Engineering the Excited-State Dynamics of 3-Aminoquinoline by

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Engineering the Excited State Dynamics of 3Aminoquinoline by Chemical Modification and Temperature Avinash Kumar Singh, Srijon Ghosh, Rajesh Kancherla, and Anindya Datta J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.6b09939 • Publication Date (Web): 28 Nov 2016 Downloaded from http://pubs.acs.org on December 6, 2016

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The Journal of Physical Chemistry

Engineering the Excited State Dynamics of 3-Aminoquinoline by Chemical Modification and Temperature

Avinash Kumar Singh, Srijon Ghosha, Rajesh Kancherla, Anindya Datta*

Department of Chemistry, Indian Institute of Technology Bombay Mumbai 400076, India e-mail: [email protected], Phone: +91 22 2576 7149, Fax: +91 22 2576 7152

a

Present address : Indian Association for the Cultivation of Science, Jadavpur, Kolkata-700032, India

1 ACS Paragon Plus Environment


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Abstract The role of the amino group in the excited state dynamics of 3-Aminoquinoline (3AQ) has been investigated, by comparison with its synthetic derivative, 3-(piperidin-1-yl)quinoline (3PQ). Absence of the amino hydrogen atoms in 3PQ eliminates, to a large extent, the complexity of the excited state processes observed in 3AQ. Polarity of the medium is found to be the most important determinant in the nonradiative rate constants of 3PQ, unlike in 3AQ where hydrogen bonding plays the most significant role. The nonradiative rate constants decrease with increase in micropolarity. This trend is opposite to what is usually observed with dipolar states. Temperature dependence of the fluorescence spectra and lifetime has been studied in order to understand this unexpected observation. An unusual red shift in the emission of 3AQ and 3PQ is observed in nonpolar media at low temperature. This is surprising, as a process involving a barrier is expected to get hindered at low temperature and be manifested in a blue shift of the spectra, due to the predominance of the locally excited state. Moreover, the variation of emission maxima of 3AQ with temperature is sigmoidal in nature, indicating the involvement of two distinct states. The counterintuitive observation of the predominance of the state with comparatively lower emission energy, at low temperature, establishes the following: the photophysics in 3AQ is dominated by a locally excited state at room temperature in nonpolar media. This state is associated with rapid flip flop of the amino group, which provides an efficient nonradiative channel of deactivation. At low temperature, this flip flop is hindered and the molecule can undergo intramolecular charge transfer (ICT), whereby the lower energy state is populated. In case of 3PQ, the ICT state is the only one present, owing to the tertiary amino group.


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Introduction The photophysics of quinoline and its derivatives has intrigued the scientific community for almost half a century.1–30 The longstanding interest in this class of molecules stems from its potential applications in the fields of sensors, dyes and medicines.31–35 The ring nitrogen of quinolines plays a crucial role in the ordering of the energy states. The lowest excited singlet state in quinoline is nπ* in non polar solvents, while it is ππ* in protic solvents. Consequently, in nonpolar solvents, the fluorescence of quinoline is quenched effectively due to a crossover from the locally excited ππ* state to the nonemissive nπ* and finally, to a triplet state by intersystem crossing (ISC).22 Low temperature phosphorescence has been found to be the primary radiative process in nonpolar solvents. In protic media, however, fluorescence is observed in addition to phosphorescence, due to the interchange of the order of energy of the nπ* and ππ* states.21,36 Introduction of functional groups into the quinoline backbone alters the photophysics significantly. Hydroxyquinolines are a prominent example in this context.8,37,38 Excited state intramolecular proton transfer in these molecules has been studied extensively.39–44 Aminoquinolines are another class of compounds that are interesting by virtue of their apparently anomalous photophysics45 and promise as photobases.2,46 Excited state dynamics of this class of molecules has not been studied in as much detail as hydroxyl- and methoxyquinolines. For other classes of molecules, like coumarins, excited state dynamics is known to be affected significantly by the introduction of the amino group.47–51 An interplay of charge transfer from the amino group and its “flip flop” motion is believed to govern the photophysics in these molecules.50



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The excited state dynamics of 3-aminoquinoline (3AQ) and their derivatives at room temperature in different solvents has been studied previously in our group.45,52 The remarkably low quantum yield in non polar solvents have been attributed to a non polar locally excited (LE) state, from which a facile nonradiative relaxation pathway is provided by the flip flop motion of amino group. The nonradiative rate constant is found to be smaller in more polar solvents and particularly so in protic solvents. These observations have been rationalized by considering an intramolecular charge transfer (ICT) process, which is favored in polar solvents and which leads to a hindrance of the flip flop motion. Hydrogen bonding in protic solvents is believed to further contribute to the hindrance to the flip flop of the amino group, leading to a greater extent of decrease in nonradiative rate and increase in fluorescence quantum yield. More recently, 3AQ has been reported to exhibit enhanced fluorescence in polymer matrices, especially when hydrogen bonding interactions are present.53 However, no direct evidence of switchover from one of these excited states to another is available. This is the primary motivation for the present work, in which we have studied the effect of chemical substitution and change in temperature on the photophysics of 3-AQ. A derivative of 3AQ has been prepared by substituting the amino hydrogen atoms to obtain 3-(piperidin-1-yl)quinoline (3PQ), with the aim to eliminate or decrease the effect of hydrogen bonding on the excited state dynamics of the fluorophore and to investigate if the chemical substitution simplifies the photophysics of the molecule. Besides, the temperature dependence of the emission spectra and lifetimes of 3AQ and 3PQ has been investigated in order to understand if the process responsible for nonradiative relaxation of 3AQ is associated with a barrier and if it is, how the interconversion of the LE and ICT states can be engineered by varying the temperature.54–56


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Materials and Methods

Materials: 3-aminoquinoline (3AQ) from Sigma Aldrich, USA was used as received. The N,Ndisubstituted derivative of 3AQ, 3-(piperidin-1-yl)quinoline (3PQ) was synthesized using Buchwald-Hartwig cross coupling of 3-bromoquinoline and piperidine using literature procedure with some modifications (Scheme 1, Supporting Information).57 The compound was recrystallized twice from n-hexane prior to the spectroscopic studies. Spectroscopy grade solvents were used for the solvatochromic studies. Spectroscopy grade 3-methylpentane (3MP) from Sigma Aldrich was used without further purification for the low temperature studies. For all the measurements the concentration of molecules in solution was maintained at ~ 10-6 molar.

Figure 1. Chemical structures of 3-aminoquinoline (3AQ) and 3-(piperidin-1-yl)quinoline (3PQ) Steady state and Time-resolved Fluorescence Experiments: UV-Visible absorption spectra Emission spectra were recorded on a JASCO-V530 spectrophotometer. The absorbance at the maximum was kept at about 0.05. were recorded on a Varian Cary Eclipse spectrofluorometer, with 5 nm bandpass for both excitation and emission monochromators. Fluorescence quantum yields were calculated using quinine in 0.5 M sulfuric acid sulfate as the reference (ϕf = 0.55) (additional details in supporting information). Fluorescence decays were recorded on Time Correlated Single Photon Counting (TCSPC) setup from IBH Horiba Jobin Yvon (FluoroCube).



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The sample was excited with 336 nm NANOLED source with a repetition rate of 1MHz (Horiba NanoLED). The IRF of the instrument was approximately 700 ps. The emission was collected at magic angle (54.7°) polarization with respect to the excitation light. Fluorescence decay traces were fitted to either single exponential or sum of exponential functions, as shown below, by iterative reconvolution using IBH DAS 6.2 software. , = ∑

(1)

Where, and are lifetime and amplitude of the ith component, respectively. Low temperature Measurements: For the low temperature steady state as well as the Time resolved measurements, the sample was taken in a cuvette and placed in an Oxford cryostat OptistatDN-V258,59 coupled with a temperature controller. The spectra and the decays were recorded on the IBH Horiba Jobin Yvon (FluoroCube) TCSPC setup, after allowing the system to stabilize at the desired temperature. The glass forming solvents 3-methypentane (3MP) and ethanol were used to dissolve the fluorophores. They were checked for impurity emission at low temperatures and were found to be clean. Quantum chemical calculations : Gaussian 09 package was used for quantum chemical calculations.60 B3LYP level of theory was used with 6-31+G(d,p) as the basis set. Relaxed scans (geometry optimization at each fixed dihedral angle) were performed for 3AQ and 3PQ at an interval of 10 degrees. Results and Discussion Comparison of the photophysics of 3AQ and 3PQ at room temperature: Steady state absorption and fluorescence spectra of 3AQ have been discussed at length in earlier publications 6 ACS Paragon Plus Environment

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from our group.45,52,61 We have reproduced the results once again, so as to perform back to back experiments for comparison with 3PQ. Briefly, the energy of the absorption maxima of 3AQ decreases linearly with the increase in the polarity of the solvent, even though the linear correlation is not very strong. Emission spectra are significantly Stokes shifted, but the Stokes shift is lesser in the nonpolar solvents than the trend expected from the Stokes shifts in polar/protic solvents. Some amount of bunching is observed for the polar protic solvents. The absorption spectra of 3PQ are almost similar to those of 3AQ in the same solvent. The emission spectra of 3PQ, however, are more significantly red shifted compared to those of 3AQ, indicating the involvement of an excited state that is more dipolar than that of 3AQ (Figure 2, Figure S1and Table S1).

Figure 2. Absorption (black) and fluorescence (blue) for 3AQ (dashed line) and 3PQ (solid line) at room temperature. The spectra of 3AQ obtained in the present experiment are in agreement with those reported in Reference (45) and are shown here for the sake of comparison with the spectra of 3PQ 7 ACS Paragon Plus Environment


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The Stokes shift for 3PQ increases with the increase in polarity of the solvent, similar to the behavior of 3AQ. This observation fortifies the contention that the emissive state in this class of molecules is more dipolar than the ground state. Plots of absorption and emission peak frequency against the orientation polarizability of the solvent (Figure S2) further support this notion. The comparatively greater extent of Stokes shift in 3PQ than in 3AQ indicates a greater polarity of the excited state, induced by the N,N-disubstitution. Indeed, the change in dipole moment upon excitation (Δ = ∗ − , where and ∗ are the ground and excited state dipole moments, respectively) is found to be 4.44 D for 3PQ, while the value is 2.20 D for 3AQ (Figure S3), from Lippert-Mataga plot.62 The data for the nonpolar and polar protic solvents have not been included in this calculation, in order to obtain a better linear correlation. If all the data points are retained, then the values are 3.20 D and 5.00 D for 3AQ and 3PQ respectively, but the fit is much poorer, presumably because the dipolar excited states are not favored in nonpolar solvents, while specific interactions with protic solvents tend to modulate the excited state beyond the realms of simple Onsager model, which is the basis of Lippert Mataga equation.63 The calculated value of Δ of 3AQ is only slightly lesser than the value of 2.34 D reported earlier, which was calculated using Bakhshiev’s formula, considering all the solvents.64 The increased value of Δ in 3PQ may be ascribed to the greater degree of intramolecular charge transfer due to the replacement of the primary –NH2 group by the N,N-disubstituted amino group, which is more electron pushing in nature. The photophysics of 3AQ is known to be governed not only by polarity, but also by the hydrogen bonding ability of the solvent. In such cases, linear solvation energy relationships like the one formulated by Kamlet and Taft are most useful.65,66 The Kamlet-Taft equation relates the


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absorption and emission peak frequency to the solvent parameters like polarity, hydrogen bond accepting ability and hydrogen bond donating ability . = ∗ + + +

(2)

Where, ν and represent peak frequencies in presence and absence of solvent respectively. ∗ , β and α represents polarity, hydrogen bond accepting ability and hydrogen bond donating ability of the solvent. , and are coefficients for respective solvent parameters. Kamlet-Taft analysis has been performed using absorption and emission peak frequencies on 3PQ as well as 3AQ by multiple regression fitting using Origin 8.0 software. For 3AQ, the value of the coefficients, obtained in the present experiment, are close to those reported earlier (Table 1). The solvent polarity, π*, is found to be the predominant factor that governs the absorption and emission maxima of 3PQ, very unlike the case in 3AQ. This observation can be rationalized by the involvement of the more dipolar excited state in 3PQ, the existence of which is established earlier in this paper. Moreover, for 3PQ, the α parameter, which indicates the hydrogen bond donating ability of the solvent, plays a more predominant role than the β parameter, which denotes the hydrogen bond accepting capability of the solvent. This is exactly opposite to the observation in 3AQ. The predominance of the α parameter in the photophysics of 3PQ can be rationalized by the absence of the H atoms bonded with the nitrogen atom, unlike in 3AQ, which makes the lone pairs on the nitrogen atoms the only candidates for participation in hydrogen bonds with the solvent molecules. Interestingly, the emission maxima have some dependence on the β parameters, which perhaps indicates the protonation of one or both of the nitrogen atoms in the excited state of the molecule.



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Table 1. Kamlet-Taft parameters for 3AQ and 3PQ Molecule 3AQ 3PQ

Spectral maxima in cm-1 Absorption Emission Absorption Emission

π*

β

α

ν0 (cm-1)

Ra

-833 (0.48) -1304 (0.43) -800 (0.70) -2295(0.66)

-779(0.45) -1324(0.43) -35(0.03) -427.32(0.12)

-123(0.07) -440(0.14) -322(0.27) -778(0.22)

29544.17 26484.98 29150.21 26484.98

0.97 0.96 0.90 0.96

a

R = Regression Coefficient. The number in parantheses are the coefficients, when their sum is normalized to unity.

Figure 3: Fluorescence decays of 3AQ (upper panel)and 3PQ (lower panel) in n-heptane (red), n-dibutylether (blue), acetone (green), 2-propanol (grey), DMSO (orange)


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The fluorescence lifetime of 3PQ generally increases with increase in the polarity of the solvent, which seems per se to be the same as that for 3AQ. However, the increase in lifetime is significantly greater than in 3AQ in case of the polar protic solvents, with the exception of methanol (Figure 3, Table S1). A more important difference is observed in the variation of the nonradiative rate constants (kNR) with the increase in the empirical micropolarity parameter, ET(30).67,68 As has been reported earlier, the kNR values of 3AQ decrease with the increase in ET(30), but the plot shows three distinct regions for polar protic, polar aprotic and apolar solvents, with DMSO being an outlier. This observation has been rationalized in terms of an interplay of solvent polarity and hydrogen bond donating and accepting abilities.45 In case of 3PQ, the trend of decrease in kNR with increase in ET(30) is found to be more prominent. Interestingly, the specific regions, observed earlier in 3AQ, are absent for 3PQ. The data in methanol and DMSO are found to be outliers in the plot (Figure 4). This observation indicates that the photophysics of the 3PQ becomes significantly more simplified than 3AQ as a result of substitution of the amino hydrogen atoms, which eliminates the possibility of the formation of hydrogen bonds with the solvent is the hydrogen bond acceptor. This inference is in line with the decreased -dependence in the Kamlet –Taft plot. The most interesting observation in this part of our study is as follows: even though ICT seems to play a more dominant role in 3PQ, the nonradiative rate constant decreases with increase ET(30). This trend is opposite to that expected for molecules with highly dipolar excited states.69 This observation reinforces our earlier proposition that the excited state dynamics of 3AQ and its analogs is an interplay of a flipflop/rotational motion of the amino group and an ICT process and that the flip-flop/rotational motion is the more effective non-radiative process of the two. So, in conditions that favor ICT and hence restrict the motion of the amino group, the effective non radiative rate decreases. Now,



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the question that remains to be addressed concerns the nature of the motion of the amino group that is associated with the efficient nonradiative process. The specific issue that we address in this context is whether or not there is a temperature dependence of the process.

Figure 4. Semilog plot of kNR vs ET(30) for 3PQ. Methanol and DMSO (red hollow circles )were excluded from the set of points used for a linear fit (R2 = 0.95).

Steady state experiments at low temperature: Fluorescence spectra of 3AQ and 3PQ have been recorded in two solvents that form transparent glass at liquid nitrogen temperature: the nonpolar solvent 3-Methylpentane (3MP, forms glass at 110 K) and the polar solvent ethanol (forms glass at 159 K).70 In ethanol, the fluorescence spectra of 3AQ as well as 3PQ undergo a gradual and monotonic blue shift (Fig. 5a, Fig. 5c, Fig. 6a). We refrain from drawing an inference from the fluorescence intensity, as it is found to depend on the position of the cryostat, which is not completely reproducible. Instead, we focus on the spectral shift and fluorescence lifetimes, which are unaffected by the position. The blue shift in the spectral maximum, 12 ACS Paragon Plus Environment

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accompanied by a red shift in the excitation spectrum, implies a smaller Stokes shift and hence, a lower polarity of ethanol at low temperature. This may be attributed to slow and therefore incomplete solvent reorientation around the highly dipolar solute in its electronic excited state. Similar observations have been reported for several other fluorophores.71,72 In case of 3MP medium, however, the trend is exactly opposite to that for ethanol. Besides, the nature of temperature dependence is different from 3AQ to 3PQ. Upon decreasing the temperature, the emission maxima exhibit a red shift rather than a blue shift, for both the fluorophores (Fig. 5b, Fig. 5d, Fig. S4, Fig. S5). The variation is linear for 3PQ, but sigmoidal for 3AQ (Figure 6b), indicating that there is a switchover from one emissive species to the other in 3AQ, upon lowering the temperature. At T< 120K, the emission maxima of 3AQ and 3PQ in 3MP (425 nm) is the same as that for 3PQ in ethanol at 77 K (Figure 6b). The red shift observed at low temperature is unusual, but has been discussed in the past by Suppan and coworkers, for compounds like 1-naphthylamine and 4-aminophthalimide.73 For compounds like cyanophenyl carbazole in non polar hexane, such unusual observation has later been rationalized in the following manner: the fluorescence spectrum of these molecules consists of dual emission from LE and ICT states. The red shift is a result of the increment in the ratio of quantum yield of the ICT state and locally excited (LE) state.74 It appears, therefore, that the emissive species for 3AQ in 3MP at low temperature is different than that at room temperature, while the species are the same at all temperatures for 3AQ in ethanol and for 3PQ in ethanol as well as 3MP. In alcohol, the ICT state is expected to be the emissive species.45 Moreover, the same ICT state persists at low temperature as well. In 3MP, the emissive species at room temperature is expected to be locally excited state, in which ICT does not take place and rapid inversion of the amino group provides the major nonradiative pathway. Upon decreasing the temperature, the inversion



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process is slowed down, indicating that it involves a barrier.75 Using quantum chemical calculations, the barrier of rotation for the exocyclic amino group in 3AQ and 3PQ is determined to be 5.3 kcal mol-1 and 3.3 kcal mol-1 respectively (Fig. S9, Fig. S10). Thus, the emissive species at low temperature is one in which ICT can take place. This proposition gains support from the identical position of the emission maxima as that in alcohol at similar temperature. Thus, the low temperature experiment lends credence to the proposal of involvement of the two species to different extents, depending on the conditions. The lack of sigmoidal variation for 3PQ indicates that substitution of the amino hydrogen atoms by alkyl groups favors the ICT state, as proposed by Pal and coworkers.50,76


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Figure 5. Steady state spectra of 3AQ in (a) ethanol and (b) 3MP and 3PQ in (c) ethanol and (d) 3MP at room temperature (298 K) and low temperature (77 K). Absorption, excitation and emission spectra are drawn in black, green and blue, respectively). Absorption spectra have not been recorded for 77K. Only excitation spectra have been reported for this temperature.



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Figure 6. Plot of peak maxima vs temperature for 3AQ (red) and 3PQ (blue). Upper panel shows variation in (a) Ethanol and lower panel shows variation in (b)3MP. For 3-Methylpentane the data points for 3AQ were fitted in sigmoidal function with R2 = 0.99 while the for 3PQ it was a linear fit with R2 = 0.98.

Time resolved fluorescence experiments at low temperature: Decrease in temperature results in slowing down of the fluorescence decays in both the solvents. However, the extent of slowing down is significantly less in ethanol (Fig. 7a, Fig. 7c) than in 3MP (Fig. 7b, Fig. 7d). Moreover, the extent of slowing down is more in 3AQ than in 3PQ in ethanol (Fig. 7a, Fig. 7c). The trend in


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the temperature dependence of time resolved fluorescence decays, in conjunction with the shift in emission maxima at low temperature, corroborates the notion of an interplay of the flip flop/ rotational motion of the amino group and ICT in the excited state dynamics of 3AQ. The significant slowing down of the decay at low temperature in 3MP is indicative of hindrance to the motion and subsequent promotion of ICT, leading to a state that has red shifted fluorescence and long lifetime. In ethanol, 3AQ and 3PQ have excited state lifetimes of 5 ns and 9.5 ns at room temperature. The longer lifetime of 3PQ may be rationalized by a greater contribution from the charge transfer state than in 3AQ, due the presence of the stronger electron pushing tertiary amino group instead of the primary amino group. At 77K, the decays are biexponential for both the molecules. For 3AQ, the lifetimes are 2.5 ns and 9.7 ns. For 3PQ, the lifetimes are 4.5 ns and 14 ns. The contribution of short and long lifetimes in the case of both the molecules is found to be similar with i.e. 0.3 and 0.7 respectively. The longer component, which is the major one, can be attributed to the molecules in which the flip flop is suppressed very efficiently and the ICT state is highly predominant. A similar assignment may be done for



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Figure 7. Fluorescence decays at 298 K (red) and 77K (blue), recorded at respective peak maxima of 3AQ in (a) Ethanol (b) 3MP and of 3PQ in (c) Ethanol and (d) 3MP. = 336 nm.


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3AQ, but the shorter component, which is even faster than the lifetime at room temperature, is surprising. In 3MP, both 3AQ and 3PQ exhibit lifetime of about 1 ns at room temperature. Upon decreasing the temperature, the decay becomes biexponential, with the emergence of a longer component, which gradually increases in magnitude to about 10 ns and in contribution to 0.63 for 3AQ and 0.41 for 3PQ. The shorter component becomes slower as well (Fig. S6, Fig. S7 and Table S2, Table S3). This is indicative of the greater contribution of ICT to the excited state dynamics at low temperature, where the motion of the amino group is hindered. Lifetime measurements have been performed at different emission wavelengths at 77K. The decays are slower at the red ends of the spectra. However, a clear rise is not observed, presumably because the rate of the conversion of the LE state to the ICT state is faster than the resolution of our instrument. Since the rise time was absent, we have not attempted to generate the time resolved emission spectra (Figure S8). Conclusion Chemical substitution at the exocyclic amino group has a profound effect on the excited state dynamics of aminoquinolines. This is evident in the enhancement of quantum yield, fluorescence lifetime and comparatively lower non radiative rate constants for 3PQ compared to those of 3AQ.

Kamlet-Taft analysis points towards the greater sensitivity of 3PQ towards solvent

polarity rather than the hydrogen bonding parameters of the solvents, which are as important as solvent polarity for 3AQ. Substitution of the amino hydrogen atoms to form the piperidine group renders a significant charge transfer character to the excited state and also decreases the effect of the hydrogen bonding ability of the solvent on the photophysics of the fluorophore. The nonradiative rate constant of 3PQ thus becomes predominantly polarity-dependent. However, the trend of its variation with micropolarity of the solvent follows the same general trend as that of 19 ACS Paragon Plus Environment


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3AQ, indicating that the competition between ICT and flip flop persists in 3PQ as well. An interesting insight into this competition is obtained in the studies performed at low temperature. The emission of both 3AQ and 3PQ exhibit an interesting thermochromism, in the nonpolar solvent 3-methylpentane (3MP). In this medium, the emission maxima of these fluorophores undergo a red shift, along with a marked increase in lifetime, upon lowering the temperature gradually to 77 K. The red shifted emission is attributed to the ICT state of these molecules. The greater extent of red shift in 3AQ than in 3PQ is rationalized by the difference in the nature of the states involved in the photophysics of the two molecules. The locally excited state in 3PQ involves a greater amount of ICT than that of 3AQ, due to the presence of the tertiary nitrogen atom. So, a crossover of states in not observed for 3PQ. On the other hand, the spectral maxima of 3AQ exhibit a sigmoidal variation with temperature, indicating a crossover from the nonpolar LE state to the polar ICT state upon lowering the temperature. The spectral maxima of both the compounds at 77 K are close to those in ethanol at the same temperature, further corroborating their assignment to the ICT state. Notably, the expected blue shift, upon decreasing the temperature, is observed in ethanolic medium for both the compounds. The lifetime of both the compounds undergo an increase upon lowering the temperature. The longer lifetimes observed in 3MP are similar to the lifetimes observed in ethanolic medium at the same temperature. These observations lend further credence to the contention that the ICT state is formed even in the nonpolar solvent, 3MP, at low temperature. The apparently surprising aspect of this inference is as follows: Usually, excited state processes that involve a barrier, get hindered at low temperature and this leads to a blue shift of the emission spectra. However for 3AQ in 3MP, ICT is feasible only when the flip-flop/flip flop of the amino group is hindered and the two segments of the molecule do not move with respect to each other. Thus, when the flip flop is hindered at


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low temperature, it becomes possible for the ICT to take place, leading to the red shift. Thus, it is shown that of the two excited state deactivation processes, ICT is the less efficient one and is associated with longer lifetimes and stronger emission.

Acknowledgement The work is supported by a SERB-DST project of AD. AKS is grateful to CSIR for fellowship. SG thanks Indian Academies of Science for a Summer Research Fellowship (SRF). Authors are grateful to Professors N. Periasamy, G. Krishnamurthy, Ranjan Das and the Department of Chemical Sciences, Tata Institute of Fundamental Research for the loan of the cryostat used for low temperature experiments. The authors are indebted to Prof. Debabrata Maiti for the use of his laboratory during the synthesis of 3PQ. Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcb Synthetic scheme, quantum yield calculation, steady state and time resolved parameters (Table S1), additional electronic spectra (Figure S1), Absorption and emission peak frequency vs orientation polarizability (Figure S2), Lippert – Mataga plot (Figure S3), fluorescence spectra and decays at different temperatures, lifetime data (Figures S4, S5, S6, S7, S8 and Tables S2, S3, S4, S5), rotational barrier (Figures S9, S10).



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Table of content graphic


Engineering the Excited-State Dynamics of 3-Aminoquinoline by

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