Synthesis and photophysical properties of chlorins bearing 0-4 distinct meso-substituents

Abstract

The presence of substituents at designated sites about the chlorin macrocycle can alter the spectral properties, a phenomenon that can be probed through synthesis. Prior syntheses have provided access to chlorins bearing distinct aryl substituents (individually or collectively) at the 5, 10, and 15-positions, but not the 20-position. A new Western half (5-phenyl-2,3,4,5-tetrahydro-1,3,3-trimethyldipyrrin) has been employed in condensation with an Eastern half (9-bromodipyrromethane-1-carboxaldehyde) followed by oxidative cyclization to give (5% yield) the zinc(II) 20-phenylchlorin. Condensation of the same Western half and a diaryl-substituted Eastern half provided (11% yield) the zinc(II) 5,10,20-triarylchlorin; demetalation with TFA followed by 15-bromination and Suzuki coupling gave the free base 5,10,15,20-tetraarylchlorin. Altogether, 10 new synthetic chlorins have been prepared. The near-UV (B) absorption band of the free base chlorins shifts bathochromically from 389 to 429 nm and that for the zinc chlorins from 398 to 420 nm as the number of meso-aryl rings is increased stepwise from 0-4. The long-wavelength (Q(y)) absorption band undergoes a bathochromic and hypochromic shift upon increase in number of meso-aryl groups. Regardless of the number and positions of the meso-aryl substituents (including "walking a phenyl group around the ring"), the respective fluorescence quantum yields (0.17 to 0.27) and singlet excited-state lifetimes (9.4 to 13.1 ns) are comparable among the free base chlorins and the same is true for the zinc chelates (0.057 to 0.080; 1.2 to 1.6 ns). Density functional theory calculations show that of the frontier molecular orbitals of the chlorin, the energy of the HOMO-1 is the most affected by meso-aryl substituents, undergoing progressive destabilization as the number of meso-aryl groups is increased. The availability of chlorins with 0-4 distinct meso-aryl substituents provides the individual stepping-stones to bridge the known unsubstituted chlorin and the meso-tetraarylchlorins.

De NovoSynthesis of Gem-Dialkyl Chlorophyll Analogues for Probing and Emulating Our Green World

“…In other words, meso-aryl groups to some extent render the chlorin more like a porphyrin. 294 This trend is observed in Figure 9. Accordingly, there is great value in the de novo synthesis of chlorins that lack the full complement of meso-aryl groups (as accrues upon derivatization of meso -tetraarylporphyrins).…”

“…A set of such structures is shown in Scheme 79. 294 The corresponding absorption spectra are shown in Figure 9. 294 The addition of each meso-aryl group causes a progressive bathochromic shift in the position of both bands but also (and perhaps surprisingly) a decrease in the relative intensity of long-wavelength absorption.…”

“…294 Reproduced by permission of The Royal Society of Chemistry (RSC) on behalf of the European Society for Photobiology, the European Photochemistry Association, and RSC (http://pubs.rsc.org/en/Content/ArticleLanding/2013/PP/c3pp50240f#!divAbstract). …”

Furan‐ and Thiophene‐Based Auxochromes Red‐shift Chlorin Absorptions and Enable Oxidative Chlorin Polymerizations

“…The effect of 10-substitution is small ( % 6nmr ed-shift for thiophene), comparable to the effect of a meso-Ph-group. [26] These differences are in line with the fact that the 3-and the 13-positionsl ie along one of the spectroscopica xes of the chlorins (Figure 1), thus, their substitutions have al argere ffect on the photophysical properties than a1 0-substitution. The simultaneous 3-thienyl, 13-formylthienyl substitution (Chl3 S 13 S-CHO )a ffords dramatically redshifted absorption (to 668 nm), andafluorescencee mission with am aximum at 680 nm.…”

Metal–Organic Covalent Network Chemical Vapor Deposition for Gas Separation

“…Despite the fact that the ZnTPC Soret band absorbs at longer wavelengths (i.e., 420 nm in n ‐hexane solution) than the Soret band of ZnTPP (i.e., 414 nm in n ‐hexane solution), the position of the Soret band of the MOCN coating is observed at a significantly shorter wavelength (i.e., 427 nm) than the evaporated ZnTPP (i.e., 437 nm) (Figure e). This 10 nm hypsochromic shift, which may also be slightly induced by the presence of new beta‐substituents to the ZnTPC formed during the iPECVD process, is indicative of a reduced stacking of the porphynoids. Indeed, the strong bathochromic shift (i.e., Δ λ = 23 nm) between the Soret band of the evaporated ZnTPP (i.e., 437 nm) and the one of ZnTPP in n ‐hexane solution (i.e., 414 nm) is indicative of a J‐type aggregation of the porpyhrins, i.e., π–π stacking driven by noncovalent interactions .…”