Corrigendum: Synthesis and Molecular Properties of Acceptor-Substituted Squaraine Dyes.

Abstract

During recent research carried out in our laboratory directed toward improved near infrared (NIR) emitters1 we became aware of the fact that some of our previously reported fluorescence quantum yield values are incorrect. Accordingly, we re-investigated a significant number of the dyes (28a, 28b, 28d, 28e, 28f, 28g, 28i, 31a, 31b, 31c, 31d) described in this manuscript with our meanwhile improved instrumentation. These results are compiled in the corrected Table 4 and the relevant parameters for the applications of these dyes in fluorescence imaging applications are illustrated in the corrected Figures 7 and 8. UV/Vis/NIR absorption spectra (top) and the respective fluorescence spectra (bottom) of the acceptor-substituted squaraines 28a (blue), 28b (green), 28g (magenta), 28i (black), 30 (turquoise), 28j (red), 31a (ochre), 31c (purple), and 31d (brown); c=1×10−5 m; CH2Cl2; T=298 K. Fluorescence quantum yield (top) and optical brightness (bottom) for fluorophores from different dye classes in relation to their respective emission wavelength. The black line illustrates the empirical trend found for the decrease of the fluorescence quantum yields in the NIR region. All data for squaraines (black squares) originate from the present work. The other classes of dyes are rylene bisimides (open circles for perylene bisimides, open pentagons for terrylene bisimides), cyanine dyes (open triangles), BODIPY dyes (open squares) and PPCy dyes (open diamonds) whose values were taken from the literature. We like to note that meanwhile a corrected value for the fluorescence standard rhodamine 800 in ethanol has been suggested (Φfl=0.252) but that we decided to go on with the previously reported value to allow for a comparison of our old and new data for the re-investigated dyes among the 28 dye series whose values are reported relative to this fluorescence standard. Because none of the newly determined values severely deviates from the previously published ones we conclude that our original data for dyes 22a-22c, 28a-28j, 29a-c and 30 are with few notable deviations (i.e. fluorescence quantum yields for 28a, 28i) still valid. For those dyes whose fluorescence is located in the NIR range, i.e. dyes 31a-d, however, the fluorescence quantum yields are by more than an order of magnitude lower than reported originally. During our studies we also noticed that the commonly utilized NIR fluorescence standard indocyanine green in dimethylsulfoxide (Φfl=0.133) is rather unstable and should not be used for extended measurement times. For this reason values provided in Table 4 for 31a–d are now determined with an Edinburgh Instruments FLS980-D2D2-ST spectrometer with integrating sphere F-M01 as absolute fluorescence quantum yields and corrected for reabsorption.4 While the absolute method for Φfl determination suffers from lower sensitivity and intrinsic reabsorption, it enables a better choice for the selection of an appropriate excitation wavelength (λex) for narrow-band absorbers, which is a huge benefit for chromophores with low Φfl, especially in the NIR region, where the liquid-nitrogen-cooled NIR photomultiplier tube (PMT) exhibits intrinsically a much lower signal-to-noise ratio than common UV-Vis PMTs.5 Squaraine λmax/ nm ϵmax/ L mol−1 cm−1 λem/ nm Φfl/ % ϵmax⋅Φfl/ L mol−1 m−1 22a[b] 688 180 000 707 57 102 600 22b[b] 663 159 000 681 80 127 200 22c[b] 710 173 000 729 75 129 750 28a[b] 625 139 000 648 66 91 740 28b[b] 684 209 000 701 36 75 200 28c[b] 684 205 000 699 34 69 700 28d[b] 694 215 000 711 51 109 650 28e[b] 695 218 000 712 54 117 700 28f[b] 699 223 000 716 55 122 650 28g[b] 702 171 000 723 76 129 960 28h[b] 718 181 000 741 80 136 000 28i[b] 719 171 000 740 66 112 860 28j 762 199 000 29a[b] 712 214 000 722 48 102 720 29b[b] 730 166 000 747 76 126 160 29c[b] 786 202 000 820 10 20 200 30[c] 759 180 000 781 13 (10) 23 400 31a[d] 870 254 000 885 0.9 2 250 31b[d] 889 259 000 901 0.7 1 800 31c[d] 893 271 000 905 0.8 2 150 31d[d] 897 271 000 911 0.8 2 150 We regret the erroneous fluorescence quantum yields published in our earlier work and apologize for all inconvenience caused to other researchers when reporting new NIR fluorophores. With regard to the comparative Figure 8 we note that the corrected values for squaraine dyes are now located well within the range of expectation compared to other dyes. Accordingly, the suitability of this class of dyes for fluorescence applications in the wavelength range >700 nm has been confirmed. The authors thank Chia-An Shen for her efforts in the re-investigation of the absorption and fluorescence properties of the series of squaraine dyes.