accuracy of the results.^51–53 Previous electronic structure computations were performed with the Turbomole program suite.^50–55 The correction for energy level order of hydrogen-bonded complexes was calculated using another exchange correlation potential, CAM-B3LYP, which was performed with the Gaussian 09 program suite.⁶² The triple-ζ valence with one set of polarization functions (TZVP) was chosen as the basis sets throughout.⁵⁴

3. Results and discussion

3.1. Steady-state spectra and electronic excitation energy calculations

The normalized steady-state absorption and fluorescence spectra of C500 in Diox and MeOH solvents are shown in Fig. 3. The absorption spectrum of C500 in Diox reveals an absorption band centered at 376 nm. In MeOH, the broad absorption band is centered at 392 nm, which is in good agreement with the results of Jain and Nad.^31,39

Fig. 3 Normalized steady-state absorption and fluorescence spectra of C500 in (A) Diox and (B) MeOH solvents. λ_exc = 400 nm. c = 50 μM.

C500 has three positions at which it can form hydrogen bonds, through carbonyl oxygen: C=O⋯H–O (Type A), amino hydrogen: N–H⋯O (Type B) and amino nitrogen: N⋯H–O (Type C) shown in Fig. 2, where Diox may only form a hydrogen bond in position B and MeOH may form three kinds of hydrogen bonds in all positions.^31,63 So we focused on the solute–solvent hydrogen-bonded complexes C500–Diox and C500–(MeOH)₃ in the gas phase to simulate the environments of C500 in Diox and MeOH solvents, respectively. The electronic excitation energies and the corresponding oscillator strengths obtained from the TDDFT calculations on the complexes C500–Diox and C500–(MeOH)₃ in the low-lying electronic excited states are listed in Table 1. One can see that the oscillator strength corresponding to the S₁ state of complex C500–Diox is the largest, which means the absorption maximum of C500–Diox is located in the S₁ state, and the calculated absorption spectrum of C500–Diox is centered at 427 nm. However with complex C500–(MeOH)₃, the S₂ state has the highest oscillator strength, hence the theoretical value of the C500–(MeOH)₃ absorption maximum is 420 nm, which also fitted with the value of 392 nm measured by experiment. Using frontier molecular orbital calculations (Table 1), it was found that both the S₁ state of C500–Diox and the S₂ state of C500–(MeOH)₃ are ICT states and the calculated S₁ state of complex C500–(MeOH)₃ is a dark state, which corresponds to an intermolecular CT state (see Section 3.3) based on ref. 40.

Table 1 Calculated electronic excitation energies (nm), the corresponding oscillation strengths and frontier molecular orbitals (FMOs) for hydrogen-bonded complexes C500–Diox and C500–(MeOH)₃ in low-lying electronically excited states using BP86 (RI) and CAM-B3LYP

Functional	C500–Diox	C500–(MeOH)3
	BP86 (RI)	BP86 (RI)	CAM-B3LYP
S1	427 (0.220)	511 (0.000)	309 (0.501)
	H → L 87.4%	H-1 → L 96.1%	H → L 68.7%
S2	350 (0.000)	420 (0.227)	264 (0.001)
		H → L 84.2%	H-1 → L 56%
S3	347 (0.080)	405 (0.000)
S4	339 (0.000)	344 (0.080)
S5	313 (0.000)	336 (0.000)
S6	298 (0.022)	313 (0.001)
S7	274 (0.110)	312 (0.000)
S8	270 (0.000)	292 (0.001)
S9	253 (0.115)	290 (0.023)

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The fluorescence maxima in MeOH (500 nm) is significantly red-shifted compared to 463 nm in Diox (Fig. 1), which is in line with ref. 39. The Stokes shift in MeOH is 108 nm which is much larger than in Diox (74 nm). The most plausible explanation for this large stokes shift is ICT from the electron-donor 7-NHEt group to the electron-acceptor CF₃.³⁹

3.2. Transient absorption measurement

The transient absorption spectra of C500 in Diox and MeOH solvents are measured with the excitation at 400 nm. The early spectral evolution of C500 in Diox (Fig. 4A) from 0 to 8.8 ps shows a strong positive band located around 430 nm and a broad negative band in the 440–650 nm region. A comparison of the spectra of the transients with the steady state absorption and fluorescence spectra of C500 in Diox and MeOH identifies the negative signal as stimulated emission and the positive signal as excited state absorption. And likewise, Fig. 4B shows the transient absorption spectrum of C500 in MeOH from 0 to 11.8 ps where the excited state absorption at 425 nm is slightly blue-shifted and the stimulated emission peak from 466–523 nm is red-shifted compared to that in Diox.

Fig. 4 Transient absorption spectra of C500 in (A) Diox and (B) MeOH solvents, registered at different time delays after excitation. λ_exc = 400 nm. c = 50 μM.

Kinetic traces for selected probe wavelengths at 426 and 480 nm of the transient absorption for C500 in Diox solvent are shown in Fig. 5A. The absorption band at 426 nm and the gain band at 480 nm grow rapidly within 1–2 ps. It shows that most of the C500 molecule located in the ground state is very rapidly converted in to a transient ICT state.

Fig. 5 Kinetics of transient absorption for C500 in (A) Diox and (B) MeOH solvents at 426 nm and 480 nm. The solid lines correspond to results of the least-squares fitting.

The kinetic curves of transient absorption for C500 in MeOH at 426 nm and 480 nm (Fig. 5B) also show a rapid increase within 1 ps which indicates conversion of the ground state C500 molecule to the transient ICT state similar to what is observed in Diox. However, unlike in Diox, the rapid absorption change at 426 nm and 480 nm is also followed by (at τ > 1.4 ps) a large shift in the stimulated emission maximum from 466 to 523 nm. This implies the existence of another intermediate state, which results in the broadening and lowering in energy of the gain band from 440–600 nm to 440–650 nm. From our calculations on the excited state of C500 (reported in the next section), we attributed this emerging second intermediate state to the twisting of the 7-NHEt group in C500 and the red-shifted fluorescence emission to the twisted intramolecular charge transfer (TICT) state of C500.

A global analysis was used where sets of kinetic curves were fitted simultaneously using exponential components with common lifetimes (Table 2). We attributed the monoexponential component with a lifetime of τ₁ ∼ 1.4 ps to ICT. The kinetic curves for C500 in MeOH were fitted to two exponentials with lifetimes of τ₁ ∼ 0.5 ps and τ₂ ∼ 8.0 ps. The biexponential model provided better fits to the experimental data, indicating that the contribution from the second component cannot be ignored. The two exponential components in MeOH were associated with ICT and ICT → TICT conversion as has been reported for coumarin 481 in ref. 64. In addition, our results are consistent with the suggestion that hydrogen bonding facilitates charge transfer mechanisms as suggested by Fiebig.⁶⁵

Table 2 Transient absorption kinetics lifetimes of C500 in Diox and MeOH solvents by global analysis

Solute	Solvent	τ1 (ps)	τ2 (ps)
C500	Diox	1.415 ± 0.2099 (14.8%)	—
	MeOH	0.4956 ± 0.1105 (22.3%)	8.065 ± 0.8228 (10.2%)

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To obtain further insight into the different ultrafast excited state deactivation schemes for C500 in Diox and MeOH solvents, we plotted 3D images (Fig. 6) of transient absorption for C500 in Diox and MeOH solvents as a function of wavelength and time delay after excitation. We attributed the red and blue regions to transient absorption and stimulated emission, respectively. ICT absorption and ICT emission were observed in both solvents (Fig. 6), whereas TICT was observed only in MeOH. The large spectral differences could be attributed to differences in hydrogen bonding interactions between C500 and Diox and MeOH.

Fig. 6 3D Image plot of transient absorption for C500 in (A) Diox and (B) MeOH solvents as a function of wavelength and time delay after excitation.

3.3. Quantum chemical calculation

In order to test the validity of the above conclusion, we obtained optimized geometric structures of the hydrogen-bonded complexes C500–Diox and C500–(MeOH)₃ in the ground state (Fig. 7). In order to obtain deeper insight into the ICT process, we plotted the frontier molecular orbitals (FMOs) of the two complexes (Fig. 8). It can be clearly seen that both complexes have a larger electron density on the 7-NHEt group and a smaller electron density on the CF₃ group in the HOMO than in the LUMO. From the results listed in Table 1, it can be seen that both the S₁ state of complex C500–Diox and the S₂ state of complex C500–(MeOH)₃ correspond to the orbital transition from HOMO to LUMO and that ICT occurs from the 7-NHEt group to the CF₃ group in C500 (Fig. 8). Similarly, the intermolecular CT state, that is, the S₁ state of complex C500–(MeOH)₃, corresponds to the orbital transition from HOMO-1 to LUMO and indicates that intermolecular CT occurs between C500 and MeOH. It is worth noting that the S₁ and S₂ states of complex C500–(MeOH)₃ correspond to the orbital transition from HOMO-1 to LUMO and HOMO to LUMO, respectively, but the energy of the S₁ state (511 nm) is lower than that of the S₂ state (420 nm). On the other hand, the intermolecular CT state is a non-fluorescent state, which is not responsible for fluorescence quenching in the complex. So it appears that the energy of the intermolecular CT state in the complex may have been underestimated by the BP86 density functional. In order to obtain better estimates of charge transfer character and energies, we used the long-range corrected density functional and recalculated electronic excitation energies and FMOs for complex C500–(MeOH)₃ using CAM-B3LYP (Table 1).^61,66,67 Through a comparative analysis of the FMOs, we found that the new S₁ state was an ICT state, which corresponded to the previously calculated S₂ state of complex C500–(MeOH)₃, whereas the new S₂ state was an intermolecular CT state, which corresponded to the previously calculated S₁ state of complex C500–(MeOH)₃. In fact, the energy of the ICT state is lower than that of the intermolecular CT state after long-range corrected density functional calculations revealing that both the ICT state of C500–(MeOH)₃ and the ICT state of the C500 monomer are located in the first electronically excited state (S₁).

Fig. 7 Optimized geometric structures of hydrogen-bonded complexes C500–Diox (left) and C500–(MeOH)₃ (right) in the ground state. (A) Top view; (B) side view.

Fig. 8 Frontier molecular orbitals (FMOs) of hydrogen-bonded complexes C500–Diox (left) and C500–(MeOH)₃ (right).

From the optimized geometries obtained from DFT and TDDFT calculations using the BP86 functional, we obtained the lengths of the intermolecular hydrogen bonds in the ground state and the ICT state for complexes C500–Diox and C500–(MeOH)₃ (Table 3). For C500–Diox, Diox can only form a hydrogen bond with C500 in position B. The length of the hydrogen bond N–H⋯O (Type B) was calculated to be 1.999 and 1.902 Å in the ground state and ICT state, respectively. The hydrogen bond of type B is observed to be strengthened slightly in the ICT state. For complex C500–(MeOH)₃, MeOH can form three kinds of hydrogen bonds in all positions, where the length of the hydrogen bond C=O⋯H–O (Type A) was calculated to be 1.905 and 1.852 Å in the ground state and ICT state, respectively. The intermolecular hydrogen bond N–H⋯O–H (Type B), which is formed between the 7-NHEt hydrogen atom and hydroxyl oxygen atom and resides out of the plane of coumarin, was calculated to be 1.930 and 1.848 Å in the ground state and ICT state, respectively. We find that the hydrogen bonds of types A and B are strengthened in the ICT state. The intermolecular hydrogen bond N⋯H–O (Type C), formed between the 7-NHEt nitrogen atom and hydroxyl hydrogen atom is nearly perpendicular to the plane of coumarin. The length of hydrogen bond type C was calculated to be 2.028 Å in the ground state and it is the longest but the weakest of the three different kinds of intermolecular hydrogen bonds according to the results of the calculations. Also the length of the hydrogen bond of type C was calculated to be 2.404 Å in the ICT state and much weaker than that in the ground state. As shown in Fig. 7, the C500–Diox is nearly planar whereas the C500–(MeOH)₃ is not. Here it should be noted that the hydrogen bonds B and C play a very important role in changing the conformation of the coumarin plane. A possible explanation for a change in the conformation is that upon optical excitation of C500, as the electron density on the 7-NHEt group decreases, the hydrogen bond of type B strengthens and the hydrogen bond of type C weakens inducing the twisting of 7-NHEt, resulting in the formation of the TICT state. We found out the TICT state by calculation (Fig. 9), and the calculated dihedral angles between the plane of coumarin and the plane of 7-NHEt were calculated to be 4.2°, 2.8° and 78.4° in the ground, ICT and TICT state, respectively. One can see that in the structure of complex C500–(MeOH)₃ in the ICT state, the 7-NHEt group is almost in the plane of coumarin, whereas in the TICT state, the 7-NHEt group undergoes a remarkable 75.6° twist and is nearly perpendicular to the plane of coumarin in the TICT state (Fig. 9). These results provide clear evidence for ICT from the 7-NHEt group to the CF₃ group and the existence of the TICT state in MeOH.

Table 3 Calculated lengths (Å) of intermolecular hydrogen bonds in the ground state and ICT state for complexes C500–Diox and C500–(MeOH)₃

Type of HB	C500–Diox	C500–(MeOH)3
	Type B	Type A	Type B	Type C
GS	1.999	1.905	1.930	2.028
ICT	1.902	1.852	1.848	2.404
ICT minus GS	−0.097	−0.053	−0.082	0.376

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Fig. 9 The twisting of 7-NHEt group in C500–(MeOH)₃ from ICT to TICT state.

3.4. Excited state deactivation mechanism

On the basis of the steady-state as well as the transient absorption results, the different deactivation mechanisms for the excited state of C500 in Diox and MeOH are proposed and are shown in Fig. 10.

Fig. 10 Different ultrafast excited state deactivation schemes for C500 in (A) Diox and (B) MeOH solvents.

In Diox and MeOH solvents, where the hydrogen bonding effects are different, dramatic effects are observed in the fluorescence and transient absorption spectra of C500. When C500 is excited in Diox, it returns to the ground state emitting fluorescence with a maximum at 463 nm as a result of efficient ICT with a lifetime of τ₁ ∼ 1.4 ps. Upon photoexcitation of the dye in MeOH, C500 very rapidly converts to the ICT state with a faster lifetime of τ₁ ∼ 0.5 ps. The results from our quantum chemical calculations show that the dihedral angle between the plane of coumarin and the plane of the 7-NHEt group changes from 4.2° (HOMO) to 2.8° (ICT). Once in the ICT state, the electron density on the 7-NHEt group decreases concomitantly with the increase in strength of hydrogen bond B and the weakening of hydrogen bond C. This leads to a rotation of the 7-NHEt group which changes the torsional angle from 2.8° to 78.4° leaving the molecule in the TICT state characterized by the τ₂ ∼ 8.0 ps component. Finally, C500 in TICT returns to the ground state with a fluorescence emission maximum at 499 nm.

4. Conclusions

We have described here the excited state dynamics of C500 in Diox and MeOH solvents through transient absorption spectroscopy. It was found that the dynamics of C500 in Diox exhibited an absorption band at 420–440 nm and a gain band at 440–600 nm with a single exponential time constant of τ₁ ∼1.4 ps. However, in MeOH, an absorption band was observed at 420–440 nm, and a gain band was observed at 450–650 nm with double exponential time constants of τ₁ ∼ 0.5 ps and τ₂ ∼ 8.0 ps. The short decay time may be associated with ICT and the long decay time may be related to the ICT → TICT conversion. We have also presented computational results at the TD-BP86/TZVP and CAM-B3LYP/TZVP level including the optimized geometric structures, energies and FMOs of solute–solvent hydrogen-bonded complexes C500–Diox and C500–(MeOH)₃ in the ground and excited states. Our results from TDDFT calculations show that, for C500–Diox, the hydrogen bond of type B is observed to be strengthened slightly in the ICT state. For C500–(MeOH)₃, while the hydrogen bonds of types A and B are strengthened, the hydrogen bond type C is weakened, resulting in the rotation of the 7-NHEt group which has nearly planar (4.2°) geometry in the ground state and nearly perpendicular (78.4°) in the TICT state. Most importantly, we have shown that the change in hydrogen bonding may facilitate ICT from the 7-NHEt group to the CF₃ group, which induces the formation of the TICT state in MeOH. Thus this study provides deeper mechanistic insights into the photochemistry of C500 in different hydrogen bonding solvents.

Acknowledgements

This work is supported by the National Basic Research Program of China (973 Program, Grant no. 2013CB922200), the National Natural Science Foundation of China (Grant no. 11174106, 10974069) and the Natural Science Foundation of Jilin Province of China (Grant no. 201115018).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra09773d

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