Dedicated to Professor Rolf Huisgen on the occasion of his 85th birthday Intramolecular [2 2] photocycloaddition of a dimeric vinylpyridine 1 through a singlet-excited species efficiently formed the corresponding syn-and anti-pyridinophanes 2 and 3. The syn-isomers were elucidated spectroscopically and by X-ray crystallography as exo,syn-configured. The high selectivity under formation of exo,syn-2 was thoroughly investigated. Consequently, an exo-outstretching effect, which is observed around the periphery of a face-to-face-oriented system between two aromatic nuclei as a transition state, on cyclobutane ring formation was discovered for the first time.Introduction. ± Rolf Huisgen is one of the pioneers working on reaction mechanisms with the most-sophisticated methods and spectroscopic techniques available in the middle of the last century. His interest has covered reactions proceeding under non-catalytic, catalytic, thermal or photochemical conditions etc. 1 ). His careful and clear formulation of scientific arguments and statements have attracted many scientists through the generations. Moreover, in carrying out kinetics and mechanistic works, his scrutiny in the analysis of results is always understood by his coworkers, by suggesting to them not to ignore any discrepancy, tiny though it may be, which is believed to lead to some new significant discovery. In this paper, we are pleased to present some mechanistic aspects of photochemical reactions arising from a discrepancy observed at the preparative level.Vinylarenes such as styrene have attracted much interest from photochemical and photophysical points of view [2]. Since our discovery of the facile intramolecular [2 2] photocycloaddition of vinylarenes towards cyclophanes in 1986, some stereoselective product formation was often observed on the cyclobutane orientation, although almost all cases show the general selectivity such as exclusive cis-cyclobutane ring formation and selective syn-arrangement of the aromatic rings. In fact, several substrates like 1-vinylnaphthalene derivatives formed an exo-directed cyclobutane ring exclusively, because they have vinyl groups that are entirely exo-directed due to the severe steric interaction between peri-H-atoms and vinyl CH 2 H-atoms [3]. The selectivity can be rationalized by the so-called NEER 2 ) principle [4].