Inactivation of Competitive Decay Channels Leads to Enhanced Coumarin Photochemistry

Abstract

In the development of photolabile protecting groups, it is of high interest to selectively modify photochemical properties with structural changes as simple as possible. In this work, knowledge of fluorophore optimization was adopted and used to design new coumarin- based photocages. Photolysis efficiency was selectively modulated by inactivating competitive decay channels, such as twisted intramolecular charge transfer (TICT) or hydrogen-bonding, and the photolytic release of the neurotransmitter serotonin was demonstrated. Structural modifications inspired by the fluorophore ATTO 390 led to a significant increase in the uncaging cross section that can be further improved by the simple addition of a double bond. Ultrafast transient absorption spectroscopy gave insights into the underlying solvent-dependent photophysical dynamics. The chromophores presented here are excellently suited as new photocages in the visible wavelength range due to their simple synthesis and their superior photochemical properties.

Keywords: ATTO 390; TICT; coumarin; fluorophore; photocage.

Figure 1

a) Selection of prominent coumarin based photocages in recent years and their reported absorption maxima. LG=leaving group. b) Structures and photophysical properties of DEACM 1 and ATTO 390.

Figure 2

a) Jablonski diagram showing relevant transitions of coumarin chromophores when excited with light. S₀: ground state, S₁: first electronically excited singlet state, S₂: second electronically excited singlet state, T: triplet state. b) General model for twisted intramolecular charge transfer (TICT) in coumarin dyes. In addition, possible hydrogen‐bonding sites are shown, that can affect non‐radiative decay.

Figure 3

a) Synthesis of compounds 7 a–d. i) TBDMS−Cl, imidazole, DCM, quant. ii) Yb(OTf)₃, acetone, 77 %. iii) iodoethane, Cs₂CO₃, MeCN, 78 %. iv) Pd/C, H₂, MeOH, 98 %. v) iodoethane, Cs₂CO₃, MeCN, 83 %. vi) TBAF, AcOH, THF, 89–94 %. vii) sodium diethyl oxalacetate, EtOH, 31–47 %,. viii) NaBH₄, MeOH, 37–62 %. TBDMS=tert‐butyldimethylsilyl. b) Evolution of new coumarin‐photocages with improved uncaging efficiency due to inhibition if intramolecular twisting. LG: leaving group, R: H/ethyl, TICT: twisted intramolecular charge transfer.

Figure 4

a) Steady‐state absorption and fluorescence spectra of compounds 7 a–d in PBS. Spectra of ATTO 390 (taken from ATTO‐tec) and compound 1 are added for comparison. b) Two‐photon absorption spectra of compounds 7 a–d. c) Stokes shifts in different solvents with different polarity.

Figure 5

Transient absorption spectra of compound 7 a in MeOH (a) and toluene (b). Time slices at 0.5 ps and 100 ps underline the dynamic stokes shifts in c) MeOH and d) toluene.

Figure 6

Synthesis of compounds 9 a–d and 11. i) CDI, DCM, 79–89 %. ii) 5‐Hydroxytryptamine hydrochloride, Et₃N, DMF, 56–77 %.

Figure 7

Exemplary photolysis curve of compound 9 b in PBS/MeOH (1 : 1). Photolysis product 7 b was identified via mass spectrometry.

Figure 8

Lifetime density analysis of 7 a (a) and 9 a (b) measured in MeOH. The ES decay is faster in compound 9 a (indicated as dashed lines), whereas the dynamics on the fs‐ to ps‐timescale do not differ. A similar pattern is seen in the TCSPC measurements (c). The fluorescence lifetimes of compounds 9 a–d are roughly 2‐fold faster. The corresponding lifetime components are given in the Supporting Information. d) Schematic representation of possible energy pathways. After excitation, an equilibrium between the locally excited state S₁ and the CT state is formed. The CT state is further stabilized by the solvent (indicated by double‐sided arrows), which increases the population of the CT state. Without this stabilization an additional channel for triplet formation becomes relevant, which then competes with the actual photochemistry.