Isomerization and Dimerization of Indocyanine Green and a Related Heptamethine Dye

Abstract

Indocyanine green (ICG) and a second heptamethine cyanine dye were studied in detail using 2D NMR spectroscopy at multiple temperatures. In addition to the all‐trans conformation, we found in both cases a small percentage of a specific β‐γ cis isomer. Exchange rates extrapolated from 2D EXSY spectra under slow exchange conditions are in agreement with that estimated from methine 1H linewidths near coalescence and with previous photo‐isomerization studies. We could also confirm our previous hypothesis that, under aerobic conditions in combination with exposure to daylight, ICG undergoes radical dimerization specifically at the γ position.

Sterically Shielded Heptamethine Cyanine Dyes for Bioconjugation and High Performance Near‐Infrared Fluorescence Imaging

“…The model is consistent with literature X‐ray crystal structures showing the meso ‐aryl ring strongly rotated out of the plane of the polyene . Close inspection of the 1 H NMR spectra for s775z (Supporting Information, Figure S3) in water reveals the heptamethine proton coupling constants ( 3 J HH ) to all be 13.5 Hz indicating a polyene chain with an all‐ trans conformation . In addition, 1 H– 1 H NOE experiments (Figure S4) identified cross relaxation between indolenine gem‐dimethyl protons and polyene protons, as well as shielding chain protons, all consistent with an all‐ trans polyene .…”

“…The predominant reaction pathway forms a strained dioxetane intermediate followed by a fragmentation cascade. A possible second minor pathway is electron transfer from the polyene to singlet oxygen leading to a dimerized dye structure …”

Water-Soluble NIR Absorbing and Emitting Indolizine Cyanine and Indolizine Squaraine Dyes for Biological Imaging

“…The higher photostability of SO 3 SQ can be attributed, in part, to the squaraine structure being devoid of double-substituted alkenes, which are known to undergo various degradation pathways with ICG. 38 In MeOH, SO 3 SQ and ICG are exceptionally stable with half-lives >24 h (Figures S12−S15). SO 3 C5 is notably less stable than ICG or SO 3 SQ in MeOH where the dye appears to be deaggregated, which suggests that the aggregate states of SO 3 C5 in water could impart some photostability since this dye is more stable than ICG in water.…”

Sterically Shielded Heptamethine Cyanine Dyes for Bioconjugation and High Performance Near‐Infrared Fluorescence Imaging

“…[14,19] Close inspection of the 1 HNMR spectra for s775z (Supporting Information, Figure S3) in water reveals the heptamethine proton coupling constants ( 3 J HH )t oa ll be 13.5 Hz indicating ap olyene chain with an all-trans conformation. [20] In addition, 1 H-1 HN OE experiments ( Figure S4) identified cross relaxation between indolenine gem-dimethyl protons and polyene protons,a sw ell as shielding chain protons,all consistent with an all-trans polyene. [21] Finally,the chemical shifts for the heptamethine b-protons and indolenine gem-dimethyl groups in 756z and s775z are substantially upfield of the analogous peaks in related heptamethine structures that do not have a meso-aryl substituent (Figure S2), reflecting strong magnetic shielding of these diagnostic protons by the face of the rotated meso-aryl ring.…”

“…[4b, 8a-c] Thep redominant reaction pathway forms astrained dioxetane intermediate followed by afragmentation cascade.Apossible second minor pathway is electron transfer from the polyene to singlet oxygen leading to adimerized dye structure. [20] Shown in Figure 1a re the results of two separate photostability studies.T he first study irradiated four different cuvettes,e ach containing as olution of dye in PBS,w ith ax enon lamp (filtered to allow wavelengths > 620 nm) and monitored for ad ecrease in the dyesa bsorption maxima band ( Figure 1a,s ee Figures S15-S18 for the entire set of spectral plots and Table S3 for quantification). Theo rder of photostabilities was observed to be s775z > 756z > UL766 > Scheme 3.…”