Kankan Bhattachayya and his students Rajdeep Chowdhury and Shyamtanu Chattoraj for their generous help in measuring the fluorescence upconversion. The results can be attributed to possible modulation of the polarity or H-bonding environment around the acceptor in the mixture.
Introduction
- Factors affecting photoinduced electron transfer
- Free energy of photoinduced electron transfer
- Substituent effect on photoinduced electron transfer
- Solvation dynamics
- Hydrogen bonding: Ground state vs excited state
- Effect of H-bonding on photoinduced electron transfer in neat electron donating
- Photoinduced electron transfer in solvent mixture: Regular and anomalous
- Aim and scope of the present work
In Chapter 5, we demonstrated that the anomalous PET behavior is preserved even in a similar polarity mixture of aniline (AN) and N,N-dimethylaniline (DMA). 20 The scope of the current project in the context of the existing literature has been discussed.
Experimental and theoretical methods
Density functional theory (DFT)
Steady-state measurements
Time-resolved fluorescence measurements
- Time correlated single photon counting (TCSPC)
- Femtosecond up-conversion measurements
Analysis of the picosecond (TCSPC) and femtosecond (up-conversion) decays
Fourier transform infrared (FTIR) spectroscopy
Estimation of quantum yield of coumarins in solvent mixtures
Chemicals
- Donors
- Solvents
Preparation of samples
- Preparation of binary mixture solution
- Preparation of fluorophore solution
- Preparation of FTIR samples
Hydrogen-Bond induced photoinduced electron transfer of coumarin 102-
Results
- Time-dependent density functional theory (TDDFT) calculations
- Steady-state absorption and emission measurements
- Time-resolved fluorescence measurements
The minimum of the S2 and S1 potentials corresponds to shorter H-bonding than in the ground state. The energy, strength and nature of the transitions in the H-bonded complex can be obtained from the TDDFT calculations. In the C102-p-Cl-phenol complex, the transitions were similar to those of the C102-phenol complex.
This increase in the relative contribution of the fast component implied that the fluorescence decay is controlled by the H-bond regulated mechanism. The fluorescence decay of the C102-phenol complex was bi-exponential with two distinct components of 610-630 ps and 2.7-1.8 ns. The contribution of the slow component was very small (10-16%), which may be due to directly excited free C102 or the C102-phenol complex with broken H-bond i.
In the presence of p-Cl-phenol, the fluorescence decay was slightly faster than phenol at the same donor concentration (Table 3.5 and Table 3.6).
Discussion
It has already been mentioned that at λex = 405 nm only the hydrogen-bonded complex is selectively excited, whereas at λex = 375 nm both the free and H-bonded complexes are excited. From this state, the complex can relax radiatively to the ground state (S0) or undergo internal conversion (IC) to the S1 (CT) state. This nonradiative IC may be the main source of fluorescence quenching, and the fast component (∼600 ps) may be due to the dynamics of the process.
Zhao et al.75 first suggested that this local CT (or PET) in the H-bonded complex can lead to fluorescence quenching and is observed by us for the first time.152 Fluorescence quenching due to PET in the absence of a H-bond has been thoroughly studied in coumarin-dimethylaniline (DMA) systems. However, the oxidation potential of the phenol derivatives is much less favorable to PET than DMA, and C102 is known to be less susceptible to PET among all other coumarins. Therefore, the fluorescence quenching through PET without an H bond could be neglected for the phenol derivatives. This may be the reason for the lack of fluorescence quenching for anisole in cyclohexane and all phenol derivatives in acetonitrile and methanol. PET in the H-bonded system could be very easy due to the proximity of the donor-acceptor and the correct orientation of the donor-acceptor compared to the non-H-bonded system.
Consequently, fluorescence quenching is observed only in the H-bonded C102-phenol or C102-p-Clphenol pairs, but not for anisole, although all have similar oxidation potentials.
Conclusion
Enhancement of H-bond mediated photoinduced electron transfer of upon
Results
- Steady-state measurements
- Time-resolved measurements
The emission spectrum of C102 showed a gradual red shift with the addition of aniline to the non-polar solvent (cyclohexane or toluene). This can be derived by plotting the emission maxima of C102 at different mole fraction against the mole fraction of aniline (Figure 4.3). The fluorescence decay of C102 measured against corresponding steady-state emission maxima at different mole fractions of aniline in cyclohexane-aniline and toluene-aniline mixtures are shown in figure 4.5 and figure 4.6, respectively.
The left panel represents the decay of C102 up to 0.075 mole fraction of aniline, which becomes progressively faster. The right panel represents the decay of C102 from 0.075 to 1.0 mole fraction of aniline, which becomes progressively slower with aniline enrichment. The left panel represents the decay of C102 up to 0.13 mole fraction of aniline, which becomes progressively faster.
Right panel represents the decay of C102 from 0.13 to 1.0 mole fraction of aniline, which becomes progressively slower with enrichment of aniline.
Discussion
- Free energy of PET in the non-H-bonded complex
- Mechanism of PET in the H-bonded complex
- Unusual modulation of PET
However, such a calculation of the free energy in the mixture is not straightforward, but can possibly be taken as an intermediate between the two pure solvents. The rate of electron transfer () at different compositions of the liquid mixtures can be estimated as. The strengthening of the H-bond in the excited state should have an important effect in facilitating PET.
Since H-bonding-assisted PET is much more effective compared to non-H-bonding PET, the strength of the H-bonding may be the main factor in controlling the rate of PET. However, it is assumed that H-bonding in a nonpolar environment is much stronger than in polar solvents due to the solvation of the donor and acceptor. The faster PET in mixed solvents than in pure aniline can also be explained on the basis of competition vs .
However, at low mole fractions of AN, the presence of a large number of the non-interacting component (cyclohexane or toluene) can disrupt the AN-AN H-bond.
Conclusion
Anomalous photoinduced electron transfer of coumarin 102-phenol in
Results
- Steady-state fluorescence measurement
- Time-resolved fluorescence measurement
- Fourier transform infrared (FTIR) measurements
Second, the emission intensity of C102 in pure AN is about half of the emission intensity in pure DMA. The pronounced red shift of the emission maximum of C102 in AN compared to regular DMA should be accredited to the additional ability of AN to form H-bonds with C102. The fluorescence decays of C102 in pure AN and DMA were single exponential with time constants of 2.5 ns and 1.4 ns, respectively.
Variation of the mean fluorescence lifetime (<τ>) and quantum yield (QY) of aniline mixture with the mole fraction of aniline, XAN. Similar to the quantum yield variation, the fluorescence decays faster with increasing mole fraction of AN. In this equation, τ and τ0 represent the lifetime of C102 in the presence and in the absence of donor.
However, in the presence of a 0.74 mole fraction of AN, this new band (1703 cm-1) became more prominent, indicating the formation of a C102-AN H-bonded complex.
Discussion
Here we have observed anomalous tendency for PET modulation of C102 with XAN in the AN-DMA mixture. It is observed that the mole fraction shifted to higher values as the polarity of the solvent increased. To understand our results, we have considered the likely states of the donor (AN) and the acceptor (C102) in the mixture (Scheme 5.1).
On the other hand, the anomalous variations of the PET rate against XAN indicated that the PET rate was much higher in the 1:1 complex than in the higher order complex. In the higher order complex, additional AN-AN H-bonding may affect the strength or relative orientation of the key C102-AN H-bond. In pure DMA, the C=O stretching frequency of C102 was found to be 1731 cm-1, indicating that all C102 remains in the free form.
The red shift and broadening of the N–H stretching band at the higher AN mole fraction compared to the lower mole fraction may be due to the formation of the AN–AN H bond.
Conclusion
Reduced fluorescence quenching of coumarin 102 at higher phenol mole
Results
- Steady-state measurements
- Time-resolved measurements
- Fourier transform infrared (FTIR) measurements
However, the absorption spectrum of C102 in the cyclohexane-anisole mixture exhibited red shifts with increasing mole fraction of anisole without any isosbestic points. In (a, b) the fluorescence intensity of C102 decreases with increasing phenol mole fractions up to a certain mole fraction, then increases with further phenol enrichment. However, the fluorescence intensity of C102 in the cyclohexane-anisole mixture decreased linearly with increasing mole fraction of anisole (Figure 6.4).
Since anisole has no H-bonding ability, the abnormal modulation of C102 fluorescence in cyclohexane-phenol and anisole-phenol mixtures may be related to the modulation of H-bonding in the mixture. By increasing the mole fraction of phenol, the average decay time of C102 gradually decreases until the same (XPhOH = 0.013) mole fraction at which the quantum yield was minimal (Figure 6.7). However, in the case of the mixture of cyclohexane and anisole, no significant change in the mean fluorescence lifetime of C102 is observed with an increase in the molar fraction of anisole (Table 6.5).
In order to investigate the nature of H-bonding in the ground state, we measured the FT-IR spectra of C102 in the C=O stretching frequency range at several mole fractions of phenol (Figure 6.8).
Discussion
These phenol-phenol H-bonds can weaken or hinder the central C102-phenol H-bond to reach the optimal state required for PET. Therefore, the competitive nature of the C102-phenol and the phenol-phenol H bond may reduce the PET quenching. To distinguish between the two possibilities (competitive H-bonding vs. polarity-induced H-bonding modulation), we have chosen anisole-phenol solvent mixture.
Since anisole does not possess H-bond donating ability, it cannot compete with the C102-phenol H-bond, but it can form H-bonds with phenol and thus reduce the phenol-phenol H-bond affinity when present in large amounts. . From this we can say that PET with the help of H-bonding has a pronounced effect on the anomalous modulation of C102 fluorescence in the two solvent mixtures - cyclohexane-phenol and anisole-phenol. FT-IR spectroscopic investigation shows that the H bond between C102-phenol is indeed modulated differently in the low-phenol and high-phenol regions in the ground state.
It can therefore be inferred that the C102-phenol H-bond becomes weaker at higher donor concentration due to self-association of several donors.
Conclusion
An inert component mixed with donor solvent can make ultrafast H-bond
Results
- Steady-state fluorescence measurements
- Time-resolved fluorescence measurements
- Fourier transform infrared (FTIR) measurements
Addition of inert co-solvent leads to different modulation of PET depending on the binding capacity of the donor. Addition of DMA to C153 dissolved in cyclohexane results in a continuous decrease in emission in. The shift in the emission maximum of C153 is indicative of the polarity modulation in the mixture, while the quenching of the fluorescence is due to PET from the donor solvent to excited C153.
The shift in the modulation emission maximum in the mixture as the fluorescence quenches is due to PET from the donor solvent to the excited C153. In the cyclohexane-DMA mixture, the fast component (2-3 ps) remains almost invariant to the amount of donor while the other component becomes faster with increasing donor mole fraction. The fast component may be due to PET from donors in the vicinity of the excited acceptor, while the slower component may arise due to diffusion of the donor within the excited state lifetime of C153.
The intensity of the band at 1736 cm-1 increases gradually with increase in the mole fraction of aniline.
Summary and conclusion
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