Helmut Jahn, the German-born architect who designed One Liberty Place, the silvery blue skyscraper that gave Philadelphia a modern skyline and an updated image of itself as a city, died Saturday after being struck by two cars while biking west of Chicago. He was 81.

Mr. Jahn, who moved to Chicago in 1966 to study architecture, was already a rising star in the mid-’80s when he was approached by the Philadelphia development company Rouse & Associates and asked to design a marquee skyscraper that would exceed Philadelphia’s traditional height limit for tall buildings. At the time, the city maintained an unwritten “gentleman’s agreement” that obliged designers to defer to City Hall’s tower and keep their high-rises shorter than the statue of William Penn — a rule that had produced a dreary skyline of flat-topped, 40-story buildings.

Mr. Jahn’s design for a jaunty skyscraper, crowned with a neo-Deco cascade of chevrons, was crucial in helping Rouse & Associates — later called Liberty Property Trust — convince hesitant Philadelphia officials to abandon the height limit. When One Liberty Place was completed in 1988, the 945-foot-tall skyscraper at 17th and Market was twice as tall as any other building in the city. That dominance, together with its distinctive crown and spire, single-handedly reoriented the Philadelphia skyline, shifting the visual center from City Hall to the west.

Liberty Place’s design was immediately hailed as a triumph, both by Philadelphia Inquirer architecture critic Thomas Hine, and critics around the country. While Mr. Jahn built signature skyscrapers in many other cities, all in the same neo-Deco style, One Liberty Place is still considered his finest high-rise. It merges the “traditional, romantic imagery of the skyscraper with the sleekness of modernism,” the New York Times critic Paul Goldberger wrote in 1987.

Mr. Jahn, who was an avid athlete his entire life, was killed Saturday while cycling through the village of Campton Hills, about 55 miles west of Chicago. According to a report in the Chicago Tribune, which quoted local police officials, Mr. Jahn failed to stop at an intersection and was struck by two vehicles coming from opposite directions. He was pronounced dead at the scene, the Tribune reported.

Born outside Nuremberg, Mr. Jahn obtained his undergraduate architecture degree at the Technical University of Munich. In 1966, he moved to Chicago to study with his celebrated compatriot, Ludwig Mies van der Rohe, at the Illinois Institute of Architecture. One of the leading architectural Modernists, Mies was known for his crisp, unadorned, almost severe towers, most famously the Seagram Building in New York.

But Mr. Jahn would eventually depart from that Miesian rigor and join a group of Chicago architects who were experimenting with historical references and more elaborate shapes. In 1967, he joined forces with architect Charles F. Murphy to found a new firm called Murphy/Jahn. Mr. Jahn’s reputation soared with his 1985 design for a bulbous, glass state office building in Chicago, later named the James R. Thompson Center. Mr. Jahn used to joke that the overtly postmodern design was a hit everywhere but in Chicago.

Fond of sharp suits and even sharper quips, Mr. Jahn became the favorite of the architectural press. He was the subject of several newspaper and magazine profiles, including one in the Wall Street Journal that caught the attention of Joseph P. Denny, who worked with Willard G. Rouse 3d at Rouse & Associates. They were putting together an architectural competition for Liberty Place and decided to include Mr. Jahn among the four invited architects.

“Helmut sent in four models and I really loved them,” Denny said in an interview Sunday.

Mr. Jahn’s flamboyant style initially made Rouse a little nervous, Denny recalled. No one had ever designed a blue building before, he said. But both developers were impressed by the architect’s ability to turn out one sketch after another.

“He drew with a Montblanc fountain pen, in brown ink, and never stopped drawing,” Denny recalled. When he and Rouse flew to Chicago for their first meeting, Mr. Jahn had wallpapered his conference room with 20 different hand-drawn designs. At the same time, Denny liked that Mr. Jahn was “cognizant of the needs of developers.”

Coming from Chicago, a city known for its rich variety of skyscraper designs, Mr. Jahn was shocked that Philadelphia insisted on limiting the height of its tallest buildings. “It looks to me as if every building is chopped off with a knife. There is no city in the world where there are so many buildings of the same height,” he told Hine in a 1985 interview.

After exploring dozens of different proposals, Rouse and Denny settled on a design that was inspired by New York’s art deco Chrysler Building, one of the world’s most famous skyscrapers. But although One Liberty Place’s crown features a similarly staggered array of chevrons, its shaft is much thicker and bulkier, with none of the setbacks that give the Chrysler tower its slim elegance.

Denny and Rouse believed that girth was needed to satisfy tenants’ demands for large floor plates. But Hine wasn’t persuaded. “The trouble with Liberty Place is that it doesn’t look tall enough,” he wrote in his 1988 review.

Even before One Liberty Place was complete, Rouse & Associates decided to proceed with a second tower on the site. At first, things went well, Denny said. But after several disputes over design changes, Mr. Jahn was fired. Rouse & Associates found another architect to complete the project. That didn’t work out, either, Denny said, and the company eventually rehired Mr. Jahn.

Those internal disputes didn’t stop Philadelphians from cheering Liberty Place and its dramatic transformation of the city skyline. In the mid-1980s, Philadelphia was just coming off a sustained period of decline, and had lost nearly 13% of its population and more than 140,000 jobs.

Mr. Jahn’s soaring blue skyscraper, shimmering like Oz on the skyline, became a symbol of hope for Philadelphia, evidence that the city’s fortunes would recover.

Speaking at a banquet organized to celebrate the completion of the building’s structure, Mayor W. Wilson Goode could hardly contain his excitement. “This is a symbol of the future of the city,” he declared.