Table 1. Spectral and physico-chemical properties of the first set of synthesised X-rhodamines (5 μM in MOPS 30 mM, 100 mM KCl) ^a .

Compound		λabsmax (nm)	λem d (nm)	Log ε (λabsmax) (M–1 cm–1)	Φ	Φ ON/ΦOFF	Dynamic range e	Dynamic range @ pH ± 1 pKa f	pKa
HR-pOH	ON b	576	587	4.78	0.55	9	34	30	8.97
	OFF c	579 f	598	4.53	0.06				±0.05
HR-mOH	ON b	578	602	4.70	0.30	3	132	75	9.33
	OFF c	577	603	4,63	0.11				±0.13
HR-oOH	ON b	581	603	4.89	0.75	3	866	460	9.68
	OFF c	579	603	4.69	0.27				±0.17
HR-Me	ON b	574	597	4.71	0.09	High	955	833	8.75
	OFF c	N/A g	598	N/A g	N/A h				±0.16
HR-OMe	ON b	577	598	4,72	0.22	8	302	N/A i	N/A i
	OFF c	576	600	4.74	0.03
HR-Cl	ON b	579	601	4.68	0.47	High	753	512	6.17
	OFF c	544	599	4.52	N/A h				±0.10
Hr-Br	ON b	581	601	4.39	0.55	High	384	291	6.51
	OFF c	544	600	4.40	N/A h				±0.10
p-Nph	ON b	581	600	4.79	0.28	6	23	9.7	8.51
	OFF c	579	602	4.34	0.05				±0.17
o-Nph	ON b	584	604	4.43	0.77	8	337	166	8.73
	OFF c	576	604	4.46	0.10				±0.17
Pip-H	ON b	578	600	4.88	0.16	4	6	3.9	8.67
	OFF c	579	601	4.84	0.04				±0.07
Pip-Alkyne	ON b	577	600	4.98	0.34	2	3	1.9	4.95
	OFF c	579	602	4.45	0.15				±0.09
Imidazole	ON b	583	609	4.80	0.40	N/A j	N/A j	N/A j	N/A j
	OFF c	584	608	N/A j	N/A j

^a ε = molar extinction coefficient, λ = wavelength, Φ = quantum yield.

^bProtonated form: pH 4.

^cDeprotonated form: pH 10.

^dExcitation at 535 nm.

^e[(I_max – I_min)/I_min] with I = fluorescence intensity.

^fFluorescence enhancement between ±1 pH unit around the pK_a value.

^gBroad absorbance.

^hNon-fluorescent.

ⁱpK_a value was too high.

^jNo pH sensitivity.