Conformer specific aggregation induced enhanced emission

Hydroxyphenyl)benzimidazol-1-ylmethyl)- benzene

4.5. Conformer specific aggregation induced enhanced emission

Chart 4.3. Cis- and trans- keto emission from unprotonated HPBI unit of BHPBI monoanion.

Figure 4.5. (A) Fluorescence and (B) absorbance spectra of BHPBI in different water-THF (v/v) mixture (a) 0% (b) 70% (c) 90% and (d) 100% water. _𝑒𝑥 = 290 nm [BHPBI] = 5 µM.

Intramolecular Proton Transfer and Conformer Specific AIEE of BHPBI

BHPBI shows a weak tautomer emission in THF (Figure 4.5.A). Upon addition of water to the THF solution the emission spectrum is blue shifted and the enol emission also emerges. At 70% water, the emission intensities at 355 nm and 430 nm are enhanced by eleven times and twenty times, respectively than those in THF. At 90% water, the enol emission completely disappears, and the 430 nm band is also replaced by the keto emission at 478 nm. The fluorescence quantum yield is also enhanced by ~ 30 times than that in THF. But in neat water, though no shift is observed, the emission intensity appears very low compared to that of 90% water. The emission intensities in THF and in water are found almost comparable though there is a huge difference in their absorbance (Figure 4.5.B).

The absorption patterns of BHPBI at different water fractions also differ from that in THF (Figure 4.5.B). In THF like other nonpolar solvents two absorption bands are observed. Conversely, at 70% water a very sharp band appears at 294 nm along with a long tailing. The band maxima is little red shifted as compared to the shorter wavelength absorption of BHPBI in THF. The longer wavelength absorbance at 322 nm in THF is diminished and buried completely underneath the 290 nm band in 70% water.

At 90% water, again two absorption bands appear with very low absorbance and a level off tailing is observed at onset. In neat water, the absorbance is found much lower though no spectral shift is observed with respect to that in 90% water. Due to the low absorbance the level of tailing is not visible but it persists in neat water also. The long tailing in 70%, 90% water and in neat water are due to Mie scattering which indicates the formation of aggregates.¹³¹

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0 10 20 30

Counts

Time (ns) a

c b (B)

0.0 0.4 0.8 1.2

270 320 370

Normalized Intensity

Wavelength (nm) a

c

b (A)

Figure 4.6. (A) Fluorescence excitation spectra of BHPBI in (a) 70% water, 𝑒𝑚 = 360 nm, (b) 70% water,

_𝑒𝑚 = 430 nm, (c) 90% water, _𝑒𝑚 = 478 nm. (B) Fluorescence decay of BHPBI in (a) THF, _𝑒𝑚 = 490 nm, (b) 70% water, _𝑒𝑚 = 400 nm, (c) 90% water, _𝑒𝑚 = 478 nm. _{𝑒𝑥 = 290 nm.}

The excitation spectra corresponding to the different emission of different aggregated structures are depicted in Figure 4.6.A. In 90% water, the excitation spectra recorded at 478 nm shows a similar nature as those of the keto emission in nonpolar solvent. The absorption band at 320 nm suggests that in 90% water, the aggregates are developed with the quasi planar conformer. The complete absence of the enol emission and the presence of only tautomer emission can be attributed to the nonexistence of the twisted conformer in these aggregates (in 90% water solution). The fluorescence decay of the aggregated tautomer emission is monoexponential with 4.2 ns lifetime and it is little longer than that in other solvents (Figure 4.6.B). The absence of short lifetime keto tautomer indicates that the torsional rotation, one of the major nonradiative decay channel of the excited keto, is prevented in the aggregates. Therefore, the enhanced emission is observed from the aggregates. In 70% water, the excitation spectral maxima recorded at 356 nm and 430 nm appear at 294 nm. This indicates that 294 nm absorbing species is a ground state precursor for both the emissions. The band at 294 nm advocates that those aggregated structures are formed with the twisted conformer in this solution.

The twisted conformer does not possess the intramolecular hydrogen bond which is a prerequisite for the ESIPT. Therefore, it emits enol emission in the aggregated state. The additional band at 430 nm matches with the anionic emission observed at higher pH in aqueous solution (Section 4.4). Therefore, it can be assigned to the anionic emission. The lifetime of the enol emission and the anionic emission in 70% water solution are 0.56 ns and 2.1 ns, respectively. Aggregation provides the restricted environment for molecules which reduce the intramolecular rotation of the molecules. This enhances the radiative

0.0E+0 5.0E+5 1.0E+6 1.5E+6 2.0E+6

320 370 420 470 520

Intensity (a.u.)

Wavelength (nm)

% Ployethylene glycol

0 100

Figure 4.7. Emission spectra of BHPBI in THF with gradual increase of polyethylene glycol, _𝑒𝑥 = 290 nm.

[BHPBI] = 5 µM.

Intramolecular Proton Transfer and Conformer Specific AIEE of BHPBI

emission. The enhancement of the normal and the tautomer emission with increase of polyethylene glycol concentration in THF solution further supports this (Figure 4.7.).

FESEM images suggest that the blue color emitting aggregates in 70% water are of rod shape with length of 0.5-1.5 µm (Figure 4.8.A). On the other hand, cyan color emitting aggregates in 90% water solution are 10-15 µm long needles (Figure 4.8.B). The longer needles infer the lesser interaction of BHPBI with solvent molecules. Hence, the intramolecular hydrogen bonding between the azole nitrogen and ‘OH’ proton are more favored. Thus, the molecules possess quasi-planar structure in the aggregate which produces the tautomer emission. The higher solvation energy in 70% water breaks the intramolecular hydrogen bond and produces the twisted conformers. Upon aggregation, this conformer formed much smaller rods. The excitation of the twisted conformer results in the enol emission as observed in other solvents. The photoexcitation also increases the acidity of the phenolic proton. Unlike in the quasi planar conformer, in the twisted conformer azole nitrogen is not in proximity to accept the proton. Therefore, the molecule transfers the proton to the solvent and produces the deprotonated HPBI unit. It results in the anionic emission along with the enol emission.

This is same as observed for the cis-H-bond complex of HPBI and its analogues, which undergoes intermolecular proton transfer to form anion in the excited state (Chapter 3).