Permanent Fluorescence Sensing Hyperbranched Dendritic Architectures

Abstract

A hyperbranched dendrimer with m-terphenyl as surface group and an anthraquinone core unit was synthesized and was found to display permanent fluorescence sensing even under highly acidic condition.

Key words

fluorescence sensing - m-terphenyl - lifetime - hyperbranched - anthraquinone

References

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10

Dendrimer 1a: mp 92-96 °C. ¹H NMR (500 MHz, CDCl₃): δ = 5.30 (s, 4 H), 7.28-7.81 (m, 32 H). ¹³C NMR (125 MHz, CDCl₃): δ = 72.1, 124.9, 125.7, 126.5, 127.4, 127.6, 128.9, 133.3, 140.9, 142.1, 153.4, 183.1. MS (FAB): m/z = 724 [M⁺]. Anal. Calcd for C₅₂H₃₆O₄: C, 86.18; H, 4.97. Found: C, 86.14; H, 4.94.

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Dendrimer 1b: mp 180-182 °C. ¹H NMR (500 MHz, CDCl₃): δ = 5.10 (s, 4 H), 5.15 (s, 8 H), 7.23-7.73 (m, 64 H). ¹³C NMR (125 MHz, CDCl₃): δ = 70.2, 125.4, 127.3, 127.4, 127.6, 128.9, 140.9, 142.2, 142.3, 184.2. MS (FAB): m/z = 1453 [M⁺]. Anal. Calcd for C₁₀₄H₇₆O₈: C, 85.89; H, 5.23. Found: C, 85.82; H, 5.18.

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Dendrimer 1c: Brown colored liquid. ¹H NMR (500 MHz, CDCl₃): δ = 5.02 (s, 4 H), 5.07 (s, 8 H), 5.15 (s, 16 H), 7.40-7.63 (m, 128 H). ¹³C NMR (125 MHz, CDCl₃): δ = 70.0, 70.2, 70.3, 101.7, 106.4, 106.5, 114.2, 127.3, 127.4, 127.6, 128.92, 128.93, 140.8, 140.9, 142.1, 142.2, 187.4. MS (FAB): m/z = 2910 [M⁺]. Anal. Calcd for C₂₀₈H₁₅₆O₁₆: C, 85.57; H, 5.36. Found: C, 85.52; H, 5.32.

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UV/Vis and Fluorescence Studies on Dendrimer 1a, 1b and 1c:
The UV/Vis spectrum of dendrimer 1a in CHCl₃ showed λ_max at 411 nm; similarly dendrimers 1b and 1c in CHCl₃ displayed absorbance at 411 nm and 413 nm, respectively. No bathochromic or hypsochromic shift could be observed on changing the solvent from CHCl₃ to MeCN. Protonation of the carbonyl chromophore in the anthraquinone moiety could lead to a change in λ_max. However, on adding HOAc, TFA up to 5 M for 1.5·10^-4 M of the dendrimer no shift in λ_max could be observed. Compound 1a exhibited a fluorescence emission band at 550 nm. Similarly dendrimers 1b and 1c showed fluorescence emission at 525 and 476 nm. Fluorescence quenching did not occur even after adding TFA to the dendrimers, which shows that the dendrimer 1a, 1b and 1c can function as permanent fluorescence sensing material even under highly acidic conditions. The UV/Vis spectra of dendrimers 1b and 1c showed λ_max at 405, 250 and 408, 258 nm, respectively. Fluorescence decay was recorded using the TCSPC method with the following setup. A diode pumped millena CW laser 532 nm was used to pump a Ti:sapphire rod in a Tsunami picosecond mode locked laser system. The 750 nm (80 MHz) output was taken from the Ti: sapphire laser and passed through a pulse picker to generate a 4 MHz pulse. The second harmonic output (375 nm) was generated by a flexible harmonic generator. The vertically polarized 375 nm laser was used to excite the sample. The fluorescence emission at the magic angle (54.7°) was dispersed in a monochromator (f/3 aperture), counted by a MCP PMT (Hamamatsu R 3809) and processed through CFD, TAC and MCA. The instrument response function for this system is ca. 52 ns; the fluorescence decay was analyzed by using the software provided by IBH (DAS-6) analysis software. The fluorescence decay of dendrimer 1a and 1b are doubly exponential with lifetimes 0.42, 1.86 and 0.46, 2.83 ns in CHCl₃, respectively, whereas the dendrimer 1c displayed triply exponential decay with lifetime 0.39, 1.68 and 6.69 ns. The lifetime and relative amplitudes are shown in the Table [1] .

Table 1 Bi- and Tri-Exponential Fluorescence Analysis of Dendrimers 1a, 1b and 1c in CHCl3

Dendrimer	Analysis	Lifetime (ns)			Amplitudes (%)
		τ1	τ2	τ3	A1	A2	A3
1a	Bi-exp.	0.42	1.86	-	65.02	34.98	-
1b	Bi-exp.	0.46	2.83	-	44.58	55.42	-
1c	Tri-exp.	0.39	1.68	6.69	8.56	36.01	55.43