Table 1. Available experimental values compared to theoretical simulations for HBO-ESIPT dyes. All emission wavelengths are expressed in nm, whereas the relative Gibbs energies are given in eV (see Fig. 3), a negative value indicating a more stable K*. We summarise the experimental observations, giving the approximated relative E* and K* intensities in the emission spectra, w and vw standing for weak and very weak contributions from the minority tautomer. CH, CHL, DCM, DIOX, HEP and 3 MP stand for cyclohexane, chloroform, dichloromethane, 1,4-dioxane, n-heptane and 3-methylpentane, respectively116.

Structures						Experiment				Theory
	R1	R2	R3	R4	Solv.	Observations	λ E* fl	λ K* fl	Ref.	λ E* fl	λ K* fl	ΔGES	ΔGES#
1	H	H	H	H	3 MP	Keto	—	ca. 480	55	—	492	–0.315	–0.146 a
2	H	H	H	H	CH	Keto	—	ca. 490	117	—	491	–0.314	–0.144
3	H	H	H	H	CHL	Keto	—	ca. 489	59	—	482	–0.303	–0.119
4	H	H	H	NH2	DCM	Keto (4/5) + enol (1/5)	442	565	62	460	593	–0.078	–0.079
						Keto	—	551	118
5	H	H	H	CHO	CHL	Keto + vw enol	ca. 405	474	119	352	455	–0.280	–0.072
6	H	H	H	CHO	DIOX	Keto	—	483	119	—	463	–0.292	–0.091
7	H	H	NEt2	H	CHL	Enol	382	—	59	377	—	0.087	0.176
						Enol	451	—	66
8	H	H	NEt2	H	DIOX	Enol	471	—	66	373	—	0.023	0.159
9	H	H	NH2	H	DCM	Keto (7/8) + enol (1/8)	376	467	62	359	458	–0.055	0.072
						Keto (3/5) + enol (2/5)	390	460	120
10	H	H	NH2	H	CHL	Keto (3/4) + enol (1/4)	390	460	120	357	464	–0.086	0.054
11	H	H	NH2	H	DIOX	Keto (3/4) + enol (1/4)	415	487	121	354	478	–0.150	0.019
12	H	H	OMe	H	CHL	Keto + vw enol	ca. 375	ca. 467	59	351	488	–0.241	–0.019
13	H	H	Me	H	3 MP	Keto	—	ca. 480	55	—	505	–0.297	–0.125
14	H	C C(CN)2	H	H	CH	Keto + w enol	ca. 425	ca. 550	122	408	586	0.083	0.074
15	H	C C(CN)2	H	H	CHL	Keto + enol	ca. 450	ca. 625	122	425	642	–0.032	0.173
16	H	CHO	H	H	CHL	Keto	—	ca. 517	59	—	500	–0.202	–0.082
17	H	COOEt	H	H	CHL	Keto	—	ca. 495	59	—	497	–0.254	–0.100
18	H	NH2	H	H	HEP	Keto + vw enol	390	500	123	376	481	–0.065	0.012
19	H	NH2	H	H	DCM	Keto (9/10) + enol (1/10)	420	485	123	399	468	0.037	0.085
20	H	COOEt	NEt2	H	CHL	Enol	421	—	59	401	—	0.188	0.245
21	H	COOEt	OMe	H	CHL	Keto + vw enol	ca. 375	ca. 482	59	365	505	–0.202	0.012
22	H	NH2	NEt2	H	DIOX	Enol	460	—	66	389	—	0.146	0.201
23	H	NO2	NEt2	H	DIOX	Keto + enol	476	656	66	434	550	0.327	0.353
24	NH2	H	NEt2	H	DIOX	Enol	440	—	66	389	—	0.129	0.216
25	NO2	H	NEt2	H	DIOX	Enol	440	—	66	435	—	0.356	0.297
26	NH2	H	H	H	HEP	Keto (9/10) + enol (1/10)	400	500	124	406	488	0.054	0.182
27	NH2	H	H	H	DCM	Enol + vw keto	450	ca. 480	124	432	474	0.076	0.197

^aA negative barrier indicates a barrierless process on the free energy surface. The transition states can nevertheless be found on the energy surface.