Materials, Methods and Instruments
3.2. Effect of temperature
can be assigned to the same MC that is present in the ground state. The fluorescence spectral characteristics of the MC of DMAPIP-b i.e. band positions and small fluorescence intensity relative to corresponding values for the neutral form, are consistent with imidazole protonated DMAPIP-b. The same behavior is also observed for DMAPIP- c, when the imidazole nitrogen atom is protonated in the excited state.
3.1.4. Conclusion
Dual fluorescence is observed in DMAPIP-b only in protic solvents. The intensity of the longer wavelength emission of DMAPIP-b increases with increasing H-bond donating capacity of the solvent but decreases in water. Experimental results suggest that the H-bonding of the solvent with pyridine nitrogen atom increases the charge flow thus facilitating the formation of the TICT state. TICT emission is enhanced in DMAPIP-b relative to DMAPIP-c. Thus the H-bonding induced emission of the DMAPIPs is strongly influenced by the position of the pyridine nitrogen. The TICT and normal emissions have different lifetimes in DMAPIP-b. This indicates that TICT state formation in DMAPIP-b is irreversible. In aqueous medium only the monocation formed by the protonation of the imidazole nitrogen is found.
3.2.1. Relative population of the isomers
Though only one isomer of DMAPIP-b was considered in section 3.1, theoretically the molecule can exits in two isomeric forms. The structures of both isomers were optimized by DFT 6-31g(d,p). The calculations suggested that isomer I is more stable than isomer II (Chart 3.2.1) and at room temperature the relative population of isomer II is negligible. But isomer II is predicted as more polar than isomer I. Therefore, solvent polarity and temperature are expected to increases the relative population of isomers. The solvation energy was calculated using following expression [233-234,223]
3
1 E 2
a µ ε −2
∆ = ε +1 (3.2.1)
where and a are the dipole moment and Onsager radius respectively and were calculated from the DFT optimized structure of isomers. The relative population obtained by Boltzmann distribution (not shown) suggested that the population of isomer II in all the solvents even at boiling temperature of the solvents is less than 1%. Therefore the presence of isomer II can be ignored. X-ray crystal structure data also showed that the molecule is present as isomer I (Appendix II). Since DMAPIP-b emits single emission in
N
N
NH
N CH3
CH3 N
HN
N
N CH3
CH3
E =-20707.08199 eV E = -20706.91737 eV g = 5.07 D g = 7.04 D
Isomer I Isomer II
Chart 3.2.1. Structures of different isomers of DMAPIP-b along with their energies in the S0 state and ground state dipole moment.
aprotic solvents and dual emission in protic solvents, effect of temperature in these two classes of solvents are discussed separately.
3.2.2. Temperature effect in aprotic solvents
The common feature in the absorption and the fluorescence spectra of DMAPIP-b in both nonpolar cyclohexane and polar acetonitrile are hypsochromical shift with increase in temperature (Figure 3.2.1). A clear isosbestic point in the absorption spectra and an isoemissive point in the fluorescence spectra are observed. The isoemissive points are in the red side of the fluorescence maxima in cyclohexane, but it on the blue side of the fluorescence maxima in acetonitrile respectively.
0 0.015 0.03 0.045 0.06
280 300 320 340 360 380 400
Wavelength (nm)
Absorbance
283 K 293 K 303 K 313 K 323 K 333 K (a)
0 400000 800000 1200000 1600000
345 395 445 495 545
Wavelength (nm)
Intensity (a.u)
333 K 323 K 313 K 303 K 293 K 283 K
283 K 293 K 303 K 313 K 323 K 333 K (a)
0 0.02 0.04 0.06
290 310 330 350 370 390
Wavelength (nm)
Absorbance
283 K 293 K 303 K 313 K 323 K 333 K 343 K (b)
0 800000 1600000 2400000 3200000
350 390 430 470 510 550
Wavelength (nm)
Intensity (a.u)
(b)
283 K 293 K 303 K 313 K 323 K 333 K
Figure 3.2.1: Absorption (left panel) and fluorescence spectra (right panel) of DMAPIP-b at different temperature in (a) cyclohexane and (b) acetonitrile.
In cyclohexane the variation in temperature, changes the relative population of the vibrational states. This can be ascertained by fact the relative absorption of the 335 nm band increases and that of the 350 nm band decreases with increase in temperature. The fluorescence spectra also suggested the change in relative population of the vibration state. An enhancement of fluorescence quantum yield was observed with rise in temperature, but the lifetime remains nearly constant with temperature change (Table 3.2.1). With rise in temperature the radiative rate increases and the nonradiative rate decrease. It appears that there is an increased cross section towards the radiative path at the higher vibrational levels. On the other hand, in acetonitrile the fluorescence spectrum undergoes a hypochromic shift. The increase of nonradiative processes related to thermal agitation in acetonitrile is evident from the decreases of fluorescence quantum yield and the lifetime with temperature (Table 3.2.1).
Table 3.2.1: Absorption maxima (λλλλmaxab, nm), fluorescence (λλλλmaxfl, nm), quantum yield (φφφφ), fluorescence lifetime (ττττ, ns), radiative rate (kr, s-1) and nonradiative rate (knr, s-1) of DMAPIP-b in cyclohexane and acetonitrile at different temperature.
T (K) λλλλmaxab λλλλmaxfl φφφφ ττττ kr (108) knr (108) Cyclohexane
293 337, 351 359, 379, 398 0.72 1.08 6.72 2.51
303 336, 351 358, 378, 395 0.73 1.09 6.69 2.51
313 336, 350 357, 377, 394 0.74 1.07 6.96 2.36
323 334, 350 356, 376, 393 0.75 1.05 7.19 2.33
Acetonitrile
293 346 408 0.76 1.58 4.85 1.48
303 344 406 0.74 1.56 4.88 1.53
313 342 405 0.73 1.54 4.77 1.73
323 341 404 0.72 1.52 4.76 1.82
3.2.2. Temperature effect on dual fluorescence
The effect of temperature on the dual emission was investigated in 1-propanol, ethanol and methanol (Figure 3.2.2). Same as in acetonitrile, the fluorescence spectra in
0 700000 1400000 2100000 2800000 3500000
355 430 505 580
Wavelength (nm)
Intensity (a.u)
283 K 293 K 303 K 313 K 323 K 333 K 343 K (a)
0 700000 1400000 2100000 2800000 3500000
355 405 455 505 555 605 655
Wavelength (nm)
Intensity (a.u)
283 K 293 K 303 K 313 K 323 K 333 K (b)
0 1500000 3000000 4500000 6000000
355 415 475 535 595
Wavelength (nm)
Intensity (a.u)
283 K 293 K 303 K 313 K 323 K (c)
Figure 3.2.2: Fluorescence spectra of DMAPIP-b at different temperature in (a) 1- propanol, (b) ethanol and (c) methanol.
1-propanol and ethanol have clear isoemissive point on the blue side of the band maxima.
In propanol an additional quasi isoemissive point is observed at 460 nm in the tail side of the band maxima. However, in all these protic solvents, dual emission is observed in all the temperatures. The quantum yield for the normal band decreases in all the protic solvents (Figure 3.2.3).
0 0.15 0.3 0.45 0.6
290 300 310 320 330
T (K-1) φφφφfΝΝΝΝ
Figure 3.2.3: Temperature dependence of quantum yield (φφφφf) of normal emission in 1- propanol ( ), ethanol ( ) and methanol ( ).
The fluorescence decay is biexponential in all the protic solvents at all the temperature. The lifetime is quenched with protic solvents and the attenuation of lifetime increases with hydrogen bonding capacity of the solvent. The lifetime of the locally excited states decreases in 1-propanol and ethanol, but increases in methanol (Figure 3.2.4a). With rise in temperature the radiative rate decreases and the non radiative rate increases for normal emission in all these solvents. The lifetime of TICT emission decreases in all three protic solvents (Figure 3.2.4b).
0.3 0.35 0.4 0.45 0.5 0.55
285 295 305 315 325 335 345
T (K-1)
τ τ τ τ (ns)
0.4 0.6 0.8 1 1.2 1.4 1.6 1.8
285 295 305 315 325 335 345
T(K-1)
τ τ τ τ (ns)
Figure 3.2.4: Temperature dependence of lifetime of normal (a) and TICT (b) emission in 1-propanol ( ), ethanol ( ) and methanol ( ).
The coupling of the locally excited state and the TICT state, though normally weak, it is strong enough for the charge transfer reaction to take place adiabatically in
(a)
(b)
single potential energy surface process. Since the orbitals involve in the radiative transition from the TICT state to the corresponding Franck-Condon state are located on different part of the molecular system (donor and acceptor) with minimum overlap, the decay is expected to posses a small transition moment, due to its forbidden nature. Upon coupling to suitable vibration, the TICT emission is predicted to barrow the intensity from the allowed higher excited states and is responsible for its appreciable quantum yield. Due to this vibronic coupling, the radiative decay of the TICT emission is temperature dependent and the major part of the TICT emission occurs from the less forbidden higher vibronic levels as hot fluorescence. Therefore, the rate constant for the formation of the TICT state depends on temperature [13]. However, it is shown that both solvent parameters viscosity and polarity, and solute parameters such twist angle of ground state, rotating volumes, molecular shape and electronic nature are found to determine the rate for this forward [13]. The kinetics of TICT emission can be explained by the generalized scheme (Scheme 3.2.1) for the TICT process as shown below [13].