Solvent-driven adiabatic trans-to-cis photoisomerization of 4-styrylquinoline

Abstract

Direct one-stage photocyclization of trans-4-styrylquinoline to dihydrobenzo[i]phenanthridine (DHBP) in n-hexane with a quantum yield of 0.013 was observed. The kinetics of the photochemical transformations and an effect of the excitation intensity on the yield of DHBP were studied. In ethanol photocyclization proceeded in two stages with intermediate formation of the cis-isomer in the ground state. These facts imply a diabatic reaction pathway for trans-to-cis photoisomerization of 4-styrylquinoline in ethanol and an adiabatic pathway in hexane.

Introduction

Photoisomerization of diarylethylenes is known to occur by one-bond-flip mechanism in fluid solutions and by hula-twist mechanism in confined media [1], [2], [3]. One-bond-flip photoisomerization in turn can follow diabatic or adiabatic pathway [4], [5]. The first one means that an excited perpendicular conformer (p*), corresponding to the minimum at 90° on the lowest excited singlet or triplet potential energy surface (PES) along the double bond torsional coordinate, is a reaction intermediate (Fig. 1a). The radiationless deactivation to the ground state PES with a maximum (p) at 90° is followed by relaxation to either cis- or trans-isomer:

$$t+hv\to t^{*}\to p^{*}\to p\to \alpha c+(1-\alpha )t$$

$$c+hv\to c^{*}\to p^{*}\to p\to \beta c+(1-\beta )t$$

where α and β are partitioning factors, and structures of the p* conformers can differ on going from c* or t* side.

Adiabatic photoisomerization takes place fully on the excited state PES with the formation of one excited isomer from another. A bright example of adiabatic reaction is “one-way” cis-to-trans photoisomerization (Fig. 1b). Many styrylanthracenes and other diarylethylenes bearing large aromatic groups with low-energy local (aromatic) excited states undergo one-way photoisomerization [6]:

$$c+hv\to c^{*}\to t^{*}\to t$$

The less common case is an adiabatic process in the reverse direction,

$$t+hv\to t^{*}\to c^{*}\to c$$

because PES is generally descending from the excited cis-isomer to the trans-one.

To the best of our knowledge there are only three examples of the singlet state t* → c* isomerization of stilbenic compounds: adiabatic photoisomerization of trans-di(1-naphthyl)ethylene (1N1N) [7], trans-1-(1-naphthyl)-2-(2-naphthyl)ethylene (1N2N) [8] and trans-1-(1′-naphthyl)-2-(3′-hydroxyphenyl)ethene (1N3PhOH) [9]. A special case is adiabatic one-way t* → c* isomerization of some aza-diarylethenes where intramolecular hydrogen bonds may stabilize cis-isomer, inducing energy inversion of the t* and c* species, for example, in l-(2′-pyridyl),2-(2″-indolyl)ethene [10], [11].

The direct experimental evidence for the adiabatic photoisomerization is the observation of an excited isomer during excitation of another geometric isomer [4], [12]. Formation of a photocyclization product – dihydrophenanthrene analogue – upon excitation of the trans-isomer is also an evidence for the adiabatic t* → c* isomerization, since cyclization can proceed only via the excited cis-isomer.

Thus, despite being a competitive side reaction with respect to photoisomerization, photocyclization however can be used to examine the reaction mechanism – diabatic or adiabatic – of the photoisomerization reaction. Namely the fact of the photocyclization product – 4a,4b-dihydrobenzo[i]phenanthridine (DHBP) – formation upon excitation of the trans-isomer of 4-styrylquinoline we used in the present report to conclude about adiabatic mechanism of the styrylquinoline trans-to-cis photoisomerization.

In the literature there are somewhat controversial data about photochemical properties of 4-styrylquinoline (4SQ) and derivatives. Galiazzo et al. [13] have marked at a qualitative level that the cis-isomer of 4SQ displays low tendency to photocyclization in apolar solvents and does not cyclize at all in polar solvents. Bartocci et al. [14] have discussed in detail fluorescence and photoisomerization reaction, but said nothing about photocyclization. Wang and Ho have described spectral and fluorescent properties of 4-(4′-dimethylaminostyryl)quinoline saying nothing about its photochemistry [15]. Earlier we have investigated the effect of substituents in the styryl fragment of 4SQ and found that nitro derivative both isomerizes and cyclizes whereas amino group prevents photocyclization [16].

In the present paper we investigated the effect of solvent polarity on photochemistry of 4SQ. This compound was found to cyclize in both apolar and polar solvents, and upon direct excitation of the trans-isomer the photocyclization proceeds as one-step reaction in apolar solvent and as two-step reaction (via cis-isomer) in polar solvent, implying adiabatic reaction pathway for trans-to-cis photoisomerization in the first case and diabatic pathway in the second case.