Absorption and fluorescence spectral studies
The steady-state electronic absorption and fluorescence spectra of PPMP were measured in 15 solvents of varying polarity at room temperature. In the absorption spectra (Fig. 2a, c), an intense band was observed with absorption maxima centered between 493 nm to 513 nm in different solvents. This band is associated with the (S1←S0) transition to the first excited singlet state. A minor absorption band was observed between 320 to 340 nm, and this is attributed to the second excited singlet state (S2←S0) electronic transition. As seen in Table 1, the absorption maxima could not be correlated to the solvent polarity.
On the other hand, the solvent's polarity significantly impacts the emission spectrum (Fig. 2b, d). PPMP exhibit positive solvatochromic behaviour. The excitation wavelength at different nanometers doesn't change the emission maxima shown in Fig. 3. The most significant solvent shift in the emission spectra was reported in acetonitrile. When the solvent changed from cyclohexane to acetonitrile, the emission maxima shifted from 467 cm− 1 to 4422 cm− 1, respectively. This behaviour is a characteristic property in push-pull systems due to the intramolecular charge transfer (ICT) phenomenon. In ICT, charge redistribution occurs from the electron donor to the acceptor system[44–47].
The substantial redshifts in the emission spectra with increasing solvent polarity suggest that the polar solvents stabilize the excited state. In a polar solvent, the orientation of solvent molecules in the excited state is considerably different from that in the ground state. Due to this, the excited state is highly polarized. Thus, we can say that this kind of fluorophore can be used as a solvent polarity sensor. In addition, we see the appearance of a shoulder for nonpolar solvents. The shoulder is attributed to a vibrational splitting of the excited state, which is more prominent in nonpolar solvents.
Table 1
Spectral parameters of PPMP in different solvents.
S.No
|
Solvents
|
ɛ
|
n
|
λmaxab (nm)
|
λmaxem (nm)
|
νa (cm− 1)
|
νf (cm− 1)
|
1
|
Hexane
|
1.89
|
1.37
|
493
|
508
|
20284
|
19685
|
2
|
Cyclohexane
|
2.02
|
1.42
|
501
|
513
|
19960
|
19493
|
3
|
1, 4 Dioxane
|
2.21
|
1.42
|
499
|
550
|
20040
|
18182
|
4
|
Toluene
|
2.38
|
1.49
|
506
|
542
|
19763
|
18450
|
5
|
Diethyl ether
|
4.20
|
1.35
|
498
|
540
|
20080
|
18519
|
6
|
Chloroform
|
4.89
|
1.44
|
513
|
566
|
19493
|
17668
|
7
|
Ethyl acetate
|
6.08
|
1.37
|
498
|
571
|
20080
|
17513
|
8
|
Tetrahydrofuran
|
7.58
|
1.40
|
502
|
575
|
19920
|
17391
|
9
|
Dichloromethane
|
8.93
|
1.42
|
509
|
590
|
19646
|
16949
|
10
|
Butan-1-ol
|
17.51
|
1.39
|
507
|
588
|
19724
|
17007
|
11
|
Acetone
|
20.7
|
1.35
|
497
|
617
|
20121
|
16207
|
12
|
Ethanol
|
24.55
|
1.36
|
498
|
600
|
20080
|
16667
|
13
|
Acetonitrile
|
35.94
|
1.34
|
497
|
637
|
20121
|
15699
|
14
|
Dimethylformamide
|
38.25
|
1.43
|
503
|
635
|
19881
|
15748
|
15
|
Dimethylsulphoxide
|
47.24
|
1.47
|
506
|
644
|
19763
|
15528
|
The ground and excited state dipole moment estimation from the solvatochromic analysis.
From the solvatochromic analysis, estimation of ground (µg) and excited state (µe) dipole moment were proposed by Bilot-Kawski[37, 38, 48] using Onsager reaction field theory by varying dielectric constant (ɛ) and refractive index (n). The ground and excited state dipole moment were estimated using equations (S5 & S6). The spectral shifts\(\stackrel{-}{\nu }\)a-\(\stackrel{-}{\nu }\)f, \(\stackrel{-}{\nu }\)a+\(\stackrel{-}{\nu }\)f, \(\stackrel{-}{\nu }\)a+\(\stackrel{-}{\nu }\)f / 2, the solvent polarity functions f1(ε,n); f2(ε,n); f3(ε,n); and \({E}_{T}^{N}\) of different solvents shown in Table S1. The solvent polarity functions f1(ε,n); f2(ε,n); f3(ε,n); and \({E}_{T}^{N}\) were calculated using equations S13, S14, S15 and S16. The fifteen solvents used varied with a dielectric constant from 1 to 47. The linear graphs of \(\stackrel{-}{\nu }\)a-\(\stackrel{-}{\nu }\)f vs f(ε,n), \(\stackrel{-}{\nu }\)a+\(\stackrel{-}{\nu }\)f vs f(ε,n)+g(n), \(\stackrel{-}{\nu }\)a-\(\stackrel{-}{\nu }\)f vs f1(ε,n), \(\stackrel{-}{\nu }\)a-\(\stackrel{-}{\nu }\)f vs f2(ε,n), \(\stackrel{-}{\nu }\)a+\(\stackrel{-}{\nu }\)f / 2 vs f3(ε,n), and \(\stackrel{-}{\nu }\)a-\(\stackrel{-}{\nu }\)f vs \({E}_{T}^{N}\) are shown in Fig. 4 respectively.
The data points were fitted to a straight line using a linear progression. The corresponding values of the slopes are shown in Table 2. In all cases, the correlation coefficient (R2) was more than 0.90, resulting in a good linearity fit, but only in the Reichardt method did the correlation coefficient (R2) correspond to 0.85. A near-perfect linearity fit could be obtained by excluding the data from solvents 1,4-dioxane, toluene, 1-butanol, ethanol, dimethyl formamide, and dimethyl sulphoxide. Typically, distinct solute-solvent interactions are needed to account for deviation from linearity[32, 33].
The ground and excited state dipole moment were calculated as 5.7 D and 16.8 D, respectively, by Bilot-Kawski theory. DFT calculations also estimated the ground state dipole moment as 7.4 D. The excited state dipole moment was evaluated by Lippert–Mataga, Bilot–Kawski (modified), Bakhshiev, and Reichardt methods using equations S10, S11, S12, and S18, respectively.
The excited state dipole moment values are presented in Table 3. Depending on the theoretical method utilized (excluding Lippert–Mataga), slight variances in µe values are observed. The Lippert–Mataga approach does not account for polarizability[49] and is known to show a substantial disparity in the value of the excited state dipole moment[25, 28, 30]. Finally, we note that the excited state is likely to be highly stabilized by polar solvents, resulting in higher dipole moments and a more polar excited state
B. Jędrzejewska et al. reported ground state dipole moment values of (Z)-4-(4-(diphenylamino) benzylidene)-2-phenyloxazol-5(4H)-one (Fig. 1a) and (1c. (4Z,4'Z,4''Z)-4,4',4''-((nitrilotris(benzene-4,1-diyl)) tris (methaneyl ylidene)) tris (2-phenyloxazol − 5(4H)-one) (Fig. 1c) as 3.52 D and 7.01 D respectively. The ground state dipole moment of the PPMP (Fig. 1b) molecule comes in between the two reported values, indicating a gradual increment of dipole moment with an increase in the number of aromatic rings
Table 2
Linear correlation data of PPMP from solvatochromic methods.
Method
|
Slope
|
Correlation Coefficient (R2)
|
Bilot–Kawski (1)
|
4649
|
0.96
|
Bilot–Kawski (2)
|
-9475
|
0.94
|
Lippert–Mataga
|
12713
|
0.93
|
Bakhshiev
|
4649
|
0.96
|
Bilot–Kawski (modified)
|
-4738
|
0.94
|
Reichardt
|
8757
|
0.85
|
Table 3
The ground and excited state dipole moments of PPMP in different methods.
Onsager
Radius a (Å)
|
Dipole moment calculated from different methods
|
|
µg a
|
µe b
|
µe c
|
µe d
|
µe e
|
µef
|
µg−Theorg
|
6.44
|
5.7
|
16.8
|
24.0
|
16.8
|
16.9
|
14.0
|
7.4
|
1D=3.33564 x 10-30 C.m. = 10-18 esu. cm
a: Bilot–Kawski method calculated from the equation (S5)
b: Bilot–Kawski method calculated from the equation (S6)
c: Lippert–Mataga method calculated from the equation (S10)
d: Bakhshiev method calculated from equation (S11)
e: Bilot–Kawski method (modified) calculated from equation (S12)
f: Reichardt method calculated from equation (S18)
g: dipole moment calculated theoretically in the ground state
Gas-phase quantum mechanical calculations
Gas phase quantum mechanical calculations were carried out using the density functional theory method based on B3LYP exchange correlations with 6-31G(d) basis sets implemented in the Gaussian09 program. First, the experimental molecular geometry of PPMP obtained from the X-ray diffraction studies was optimized in its ground state. Figure 5 represents molecular orbitals and electrostatic potential maps of PPMP. To understand the electronic structure of PPMP, we used the theoretically optimized coordinates for analyses in terms of molecular orbitals. After that, the isosurfaces of the HOMO and LUMO, shown in Fig. 5a for the PPMP, were further studied using the Gauss view visualization program. Unlike the experiments in various solvents, we note that the DFT studies are performed in a vacuum.
We found that the energy gap between HOMO (-5.21 eV) and LUMO (-2.58 eV) is 2.63 eV (or ~ 471 nm), which matches reasonably well with the experimental λmax (around 500 nm). Isosurfaces belonging to HOMO and LUMO provide a tentative understanding of charge density distribution in the ground and excited states, respectively, suggesting that PPMP could have an intramolecular charge transfer. Upon excitation, charge densities shift predominantly from phenylamino (HOMO) to the molecule's 2-phenyloxazolone (LUMO) portion. Finally, the Onsager cavity radius, an essential property of the PPMP required for evaluating the dipole moments, was also calculated theoretically with Gaussian 09 program (Onsager cavity radius = 6.44 Å).